1 Introduction
Nitrogen is foundational to life and modern agriculture; its biogeochemical cycling is critical for maintaining ecosystem function and global food security. However, since the Industrial Revolution, anthropogenic activities have fundamentally reshaped the nitrogen cycle at a planetary scale.[1-3] Human interventions have more than doubled the global flux of reactive nitrogen entering the Earth system, severely disrupting natural balances. The excessive application of synthetic fertilizers, coupled with industrial emissions and domestic wastewater, has injected massive quantities of reactive nitrogen—primarily as nitrate (NO3−)—into aquatic environments. This influx drives widespread eutrophication, groundwater contamination, and ecosystem degradation worldwide.[4-8]
This environmental challenge is particularly acute in China, where NO3− contamination of surface water and groundwater has reached critical levels. In many northern regions—spanning urban centers like Beijing to agricultural hubs like the Chencang District of Baoji City—NO3− concentrations in water bodies routinely exceed the national drinking water standard (45 mg•L−1) by a factor of one to three. Such severe pollution urges the need for effective environmental remediation strategies.
Concurrently, modern society relies heavily on ammonia (NH3) as a vital fertilizer feedstock, chemical precursor, and emerging energy carrier; consequently, over 200 million tons of NH3 are synthesized annually via the Haber-Bosch process. This traditional industrial route requires extreme operating conditions (400—500 ℃, 150—300 atm), consumes approximately 1%—2% of the global energy supply, and emits roughly 2.86 tons of CO2 per ton of NH3 produced.[4,9-10] Ultimately, this presents a striking paradox: humanity expends massive amounts of energy to activate highly inert nitrogen gas (N2), only to expend further resources later to remove the resulting surplus of mobile, reactive nitrogen (such as NO3−) from the environment.
A more coherent, resource-efficient nitrogen economy would treat NO3− pollution as a feedstock. Converting ubiquitous nitrogen-containing contaminants directly into value-added nitrogen products via low-carbon processes couples remediation with resource recovery and reframes nitrogen management from extraction and end-of-pipe treatment to closed-loop, value-added recycling.[11]
Within this context, the electrochemical nitrate reduction reaction (eNO3RR) has gained momentum as a platform technology. Using renewable electricity, eNO3RR can selectively convert aqueous NO3− to NH3 or other nitrogen products under ambient conditions.[12] Relative to direct electrochemical nitrogen reduction (eNRR), NO3− reduction is thermodynamically more accessible: the N-O bond energy (≈204 kJ•mol−1) is far lower than the N≡N bond energy (≈941 kJ•mol−1), and the high solubility of NO3− in H2O supports efficient reactant delivery to catalytic interfaces.[13-14] Beyond fundamental advantages, eNO3RR also aligns naturally with distributed treatment infrastruc-tures that wastewater plants and agricultural runoff collection points could become modular, on-site NH3 production nodes, advancing both the green NH3 economy and H2O protection.[15]
Catalyst design is the central bottleneck. NO3−-to-NH3 conversion is a multi-step, proton-electron coupled process involving 8 electrons (e−) and 9 protons (H+) and a dense network of competing pathways that can yield NH3, N2, NO2−, and N2O.[14,16] Practical deployment, therefore, demands electrocatalysts that combine high activity, high NH3 selectivity (Faradaic efficiency, FE), durability, and tolerance to complex H2O matrices.[17] This need is especially acute under near-neutral conditions, which predominate in real H2O-treatment settings.[18] At neutral pH, the low nucleophilicity and weak binding affinity of NO3− often translate to diminished NH3 yield and selectivity,[19] intensifying the challenge of steering multi-step kinetics while suppressing side reactions. Achieving high activity, selectivity, and stability in neutral media is thus pivotal for translating eNO3RR from laboratory studies to field-relevant operation.
Metal-organic frameworks (MOFs), an emerging family of crystalline porous materials, offer an unusually versatile design space for this problem. Their ordered and tunable pore architectures can concentrate reactants and facilitate mass transport, while their high surface area[20] can expose abundant active sites.[21-25] Most importantly, MOFs enable atomic-level "programmability": by choosing metal nodes and organic linkers, one can construct well-defined active motifs spanning single atoms, bimetallic sites, and nanoclusters, while functionalizing pore surfaces to tune microenvironments and intermediate adsorption.[26-29] These attributes are particularly attractive for eNO3RR, where catalytic performance hinges on orchestrating a multi-step proton-electron transfer sequence—especially under neutral conditions where H+ availability, interfacial H2O activation, and suppression of competing pathways become decisive.
Accordingly, this review summarizes recent progress, design strategies, and mechanistic understanding of MOF-based materials for eNO3RR in neutral electrolytes. The fundamentals of eNO3RR were first outlined, including reaction pathways and performance metrics. MOF-based electrocatalysts were classified into pristine MOFs, MOF composites, MOF-derived materials, and MOF-based single-atom catalysts to discuss the structure-activity relationships governing activity, selectivity, and stability. Finally, we distill key challenges and propose research directions to enable efficient, robust NO3− conversion and practical resource-recovery technologies.
2 Reaction Mechanism and Influencing Factors of eNO3RR under Neutral Conditions
eNO3RR proceeds through multiple pathways and can produce diverse products; its kinetics and selectivity are strongly modulated by electrolyte pH. Neutral conditions (pH≈7) best reflect natural H2O and many industrial wastewater streams, and they offer practical advantages (safer operation, lower corrosivity, broader compatibility). At the same time, neutral media impose distinct constraints, including limited H+ availability, lower NH3 yield and selectivity, competition with hydrogen evolution (HER), and stability challenges for many catalysts. This section focuses on the mechanistic features of neutral eNO3RR and the factors that most strongly shape performance.
2.1 Reaction Mechanism and Pathways under Neutral Conditions
Overall reduction of NO3− to NH3 requires 8 e- and 9 H+ (Eq. 1). Compared with eNRR, eNO3RR has a wider reaction potential window (Figure 1a). In neutral electrolytes, H+ is supplied predominantly through H2O rather than abundant free H+, shifting both the thermodynamics and kinetics of proton-coupled electron transfer (PCET) compared with strongly acidic media.[30]
The conversion of NO3− to NH3 proceeds through a sequence of deoxygenation and hydrogenation steps, which the following reactions can describe:
A widely accepted mechanism divides the conversion of NO3− to NH3 into two coupled stages: (i) the stepwise deoxygenation of NO3− and (ii) the subsequent hydrogenation of adsorbed nitrogen-oxygen (*N-O) intermediates. Initially, NO3− adsorbs onto the catalyst surface and undergoes reduction via the NO3−→NO2−→*NO sequence.[31-32] In acidic media, this initial NO3−-to-NO2− transition is frequently identified as the rate-determining step (RDS) (Eq. 3).[33-34] The intermediate *NO2− is then further deoxygenated to *NO (Eq. 4). Thereafter, the adsorbed *NO undergoes successive hydrogenation—passing through intermediates such as *NOH and *NH2OH—to ultimately yield NH3 (Figure 1b).
However, under neutral conditions, the reaction dynamics shift fundamentally. Unlike acidic electrolytes, where free H+ is abundant, neutral media rely on H2O dissociation (the Volmer step: H2O+e-→*H+OH-) as the primary H+ source. This sluggish interfacial H+ supply significantly increases the energy barrier for the *NO→*NOH conversion. Consequently, the hydrogenation of *NO (Eq. 5) becomes kinetically limiting, shifting the RDS from the initial deoxygenation phase to the subsequent hydrogenation step, which may also reroute selectivity among competing reaction pathways.[35] This mechanistic shift is corroborated by recent density functional theory (DFT) calculations, which demonstrate a marked increase in the free-energy barrier for *NO hydrogenation under H+-limited conditions (see Section 2.5 for details). The constrained H+ availability in neutral media can also disfavor rapid hydrogenation and, in some cases, increase the kinetic competitiveness of N-N coupling among NO-derived intermediates to form N2O or N2.[36] At the same time, neutral conditions may promote the accumulation of NO2− either on the electrode surface or in solution. This is problematic because NO2− is substantially more toxic than NO3− and can be slow to reduce further under many conditions.[37] Thus, accelerating *NO2− conversion—while maintaining controlled hydrogenation of *NO—is central to achieving high NH3 selectivity and high FE in neutral eNO3RR.
In essence, neutral eNO3RR is governed by a delicate choreography between deoxygenation and H2O-mediated hydrogenation, making catalyst control over *NO2− turnover and *NO hydrogenation the decisive lever for NH3 selectivity. The mechanistic understanding above mainly relies on traditional electrochemical analyses and theoretical calculations. However, recent in situ/operando studies have shown that MOF-based catalysts can undergo dynamic changes during reactions, and their actual active sites are different from those in the original structure. This will be discussed in detail in Section 3.9.
2.2 Key Influencing Factors Governing Neutral eNO3RR
In neutral eNO3RR, the fundamental tension is between enabling sufficient H+/e− delivery for NO3− hydrogenation and suppressing HER.[40] Although neutral conditions often raise the HER overpotential and widen the thermodynamic window available for NO3− reduction,[31] many catalytic surfaces still sustain appreciable HER currents, eroding NH3 selectivity. Rational catalyst design, therefore, must simultaneously (i) suppress surface active hydrogen (*H) formation and/or *H recombination, and (ii) promote H2O activation and the hydrogenation of oxygenated nitrogen intermediates.
Representative strategies include: (1) tuning the generation-consumption balance of *H;[31] (2) controlling local H+ accessibility and interfacial H2O structure via ligand engineering (e.g., introducing carboxyl groups);[41] and (3) modulating the electronic structure of active sites through doping and defect engineering to lower barriers along the NO3−-reduction pathway while weakening H adsorption to suppress HER.[42]
Ultimately, the best neutral-pH catalysts are those that treat H2O not as a passive solvent but as an actively managed H+ reservoir—activating it when needed, while keeping HER kinetically disfavored.
2.3 Effects of pH on Selectivity, Faradaic Efficiency, and Catalyst Stability
Electrolyte pH regulates eNO3RR by shaping reactant/intermediate speciation, interfacial charge, and H+ availability, thereby impacting selectivity, FE, and catalyst durability.
Selectivity. In strongly acidic media, abundant H+ can accelerate hydrogenation but often intensifies HER. In strongly alkaline media, high OH− can impede NO3− adsorption and may promote catalyst oxidation or surface reconstruction.[43] Neutral conditions provide a practical compromise: they can support NO3− adsorption (often favored by moderately positive surface charge) while avoiding the extremes that drive HER or oxidative degradation.[44] For MOFs, adjusting the isoelectric point via ligand engineering can render frameworks positively charged at neutral pH, strengthening electrostatic attraction toward anionic NO3- and enhancing interfacial reactant enrichment.[45]
Faradaic efficiency. Achieving high FE at neutral pH requires simultaneous suppression of HER and avoidance of NO2− accumulation. MOFs with well-defined active motifs—such as single-atom M-N4 sites[46] or synergistic bimetallic site[47]—can tune adsorption energetics to lower the barrier for *NO hydrogenation while weakening H binding, thereby preserving NO3−-reduction selectivity across broad potential windows.
Stability. Neutral aqueous media can be both protective and erosive for MOFs. They avoid the severe corrosion characteristic of strong acids/bases, yet dissolved O2/CO2 and reactive intermediates can drive gradual oxidation, ligand exchange, or coordination erosion over time.[48] Robust MOFs—e.g., frameworks built from stable Zr6 clusters and chemically resilient linkers such as phenazine-based motifs—are therefore essential for sustained operation in hydrated neutral environments.[49-50]
Overall, efficient neutral eNO3RR demands catalyst design at multiple scales: atomic-level active sites that balance NO3− activation with H2O dissociation; materials-level conductivity and structural robustness; and system-level reactor optimization to mitigate mass-transfer limitations. These requirements naturally motivate MOF designs featuring well-defined catalytic centers, effective H+/H2O management, high chemical stability, and hierarchical porosity. Thus, pH is not merely an operating parameter; under neutral conditions, it defines the mechanistic bottlenecks, the selectivity landscape, and the durability constraints that MOF design must satisfy simultaneously.
2.4 Comparative Analysis of Acidic, Neutral, and Alkaline Conditions
As outlined in Section 2.3, electrolyte pH profoundly influences selectivity, FE, and overall stability of the eNO3RR. It does so by actively regulating reactant speciation, interfacial charge, and H+ supply dynamics. The pH not only dictates the source and availability of H+—relying on abundant free H+ in acidic media versus H2O dissociation in neutral or alkaline environments—but it also modulates the adsorption behavior of reaction intermediates, the severity of the competing HER, and the surface chemical state of the catalyst.[51]
Recent studies demonstrate that through rational structural design, catalysts can achieve excellent NO3- reduction performance across a broad pH spectrum.[52] Drawing on insights from both non-MOF model catalysts and emerging MOF systems, we compare reaction characteristics and material optimization strategies under acidic, neutral, and alkaline conditions. Ultimately, this comparison provides a comprehensive, cross-pH perspective to guide the future design of highly efficient MOF-based catalysts.
2.4.1 Acidic Conditions
In acidic environments, an abundant H+ supply facilitates rapid reaction kinetics but also renders the competing HER highly aggressive.[53] At low pH, undissociated HNO3 emerges as the dominant active species, dictating a reduction pathway distinct from that of the NO3- anion. For instance, electrochemical studies reveal that Pd-modified Cu electrodes exhibit prominent NO3- reduction peaks in 0.1 mol•L−1 HClO4, whereas this activity is markedly suppressed under alkaline conditions.[54] Consequently, the primary catalyst design strategy in acidic media centers on suppressing HER while maintaining high NO3- conversion activity. Noble-metal catalysts (such as Pt, Pd, Rh, and their alloys) are frequently employed to steer the reaction pathway, leveraging their strong binding affinity for nitrogenous intermediates.[54] Alternatively, catalytic performance can be optimized by tuning the electronic structure and lattice strain via heteroatom doping. A notable example is Ru-doped TiO2 nanoarrays, which deliver outstanding acidic eNO3RR performance across a broad NO3- concentration range of 0.1—6 mol•L−1.[53]
While acidic conditions enable high reaction rates and favorably yield NH4+ salts[53]—which are highly convenient for direct utilization as fertilizer—they also present significant drawbacks. The harsh acidic environment poses a severe threat to the chemical stability of the catalysts. MOF materials are particularly vulnerable in these regimes, as they are prone to structural degradation via node protonation or ligand hydrolysis, fundamentally limiting their direct application in low-pH electrolytes.
2.4.2 Alkaline Conditions
Under alkaline conditions, the competing HER is significantly suppressed. However, high-pH environments introduce new challenges: abundant OH- ions compete with NO3- for active sites, diminishing NO3- surface coverage. Simultaneously, the severe proton deficiency dictates that NO3--to-NH3 conversion must rely on sluggish water dissociation to generate*H.[55] Consequently, NO3- reduction is kinetically hindered in alkaline media compared to acidic or neutral regimes. To overcome these barriers, bimetallic synergistic catalysis is frequently employed. For example, in Cu-GO@NF systems, Cu provides NO3- adsorption sites while Ni facilitates H2O dissociation.[55] Alternatively, modulating the electronic structure via halogen coordination can accelerate the generation and desorption of *H.[56]
Despite achieving industrial-scale current densities (>2 A•cm-2) and robust catalytic stability,[56] alkaline electrolytes remain fundamentally incompatible with the natural pH of most real-world water bodies. The requisite pH pretreatment significantly increases process complexity and operational costs. Furthermore, as Ho et al.[57] demonstrated, harsh alkaline conditions can induce catalyst corrosion, structural degradation, and severe fouling, ultimately restricting their practical deployment.
2.4.3 Neutral Conditions
Under neutral conditions, protons are predominantly sourced from H2O dissociation rather than free H+, which severely restricts the kinetics of PCET steps. Consequently, overall reaction kinetics are sluggish, increasing the risk of toxic NO2- accumulation. Furthermore, the long-term stability of many MOF materials in these aqueous environments remains a significant challenge that requires further optimization. Despite these kinetic and stability challenges, neutral electrolytes closely mirror the pH of most natural and polluted water bodies. Therefore, mastering neutral-pH eNO3RR represents a critical frontier that bridges fundamental laboratory research and practical, real-world environmental remediation.[57] To navigate these complexities, catalyst design for neutral conditions must simultaneously satisfy four stringent criteria: (1) Promote H2O activation to efficiently supply *H species; (2) Accelerate *NO2- conversion to prevent the accumulation of toxic intermediates; (3) Suppress the competing HER to maintain high selectivity; (4) Ensure long-term structural durability in real-world water matrices.
Representative structural optimization strategies that address these criteria—including single-atom anchoring, bimetallic synergy, conductive composite fabrication, and derivative engineering—are discussed in detail in the following section.
2.5 Theoretical Insights from DFT Calculations in eNO3RR
DFT calculations have emerged as indispensable tools for unraveling the complex reaction network of the eNO3RR and guiding rational catalyst design. By evaluating the free-energy changes of elementary steps, DFT identifies rate-determining steps, elucidates pH-dependent kinetics, and establishes reliable descriptors for catalytic performance. Recently, the role of DFT in eNO3RR research has evolved from post-hoc experimental rationalization to proactive performance prediction. This paradigm shift provides a robust theoretical framework for analyzing the in situ dynamic behavior and driving the targeted structural optimization of MOF-based catalysts.[58-59]
2.5.1 Elucidating Reaction Pathways and Rate-Determining Steps
The core value of DFT calculations lies in quantifying the Gibbs free energy change (ΔG) for each elementary step within a multi-step reaction network. This allows researchers to pinpoint the transition state with the highest energy barrier—namely, RDS.[60] Because eNO3RR is a highly complex process involving 8 e− and 9 H+, proposed reaction pathways often vary across studies. DFT provides the theoretical rigor necessary to resolve these controversies and identify the dominant pathway.
For instance, Guo et al.[61] utilized systematic DFT calculations to evaluate the complete NO3--to-NH3 conversion pathway across three low-index Cu facets: Cu(111), Cu(100), and Cu(110). Both thermodynamic and kinetic analyses supported the following optimal pathway: NO3- → *NO3→*NO2→*NO→*NOH→*NHOH→*NH→*NH2→ *NH3→NH3(g). Crucially, this established the *NO→*NOH transition as the pivotal hydrogenation step.
Furthermore, DFT elucidates the significant influence of electrolyte pH on the RDS. Under proton-limited neutral conditions, the free-energy barrier for the *NO→*NOH step increases markedly, effectively becoming the kinetic bottleneck. Similarly, Zhang et al.[58] constructed Gibbs free energy thermodynamic phase diagrams for surface species on RuCo alloy catalysts, deducing that the overpotential-determining step (PDS) is the *NO-to-*N transformation. Their calculations revealed that if N-O bond cleavage relies entirely on an electrochemical process, the required overpotential would far exceed that observed experimentally. This theoretical prediction directly validates the necessity of a coupled catalytic/electrocatalytic relay mechanism.
Ultimately, DFT can also decode how crystal facet engineering regulates the RDS and identifies key reaction intermediates in MOF systems.[47,62] Consequently, these computational methods have solidified their role as an essential tool for unraveling complex eNO3RR reaction pathways, identifying RDS and PDS, interpreting pH-dependent reaction kinetics, and validating synergistic catalytic mechanisms.
2.5.2 Establishing Activity Descriptors for Catalyst Screening
Another core value of DFT calculations is that they can extract key physical quantities as descriptors from complex reaction networks, establish quantitative correlations between computation and experiment, and thereby enable high-throughput screening and performance prediction of catalysts. For eNO3RR, the most representative descriptors mainly include Gibbs free energy of nitrate adsorption energy (∆G*NO3), Gibbs free energy of hydrogen adsorption (∆G*H), and limiting potential (UL).
∆G*NO3 is the primary descriptor governing reaction initiation. Through DFT calculations on transition metals, Liu et al.[63] demonstrated that the adsorption strengths of O and N atoms can serve as reliable descriptors for the overall activity and selectivity of electrocatalysts toward NO3- reduction. Extremely weak NO3- adsorption hinders reactant activation, whereas excessively strong adsorption can lead to active-site poisoning and inhibit subsequent reaction steps. Building on this thermodynamic framework, researchers constructed a volcano-type plot for NO3- reduction activity, successfully predicting the high activity and selectivity of alloy catalysts such as Fe3Ru and Fe3Ni.[63] Furthermore, Karamad et al.[64] elucidated why Cu exhibits the most outstanding NO3- reduction activity among ten evaluated transition metals, noting that its moderate adsorption strength positions it precisely near the apex of the volcano curve.
∆G*H correlates directly with the competitive dynamic between the eNO3RR and HER. Under neutral conditions characterized by a limited H+ supply, the binding affinity of the catalyst for *H dictates the bifurcation of the reaction pathway. If ∆G*H is near zero, the catalyst surface strongly favors the HER, unnecessarily consuming e- and occupying active sites required for NO3- reduction. Conversely, if ∆G*H deviates appropriately from zero, the HER can be effectively suppressed, redirecting *H toward the hydrogenation of nitrogenous intermediates.[59,65] In their investigation of single-atom catalysts, Wu et al.[65] established a linear correlation between ∆G*NO3 and the d-band center, revealing how symmetry breaking—induced by heteroatom doping—modulates the adsorption behavior of intermediates by tuning the orbital splitting energy. This insight provides a foundational theoretical basis for regulating HER competition through deliberate ligand engineering.
UL serves as a comprehensive thermodynamic descriptor reflecting the intrinsic activity of a catalyst. It is defined as the potential corresponding to the maximum free-energy increase among all elementary steps within the reaction pathway. A more positive UL, corresponding to a smaller negative value, indicates that the catalyst can drive the reaction at a lower overpotential, thereby achieving superior energy efficiency.[66] For example, Chen et al.[67] systematically screened diatomic catalysts anchored on biphenylene and identified five distinct catalysts, including Mo2, Ru2, and Rh2, that exhibited limiting potentials ranging from −0.40 V to −0.16 V, demonstrating exceptional intrinsic catalytic activity.
2.5.3 Understanding pH-Dependent Reaction Energetics
The pH of the electrolyte not only determines the form of the H+ source but also profoundly affects the thermodynamic driving force and kinetic energy barrier of each elementary step in eNO3RR. The integration of DFT with the computational hydrogen electrode (CHE) model has enabled the resolution of pH-dependent reaction energetics at the atomic scale, providing a robust theoretical tool for understanding reaction bottlenecks, particularly under neutral conditions.
The fundamental principle of the CHE model is to relate the chemical potentials of protons and e− to gaseous H2 under standard hydrogen electrode (SHE) conditions. Within this framework, the free-energy change of any electrochemical PCET step (*A+H++e-→*AH) can be expressed as ΔG =ΔG⁰+kT·ln[H+]−eU, where ΔG⁰ is the ground-state energy calculated from the gas-phase mechanism, the [H+] term directly introduces pH dependence, and U represents the applied potential.[68] This mathematical treatment enables DFT to simulate reaction free-energy profiles across a range of pH values and accurately predict the pH-dependent behavior of UL.
Applying this methodology, Kattel et al.[51] systematically revealed the regulatory effect of pH on the reaction pathway in their investigation of Fe-N-C single-atom catalysts. Their DFT calculations demonstrated that the most favorable pathway for NO3- reduction to NH3 on the Fe-N-C surface is the *NHO-mediated route, and the free-energy changes associated with this pathway exhibit strong pH dependence. Crucially, the researchers discovered that the PDS for both eNO3RR and the competing HER vary with pH, demonstrating distinct pH-dependent trends. This computational finding directly elucidates the experimental observation that HER competition is most intense under alkaline conditions, whereas the eNO3RR maintains higher selectivity in acidic and neutral environments. Ultimately, combining DFT with the CHE model and advanced interface-simulation methods enables researchers to gain a comprehensive understanding of how pH regulates reaction pathways, rate-determining steps, and overall catalyst stability at the atomic level.
2.5.4 Guiding Rational Design of Ligand and Metal-Node Engineering
The most central application value of DFT calculations lies in their predictive power. By establishing clear structure-activity relationships, DFT provides verifiable theoretical hypotheses for experimental synthesis, thereby guiding the targeted design of catalysts. The rational screening of metal nodes represents the most direct manifestation of DFT-guided catalyst design. By calculating intrinsic parameters, such as the d-band center and the adsorption energy of different metal centers, theoretical simulations can predict optimal active components before experimental synthesis. For example, Yu et al.[69] identified four highly promising catalysts—including Nb-N4-TEP and Mo-N3C1-TEP—from 56 distinct TM-N4/N3C1-TEP configurations through machine-learning-assisted, high-throughput DFT screening. A SHAP analysis revealed that the cross descriptor (ϕ1), adsorbate electronegativity (ϕ2), and transition metal charge (QTM) are the key factors governing the stable adsorption of NO3−. This methodology provides a powerful paradigm for establishing interpretable catalytic models.
Electronic effect modulation via ligand engineering represents another core dimension of DFT-guided MOF design. Substituents on the ligand can tune the e− density of metal centers through inductive and conjugative effects, thereby influencing the adsorption strength of key reaction intermediates. DFT calculations successfully quantify both the transfer pathway and the magnitude of these electronic effects, allowing researchers to predict how catalytic performance will trend following different functional group modifications.
In a study of photocatalytic NO3- reduction over Cu-doped NH2-MIL-125, DFT calculations elucidated the deep-tuning mechanism by which the ligand environment modulates the electronic structure of the metal centers.[70] Theoretical simulations indicated that Cu2+ doping induced directional charge transfer within the Ti-O-Cu structure, significantly enhancing the electron density at the Cu active sites. More importantly, combining DFT calculations with temperature-programmed desorption (TPD) experiments confirmed that this doping induced p-d orbital hybridization between the Cu 3d orbitals and the O 2p orbitals of the NO3− intermediate. This orbital interaction promoted the formation of a highly stable bidentate adsorption configuration of NO3− on Cu sites, thereby lowering the activation energy barrier for key reaction intermediates. This work clearly demonstrates how DFT calculations, by resolving orbital interactions, provide atomic-scale theoretical insights into the electronic effects of ligand modification.
In summary, by establishing quantitative correlations among ligand electronic structure, metal center charge density, and catalytic performance, DFT calculations are driving the design paradigm for MOF-based catalysts away from empirical trial-and-error and toward precise, atomic-level construction. As this methodology continues to mature, the deep integration of machine learning and high-throughput computing with this theory-guided research mode is expected to drastically accelerate the discovery of novel MOF electrocatalysts.
2.6 Evaluation Framework for eNO3RR Catalysts
Following the execution of catalytic experiments, the catalyst's performance is rigorously evaluated by analyzing key parameters, such as the composition of reaction products and the evolution of the catalytic state. This analytical phase is crucial for elucidating the underlying reaction mechanisms and providing a theoretical foundation for the design of more efficient catalysts. The most common evaluation indicators employed in this context include FE, selectivity, stability, turnover frequency, and partial current density.
FENH3 represents the percentage of the total electric charge passed through the electrode that is specifically used to produce NH3. It serves as a direct measure of e− utilization efficiency and is calculated using Eq. 10:[71]
In this equation, n represents the eight e− required to generate a single NH3 molecule, CNH3 is the measured molar concentration of NH3, V is the electrolyte volume in the cathode compartment, F is the Faraday constant (96485 C• mol−1), and Q denotes the total electric charge passed through the circuit.
While FENH3 reflects the e− distribution, chemical selectivity provides insight into the distribution of the consumed reactants. For eNO3RR, researchers must account for the selectivity of the target product NH3 as well as byproducts such as NO2− and various gaseous species, including N2, N2O, and NO. The selectivity for NH3 (FENH3) is defined as the ratio of the amount of NH3 produced to the total amount of NO3− consumed during the process, Eq. 11:[71]
Here, C0 (mol•L−1) and ${C}_{{\mathrm{N}\mathrm{O}}_{3}^{-}}$ (mol•L−1) represent the initial and post-electrolysis concentrations of NO3−, respectively. This distinction is vital because a catalyst may exhibit high e− efficiency for a specific pathway yet still suffer from poor reactant conversion.
The stability of a catalyst refers to its ability to maintain high activity, selectivity, and structural integrity under prolonged electrochemical operation. In techniques such as chronoamperometry or chronopotentiometry, stability is typically assessed by monitoring the retention of the FE over time.[72-73] Additionally, CV stability is evaluated by observing the degree of shift in polarization curves after thousands of potential scans. Beyond electrochemical metrics, the material's structural stability is verified by post-reaction characterization using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microscopy (SEM/TEM).[74-77] These comparisons allow researchers to determine whether the catalyst has undergone a crystal-structure collapse, changes in metal valence states, morphological modifications, or the leaching of active components.
Turnover frequency (TOF) is arguably the most direct indicator of catalytic efficiency at the individual active-site level, as it quantifies the number of NH3 molecules generated per active site per unit time, typically expressed in s−1 or h−1. By eliminating the variables of specific surface area and catalyst loading, TOF truly reflects the material's intrinsic catalytic capability.[78] In MOF catalysts, where the structure of the active sites is well-defined, TOF values enable researchers to directly correlate catalytic performance with specific coordination environments and electronic structures. The TOF is generally calculated as follows, Eq. 12:[71,79]
In this formula, nM (mol) represents the molar quantity of the metal element on the electrode, determined by the metal content and the total catalyst loading, while t represents the duration of the reaction.
Additionally, the partial current density (jNH3) measures the portion of the total current used for NH3 production per unit of geometric electrode area (Eq. 13). It serves as a metric for the intrinsic activity of the catalyst, indicating how rapidly the system can convert NO3− to NH3 within a specific spatial footprint:
In this expression, i is the total electrolysis current and A is the geometric area of the cathode. A high jNH3 is a critical prerequisite for industrial applications, as it is directly tied to achieving the high space-time yields required for large-scale NH3 production.[80]
3 Catalytic Performance and Design Strategies of MOFs in Neutral eNO3RR
The performance of eNO3RR catalysts under neutral conditions depends critically on how effectively they balance a set of competing requirements: strong enough NO3− adsorption, efficient multi-e−/H+ transfer, suppression of HER, and structural robustness in aqueous media. MOFs are uniquely positioned to address these demands because their metal nodes, organic linkers, and pore environments can be tailored almost independently of one another. In this section, we discuss how different MOF design strategies translate into improved NO3−-to-NH3 activity and selectivity in neutral electrolytes.
3.1 Single-Metal MOF Catalysts
Single-metal MOFs contain a single type of metal ion or cluster as the catalytic center. Their structural clarity and compositional simplicity make them valuable model platforms for probing structure-activity relationships and interrogating neutral eNO3RR mechanisms.[81] By tuning the coordination environment and spatial arrangement of metal nodes, one can modulate the kinetics of NO3− adsorption, activation, and hydrogenation.[44] Current optimization strategies for single-metal MOFs in neutral eNO3RR can be grouped into four themes.
3.1.1 Single-Atom Anchoring
To rigorously analyze single-metal MOF catalysts, it is essential to distinguish between conventional single-metal MOFs and MOF-based single-atom catalysts, as these two classes possess fundamentally distinct structural ori- gins.[82-83] In conventional single-metal MOFs—such as Cu-BDC and Zn-MOF—the catalytically active centers derive directly from the intrinsic metal nodes or clusters that constitute the periodic framework.[81,84] Conversely, MOF-based single-atom catalysts utilize the highly ordered, porous MOF architecture as an anchoring scaffold, in which isolated heterometal atoms are introduced and stabilized via post-synthetic modification. In these systems, the MOF acts as a structurally tunable support, while the atomically dispersed metals serve as the primary catalytic centers.[83] This synergy integrates the high crystallinity of MOFs with the maximum atom-utilization efficiency of single-atom catalysts, providing a unique materials platform for the precise tailoring of coordination environments and electronic structures under neutral conditions.
The primary advantage of the single-atom anchoring strategy is that, by immobilizing isolated metal atoms on organic ligands or at node defects, catalysts can achieve nearly 100% metal-atom utilization while maintaining uniform, well-defined active sites.[85-87] Within the context of neutral eNO3RR, these materials effectively modulate the strength of NO3− adsorption, lower the hydrogenation energy barriers for key intermediates, and suppress the competing HER.[82,85] By leveraging the architectural precision of the MOF host, researchers can engineer specific electronic states that favor the 8-e− reduction pathway to NH3 over the 2-e− pathway to H2, thus significantly enhancing the FE of the process.
A recent example is Zhang and co-workers’ bipyridine-anchored copper single-atom catalyst (Cu-SA/UiO-bpy), identified as an optimal candidate through combined DFT screening and experimental validation[59] (Figure 2a). Using a stable Zr-based UiO-bpy scaffold, Cu atoms were installed on bipyridine N sites via post-synthetic metalation, yielding a well-defined single-atom system (Figure 2b). Under neutral conditions, the catalyst delivered a FENH3 of 98.1% and an NH3 yield of 7.4 mg•h−1•cm−2, while maintaining activity and stability over a wide pH range. Mechanistically, the [Cubpy] sites were proposed to promote interfacial H2O dissociation, generating reactive hydrogen species that drive the selective hydrogenation of NO3−-derived intermediates.
Figure 2 (a) Limiting potentials for NO3RR and HER illustrating the NO3RR selectivity of TM-SAs/UiO-bpy. (b) Schematic illustration of the synthesis of Cu-SA/UiO-bpy samples. Reproduced with permission,[59] Copyright 2025, Wiley-VCH. (c) View of the structure of the cluster node in UiO-66-Cu incorporating a single Cu(II) center at the defect site. Color scheme: Cu, orange; O, red; C, grey; H, white. (d) Gibbs free energy diagram of reaction coordinates for conversion of NO3− to NH3 over Cu-, Fe- and Mn-SAC. Reproduced with permission,[88] Copyright 2024, Springer |
In 2024, Yang, Martin, and co-workers exploited intrinsic OH/OH2 defect sites in UiO-66 as coordination anchors to construct a family of single-atom catalysts (M-SACs; M=Mn, Fe, Co, Ni, Cu, Zn, Mo) for NO3−-to-NH3 conversion[88] (Figure 2c). The approach enabled high dispersion and stable immobilization of metal atoms. Among the series, CuSAC and FeSAC delivered particularly high performance, with reported NH3 yields of 30.0 and 29.0 mg•h−1•cm−2 and FENH3 exceeding 96% at −1.0 V vs. RHE, underscoring the ability of isolated metal centers to stabilize key intermediates and reduce energetic barriers along the reaction network (Figure 2d).
In summary, the core design principle of MOF-based single-atom catalysts is to precisely regulate the coordination environment at anchoring sites, enabling the active centers to simultaneously bind NO3−-derived intermediates effectively and modulate the H2O activation process. In this way, the structural advantages of atomic dispersion are translated into precise control over the reaction pathway.
3.1.2 Metal Cluster Design
Metal clusters, spanning several to dozens of atoms, occupy a regime between single atoms and nanoparticles. Their distinctive geometries, quantum-size effects, and tunable electronic structures can create dense interfacial active sites and facilitate multi-e− processes.[89-90] Compared with single atoms, intracluster metal-metal interactions can reshape adsorption energetics via orbital hybridization, while delocalized electronic states can support multi-e− transfer.[91-92] Compared with larger nanoparticles, atomically precise clusters offer higher atomic efficiency and more uniform active sites; their size compatibility with MOF pores also favors spatial confinement and stable anchoring.[93-94] In 2025, Zuo et al.[89] assembled highly symmetric coordination metal-organic polymers (MOPs) from square-planar tetranuclear clusters [M4(μ4-O)(CO2)8] (M=Mn, Fe, Co) and TATB linkers (Figure 3a), leveraging the square-planar geometry to increase site density, reduce steric constraints, and improve transport. Co-TATB achieved ca. 98% FENH3 across a broad potential range and maintained strong stability at industrially relevant current densities in a flow cell.
Figure 3 (a) Synthesis Strategy of M-TATB. (b) ESP surface of the Co-TATB model. (c) Optimized adsorption configurations of various reaction intermediates for Co-TATB along the N-terminal pathway. (d) Contour mapping of in situ electrochemical ATR-FTIR spectra of the Co-TATB electrode across a range of applied potentials. Reproduced with permission,[89] Copyright 2025, American Chemical Society |
Both in situ spectroscopy and theoretical calculations have elucidated the operative reaction pathway, revealing how the symmetric cluster architecture facilitates favorable adsorption and subsequent hydrogenation sequences. Specifically, the square-pyramidal coordination environment—characterized by C4v symmetry—of each Co center within the tetranuclear [Co4(μ4-O)(CO2)8] unit offers significant geometric and electronic advantages. Geometrically, this configuration minimizes steric hindrance around the metal sites, thereby promoting the approach of NO3− ions and stabilizing *H intermediates. Electronically, projected density of states (PDOS) analysis indicates a robust orbital overlap between the Co 3d and NO3− 2p states near the Fermi level, while in Figure 3b, electrostatic potential (ESP) mapping reveals highly positive Co sites (+42.10 kcal•mol−1) that exert a strong attraction on nucleophilic NO3−.
Charge-transfer calculations further indicate that each Co center donates approximately 0.40 e- to the adsorbed NO3- species, thereby weakening the N-O bonds and initiating deoxygenation. Crucially, DFT free-energy calculations distinguish between two competing pathways: the O-end route, which requires a prohibitive 4.76 eV barrier for the *ONH→*ONH2 transition, and the N-end pathway (*NO →*NOH→*NH→*NH2→*NH3) (Figure 3c), which proceeds through energetically favorable or exothermic steps. This mechanistic preference is corroborated by in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, as shown in Figure 3d, which detects key intermediates such as *NO at approximately 1515 cm−1 and *NH4+ at ca. 1459 cm−1, while showing negligible signals for *ONH species. Consequently, the square-planar tetranuclear motif provides a high density of accessible Co sites and electronically tunes them to steer the reaction toward the kinetically favorable N-end hydrogenation sequence, effectively suppressing both toxic *NO2− accumulation and the competing HER.
Meanwhile, Cao et al.[95] applied a biomimetic microenvironment strategy in a trinuclear Cu-cluster MOF (DiMe-Cu3-MOF) by introducing methyl groups to stabilize the key *NO2 intermediate via weak interactions, thereby lowering the barrier to NH3 formation and suppressing side pathways. The catalyst achieved a FENH3 of 95% and a production rate of 401 μg•h−1•cm−2 at an industrial-scale current density (−950.6 mA•cm−2), highlighting how subtle noncovalent tuning can influence selectivity under demanding conditions.
Collectively, cluster-based MOF catalysts demonstrate that high selectivity can be achieved through cooperative metal-metal ensembles, provided that the framework fixes their geometry, maintains the accessibility of active sites, and prevents cluster structural evolution under applied bias.
3.1.3 Functionalization Modification
Chemical functionalization of linkers or nodes directly reshapes the electronic structure, coordination environment, and interfacial microenvironment of MOFs—often resulting in improved intrinsic activity and selectivity.[44,89,96] Under neutral conditions, where proton availability is limited, and HER competition remains acute, functionalization can serve as a precise tool to (i) modulate the e− density of metal centers, (ii) enhance interfacial water activation, and (iii) introduce proton-relay motifs, thereby promoting the PCET steps essential for selective NO3--to-NH3 conversion.[47,97]
A prominent strategy is to introduce e−-withdrawing substituents to increase the electrophilicity of metal sites, thereby enhancing NO3- adsorption and facilitating its subsequent reduction.[98] In 2023, Zhi and co-workers demonstrated this by incorporating carbonyl groups into a Cu-based metal-organic polymer derived from 1,2,4,5-tetraaminobenzene (BTA).[99] As illustrated in Figure 4a, 4b, the resulting CuTABQ exhibited a significantly higher FENH3 of 97.7% and an NH3 yield of 18.69 mg•h−1•mgcat−1 (Figure 4c, 4d), outperforming the non-carbonylated CuBTA (FENH3=85.1%). This improvement was attributed to the e−-deficient Cu centers, which enhanced NO3− binding and promoted the hydrogenation sequence.
Figure 4 The ESP for (a) CuBTA and (b) CuTABQ. (c) LSV curves of different catalysts in 0.5 mol•L−1 K2SO4 with and without 50 mmol•L−1 NO3−. (d) FEs of CuBTA, CuBTAQ and ligands. Reproduced with permission [99], Copyright 2023, Wiley-VCH. (e) The coordination geometry of Fe3+ and the chain-type Fe-pyrazole metal cluster. (f) Two-fold interpenetrated Fe-pyNDI. Reproduced with permission [100], Copyright 2024, Wiley-VCH |
Beyond electronic tuning, functionalization can also be used to construct modular architectures that integrate catalytic units with conductive or stabilizing motifs. In 2024, Kitagawa and co-workers designed a porous coordination polymer (Fe-pyNDI) featuring chain-like Fe-pyrazole clusters as catalytic units and π-stacked naphthalene diimide (NDI) columns as conductive/stabilizing modules[100] (Figure 4e, 4f). This "modular design" endowed the material with remarkable stability across a wide pH range (1—13) and semiconducting behavior (1.1 × 10−6 S•cm−1), while delivering an NH3 production rate of 14677 μg•h−1•mgcat−1 and an FENH3 of 87%. Operando X-ray absorption spectroscopy revealed that Fe3+ was partially reduced to Fe2+ during catalysis, suggesting Fe2+ as the active species within the stable framework.
Perhaps most pertinent to neutral-pH operation is the functionalization that directly addresses the sluggish proton supply. In 2024, Wang et al.[44] designed a Co-based MOF (HUST-38) with coordinated water molecules strategically retained within the framework. This aqueous microenvironment enhanced water dissociation, enriched the local concentration of active hydrogen species, and lowered the energy barriers for NO3− adsorption and hydrogenation, while effectively suppressing HER. As a result, HUST-38 achieved an FENH3 of 95.7% and an NH3 yield of 13.38 mg•h−1•mgcat−1 at 0.6 V vs. RHE in neutral electrolyte, substantially outperforming its anhydrous analog HUST-39. Collectively, these examples illustrate how functionalization—whether through e−-withdrawing groups, modular conductive components, or embedded H2O networks—can be tactically employed to overcome by precisely tuning the electronic structure, improving charge transport, and facilitating H+ delivery, such modifications enable MOFs to orchestrate the multi-step PCET sequence required for efficient and selective NO3−-to-NH3 conversion.
3.1.4 Structural Regulation
Beyond chemical tuning, altering MOF morphology and dimensionality can markedly improve catalytic performance. Transforming bulk MOFs into low-dimensional architectures—2D nanosheets, 1D rods, etc.—can increase exposure of active sites, accelerate reactant/H+ transport, and shorten e− pathways.[101-103] When combined with dynamic operation (e.g., pulsed electrolysis), low-dimensional MOFs can also regulate interfacial intermediate coverage and conversion kinetics.[81]
In 2025, Xu and co-workers constructed a 2D layered Cu-based MOF (Cu-BDC) that enabled efficient NO3−/NO2− reduction by exposing five-coordinate Cu(II) centers and forming an e−-rich interface via coordination with DMF.[84] The architecture improved adsorption/activation of NO3−/NO2− and supported favorable e− transfer, delivering FEs of 80.9% (NO2− reduction) and 67.42% (NO3− reduction), with NH3 yields of 71.20 and 130.16 μmol•h−1• cm−2 at −0.746 and −0.946 V vs. RHE, respectively.
In the meantime, Jiang et al.[81] developed a rod-shaped conductive Zn-MOF designed to suppress HER by leveraging Zn’s low e− affinity and filled orbitals and introduced pulsed electrolysis (EH=−1.1 V, EL=−0.6 V vs. RHE; tH=tL=5 s) to regulate *NO2− accumulation and conversion. Under optimized pulsing, the Zn-MOF achieved an NH3 yield of 461.1 μmol•h−1•mgcat−1 and FENH3 of 80.5%, substantially exceeding constant-potential operation. These studies highlight how dimensional control and dynamic electrochemical protocols can work together to reshape interfacial reaction landscapes in neutral eNO3RR.
In practical terms, structural regulation is a transport strategy as much as a site strategy: it aligns pore accessibility, e− conduction, and intermediate residence time with the kinetic bottlenecks characteristic of neutral electrolytes. Overall, single-metal MOFs demonstrate that high selectivity at neutral pH is attainable when active-site identity, microenvironment, and transport geometry are designed as a unified catalytic system rather than optimized in isolation.
Although single-metal MOFs provide a clear structure-performance relationship, their long-term stability in aqueous media remains limited due to the hydrolysis of metal nodes and the oxidative sensitivity of ligands. Their structural stability in neutral media can be greatly enhanced by incorporating high-connectivity nodes (such as Zr6 clusters) or hydrophobic ligands,[48,104] as detailed in Section 3.8.
In summarizing and comparing the performance of current single-metal MOF catalysts toward the neutral eNO3RR, a clear intrinsic correlation emerges between the metal center's electronic structure and its catalytic efficacy. Among the various reported single-metal MOFs, Cu-based systems exhibit the most outstanding overall performance, frequently achieving nearly 100% FENH3 alongside exceptionally high yield rates. Notable examples include the previously discussed Cu-SA/UiO-bpy, CuTABQ, and DiMe-Cu3-MOF.[59,95,99] This superior performance is rooted in the unique electronic configuration of Cu; DFT calculations in Figure 5a reveal that Cu has a moderate d-band center. This electronic "sweet spot" confers an appropriate adsorption strength toward key reaction intermediates such as *NO, effectively avoiding both active-site poisoning from excessively strong binding and insufficient activation from overly weak interactions. Consequently, Cu sits favorably at the apex of the activity volcano plot.[63-64] Furthermore, the intrinsically high overpotential for HER on Cu surfaces is a distinct advantage under neutral, H+-limited conditions, as it suppresses parasitic HER and directs reactive hydrogen toward hydrogenating nitrogen-containing intermediates.[59]
Figure 5 (a) Projected crystal orbital Hamilton population (pCOHP) of NO3 and NO before and after adsorption on Cu-SA/UiO-bpy, respectively. Reproduced with permission,[59] Copyright 2025, Wiley-VCH. (b) Fe K-edge XANES spectra of Fe-pyNDI at different potentials. (c) The variation curve of the average Fe oxidation state with potential derived from XANES data fitting. Reproduced with permission.[100] Copyright 2024, Wiley-VCH |
By contrast, while Co-based MOFs also exhibit impressive FENH3, their catalytic mechanism relies more heavily on the redox flexibility of the Co centers (Co2+/Co3+) to drive PCET.[89] However, their ability to regulate reactive hydrogen under neutral conditions is generally weaker than that of Cu, leading to more pronounced HER competition. Fe-based MOFs, such as Fe-pyNDI, often exhibit significant dynamic structural evolution during the reaction.[100] Operando X-ray absorption spectroscopy (XAS) studies indicate that Fe3+ partially converts to Fe2+ under reductive potentials (Figure 5b, 5c), where these in situ-generated active sites operate within the stable framework. Nevertheless, the high oxophilicity of iron can lead to the over-stabilization of O-containing intermediates, which may hinder the subsequent hydrogenation kinetics. Zn-based MOFs typically exhibit relatively weak NO3- activation ability on their own, often requiring external regulation strategies—such as pulsed electrolysis—to optimize the reaction pathway.[81] Nonetheless, in bimetallic systems, Zn can serve as an ideal synergistic adsorption site, facilitating a "divisional cooperation" catalytic mode when paired with a second metal.[47]
Accordingly, it can be concluded that the core design principle for high-performance single-metal MOF catalysts lies in precisely tuning the coordination environment to optimize the metal center's electronic structure toward an ideal state. This state must enable efficient NO3− activation while maintaining a moderate level of binding to *H. For neutral eNO3RR, Cu remains the most effective single-metal active center to date, owing to its balanced d-band center, favorable *NO binding energy, and intrinsic ability to suppress the HER. In contrast, metals such as Co and Fe can compensate for their insufficient intrinsic activity under specific conditions through dynamic valence evolution or synergistic interactions. These insights provide robust electronic structure guidelines for the subsequent design of multimetallic MOFs, the application of ligand engineering, and the precise regulation of the reaction microenvironment.
3.2 Bimetallic/Multimetallic MOF Catalysts
Under neutral conditions, single-metal sites often struggle with complex branching pathways, persistent HER competition, and the accumulation of intermediates (notably NO2−), This issue will be elaborated on in Section 3.7. Incorporating additional metal elements to build bimetallic/multimetallic MOFs can create synergistic effects, tune electronic structures, and enable cascade catalysis across sequential steps.[105-106]
3.2.1 Bimetallic Node Synergy
While single-metal MOFs provide clear structure-activity insights, they often face inherent trade-offs under neutral conditions: a site optimized for NO3− adsorption may be less effective for hydrogenation, and suppressing HER while maintaining efficient proton coupling remains challenging. Introducing a second metal to form bimetallic nodes within the same secondary building unit can break this constraint. By enabling short-range electronic coupling and redistributing charge density, bimetallic nodes allow the adsorption energetics of NO3− and its intermediates (NO2, NO) to be tuned synergistically, while assigning distinct mechanistic roles to each metal—effectively enabling "division of labor" across the multi-step reduction cascade.[46,107-108]
A representative example is the redox-active MOF Zn5-NiS4TP reported by Zuo and co-workers, which explicitly implements a cascade-catalysis design[47] (Figure 6a—6c). Here, pentanuclear Zn5 clusters preferentially adsorb and reduce NO3− to NO2−, while adjacent ferredoxin-like [NiS4] units subsequently reduce NO2− to NH3. The thiophene-based linkers further enhance electronic conductivity (Figure 6d), thereby ensuring efficient e− flow between the two functional sites. This cooperative workflow minimizes the accumulation of toxic NO2−—a common bottleneck in neutral media—and delivers an NH3 production rate of 23477 μg•h−1•mgcat−1 with an FE of 92.87% in neutral electrolyte.
Figure 6 Crystal structure of the ligand (a), view of (b) Zn5 cluster and the (c) network of Zn5-NiS4TP. (d) The concept of the catalytic mechanism of eNO3RR by Zn5-NiS4TP. Reproduced with permission,[47] Copyright 2024, Wiley-VCH. (e) Structure of Cu node, In node, and crystal structure for InCu-MOF. (f) EDS mapping of InCu-MOF: Cu (orange), In (cyan), and N (blue). Reproduced with permission,[108] Copyright 2025, Tsinghua University Press |
Meanwhile, Sun et al.[108] constructed a doubly interpenetrated InCu-MOF featuring high-density In-Cu bimetallic sites (Figure 6e, 6f). Electronic interaction between In and Cu was found to enhance NO3− adsorption and promote its hydrogenation. Even at a low NO3− concentration (300 ppm)—a condition relevant to many real wastewater streams—the catalyst achieved an FENH3 of 82% and an NH3 yield of 892 μg•h−1•mgcat−1 at −1.0 V vs. RHE, outperforming its single-metal counterparts. This result underscores how bimetallic synergy can not only improve intrinsic activity but also extend the operating window to more realistic, dilute NO3− concentrations.
These examples demonstrate that bimetallic-node design is most powerful when it deliberately allocates complementary functions: one metal may be tailored for early deoxygenation steps (e.g., NO3− → NO2−), while the other facilitates subsequent hydrogenation (e.g., NO2−→ NH3). Such a "two-site" strategy effectively decouples the competing energetic demands that often limit single-metal intermediate accumulation, enhances selectivity, and achieves robust NO3−-to-NH3 conversion under neutral conditions.
3.2.2 Metal Cluster/Nanoparticle Embedding
Embedding metal clusters or nanoparticles within MOF pores or on MOF surfaces creates heterogeneous interfaces that combine MOF porosity with highly active metallic domains.[81] Such interfaces can induce electronic interactions between the embedded phase and the framework, tune intermediate adsorption, and facilitate coupled proton-electron transfer.[98,109]
In 2025, Chen and co-workers incorporated Cu into Co-MOF-74 via a simple immersion method to form Cu@Co-MOF-74.[110] The bimetallic configuration was proposed to leverage Cu for NO3− adsorption/conversion and Co for hydrogenation, thereby enabling a synergistic pathway and an improved electronic structure. At −0.88 V vs. RHE, the catalyst achieved an FENH3 of 84.01% and a reported NH3 yield of 307.98 mmol•h−1•mgcat−1, demonstrating the practicality of post-modification to create bimetallic synergy within established MOF scaffolds.
In this embedding paradigm, the MOF serves as both a structured electrolyte interface and a stabilizing matrix, while the embedded metallic phase provides the high-turnover sites required for sustained operation.
3.2.3 Heterogeneous Single-Atom Doping for Synergistic Site Construction
Beyond bimetallic nodes, introducing a second metal as isolated dopant atoms into a host MOF provides an atomic-scale route to heterojunction-like coupling. Such catalysts preserve the uniform coordination environment of single atoms while enabling strong, short-range electronic interactions that can steer the energetics of intermediates and reaction branching.[83,111] The concept parallels cooperative units such as Fex/Cu-N@CF, in which adjacent metals cooperatively modulate adsorption and PCET barriers.[112]
In 2024, Kitagawa et al.[113] doped atomically dispersed Ru3+ into crystalline Co-pyNDI, producing a Ru-Co-pyNDI catalyst (Figure 7a). The Ru dopants induced local distortion and electronic coupling, shifting the Co d-band center downward (Figure 7b), optimizing intermediate adsorption/desorption, and enhancing electrochemical NH3 synthesis performance (Figure 7c). This study exemplifies how atomic-scale heterometallcoupling within PCP/MOF frameworks can be used to engineer catalytic energetics with high precision. In brief, heterogeneous single-atom doping turns proximity into functionality: when two metals communicate electronically at angstrom distances, selectivity can be tuned without sacrificing the definability that makes MOFs mechanistically instructive. Overall, multimetallic MOFs provide an explicit route to "division of labor" in NO3- reduction, offering a rational means to overcome NO2− accumulation and HER competition by distributing elementary steps across cooperative sites.
In addition to the synergistic catalytic advantages of bimetallic node catalysts, their long-term stability in aqueous media can be enhanced through electronic structure regulation and node engineering. Compared with MOFs with single-metal sites, charge transfer between the two metals can effectively tune the e− density at metal centers and strengthen the covalency of metal-ligand bonds, thereby suppressing metal-node leaching caused by hydrolysis. Alternatively, highly connected bimetallic clusters can be used to construct rigid frameworks, thereby raising the energy barrier to coordination bond cleavage and significantly improving structural robustness.[108] The pristine MOFs employed for the eNO3RR under neutral conditions in recent years, along with their corresponding performance, have been compiled in Table 1 for reference purposes.
Table 1 Summary of reported pristine MOFs electrocatalysts for eNO3RR in recent years |
| Electrocatalyst | Electrolyte | NH3 yield | NH3 Selectivity | FENH3 | Optimal potentials | Stability | Ref. |
|---|---|---|---|---|---|---|---|
| Cu-BDC | 0.5 mol•L−1 Na2SO4 + 0.05 mol•L−1 NaNO3 | 130.16 μmol•h−1•cm−2 | — | 67.42% | −0.946 V | 5 r | [84] |
| Zn-MOF | 0.1 mol•L−1 Na2SO4 + 500 ppm KNO3 | 461.1 μmol•h−1•mgcat−1 | — | 80.50% | −1.1 V | 10 r | [81] |
| Zn5-NiS4TP MOF | 0.05 mol•L−1 K2SO4 + 0.5 mol•L−1 KNO3 | 1378.52 μmol•h−1•mgcat−1 | 96.20% | 92.87% | −1.3 V | 20 r | [47] |
| InCu-MOF | 0.5 mol•L−1 K2SO4 + 300 ppm KNO3 | 52.37 μmol•h−1•mgcat−1 | — | 82% | −1.0 V | 5 r | [108] |
| Co-TATB | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 378.8 mmol•h−1•gcat−1 | — | 98% | −1.0 V | 8 r | [89] |
| Co-bpta-btc | 0.05 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 604.78 μmol•h−1•mgcat−1 | — | 83.3% | −1.0 V | 12 h | [114] |
| HUST-38 | 0.5 mol•L−1 K2SO4 + 0.1 mol•L−1 NO3- | 785.63 μmol•h−1•mgcat−1 | 92.7% | 95.7% | −0.6 V | 96 h | [44] |
| agCu(Melm)2-CC | 0.5 mol•L−1 K2SO4 + 75 mmol•L−1 KNO3 | 1.20 mmol•h−1•cm−2 | — | 91% | −1.8 V | 16 h | [115] |
| DiMe-Cu3-MOF | 0.5 mol•L−1 K2SO4 + 50 mmol•L−1 KNO3 | 23.54 μmol•h−1•cm−2 | — | 95% | −1.55 V | 12 h | [95] |
| Fe-pyNDI | 0.5 mol•L−1 K2SO4 + 0.1 mol•L−1 KNO3 | 861.78 μmol•h−1•mgcat−1 | — | ≈90% | −1.2 V | 10 r | [100] |
| Cu-CA | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 186.72 μmol•h−1•mgcat−1 | — | 90.3% | −0.9 V | 10 r | [116] |
| NJUZ-2 | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 253.7 mmol•h−1•gcat−1 | — | 98.4% | −0.8 V | 70 r | [117] |
| NJUZ-3 | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 214.5 mmol•h−1•gcat−1 | — | 98.8% | −0.8 V | — | [117] |
| Cu-BTA | 0.5 mol•L−1 Na2SO4 + 100 ppm KNO3 | 133.87 μmol•h−1•cm−2 | 79.46% | — | −1.2 V | 20 r | [118] |
| CuNi0.75-MOF/NF | 0.5 mol•L−1 Na2SO4 + 0.1 mol•L−1 NaNO3 | 3.04 mmol•h−1•cm−2 | — | 95.88% | −1.0 V | 10 r | [74] |
Note: The optimal potentials are all referenced to the Reversible Hydrogen Electrode (RHE). For ease of comparison, the yield data have been converted to molar units (MNH3=17.03 g•mol−1). The number of cycles (r, run) and operation duration (h, hour) are selected based on stability data. |
3.3 Conductive MOF
Despite the profound advantages of MOFs in structural designability and well-defined active sites, their notoriously low intrinsic conductivity—typically below 10−10 S•cm−1—has historically limited their broader electrocatalytic utility. Under the high-current-density regimes required for neutral eNO3RR, these sluggish e− transport kinetics result in suboptimal active-site utilization, aggravated ohmic polarization, and ultimately, a limited practical NH3 yield.[119] The development of conductive MOFs (cMOFs) has emerged as a transformative strategy to circumvent this conductivity bottleneck. By leveraging charge-transport mechanisms enabled by extended conjugated backbones or strong π-π stacking interactions, cMOFs can enhance conductivity into the semiconducting or even metallic regimes, reaching values between 10−3 and 102 S•cm−1.[120] This dramatic improvement in electronic mobility enables highly efficient electrocatalytic conversion, positioning cMOFs as a superior platform for the high-rate production of NH3.
3.3.1 Intrinsically Conductive MOFs
A primary strategy to overcome the insulating nature of traditional MOFs is to construct intrinsically cMOFs directly. This is typically achieved by utilizing extended π-conjugated ligands—such as hexahydroxytriphenylene, hexaaminotriphenylene, and their derivatives—to form two-dimensional layered structures with transition metal ions, or by introducing redox-active ligands that facilitate robust charge delocalization.[121-124] Recently, Zhang and Long realized the precise construction of diatomic sites within these conductive frameworks. This breakthrough provided an ideal platform for elucidating the "relay catalysis" mechanism inherent in eNO3RR.[120] The researchers synthesized a series of structurally well-defined cMOFs, CuxNiy-DBCO (Figure 8a), featuring a two-dimensional (2D) conjugated architecture that endows the material with excellent intrinsic conductivity (2.1×102 S•m−1) and robust aqueous stability. Among these variations, the optimal Cu98.5Ni1.5-DBCO catalyst achieved nearly 100% SNH3 and a 98.5% FENH3. Furthermore, it demonstrated an outstanding NH3 production rate of 200.7 mg•h−1•mgcat−1 at an industrial-level current density of 752 mA•cm−2. Beyond pure electrocatalysis, this conductive MOF was successfully integrated into a Zn-NO3− battery system, delivering a competitive power density of 35.6 mW•cm−2 and effectively highlighting its broader application potential in advanced energy conversion and storage technologies (Figure 8b).
Figure 8 (a) Schematic illustration of the preparation of CuxMy-DBCO MOFs catalysts. (b) Polarization curves and power density of Zn-NO3− batteries composed of Cu98.5Ni1.5-DBCO.[120] Copyright 2025, American Chemical Society. (c) SEM images of Cu-MOF/Co-MOF@NF. (d) Cu 2p spectra of Cu-MOF/Co-MOF@NF compared with Cu-MOF@NF and (e) Co 2p spectra compared with Co-MOF@NF.[126] Copyright 2025, American Chemical Society |
3.3.2 Conductive MOF Composites
Beyond the development of intrinsically conductive frameworks, the hybridization of conventional MOFs with highly conductive substrates represents a highly effective approach to enhancing e−-transport efficiency.[125-126] For instance, Zhu et al.[126] recently reported a Cu-MOF/Co-MOF composite nanoflower architecture grown in situ on a Ni foam substrate (Figure 8c). XPS confirmed that significant interfacial e- transfer occurs between the Cu and Co centers, resulting in a unique electronic redistribution that optimizes the reaction landscape. (Figure 8d, 8e) This synergy effectively bifurcates the catalytic process: the Cu sites primarily facilitate the initial adsorption and reduction of NO3−, while the adjacent Co sites promote the subsequent selective conversion toward NH3.
At a potential of −0.5 V vs. RHE, this composite catalyst achieves an SNH3 of 93.3% and a yield of 318.5 μg•h−1•cm−2. These metrics represent improvements of 8.5% and 39.6% over the performance of the individual Cu-MOF and Co-MOF counterparts, respectively. Such findings demonstrate that interfacial electronic coupling, when combined with a macroscopic conductive support such as Ni foam, can overcome the inherent transport limitations of MOFs to deliver high-efficiency performance in neutral eNO3RR.
3.3.3 Perspectives on the Synergistic Design of Conductive MOFs and MOF-Based Composites
Based on the progress outlined above, two distinct yet parallel technical trajectories have emerged to address the electronic-transport limitations inherent in MOF-based electrocatalysts. The first involves the molecular-level engineering of intrinsically conductive MOFs, in which charge delocalization is achieved through extended conjugated frameworks.[120,123-124] The second involves composite-level integration, where conventional MOFs are coupled with highly conductive substrates—such as Ni foam, carbonaceous materials, or MXenes—to facilitate efficient charge injection across heterogeneous interfaces.[125,127] Each approach offers unique advantages: intrinsically conductive MOFs provide highly defined active sites that serve as ideal platforms for fundamental mechanistic investigations, whereas the composite strategy offers broader universality and seamless compatibility with established industrial electrode fabrication techniques.
Future research is likely to focus on the convergence of these two strategies—specifically, the integration of intrinsically conductive MOFs within three-dimensional (3D) porous substrates. Such hybrid architectures are expected to push the boundaries of synergistic optimization by simultaneously enhancing e− transport and mass-transfer kinetics. Furthermore, the development of conductive MOFs provides a superior model system for in situ and operando characterization. Their structurally ordered and conductive networks enable precise detection of evolving reaction intermediates using advanced techniques such as XAS and Raman spectroscopy. This high-resolution experimental insight is critical for deepening the mechanistic understanding of neutral eNO3RR and establishing the design rules for the next generation of high-performance catalysts.
3.4 MOF Composites
Despite their architectural advantages, many MOFs suffer from limited electrical conductivity and, in certain electrolytes, insufficient chemical robustness—constraints that can limit catalytic utilization at meaningful current densities.[120] Integrating MOFs with conductive substrates (nickel foam, carbon cloth, carbon paper, etc.) can markedly improve electrode conductivity and mechanical/chemical stability.[128-130] Such composites retain MOF porosity and tunable active sites[131] while enabling rapid e− transport and improved NO3- conversion kinetics at the electrode interface.
In 2025, Zhao and co-workers fabricated a 3D porous layered Cu-TCPP/Cu2O/CF electrode by in situ growing Cu2O nanoparticles and 2D Cu-porphyrin MOF (Cu-TCPP) nanosheets on Cu foam (CF) via a two-step hydrothermal method[30] (Figure 9a). The conductive CF scaffold provided efficient e− highways, while the Cu-TCPP/Cu2O tandem arrangement partitioned function: Cu-TCPP favored NO3- adsorption/activation, and Cu2O promoted deeper hydrogenation, collectively boosting both efficiency and selectivity for NH3 formation. This architecture highlights how MOF composites can be designed as multifunctional tandem catalytic systems.
Figure 9 (a) Schematic of the preparation of Cu-TCPP/Cu2O/CF 3D tandem electrode synthesized on CF. Reproduced with permission,[30] Copyright 2025, American Chemical Society. (b) Schematic illustration of the preparation process of the MOF@MXene nanosheets as a flexible electrode via permeation-mediated strategy. Reproduced with permission,[134] Copyright 2022, AAAS. (c) Schematic diagram of the "ship-in-a-bottle" strategy for encapsulating ZIF-67 into HMCS. Reproduced with permission,[135] Copyright 2025, Wiley-VCH |
2D conductors, such as MXenes and graphdiyne, further expand the design space for composites. Their high conductivity, abundant functional groups, and layered structures enable multidimensional interfaces with MOFs, improving charge transport, reactant delivery, and intermediate stabilization.[132-133] MXene’s metallic conductivity and hydrophilic surface can accelerate e− transfer and facilitate NO3− diffusion/adsorption in neutral electrolytes[130,134] (Figure 9b). Graphdiyne, with sp-hybridized carbon and intrinsic porosity, can act as an electronic bridge and microenvironment modulator, tuning the electronic properties of MOF active centers and stabilizing reaction intermediates.[133] These composite interfaces can expand the electrochemically active area and reshape pathways via interfacial engineering, enabling high FENH3 and NH3 yield under neutral conditions.
In 2025, He and colleagues introduced a biomimetic nano-organelle concept by encapsulating ZIF-67 within hollow mesoporous carbon spheres using a "ship-in-a-bottle" approach[135] (Figure 9c). The outer carbshell enhanced conductivity and mass transfer, while the internal confined cavity tuned the reaction microenvironment, suppressed HER, and promoted highly selective NO3−-to-NH3 conversion—underscoring how spatial confinement can be deliberately engineered to control selectivity in neutral media.
Beyond carbon-based conductors, MOFs can be hybridized with metal oxides, layered double hydroxides (LDHs), phosphides, and alloys to exploit complementary functionalities.[136-137] LDH/MOF composites can leverage their hydroxyl-rich surfaces and ion-exchange capacity to enhance H+ transport and the adsorption of intermediates.[138] Metal phosphides (e.g., CoP, Ni2P) can provide metal-like conductivity and strong electronic interactions, thereby lowering hydrogenation barriers.[139] Alloy nanoparticles can tune the e− density via bimetallic effects, thereby improving the selective conversion of key nitrogen intermediates.[140] Such multifunctional hybrids broaden the operating envelope of MOFs for neutral eNO3RR and provide a flexible toolkit for durability and selectivity engineering.
Overall, MOF composites translate molecular-level site tunability into device-relevant electrodes by solving conductivity and mass-transfer bottlenecks—often the decisive constraints under neutral, high-current operation.
Composite integration with conductive substrates not only enhances the e−-transport capability of MOFs but also significantly suppresses the leaching of active components and structural collapse through physical anchoring and chemical coupling. Conductive media such as carbon materials and MXene can serve as protective layers, mitigating nucleophilic attack by H2O molecules on metal-coordination bonds and thereby enhancing the operational stability of MOFs in neutral electrolytes. However, to date, the long-term compatibility of composite interfaces and their anti-poisoning ability in real water matrices still requires further validation.
3.5 MOF Derivatives
While pristine MOFs and composites offer exceptional structural tunability, many remain limited by poor electronic conductivity and the risk of chemical transformation during long-term operation in neutral aqueous electrolytes, particularly at high current densities.[141] MOF-derived materials—prepared by controlled pyrolysis using MOFs as precursors or self-sacrificial templates—have therefore emerged as a powerful route to high-performance eNO3RR catalysts.[142] These materials can inherit MOF porosity and uniform metal distributions while generating conductive carbon matrices and robust active phases, achieving a practical balance among conductivity, site accessibility, mass transport, and stability.[143-145] Depending on the targeted features (e.g., conductive carbon vs. vacancy-rich oxides), pyrolysis can be conducted under either inert or oxidizing atmospheres.
3.5.1 Pyrolytic Carbonization
Carbonization under an inert atmosphere converts organic linkers into conductive porous carbon while transforming metal nodes into nanoparticles, alloys, or atomically dispersed species anchored within the carbon matrix.[146-147] The resulting carbon framework improves charge transport and can suppress metal aggregation/leaching through encapsulation, enhancing durability.[148]
A study by Luo et al.[149] further revealed the regulatory effect of the state of metal particles on catalytic performance. Using Co-based MOFs as the precursor, they prepared the low-crystallinity Co-N-C-500 catalyst by controlling the carbonization temperature. Its small, well-dispersed cobalt nanoparticles (approximately 14.87 nm) provide abundant active sites, and coordinatively unsaturated Co centers promote NO3−-to-NO2− conversion, thereby endowing the catalyst with high activity and stability in the NO3− reduction reaction for NH3 synthesis. This catalyst achieves excellent NH3 yield and FE in a neutral electrolyte and has been successfully applied in ZnNO3 batteries to simultaneously achieve NO3− conversion and electrical energy output. Therefore, carbonized MOF-derived materials achieve a practical balance: they not only retain the site-distribution characteristics imparted by MOFs but also possess the electronic stability required for NO3- conversion at high current densities.
3.5.2 Oxidative Calcination
For example, Wei et al.[152] used hollow Cu-Co Prussian blue analogs as precursors to prepare a hollow cage structure (Cu-Co HOC) composed of ultrasmall CuO/Co3O4 heteronanoparticles via calcination. This material exhibited remarkable performance in the eNO3RR, achieving a FE of 90.68% for NH3 at −0.5 V vs. RHE. Further studies revealed that electrochemically induced reconstruction forms synergistic active sites between Cu and Co3O4, which can facilitate the continuous conversion of NO3− and NO2−, thus enabling the highly efficient production of NH3. This case demonstrates that the conversion of MOFs to oxides can not only regulate the microstructure and defect chemistry of materials, but also construct multi-interface synergistic active centers, thereby further enhancing the activity and selectivity of eNO3RR under neutral conditions. In oxide-derived materials, defect and interface engineering represent the core design strategies: oxygen vacancies, heterojunctions and dynamic reconstruction processes can synergistically optimize the reaction pathway without complete reliance on the intrinsic electrical conductivity of the materials. Therefore, MOF-derived materials extend the structural designability of MOFs to more conductive and stable oxide systems, providing an effective approach for translating structural innovation into practical current-density applications.
While the fabrication of MOF-derived catalysts necessitates sacrificing the pristine framework's long-range crystallinity, the resulting carbon-encapsulated architectures offer a substantial leap in electrochemical stability. During the thermal transformation process, the in situ generated graphitic carbon layers physically isolate metal nanoparticles, effectively preventing the detrimental agglomeration and leaching that often plague surface-active species. Simultaneously, this carbonaceous shell acts as a protective buffer, mitigating the corrosive impact of the electrolyte on the embedded active sites. Such structural robustness allows these derivatives to maintain stable performance over extended durations—often exceeding tens of hours—even under the rigorous conditions of high-current-density operation. These anti-leaching and anti-reconstruction mechanisms align closely with fundamental design principles such as hydrophobic barrier formation and kinetic shielding. Consequently, the transition to MOF-derived materials represents a strategic "trade-off," where structural transformation is intentionally exchanged for significantly enhanced catalytic longevity.
On this basis, MOF composites and MOF derivatives are included together in Table 2 for summary and reference.
Table 2 Recently explored MOF composites and MOF-derived catalysts |
| Electrocatalyst | Electrolyte | NH3 yield | NH3 Selectivity | FENH3 | Optimal potentials | Stability | Ref. |
|---|---|---|---|---|---|---|---|
| Cu/SO3-MOF-808 | 0.5 mol•L−1 Na2SO4 + 0.1 mol•L−1 NaNO3 | 0.383 mmol•h−1•mgcat−1 | 95.6% | 87.5% | −1.19 V | — | [153] |
| CoP/Zn-ZIF | 0.5 mol•L−1 Na2SO4 + 0.1 mol•L−1 NaNO3 | 0.9 mmol•h−1•cm−2 | — | ≈97% | −1.0 V | 30 r | [154] |
| Cu3P-a | 0.5 mol•L−1 K2SO4 + 50 ppm KNO3-N | 31.86 μmol•h−1•cm−2 | 83.62% | 90.95% | −0.5 V | 8 r | [155] |
| FeNx-PC-Fe NPs | 0.5 mol•L−1 Na2SO4 + 1.0 mol•L−1 NaNO3 | 1.77 ± 0.06 mmol•h−1•mgcat−1 | — | 93.5% | −1.0 V | 120 h | [156] |
| AuPt@ZIF-8 | 0.5 mol•L−1 Na2SO4 + 50 ppm NaNO3 | 7.72 μmol•h−1•mgcat−1 | 96% | (95.8 ± 3.0)% | −0.5 V | 6 r | [140] |
| Ni-MOFs@HsGDY@Cu | 0.5 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 0.321 mmol•h−1•cm−2 | — | 95.5% | −0.11 V | 12 r | [157] |
| CuCeOx/C | 0.5 mol•L−1 Na2SO4 + 0.5 mol•L−1 NaNO3 | — | 73.40% | 75.5% | −1.29 V | 1 h | [158] |
| Cu-Co HOC | 0.5 mol•L−1 K2SO4 + 50 ppm KNO3-N | 0.022 mmol•h−1•cm−2 | — | 90.68% | −0.5 V | — | [152] |
| UiO-CuZn | 0.5 mol•L−1 Na2SO4+ 200 ppm NaNO3 | 228.11 μmol•h−1•mgcat−1 | 95.2% | 92.6% | −1.0 V | — | [159] |
| UiO-66-derived M-SAC | 1 mol•L−1 KNO3 | 1.76 mmol•h−1•cm−2 | — | >96% | −1.0 V | — | [88] |
| Fe-S-Z-C | 0.02 mol•L−1 Na2SO4 + 100 mg•L−1 NO3--N | — | — | 93.9% | −0.47 V | 6 r | [160] |
| Cu-Ru@C | 0.1 mol•L−1 Na2SO4 + 50 μg•mL−1 NaNO3 | 99.84 μmol•h−1•mgcat−1 | 84.70% | 90.4% | −0.9 V | 25 h | [161] |
| Cu-Co3O4/NF | 0.2 mol•L−1 K2SO4 + 2 mmol•L−1 KNO3 | 0.076 mmol•h−1•cm−2 | — | 92.4% | −0.53 V | 6 r | [162] |
| NiPC | 0.5 mol•L−1 Na2SO4 + 200 mg•L−1 KNO3 | 108.04 μmol•h−1•cm−2 | 99.04% | 96.68% | −1.0 V | — | [139] |
| NiCoO2@Cu | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 NaNO3 | 348.82 μmol•h−1•cm−2 | 99.7% | 94.2% | −0.7 V | 33 h | [127] |
| Co-N-C-500 | 0.05 mol•L−1 K2SO4 + 0.05 mol•L−1 KNO3 | 66.93 μmol•h−1•cm−2 | 74% | 86.8% | −0.6 V | 30 h | [149] |
| Co3O4@MoS2 | 0.1 mol•L−1 Na2SO4 + 0.2 mol•L−1 KNO3 | 266.55 μmol•h−1•mgcat−1 | — | 52.69% | −0.64 V | 10 r | [163] |
The optimal potentials are all referenced to the Reversible Hydrogen Electrode (vs. RHE). For ease of comparison, the yield data have been converted to molar units (MNH3=17.03 g•mol−1). The number of cycles (r, run) and operation duration (h, hour) are selected based on stability data. |
3.6 Reactor Engineering for MOF-based eNO3RR
While the strategic design of atomic-level active sites and the implementation of conductive composite frameworks have significantly bolstered the intrinsic activity of MOF-based materials, these advancements at the material level are insufficient to ensure sustained, long-term stability at current industrial-scale densities. Beyond the catalyst itself, macroscopic considerations—specifically, mass-transfer optimization and reactor-level electrode-configuration engineering—are equally paramount.
In traditional, static H-type electrolytic cells, the transport of reactants is governed primarily by diffusive mass transfer, with the diffusion layer often extending to several hundred micrometers in thickness. This substantial transport barrier severely restricts the attainable limiting current density and frequently leads to the localized accumulation of reaction intermediates, such as NO2-, on the electrode surface. Such accumulation not only creates a chemical bottleneck but also risks degrading the system's overall selectivity by promoting undesired side reactions or site poisoning.
3.6.1 Diffusion Layer Regulation and Mass Transfer Enhancement
The diffusion layer represents the critical interfacial region where reactant concentration gradients determine the flux of chemical species. Its thickness is the primary lever for controlling the rate of reactant supply to the active sites; thus, minimizing this layer is essential for accelerating overall reaction kinetics. However, the role of the diffusion layer extends beyond simple mass transfer—it is intrinsically coupled to the structure of the electric double layer (EDL), and together they dictate the local electrochemical microenvironment at the interface.[164]
Theoretical modeling has been instrumental in elucidating this coupled mass-transfer-kinetic mechanism. Lees et al.[164] developed a multiphysics framework coupling the Stern layer, diffusion layer, and diffusion boundary layer to investigate how the EDL influences NO3− reduction. Their findings highlight that the point of zero charge (PZC) of surface species is a decisive factor in interfacial electric field distribution. Specifically, a negative PZC allows the electrode to maintain a positive surface charge over a broader potential range, thereby enhancing the flux of NO3− toward the surface and increasing the limiting current density. This suggests that NO3− reduction performance cannot be accurately captured by simplified diffusion models; rather, it requires a unified framework that accounts for the complex interplay between ion migration and intrinsic kinetics.
Furthermore, the diffusion layer thickness exerts a differential regulatory effect on both reactivity and selectivity. Through integrated experimental and simulation studies, Guo et al.[165] elucidated these dynamics in flow electrolysers. Their research demonstrated that while the diffusion layer thickness and background electrolyte concentration primarily govern eNO3RR activity, the interfacial pH is the primary modulator of selectivity. Reducing the diffusion layer thickness significantly increases the NO3− and NH3 fluxes. Concurrently, shifts in interfacial pH alter the protonation pathways of intermediates, determining the final ratio of NH3 to NO2−. By leveraging this understanding, researchers utilized pulsed potentials to periodically refresh the interfacial environment and lower the local pH, successfully tripling the selectivity for NH3 over NO2−. This proves that diffusion layer regulation is an active tool for tailoring the reaction microenvironment rather than a passive engineering constraint.
At the macro-scale, the reactor channel geometry further modulates these interfacial dynamics by regulating fluid flow characteristics. Sun et al.[166] analyzed a 3D serpentine-channel flow electrolyser, finding that the total mass transfer flux is a synergistic product of convective and diffusive components, which can differ by several orders of magnitude. The dominant mechanism shifts across the reactor: convective transport prevails near the inlet, while diffusion limitations become the bottleneck in deeper sections of the channel. For MOF-based electrodes, reactor design must be precisely calibrated to the material's porous architecture. The micropores inherent to MOFs can themselves become diffusion bottlenecks; if convective mass transfer at the reactor level is insufficient, active site utilization within the framework drops precipitously.
Ultimately, for MOF-based catalysts, the high specific surface area is a "double-edged sword" that may introduce mass-transfer resistance due to nanoconfinement effects. When MOF pore sizes approach the thickness of the EDL, overlapping double layers can fundamentally alter ion transport behavior. Therefore, the realization of high-performance eNO3RR systems requires a multi-scale optimization strategy: convection enhancement at the reactor scale, porous-structure engineering at the electrode scale, and regulation of the EDL at the molecular interface.
3.6.2 Electrode Configuration Optimization
The architectural configuration of electrodes serves as a primary determinant of mass transfer efficiency, often acting as the bottleneck between laboratory-scale potential and industrial-scale reality. Gas diffusion electrodes have set a high bar in this regard by establishing a gas-liquid-solid three-phase interface, which effectively truncates reactant diffusion pathways—a strategy that has already proven transformative in fields such as CO2 electrolysis.[167] When applied to liquid-phase NO3− reduction, the utilization of 3D porous substrates, such as Ni foam or carbon paper, enables the direct in situ growth of MOF catalysts onto conductive skeletons, thereby forming self-supporting integrated electrodes. This monolithic approach bypasses the typical pitfalls of conventional polymer binders, such as unintended pore blockage and increased internal resistance, while simultaneously enhancing active-site accessibility by shortening the effective diffusion distance for NO3− species.[125-126,168]
A compelling recent example of this engineering strategy is the work of Wu et al.,[169] who developed an integrated electrode featuring a FeCo-gallate MOF partially coating a Ni foam skeleton. By strategically depositing the MOF only on specific portions of the Ni foam, they allowed the uncoated regions to function as an integrated gas diffusion layer. This design achieves a sophisticated functional coupling between the catalyst layer and the transport channels, thereby substantially mitigating both ohmic and mass-transfer resistance. The 3D porous framework facilitates rapid convective transport of the electrolyte, while the thin, localized MOF layer ensures that the FeCo active sites remain fully accessible to the reactive environment.
The practical implications of this design are underscored by its performance in an anion exchange membrane (AEM) electrolyser. The electrode required only 1.63 V to achieve an industrial-scale current density of 1 A•cm−2 and maintained stable operation for over 800 hours. Such metrics represent a significant departure from standard coated electrodes, which often suffer from delamination or transport limitations at these magnitudes. These results confirm that the monolithic integration of MOFs with 3D porous substrates can effectively overcome the mass-transfer barriers inherent to traditional configurations, providing a viable blueprint for high-activity, high-durability catalysts in the neutral eNO3RR process.
3.6.3 Coupling with Renewable Energy
A higher-level objective in current reactor design is the seamless integration of electrochemical systems with renewable energy sources, such as solar power, to facilitate distributed NH3 recovery directly at wastewater treatment sites. Wang et al.[170] recently demonstrated a sophisticated three-chamber porous solid electrolyte reactor that exemplifies this approach. By directing treated water from the cathode chamber through a central porous solid electrolyte layer, the system establishes a shuttling cycle for alkali metal cations, thereby enabling highly efficient NO3− reduction without the need for additional supporting electrolytes.
When processing industrial wastewater with a typical NO3− concentration of 2000 ppm, this reactor configuration consistently achieves a FE exceeding 90% and a practical current density greater than 100 mA•cm−2. Critically, this setup produces high-purity gaseous NH3 and purified water simultaneously, eliminating the requirement for subsequent electrolyte recovery steps and providing a closed-loop solution for nitrogen management.
The coupling of such advanced reactors with solar photovoltaics represents a pivotal milestone toward sustainable, solar-driven NH3 production.[171-172] Recently, Guo et al.[171] reported a dynamic photovoltaic-electrolysis coupling system utilizing a CuP/CoF bifunctional catalyst, which demonstrated remarkable durability, with over 1000 hours of stable operation in an anion-exchange membrane electrolyser. When integrated with high-efficiency silicon solar modules—boasting efficiencies exceeding 25%—the system operated reliably for 50 hours under simulated solar irradiation, yielding a solar-to-fuel conversion efficiency of 5.92%. A comprehensive techno-economic model based on these experimental results indicates that the levelized cost of NH3 could be reduced to as low as $0.93 per kg in a 1 MW-scale solar NH3 production facility. This demonstrates that decentralized, renewable-driven nitrogen recovery is not just a laboratory curiosity but is quickly approaching the threshold of genuine economic feasibility.
From a broader systems-level perspective, future research must pivot from the isolated evaluation of single-catalyst performance toward the design of holistic, end-to-end resource recovery processes. This comprehensive approach must encompass every stage of the lifecycle, beginning with upstream NO3- preconcentration and moving through electrochemical conversion to final downstream NH3 separation and purification. The true advancement of eNO3RR toward practical industrial applications will only be realized through the synergistic optimization of materials innovation, reactor engineering, and the robust integration of renewable energy systems. This shift in focus ensures that we move beyond "paper-based" efficiency toward a technology that can genuinely decentralize the global nitrogen cycle.
3.7 Addressing the Challenge of NO2− Accumulation
Although the ultimate objective of the eNO3RR is to convert NO3− into valuable NH3, the reaction pathway is inherently complex. It often leads to the production of intermediate species, most notably NO2−. It is important to emphasize that NO2− is significantly more toxic than NO3−; it can bind to hemoglobin, inducing methemoglobinemia, and serves as a direct precursor to carcinogenic nitrosamines.[173-176] Consequently, the World Health Organization (WHO) has established a stringent limit for NO2− in drinking water of 3 mg• L−1, in sharp contrast to the 50 mg•L−1 threshold permitted for NO3−.[177] Under neutral pH conditions, the scarcity of available H+ results in sluggish kinetics for the subsequent reduction of NO2− to *NO, presenting a unique and formidable challenge for achieving high selectivity in neutral-media eNO3RR.
The accumulation of NO2− is often attributed to the inherent difficulty of a single catalytic site efficiently managing multiple, disparate reaction steps simultaneously. To address this, designing a catalyst with two distinct functional sites that operate collaboratively has emerged as an ideal "cascade catalysis" strategy. For example, as discussed in the context of bimetallic systems, the Zn5-NiS4TP MOF employs a spatial-separation approach: the Zn5 cluster facilitates the initial deoxygenation of NO3− to NO2−, while the NiS4 unit specifically catalyzes the subsequent reduction of the resulting NO2− intermediates. This functional synergy effectively prevents NO2− from desorbing from the active sites and leaching into the bulk solution.[47] Based on established research, metals such as Cu, Sn, and In are highly suitable for the initial deoxygenation step. In contrast, noble metal sites like Pd, Pt, and Rh are superior for reducing NO2− to NH4+.[178-179]
Beyond structural modifications at the catalyst level, regulating the interfacial microenvironment—specifically the local pH gradient—offers a novel strategy to suppress NO2− accumulation. Under near-neutral conditions, the eNO3RR process naturally consumes protons, leading to a localized increase in pH and the formation of an alkaline microenvironment at the electrode surface. Through in situ monitoring with a rotating ring-disk electrode (RRDE), Koper et al.[180] observed that the local pH on Cu electrode surfaces increases significantly in neutral phosphate buffers, and that this gradient may support higher reaction rates. This suggests that researchers can actively tailor the local environment; for instance, creating a slightly acidic microenvironment can promote the H+-dependent reduction of NO2− while the bulk solution remains neutral, thereby accelerating the conversion of intermediates and minimizing their accumulation.[181]
In scenarios where a purely electrochemical process struggles to manage NO3− and to prevent the accumulation of intermediates in complex water matrices, integrating electrochemical systems with biological treatment technologies offers a viable "electro-biochemical" hybrid option. The core design philosophy of such systems involves leveraging the rapid reaction rates and high pollutant tolerance of electrochemical processes for primary reduction, followed by the exceptional selectivity of biological processes to polish the remaining intermediates. This combined approach effectively addresses severe NO2− accumulation by leveraging microbial pathways to ensure complete and selective conversion of nitrogenous species.[182] This multi-technology integration represents a holistic shift toward more resilient and efficient water treatment frameworks.
3.8 Stability Mechanisms and Design Principles of MOFs in Neutral Media
The transition from laboratory curiosity to industrial reality hinges on one uncompromising factor, which is long-term stability under rigorous operation.[183-184] While the porousity of MOF-based catalysts enables exceptional control over the reaction microenvironment, this same architectural flexibility often proves fatal flaw when the materials are submerged in the harsh emvironment of industrial wastewater. In neutral aqueous electrolytes, these frameworks are beset by multiple degradation pathways, ranging from simple hydrolysis of metal-ligand bonds to more insidious electrochemical reconstruction.[184] Furthermore, the oxidative potential of the environment can trigger ligand oxidation, while localized pH fluctuations—previously discussed as a lever for selectivity—can inadvertently promote metal node leaching or the total collapse of the crystalline framework.
Understanding these instability mechanisms is not merely a diagnostic exercise; it is the prerequisite for moving beyond "trial-and-error" synthesis. Establishing clear structure-stability relationships allows researchers to predict how specific metal-organic coordination environments will behave under the polarized conditions of eNO3RR. For instance, the strength of the coordination bond, the hydrophobicity of the ligand, and the overall topology of the framework all dictate whether a catalyst survives for minutes or months. Only by designing MOFs that can maintain their active-site configuration despite the mechanical and chemical stresses of high-current-density operation can we hope to bridge the gap between academic benchmarks and the multi-thousand-hour durability required for industrial NH3 recovery.
3.8.1 Key Mechanisms of MOF Destabilization
The structural destabilization of MOFs essentially originates from the cleavage of metal-ligand coordination bonds within the crystalline lattice.[184-185] Under the specific operating conditions of neutral eNO3RR, this degradation is primarily driven by several distinct but interconnected mechanisms, the most ubiquitous of which is hydrolysis. As a highly polar solvent, H2O readily attacks the coordination bonds between metal nodes and organic ligands, leading to the gradual decomposition of the framework.[184,186] For example, MOF-5, formulated as Zn4O(BDC)3, is a milestone material in the field that nonetheless undergoes rapid hydrolysis upon exposure to H2O, resulting in the immediate cleavage of Zn-O bonds and structural collapse.[184-185] The thermodynamic driving force for this hydrolysis stems from the direct competition between H2O molecules and metal centers for coordination. H2O molecules can effectively displace organic ligands from coordination sites; the resulting intermediates then undergo H+ transfer, ultimately culminating in complete cleavage of the coordination bond.[186] Although the bulk H+ concentration is relatively low at neutral pH, the persistent nucleophilic attack by H2O molecules on these vulnerable coordination bonds remains a formidable threat to the catalyst's structural integrity.
Beyond hydrolysis, the electrochemical redox environment introduces the severe risks of ligand oxidation and metal node leaching. During eNO3RR, organic ligands may be attacked by reactive oxygen species (ROS) or radical intermediates—such as hydroxyl (•OH) or hydroperoxyl (•OOH) radicals—generated from dissolved O2 under cathodic potentials. These highly reactive species can oxidize sensitive functional groups within the organic linkers, including amine and thioether moieties, inevitably leading to the structural degradation of the ligands and a fatal loss of their coordination capability.[184] Concurrently, the continuous application of electrochemical polarization exacerbates the dissolution and loss of metal ions from the MOF nodes into the surrounding electrolyte, which acts as a primary catalyst for activity degradation. Under reductive cathodic potentials, these metal nodes are susceptible to being reduced to lower valence states. This chemical reduction inherently weakens their binding to the organic ligands, thereby facilitating their rapid leaching into solution. This vulnerability is particularly acute for frameworks constructed from low-valent transition metals, such as Zn2+ and Cu2+, when operating in aqueous media.[185] The leaching of these metal nodes not only depletes the population of catalytically active sites but also precipitates the macroscopic collapse of the entire framework, abruptly terminating the material's catalytic lifespan.
3.8.2 Structure-Stability Relationship and Regulation Strategies
A complex interplay of thermodynamic and kinetic factors governs the stability of MOFs in aqueous media. Thermodynamic stability primarily depends on the absolute strength of the metal-ligand coordination bonds, while kinetic stability is dictated by the activation energy barrier required for coordination bond cleavage and the physical accessibility of H2O molecules to the metal nodes.[187] Based on a fundamental understanding of these influencing factors, researchers have developed various rational design strategies to systematically improve the aqueous durability of MOFs.
(1) Coordination Bond Enhancement Strategy Based on hard-soft acid-base (HSAB) Theory
Pearson’s HSAB theory provides a foundational framework for designing robust coordination bonds: the strongest coordination interactions typically occur between hard acids and hard bases, or, conversely, between soft acids and soft bases.[94,188] Consequently, highly stable MOFs are usually constructed using one of two primary combinations: high-valence metal ions (acting as hard acids, e.g., Zr4+, Al3+, Cr3+) paired with carboxylate ligands (hard bases), or low-valent metal ions (acting as soft acids, e.g., Zn2+) paired with azole-based ligands (soft bases).[189]
However, recent studies have revealed that thermodynamic coordination bond strength is not the sole determinant of aqueous stability. Cu2TBAPy, reported by Gu et al.,[189] remarkably defies standard HSAB theory predictions. In this architecture, each Cu cluster is shielded by two hydrophobic pyrene cores, forming a sandwich-like structure that effectively prevents H2O molecules from approaching the Cu-O coordination bonds. As a result, the material maintains its structural integrity under extreme pH conditions (pH 0—13) and can remain stable in H2O at room temperature for up to 240 days. This breakthrough highlights that alongside thermodynamic bond strength, the deliberate design of kinetic barriers is equally critical.
(2) High-Connectivity Node Design Strategy
The coordination number between metal nodes and organic ligands exerts a decisive influence on the macroscopic stability of MOFs. Highly connected frameworks form rigid, 3D networks that significantly increase the activation energy barrier for coordination bond cleavage.[190] For instance, canonical Zr-MOFs (such as UiO-66) utilize Zr6 clusters connected to 12 distinct carboxylate ligands, yielding a highly connected framework endowed with exceptional hydrothermal stability.
Recently, the tsn-MOF-1-Fe3 framework, developed by Chen et al.,[191] was constructed utilizing 9-connected Fe3(μ3-O) trinuclear nodes. Remarkably, it retains an ultrahigh specific surface area of 5100 m2•g−1 following conventional solvent activation and exhibits robust structural stability in both highly acidic and alkaline aqueous solutions. This achievement directly validates the principle that maximizing node connectivity can exponentially enhance framework robustness.
(3) Hydrophobic Pore Environment Engineering Strategy
By deliberately introducing hydrophobic functional groups or constructing an intrinsically hydrophobic pore environment, the physical contact between water molecules and vulnerable coordination bonds can be kinetically blocked, thereby significantly improving the aqueous stability of the MOF.[187] The core objective of this strategy is to elevate the energy barrier that water molecules must overcome to mount a nucleophilic attack. For example, the pristine bio-MOF CaSyr-1 rapidly degrades into a non-porous CaSyr-2 phase upon exposure to water. However, following hydrophobic post-synthetic modification with stearic acid, its kinetic stability in aqueous media is markedly enhanced, and the rate of structural degradation is substantially slowed.[187] Similarly, integrating hydrophobic functional groups into ZIF-8 coatings effectively repels the ingress of corrosive ions through microporous defects, thereby markedly enhancing the coating's operational stability.[192]
(4) Kinetic Stabilization and Steric Hindrance Strategy
Introducing steric hindrance to artificially inflate the activation energy barrier for coordination bond cleavage is another highly effective approach to boosting kinetic stability. DFT studies by Keshavarz et al.[190] revealed a counterintuitive mechanism: the exceptional kinetic stability of MOF-303 does not originate from a hydrophobic environment, but rather from the formation of an extended, dense hydrogen-bonding network between intruding H2O molecules and the framework's polar ligands. This synergistic hydrogen-bonding effect effectively insulates the M-O bonds and prevents pore collapse. This crucial finding challenges the conventional dogma that hydrophobicity is the exclusive pathway to stability, revealing instead that H2O molecules themselves gather to act as structural stabilizers via organized hydrogen-bonding networks.
(5) Ligand Rigidity and Framework Densification Strategy
Employing rigid ligands to construct high-density, inflexible frameworks can effectively suppress localized structural distortions, thereby preserving long-term stability.[193] A highly rigid coordination environment restricts the vibrational and rotational degrees of freedom, stabilizing the overall MOF architecture and minimizing the spatial distance between functional ligands. Comparative studies have demonstrated that TTFB-DAT-Zn-MOF, which features a highly rigid coordination environment, exhibits vastly superior structural stability compared to its more flexible counterpart, TTFBCAT-Zn-MOF.[193] Furthermore, the specific pores of anisotropic, flexible MOFs can be intentionally "rigidified" by installing supplementary bridging ligands during post-synthetic modification, a technique that significantly fortifies the overall mechanical and chemical stability of the framework.[194]
3.9 Dynamic Evolution and Real Active Sites of MOF-based Catalysts
3.9.1 Dynamic Evolution under Electrochemical Conditions
With the rapid advancement of in situ and operando characterization techniques, researchers have increasingly realized that many MOFs do not retain their pristine crystalline architectures under realistic electrocatalytic operating conditions. Instead, they frequently undergo profound dynamic transformations, including surface reconstruction, valence-state fluctuations of the metal nodes, partial ligand dissociation, or even complete morphological conversion into metal (oxy)hydroxides and nanoparticles.[195] This phenomenon raises a fundamental mechanistic question: are the MOFs we meticulously design and synthesize the genuine catalysts for the NO3--to-NH3 conversion, or do they merely serve as precatalysts that generate the true active sites in situ under applied bias? This ambiguity is particularly pronounced for eNO3RR conducted in neutral media, where prolonged electrochemical polarization, the strong adsorption of reactive intermediates, and drastic localized pH fluctuations can collectively disrupt the relatively fragile metal-ligand bonds. If this dynamic evolution is neglected—and mechanistic inferences are drawn solely from the pristine, pre-reaction crystallographic data—researchers risk deriving fundamentally misleading structure-activity relationships.
Empirical studies have definitively confirmed that such dynamic evolution is prevalent in MOF-based materials during eNO3RR. For instance, in the previously discussed Fe-pyNDI framework reported by Kitagawa et al.,[100] operando XAS revealed that a significant fraction of the Fe3+ nodes was electrochemically reduced to Fe2+ upon application of cathodic potential, even while the macroscopic framework remained structurally intact. This critical finding implies that the genuine catalytically active centers are likely to be these in situ generated Fe2+ species rather than the original Fe3+ nodes. In this context, the MOF acts as a "dynamic platform" that securely anchors these transient active sites while maintaining a highly tailored coordination microenvironment. Another illustrative example is the Co-TATB system reported by Zuo et al.[89] By coupling in situ Raman spectroscopy with theoretical calculations, the authors meticulously tracked the potential-dependent adsorption configuration of the *NO intermediate on the catalyst surface. Their work elucidated how the symmetric cluster architecture dynamically stabilizes key intermediates, thereby facilitating subsequent kinetically demanding hydrogenation steps. Ultimately, it is evident that dynamic structural and electronic evolution is an intrinsic property of MOF-based eNO3RR catalysts—one that must be rigorously integrated into both experimental characterization and theoretical modeling.
3.9.2 In situ/Operando Characterization Techniques
To unravel the dynamic structural evolution of catalysts and definitively identify their genuine active sites during the reaction process, researchers have developed and deployed a variety of sophisticated in situ and operando characterization techniques. These advanced methodologies enable the real-time monitoring of catalyst structural transformations, the transient adsorption behavior of reaction intermediates, and the dynamic distribution of chemical products. Consequently, they provide the critical experimental evidence required to establish accurate, operationally relevant structure-activity relationships.
(1) In situ X-ray Absorption Spectroscopy (XAS)
In situ XAS—comprising both X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS)—can simultaneously probe the electronic valence state of active sites and their local coordination environment, including bond lengths and coordination numbers. This dual capability makes XAS one of the most powerful analytical tools for identifying genuine active centers. In the specific context of MOF-based eNO3RR, in situ XAS has been instrumental in tracking the dynamic evolution of metal nodes. Beyond the previously discussed Fe-pyNDI system,[100] in situ XAS has been employed to investigate a series of metal single-atom catalysts (M-SACs), revealing critical valence state fluctuations and assessing the stability of different metal centers under applied potentials.[88] Drawing parallels from non-MOF systems, Beatriz et al.[196] from the Fritz-Haber Institute integrated quasi-in situ XPS with operando XAS to reveal that Cu predominantly exists in the metallic state (Cu0) during the complete NO3--to-NH3 conversion over Cu2O nanocubes. Conversely, CuI species only dominate within the specific potential window corresponding to the 2-e− reduction of NO3--to-NO2-. This pivotal finding provides a vital reference for understanding the dynamic reconstruction of Cu-based active sites. It strongly suggests that analogous MOF-based catalysts may undergo similar in situ reduction pathways.
(2) In situ Raman Spectroscopy
In situ Raman spectroscopy provides crucial vibrational "fingerprint" information on surface-adsorbed species while simultaneously monitoring structural changes within the catalyst framework. Because it is highly applicable in aqueous environments, Raman spectroscopy is exceptionally suited for the real-time tracking of surface reactions during eNO3RR. For instance, Zhu et al.[197] utilized in situ Raman spectroscopy to investigate the dynamic reconstruction of nanoporous Cu-Co alloys during NO3- reduction. Their analysis revealed that the Cu-Co alloy undergoes an in situ surface reconstruction to form a Cu/CoOOH heterojunction, driven by chemical and electrochemical equilibria with NO3- species in solution. This newly formed heterojunction exhibits a significantly stronger binding affinity for the *NO2 intermediate, thereby lowering the thermodynamic energy barrier for subsequent NO2H formation. Similarly, in the evaluation of Cu/a-CeOx catalysts, Li et al.[198] utilized in situ Raman alongside in situ XRD to definitively confirm that initial Cu2O species are completely converted to metallic Cu0 under reaction conditions, verifying that the true active species is the zero-valent metal rather than the pristine cuprous oxide phase.
(3) In situ X-ray Diffraction (XRD)
In situ XRD facilitates the continuous monitoring of crystallographic evolution and the emergence of new phases under reaction conditions. This technique is indispensable for determining whether a highly ordered MOF collapses into an amorphous state or transforms into a new, stable crystalline phase. Among notable related studies, Li et al.[199] used Cu2O as a model electrocatalyst to systematically elucidate the in situ reconstruction mechanisms of Cu-based oxides during NO3- reduction, employing in situ electrochemical XAFS and XRD. Their work revealed that the stable operation of the catalyst is governed by a delicate "2D electrochemical window" defined by the applied potential and the local NO3- concentration. Under high NO3- concentrations and appropriate cathodic bias, a dynamic equilibrium is established between the loss of lattice oxygen and its continuous replenishment from incoming NO3- ions. In stark contrast, under conditions of low NO3- concentration and highly negative potentials, the catalyst undergoes continuous lattice oxygen depletion and extensive reduction of Cu+ to Cu0, ultimately resulting in an irreversible phase transformation and complete morphological collapse. The conceptualization of this "2D electrochemical window" provides a robust analytical framework for predicting and understanding the operational stability boundaries of MOF-based catalysts in neutral media.
(4) Differential Electrochemical Mass Spectrometry (DEMS)
DEMS enables continuous, online quantitative detection of volatile products generated during electrochemical reactions. For eNO3RR, DEMS enables real-time monitoring of gaseous effluents—such as N2, N2O, and NO—thereby facilitating comprehensive analysis of the reaction pathway and precise quantification of selectivity toward undesired side reactions. Although the integration of DEMS into MOF-based eNO3RR is not yet widespread, its mature application in other electrocatalytic domains has thoroughly demonstrated its immense analytical potential. For example, DEMS can quantitatively distinguish the direct competition between NH3 production and the evolution of N2/N2O byproducts, thus experimentally validating the completeness of proposed theoretical reaction pathways.[126,200-201] Coupling DEMS with structural probes like XAS and Raman spectroscopy enables holistic, full-process tracking—from the initial atomic-level structural evolution of the catalyst to the final macroscopic distribution of reaction products.
(5) Other in situ Techniques
Beyond the primary methodologies detailed above, a suite of supplementary in situ techniques provides vital corroborating data. In situ XPS is routinely employed to continuously monitor surface elemental valence states under applied bias.[126] Meanwhile, in situ FTIR spectroscopy excels at identifying the specific adsorption configurations of key nitrogen-containing intermediates, including NO3-, NO2- and *NH2.[74,162] Additionally, advanced techniques such as in situ synchrotron X-ray fluorescence (SXRF) can clearly track the microscopic migration and potential aggregation of single-atom catalysts under rigorous reaction conditions.[202] It is the synergistic, combined application of these diverse in situ techniques that makes it possible to construct a complete, unambiguous mechanistic picture of the eNO3RR process.
In summary, in situ and operando characterization techniques have firmly established themselves as the core analytical tools required for revealing the dynamic evolution of MOF-based catalysts, definitively identifying true active sites, and clarifying complex reaction mechanisms. In future research paradigms, the strategic integration of multiple in situ methodologies should be pursued to achieve multi-dimensional, full-process tracking—spanning from the atomic scale (XAS) to the molecular scale (Raman, IR) and ultimately culminating in macroscopic product distribution (DEMS).
4 Comparative Assessment with Benchmark Non-MOF Catalysts
The preceding sections have illustrated that MOF-based materials exhibit excellent structural tunability and immense catalytic potential for eNO3RR under neutral conditions. To objectively evaluate their feasibility for practical, industrial-scale applications, we have collated recent research findings on state-of-the-art non-MOF electrocatalysts and conducted a rigorous comparative analysis against the aforementioned MOF-based systems. This comparison aims to delineate the relative strengths and inherent limitations of these two distinct categories of catalytic materials.
4.1 Activity and Stability
In terms of FE, the performance metrics of state-of-the-art MOF-based catalysts are fully comparable to those of premier non-MOF catalysts. As shown in Table 3, MOF systems such as Cu-SA/UiO-bpy (FE=98.1%),[59] Co-TATB (FE=98%),[89] and NJUZ-2&3 (FE=98.8%)[117] approach the theoretical maximum for NH3 selectivity. These values stand toe-to-toe with benchmark non-MOF systems, including Au/Cu single-atom alloys (FE=99.69%) and Ni1Cu single-atom alloys (FE≈100%).[203-204] This confirms that MOFs can successfully engineer electronic and coordination microenvironments that are equally conducive to the highly selective conversion of NO3- to NH3.
Table 3 Performance and efficacy comparison between non-MOF catalysts and MOF-based catalysts in neutral electrolytes |
| Electrocatalyst | Electrolyte | NH3 yield | FENH3 | Optimal potentials | Stability | Cost Evaluation | Ref. |
|---|---|---|---|---|---|---|---|
| Au/Cu SAA | 0.5 mol•L−1 Na2SO4+100 ppm of NO3−−N | 0.193 mmol•h−1•cm−2 | 99.69% | −0.8 V | 45 h | High | [203] |
| Bi/Cu2O | 0.5 mol•L−1 K2SO4+1000 ppm NO3− | 0.150 mmol•h−1•mgcat−1 | 99.2% | −0.8 V | 14 h | Low | [208] |
| Ni1Cu-SAA | 0.5 mol•L−1 K2SO4+200 ppm NO3−−N | 0.326 mmol•h−1•cm−2 | ≈100% | −0.55 V | ≈8.89 h | Medium | [204] |
| Rh@Cu-0.6% | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 1.27 mmol•h−1•cm−2 | 93 % | −0.2 V | 30 h | High | [206] |
| Fe-N-C | 0.1 mol•L−1 K2SO4 +0.5 mol•L−1 KNO3 | 0.46 mmol•h−1•cm−2 | 75% | −0.66 V | 20 r | Medium | [208] |
| P-Cu0.51Ni0.49 | 0.5 mol•L−1 K2SO4 + 50 ppm KNO3 | 94.94 μmol•h−1•cm−2 | 98.38% | −0.55 V | 10 h | Low | [209] |
| hcp/fcc Cu10Ni₉₀ | 0.5 mol•L−1 K2SO4 + 100 mmol•L−1 KNO3 | 3.37 mmol•h−1•mgcat−1 | 98.1% | −0.7 V | 12 r | Low | [210] |
| Cl-Cu | 0.1 mol•L−1 K2SO4 + 50~200 mg•L−1 NO3−−N | 4.32 μmol•h−1•cm−2 | 82.5% | −0.65 V | >100 h | Low | [207] |
| Cu-SA/UiO-bpy | 0.5 mol•L−1 K2SO4 + 0.1 mol•L−1 KNO3 | 434.5 μmol•h−1•cm−2 | 98.1% | −0.8 V | 10 r | Medium | [59] |
| NJUZ-2 | 0.1 mol•L−1 Na2SO4 + 0.1 mol•L−1 KNO3 | 253.7 mmol•h−1•mgcat−1 | 98.8% | −0.8 V | 70 h | Medium | [117] |
| Zn5-NiS4 TP MOF | 0.05 mol•L−1 K2SO4 + 0.5 mol•L−1 KNO3 | 1.348 mmol•h−1•mgcat−1 | 92.87% | −1.3 V | 20 r | High | [47] |
| FeNx-PC-Fe NPs | 0.5 mol•L−1 Na2SO4 + 1.0 mol•L−1 NaNO3 | 1.77±0.06 mmol•h−1•mgcat−1 | 98.4% | −1 V | 120 h | Medium | [156] |
| CuNi0.75-MOF/NF | 0.5 mol•L−1 Na2SO4 + 0.1 mol•L−1 NaNO3 | 3.04 mmol•h−1•cm−2 | 95.88% | −1 V | 10 r | Low | [74] |
| HUST-38 | 0.5 mol•L−1 K2SO4 + 0.1 mol•L−1 NO3− | 0.785 mmol•h−1•mgcat−1 | 95.7% | −0.6 V | 96 h | Medium | [44] |
The "Cost Evaluation" section is assessed based on the total cost, including synthesis and replacement of the material. For ease of comparison, the yield data have been converted to molar units. (MNH3=17.03 g•mol−1). The number of cycles (r, run) and operation duration (h, hour) are selected based on stability data. |
However, a different picture emerges when comparing mass-normalized activity (i.e., NH3 yield per milligram of catalyst). The highest reported values for non-MOF catalysts, such as the hcp/fcc Cu10Ni90 alloy (57.4 mg•h−1• mgcat−1),[205] exceed those of most pristine MOFs by a factor of two or more. Even the most highly active MOF-based systems—such as Zn5-NiS4TP (23.5 mg•h−1•mgcat−1)[47] and the MOF-derived FeNx-PC-Fe NPs (30.1 mg•h−1• mgcat−1)[156]—remain below this benchmark. This disparity likely reflects the inherently higher electrical conductivity and the optimal metal loading achievable in alloy- and carbon-based non-MOF catalysts, which facilitate more efficient charge transfer and maximize active-site utilization per unit mass.
Conversely, when catalytic activity is normalized to the electrode's geometric area, MOF-based catalysts demonstrate a distinct and compelling advantage. For instance, the CuNi0.75-MOF/NF electrode achieves an exceptional area-specific yield of 51.78 mg•h−1•cm−2,[74] significantly surpassing the best area-normalized values reported for non-MOF catalysts (e.g., Rh@Cu-0.6% at ≈21.6 mg•h−1•cm−2).[206] This suggests that the porous architecture and tunable morphology of MOFs—particularly when grown in situ on conductive 3D substrates like Ni foam—enable extraordinarily high catalyst loadings and efficient mass transport. This translates directly into superior performance per unit of geometric area, arguably the most critical metric for the practical sizing and design of industrial electrolysers.
Stability remains the most critical hurdle to the industrial deployment of these catalysts. In the cycling and stability tests conducted across relevant studies, most MOF-based materials exhibit performance comparable to their non-MOF counterparts over short durations, typically maintaining stable catalytic activity for 8 to 24 hours. The MOF with the longest reported continuous stability to date is HUST-38, which maintained its performance for 96 hours at an applied potential of −0.6 V.[44] MOF-derived materials generally exhibit slightly superior durability; for instance, the FeNx-PC-Fe NPs architecture successfully passed a 120-hour stability test, a performance fully on par with high-end non-MOF catalysts.[156,207] Nevertheless, most highly active MOF-based materials still lack empirical stability data for extended, multi-hundred-hour durations. Their long-term viability under the grueling operational conditions required for industrialization remains to be rigorously verified. Overall, the structural integrity of MOF units—specifically, the metal-ligand bonds—must be further strengthened against sustained electrochemical polarization in aqueous media to reliably withstand industrial production conditions.
4.2 Economic and Scalability Considerations
Beyond raw catalytic performance, the scalability potential and economic viability of MOF-based catalysts demand rigorous evaluation. Setting aside the use of noble metals—which universally cause a sharp surge in mass-production costs regardless of the support material—non-MOF catalysts generally benefit from relatively facile and highly scalable synthetic routes. For example, CuNi alloys can be mass-produced via room-temperature electrodeposition or simple thermal reduction, while Fe-N-C materials are typically prepared through the one-step pyrolysis of inexpensive precursors.[208] In stark contrast, the fabrication of the high-performance MOFs summarized previously relies heavily on solvothermal synthesis, which requires elevated temperatures, extended reaction times, and the extensive use of organic solvents such as DMF. Furthermore, some architectures require intricate post-synthetic modification steps, such as metalation or ligand exchange. Collectively, these factors contribute to a substantially higher projected per-unit-mass production cost for MOF-based catalysts.
However, a direct economic comparison based solely on raw synthesis expenses is overly simplistic and potentially misleading. We argue that the ultimate economic evaluation metric for the electrochemical conversion of NO3- to NH3 must be the levelized cost of NH3 production (USD•kg−1 NH3). This holistic metric integrates multiple interconnected factors, including the initial catalyst price, operational replacement frequency, system energy consumption, and overall Faradaic efficiency. Viewed through this lens, the ability of certain MOF-based integrated electrodes to deliver exceptionally high area-specific yields (e.g., the aforementioned 51.78 mg•h−1•cm−2 for CuNi0.75-MOF/NF) could directly translate into smaller electrolyser footprints and vastly reduced capital expenditures for plant infrastructure, effectively offsetting the higher initial catalyst costs. Furthermore, for specialized applications that demand ultra-high selectivity or the ability to sustain industrial-scale current densities (e.g., DiMe-Cu3-MOF operating at 950 mA• cm−2 with 95% FE),[95] the unique, tailored capabilities of MOFs may fully justify their premium synthetic price tag.
5 Current Challenges and Future Outlook
5.1 Challenges for Large-Scale Application
Although MOF-based catalysts have made remarkable progress in laboratory-scale eNO3RR, translating these foundational research findings into practical, industrial-scale applications still face severe engineering and scale-up challenges.
First, the reproducible, large-scale synthesis of complex MOFs remains a significant hurdle. Conventional solvothermal methods—while highly effective for yielding pristine, high-quality crystals in an academic setting—typically require prolonged reaction times, elevated temperatures and pressures, and substantial volumes of toxic organic solvents. These requirements render them both economically and environmentally unsustainable for industrial-scale production.[211-212] Scalable synthetic approaches, such as mechanochemistry, continuous-flow chemistry, room-temperature precipitation, and microwave-assisted synthesis, have been successfully applied to the synthesis of prototype MOFs, including ZIF-8, HKUST-1, and UiO-66.[213] However, for the more complex, diverse range of functionalized MOFs specifically developed for eNO3RR, critical metrics such as batch-to-batch reproducibility, high product yield, and overall cost-effectiveness remain completely unproven on scale. Establishing standardized, "green," and economically viable synthesis routes is an absolute prerequisite for their eventual industrial adoption.
Second, the fabrication of binder-free electrodes is essential to fully preserve MOF functionality at scale. Traditional electrode preparation involves mechanically mixing MOF powders with inert polymeric binders (e.g., Nafion or PTFE) and subsequently coating them onto current collectors. This brute-force approach frequently results in severe pore blockage, drastically reduced active-site accessibility, and a substantial increase in charge-transfer resistance.[214] To overcome these physical limitations, advanced fabrication strategies—such as the in situ growth of MOFs directly onto conductive substrates (e.g., carbon paper or Ni foam) or the construction of self-supported monolithic MOF architectures—have been actively developed.[115,212] Notably, integrating MOFs with highly conductive scaffolds, such as carbon nanotube networks, enables scalable, binder-free electrode fabrication while maintaining the framework's structural integrity and significantly enhancing macroscopic e− transport.[214]
Finally, the long-term operational stability of these materials under authentic, rigorous working conditions must be comprehensively evaluated. Although neutral aqueous electrolytes are generally milder than strongly acidic or highly alkaline media, the structural degradation of MOFs can still occur relentlessly via hydrolysis, ligand oxidation, or the cathodic leaching of metal nodes. Furthermore, as previously discussed, recent studies have revealed that many pristine MOF materials undergo profound surface reconstruction under applied cathodic potentials, frequently transforming into metal (oxy)hydroxides or metallic nanoparticles, which serve as the actual active centers during the reaction.[195] Consequently, future research must not only report the initial, transient catalytic performance of these materials but must also rigorously employ in situ characterization techniques—such as XAS and XRD—to dynamically track structural evolution. Establishing a definitive correlation between this structural evolution and long-term catalytic durability is the only viable path forward for designing robust MOF architectures capable of sustained industrial operation.
5.2 Future Outlook
Progress will likely depend on the atomic-level precision of design, guided by realistic operating constraints. Developing intrinsically stable frameworks—e.g., using high-valent metal clusters (Zr, Cr) and/or hydrophobic motifs to resist H2O-induced erosion—remains a foundational direction. Mechanistically, greater reliance on in situ/operando characterization (X-ray Absorption Fine Structure, XAFS and Attenuated Total Reflectance-Fourier Transform Infrared, ATR-FTIR, etc.) is essential to capture transient intermediates and track dynamic structural evolution under neutral polarization, enabling identification of true active centers and rate-limiting steps. Application-oriented studies should move beyond model electrolytes to quantify performance, toxin tolerance, and regeneration in real H2O. At the system level, pairing cathodic NO3− reduction with anodic value-added oxidation offers an attractive route to reduce cell voltage and improve energy efficiency. Finally, integrating high-throughput computation, machine learning, and synthesis-testing loops may accelerate discovery of optimal MOF chemistries and architectures for neutral eNO3RR, helping move the technology from laboratory proof-of-concept toward practical H2O remediation and green NH3 production.
5.3 Industrialization Roadmap
As detailed above, translating advanced laboratory research into a commercially viable industrial process is a multifaceted challenge that requires the seamless integration of materials innovation, reactor engineering, and system-level optimization. Drawing on recent progress in MOF-based eNO3RR and established expertise in industrial electrochemistry,[215] we propose a phased technology roadmap to bridge the gap between academic achievements and practically deployable technologies (Figure 10).
In the near-term (2026—2028), the focus must be on lab-scale validation under industrially relevant conditions. Priority should be given to demonstrating MOF-based catalysts in flow-cell configurations at current densities exceeding 100 mA•cm−2, with an NH3 FE>90% over at least 100 hours of continuous operation.[216] This phase entails several critical milestones, including the establishment of standardized protocols for evaluating stability and selectivity in the presence of common coexisting ions such as Cl−, SO42−, PO43−. Simultaneously, the synthesis of the most promising MOF candidates, such as Zr-MOFs or cMOFs, must be scaled up to hundred-gram batches with reproducible quality, while operando techniques are employed to definitively validate the true active species under high-current-density operation.
Transitioning to the mid-term (2028—2030), research will shift toward pilot-scale integration and wastewater validation. Building upon the achievement of preliminary goals, the construction of membrane electrode assembly (MEA) electrolysers with MOF-based cathodes will be pursued, targeting an NH3 yield of >5 mmol•h−1•cm−2 and a specific power consumption of <30 kWh•kg−1 NH3.[215] It is also imperative to verify the long-term stability of the catalyst (>1000 hours) in authentic wastewater systems—such as agricultural runoff or industrial effluents—containing organic substances and competing anions. During this period, comprehensive techno-economic analysis (TEA) and life cycle assessment (LCA) will be conducted to benchmark the technology against the traditional Haber-Bosch NH3 synthesis process in terms of production cost (USD• kg−1 NH3) and total carbon footprint.[217]
Ultimately, the long-term (2030—2035) vision centers on distributed NO3--upcycling demonstrations to deploy modular, renewable-energy-driven eNO3RR systems at point-source pollution sites. Key milestones at this stage include integrating MOF-based electrolysers with renewable energy sources, such as solar photovoltaics and wind power, to support autonomous, off-grid operation.[215] Furthermore, the systems should achieve an NH3 recovery efficiency of >80% in wastewater with NO3− concentrations ranging from 100—1000 ppm NO3−−N. To further maximize energy efficiency and economic benefits, integrated processes coupling cathodic NO3- reduction with anodic high-value-added oxidation reactions, such as alcohol or biomass oxidation, will be developed. This roadmap, while ambitious, is grounded in the rapid progress of MOF design and electrochemical engineering. Achieving these milestones will require close collaboration among synthetic chemists, electrochemical engineers, and sustainability analysts—transforming NO3− pollution from an environmental liability into a distributed resource for green NH3 production.
6 Conclusions
MOFs offer a distinctive and powerful materials platform for electrocatalytic NO3−-to-NH3 conversion under neutral conditions. Their tunable architectures, high surface areas, and capacity to host well-defined active motifs—combined with diverse functionalization and compositing strategies—enable targeted control over NO3− adsorption/activation, PCET kinetics, and suppression of competitive HER and NO2− accumulation. By carefully designing metal nodes, organic linkers, and pore microenvironments, MOF-based catalysts can meet the simultaneous requirements of high activity, high selectivity, and robust stability in neutral aqueous media. Although challenges persist—especially long-term durability, performance in complex real H2O, and scalable manufacturing—continued mechanistic clarification and materials engineering, supported by operando analytics and interdisciplinary system design, position MOF-enabled eNO3RR as a realistic bridge between NO3− pollution control and the emerging green NH3 economy. If these advances converge, electrocatalytic NO3− upcycling could become a practical lever to close the nitrogen cycle and advance sustainable resource recovery. Ultimately, the most compelling promise of MOF-enabled neutral eNO3RR is its conceptual elegance: it turns an environmental liability into a chemical asset through increasingly precise, testable, and scalable catalyst design principles.
(Cheng, B.)