固体氧化物燃料电池运行初期电化学性能演变机制

doi:10.6023/A21020065

摘要/Abstract

固体氧化物燃料电池(solid oxide fuel cell, SOFC)是一种清洁高效的发电装置, 其运行中的性能衰减受到众多因素的影响, 如何分辨这些因素的贡献、并在此基础上确定衰减的主导因素, 对于更有针对性地提升SOFC的运行寿命至关重要. 广泛的测试结果表明, SOFC的电化学性能在运行初期(约前100 h)会发生最为显著且复杂的变化, 但是对其演变机制还没有形成清晰的认识. 本工作基于电化学阻抗谱(electrochemical impedance spectroscopy, EIS)分析方法分别研究了工业尺寸电池(100 cm²)和纽扣电池(0.45 cm²)在运行初期的电化学性能演变规律, 得到了关于电池性能和电极微观结构演变机制的一致性规律. 电池在运行初期依次经历活化阶段和老化阶段: 活化阶段阳极孔隙率增加、气相扩散过程改善, 电池性能逐渐上升; 老化阶段阳极Ni颗粒发生烧结团聚, 有效三相界面长度显著降低, 阳极界面反应过程劣化, 电池性能逐渐下降. 各阶段的持续时间、性能变化幅度会受到电池制备工艺的影响. 本工作提出的运行初期电化学性能演变机制能够为进一步研究SOFC的长期稳定性奠定基础.

关键词: 固体氧化物燃料电池, 稳定性, 衰减机理, 电化学阻抗谱, 弛豫时间分布

Solid oxide fuel cell (SOFC) is a clean and efficient power generation device, which has been widely used in recent years. However, its operation lifetime still needs to be improved to meet the requirements for further commercialization. The performance degradation of SOFCs is affected by many factors. It is very important to distinguish these factors and determine the dominant factor(s) for improving the operation lifetime in a more efficient manner. In this paper, the initial operation stage (<100 h) of SOFCs is mainly focused, in which the electrochemical performance changes most significantly. The electrochemical impedance spectroscopy (EIS) was periodically monitored during the operation process. Individual electrode processes of button cells and industrial-size cells were distinguished via distribution of relaxation time (DRT) method and subsequent equivalent circuit model (ECM) fitting. Through detailed time-varying analysis of EIS, the changes of different electrode processes in the initial-stage operation were obtained, and the evolution mechanism of electrochemical performance was proposed. After reduction, SOFCs go through activation stage and aging stage in turn: i) In the activation stage, the anode porosity increases and the gas-phase diffusion process is enhanced, resulting in the improvement of cell performance. ii) In the aging stage, the Ni particles in the anode agglomerate, so the effective three phase boundary (TPB) density decreases, which leads to the degradation of anode charge transfer reaction and the drop of cell performance. This evolution mechanism of electrochemical performance was partially confirmed by detailed microstructure characterization. It should be mentioned that the duration of each stage and the performance change in each stage can be affected by the initial electrode structure. Therefore, in previous studies, it was observed that some cells did not show activation process during initial-stage operation, but directly entered the aging stage. Besides, during longer-term operation of the cell (>100 h), we speculate that the dominant degradation mechanism is the continuous increase of the anode interface reaction-related resistance, which is caused by the agglomeration of Ni particles. However, since the anode structure is gradually stable, the degradation rate of cell performance decreases with respect to the operation time.

Key words: solid oxide fuel cell, stability, degradation mechanism, electrochemical impedance spectroscopy, distribution of relaxation time

引用此文

吕泽伟, 韩敏芳, 孙再洪, 孙凯华. 固体氧化物燃料电池运行初期电化学性能演变机制[J]. 化学学报, 2021, 79(6): 763-770.

Zewei Lyu, Minfang Han, Zaihong Sun, Kaihua Sun. Evolution of Electrochemical Characteristics of Solid Oxide Fuel Cells During Initial-Stage Operation[J]. Acta Chimica Sinica, 2021, 79(6): 763-770.

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图/表 9

图1. (a)工业尺寸电池初期稳定性测试结果; (b)活化后电池在不同温度下的输出性能; (c)电池在还原后(4 h)、活化后(20 h)以及老化后(44 h)的输出性能; 以及电池活化前后EIS比较: (d) Nyquist图, (e) Bode图, (f) DRT计算结果. 稳定性测试中阳极H2流量为1 L?min-1, 阴极空气流量为3 L?min-1; IV、EIS测试中阳极H2流量为3 L?min-1, 阴极空气流量为6 L?min-1.

Figure 1. (a) The initial stability test results of industrial size cell. (b) The IV curves of the activated cell at different temperatures. (c) The cell performance after reduction (4 h), activation (20 h) and aging test (44 h). Comparison of EIS before and after the activation process: (d) Nyquist plots, (e) Bode plots, (f) DRT calculation results. The H2 flow fed to anode was 3 L?min-1 and the air flow fed to cathode was 6 L?min-1 during the measurements of IV and EIS characteristics. While for the stability tests, the H2 flow fed to anode was 1 L?min-1, and the air flow fed to cathode was 3 L?min-1.

图2. (a)纽扣电池初期稳定性运行结果. (b)电池运行中不同时刻测得的IV曲线

Figure 2. (a) The initial stability test results of a button cell. (b) The IV curves of the cell measured at different operation time

图3. 电池运行中不同时刻测得的EIS: (a) Nyquist图, (b) Bode图; 运行中不同时刻的DRT计算结果: (c) 4~25 h, (d) 25~63 h

Figure 3. EIS plots of the cell measured at different operation time: (a) Nyquist plots, (b) Bode plots. Comparison of DRT results calculated at different operation time: (c) 4~25 h, (d) 25~63 h

图4. 电池运行中不同时刻EIS等效电路拟合结果

Figure 4. The equivalent circuit fitting results of EIS measured at different times during cell operation.

图5. 不同运行阶段电池电解质-阳极界面附近的微观形貌: (a)未还原, (b)还原4 h, (c)还原4 h、恒电流运行20 h, (d)还原4 h、恒电流运行60 h

Figure 5. The morphology of anode-electrolyte interface in different operation stages of the cell: (a) before reduction, (b) reduction for 4 h, (c) reduction for 4 h and galvanostatic operation for 20 h, (d) reduction for 4 h and galvanostatic operation for 60 h

图6. 不同运行阶段阳极功能层微观形貌: (a)还原4 h, (b)还原4 h、恒电流运行20 h, (c)还原4 h、恒电流运行60 h

Figure 6. The morphology of anode functional layer in different operation stages of the cell: (a) reduction for 4 h, (b) reduction for 4 h and galvanostatic operation for 20 h, (c) reduction for 4 h and galvanostatic operation for 60 h

图7. SOFC运行初期Ni-YSZ阳极微观结构演变机制

Figure 7. Evolution of Ni-YSZ anode morphology in initial-stage operation of SOFC

图8. 本文中使用的工业尺寸电池、纽扣电池以及电池截面微观形貌

Figure 8. The industrial-size cell and button cells used in this study, and the cross-section SEM image of the industrial-size cell

Peak	P₀	P₁	P₂	P₃	P₄	P₅
τ/s^a	<1×10^-5	2×10^-5~1×10^-4	1×10^-4~1×10^-3	1×10^-3~8×10^-3	9×10^-3~9×10^-2	1×10^-1~2×10⁰
Process	Calculation deviation caused by inductance	Ionic transport in anode bulk	Anode charge transfer reaction	O₂ surface exchange & diffusion in cathode	Anode gas-phase diffusion	Anode gas conversion and diffusion

表1. DRT中各特征峰对应的电极过程

Table 1. The electrode process corresponding to each characteristic peak in DRT

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