Evolution of Electrochemical Characteristics of Solid Oxide Fuel Cells During Initial-Stage Operation

doi:10.6023/A21020065

Abstract

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

Cite this article

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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Fig. & Tab. 9

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.

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

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

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

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

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

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

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

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

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