Mitigation of chromium poisoning in solid oxide fuel cell cathodes
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Citation
Abstract
Solid oxide fuel cells (SOFCs) have several advantages as electrochemical energy generation devices, including high energy conversion efficiency, easily sequesterable emissions, and high fuel flexibility. However, durability issues still limit the commercial viability of SOFCs. At high temperature operating conditions, volatile hexavalent chromium oxide/hydroxide species are formed from the thermally grown oxide (TGO) scale on stainless-steel interconnects. These vapor phase species are reduced and deposited on the electrochemically active areas in the cathode. This phenomenon, known as ‘chromium poisoning,’ is one of the primary reasons for long-term degradation of SOFC performance. In this study, two strategies were explored to mitigate Cr poisoning.
The first strategy is ‘electrochemical cleaning’, based on periodic reversible operation in a solid oxide electrolyser cell (SOEC) mode, which mitigates the deleterious effects of chromium poisoning by oxidizing the deposits species back into the vapor phase. Electrochemical cleaning of cells with lanthanum strontium manganite (LSM) cathodes showed that under the conditions tested, deposited Cr2O3 can be effectively removed, significantly reversing the deleterious effects of chromium poisoning. However, Cr-Mn spinel deposits that were also formed at the cathode/electrolyte interface remain. In order to demonstrate that electrochemical cleaning can be carried out periodically without damaging the cell, cells were subjected to three cycles of repeated poisoning and cleaning. It was found that a similar level of degradation and recovery happened after each cycle. After the third cycle, the cell still maintained the initial level of power output, indicating that periodic electrochemical cleaning under mild electrolytic conditions is an effective method to reverse chromium poisoning without damaging the cell.
The second strategy explored was the deposition of protective coatings on the stainless-steel interconnects. Deposition of spinel coatings by DC and AC electrophoretic deposition (EPD) on flat and porous SUS 430 stainless steel substrates was studied. The required variations in deposition parameters to obtain uniform EPD coatings on porous substrates with increasing geometrical complexity were explored. Four spinel compositions for these coatings were studied; CuMn2O4, CuNi0.2Mn1.8O4, MnCo2O4, and MnFe0.34Co1.66O4. The coatings were evaluated based on multiple criteria; including phase stability, microstructural stability, conductivity, Cr gettering ability, ability to act as a diffusion barrier to outward chromium and inward oxygen diffusion at 700ºC and 800ºC in dry and humid air, and the ability to limit the increase in the area specific resistance (ASR) during high temperature oxidation exposures. The results showed that CuNi0.2Mn1.8O4 was the best candidate for the coatings.