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Important contribution to understand degradation of solid electrolysers (SOEC)

Enhancing the amount of regenerative electricity (solar, wind) leads to the need for further storage possibilities for the surplus electricity (Germany first half year 2018: > 40% regeneratives). The storage could be realized in classical batteries or with electrolysers generating high-grade, useful hydrogen (and also, if necessary, pure oxygen). The high-temperature electrolysis offers the additional possibility to work in co-electrolysis mode, thereby transforming water vapor and carbon dioxide to hydrogen and carbon monoxide, a so-called synthetic gas (syngas). This syngas is a valuable basis of fuels or pre-chemicals.

At Jülich Research Center, an SOEC stack was recently operated for more than 18,000h. This is the longest SOEC duration reported so far. The stacks showed, in comparison to the fuel cell mode, an enhanced voltage degradation. Within a detailed post-mortem analysis, the stack was characterized and the reason for the higher degradation was found.

Originating from electrochemical/chemical/physical processes that are not fully understood up to now, the catalytically active nickel phase in the fuel electrode is removed. Due to this removal, the electrochemically active electrode zone shifts to the more coarse support, thereby enhancing the ohmic resistance (= higher degradation). The nickel depletion is not uniformly distributed over the entire cell area. The nickel is nearly completely depleted from the electrode in the region close to the fuel inlet, while this depletion is restricted to the first third of the electrode facing the electrolyte at the fuel-out area.

This nickel depletion is a fundamental degradation process which needs to be fully understood to mitigate or minimize it. This counts for both alternative materials development and operating conditions.

Original publications: