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This paper deals with degradation mechanisms of Ni-YSZ electrodes for solid oxide cells, mainly solid oxide electrolysis cells (SOECs), but also to some extent solid oxide fuel cells (SOFCs). Analysis of literature data reveals that several apparently different and even in one case apparently contradicting degradation phenomena are a consequence of interplay between loss of contact between the Ni-YSZ (and Ni-Ni particles) in the active fine-structured composite fuel electrode layer and migration of Ni via weakly oxidized Ni hydroxide species. A hypothesis that unravels the apparent contradiction and explains qualitatively the phenomena is presented, and as a side effect, light has been shed on a degradation phenomenon in solid oxide fuel cells (SOFCs) that has been observed during a decade.

Carbon formation during co-electrolysis of H 2 O and CO 2 in Ni-YSZ supported Solid Oxide Electrolysis Cells (SOECs) may occur, especially at high current density and high conversion. In order to evaluate the carbon formation limits, five galvanostatic tests were performed in this work at electrolysis current densities from 1.5 to 2.25 A/cm 2 and reactant (H 2 O + CO 2 ) conversion of up to 67%. Delamination and carbon nano-fibers were observed at the Ni-YSZ|YSZ interface for two cells with a dense microstructure operated at electrolysis current densities of 2.0 and 2.25 A/cm 2 and a conversion of 59% and 67% respectively. Based on the nominal gas composition, carbon formation only occurs, according to equilibrium thermodynamics, at a reactant conversion above 99%. Therefore, the observed carbon formation is ascribed to a change in gas composition due to the diffusion limitations within the Ni-YSZ support and the active Ni-YSZ electrode. Carbon nano-fibers were only observed close to the YSZ electrolyte, indicating a very reducing atmosphere and a large over-potential gradient in the active electrode, being highest at the interface to the bulk electrolyte and decreasing toward the Ni-YSZ support.

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