X-ray nano-spectroscopy study of two solid oxide cells operated for long-term in steam electrolysis

Abstract

This study addresses the degradation mechanisms of porous fuel electrode from an electrode (Ni/YSZ) and an electrolyte (Ni/CGO)-supported cell operated for long-term in steam electrolysis mode. Using a nanobeam, spatially resolved X-ray fluorescence and X-ray absorption spectroscopy are combined to acquire high-resolution compositional maps and to probe intra-granular changes with the evolution of the Ni chemical state. This experimental work highlights oxidized states of nickel (i.e. NiO/Ni(OH)2 species) in the case of Ni/CGO while for Ni/YSZ the chemical state is metallic. In both types of fuel electrodes, the Ni depletion occurs and the depletion length is reduced to around 1 μm in the case of Ni/CGO after 23,000 h in contrast to around 4 μm in the case of Ni/YSZ after 6100 h. In the case of Ni/CGO, the coating of Ni particles by the diffusion of Gd and Ce from the CGO contact layer prevents the Ni migration while in the case of Ni/YSZ the Ni migration is not suppressed. These results are in accordance with in-situ impedance data where the main difference observed between Ni/YSZ and Ni/CGO fuel electrodes is a deactivation of the fuel electrode as non-ohmic losses for Ni/YSZ in contrast to Ni/CGO.