Degradation of Solid Oxide Cells during Co-Electrolysis of H2O and CO2: Carbon Deposition under High Current Densities

Abstract

During co-electrolysis of H2O and CO2 using solid oxide cells (SOCs) the risk of carbon deposition in the Ni-YSZ electrode under high current densities (~ 2.0 A/cm2) was studied in this work. Five galvanostatic tests were performed at current density between 1.5 and 2.25 A/cm2 and the average conversions of the reactants were no more than 66.8 %. Ni-YSZ electrode delamination and carbon nano-fibers could be observed after test at the Ni-YSZ | YSZ electrolyte interface for two of the cells. Thermodynamic calculation shows that the reactant conversion needed for carbon formation is above 99 %, far above the experimental conversions. The observed carbon formation may be caused by the gas diffusion limitations at high current densities. Carbon nano-fibers were only observed close to the YSZ electrolyte indicating a large over-potential gradient at the TPBs close to the electrolyte.