Elementary reaction modeling of reversible CO/CO2 electrochemical conversion on patterned nickel electrodes

CO/CO2 are the major gas reactant/product in the fuel electrode of reversible solid oxide cells (RSOC). This study proposes a two-charge-transfer-step mechanism to describe the reaction and transfer processes of CO-CO2 electrochemical conversion on a patterned Ni electrode of RSOC. An elementary reaction model is developed to couple two charge transfer reactions, C(Ni)+O2−(YSZ) ↔ CO(Ni)+(YSZ) +2e− and CO(Ni)+O2−(YSZ) ↔ CO2(Ni)+(YSZ)+2e−, with adsorption/desorption, surface chemical reactions and surface diffusion. This model well validates in both solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes by the experimental data from a patterned Ni electrode with 10 μm stripe width at different pCO (0-0.25 atm), pCO2 (0-0.35 atm) and operating temperature (600-700 °C). This model indicates SOEC mode is dominated by charge transfer step C(Ni)+O2−(YSZ)↔CO(Ni)+(YSZ) +2e−, while SOFC mode by CO(Ni)+ O2−(YSZ)↔CO2(Ni)+(YSZ)+2e− on the patterned Ni electrode. The sensitivity analysis shows charge transfer step is the major rate-determining step for RSOC, besides, surface diffusion of CO and CO2 as well as CO2 adsorption also plays a significant role in the electrochemical reaction of SOEC while surface diffusion of CO and CO2 desorption could be co-limiting in SOFC.