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Non-faradaic electrochemical modification of catalytic activity (NEMCA) by the induction of an external electric field can improve catalyst performance. Previous reports individually investigated the effects of direct electric-field induction or co-adsorbed oxygen atoms for each as the possible activation factor of the NEMCA mechanism. We investigated the mechanism of NEMCA in CO2methanation in a solid-oxide cell (SOC) using density functional theory (DFT). DFT calculations were performed for the hydrocarbon species on Ni(111) with a directly applied electric field and co-adsorbed oxygen atoms, and we compared them to clarify which effect is the dominant factor in the NEMCA mechanism over CO2methanation based on practical SOC system conditions. The rate-determining steps (RDSs) of CO2methanation were discussed based on detailed kinetic simulations. We found that the direct effects of the electric field on surface adsorption differed for each intermediate and that all RDSs accelerated with the application of the electric field. As the number of co-adsorbed oxygen atoms increased, all intermediates adsorbed less strongly on the surface, CHO and CO2dissociations decelerated, and CH4desorption accelerated. The effect of the co-adsorbed oxygen atoms on the kinetic energy is larger than that of direct electric field induction. Detailed kinetic simulations revealed that overall CO2methanation accelerates in solid-oxide electrolysis cell mode (with decreasing oxygen coverage).