Investigation on the reaction mechanism of solid oxide co-electrolysis with different inlet mixtures based on the comparison of CO2 electrolysis and H2O electrolysis

•The different patterns of total resistance with current for different mixture were emphasized.•The causes for the different total resistance patterns were explored.•The electrode catalytic activity influenced significantly the activation resistance pattern.•The mechanism of co-electrolysis was explored based on the resistance patterns.•The contribution of electrochemical reduction of CO2 to the production of CO was experimentally clarified.