Perovskite oxides are popular as cathode materials of solid oxide electrolysis cells,because of their good redox stability and high resistance to coke formation.Unexpectedly,a negative effect of Ni doping is found on Sr2Fe1.5-xNixMo0.5O (x=0,0.05,0.1,0.2) cathode for pure CO2 electroreduction at 800 ℃,although Ni is highly active for CO2 electroreduction.The CO2 electroreduction performance degrades with the increase of Ni doping amount.Various characterization techniques are used to disclose the negative effect.Ni doping decreases the perovskite stability under electroreduction conditions,Fe and Ni cations in the B-site are reduced to metal nanoparticles and SrCO3 forms on the surface of the perovskite.The phase instability results from the weaker Ni-O bond.Although the Fe-Ni nanoparticles are in favor of the CO2 electroreduction,too much SrCO3 and carbon deposition block the charge transfer and diffusion of oxygenous species on the cathode surface.