Electrochemical CO 2 reduction into chemicals and fuels holds great promise for renewable energy storage and carbon recycling 1–3 . Although high-temperature CO 2 electroreduction in solid oxide electrolysis cells is industrially relevant, current catalysts have modest energy efficiency and a limited lifetime at high current densities, generally below 70% and 200 h, respectively, at 1 A cm − 2 and temperatures of 800 °C or higher 4–8 . Here we develop an encapsulated Co–Ni alloy catalyst using Sm 2 O 3 -doped CeO 2 that exhibits an energy efficiency of 90% and a lifetime of more than 2,000 h at 1 A cm − 2 for high-temperature CO 2 -to-CO conversion at 800 °C. Its selectivity towards CO is about 100%, and its single-pass yield reaches 90%. We show that the efficacy of our catalyst arises from its unique encapsulated structure and optimized alloy composition, which simultaneously enable enhanced CO 2 adsorption, moderate CO adsorption and suppressed metal agglomeration. This work provides an efficient strategy for the design of catalysts for high-temperature reactions that overcomes the typical trade-off between activity and stability and has potential industrial applications.