Acidic-alkaline tandem system for durable CO₂-CO-C₂

Tandem carbon dioxide reduction reaction (CO₂RR) to multicarbon products (CO₂-CO-C₂₊) such as ethylene represents a pivotal strategy for carbon valorization. However, conventional alkaline or neutral tandem systems limit the carbon dioxide utilization and stability due to severe (bi)carbonate formation. Here, we design a durable acidic-alkaline tandem system to efficiently synthesize C₂₊ products, especially ethylene. In the acidic CO₂RR step, a catalyst with precisely controlled metal site distance was used to regulate *COOH adsorption. Typically, a 99.7 ± 0.4% carbon monoxide Faradaic efficiency (FE) at 1.4 amperes per square centimeter can be achieved using a catalyst with 0.5-nanometer nickel sites, surpassing the previously reported metal catalysts. When this catalyst was integrated into the acidic-alkaline tandem system and coupled with the alkaline carbon monoxide electroreduction, the ethylene and C₂₊ FE reached 75.3 ± 1.2 and 92.5 ± 1.3% at 10 amperes, respectively, and remained stable for 400 hours. Our work provides solutions for the durable conversion of carbon dioxide to C₂₊ products from catalyst and electrolyzer design.