A single device combining the functions of a CO 2 electrolyzer and a formate fuel cell is a new option for carbon-neutral energy storage but entails rapid, reversible and stable interconversion between CO 2 and formate over a single catalyst electrode. We report a new catalyst with such functionalities based on a Pb–Pd alloy system that reversibly restructures its phase, composition, and morphology and thus alters its catalytic properties under controlled electrochemical conditions. Under cathodic conditions, the catalyst is relatively Pb-rich and is active for CO 2 -to-formate conversion over a wide potential range; under anodic conditions, it becomes relatively Pd-rich and gains stable catalytic activity for formate-to-CO 2 conversion. The bifunctional activity and superior durability of our Pb–Pd catalyst leads to the first proof-of-concept demonstration of an electrochemical cell that can switch between the CO 2 electrolyzer/formate fuel cell modes and can stably operate for 12 days.
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