Durable CO 2 conversion in the proton-exchange membrane system.

Electrolysis that reduces carbon dioxide (CO 2 ) to useful chemicals can, in principle, contribute to a more sustainable and carbon-neutral future 1-6 . However, it remains challenging to develop this into a robust process because efficient conversion typically requires alkaline conditions in which CO 2 precipitates as carbonate, and this limits carbon utilization and the stability of the system 7-12 . Strategies such as physical washing, pulsed operation and the use of dipolar membranes can partially alleviate these problems but do not fully resolve them 11,13-15 . CO 2 electrolysis in acid electrolyte, where carbonate does not form, has therefore been explored as an ultimately more workable solution 16-18 . Herein we develop a proton-exchange membrane system that reduces CO 2 to formic acid at a catalyst that is derived from waste lead-acid batteries and in which a lattice carbon activation mechanism contributes. When coupling CO 2 reduction with hydrogen oxidation, formic acid is produced with over 93% Faradaic efficiency. The system is compatible with start-up/shut-down processes, achieves nearly 91% single-pass conversion efficiency for CO 2 at a current density of 600 mA cm -2 and cell voltage of 2.2 V and is shown to operate continuously for more than 5,200 h. We expect that this exceptional performance, enabled by the use of a robust and efficient catalyst, stable three-phase interface and durable membrane, will help advance the development of carbon-neutral technologies.