Unravelling the carbonate issue through the regulation of mass transport and charge transfer in mild acid.

The electrochemical CO 2 reduction reaction (CO 2 RR) triggered by renewable electricity provides a promising route to produce chemical feedstocks and fuels with low-carbon footprints. The intrinsic challenge for the current CO 2 RR electrolyzer is the carbonate issue arising from the reaction between hydroxide and CO 2 . Acid CO 2 RR electrolyzers, in principle, can effectively solve the carbonate formation, but it remains inevitable practically. In this work, we thoroughly investigated the electrode processes of the CO 2 RR on the benchmark Ag catalyst in mild acid. The root of the carbonate issue arises from the imbalanced supply-consumption rate of protons-the electron transfer vs. mass transport. Regulating the hydrodynamics substantially reduces the proton diffusion length by 80%, increasing the single-pass carbon utilization efficiency of CO 2 -to-CO to 44% at -100 mA cm -2 . The fundamental difference between mass transport and electron transfer on the spatial and temporal scale still leads to unavoidable carbonate formation. Future work to design intrinsically active catalysts in strong acid or metal-cation-free media is critical to solving the carbonate issue.