Cation Buffering Mitigates Potential Oscillation for Efficient Acidic Ethylene Electrosynthesis

Electrochemical CO 2 reduction (CO 2 RR) to ethylene in acidic electrolytes offers a compelling route toward high single‐pass conversion efficiency by suppressing carbonate formation; however, at industrially relevant current densities, severe potential oscillations induce excessive alkali‐cation accumulation, triggering carbonate precipitation that deactivates Cu catalysts and compromises C‐C coupling. Here, we identify potential oscillation‐driven cation enrichment as a critical yet overlooked limitation in acidic CO 2 RR and introduce an alkali‐cation buffering strategy to address this challenge. By integrating a C 3 N 4 nanosheet buffering layer onto Cu cube catalysts, the local K + concentration in the Helmholtz layer is dynamically regulated, suppressing carbonate formation and stabilizing the electrochemical potential under high current operation. Molecular dynamics simulations reveal that the C 3 N 4 layer reduces K + concentration fluctuations across a wide potential window, while operando Raman and attenuated total reflection infrared spectroscopy confirm diminished carbonate accumulation and enhanced stabilization of C‐C coupling intermediates. As a result, the C 3 N 4 ‐Cu catalyst achieves an ethylene Faradaic efficiency of ∼66% and maintains operation for over 50 h at 700 mA cm −2 in strongly acidic electrolytes (pH ∼ 1). This work offers a general interfacial engineering principle to reconcile high‐rate operation, catalyst stability, and product selectivity, providing a viable pathway toward industrial‐scale acidic CO 2 RR.