Dynamic assembly of interfacial organic cations enables highly stable and selective CO ₂ electroreduction in acid

Acidic CO 2 electroreduction offers enhanced carbon utilization efficiency compared to neutral/alkaline systems but faces dual challenges of hydrogen evolution dominance and salt precipitation–induced instability when using conventional metal cation electrolytes. Here, we report a tetramethylammonium (TMA + ) cation–mediated strategy that simultaneously achieves exceptional selectivity and stability toward CO production over Au nanoparticles, a near-unity Faradaic efficiency at an industrial-level current density, while maintaining continuous operation for 2600 hours without salt precipitation. Through in situ electrochemical atomic force microscopy, we directly visualize the potential-dependent dynamic assembly of TMA + into multilayered structures within the electric double layer, a phenomenon previously predicted theoretically. The assembled TMA + layers disrupt the hydrogen-bond network for proton transport, and meanwhile, their hydrophobicity and the high TMA + -bicarbonate solubility prevent salt precipitation, which leads to both substantial selectivity and stability. This work not only introduces an advanced paradigm for propelling the CO 2 electrolysis toward practical applications but also broadens our understanding of the cation effect.