LAUSR

Achieving selective electrochemical CO2 reduction reaction (CO2RR) in strong acid holds potential to resolve the "carbonate formation" problem yet is hindered by the competing hydrogen evolution reaction (HER). The interplay between different hydrogen sources (i.e., H+ vs H2O) and its impact on CO2RR selectivity in acidic electrolytes remains poorly understood. Herein, we introduce an immobilized N-containing azole layer (phTA) onto Bi catalysts to dynamically regulate the interfacial hydrogen source in strong acid (pH 0.4). Combining experimental and modeling approaches, we reveal a dual mechanism dependent on local interfacial conditions. At lower potentials with high local [H+], the protonated phTA layer (NH+ sites) serves as a proton relay while electrostatically shielding bulk H+ diffusion. Conversely, at higher potentials with low local [H+], less protonated phTA disrupts the interfacial hydrogen-bond network, impeding Grotthuss-type proton transport, thus suppressing HER through hydrogen source shifts toward H2O. As a result, the faradaic efficiency for formic acid (FEHCOOH) on Bi-phTA significantly increases to 36% at -300 mA cm-2 (pH 0.4) compared to <10% for bare Bi. This work demonstrates the controlled manipulation of the hydrogen source via a dynamically responsive organic layer, highlighting the importance of managing interfacial hydrogen species for enhancing the CO2RR in acid.