Immobilized Azole Layer Tunes Interfacial Hydrogen Source for CO2 Electroreduction in Strong Acid

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 (FE_HCOOH) 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.