Decoupling Electric-Field and Short-Range Cation Effects in Acidic CO₂ Electroreduction

Alkali cations play vital roles in suppressing hydrogen evolution and enabling CO₂ reduction reaction (CO₂RR) in acidic media, yet whether their identity-dependent promotion originates from interfacial electric fields or short-range interactions remains under debate. Here, we decouple these contributions by combining dielectric-aware generalized modified Poisson-Nernst-Planck (GMPNP) modeling, in situ vibrational Stark analysis, and differential electrochemical mass spectrometry (DEMS). Low-coverage 4-mercaptobenzonitrile (MBN) on Au and Ag serves as an inert vibrational Stark probe of the local field. Both modeling and spectroscopy show that adding alkali cations strengthens the interfacial field under cathodic polarization, whereas replacing Li⁺ with K⁺ produces undetectable field change. Nevertheless, K⁺ enhances CO formation by up to five-fold relative to Li⁺, indicating the existence of cation-specific short-range effects. Increasing the cation concentration from 10 mM to 100 mM enhances both the field and CO formation without increasing the outer-Helmholtz-plane (OHP) cation concentration. Interfacial water spectra further reveal that K⁺ disrupts hydrogen bonding and undergoes greater partial dehydration under CO₂RR conditions. Thus, cation concentration governs a general field effect, whereas cation identity controls local water environment and short-range interactions.