S‐Vacancy‐Induced “Proton Fence Effect” Enables Selectivity Switching between CH ₄ and CO in Photo‐Assisted CO ₂ Electroreduction

Upcycling CO₂ into high-value C₁ products is impressive for achieving carbon neutrality and energy sustainability, while rational modulation of C₁ product selectivity is one of the biggest challenges in electrocatalytic CO₂ reduction reaction (eCO₂RR) due to the competing reaction pathways and thermodynamic limitation. Here, we showcase a "proton fence" strategy enabled by in situ adsorbed *OH on sulfur vacancies (S_V) to ultraselectively switch the C₁ product between CH₄ and CO during CO₂RR, with Faraday efficiency of 93.6% and 95.3%, respectively. In situ measurements uncover that the photo-generated holes counteract Cu²⁺ electroreduction to retain the intact structure of CuInS₂/CuS, while *OH dissociated from water can spontaneously anchor toward S_V to hinder the local proton migration, completely circumventing multiproton products. Meanwhile, the preferential desorption of *CO from Cu centers adjacent to the *OH-anchored S_V renders the exclusive formation of CO. In the absence of S_V, *CO can be further hydrogenated in a lower free energy/even spontaneously to afford CH₄. The proposed proton confinement effect furnishes a promising reference for the selectivity control of eCO₂RR, and the photo-assisted electroreductive protocol demonstrates a paradigm of in situ stabilization of electron-intolerant catalytic structures.