S-Vacancy-Induced "Proton Fence Effect" Enables Selectivity Switching between CH and CO in Photo-Assisted CO Electroreduction.

Upcycling CO into high-value C products is impressive for achieving carbon neutrality and energy sustainability, while rational modulation of C product selectivity is one of the biggest challenges in electrocatalytic CO reduction reaction (eCORR) 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) to ultraselectively switch the C product between CH and CO during CORR, with Faraday efficiency of 93.6% and 95.3%, respectively. In situ measurements uncover that the photo-generated holes counteract Cu electroreduction to retain the intact structure of CuInS/CuS, while *OH dissociated from water can spontaneously anchor toward S 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 renders the exclusive formation of CO. In the absence of S, *CO can be further hydrogenated in a lower free energy/even spontaneously to afford CH. The proposed proton confinement effect furnishes a promising reference for the selectivity control of eCORR, and the photo-assisted electroreductive protocol demonstrates a paradigm of in situ stabilization of electron-intolerant catalytic structures.