Photoelectrochemical (PEC) CO2 reduction has received considerable attention given the inherent sustainability and simplicity of directly converting solar energy into carbonbased chemical fuels. However, complex photocathode architectures with protecting layers and cocatalysts are typically needed for selective and stable operation. We report herein that bare CuIn0.3Ga0.7S2 photocathodes can drive the PEC CO2 reduction with a benchmarking 1 Sun photocurrent density of over 2 mA/cm(2) (at -2 V vs Fc(+)/Fc) and a product selectivity of up to 87% for CO (CO/all products) production while also displaying long-term stability for syngas production (over 44 h). Importantly, spectroelectrochemical analysis using PEC impedance spectroscopy (PEIS) and intensity-modulated photocurrent spectroscopy (IMPS) complements PEC data to reveal that tailoring the proton donor ability of the electrolyte is crucial for enhancing the performance, selectivity, and durability of the photocathode. When a moderate amount of protons is present, the density of photogenerated charges accumulated at the interface drops significantly, suggesting a faster charge transfer process. However, with a high concentration of proton donors, the H2 evolution reaction is preferred.

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