Proton Environment Optimization Enhanced Asymmetric C₁-C₂ Intermediate Coupling for CO₂ Reduction to n-Propanol

The electrochemical conversion of CO₂ into value-added n-propanol (n-PrOH) product is pivotal for establishing a sustainable carbon cycle. However, the process is severely hindered by the kinetically sluggish and uncontrolled asymmetric C₁-C₂ coupling step. Here, we develop a proton modulation strategy to optimize the CO₂-to-C₃ pathway. Structural engineering of the gas diffusion electrode modulates the H₂O network environment at the triphasic interface, establishing a gradient H⁺ concentration environment that avoids the direct hydrogenation of *C₂ intermediates. The Faradaic efficiency (FE) for n-PrOH reaches 21% at a partial current density of 167.5 mA cm^-2 in a flow cell. In the membrane electrode assembly, the FE_n-PrOH remains 19% at a partial current density of 132.9 mA cm^-2. Mechanistic investigations identify *OCH₂CH₃ as the pivotal intermediate in n-PrOH formation. The simultaneous reduction of the formation energy barriers for both the C₁-C₁ and C₁-C₂ coupling rate-determining steps drives C₃ product formation. This work highlights the significant role of proton environment engineering as a crucial parameter in the selection of the C₃ product pathway.