Cu/In interfacial proton relay enables efficient CO2-to-ethanol photoreduction in a ternary heterojunction.

Selectively converting carbon dioxide (CO2) to ethanol via photocatalysis offers a sustainable means of storing solar energy in chemical bonds. However, its efficiency is limited by an inadequate proton supply from water oxidation to drive the carbon‑carbon (CC) coupling step. This inefficiency leads to poor ethanol selectivity and promotes a competing hydrogen evolution side reaction, ultimately restricting overall system performance. Here, we designed a ternary In(OH)3-Cu2O/PCN heterojunction as a functionally partitioned catalyst: Polymeric carbon nitride (PCN) acts as the photoelectron donor, cuprous oxide (Cu2O) acts as the CO2 reduction center, and trace indium hydroxide (In(OH)3) is dedicated to water oxidation. Density Functional Theory (DFT) calculations reveal that water dissociation on In(OH)3 generates active hydrogen species (*H), which are subsequently transferred to adjacent Cu sites with a low migration barrier. This interfacial proton relay mechanism avoids the accumulation of high-energy *H intermediates on the active sites, which suppresses the competing hydrogen evolution reaction. Correspondingly, the energy barrier for the crucial adsorbed carbon monoxide (*CO) dimerization step is lowered from 0.93 eV to 0.78 eV. The catalyst demonstrates superior activity to most documented photocatalysts for CO2-to-ethanol conversion, operating under sacrificial agent-free conditions, achieving an ethanol production rate of 125.4 ± 5.1 μmol g-1 h-1 and a selectivity of 90.4% ± 2.8%. These findings highlight the promise of functionally partitioned architectures in steering complex multi-step photocatalytic transformations toward valuable multi‑carbon products.