Water‐Activated Proton Transfer Mediated by Subnanoscale CeO ₂ ‐Cu ₂ O Interfaces for Near‐Unity CH ₄ Selectivity in Photothermal CO ₂ Reduction

ABSTRACT Water‐mediated photothermal CO 2 reduction to CH 4 is promising for renewable energy storage and carbon neutrality, but its selectivity and efficiency are limited by sluggish water dissociation that serves as a critical proton source and the demanding eight‐electron transfer barrier of CH 4 formation. These issues lead to insufficient protons for *CO hydrogenation. Herein, we engineer subnanoscale CeO 2 ‐Cu 2 O heterostructured subnanowires with densely enriched CeO 2 ‐Cu 2 O interfaces to overcome this proton bottleneck. Experimental and theoretical simulation results demonstrate that these interfaces accelerate H 2 O dissociation and create a “proton‐rich microenvironment,” which boosts the proton supply rate to match the kinetic demand of *CO hydrogenation. This directs the protonation of *CO to *CHO instead of CO desorption; these intermediates then convert to *CH 2 O and *CH 3 O, achieving over 99% CH 4 selectivity and a record yield of 2818 μmol g −1 h −1 . This yield is 15‐fold higher than that of bulk and nanoparticle counterparts, underscoring the key role of subnanoscale interface enrichment in optimizing multielectron reactions.