Engineering interfacial H-bond networks via orbital hybridization on CuBiO_2−x to boost proton-coupled electron transfer for CO₂-to-HCOOH electrosynthesis

The electrochemical CO₂ reduction reaction (CO₂RR) to formic acid (HCOOH) is constrained by slow kinetics and limited selectivity due to inefficient proton-coupled electron transfer (PCET). Herein, we synthesized Cu-doped BiO_2−x (CuBiO) nanosheets to facilitate proton transfer through reconstruction of the hydrogen-bond (HB) network, thereby accelerating the PCET process. The in-situ measurements reveal that part of the 4-coordinated hydrogen-bonded water (4-HB·H₂O) transforms into 2-coordinated hydrogen-bonded water (2-HB·H₂O) over CuBiO during CO₂RR. This reconstruction forms a linear proton transport pathway which efficiently promotes proton transport during PCET steps and concurrently inhibits the competing hydrogen evolution reaction. Density functional theory (DFT) calculations further elucidate that the Cu doping not only facilitates water dissociation into protons while inhibiting proton dimerization, but also enhances CO₂ activation and reduces the energy barrier for of *CO₂ → *OCHO. Ultimately, the CuBiO-6 displays highly efficient conversion of CO₂ to HCOOH with a Faradaic efficiency (FE) of 92.4% and maintains stable operation for over 22 h. These findings provide a novel strategy to accelerate the PCET through regulation of interfacial HB network towards efficient CO₂RR to HCOOH.