Engineering interfacial H-bond networks via orbital hybridization on CuBiO _{2− <em>x</em>} to boost proton-coupled electron transfer for CO ₂ -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 the proton transfer of reconstructed the hydrogen-bond (HB) network to accelerate 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 promoting proton transport during PCET steps and concurrently inhibiting 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 faradaic efficiency (FE) of 92.4% and maintains stable operation for over 22 hours. These findings provide a novel strategy to accelerate the PCET through regulation of interfacial HB network towards efficiently CO₂RR to HCOOH.