Protonation pathway for CO₂ reduction mediated by coordinated H₂O on active sites

Electrochemical CO₂ reduction to fuels necessitates multiple steps of electron and proton transfer. As one of the primary proton donors for CO₂ reduction, H₂O is essential to regulating the reaction pathway and product selectivity. However, the coordinating nature of H₂O and its influence are poorly understood due to challenges in identifying the precise structure of catalytic sites. Here, we employ a well-defined bismuth-based metal-organic framework material with molecularly precise active sites and ordered microporosity for in-depth mechanistic studies. The coordinated H₂O on active sites promotes the adsorption of CO₂ and alleviates the sluggish proton supply via a protonated carbonic acid pathway involving a surface hydride transfer, which significantly promotes the adsorption and activation of CO₂. This results in a 99% selectivity for CO₂ reduction to formic acid with a high turnover frequency of 21.1 s^-1. Leveraging the advantages of crystalline coordination frameworks in electrocatalysis, this work fills up a lacuna in our understanding of CO₂ hydrogenation/reduction and reinforces the importance of H₂O to the advanced design of catalytic systems.