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. H2O, a primary proton donor, governs CO2 reduction pathways and regulates selectivity, yet its coordination effects remain elusive. By exploiting the long-range structural order and porosity of crystalline coordination frameworks, this work uncovers how coordinated H2O influences CO2 reduction.