Surface Hydrogen Mediated Formate Formation in CO ₂ Electroreduction

The electrochemical CO2 reduction to formate offers a promising route for renewable energy storage. While Sn-based oxides are known for their high formate selectivity, the atomic-scale factors governing their performance remain unclear, and the formate formation mechanism is still debated. Herein, we synthesized a series of Sn-based oxide catalysts with tailored coordination environments via the controlled pyrolysis of a Sn6O4(OH)4 precursor. We found that while the selectivities for H2 and CO evolution follow established trends based on the Gibbs free energy for *H and *COOH formation, the formate selectivity deviates from the conventional descriptor, the Gibbs free energy for *OCHO formation. Instead, we propose an alternative mechanism where an adsorbed *H species acts as a proton donor to stabilize the key formate precursor. This role of *H as a proton donor establishes the Gibbs free energy for *H formation as a critical descriptor for formate selectivity, providing a fresh design principle for developing selective C1 catalysts.