Synergistic catalysis between In single atoms and In nanoparticles for highly selective electrocatalytic CO₂ reduction to formate with high current densities

The practical realization of the electrocatalytic reduction of CO₂ to formate is limited by the lack of suitable highly active and selective electrocatalysts, particularly candidates compatible with operation at high current densities. Herein, we report a dual-active site electrocatalyst consisting of In single atoms and In nanoparticles supported on N, S-codoped porous carbon (In-NSC/NPs), which enables a remarkable formate faradaic efficiency (FE) of 92% with a large absolute partial current density for formate of up to 1.1 A cm^-2. By using membrane electrode assembly cells, a formate FE exceeding 90% and an energy conversion efficiency of over 44% are attainable within a wide cell voltage range of 2.4-3 V. The maximum formate generation rate reaches 10.5 mmol cm^-2 h^-1 at a cell voltage of 2.9 V. By coupling with anodic glycerol oxidation, the formate yield rate in a full electrolytic cell is significantly improved to 23.2 mmol cm^-2 h^-1 while using the same reaction conditions as the standard anodic oxygen evolution reaction. A combination of control experiments and <i>in situ</i> characterization methods reveals that In nanoparticles facilitate the generation of the *OCHO and the subsequent hydrogenation step to generate formate while the In single atoms boost H₂O dissociation. The generated *H migrates to the surface of the In nanoparticles, increasing the proton concentration and promoting the hydrogenation reaction.