High current density electroreduction of CO₂ into formate with tin oxide nanospheres

In this study, we demonstrate three-dimensional (3D) hollow nanosphere electrocatalysts for CO₂ conversion into formate with excellent H-Cell performance and industrially-relevant current density in a 25 cm² membrane electrode assembly electrolyzer device. Varying calcination temperature maximized formate production via optimizing the crystallinity and particle size of the constituent SnO₂ nanoparticles. The best performing SnO₂ nanosphere catalysts contained ~ 7.5 nm nanocrystals and produced 71-81% formate Faradaic efficiency (FE) between -0.9 V and -1.3 V vs. the reversible hydrogen electrode (RHE) at a maximum formate partial current density of 73 ± 2 mA cm_geo^-2 at -1.3 V vs. RHE. The higher performance of nanosphere catalysts over SnO₂ nanoparticles and commercially-available catalyst could be ascribed to their initial structure providing higher electrochemical surface area and preventing extensive nanocrystal growth during CO₂ reduction. Our results are among the highest performance reported for SnO₂ electrocatalysts in aqueous H-cells. We observed an average 68 ± 8% FE over 35 h of operation with multiple on/off cycles. In situ Raman and time-dependent X-ray diffraction measurements identified metallic Sn as electrocatalytic active sites during long-term operation. Further evaluation in a 25 cm² electrolyzer cell demonstrated impressive performance with a sustained current density of 500 mA cm_geo^-2 and an average 75 ± 6% formate FE over 24 h of operation. Our results provide additional design concepts for boosting the performance of formate-producing catalysts.