High current density electroreduction of CO into formate with tin oxide nanospheres.

In this study, we demonstrate three-dimensional (3D) hollow nanosphere electrocatalysts for CO conversion into formate with excellent H-Cell performance and industrially-relevant current density in a 25 cm membrane electrode assembly electrolyzer device. Varying calcination temperature maximized formate production via optimizing the crystallinity and particle size of the constituent SnO nanoparticles. The best performing SnO 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 at -1.3 V vs. RHE. The higher performance of nanosphere catalysts over SnO nanoparticles and commercially-available catalyst could be ascribed to their initial structure providing higher electrochemical surface area and preventing extensive nanocrystal growth during CO reduction. Our results are among the highest performance reported for SnO 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 cm electrolyzer cell demonstrated impressive performance with a sustained current density of 500 mA cm 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.