Unveiling the synergistic effect of multi-valence Cu species to promote formaldehyde oxidation for anodic hydrogen production

The ultra-low potential oxidation of aldehydes is a conspicuous replacement for oxygen evolution reaction due to its efficiency in lowering the energy required for hydrogen production and produc-ing H2 on dual sides of the electrolyzer. However, the activity origin of catalysts and the reaction mechanism remains unclear. Herein, a formaldehyde oxidation reaction (FOR) with high current density (165 mA cm -2 at 0.35 VRHE) is realized on partially reduced CuO on Cu foam (CuxO@CF) electrocatalysts. In situ characterizations and density functional theory calculations identified and investi-gated the existence of Cu0 and Cu+ and the synergistic effect for FOR and corroborate the reaction pathway. The results reveal that Cu0 can facilitate the reaction energy of HOCH2O* + HO*, and Cu+ is more favorable for C-H bond cleavage. Meanwhile, H atoms in the produced H2 are verified to be entirely from HCHO. This work provides theoretical guidance for designing Cu-based electrocatalysts for FOR.