Ultrathin\nCarbon Shell Protecting Copper Sites to\nBoost Anodic Hydrogen Production via Low-Potential Formaldehyde Oxidation

The\noxidation of aldehydes on a copper-based electrocatalyst within\na small potential window can produce hydrogen at the anode, thus offering\na bipolar hydrogen production system. However, the inherent activity\nand stability of Cu-based electrocatalysts for aldehyde oxidation\nare still not satisfactory in practical application. Herein, by coating\nan ultrathin carbon shell on the copper sphere, an effective and stable\nformaldehyde oxidation reaction (FOR) can be realized to produce H<sub>2</sub> at a very low potential. FOR needs only a potential of 0.13\nV (vs RHE) to reach a current density of 100 mA cm<sup>–2</sup>. By coupling FOR with hydrogen evolution reaction (HER), hydrogen\nis generated simultaneously at both the cathode and the anode. The\nFaraday efficiency of H<sub>2</sub> at the bipolar state is close\nto 100%. In a flow cell, it needs a low cell voltage of 0.1 V to reach\na current density of 100 mA cm<sup>–2</sup>. Moreover, it can\nbe operated steadily for more than 30 h at high current density. The\ncarbon shell acts as an armor to protect the Cu(0) sites, avoid the\noxidation of copper, and keep the catalyst activity for a long time\nin the electrolytic process. Experimental and theoretical calculation\nresults indicate that electron transfer occurs at the interface between\nthe copper core and ultrathin carbon shell. The ultrathin carbon-coated\nCu reduces the reaction energy barrier, making the C–H bond\nmore easily fractured and facilitating H coupling to generate H<sub>2</sub>. This study provides a basic principle for the design of\ncopper-based electrocatalysts with long durability and activity.