Ultrathin Carbon Shell Protecting Copper Sites to Boost Anodic Hydrogen Production via Low-Potential Formaldehyde Oxidation.

The oxidation of aldehydes on a copper-based electrocatalyst within a small potential window can produce hydrogen at the anode, thus offering a bipolar hydrogen production system. However, the inherent activity and stability of Cu-based electrocatalysts for aldehyde oxidation are still not satisfactory in practical application. Herein, by coating an ultrathin carbon shell on the copper sphere, an effective and stable formaldehyde oxidation reaction (FOR) can be realized to produce H₂ at a very low potential. FOR needs only a potential of 0.13 V (vs RHE) to reach a current density of 100 mA cm^-2. By coupling FOR with hydrogen evolution reaction (HER), hydrogen is generated simultaneously at both the cathode and the anode. The Faraday efficiency of H₂ at the bipolar state is close to 100%. In a flow cell, it needs a low cell voltage of 0.1 V to reach a current density of 100 mA cm^-2. Moreover, it can be operated steadily for more than 30 h at high current density. The carbon shell acts as an armor to protect the Cu(0) sites, avoid the oxidation of copper, and keep the catalyst activity for a long time in the electrolytic process. Experimental and theoretical calculation results indicate that electron transfer occurs at the interface between the copper core and ultrathin carbon shell. The ultrathin carbon-coated Cu reduces the reaction energy barrier, making the C-H bond more easily fractured and facilitating H coupling to generate H₂. This study provides a basic principle for the design of copper-based electrocatalysts with long durability and activity.