Shrinking the Hydrogen Overpotential of Cu by 1 v and Imparting Ultralow Charge Transfer Resistance for Enhanced H₂ Evolution

Copper and its oxides are among the best electrocatalysts for the electrochemical conversion of CO₂ to value-added small organics because of its high hydrogen overvoltage, making the hydrogen evolution reaction (HER) a poor side reaction. Here we report an interesting finding that turned the nature of surface-oxidized Cu upside down in electrochemical H₂ evolution. It is commonly known that the electrochemical reactivity of a metal ion is highly sensitive to the anion to which it is coordinated in the electrolyte. In the case of Cu, when it is in the form of copper oxide, the hydrogen overvoltage is huge. Nonetheless, we found that when Cu is in coordination with Se^2- ions as Cu₂Se, the hydrogen overvoltage was shrunken by ∼1 V, imparting ultralow charge transfer resistance (R_CT) that varied from 0.32 to 0.61 Ω depending on the means by which selenization was carried out. Selenization was done by two different methods. In one method, conventional stirring was employed to selenize Cu foam in a preheated NaHSe solution at 90 °C for 20 min. In another method, hydrothermal treatment was employed to selenize Cu foam with NaHSe solution at 120 °C for 1 h. The wet-chemical method yielded honeycomb-like hierarchical arrays of Cu₂Se sheets on Cu foam (designated as Cu₂Se-ch/Cu), and the hydrothermal method yielded a uniform array of spiky rods of Cu₂Se (designated as Cu₂Se-ht/Cu). The HER electrocatalytic studies carried out in 0.5 M H₂SO₄ showed that Cu₂Se-ch/Cu and Cu₂Se-ht/Cu had similar kinetics, with Tafel slopes of 32 to 35 mV dec^-1, which is closer to the state-of-the-art Pt/C. Interestingly, the Cu₂Se-ch/Cu delivered a total kinetic current density of -1200 mA cm^-2 when polarized up to -0.85 V vs RHE, whereas Cu₂Se-ht/Cu delivered a maximum of -780 mA cm^-2 only.