Enhanced catalytic activity of SOx-incorporated graphene for the hydrogen evolution reaction

Abstract

Sulfur defects have been used to enhance the catalytic activity of carbon-based nanostructures in the hydrogen evolution reaction (HER). This is accomplished by increase in the hydrogen adsorption ability due to the large size of sulfur atoms, which increases the sp³ character of graphene carbons. However, the effect of sulfur oxidation on the HER activity has not yet been discussed, even though sulfur can easily be oxidized to sulfur oxide (SO_x) in acidic environments, which may decrease the metallicity of graphene by opening the bandgap. Herein, we systematically investigate the HER activity of SO_x-incorporated graphene, SO_x@G (x = 2, 3 or 4), based on electronic, thermodynamic and kinetic viewpoints. Our results reveal that SO₃@G on the basal plane has superior HER catalytic activity due to its metallic nature and ability to stabilize adsorbed hydrogen (H*) which results from electrostatic interactions between SO₃ and H* in an intermediate state. As a thermodynamic descriptor of the HER activity, the hydrogen binding Gibbs free energy (ΔG^Volmer_H*) is calculated to be −0.04 eV for SO₃@G in the Volmer step. In addition, the activation energies in both the Volmer and Tafel steps (ΔE^Volmer_a and ΔE^Tafel_a, respectively) of SO₃@G show the lowest energy barriers among SO_x@G, taking values of 0.005 eV and 0.14 eV, respectively. Thus, these values can be used as kinetic descriptors and are close to those of conventional Pt catalysts.