Enhancing Hydrogen Evolution Reaction Activities of 2H-Phase VS2 Layers with Palladium Nanoparticles.

ABSTRACT

Effective hydrogen (H2) production with surface engineering of less active catalysts by an innovative approach is followed here. In this work, a non-noble 2H phase of VS2 layers, which showed poor activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, was made highly active by decorating palladium (Pd) nanoparticles (NPs) over VS2 layers. A density functional theory (DFT) study confirmed the successful binding of Pd with VS2, and the bond length in a Pd4 tetrahedron was measured to be 2.60 Å. In VS2-Pd, Pd as a Pd4 tetrahedron is pointed toward the VS2 layer, and the calculated Pd-S bond distance is 2.42 Å with some expansion of three Pd-Pd bonds (2.85 Å). From the density of states, it was confirmed that the band gap was too high for VS2 (0.2 eV; 2H phase) and was reduced to nearly zero in VS2-Pd (0.05 eV). In the electrocatalytic HER part, the obtained ΔGH values from DFT were 0.05, -0.45, and 0.22 eV for VS2/Pd4, Pd4, and VS2, respectively, which imply that VS2-Pd4 had improved HER activity compared to pristine VS2 and Pd4. A concentration-dependent study was carried out with molar ratios of Pd at 0.01, 0.05, and 0.1 M with VS2 layers. From the HER polarization study, VS2-Pd (0.05 M) showed an overpotential of 157 mV at 20 mA cm-2, which is 373 mV less than only VS2 with a Tafel slope of 75 mV dec-1 with overwhelming stability. These highly promising results will be interesting to make less active stable phases by incorporating metal NPs for efficient and stable H2 production.