First-principles Studies on the Stability and Activity of CoS₂ Surfaces for Hydrogen Evolution Reaction

Among the many new energies, the hydrogen energy relying on water splitting is a promising alternative energy strategy. Therefore, highly active, low-cost and environment-friendly electrocatalysts for hydrogen evolution reaction (HER), such as transition metal dichalcogenides, are highly desirable and attracting great attention. In this work, we studied the electronic, structural, and energetics of atomic hydrogen on several CoS₂ surfaces including stoichiometric and non-stoichiometric (001), (110), (111) and (210) surfaces by using first principles calculations. The free energy of adsorbed H on each structure, ΔG_H, was calculated to predict its HER activity. It was found that CoS₂(001) is the most stable facet among these surfaces with a lowest surface energy of 0.370 eV Å^-2, while (111) has the highest surface energy of 1.034 eV Å^-2. The best site of HER is on the S2 site of (111) surface with a lowest ΔG_H = -0.057 eV. Detailed analyses show that the anions or S-S dimers have a great influence on HER performance of CoS₂ surfaces. For example, a single S-defect on (001) surface significantly improve the HER performance of the outmost S site on surface. Our theoretical results provide useful guidelines for the design of cobalt sulfide compounds for HER catalysts.