First-principles investigation of the hydrogen evolution reaction of transition metal phosphides CrP, MnP, FeP, CoP, and NiP.

We comprehensively investigated the hydrogen evolution reaction (HER) activity of a series of transition metal phosphides (MPs) (M = Cr, Mn, Fe, Co, and Ni) using first-principles calculations. The free energy difference was calculated for possible sites on the surface to pinpoint the reactive sites and the associated catalytic activities. We found that the chemical properties of these considered MPs are different from those of WP, including CrP which has the same electronic configuration as WP but was shown not to be a good electrocatalyst. Different reactive sites other than WP were predicted, and notably, unlike WP, phosphorus can participate/catalyze the HER on the considered MP. Among these MPs, there are more active sites on FeP, CoP, and NiP than CrP and MnP. Our electronic structure analysis suggests that the spin polarization is critical in determining the hydrogen adsorption and hence the HER performance. We further explored the HER of metal- or phosphorus-deficit MPs, as samples can be grown under different conditions. In particular, phosphorus-deficit FeP, CoP, and NiP were found to have enhanced HER performance, with either better catalytic activities or more active sites. Therefore, we proposed that controlling of these defects can be an effective approach to tune the HER catalytic ability of these MPs. It can serve as the design principle to synthesize new MP based electrocatalysts.

First-principles investigation of the hydrogen evolution reaction on different surfaces of pyrites MnS2, FeS2, CoS2, NiS2.

We theoretically investigated hydrogen evolution reaction (HER) on the XRD observed (100), (110), (111), and (210) surfaces of pyrite structure CoS2. The random structure searching method was employed in this work to thoroughly and less-biasedly identify the active sites for each considered surface. We calculated the free energy of hydrogen adsorption, and found that (110) and (210) surfaces are more active than the conventionally assumed (100) facet. While the lowest energy active site on the (100) and (210) surfaces is the five-coordinated transition metal site that is commonly seen in other HER catalysts, the lowest energy active site on the (110) surface is the two-coordinated S site, which is a S tetrahedron with two corners missing. Besides those lowest energy active sites, both (110) and (210) have more than one species of active site on the surface, including not fully coordinated transition metals and sulfur. We further explored the reaction for MnS2, FeS2, and NiS2, and analyzed the density of states. Our results showed both CoS2 and NiS2 (110) and (210) surfaces are catalytically reactive for HER.