Relationship between the Surface Reconstruction of Nickel Phosphides and Their Activity toward the Hydrogen Evolution Reaction

Nickel phosphides (NixPy) are a class of materials that are made out of earth abundant elements and have shown relatively high hydrogen evolution reaction (HER) activity. Here, we perform first-principles density functional theory (DFT) calculations to systematically investigate the stoichiometric and nonstoichiometric surface reconstructions of six different NixPy, i.e., Ni3P, Ni12P5, Ni2P, Ni5P4, NiP2, and NiP3, under electrochemical conditions and to illustrate the implications of such reconstructions for the catalytic activity toward HER. Our results can explain a broad range of experimental observations on the HER activity of NixPy in a unified framework. For the majority of cases, our protocol can closely reproduce the experimentally measured overpotential trends in the literature, which validates its usefulness in generating physical insight into the surface phenomena responsible for HER activity. We find that, among the NixPy studied here, Ni3P and Ni5P4 are the most active catalysts toward HER in acid, whereas Ni5P4 performs the best compared to other NixPy in base, in agreement with previous experimental reports. We show that P-vacancy formation in base renders the Ni-rich NixPy (Ni3P, Ni12P5, Ni2P, and Ni5P4) worse performers in base when compared to their activity in acid and hence propose that introducing nonmetals, which are less prone to dissolution, can improve their catalytic performance. In terms of active site design, we find Ni3 hollow sites bind H too strongly and surface P sites with P-Ni bonds bind H too weakly. On the other hand, we identify that surface P sites with P-P bonds offer the best catalytic performances, and therefore, we predict that active site engineering to maximize the abundance of such surface motifs can further improve the HER activity. Moreover, we unravel the nature of H binding across the material class for different binding motifs via electronic structure theory analysis. The chemical insight we provide in this work can help rationalize the search for materials composed of inexpensive earth abundant elements that provide improved HER catalytic activity.