No abstract

Optimizing catalysts for the hydrogen evolution reaction (HER) is a critical step toward the efficient production of H2(g) fuel from water. It has been demonstrated experimentally that transition-metal phosphides, specifically nickel phosphides Ni2P and Ni5P4, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through density functional theory with thermodynamics, the aqueous reconstructions of Ni2P­(0001) and Ni5P4(0001)/(0001̅), and we find that the surface P content on Ni2P­(0001) depends on the applied potential, which has not been considered previously. At −0.21 V ≥ U ≥ −0.36 V versus the standard hydrogen electrode and pH = 0, a PH x -enriched Ni3P2 termination of Ni2P­(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow sites. On the other hand, Ni5P4(0001̅) does not favor additional P. Instead, the Ni4P3 bulk termination of Ni5P4(0001̅) is passivated by H at both the Ni3 and P3-hollow sites. We also found that the most HER-active surfaces are Ni3P2+P+(7/3)H of Ni2P­(0001) and Ni4P3+4H of Ni5P4(0001̅) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calculating the reaction free energies of the associated elementary steps, we calculate that the overpotential on the Ni4P3+4H surface of Ni5P4(0001̅) (−0.16 V) is lower than that of the Ni3P2+P+(7/3)H surface of Ni2P­(0001) (−0.21 V). This is due to the abundance of P3-hollow sites on Ni5P4 and the limited surface stability of the P-enriched Ni2P­(0001) surface phase. The trend in the calculated catalytic overpotentials, and the potential-dependent bulk and surface stabilities explain why the nickel phosphides studied here perform almost as well as Pt, and why Ni5P4 is more active than Ni2P toward HER, as is found in the experimental literature. This study emphasizes the importance of considering aqueous surface stability in predicting the HER-active sites, mechanism, and overpotential, and highlights the primary role of P in HER catalysis on transition-metal phosphides.

The activity of NiP catalysts for the hydrogen evolution reaction (HER) is currently limited by strong H adsorption at the Ni-hollow site. We investigate the effect of surface nonmetal doping on the HER activity of the NiP termination of NiP(0001), which is stable at modest electrochemical conditions. Using density functional theory (DFT) calculations, we find that both 2 p nonmetals and heavier chalcogens provide nearly thermoneutral H adsorption at moderate surface doping concentrations. We also find, however, that only chalcogen substitution for surface P is exergonic. For intermediate surface concentrations of S, the free energy of H adsorption at the Ni-hollow site is -0.11 eV, which is significantly more thermoneutral than the undoped surface (-0.45 eV). We use the regularized random forest machine learning algorithm to discover the relative importance of structure and charge descriptors, extracted from the DFT calculations, in determining the HER activity of NiP(0001) under different doping concentrations. We discover that the Ni-Ni bond length is the most important descriptor of HER activity, which suggests that the nonmetal dopants induce a chemical pressure-like effect on the Ni-hollow site, changing its reactivity through compression and expansion.

We report microcrystalline Ni3P as a noble-metal-free electrocatalyst for the H2 evolution reaction (HER) with high activity just below those of Ni5P4 and Pt, the two most efficient HER catalysts known. Ni3P has previously been dismissed for the HER, owing to its anticipated corrosion and its low activity when formed as an impurity in amorphous alloys. We observe higher activity of single-phase Ni3P crystallites than for other nickel phosphides (except Ni5P4) in acid, high corrosion tolerance in acid, and zero corrosion in alkali. We compare its electrocatalytic performance, corrosion stability, and intrinsic turnover rate to those of different transition-metal phosphides. Electrochemical studies reveal that poisoning of surface Ni sites does not block the HER, indicating P as the active site. Using density functional theory (DFT), we analyze the thermodynamic stability of Ni3P and compare it to experiments. DFT calculations predict that surface reconstruction of Ni3P (001) strongly favors P enrichment of the Ni4P4 termination and that the H adsorption energy depends strongly on the surface reconstruction, thus revealing a potential synthetic lever for tuning HER catalytic activity. A particular P-enriched reconstructed surface on Ni3P­(001) is predicted to be the most stable surface termination at intermediate P content, as well as providing the most active surface site at low overpotentials. The P adatoms present on this reconstructed surface are more active for HER at low overpotentials in comparison to any of the sites investigated on other terminations of Ni3P­(001), as they possess nearly thermoneutral H adsorption. To our knowledge this is the first time reconstructed surfaces of transition-metal phosphides have been identified as having the most active surface site, with such good agreement with the experimentally observed catalytic current onset and Tafel slope. The active site geometry achieved through reconstruction identified in this work shows great similarity to that reported for Ni2P­(0001) and Ni5P4(0001) facets, serving as a general design principle for the future development of even more active transition-metal phosphide catalysts and further climbing the volcano plot.

The control of oxygen vacancy (V O ) formation is critical to advancing multiple metal-oxide-perovskite-based technologies. We report the construction of a compact linear model for the neutral V O formation energy in ABO 3 perovskites that reproduces, with reasonable fidelity, Hubbard-U-corrected density functional theory calculations based on the state-of-the-art, strongly constrained and appropriately normed exchange-correlation functional. We obtain a mean absolute error of 0.45 eV for perovskites stable at 298 K, an accuracy that holds across a large, electronically diverse set of ABO 3 perovskites. Our model considers perovskites containing alkaline-earth metals (Ca, Sr, and Ba) and lanthanides (La and Ce) on the A-site and 3d transition metals (Ti, V, Cr, Mn, Fe, Co, and Ni) on the B-site in six different crystal systems (cubic, tetragonal, orthorhombic, hexagonal, rhombohedral, and monoclinic) common to perovskites. Physically intuitive metrics easily extracted from existing experimental thermochemical data or via inexpensive quantum mechanical calculations, including crystal bond dissociation energies and (solid phase) reduction potentials, are key components of the model. Beyond validation of the model against known experimental trends in materials used in solid oxide fuel cells, the model yields new candidate perovskites not contained in our training data set, such as (Bi,Y)(Fe,Co)O 3 , which we predict may have favorable thermochemical water-splitting properties. The confluence of sufficient accuracy, efficiency, and interpretability afforded by our model not only facilitates high-throughput computational screening for any application that requires the precise control of V O concentrations but also provides a clear picture of the dominant physics governing V O formation in metaloxide perovskites.