Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces

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.

Phase‐Controlled Synthesis of Nickel Phosphide Nanocrystals and Their Electrocatalytic Performance for the Hydrogen Evolution Reaction

“…The (0 001 ) surfaceo fN i 5 P 4 provides the P 3 -hollow site, which offersn early optimal hydrogen binding, thus Ni 5 P 4 (0 001 )has better HER activity than Ni 2 P(0 001). [19] The high specific surface area of our obtained Ni 5 P 4 nanocrystals is another factor to the high HER performance. Owing to the small size of the nanocrystals and even distribution on the carbon supports, more actives ites are exposed.…”

“…Ni 2 P(0 0 0 1) and Ni 5 P 4 (0 0 0 1)/(0 0 0 ) are both HER‐active surfaces. The (0 0 0 ) surface of Ni 5 P 4 provides the P 3 ‐hollow site, which offers nearly optimal hydrogen binding, thus Ni 5 P 4 (0 0 0 ) has better HER activity than Ni 2 P(0 0 0 1) . The high specific surface area of our obtained Ni 5 P 4 nanocrystals is another factor to the high HER performance.…”

Structure–Activity Relationships for Pt‐Free Metal Phosphide Hydrogen Evolution Electrocatalysts

“…Hydrogen absorption on the different crystallographic faces of TMP crystals has been intensively characterized computationally . For example, Jaramillo et al.…”

“…Based on their data, the authors rationally developed superior bi‐metallic catalysts Fe 0.5 Co 0.5 P . Phosphorus‐rich termination of the Ni 2 P and Ni 5 P 4 catalysts was shown to be crucial to reduce the overpotential and provide optimal absorption energy . Rappe et al.…”

“…have computationally shown that P−H enriched termination of Ni phosphides are the most stable one. Moreover, they have demonstrated that P is a crucial active site for weak H adsorption in contrast to commonly assumed Ni‐based adsorption active sites. In the computational studies, regular terminations of the crystal structure, with no substantial surface restructuring under reaction conditions, have been assumed to reduce computational costs.…”