Title: 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 H ₂(g) fuel from water. It has been demonstrated experimentally that transition-metal phosphides, specifically nickel phosphides Ni ₂P and Ni ₅P ₄, 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 Ni ₂P(0001) and Ni ₅P ₄(0001)/(000$$ \overline{1}\ $$), and we find that the surface P content on Ni ₂P(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 Ni ₃P ₂ termination of Ni ₂P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni ₃P ₂ surface is instead passivated by H at the Ni ₃-hollow sites. On the other hand, Ni ₅P ₄(000$$ \overline{1}\ $$) does not favor additional P. Instead, the Ni ₄P ₃ bulk termination of Ni ₅P ₄(000$$ \overline{1}\ $$) is passivated by H at both themore » Ni ₃ and P3-hollow sites. We also found that the most HER-active surfaces are Ni ₃P ₂+P+(7/3)H of Ni ₂P(0001) and Ni ₄P ₃+4H of Ni ₅P ₄(000$$ \overline{1}\ $$) 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 Ni ₄P ₃+4H surface of Ni ₅P ₄(000$$ \overline{1}\ $$) (-0.16 V) is lower than that of the Ni ₃P ₂+P+(7/3)H surface of Ni ₂P(0001) (-0.21 V). This is due to the abundance of P ₃-hollow sites on Ni ₅P ₄ and the limited surface stability of the P-enriched Ni ₂P(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 Ni ₅P ₄ is more active than Ni ₂P 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.« less

Authors:

Wexler, Robert B.  ^[1];

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Martirez, John Mark P. ^[2]; Rappe, Andrew M.  ^[1]

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+ Show Author Affiliations

1. Univ. of Pennsylvania, Philadelphia, PA (United States). Dept. of Chemistry
2. Princeton Univ., NJ (United States). Dept. of Mechanical and Aerospace Engineering

Publication Date:

2017-10-02

Research Org.:

Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)

Sponsoring Org.:

USDOE; US Department of the Navy, Office of Naval Research (ONR)

OSTI Identifier:

1479670

Grant/Contract Number:  

N00014-17-1-2574

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Volume: 7; Journal Issue: 11; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 08 HYDROGEN; electrocatalysis; hydrogen evolution; metal phosphides; nickel phosphides; density functional theory; aqueous surface phase diagram