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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:
ORCiD logo [1];  [2]; ORCiD logo [1]
  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:
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

Citation Formats

Wexler, Robert B., Martirez, John Mark P., and Rappe, Andrew M. Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces. United States: N. p., 2017. Web. doi:10.1021/acscatal.7b02761.
Wexler, Robert B., Martirez, John Mark P., & Rappe, Andrew M. Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces. United States. https://doi.org/10.1021/acscatal.7b02761
Wexler, Robert B., Martirez, John Mark P., and Rappe, Andrew M. Mon . "Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces". United States. https://doi.org/10.1021/acscatal.7b02761. https://www.osti.gov/servlets/purl/1479670.
@article{osti_1479670,
title = {Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces},
author = {Wexler, Robert B. and Martirez, John Mark P. and Rappe, Andrew M.},
abstractNote = {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)/(000$ \overline{1}\ $), and we find that the surface P content on Ni2P(0001) depends on the applied potential, which has not been considered previously. At -0.21 V ≥ U ≥ -0.36 V versus the standard hydrogen electrode and pH = 0, a PHx-enriched Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow sites. On the other hand, Ni5P4(000$ \overline{1}\ $) does not favor additional P. Instead, the Ni4P3 bulk termination of Ni5P4(000$ \overline{1}\ $) is passivated by H at both the Ni3 and P3-hollow sites. We also found that the most HER-active surfaces are Ni3P2+P+(7/3)H of Ni2P(0001) and Ni4P3+4H of Ni5P4(000$ \overline{1}\ $) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calculating the reaction free energies of the associated elementary steps, we calculate that the overpotential on the Ni4P3+4H surface of Ni5P4(000$ \overline{1}\ $) (-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.},
doi = {10.1021/acscatal.7b02761},
url = {https://www.osti.gov/biblio/1479670}, journal = {ACS Catalysis},
issn = {2155-5435},
number = 11,
volume = 7,
place = {United States},
year = {2017},
month = {10}
}

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Works referencing / citing this record:

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