First-Principles Mechanistic Insights into the Hydrogen Evolution Reaction on Ni2P Electrocatalyst in Alkaline Medium

Abstract

Nickel phosphide (Ni 2 P) is a promising material for the electrocatalytic generation of hydrogen from water. Here, we present a chemical picture of the fundamental mechanism of Volmer-Tafel steps in hydrogen evolution reaction (HER) activity under alkaline conditions at the (0001) and (1010) surfaces of Ni 2 P using dispersion-corrected density functional theory calculations. Two terminations of each surface (Ni 3 P 2 -and Ni 3 P-terminated (0001); and Ni 2 P-and NiP-terminated (1010)), which have been shown to coexist in Ni 2 P samples depending on the experimental conditions, were studied. Water adsorption on the different terminations of the Ni 2 P (0001) and (1010) surfaces is shown to be exothermic (binding energy in the range of 0.33−0.68 eV) and characterized by negligible charge transfer to/from the catalyst surface (0.01−0.04 e − ). High activation energy barriers (0.86−1.53 eV) were predicted for the dissociation of water on each termination of the Ni 2 P (0001) and (1010) surfaces, indicating sluggish kinetics for the initial Volmer step in the hydrogen evolution reaction over a Ni 2 P catalyst. Based on the predicted Gibbs free energy of hydrogen adsorption (∆G H *) at different surface sites, we found that the presence of Ni 3 -hollow sites on the (0001) surface and bridge Ni-Ni sites on the (1010) surface bind the H atom too strongly. To achieve facile kinetics for both the Volmer and Heyrovsky-Tafel steps, modification of the surface structure and tuning of the electronic properties through transition metal doping is recommended as an important strategy.and Ni 2 P [18-20] have been regarded as very promising electrocatalysts for the HER owing to their low-cost, appropriate electronic structures, and high electrochemical stability. Nickel phosphide (Ni 2 P) in particular is an emerging catalyst for hydrogen evolution reactions [20][21][22][23], hydrodesulfurization (HDS) [24,25], hydrodenitrogenation (HDN) [26][27][28], hydrodeoxygenation (HDO) [29], hydrodechlorination (HDCl) [30], and water-gas shift reactions [31]. Ni 2 P is considered an attractive alternative to noble metal catalysts for HER [20] because both of its constituent elements nickel and phosphorous are cheap, abundant, and non-toxic, which makes Ni 2 P a promising cost-effective material for scalable renewable energy conversion systems.Nickel phosphides are stable and durable in strong acid and alkali conditions, prolonging the turnover number (TON) and lifetime of the catalyst and hence can achieve enhanced HER efficiency [32,33]. The Ni 2 P (0001) surface is the most studied facet for HER activity owing to its comparable predicted hydrogen evolution activity to that of hydrogenase [22,[34][35][36]. Earlier investigations have considered HER activity in acidic medium over Ni 2 P catalysts, whereby the Volmer-Tafel mechanism (H + (aq) → H * , 2H * → ↑H 2 ) involves only characterizing the Gibb's free energy hydrogen adsorption to the catalyst surface [22,23]. There exist limited studies of the HER activity of Ni 2 P under alkaline c...

First-Principles Surface Characterization and Water Adsorption of Fe3P Schreibersite

“…This is in contrast to the upward and downward initial configurations, where both hydrogen sites were either above or below the oxygen site. DFT calculations of water adsorption on nickel phosphide Ni 2 P produced binding energies in the range 0.33−0.68 eV and similar relaxed geometries (i.e., planar configurations) of water molecules, 60 in good agreement with our findings despite differences in elemental composition and crystal structure.…”

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

“…Most of the theoretical modeling has been focused on the Volmer reaction in acidic aqueous solution, where the proton donor is typically presumed to be a partially solvated hydronium ion (H 3 O + ) in the form of a cationic water cluster, such as the Zundel (H 5 O 2 + ) or Eigen (H 9 O 4 + ) cation. However, a few theoretical studies have examined the Volmer reaction in alkaline solution. ,− ,− These particular studies include electronic structure calculations of the potential-dependent activation energies using local reaction center models ,,,, and various theoretical calculations based on model Hamiltonians and interfacial electron transfer theories. ,,, The most notable feature of the HER in alkaline conditions is that it exhibits sluggish kinetics with an almost two orders of magnitude reduction in activity, relative to acidic media . This phenomenon is still under active investigation and constitutes one of the unresolved puzzles in modern electrochemistry. , In contrast to acidic solutions, the proton donor for the Volmer reaction in basic solutions is expected to be the neutral water molecule, H 2 O, which undergoes the cleavage of the very strong covalent H–OH bond to produce the hydroxide ion, OH – , upon proton transfer to the metal electrode: As a result, the elementary step involves a neutral reactant species and a negatively charged product species, and the Volmer reaction follows distinct pathways from those observed in acidic solution.…”

The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

“…39 An approach embracing the often overlooked complexity of TMP surfaces and the diversity of surface adsorption sites can help us to better understand key active sites and energies for the adsorption of H. Most importantly, due to the large spread of the HBE, justification of the choice of the computation slab structure and validation from experimental results are recommended. Ni3 hollow site 14,31,34,35,38,[40][41][42] ; turquoise triangle: Ni-P bridge site 34,38,40,41 ; green square: P-top site 38,40,41 ; red dot: Co-bridge site 14,[43][44][45] ; pink square: P-top site 43,44 ; orange triangle: Co-P bridge 43 ; yellow diamond: Co hollow site 43 .…”