W Doping in Ni12P5 as a Platform to Enhance Overall Electrochemical Water Splitting

Abstract

Bifunctional electrocatalysts for efficient hydrogen generation from water splitting must overcome both the sluggish water dissociation step of the alkaline hydrogen evolution half-reaction (HER) and the kinetic barrier of the anodic oxygen evolution half-reaction (OER). Nickel phosphides are a promising catalysts family and are known to develop a thin active layer of oxidized Ni in an alkaline medium. Here, Ni₁₂P₅ was recognized as a suitable platform for the electrochemical production of γ-NiOOH─a particularly active phase─because of its matching crystallographic structure. The incorporation of tungsten by doping produces additional surface roughness, increases the electrochemical surface area (ESCA), and reduces the energy barrier for electron-coupled water dissociation (the Volmer step for the formation of H_ads). When serving as both the anode and cathode, the 15% W-Ni₁₂P₅ catalyst provides an overall water splitting current density of 10 mA cm^-2 at a cell voltage of only 1.73 V with good durability, making it a promising bifunctional catalyst for practical water electrolysis.

Keywords: DFT calculations; nickel phosphide; oxygen evolution reaction; structure-function relationship; γ-NiOOH.

Figure 1

(A–D) TEM images of the as-synthesized nanoparticles and their corresponding size distribution plots (E).

Figure 2

(A) X-ray diffraction (XRD) patterns of the synthesized nanoparticles. The red asterisk refers to the signal from the Si wafer. (B) Enlarged view of the major diffraction peaks in (A), showing a shift toward lower 2θ values. At the top, a ball-and-stick model of the unit cell of tetragonal Ni₁₂P₅ is shown (purple: P, cyan: Ni).

Figure 3

Electrochemical HER performances of pristine and W-doped nickel phosphide catalysts in alkaline solution (1.0 M KOH): (A) LSV polarization curves, (B) overpotentials at a current density of 10 mA cm^–2, (C) Tafel plots, (D) electrochemically active surface area (ECSA), (E) Nyquist plots obtained at an overpotential of η = 200 mV for the HER. Figure S9 shows the equivalent circuit, and (F) stability test for the 15% W-Ni₁₂P₅ catalyst.

Figure 4

Electrochemical performances of pristine and W-doped nickel phosphide catalysts for the OER: (A) LSV polarization curves in 1.0 M KOH at a scan rate of 5 mV s^–1, (B) overpotentials at a current density of 10 mA cm^–2, (C) Tafel plots, and (D) Nyquist plots of the W-doped Ni₁₂P₅ samples along with pristine Ni₁₂P₅ at an overpotential of η = 340 mV for the OER.

Figure 5

XPS spectra of (A) Ni and (B) W after 200 CV cycles of the OER. (C, D) HRTEM and STEM images of 15W-Ni₁₂P₅ after 200 CV cycles of the OER. (E, F) Element mappings of the same nanoparticle as in (D). The scale bar for (D–F) is 20 nm.

Figure 6

(A) Polarization curves of the 15%W-Ni₁₂P₅ electrolyzer in 1 M KOH at 5 mV s^–1 (in a two-electrode system). (B) Chronopotentiometry curve of water electrolysis at a 2 V bias voltage in 1 M KOH; the inset shows a photograph of H₂ and O₂ bubbles on the surface of the electrodes during the electrolysis process.

Figure 7

(A) Ball-and-stick models for W-doped γ-NiOOH with a single W dopant atom. Ni in cyan, W in blue, O in red, and H in white. W-doped γ-NiOOH may adopt three configurations: (1) bare WO₆ octahedron, (2) WO₆ with adsorbed H-atoms and three H vacancies at the NiO₆H₃ octahedra, or (3) a single Ni vacancy. (B) Cumulative free energies, ΔG, for a single-site associative reaction mechanism of the OER using γ-NiOOH doped by a single W atom. A is a pristine undoped layer; B is the structure depicted in model 1, where the bare WO₆ octahedron serves as the reaction site; C is the structure depicted in model 2, where the WO₆H₃ octahedron serves as the reaction site; D is the structure depicted in model 3, where the WO₆H₃ octahedron serves as the reaction site; and E is the structure depicted in model 3, where the Ni vacancy serves as the reaction site. (C) Map of the electron density redistribution Δρ after the adsorption of an O atom to the NiO₃H₃ octahedron in pristine γ-NiOOH. (D) The same after the adsorption of an O atom to the WO₃H₃ octahedron in doped γ-NiOOH. (E) Difference, ΔΔρ, between (D) and (E), which shows a diminished electron density in the vicinity of the O atom adsorbed on the WO₃H₃ site (i.e., intermediate reaction complex at step IV in model 2).