Platinum-Nickel Nanowires with Improved Hydrogen Evolution Performance in Anion Exchange Membrane-Based Electrolysis

Abstract

Platinum-nickel (Pt-Ni) nanowires were developed as hydrogen evolving catalysts for anion exchange membrane electrolyzers. Following synthesis by galvanic displacement, the nanowires had Pt surface areas of 90 m² g_Pt^-1. The nanowire specific exchange current densities were 2-3 times greater than commercial nanoparticles and may benefit from the extended nanostructure morphology that avoids fringe facets and produces higher quantities of Pt{100}. Hydrogen annealing was used to alloy Pt and Ni zones and compress the Pt lattice. Following annealing, the nanowire activity improved to 4 times greater than the as-synthesized wires and 10 times greater than Pt nanoparticles. Density functional theory calculations were performed to investigate the influence of lattice compression and exposed facet on the water-splitting reaction; it was found that at a lattice of 3.77 Å, the (100) facet of a Pt-skin grown on Ni₃Pt weakens hydrogen binding and lowers the barrier to water-splitting as compared to pure Pt(100). Moreover, the activation energy of water-splitting on the (100) facet of a Pt-skin grown on Ni₃Pt is particularly advantageous at 0.66 eV as compared to the considerably higher 0.90 eV required on (111) surfaces of pure Pt or Pt-skin grown on Ni₃Pt. This favorable effect may be slightly mitigated during further optimization procedures such as acid leaching near-surface Ni, necessary to incorporate the nanowires into electrolyzer membrane electrode assemblies. Exposure to acid resulted in slight dealloying and Pt lattice expansion, which reduced half-cell activity, but exposed Pt surfaces and improved single-cell performance. Membrane electrode assembly performance was kinetically 1-2 orders of magnitude greater than Ni and slightly better than Pt nanoparticles while at one tenth the Pt loading. These electrocatalysts potentially exploit the highly active {100} facets and provide an ultralow Pt group metal option that can enable anion exchange membrane electrolysis, bridging the gap to proton exchange membrane-based systems.

Figure 1

(a) HER–HOR mass (red) and site-specific (blue) exchange current densities of as-synthesized Pt–Ni nanowires in RDE half-cells as a function of Pt content (x-axis). The mass (solid red) and site-specific (dashed blue) activities of Pt/HSC are provided as horizontal lines. (b) Pt ECAs (green) of as-synthesized Pt–Ni nanowires as a function of Pt content (x-axis). (c) Linear sweep voltammograms and (d) Butler–Volmer plots of as-synthesized Pt–Ni nanowires with the Nernstian diffusion limited overpotential (η_d, dashed line).

Figure 2

(a) HER–HOR mass (red) and site-specific (blue) exchange current densities of hydrogen annealed Pt–Ni nanowires in RDE half-cells as a function of annealing temperature (x-axis). The mass (solid red) and site-specific (dashed blue) activities of Pt/HSC are provided as horizontal lines. (b) Pt ECAs (green) of hydrogen annealed Pt–Ni nanowires as a function of annealing temperature (x-axis). (c) Linear sweep voltammograms and (d) Butler–Volmer plots of as-synthesized Pt–Ni nanowires with the Nernstian diffusion limited overpotential (η_d, dashed line).

Figure 3

(a) Microscopy of Pt–Ni nanowires hydrogen annealed to 275 °C, including dark-field imaging, bright-field imaging, and EDS of Pt (red) and Ni (green). (b,c) XRD patterns of Ni nanowires (Ni) and Pt–Ni nanowires, as-synthesized (Pt–Ni), hydrogen annealed to 275 °C ex situ (275 °C) and following electrochemical conditioning (275 °C *), and acid leached ex situ (Acid) and following electrochemical conditioning (Acid *). (d) Cyclic voltammograms of Pt–Ni nanowires (as-synthesized, hydrogen annealed, and acid treated) and Pt/HSC in a germanium (solid) and tellurium (dashed) containing electrolyte. (e) Approximation of exposed Pt facets for Pt–Ni nanowires (as-synthesized, hydrogen annealed, and acid treated) and Pt/HSC based on germanium and tellurium data.

Figure 4

(a) Exposed facets of a thick Pt-skin (three layers of Pt) on the Ni₃Pt subsurface. Pt atoms are in gray and Ni atoms are in green. (b) Adsorption strength of OH vs the adsorption strength of H on pure Pt and the Pt-skins grown on the Ni₃Pt subsurface. Because of the space constraints on the graph, “Pt–Ni” refers to the Ni₃Pt subsurface with a Pt-skin, whereas Pt refers to the pure Pt surface. Data point markers are categorized by the facets: green indicates the (111) facet; blue—(100); and red—(110).

Figure 5

cNEB calculations of the water-splitting reaction of the pure Pt surface (red) vs the Pt–Ni surface (green) at a lattice constant of 3.77 Å for the (100) (left) and (111) facets (right). The mechanistic pathway is visualized below the plot of the reaction coordinate, summarizing the initial/final states, and activation energy (E_A) at the transition state (the highest point in the barrier calculation) is displayed. Pt atoms are in gray, Ni atoms are in green, O atoms are in red, and H atoms are in yellow.

Figure 6

cNEB calculations of the proton-hopping mechanism on the (100) facet of the pure Pt surface (red) vs the Pt–Ni surface at a lattice constant of 3.77 Å (green). The mechanistic pathway is visualized below the plot of the reaction coordinate, summarizing different key points: initial/final states, activation energy (E_A) at the transition state (the highest point in the barrier calculation), and various sites that hydrogen can hop to. Pt atoms are in gray, Ni atoms are in green, O atoms are in red, and H atoms are in yellow.

Figure 7

(a) HER–HOR mass (red) and site-specific (blue) exchange current densities of as-synthesized (Pt–Ni), hydrogen annealed (275 °C), and acid leached (Acid) Pt–Ni nanowires and Pt/HSC in RDE half-cells. The site-specific (dashed blue) activity of polycrystalline Pt is provided as a horizontal line. (b) Pt ECAs of as-synthesized (Pt–Ni), hydrogen annealed (275 °C), and acid leached (Acid) Pt–Ni nanowires and Pt/HSC as a function of cycles in the potential range −0.2 to 0.2 V versus RHE. (c) Linear sweep voltammograms and (d) Butler–Volmer plots of as-synthesized (Pt–Ni), hydrogen annealed (275 °C), and acid leached (Acid) Pt–Ni nanowires and Pt/HSC with the Nernstian diffusion limited overpotential (η_d, dashed line).

Figure 8

(a) MEA polarization curves, (b) Tafel plots, and (c) impedance data of acid leached Pt–Ni nanowires, Pt/HSC, and Ni nanoparticles. MEAs were tested in electrolysis mode with Co anodes (0.4 mg_Co cm^–2), NREL Gen 2 perfluorinated AEMs and ionomers (ionomer to catalyst ratio of 0.22), Toray transport layers, and Ni flow fields. Cathode catalyst layers were sprayed to loadings of 0.1 (Pt–Ni, Pt/HSC) and 0.2 (Ni) mg_M cm^–2. Tafel plots in (b) were corrected for high frequency resistance and impedance experiments in (c) were completed at 0.01 A cm^–2. (d) Breakdown of Ohmic, transport, and kinetic losses for the acid leached Pt–Ni nanowire MEA.