Composite Ni/NiO-Cr2O3 Catalyst for Alkaline Hydrogen Evolution Reaction

Abstract

We report a Ni-Cr/C electrocatalyst with unprecedented mass-activity for the hydrogen evolution reaction (HER) in alkaline electrolyte. The HER kinetics of numerous binary and ternary Ni-alloys and composite Ni/metal-oxide/C samples were evaluated in aqueous 0.1 M KOH electrolyte. The highest HER mass-activity was observed for Ni-Cr materials which exhibit metallic Ni as well as NiO _x and Cr₂O₃ phases as determined by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analysis. The onset of the HER is significantly improved compared to numerous binary and ternary Ni-alloys, including Ni-Mo materials. It is likely that at adjacent Ni/NiO _x sites, the oxide acts as a sink for OH_ads, while the metallic Ni acts as a sink for the H_ads intermediate of the HER, thus minimizing the high activation energy of hydrogen evolution via water reduction. This is confirmed by in situ XAS studies that show that the synergistic HER enhancement is due to NiO _x content and that the Cr₂O₃ appears to stabilize the composite NiO _x component under HER conditions (where NiO _x would typically be reduced to metallic Ni⁰). Furthermore, in contrast to Pt, the Ni(O _x )/Cr₂O₃ catalyst appears resistant to poisoning by the anion exchange ionomer (AEI), a serious consideration when applied to an anionic polymer electrolyte interface. Furthermore, we report a detailed model of the double layer interface which helps explain the observed ensemble effect in the presence of AEI.

Figure 1

Diagnostic CV scans (50 mV/s) showing H_upd (+50 mV cathodic limit) and H_opd (−100 mV cathodic limit) on 50 μg_Pt/cm² Pt/C (a) and 250 μg_Pt/cm² unsupported Pt black (b) in the presence of Nafion vs AS-4 ionomer binders (incorporated during catalyst ink formulation). Scans collected in RDE cell with argon-purged 0.1 M KOH at 23 °C and 0 rpm.

Figure 2

CV scans (50 mV/s) showing HER activity on Pt/C (50 μg_Pt/cm²) and unsupported Pt black (250 μg_Pt/cm²) in the presence of Nafion vs AS-4 ionomer binder. Scans collected in RDE cell with argon-purged 0.1 M KOH at 23 °C and 2500 rpm.

Figure 3

Qualitative representation of electrostatic effects of AEI on φ1 and φ2. The condition studied previously under MOR conditions is represented in the top panel, while the condition studied in this paper is represented in the bottom panel. In both situations the chemical potential of the AEI (μAEI) increases the effective potentials at the IHP (φ1) and the OHP (φ2).

Figure 4

Steady-state HER chronoamperometry: 50 mV step size with 60 s hold-time/step in 0.1 M KOH (Ar-purged) at 50 °C and 2500 rpm. Catalyst loading is 50 μg(metal)/cm² with 15 wt % AS-4 used as binder in catalyst layer.

Figure 5

CV showing HER kinetics of Pt (black), Ni–Cr/C (orange), 60% Ni/C (blue), and 60% Cr-Ox/C (red) catalysts in 0.1 M KOH (Ar-purged) at 50 °C and 2500 rpm with Nafion vs AS-4 binder tested in RDE cell. Ni–Cr/C (orange) is the only sample that exhibits increased HER activity in the presence of AEI. All samples were prepared with 50 μg/cm² total metal loading.

Figure 6

XRD of 60% Ni–Cr/C sample. NiCr₂O₄ is mixture of NiO and Cr₂O₃. Peak positions of standards retrieved from ICDD ref: 01-070-0989 (Ni), 01-085-0936 (NiCr₂O₄), 00-004-0835 (NiO), 00-006-0504 (Cr₂O₃).

Figure 7

XANES (left) and FT-EXAFS (right) data at the Cr K edge collected on Ni–Cr/C electrodes. The data for Cr₂O₃ standard are also included for comparison purposes.

Figure 8

XANES (left) and FT-EXAFS (right) data at the Ni K edge collected on Ni–Cr/C electrodes. The XANES of Ni(OH)₂ standard is also included for comparison purposes.

Figure 9

EXAFS fitting results from data collected from “original” Ni–Cr/C sample. Ni-edge fitting confirms mixture of metallic Ni⁰ and Ni-oxide. While Cr-edge fitting excludes the presence of NiCr₂O₄ since the Ni–metal bond (2.98 Å) is in agreement with that in Ni-oxide (2.95 Å in NiO_x) but much smaller than the Ni–metal bond distance (3.42 Å) in NiCr₂O₄.

Scheme 1

Proposed Interface Model for Alkaline HER on Pure Metal and M/M_Ox Surfaces^{a a}AEI represented by archetypical quaternary ammonia polysulfone structure in OH-exchanged form. Metal represented by fcc unit cell model and metal oxide represented by NiO unit cell model.