Boride-derived oxygen-evolution catalysts

Abstract

Metal borides/borates have been considered promising as oxygen evolution reaction catalysts; however, to date, there is a dearth of evidence of long-term stability at practical current densities. Here we report a phase composition modulation approach to fabricate effective borides/borates-based catalysts. We find that metal borides in-situ formed metal borates are responsible for their high activity. This knowledge prompts us to synthesize NiFe-Boride, and to use it as a templating precursor to form an active NiFe-Borate catalyst. This boride-derived oxide catalyzes oxygen evolution with an overpotential of 167 mV at 10 mA/cm² in 1 M KOH electrolyte and requires a record-low overpotential of 460 mV to maintain water splitting performance for over 400 h at current density of 1 A/cm². We couple the catalyst with CO reduction in an alkaline membrane electrode assembly electrolyser, reporting stable C₂H₄ electrosynthesis at current density 200 mA/cm² for over 80 h.

Fig. 1. Results of DFT calculations for NiFe-Boride catalysts.

Pourbaix diagram of (a) NiFe and (b) NiFe-Boride; (c) The cell configurations of NiB₄O₇ and FeBO₃; (d) The schematic illustration of the 4-step OER pathway; (e) Predicted OER reaction energy diagram for NiB₄O₇, FeBO₃, γ-NiOOH, and γ-NiFeOOH in the alkaline electrolyte at 1.23 V vs. RHE.

Fig. 2. Characterization of the morphology and composition of NiFe-Boride catalyst.

(a) Elemental mapping analysis, showing the uniform, uncorrelated spatial distribution of Ni, Fe, B and O. (b) XRD patterns. (c) B 1 s XPS spectrum of fresh NiFe-Boride sample. The Ni and Fe K-edge XANES (d) and (e) and EXAFS (f) and (g) data for fresh NiFe-Boride and control samples.

Fig. 3. Ex/in-situ characterization of as-prepared NiFe-Boride catalyst.

a In-situ SRXRD patterns of the catalyst during the OER test in 1 M KOH electrolyte; The in-situ Ni and Fe K-edge XANES (b) and (c) and EXAFS (d) and (e) data of NiFe-Boride during the LSV process in 1 M KOH aqueous electrolyte. The in-situ applied voltages before and during the OER are 1.2 and 1.4 V vs. RHE, respectively. f Ni 2p and (g) Fe 2p XPS spectra for fresh and post-OER samples.

Fig. 4. Performance of NiFe-Boride catalyst and controls in a three-electrode configuration in 1M KOH aqueous electrolyte.

a OER LSV polarization curves for catalysts loaded on Ni foam without iR correction. b The corresponding Tafel plot of catalysts. c Comparison of Tafel slope and overpotential required to achieve 10 mA/cm², with references all measured in alkaline medium^–. d EIS data for NiFe-Boride and controls in three-electrode configuration. The data were collected at 1.45 V vs. RHE. The inset provides the equivalent circuit: R_s series resistance, R_ct charge-transfer resistance, and CPE constant-phase element related to the double-layer capacitance. e Chronopotentiometric curves obtained from the NiFe-Boride catalyst on Ni foam electrode at a constant current density of 20 mA/cm². f Comparison of stability at different current density, with references all measured in alkaline medium^–,,. g Operating voltage and O₂ FE at constant 1 A/cm² current density in a three-electrode configuration in 1 M KOH aqueous electrolyte. h The operating voltage and ethylene FE were monitored at constant 200 mA/cm² in a membrane-electrode assembly device. NiFe-Boride and IrO₂ supported on titanium felt were used as the anodes. The high surface area Cu catalyst on hydrophobic carbon paper acted as cathode. Humidified CO was flowed through the gas channels in the cathode, and 2 M aqueous KOH solution was flowed through channels in the anode.