Nanostructured Ni-Cu Electrocatalysts for the Oxygen Evolution Reaction

Abstract

Ni-based materials are promising electrocatalysts for the oxygen evolution reaction (OER) for water splitting in alkaline media. We report the synthesis and OER electrocatalysis of both Ni-Cu nanoparticles (20-50 nm in diameter) and Ni-Cu nanoclusters (<20 metal atoms). Analysis of mass spectral data from matrix-assisted laser desorption/ionization and electrospray ionization techniques demonstrates that discrete heterobimetallic Ni-Cu nanoclusters capped with glutathione ligands were successfully synthesized. Ni-Cu nanoclusters with a 52:48 mol % Ni:Cu metal composition display an OER onset overpotential of 50 mV and an overpotential of 150 mV at 10 mA cm^-2, which makes this catalyst one of the most efficient nonprecious metal OER catalysts. The durability of the nanocluster catalysts on carbon electrodes can be extended by appending them to electrodes modified with TiO₂ nanoparticles. Infrared spectroscopy results indicate that the aggregation dynamics of the glutathione ligands change during catalysis. Taken together, these results help explain the reactivity of a novel class of nanostructured Ni-Cu OER catalysts, which are underexplored alternatives to more commonly studied Ni-Fe, Ni-Co, and Ni-Mn materials.

Keywords: bimetallic; electrocatalysts; nanoclusters; oxygen evolution reaction.

FIGURE 1.

Positive-mode MALDI-TOF mass spectrum of the as-synthesized 52:48 mol % Ni-Cu nanoclusters (A). Calculated (B, top panel) isotopic pattern and experimental mass spectrum obtained from high resolution ESI mass spectrometry (B, bottom panel) of the nanoclusters.

FIGURE 2.

Linear sweep voltammograms at 10 mV s⁻¹ of the oxygen evolution reaction in 1 M NaOH using a glassy carbon working electrode modified with a mixture of Ni-Cu bimetallic nanoparticles (NPs, A) or nanoclusters (NCs, B), Vulcan XC-72, and PVDF. Ni-Cu bimetallic NPs and NCs with various molar ratios were tested (colored lines) along with pure Ni (black lines) and pure Cu (Figure S11, ESI) NPs and NCs. Current densities are reported against the geometric electrode area.

FIGURE 3.

Plots of onset overpotentials (A) and overpotentials at 10 mA cm⁻² (B) for the oxygen evolution reaction in 1 M NaOH using various compositions of Ni-Cu bimetallic nanoclusters (NCs, black) or nanoparticles (NPs, red) on a glassy carbon working electrode.

FIGURE 4.

Chronopotentiometry curves of carbon (black line), TiO₂ nanoparticles on carbon (red line), Ni-Cu nanoclusters with PVDF on carbon (blue line), and Ni-Cu nanoclusters on TiO₂ nanoparticles on carbon (green line) electrodes at a current density of 10 mA cm⁻² in 1 M NaOH.

FIGURE 5:

XRD spectra of the 52:48 mol % Ni-Cu nanoclusters before (A) and after (B) OER catalysis.

FIGURE 6.

Normalized FTIR spectra of the 52:48 mole % Ni-Cu nanoclusters before (blue) and after (gray) catalysis along with the spectrum of glutathione (orange).