Tungsten-Doped NiFe-Layered Double Hydroxides as Efficient Oxygen Evolution Catalysts

Abstract

Electrochemical water splitting proves critical to sustainable and clean hydrogen fuel production. However, the anodic water oxidation reaction—the major half-reaction in water splitting— has turned into a bottleneck due to the high energy barrier of the complex and sluggish four-electron transfer process. Nickel-iron layered double hydroxides (NiFe-LDHs) are regarded as promising non-noble metal electrocatalysts for oxygen evolution reaction (OER) catalysis in alkaline conditions. However, the electrocatalytic activity of NiFe-LDH requires improvement because of poor conductivity, a small number of exposed active sites, and weak adsorption of intermediates. As such, tremendous effort has been made to enhance the activity of NiFe-LDH, including introducing defects, doping, exfoliation to obtain single-layer structures, and constructing arrayed structures. In this study, researchers controllably doped NiFe-LDH with tungsten using a simple one-step alcohothermal method to afford nickel-iron-tungsten layered double hydroxides (NiFeW-LDHs). X-ray powder diffraction analysis was used to investigate the structure of NiFeW-LDH. The analysis revealed the presence of the primary diffraction peak corresponding to the perfectly hexagonal-phased NiFe-LDH, with no additional diffraction peaks observed, thereby ruling out the formation of tungsten-based nanoparticles. Furthermore, scanning electron microscopy (SEM) showed that the NiFeW-LDH nanosheets were approximately 500 nm in size and had a flower-like structure that consisted of interconnected nanosheets with smooth surfaces. Additionally, it was observed that NiFeW-LDH had a uniform distribution of Ni, Fe, and W throughout the nanosheets. X-ray photoelectron spectra (XPS) revealed the surface electronic structure of the NiFeW-LDH catalyst. It was determined that the oxidation state of W in NiFeW-LDH was +6 and that the XPS signal of Fe in NiFeW-LDH shifted to a higher oxidation state compared to NiFe-LDH. These results suggest electron redistribution between Fe and W. Simultaneously, the peak area of surface-adsorbed OH increased significantly after W doping, suggesting enhanced OH adsorption on the surface of NiFeW-LDH. Furthermore, density functional theory (DFT) calculations indicated that W(VI) facilitates the adsorption of H2O and O*-intermediates and enhances the activity of Fe sites, which aligns with experimental results. The novel NiFeW-LDH catalyst displayed a low overpotential of 199 and 237 mV at 10 and 100 mA∙cm−2 in 1 mol∙L−1 KOH, outperforming most NiFe-based colloid catalysts. Furthermore, experimental characterizations and DFT+U calculations suggest that W doping plays an important role through strong electronic interactions with Fe and facilitating the adsorption of important O-containing intermediates.