Bifunctional Tungsten-Doped Ni(OH)2/NiOOH Nanosheets for Overall Water Splitting in an Alkaline Medium

Abstract

The development of a cost-effective and proficient bifunctional electrocatalyst is highly fascinating. Herein, we have synthesized a tungsten (W6+)-doped vertically grown nanosheet-like structure of Ni(OH)2/NiOOH on carbon cloth for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity in KOH solution. Doping with W6+ ions in Ni(OH)2/NiOOH is performed by electrodeposition, followed by the hydrothermal method. Various amounts of the dopant (W6+) are used to confirm the role of W, but the W0.1Ni(OH)2/NiOOH nanosheet shows the highest efficiency in electrocatalysis. The surface composition and the oxidation state of the developed electrocatalyst are confirmed by inductively coupled plasma atomic emission spectroscopy and X-ray photoelectron spectroscopy analyses. After doping, the lattice suffers a tensile strain, which is confirmed by Raman and X-ray powder diffraction analyses. Field emission scanning electron microscopy and transmission electron microscopy analyses confirm the nanosheet morphology of W0.1Ni(OH)2/NiOOH. The electrocatalyst, W0.1Ni(OH)2/NiOOH, has a lower value of overpotential of 56 and 293 mV to obtain current densities of 10 and 50 mA/cm2 for HER and OER, respectively, in a basic medium. The corresponding Tafel slope values are 63.5 and 48.2 mV dec–1 for HER and OER, respectively. In W0.1Ni(OH)2/NiOOH, the W6+ ion is a d0 system that behaves as a strong Lewis acid and helps in electron pulling from Ni2+ ions, which facilitates the formation of Ni3+ ions as an active site for HER and OER. The electron pulling nature of the W6+ ion is further confirmed from Bader’s charge analysis. Moreover, the synergistic effect between Ni2+ and W6+ ions plays an important role in a higher electrocatalytic efficiency. Density functional theory calculations revealed an increase in the Gibbs free energy of H adsorption in the presence of W, suggesting an enhanced HER activity for W0.1Ni(OH)2/NiOOH.