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 Xray 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.