Composite shell empowered crystalline-amorphous NiO/NiWO<sub>4</sub>-rGO core-shell electrocatalyst for efficient water electrocatalysis

Abstract

Nickel-based materials exhibit excellent electrochemical water splitting activity; however, their inferior mass transport limits further improvement in catalytic performance. Herein, we report a composite core-shell material consisting of spherical nanoparticles of NiWO4 and rGO sheets coated on crystalline NiO for overall water splitting in an alkaline medium. The macropores created from a uniform coating of spherical nanoparticles with rGO sheets impart high porosity and short diffusion passages, facilitating fast electrolyte flow and thereby enhancing mass transport capability. Benefiting from the excellent mass transport due to mesoporosity, NiO/NiWO4-rGO required an overpotential of 270 mV to achieve a current density of 50 mA cm-2 for OER and 54 mV to achieve a current density of -10 mA cm-2 for HER. A Tafel slope of 82 and 58 mV dec-1 for OER and HER was observed for NiO/NiWO4-rGO, respectively. Overall water splitting devices fabricated using NiO/NiWO4-rGO as an anode and cathode require a cell voltage of 1.59 V to enable a current density of 50 mA cm-2 with stability for over 50 h indicating a favorable morphological modulation at the interface of NiWO4-rGO shell structure coated on a crystalline NiO core, which lowers the overpotential requirement. The assembled water-splitting device performs water splitting 10 M KOH and requires only 1.55 V to reach the current density of 50 mA cm-2. Our density functional theory (DFT) calculations reveal the free energy profiles of hydrogen adsorption, guiding the experimental optimization of catalysts for efficient HER and OER. Furthermore, a seawater electrocatalysis device assembled using NiO/NiWO4-rGO required only 1.77 V to reach 50 mA cm-2 current density with stability over 50 h. This confirms that NiO/NiWO4-rGO is a potential material for industrial and practical water splitting.