Architecting Hollow\nSpherical NiCo<sub>2</sub>S<sub>4</sub>@rGO Bifunctional Electrocatalyst-Loaded\nQuaternized Poly(2,6-dimethyl-1,4-phenylene\nOxide)-Based Membrane Electrode Assembly for an Alkaline Water Electrolyzer

In alkaline media for overall water\nsplitting with instantaneous\noxygen evolution reaction (OER) and hydrogen evolution reaction (HER),\nwe urgently required a noble metal free bifunctional electrocatalyst.\nWe report a simple protocol for the preparation of NiCo<sub>2</sub>S<sub>4</sub>@rGO spherical hollow spheres, as a bifunctional electrocatalyst.\nThe <i>N</i>-methyl imidazole-grafted poly 2,6-dimethyl-1,4-phenylene\noxide (PPO)-based alkaline membrane (QimPPO) was prepared as a substrate\nfor direct electrocatalyst loading and construction of membrane electrode\nassembly (MEA). The reported electrocatalyst (NiCo<sub>2</sub>S<sub>4</sub>@rGO hollow spheres) exhibited a shape/morphology-dependent\nelectrocatalytic activity for water splitting (simultaneous OER/HER)\nin an alkaline environment. The NiCo<sub>2</sub>S<sub>4</sub>@rGO\nhollow spheres exhibited a current density of 10 mA cm<sup>–2</sup> at 1.61 and 0.22 V for the OER and HER, respectively. In an alkaline\nmedium, the reported MEA showed superior performance in comparison\nwith precious (Pt/Ir) or nonprecious metal-based electrocatalysts.\nThe electrolyzer performance was assessed at an applied current density\nof 400 mA cm<sup>–2</sup> for 70 h. An optimized MEA will mark\na significant milestone for an advanced alkaline water electrolysis\nand corresponding HER/OER processes. The reported results will facilitate\nthe advancement of research in designing a meticulously structured\nbifunctional electrocatalyst for eco-friendly H<sub>2</sub>/O<sub>2</sub> production via water electrolysis.