Hierarchical coupling effect in hollow Ni/NiFe₂O₄-CNTs microsphere via spray-drying for enhanced oxygen evolution electrocatalysis

Design and fabrication of cost-effective transition metal and their oxides-based nanocomposites are of paramount significance for metal-air batteries and water-splitting. However, the traditional optimized designs for nanostructure are complicated, low-efficient and underperform for wide-scale applications. Herein, a novel hierarchical framework of hollow Ni/NiFe₂O₄-CNTs composite microsphere forcibly-assembled by zero-dimensional (0D) Ni/NiFe₂O₄ nanoparticle (< 16 nm) and one-dimensional (1D) self-supporting CNTs was fabricated successfully. Benefitted from the unique nanostructure, such monohybrids can achieve remarkable oxygen evolution reaction (OER) performance in alkaline media with a low overpotential and superior durability, which exceeds most of the commercial catalysts based on IrO₂/RuO₂ or other non-noble metal nanomaterials. The enhanced OER performance of Ni/NiFe₂O₄-CNTs composite is mainly ascribed to the increased catalytic activity and the optimized conductivity induced by the effects of strong hierarchical coupling and charge transfers between CNTs and Ni/NiFe₂O₄ nanoparticles. These effects are greatly boosted by the polarized heterojunction interfaces confirmed by electron holography. The density functional theory (DFT) calculation indicates the epitaxial Ni further enriches the intrinsic electrons contents of NiFe₂O₄ and thus accelerates absorption/desorption kinetics of OER intermediates. This work hereby paves a facile route to construct the hollow composite microsphere with excellent OER electrocatalytic activity based on non-noble metal oxide/CNTs.