Superior oxygen evolution reaction performance of NiCoFe spinel oxide nanowires in situ grown on β-Ni(OH)2 nanosheet-decorated Ni foam: case studies on stoichiometric and off-stoichiometric oxides

Abstract

In this work, stoichiometric [NiCo_(2−x)Fe_xO₄ (x = 0.125, 0.25)] and Co-excess off-stoichiometric [Ni_0.75Co_(2.25−x)Fe_xO₄] NiCoFe spinel oxide nanostructures were self-grown on a β-Ni(OH)₂ nanosheet-decorated Ni foam (NF) substrate and the heterostructural nanocomposites thus obtained were explored for their oxygen evolution reaction (OER) performance. A NiCoFe spinel oxide phase with interesting hierarchical porous nanowire morphology was obtained after annealing NiCoFe hydroxide-carbonate derivatives at 350 °C. As the hierarchical porous nanowire architecture is binder-free, these heterostructural nanocomposite materials have great potential for enhanced mass-transport, improved conductivity and excellent electrochemical stability even at a current density as high as 250 mA cm⁻². The synergistic effects originating from structural and compositional advantages empowered NiCo_1.75Fe_0.25O₄@NiO@NF nanocomposites to show superior OER performance owing to the higher proportion of Ni³⁺/Ni²⁺ and Co²⁺/Co³⁺ ions on active octahedral sites and enhanced ferromagnetic double exchange interactions primarily triggered by oxygen vacancies. The electrochemical studies demonstrated that the nanocomposites showed exceptionally high electrocatalytic activity with an ultralow overpotential value of 272 (±5) mV at 100 mA cm⁻² and a small Tafel slope of 54 mV dec⁻¹. Interestingly, with this strategy of decorating NF with β-Ni(OH)₂ nanosheets, we observed a high conservation rate of catalytic activity (99%) for heterostructural nanocomposites even after 24 h of continuous electrolysis at 50 mA cm⁻². However, the electrocatalysts deposited directly on the substrate showed very poor durability with inferior OER performance owing to the sacrificial nature of NF in the presence of Fe³⁺ions. This work thus proposes a new direction towards the development of highly efficient, durable and inexpensive OER electrocatalysts suitable for high current density applications.