In situ formation of Mn4+ over oxygen vacancy rich spongy manganese–cobalt oxide through sodium and NiP incorporation for the electrocatalytic oxygen evolution reaction

Abstract

A spongy MnCo₂O₄ catalyst system with suppressed Jahn–Teller distortion and enhanced OH adsorption was developed through the incorporation of sodium into MnCo₂O₄ and its deposition with an NiP coating as an electrode for the oxygen evolution reaction. Mn³⁺/Mn⁴⁺ redox couples, enhanced redox properties and an electronic environment with an optimum concentration of oxygen vacancies were tailored through the modulation of Na content in the defective MnCo₂O₄ system, followed by electroless deposition. Modified sol–gel derived spongy MnCo₂O₄ was calcined using a sodium source to generate electron rich Mn and Co species with good conductivity due to the diffusion of sodium into oxygen vacancies. The amorphous–crystalline combination of the active phases and Mn³⁺/Mn⁴⁺ formation via the interaction of NiP with Na–MnCo₂O₄ in the electrode suppressed Jahn–Teller distortion to provide an optimal electronic environment for accelerated electrocatalytic activity during the OER by increasing the kinetics of oxygen desorption. The Na–MnCo₂O₄/NiP electrode exhibits low charge transfer resistance (R_ct = 309 Ω) at open circuit potential and a electrocatalytically active surface area of 19.75 cm². The electrode has a relatively low OER overpotential of 214 mV at 10 mA cm⁻² and a Tafel slope of 70.2 mV dec⁻¹. Na–MnCo₂O₄/NiP possesses high stability even after 1000 cycles of cyclic voltammetry. The material design strategy introduced in the present work has the potential to develop similar systems with suitable components to achieve excellent OER and other electrochemical performances and provides valuable insight into catalytic electrode characteristics and reaction kinetics.