Engineering nanostructured spinel ferrites by co-substitution for total water electrolysis by preferential exposure of metal cations on the surface

Abstract

Single phasic magnetite samples co-substituted with cobalt and nickel, with the formula Co_xNi_(0.4−x)Fe^II_0.6Fe^III₂O₄ (x = 0, 0.1, 0.2, 0.3, and 0.4) were synthesized in the nanoregime via a co-precipitation technique. Being an inverse spinel, magnetite will preferentially expose the octahedral sites and make metal cations available on the surface, which will play a conducive role in both hydrogen evolution (HER) and oxygen evolution (OER) reactions. We demonstrate that the partial substitution of Fe²⁺ (either by Ni²⁺ or Co²⁺ ions) on the octahedral sites of the inverse spinel structure of Co_xNi_(0.4−x)Fe^II_0.6Fe^III₂O₄ significantly enhance the bifunctional electrocatalytic activity of the magnetite samples in an alkaline medium. The spinel ferrite with the formula Co_0.2Ni_0.2Fe^II_0.6Fe^III₂O₄ exhibits outstanding bifunctional electrocatalytic activity in 1 M KOH with the lowest onset overpotential (ƞ_OER = 190 and ƞ_HER = 200 mV), small overpotential at η₁₀ (OER = 270 mV and HER = 275 mV), excellent kinetics (Tafel slopes, b_OER = 44 mV dec⁻¹ and b_HER = 99 mV dec⁻¹), and high durability (>10 h). Furthermore, Co_0.2Ni_0.2Fe^II_0.6Fe^III₂O₄ can serve as both cathode and anode for the overall water-splitting reaction, and delivered a current density of 10 mA cm⁻² at a very low cell voltage of 1.72 V with excellent stability (>10 h at 10 mA cm⁻²). Thus, this work provides a lucid approach to engineer a highly efficient non-noble transition metal-based electrocatalyst for renewable energy applications via simple micro-structural and surface engineering.