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

Single phasic magnetite samples co-substituted with cobalt and nickel, with the formula Co(x)Ni((0.4-x))FeII0.6FeIII2O(4)(x= 0, 0.1, 0.2, 0.3, and 0.4) were synthesized in the nanoregimeviaa 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 Ni(2+)or Co(2+)ions) on the octahedral sites of the inverse spinel structure of Co(x)Ni((0.4-x))FeII0.6FeIII2O(4)significantly enhance the bifunctional electrocatalytic activity of the magnetite samples in an alkaline medium. The spinel ferrite with the formula Co(0.2)Ni(0.2)FeII0.6FeIII2O(4)exhibits outstanding bifunctional electrocatalytic activity in 1 M KOH with the lowest onset overpotential (n(OER)= 190 andn(HER)= 200 mV), small overpotential at eta(10)(OER = 270 mV and HER = 275 mV), excellent kinetics (Tafel slopes,b(OER)= 44 mV dec(-1)andb(HER)= 99 mV dec(-1)), and high durability (>10 h). Furthermore, Co(0.2)Ni(0.2)FeII0.6FeIII2O(4)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 applicationsviasimple micro-structural and surface engineering.