Engineering Nanostructured Spinel Ferrites by co-substitution for Total Water Electrolysis by Preferential Exposure of Metal Cations on the Surface
Single phasic nanosized magnetite co-substituted with cobalt and nickel having the formula CoxNi(0.4-x)Fe0.6IIFe2IIIO4 (x=0, 0.1, 0.2, 0.3, and 0.4) were synthesized in the nanoregime using a co-precipitation technique. Magnetite being an inverse spinel will preferentially expose octahedral sites and make available metal cations on the surface which will play a conducive role in both hydrogen evolution (HER) and oxygen evolution (OER) reactions. We demonstrate that partial substitution of Fe2+ either by Ni2+ or Co2+ ions on the octahedral sites of the inverse spinel structure of CoxNi(0.4-x)Fe0.6IIFe2IIIO4 was found significantly enhancing the bifunctional electrocatalytic activity of the magnetite NPs in alkaline media. The spinel ferrite having the formula Co0.2Ni0.2Fe0.6IIFe2IIIO4 exhibits outstanding bifunctional electrocatalytic activity in 1 M KOH with lowest onset overpotential (ƞOER= 190, and ƞHER = 200 mV), small overpotential at η10(OER = 270 mV and HER = 275 mV), excellent kinetics (Tafel slopes, bOER = 44 mV dec-1 and bHER = 99 mV dec-1), and high durability (˃ 10 h). Furthermore, this Co0.2Ni0.2Fe0.6IIFe2IIIO4 catalyst can serve as both cathode and anode for overall water-splitting and delivered a current density of 10 mA cm-2 at a quite 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 highly efficient non noble transition metal based electrocatalyst for renewable energy applications via simple micro-structural engineering.