The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis

Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV_2–<i>x</i>Fe_<i>x</i>O₄ nanoparticles with 0 ≤ <i>x</i> ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV₂O₄ is known to exhibit a high conductivity, while in CoFe₂O₄ the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV_1.5Fe_0.5O₄, shows an overpotential for the OER of ∼300 mV for 10 mA cm^–2 with a Tafel slope of 38 mV dec^–1 in alkaline electrolyte. DFT+<i>U</i>+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV_2–<i>x</i>Fe_<i>x</i>O₄ that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V³⁺ → V²⁺. The high catalytic activity, facile synthesis, and low cost of the CoV_2–<i>x</i>Fe_<i>x</i>O₄ nanoparticles render them very promising for application in bifunctional electrocatalysis