Influence of the cobalt content in cobalt iron oxides on the electrocatalytic OER activity

Sub 10 nm cobalt ferrite CoxFe3−xO4 (x ≤ 1.75) nanoparticles and cobalt-rich wustite (Cox/3Fe(1−x)/3)O nanoparticles (x ≥ 2) were synthesized in a solvothermal approach and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) as well as energy dispersive X-ray spectroscopy (EDX), IR, Raman, and 57Fe-Mossbauer spectroscopy. Their electrocatalytic activity in the oxygen evolution reaction (OER) was evaluated and the active state formation was tracked by operando X-ray absorption spectroscopy (XAS). Our studies demonstrate that the cobalt-rich wustite (Cox/3Fe(1−x)/3)O nanoparticles underwent a phase-transformation into the spinels CoxFe3−xO4 (x ≥ 2) under the applied OER conditions. The overpotential η10 at 10 mA cm−2, serving as a benchmark for the OER activity of the cobalt ferrite nanoparticles in alkaline media, was lower than that of magnetite Fe3O4 even with low cobalt concentrations, reaching a minimum of 350 mV for Co2.25Fe0.75O4 with a Tafel slope of 50 mV dec−1. Finally, we identified that the catalytic activity is linked to the nanoparticle size as well as to the degree of Co redox activity and change in coordination during OER.