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 Fe-57-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 eta(10) 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.

Automated summary

The cobalt-rich wustite nanoparticles transformed into spinels under OER conditions. The electrocatalytic activity of cobalt ferrite nanoparticles in alkaline media was found to be better than that of magnetite, and is related to nanoparticle size, cobalt redox activity, and coordination change during OER.