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

Abstract

Sub 10 nm cobalt ferrite Co_xFe_3−xO₄ (x ≤ 1.75) nanoparticles and cobalt-rich wüstite (Co_x/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-Mössbauer 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 wüstite (Co_x/3Fe_(1−x)/3)O nanoparticles underwent a phase-transformation into the spinels Co_xFe_3−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 Co_2.25Fe_0.75O₄ 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.