Unraveling OER kinetics in multicationic spinel oxides: Challenges and insights from Tafel slope analysis

A multicationic spinel, (Co 0.25 Fe 0.25 Mn 0.25 Zn 0.25 ) 3 O 4 , is studied as an (electro)catalyst for the oxygen evolution reaction. It is compared with Co 3 O 4 , and the (electro)catalytic benefits of multiple cations in the spinel lattice are then demonstrated. A considerable increase in the lattice parameter of the multicationic oxide is observed through structural analysis, accompanied by increased local structural disorder. In addition, X-ray photoelectron spectroscopy reveals the presence of additional oxidation states, indicating a more complex electronic environment. The multicationic oxide exhibited, by linear sweep voltammetry, a reduction in overpotential of ∼23 mV at 10 mA cm −2 and of ∼102 mV at 100 mA cm −2 . For the multicationic composition, the rate-determining step involves a chemical step subsequent to the first electron transfer step, as indicated by the Tafel slopes of ∼54–65 mV dec -1 , while Co 3 O 4 exhibits a higher Tafel slope of ∼74–85 mV dec -1 , which indicates a slower electron transfer step for the Co 3 O 4 sample. This change is in line with the different relaxation frequencies obtained by electrochemical impedance spectroscopy. According to chronoamperometry measurements, an overpotential of ∼292 mV is obtained at 17 mA cm −2 for the multicationic oxide, attributed to the cooperative electronic effects of active centers, which optimize the surface adsorption of intermediates.