Effect of partial substitution of Cr on electrocatalytic properties of CoFe_2O_4 towards O_2-evolution in alkaline medium

摘要

Some nanocrystalline ternary ferrites (～30—40 nm) having molecular formula, CoFe_(2-x)Cr_xO_4(0 ≤ x ≤ 1.0) and face centered cubic structure were investigated by electronic, EPR, magnetization, impedance and Tafel polarization techniques. Results indicate that the combination of antiferromagnetic superexchange interactions of the spinel lattice determines the Curie temperature (T_c) value. The AB interaction dominates over the sublattice interactions, and Fe~(3+) occupies A sites, and Cr~(3+) and Co~(2+) ions occupy octahedral sites because of large crystal field stabilization energy. The saturation magnetization (σ_s) coercivity (H_c), remnant field (H_r) and electrical resistivity are observed to decrease with the progressive replacement of Fe~(3+) ions (d~5, five unpaired electrons) by Cr~(3+) ions (d~3, three unpaired electrons). However, the electrocatalytic activity of the oxides towards the oxygen evolution reaction (OER) in 1 M KOH at 25 ℃ increases with the increase in x; the optimum improvement in the apparent electrocatalytic activity being with 1.0 mol Cr. At low overpotentials, the OER on substituted compounds displayed a Tafel slope of b = 50 ± 5 mV decade~(-1) and the reaction order with respect to OH~- concentration as unity, regardless of the composition of the oxide catalyst. It seems that the electrocatalytic activity and saturation magnetization/Curie temperature for the oxide are inversely related.