Electrocatalytic activity of spinel related cobaltites MxCo3-xO4 (M=Li, Ni, Cu) in the oxygen evolution reaction

A comparative study of the electrochemical and physicochemical behaviour of binary cobalt oxides with spinel structure MxCo3-xO4 (M=Li, Ni, Cu) was performed in order to elucidate the effect of the cation distribution in the crystal lattice on the electrocatalytic activity of the oxides studied in the oxygen evolution reaction in alkaline media. Teflon-bonded electrodes were studied whose true electrochemical surface area was estimated from the double layer capacitance. It was found that the activity of the spinels increases relative to that of Co3O4 in the order: Co3O4<NixCo3-xO4 much less than CuxCo3-xO4<LixCo3-xO4. A relation was found between the activity and the dopant content. The enhanced activity is determined by two inter-related factors: the cation distribution in the two types of spinel site (8a) and (16d) and the number and energetics of the active sites formed on the surface in the anodic peak potential range preceding the oxygen evolution potential. Tafel slopes between 2.3RT/F and 2.3(2RT/3F) at Co3O4, CuxCo3-xO4 and NixCo3-xO4 electrodes and 2.3(RT/2F) at LixCo3-xO4 electrodes were observed; the slopes decrease with increasing dopant content. The reaction order with respect to OH- concentration was nearly two. The data are consistent with two mechanisms: (i) Bockris's electrochemical oxide path with the second step, MOH+OH--->MO+H2O+e(-), as rate determining for Co3O4, CuxCo3-xO4 and NixCo3-xO4 and with the third step, 2MO-->2M+O-2, as rate determining for LixCo3-xO4; (ii) Krasil'shchikov's path with the third step, MO--->MO+e(-), as rate determining for the first three spinels and with the fourth step, 2MO-->2M+O-2, as rate determining for the Li doped oxide (M=Co4+). The decrease of the Tafel slopes with increasing dopant content is assumed to be due to the formation of active sites with larger charge transfer coefficients. (C) 1997 Elsevier Science S.A.