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 behavio ur of binary cobalt oxides with spinel structure MxCo3-xO4 (M=Li, Ni, Cu) was performed in order to elucidate the effect of the cation distr ibution in the crystal lattice on the electrocatalytic activity of the oxides studied in the oxygen evolution reaction in alkaline media. Te flon-bonded electrodes were studied whose true electrochemical surface area was estimated from the double layer capacitance. It was found th at 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 re lation was found between the activity and the dopant content. The enha nced activity is determined by two inter-related factors: the cation d istribution in the two types of spinel site (8a) and (16d) and the num ber and energetics of the active sites formed on the surface in the an odic peak potential range preceding the oxygen evolution potential. Ta fel slopes between 2.3RT/F and 2.3(2RT/3F) at Co3O4, CuxCo3-xO4 and Ni xCo3-xO4 electrodes and 2.3(RT/2F) at LixCo3-xO4 electrodes were obser ved; the slopes decrease with increasing dopant content. The reaction order with respect to OH- concentration was nearly two. The data are c onsistent with two mechanisms: (i) Bockris's electrochemical oxide pat h 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 wit h the third step, MO--->MO+e(-), as rate determining for the first thr ee 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 wit h increasing dopant content is assumed to be due to the formation of a ctive sites with larger charge transfer coefficients. (C) 1997 Elsevie r Science S.A.