Abstract
A systematic investigation was made to correlate the electrocatalytic properties of perovskites, ABO(,3) (A is a lanthanide, B is a first row transition metal), for oxygen evolution with their electronic structures. The perovskites were prepared by high temperature solid state chemical reactions. Electrodes were made by pressing powders into pellets, followed by a sintering process. The surface areas of powders (by BET) and electrodes (by double layer changing curves) were (TURN) 15 m('2) g('-1) and 100-1000 in terms of roughness factor, respectively. The Hall effect and impedance measurements showed that the electrodes were all p-type semiconductors with carrier densities of 10('17)-10('18) cm('-3), mobilities of (TURN) 30 cm('2) V('-1) sec('-1) and flat-band potentials of 0.2-0.5 V (vs. NHE) in 1 M NaOH. A paramagnetism, a weak ferromagnetism and a strong ferromagnetism were observed on LaNiO(,3), La(,0.9)Sr(,0.1)CoO(,3), and LaMnO(,3), respectively. The acid/base titration experiments on powders showed a high coverage of OH('-) species with zpc of 7-9 (pH). The XPS analysis on the La(,0.9)Sr(,0.1)CoO(,3) electrode showed that the untreated surface consisted largely of Co('II) and Co('III); upon anodic polarization the surface became entirely Co('III). The Tafel slopes of oxygen evolution were 2RT/3F, RT/F. and 2RT/F for nickelates, cobaltates and ferrites or manganites, respectively. The reaction order with respect to OH('-) was close to unity. It was concluded, based on the kinetic parameters and an examination of various rate correlations, that a common mechanism for oxygen evolution occurs on perovskites, i.e., the electrochemical adsorption of OH('-), followed by the rate-determining electrochemical desorption of OH, forming H(,2)O(,2) as an intermediate.The electrocatalytic activity increases with the increase of number of d-electrons, which is associated with the decrease of the M('z)-OH bond strength. A qualitative MO model suggests that the occupancy of the antibonding orbitals maximizes the catalytic activity. Thus, LaNiO(,3) (d('7)) gives the fastest oxygen evolution rate.
Otagawa, Takaak (1983). Electrocatalysis of oxygen evolution of perovskite-type oxides. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -537929.