Influence of the electrode history and effects of the electrolyte composition and temperature on O2 evolution at β-PbO2 anodes in acid

An investigation is reported on O-2 electrogeneration at beta-PbO2 electrodes in HClO4, H2SO4 and CF3SO3H (TFMSA). using steady-state and impedance measurements. It is established that desorption of reaction intermediates is rate limiting in all the cases examined, and it is proposed that elementary reactions leading to oxygen evolution take place at a surface consisting of crystal line (PbO2) and hydrous [PbO(OH)(2)] zones. Contribution of the latter becomes particularly pronounced at high potentials, probably on account of PbO2 dissolution and re-deposition in the form of Pb(OH)(4) that subsequently evolves to give PbO(OH)(2). This process is indicated as a possible cause of an inflexion point in the Tafel plots between 1.95 and 2.0 V SCE. Different important factors are found to affect the electrochemical process: (i) pre-treatment of the electrode, which makes electrodes more active by reason of an increase of the hydrous zones; (ii) the temperature at which electrodeposition of PbO2 is carried out has an effect on the hydration state of the surface and hence the electrocatalytic activity; (iii) electrolyte anions, particularly SO42- and CF3SO3-, are adsorbed and inhibit both water discharge and desorption of reaction intermediates; besides, they undergo oxidation at the more positive potentials; (iv) fluoride added to the electrolyte is strongly adsorbed and suppresses and/or modifies the structure of the hydrous layer with the consequence that water discharge is inhibited in the lower potential range and, prevalently at the more positive potentials, desorption of oxygen intermediates is strongly retarded; and (v) a low electrolyte temperature also inhibits 02 evolution mainly by inhibition of the desorption of reaction intermediates in the whole potential range examined, in contrast to the selective effect of fluoride. (C) 2002 Elsevier Science B.V. All rights reserved.