PROPERTIES OF THIN, HYDROUS PD OXIDE-FILMS

Thin, hydrous (dispersed) Pd oxide films (beta-oxide) formed electroch emically at polycrystalline Pd electrodes in basic solutions have been studied using a range of electrochemical techniques as well as transm ission electron microscopy (TEM). These films have been formed by mult i-cycling potential methods to maximum estimated thicknesses of ca. 10 0nm. The hydrous oxide Nm forms only when a potential of 2V is exceede d, and electrochemical evidence indicates that it exists in the form o f islands or strands lying above the compact Pd alpha-oxide film. Base d on its -90mV pH dependence during reduction, the beta-oxide film is suggested to have the following composition: PdO2(OH-)(2) .(Na+)(2) .( n+2)H2O. Impedance studies have indicated that the film is non-conduct ing, and an equivalent circuit very similar to that for the reduced no n-conducting forms of Ir oxide and polyaniline films applies to the Pd beta-oxide film material. The TEM examination of cross-sections of th e oxide films confirms that it is highly porous and dispersed in natur e, with pore diameters up to 2 to 3 nm and with a very low density, es timated on the basis of its measured thickness and charge density. (C) 1997 Elsevier Science S.A.