Properties of thin, hydrous Pd oxide films

Thin, hydrous (dispersed) Pd oxide films (beta-oxide) formed electrochemically at polycrystalline Pd electrodes in basic solutions have been studied using a range of electrochemical techniques as well as transmission electron microscopy (TEM). These films have been formed by multi-cycling potential methods to maximum estimated thicknesses of ca. 100nm. The hydrous oxide Nm forms only when a potential of 2V is exceeded, and electrochemical evidence indicates that it exists in the form of islands or strands lying above the compact Pd alpha-oxide film. Based 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-conducting, and an equivalent circuit very similar to that for the reduced non-conducting forms of Ir oxide and polyaniline films applies to the Pd beta-oxide film material. The TEM examination of cross-sections of the oxide films confirms that it is highly porous and dispersed in nature, with pore diameters up to 2 to 3 nm and with a very low density, estimated on the basis of its measured thickness and charge density. (C) 1997 Elsevier Science S.A.