An Examination of the Electrochemical Behavior of Palladium Electrodes in Acid

Although the basic electrochemical response of palladium in aqueous acid media is frequently considered as being rather similar to that of platinum (apart from a region at low potentials where palladium displays unusual behavior due to the absorption/desorption of large quantities of hydrogen and its isotopes), there are significant differences between these two metals. Palladium is of higher reactivity: it is more prone to dissolution, hydrous oxide growth, formation of the higher oxide, and possibly oxygen absorption into the outer layers of the metal lattice. As a catalyst for oxygen gas reduction at the air cathode of an acid fuel cell Pd is prone to dissolution and formation of inhibiting hydrous oxide species, i.e. , its performance decays significantly with time at the potential region of interest, ca. 0.8 V (RHE). Thick hydrous oxide films may be grown on Pd in acid, at temperatures up to 40°C, by the potential cycling technique; unlike Pt, dissolution tends to predominate at higher temperatures. The reduction behavior of such films is also more complex than in the case of Pt. The presence of a curious, widely ignored but independently reported, minor reversible response just above 0.2 V (RHE) in cyclic voltammograms recorded for Pd in acid was confirmed; the significance of this feature will be considered in more detail in a forthcoming paper.