Electrochemical behaviour of palladium electrode: Oxidation, electrodissolution and ionic adsorption

In electrochemistry, Pourbaix diagrams also known as potential-pH diagrams map out the regions of stability of metals as well as the regions of possible existence of stable compounds. The stable states other than the metallic one are macroscopic compounds, typically oxides and hydroxides, or their hydrated forms. Yet, several noble metals possess the ability to form thin surface oxides, to adsorb anions, and to adsorb or to absorb hydrogen in the potential range of water stability. Palladium is a unique noble metal owing to its ability to adsorbed and to absorb H, as well as to form compact and stable surface oxides. Anodic polarization of Pd results in the formation of a surface oxide. Thus, a great majority of anodic electrode processes takes place not at a metallic surface but at a surface covered with an oxide layer. Although thin surface oxides reveal metallic conductivity, the presence of O-containing species affects the electron transfer kinetics and the adsorption behaviour of reactants, intermediates, and products. Thus, the electrocatalytic properties of Pd electrodes, and the mechanism and kinetics of the reaction under consideration are strongly affected by the oxide layer, its thickness, chemical composition (Pd oxidation state and nature of O-containing species) and 3D structure, degree of oxide hydration, and electronic properties. This contribution presents an overview of the current understanding of electro-oxidation of Pd in aqueous acidic and basic electrolytes. In particular, it describes the formation of Pd oxides under various experimental conditions and discusses their chemical and physical nature. It examines the reduction of O-containing species present on Pd electrodes as well as the adsorption of anions and cations. Since Pd can undergo electrodissolution that results in material loss, the process is analyzed in relation to experimental parameters. Recent developments in the electrochemical behaviour of single-crystal Pd electrodes are discussed. Electrochemical parameters such as the potential of zero charge, potential of zero total charge and potential of zero free charge are of importance to the structure of the electric double layer; thus, their values are discussed in relation to the absence/presence of Pd surface oxides. Finally, various experimental procedures commonly used to determine the real surface area of Pd electrodes are outlined and compared.