Hydride phase formation in carbon supported palladium nanoparticle electrodes investigated using in situ EXAFS and XRD

Abstract

In situ EXAFS (extended X-ray absorption fine structure), in situ XRD (X-ray diffraction) and electrochemical studies have been used to investigate the palladium hydride phases of carbon supported palladium nanoparticles as a function of applied potential. Electrochemical investigations showed an increase in the hydrogen to palladium ratio with an increasingly negative potential. The H/Pd ratio could be divided into four distinct regions, which described the palladium hydride phase present; the α-phase, a mixture of the α- and β-phases, the β-phase, and a hyperstoichiometric region. The β-hydride phase stoichiometry obtained from the electrochemical data corresponded to PdH. However, the composition obtained from the lattice expansions observed from the in situ EXAFS and XRD, 3.3% and 3.8%, correspond to compositions of PdH_0.59 to PdH_0.68. The excess hydrogen and hyperstoichiometric amounts found at more negative potentials are attributed to either spillover on to the carbon support or trapping and subsequent reoxidation of H₂ in the porous electrode structure.