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

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 PdH0.59 to PdH0.68. 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.