Observations in the H-Pd{111} system via stress and EQCM measurements

The under-potential deposition (UPD) absorption of hydrogen in thin film palladium (45-75 nm) was investigated electrochemically utilizing both EQCM and thin film stress measurement techniques. The total frequency change during H absorption (desorption) as measured by EQCM is attributed to both an increase (decrease) in mass, Δf _m, as well as the associated thin film stress evolution, Δf _s. In order to deconvolute these two contributions _, the changes in the Pd{111} thin-film stresses were measured directly by a high-resolution cantilever curvature-based stress monitoring technique. Mass incorporation and stress generation due to hydrogen absorption were recorded during cyclic voltammetric scans of increasingly cathodic potentials as well as during chronoamperometric holds in the hydrogen UPD region. The combination of stress measurements and mass uptake of hydrogen in the purely elastic regime provided a measure of a biaxially constrained partial molar volume of hydrogen in the Pd thin films, V ^biax _H-Pd = 0.44 cm ³mol. Additionally, the occurrence of hysteresis and dramatic changes in slope of stress vs. H concentration during loading and unloading of thin films is explained in the context of elastic-plastic transitions that occur in the film.