Direct assessment from cyclic voltammetry of size effect on the hydrogen sorption properties of Pd nanoparticle/carbon hybrids

The hydrogen sorption properties of palladium particles synthesized with different sizes (2, 6 and 18 nm) dispersed on high surface area graphite powders are studied by both solid/gas and electrochemical methods. We demonstrate that we can directly evaluate by cyclic voltammetry the influence of the particle size on Pd hydride phase formation despite the ohmic losses and capacitive currents associated with powder electrodes. The size-dependent Pd hydride compositions calculated from electrochemical signals and pressure-composition-isotherms under ambient conditions are in good agreement and show the same trend: as the Pd particle size is reduced, the hydrogen content increases in the α phase and decreases in the β phase. Changes in electrochemical signals during cycling are observed when the potential domain covers the Pd oxide region, corresponding to an increase of the mean size of the smallest particles from 2 to ∼12 nm (as confirmed by ex situ X-ray diffraction) whereas no modification occurs if it is restricted to the hydrogen region. This highlights that voltammograms can be used to readily detect in situ size modifications of Pd nanomaterials.