Direct observation of phase change accommodating hydrogen uptake in bimetallic nanoparticles

Hydrogen is a promising alternative to fossil fuel, however storing it efficiently poses challenges. One promising solution is to adsorb hydrogen on solid materials demonstrating quasi-molecular bonding with hydrogen. The hydrogen adsorption energy can be tuned by changing the morphology or stoichiometry of bimetallic nanoparticles. Here we used complementary techniques to unveil the chemical compositional and morphological transformation undergone by PdxNi100-x nanoparticles during H2 adsorption. Our findings reveal NiO-rich shell and Pd-rich core as confirmed by X-ray photoelectron spectroscopy, X-ray scattering, and electron energy loss spectroscopy. During hydrogen adsorption, which mainly occurs on Pd atoms, the Pd-rich core fragments into small pockets, increasing its surface area. This process is more pronounced for nanoparticles with lower Pd loading, emphasizing the role of NiO. These results shed light on the atomic changes occurring in the PdxNi100-x nanoparticles during hydrogen adsorption and can be applicable on multi-metallic systems to improve hydrogen storage properties.