Metal hydrides are a promising material for hydrogen storage, given their relatively low cost, abundance and high weight percent hydrogen absorption (e.g. 7.6 wt. % for magnesium hydride). However, to date, commercial application of metal hydrides has been limited by the relatively slow absorption and desorption kinetics, requiring high pressure and high temperature, respectively. A first approach for enhancing the properties of metal hydrides is nanostructuring, with the associated increase in surface area and reactivity that this inherently creates. Pd nanoparticles on Mg nanofilms may increase the efficiency of bulk Mg and offer an attractive alternative to bulk Pd for catalysis and hydrogen storage.
In this work, the diffusion of H in Mg nanofilms decorated with Pd nanoparticles is investigated by DFT using the VASP ab initio simulation package. Models of Pd nanoparticle/Mg film interfaces and their variations, i.e. Pd(H)/Mg(O)Pd and PdH/MgH2, are examined and the specific reconstructions of the nanoparticle/nanofilm interface arising from the strain are deduced. The diffusion path of a H atoms starting from the nanoparticle region and ending deep in the nanofilm region is investigated and the respective diffusion barriers are extracted. The Pd nanoparticle is found to facilitate the diffusion of H in the first layers of Mg. This barrier is even smaller for PdH, suggesting that the diffusion is dependent on the H content of PdHx. Finally, H diffusion is prohibited in MgO for both Pd and PdH, suggesting that the nanofilm surfaces should not be oxidized before the nanoparticle deposition, and significantly suppressed through previously formed MgH2 domains.