Metal hydrides are promising materials for the solid-state hydrogen storage to replace current high-pressure storage of hydrogen. For this technology to become commercially competitive, however, high dehydrogenation temperature originated from large reaction enthalpies and sluggish kinetics should be resolved. Nanoconfinement engineering has shown potential to advance the design of metal hydrides. Metal hydride particles under nanoconfinement exhibit improved thermodynamics and kinetics, but elucidating a definitive relationship between the properties of nanoconfined metal hydrides and their resulting impacts on hydrogen storage performance is challenging due to the combinatorial effects of multiple factors: Nanosizing of metal hydride particles, catalytic reactivity of confining media, and chemical and mechanical interactions between those two. In this presentation, using palladium (Pd) nanoparticles encapsulated in reduced graphene oxide (rGO) as a model system, we isolated and quantified the various chemomechanical impacts of encapsulation on metal hydrides for hydrogen storage. By combining high-resolution transmission electron microscopy (HRTEM), first-principles calculations, and microelasticity, we identified the mechanical strain exerted by rGO on Pd and hydrogenated Pd (PdHx) nanoparticles, 4.96% and 2.99% of in-plane strain before and after hydrogenation. Calculated mechanical properties of nanoparticles concluded that the reaction enthalpy increases by 0.9-2.4 kJ/mol H2 due to the strain energy. Integrating the predicted enthalpy change from chemical interaction with rGO (1.6 kJ/mol H2) and nanosizing effect (≤ 0 kJ/mol H2), the net reaction enthalpy change excellently agrees with the enthalpy increase of 3.7 kJ/mol H2 from PCT measurement. In addition, the mechanical strain on Pd nanoparticles aids the accelerated nucleation of the hydride, improving the uptake kinetics. Our findings provide an understanding of individual factors in nanoconfined metal hydrides, enabling to devise potential engineering strategies to concurrently tune the reaction thermodynamics and kinetics.

Decomposition and quantification of nanosizing, mechanical stress, and chemical interaction effects of reduced graphene oxide encapsulation on hydrogen storage performance of Pd nanoparticles

ACS Fall 2021

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