Study of the hydrogen storage properties of palladium encapsulated by reduced graphene oxide

Abstract

A deeper understanding of solute-induced phase transformation is important for host-metal hydride systems, in which structural changes alter the thermodynamics and the kinetics behaviors governing hydrogen storage properties. Although a lot of research has shown distinct hydride phase transformation associated with interactions between host materials and metal hydrides, an extrinsic structural effect of host matrix on the metal lattice has not been identified, hindering practical applications of other metal hydride systems. Future design of a host-metal hydride systems, therefore, significantly relies on establishing the nature of structure-property relationships. Here, we prepared palladium (Pd) nanoparticles encapsulated by reduced graphene oxide (rGO) sheets (rGO-Pd) and elucidated that the compressive stress driven by rGO sheets during phase transformation causes elasto-plastic contributions, opening a hysteresis gap. Moreover, the constrained rGO-Pd system showed significantly increased desorption enthalpy and entropy, indicating mechanochemical changes compared to the simple phase transformation of single nanoparticles. $\textit{Ab initio}$ computations show that under hydrostatic compression above 1 % the vibrational entropy of Pd and $PdH_{x}$ decreased at different rate, inducing an increased reaction entropy. Our detailed study demonstrates that rGO encapsulation presents significant constraints to the inner nanoparticles, which provide the possibility to tune hydrogen storage properties of various confined metal hydride systems.