Fundamental Insights into the Structural Effect of Graphene Encapsulation Layer on Hydrogen Storage Properties of Pd Nanocubes

Abstract

A comprehensive understanding of the role of hydrogen-driven phase transformation in metal hydride systems is important to optimize hydrogen storage performance. Since the phase transformation dictates the reaction thermodynamics of metal hydrides, a new material design based on the confinement of nanostructured metal hydrides into a host matrix has been extensively studied to tune hydriding phase transformation by controlling strain dynamics. Through this structural engineering, the mechanical stress builds up in the nanocomposites and their hydrogen sorption thermodynamics can be significantly altered. However, establishing the correlation between the strain and hydriding function is challenging because the fundamental roles of the mechanical stress from the host matrix have not yet been clarified, limiting further application to various metal hydride systems. Here, Pd nanocubes encapsulated by reduced graphene oxide (rGO) layers were selected as a model platform, and the role of rGO encapsulation on the hydrogen sorption thermodynamics of Pd nanocubes was demonstrated. During phase transformation, rGO-driven mechanical constraints impose restricted volume expansion on Pd nanoparticles, which subsequently leads to a wider hysteresis gap under isothermal conditions. These mechanical constraints are also shown to increase the reaction enthalpy and entropy during hydrogen uptake and release, suggesting that the mechanical stress from the matrix takes a significant part in governing hydrogen sorption thermodynamics despite nanoscaled metal hydrides. Our results provide fundamental insights into the hydrogen-driven phase transformation in the confined metal hydride systems for thermodynamic tuning.