Enabling hydrogen storage under low-purity hydrogen condition by encapsulating Mg nanocrystals with reduced graphene oxide sheets

Abstract

Hydrogen is commonly generated as a gas mixture, containing impurities such as CO2 and H2O, during the fossil fuel reforming process. In order to store and utilize hydrogen as an energy source, it is crucial to achieve a state of ultra-high purity through a purification process. However, the conventional methods for hydrogen purification and storage are both costly and energy-intensive. To approach the hydrogen economy, it is necessary to simplify the current purification process and develop efficient storage systems. Here, we designed a nanocomposite of magnesium (Mg) nanocrystals encapsulated by reduced graphene oxide (rGO) sheets for hydrogen storage under the low-purity hydrogen environments. The rGO layers serve as a gas barrier that allows a selective penetration of hydrogen molecules through the defects, which leads Mg nanocrystals to react with hydrogen without the influence of other gas impurities such as CO2. We investigated the hydrogen sorption performance of rGO/Mg nanocomposites under the low-purity hydrogen conditions by varying the rGO content. Increasing the amount of rGO contributes to the formation of highly graphene-laminated structure, encapsulating Mg crystals in the nanocomposites. It results in the delayed diffusion of other gas molecules, while preserving hydrogen uptake under the low-purity hydrogen condition. Furthermore, since the diffusivity of gas species penetrating the rGO sheet is greatly affected by temperature conditions, the hydrogen storage performance was evaluated by lowering the operating temperature through transition metal doping to increase the selectivity of hydrogen molecules. This study demonstrates the possibility of combining hydrogen purification and storage system by enhancing the oxidation resistance of metal-based hydrogen storage materials.