Mechanism of gas barrier improvement of graphene/polypropylene nanocomposites for new-generation light-weight hydrogen storage

Hydrogen is a promising energy source for the low-carbon economy but light-weight materials with excellent gas barrier properties are needed for its transport and storage. Nanocomposites can deliver such properties, using two-dimensional materials to provide a barrier within the polymer matrix. Herein, we have investigated the optimal characteristics of graphene-based fillers for their use as a hydrogen barrier in a polymer matrix. We employed reduced graphene oxide (rGO) and graphene nanoplatelets (GNPs) that presented distinct filler morphologies and melt-mixed them separately into isotactic polypropylene (PP). A reliable characterisation approach was developed to evaluate the effective aspect ratio of 2D nanomaterials within a polymer matrix by combining BET surface area for the effective thickness and microscopic measurement for lateral size on the microstructure of the nanocomposite samples. The effective aspect ratio values were evaluated to be 153 ± 85 for the rGO and 14 ± 6 the GNPs. Therefore, the rGO outperformed the GNPs in improving both the mechanical and H2 barrier properties of their nanocomposites. The modulus of PP was increased from 1.2 GPa to 2.3 GPa by the addition of 0.9 vol% of the rGO, whereas GNP-filled PP only reached 1.8 GPa by the addition of a higher loading of the GNP (4.6 vol%). The H2 permeability of PP-rGO nanocomposites compared to pure PP was reduced by 40% at 0.9 vol% loading, compared to PP-GNP samples which had a 30% reduction at 4.6 vol% loading. Nielson's model was employed to analyse the results, showing an effective aspect ratio of rGO and GNP to be 165 and 20 respectively, consistent with the characterisation results.