The on-board, high-capacity, facile, and reversible storage of hydrogen fuel is one of several significant challenges for hydrogen-fueled vehicles. While gas-adsorbant metal-organic framework structures display high H2 uptake, the volume of H2 they can hold is insufficient due to the material’s low density. The densities of carbon materials are sufficiently high but their capacity for H2 adsorption is low. Strategies for increasing the H2-uptake capacity of carbon materials include surface modification with heteroatoms or functional groups in order to polarize the H2 molecules. Pillared graphene’s potential for H2 storage has been demonstrated with theoretical calculations. In addition, recent experiments show that H2 uptake by thermally exfoliated graphene (TEG) increases linearly with surface area. Recently, C. Kittrell and J.M. Tour of Rice University, K.J. O’Neill of the National Renewable Energy Laboratory (NREL), and co-researchers hypothesized that H2 uptake in TEG could be improved by engineering nanospaces; carbon scaffolds could be created by insertion of molecular spacers between graphene sheets.
As reported in the February 22nd issue of Chemistry of Materials (DOI: 10.1021/cm1025188; p. 923), the researchers used a modified procedure to make the TEG, in which most of the oxygen was removed by reduction at elevated temperature. Dispersion and functionalization of the graphene sheets were achieved by reaction with either a diazonium compound or t-butylaniline in the super acids chlorosulfonic acid or oleum (see Scheme). Cross-links between the graphene sheets were obtained with the diazonium reaction in oleum (see Scheme).
The researchers monitored the degree of functionalization with x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Scanning electron micrographs of the functionalized TEG show that the aryl groups serve as spacers, and open up nanometer-sized interstitial gaps between the graphene planes where H2 can adsorb, in contrast to the original TEG sheets, which appear as disordered, fluffy conglomerates.
Another factor leading to increased H2 adsorption is the charge transfer generated by the sulfonic acid groups. The H2 uptake and surface areas were measured for TEG and functionalized TEG with a custom-built, high-precision system at NREL. The H2 uptake (measured as a weight-percent per 500 m2/g at 77 K and 2 bar) for the functionalized TEG samples were higher than the H2 uptake for TEG by 60%, 40%, 40%, and 10% for 1a, 1b, 2a, and 2b, respectively (see Scheme). The researchers said that they expect the H2 uptake by the functionalized TEGs to be substantially higher at higher pressures, and that the “hydrogen uptake enhancement of TEG by organic cross-linking demonstrates a possible route to further increase the hydrogen storage capacity of graphene materials.”
Functionalization and cross-linking of thermally exfoliated graphene (TEG) sheets were performed in chlorosulfonic acid and in oleum. The degree of functionalization in chlorosulfonic acid is higher than in oleum. Reproduced with permission from Chem. Mater. 23 (4)(2011) DOI: 10.1021/cm1025188; p. 923. © 2011 American Chemical Society.
