New thermoplastic-graphene nanoribbon composite could offer lighter, more effective natural gas storage for vehicles
|An electron microscope image shows graphene nanoribbons embedded in a block copolymer. Image by Changsheng Xiang. Click to enlarge.|
A new composite material created at Rice University is nearly impervious to gas and may lead to lighter and more efficient storage of compressed natural gas for vehicles. By adding modified, single-atom-thick hexadecyl-functionalized low-defect graphene nanoribbons (HD-GNRs) to thermoplastic polyurethane (TPU), the Rice lab of chemist James Tour made the resulting material far more impermeable to pressurized gas and far lighter than the metal in tanks now used to contain the gas.
In an open access paper in the journal ACS Nano, Tour and his colleagues at Rice and in Hungary, Slovenia and India reported that nitrogen gas effective diffusivity of the TPU was decreased by three orders of magnitude with only 0.5 wt% GNRs.
Graphene, the two dimensional atomically thin carbon framework, is an impermeable material, in addition to its possessing intriguing electrical, mechanical and thermal properties. Graphene can be either derived from a top-down method such as mechanical exfoliation, or from bottom-up chemical vapor deposition methods. However, neither of the two approaches have yet been scaled to large quantities as needed for composite applications.
...The concept of adding impermeable fillers into a polymer matrix is to create tortuous paths for the gas molecules that are attempting to travel through the film. Most work to date has focused upon the filler’s aspect ratio or configuration within the polymer matrix affects gas barrier properties. Graphene nanoribbons (GNRs) might be preferred gas barriers in composites since they, unlike GO, are stable to water, and they can be edge-functionalized to improve processability without sacrificing the integrity of the basal planes.
In this work, we used hexadecylated GNRs (HD-GNRs) produced from in situ intercalation of Na/K alloy into multi-walled carbon nanotubes (MWCNTS), followed by quenching with 1-iodohexadecane. The hexadecyl groups on the edges make the ribbons easily dispersed in organic solvents. The resulting, somewhat foliated HD-GNRs render the composite to be highly impermeable to gases.—Xiang et al.
Tour developed a breakthrough “unzipping” technique for turning multi-walled carbon nanotubes into GNRs, first revealed in Nature in 2009; the technology has been licensed for industrial production.
The researchers, led by Rice graduate student Changsheng Xiang, produced thin films of the composite material by solution casting GNRs treated with hexadecane and TPU, a block copolymer of polyurethane that combines hard and soft materials. The overlapping 200- to 300-nanometer-wide ribbons dispersed so well that they were nearly as effective as large-sheet graphene in containing gas molecules. The GNRs’ geometry makes them far better than graphene sheets for processing into composites, Tour said.
They tested GNR/TPU films by putting pressurized nitrogen on one side and a vacuum on the other side. For films with no GNRs, the pressure dropped to zero in about 100 seconds as nitrogen escaped into the vacuum chamber. With GNRs at 0.5%, the pressure didn’t budge over 1,000 seconds, and it dropped only slightly over more than 18 hours.
Stress and strain tests also found that the 0.5% ratio was optimal for enhancing the polymer’s strength.
The idea is to increase the toughness of the tank and make it impermeable to gas. This becomes increasingly important as automakers think about powering cars with natural gas. Metal tanks that can handle natural gas under pressure are often much heavier than the automakers would like.—James Tour
The Air Force Research Laboratory through the University Technology Corp., the Office of Naval Research MURI graphene program and the Air Force Office of Scientific Research MURI program supported the research.
Changsheng Xiang, Paris J Cox, Akos Kukovecz, Bostjan Genorio, Daniel P Hashim, Zheng Yan, Zhiwei Peng, Chih-Chau Hwang, Gedeng Ruan, Errol L. G. Samuel, Parambath M Sudeep, Zoltan Konya, Robert Vajtai, Pulickel M Ajayan, and James M. Tour (2013) “Functionalized Low Defect Graphene Nanoribbons and Polyurethane Composite Film for Improved Gas Barrier and Mechanical Performances” ACS Nano doi: 10.1021/nn404843n