More effective compressed natural gas storage

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2014

New composite prepared with a graphene derivative displays good impermeability to gas and improved mechanical properties. 

KEY CONCEPTS
• Natural gas is becoming a more attractive fuel for use in automobiles because of its increasing availability.  But compressed natural gas can only be stored in bulky steel tanks that can limit fuel economy.
• Plastic resins represent a lightweight alternative, but many options do not prevent natural gas from leaking.
• A new composite prepared from a representative plastic knows as TPU and hexadecylated graphene nanoribbons exhibits good impermeability to gas and improved mechanical preperties.

THE INCREASING AVAILABILITY OF NATURAL GAS in the U.S. is leading to its greater use in a number of applications. The chemical industry views natural gas as a cheaper source of raw materials such as ethylene. In the automotive industry, attention is now being paid to developing more efficient natural gas engine vehicles.

The U.S. Energy Information Administration estimates that natural gas production will increase from 23 trillion cubic feet in 2011 to 33 trillion cubic feet in 2040 (1). This agency attributes almost all of the growth to be originating from shale gas.

But there are problems with storing natural gas in automobiles. Bulky steel tanks are needed to make sure that compressed natural gas at pressures up to 3600 psi does not leak into the environment.

James Tour, professor of chemistry at Rice University in Houston, says, “The need for large natural gas tanks means that drivers have limited storage and also the weight of the vehicle limits fuel economy benefits. We spoke with taxi cab drivers in California who complained about filling up with natural gas three times a day and having very limited trunk space because that’s where the bulky natural gas tank needs to be placed in the vehicle.”

Research continues to take advantage of the outstanding physical properties of the two-dimensional layer of carbon atoms organized into hexagonal structures, which is known as graphene. In a recent TLT article, a coating of graphene monolayers known as nanodrapes on a rough metal surface significantly boosted water repellency (2). The performance benefits of graphene were seen through images taken of the interaction of water droplets with the graphene coated surface.

Tour indicates that there is need for a more effective, lightweight material to be used for storing and transporting natural gas. He says, “During natural gas extraction, flexible pipe is needed, but leakage creates problems where much of the natural gas goes into the air and is lost.”

Polymer resins are being used to transport and store natural gas. In fact, Tour indicates that high-density polyethylene (HDPE) is the main choice and is pretty good, so Tour chose a more leaky system for his tests: thermoplastic polyurethane (TPU).

Tour says, “TPU has a problem with gas permeability. That is why it was chosen. If we could make TPU stop leaking, then we could improve most any plastic.”

If TPU could be modified in the appropriate manner, then it might become less gas permeable. Such an approach has now been developed using a graphene derivative.

HEXADECYLATED GRAPHENE NANORIBBONS
Tour and his fellow researchers modified TPU by treating the thermoplastic resin with hexadecylated graphene nanoribbons (HD-GNRs) to produce a composite that displays good impermeability to gas and improved mechanical properties. He says, “We found that by derivatizing graphene nanoribbons, we are able to make it 1,000 times more difficult for gas to escape from a container using the TPU/HD-GNR composite.”

HD-GNR is prepared by intercalation of sodium/potassium alloy into multiwalled graphene nanoribbons followed by treatment with 1-iodohexadecane. Solution casting was used to prepare a film of the TPU/HD-GNR composite. The concentrations of the HDGNR in the TPU range from 0.05 to 5 wt percent.

Tour says, “We chose HDGNR because this material contained very few defects as compared to other options such as graphene oxide, and HD-GNR is much more dispersible in organic materials than is graphene oxide. The defects in graphene oxide mean that there are holes in this material where gas can escape.”

Raman spectroscopy showed that the G/D ratio of HD-GNR, which is a measure of the ratio of sp² to sp³ carbons, is much higher than is seen with graphene oxide. A high ratio indicates a well established graphene structure (that requires sp² carbons) with few defects is present in HD-GNR.

The researchers evaluated the dispersibility of HD-GNR versus graphene nanoribbons (GNR) by preparing 1 milligram per milliliter solutions in chloroform. After sonication for five minutes, the graphene nanoribbons started to precipitate after 10 minutes, while the HD-GNR dispersion was stable for two days.

TPU is a thermoplastic with hard segments composed of aromatic diisocyanates and soft segments prepared from polyethers. Changsheng Xiang, a Rice graduate student and lead author on the work, says, “Addition of the HD-GNR causes a phase separation of the composite due to the interdomain interface and related free energy and entropy changes. Due to the good dispersion in organic solvent, the HDGNR is able to disperse throughout the polymer matrix, leading to a strengthening of the mechanical properties of the resulting composite.”

Static tensile tests and dynamic mechanical analysis were used to evaluate the mechanical properties of the TPU/ HD-GNR composite.

Gas permeability testing was conducted by determining how long it took for a specific concentration of gas to penetrate TPU/HD-GNR films. Initial testing with nitrogen gas showed that permeability was not reduced until the concentration of the HD-GNR in the composted had reached 0.5 wt percent. No gas penetrated through the film after testing for 1,000 seconds. In contrast, gas took 100 seconds to pass through the TPU control and 500 seconds to pass through a composite with 0.1 wt percent HD-GNRs.

Tour says, “We believe that at the 0.5 wt percent threshold concentration, sufficient HDGNR is present in a percolated network through the composite to block all gas molecules from getting through the film.”

Figure 2 shows an image of a cross section of the composite. The HD-GNRs are seen as white dots dispersed through the composite.


Figure 2. This cross sectional image of the TPU composite shows hexadecylated graphene nanoribbons as white dots dispersed through the material. The nanoribbons act to prevent gas leakage, which means they can be used to help lighter weight plastics store and transport compressed natural gas. (Courtesy of Rice University)

The researchers did similar testing and had similar results with carbon dioxide. Tour says, “Since methane is a larger molecule than nitrogen, we believe that the TPU/HD-GNR composite should also be effective.”

Future work will focus on preparing a composite with HDPE, which is a better material to work with than TPU. Additional information can be found in a recent article (3) or by contacting Tour at tour@rice.edu.

REFERENCES
1. Click here.
2. Canter, N. (2013), “Increasing Water Resistance of Rough Surfaces,” TLT, 69 (12), pp. 12-13.
3. Xiang, C., Cox, P., Kukovecz, A., Genorio, B., Hashim, D., Yan, Z., Peng, Z., Hwang, C., Ruan, G., Samuel, E., Sudeep, P., Konya, Z., Vajtai, R., Ajayan, P. and Tour, J. (2013), “Functionalized Low Defect Graphene Nanoribbons and Polyurethane Composite Film for Improved Gas Barrier and Mechanical Performances,” ACS Nano, 7 (11), pp. 10380-10386.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.