As an alternative automotive fuel, natural gas, or methane, doesn't get a lot of attention. Millions of environmentally friendly, natural gas–powered vehicles cruise the world’s roads, but they still account for just a tiny fraction of new autos sold. In part that’s because they require bulky and expensive high-pressure tanks to store enough of the fossil fuel to meet drivers’ demands. Now, researchers have come up with a new material that’s able to store a large volume of methane at low pressure. If they can figure out how to make large quantities of the stuff, the material could spark the development of high-capacity gas tanks and propel wider adoption of natural gas–powered vehicles.
Natural gas has some decided benefits as a fuel. Abundant underground in oil-rich geological formations, it typically costs less than gasoline for an equivalent amount of energy. Per mile of travel, methane also produces about one-third less climate-warming carbon dioxide than gasoline and diesel. But natural gas also has a big downside: It’s far less dense than liquid gasoline, so it takes up far more space. One liter of gasoline contains as much energy as 1000 liters of natural gas at ambient temperature and pressure. Fuel suppliers whack that enormous volume down by compressing the gas to about 250 times atmospheric pressure, or 250 bar. But containing the high-pressure gas requires specialized tanks that cost thousands of dollars and still fill up much of the trunk of a car.
As an alternative to high-pressure tanks, researchers have created a variety of porous, spongelike crystalline materials that can soak up methane at a modest pressure and then release it when the pressure is reduced. One such family are materials called metal organic frameworks (MOFs), which are made up of metal atoms connected by organic linkers. One of the best such absorbents was a MOF created by researchers in Hong Kong, China, in 1999 and dubbed HKUST-1. It has been shown to store 180 cubic centimeters of gaseous methane per cubic centimeter of absorbent. That’s not bad, but it’s still well short of the target volume-to-volume ratio, or v/v, of 263 set by the U.S. Department of Energy (DOE), an amount equivalent to compressing natural gas to 250 bar at 25°C.
Now, the DOE target is in sight. Researchers led by David Fairen-Jimenez, a chemist at the University of Cambridge in the United Kingdom, have come up with a simple way to make HKUST-1 more dense and increase its v/v to 259, essentially meeting the DOE target for the first time. “Everything started with a mistake from one of the Ph.D. students in the lab,” Fairen-Jimenez says. The group, he explains, was trying different recipes for converting their HKUST-1, which is normally a powder of tiny crystalline particles, into something called a sol-gel, a continuous network of those particles. Fairen-Jimenez’s student, Tian Tian, suspended a batch of HKUST-1 particles in an ethanol-based solution and put the solution in a centrifuge to drive out most of the solvent. He then planned to put the vials in an oven to dry, but accidentally left one vial under the vacuum hood overnight. That allowed the ethanol to evaporate out slowly, which turned out to be the key. The next morning Tian realized he was left with a highly dense HKUST-1 material. A few further tweaks to the recipe gave them the best methane storage absorbent made to date, the group reports today in Nature Materials.
The simple approach is “very clever,” says Omar Yaghi, a chemist and MOF expert at the University of California, Berkeley. The new material can adopt different shapes, he notes, so gas tank manufacturers may be able to use it to replace conventional cylindrical tanks with rectangular ones, which make better use of a car’s limited storage capacity. Still, Yaghi says, researchers have a way to go before the stuff will be ready for the assembly line. “They need to get their [materials] to be a lot larger.”
Fairen-Jimenez says his team has already succeeded in scaling up production from single gram quantities to 100 grams. He has also recently formed a company called Immaterial Labs to commercialize the new MOFs. If they succeed it could help give natural gas vehicles some new momentum to zip past their gasoline-powered rivals.