Volume 2, Issue 16 November 2005
Driving Hydrogen Car Research
While eco-friendly hybrid automobiles gain popularity, researchers are already developing cars with no emissions at all. Powered by hydrogen fuel cells, future automobiles may travel long distances with only water dribbling out of the exhaust pipe. The path to the hydrogen economy isn't smoothly paved though. One big question is whether a safe and practical hydrogen storage system can be built to store enough fuel for long journeys. To that end, UC Berkeley chemist Jeff Long is developing novel nanomaterials for tomorrow's hydrogen fuel tanks.
In 2002, Jeffrey R. Long was included in Technology Review magazine's TR100, "a list of 100 innovators 35 or younger whose technologies are poised to make a dramatic impact on our world."
"Currently, hydrogen must be stored at low temperature or high pressure, requiring specialized, heavy, and awkward containers that take up a lot of volume," says Long, who is also a researcher with Lawrence Berkeley National Laboratory's Materials Science Division. "That's a problem when you want to have room for passengers and luggage."
Hydrogen storage is a challenge that Long and his collaborators hope to solve through synthetic inorganic chemistry. Indeed, Long's specialty is controlling the chemical structure of compounds to create new materials with specific physical properties. Since May, he's been leading a project with eight UC Berkeley and Berkeley Lab scientists to study, synthesize, and test materials that could store large amounts of hydrogen at ambient temperature and pressure. The effort was funded with $4.5 million from the Department of Energy as part of the agency's broader initiative to make &qot;hydrogen fuel cell vehicles and refueling stations available, practical, and affordable for American consumers by 2020."
While scientists are still exploring how to produce hydrogen cleanly and cheaply, the element has great promise as a renewable source of energy. When combined with oxygen in a fuel cell, hydrogen produces electrical energy. The only waste product is plain water. Eventually, hydrogen fuel cells could become a common power source for laptop computers, cell phones, and even homes.
A graphical visualization of hydrogen molecules flowing into a single cube, one of many in a metallo-organic framework.
"Hydrogen is a very lightweight molecule, so the amount of energy you can get out of hydrogen with a fuel cell is very high," Long says. "But if you have to add a lot of weight to store and transport the hydrogen, you're essentially reducing its energy content."
Gaseous hydrogen must be contained in high pressure cylinders while liquid hydrogen requires extreme cooling, he explains. Long's approach to hydrogen storage leverages the unique material characteristics that emerge at the nanoscale. The hydrogen storage material he's developing consists of a three-dimensional lattice of tiny hollow cubes, each capable of storing eight hydrogen molecules inside. That may not sound like much, but nanomaterials like this lattice have huge surface areas with respect to their volume, sometimes as much as several thousand square meters per gram. A gas tank filled with this material in a powder form could potentially hold enough hydrogen for long range drives between refills, Long says.
Synthesized in a solution reaction, each cube in the lattice consists of metal corners with edges of organic molecules. Hydrogen molecules are then introduced into the framework where they bind to the metal sites inside the cubes. The key though is getting the binding energy just right so that tiny swings in pressure are all that's needed to push the hydrogen in or pull it out.
"If the binding energy is too low, it'll be difficult to keep the hydrogen inside," Long says. "But if it's too high, you won't be able to get it out without heating up the material. You don't want to spend all day filling your tank and you certainly can't be waiting when you push down the accelerator."
Right now, Long and his colleagues are testing various metals to find one with the most desirable binding energy. Long's colleague, chemistry professor Martin Head-Gordon, is helping predict which metals may provide the best results.
"Forming the cubes isn't easy, so the predictions help limit the trial-and-error," Long says.
The researchers stress that the perfect material is still a long way down the road. And even if they demonstrate that the cubes are an effective storage method, they must then determine whether the synthesis process could be commercialized.
"We want these materials to be very cheap so that the cost of using hydrogen can be comparable to that of gasoline," Long says. "There are still many scientific issues that need to be addressed, but I believe that in the long term hydrogen is the ultimate fuel."