Hydrogen seems like the perfect fuel. By weight it packs more punch than any other fuel. It can be made from water, meaning supply is almost limitless, in principle. And when burned or run through a fuel cell, it generates energy without any carbon pollution. But hydrogen takes up enormous volume, making it impractical to store. Compressing it helps, but is expensive and essentially turns hydrogen storage tanks into high-pressure explosives.
Now, a molecular sponge made of organic compounds and cheap aluminum promises a practical solution, holding significant amounts of hydrogen at low pressures. Described in a paper accepted last week at the Journal of the American Chemical Society (JACS), it is the latest in a series of promising metal-organic frameworks (MOFs), and it suggests that the materials could be close to a mass market application, serving as fuel depots for backup power sources at industrial operations.
“For the first time ever, we have sorbents that are potentially cheaper than compressed gas in a realistic application,” says Hanna Breunig, a chemical engineer at Lawrence Berkeley National Laboratory who helped analyze the economics of using the aluminum MOF. To Omar Farha, a chemist at Northwestern University who wasn’t involved in the new work, it shows that “this field is progressing at a really incredible speed.”
Hydrogen is already in wide use as an industrial chemical, and storage has been a long-standing problem. The primary solution to date has been to compress hydrogen at up to 700 bar, some 50 times the pressure of an outdoor grill’s propane tank. But the high-pressure tanks are costly, and energy-guzzling compressors are needed to fill them. And even then, a liter of hydrogen compressed to 700 bar stores less than one-fifth of the energy of a liter of gasoline.
Some researchers are exploring storing hydrogen in underground caverns carved out of salt formations—but that geology is rare, and subterranean microbes might eat up the hydrogen. Compounds such as metal hydrides or ammonia can store hydrogen chemically. But these compounds must undergo reactions to unshackle the hydrogen, and recharging the material can be difficult.
MOFs are now emerging as an alternative. These porous solids look like molecular Tinkertoys. Metal atoms serve as hubs that are tied together with organic linkers— carbon-bearing molecular chains. The result is a chemical cage with passageways and voids that trap gases injected under mild pressures. When the pressure is lifted, the hydrogen flows back out.
In 2014, Jeffrey Long, a chemist at the University of California (UC), Berkeley, and his colleagues reported a nickel-based MOF that could store a record amount of hydrogen: 23 kilograms per cubic meter, about half as much as a high-pressure tank, but without the danger and expense of added pressure.
An MOF not only needs to soak up lots of hydrogen; it must also release it easily. The ideal binding strength—measured as the heat of absorption—is between 15 and 25 kilojoules per mole of hydrogen (kJ/mol). Below that range the grip is too loose, and the natural energy of hydrogen is enough for it to wriggle free of the cage. Above that range, the grip is too tight, and the system must be heated to push hydrogen out. “It’s like a Goldilocks zone,” says Hayden Evans, a chemist at the National Institute of Standards and Technology.
The nickel-based MOF has a near ideal binding energy of 14 kJ/mol, because the nickel atoms attract the slightly polar hydrogen molecule through weak electrostatic forces, Long explains. Baker Hughes, an offshoot of General Electric, is exploring using the material to store hydrogen and carbon dioxide captured from industrial furnaces.
In 2021, Long and colleagues followed up with a vanadium-based MOF that grabs hydrogen molecules more tightly at 21 kJ/mol, in the heart of the Goldilocks zone. But these MOFs store less hydrogen than their nickel-based cousins, because only a subset of the vanadium atoms have the right number of positive charges to attract hydrogen.
Competition is now rising from aluminum, which costs just over 1/10 as much as nickel and 1/13 as much as vanadium. In 2022, Anthony Cheetham, a UC Santa Barbara chemist, and his colleagues laid the groundwork when they reported an aluminum-based MOF that looked promising for capturing carbon dioxide. The aluminum MOF requires less energy to release the captured carbon than more conventional liquid carbon-capture compounds, and its small pore size naturally excludes nitrogen gas, the primary component of the atmosphere.
Now, in the JACS paper, Cheetham, Evans, and colleagues have tested the aluminum MOF for hydrogen storage. It stores just two-thirds as much gas as the nickel MOF. And with a binding energy of just 8.6 kJ/mol, it must be chilled to about –100°C to store its maximum amount of hydrogen. Nevertheless, Cheetham says the raw materials are so cheap that he expects the MOF to cost just $2 per kilogram. That would easily beat a $10 per kilogram community goal for the production cost of MOFs made from nickel and other more expensive metals. “It’s hard to imagine any MOF being as cheap as this one,” says Zeric Hulvey, who heads hydrogen storage at the U.S. Energy Department’s Office of Energy Efficiency and Renewable Energy.
The modest storage capacity means the new MOF isn’t likely to work for storing hydrogen in fuel cell vehicles, where volume and weight are critical constraints. However, the combination of low cost, mild operating pressures, and a manageable cooling requirement already points to a practical use, Hulvey says: storing hydrogen for backup power sources that could replace diesel generators at industrial operations, such as data centers.
Cheetham says he and his team want to show they can produce the aluminum MOF in large amounts, beyond the kilograms they can already synthesize. They’re also working on tweaking the structure of the MOF, spiking it with small amounts of iron and other metals to see whether they can further increase its hydrogen storage capacity. Cheetham says: “There is lots of chemistry to explore.”
