CAGING H2

IN AAAS SCIENCE, September 28, 2023, Robert F. Service writes, “Chemical Cages Could Store Hydrogen, Expand Use of Clean-burning Fuel.”

Service says, “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,” Service continues, “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.” 

An MOF (metal-organic framework) could store hydrogen in voids. Image by Craig Brown/National Institute of Standards and Technology from Science, September 29, 2023.

Molecular Sponges. “Now,” Service says, “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.”

Alas, Not in Fuel-Cell Vehicles. All this caught my eye because of my enthusiasm for hydrogen-powered fuel-cell cars and trucks.

However, Service is told, “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.”

Nevertheless, “serving as fuel depots” is a major achievement. Here are tidbits gleaned from Service’s article.

Today’s Technology. “Hydrogen is already in wide use as an industrial chemical,” Service notes, “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.”

Ignoring aspects of pollution (an Earth-threatening proposition), you can see why fossil fuels persisted. 

MOFs to the Rescue. Service writes, “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.”

Goldilocks Zone of Release. “An MOF not only needs to soak up lots of hydrogen,” Service says, “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.”

Different Cage Materials. A nickel-based MOF has a binding energy of 14 kJ/mol. Research continues, though: Service reports, “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.”

Aluminum Cages. “Competition is now rising from aluminum,” Service writes, “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.” 

But what about hydrogen rather than carbon dioxide?

Chilling Out Goldilocks. Service describes, “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.”

Bargain MOFs. Service notes, “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.”

Work to be Done. Upscaling the aluminum MOF is one research area. Another is spiking it with small amounts of iron and other metal to enhance its storage capacity. Cheetham says, “There is lots of chemistry to explore.”

And, potentially, lots of hydrogen to store. ds 

© Dennis Simanaitis, SimanaitisSays.com, 2023