New structures pave way for higher-capacity hydrogen fuel cells

New metal organic frameworks may boost the performance of hydrogen fuel cells to new heights, new research suggests.

A hydrogen fuel cell is a zero-harmful-emissions power source that acts like a cross between a battery and a gas tank. It employs hydrogen as the fuel and harnesses the reaction between hydrogen and oxygen to produce electricity. The only “emission” is water.

One limitation of this technology is the ability to store sufficient quantities of hydrogen onboard. The researchers have identified ways to cram more hydrogen than ever before into small storage structures called metal-organic frameworks, increasing the energy density, and, as a result, the projected driving range of a fuel cell vehicle.

Metal organic frameworks, or MOFs, are designer materials comprised of metal ions coupled with organic molecules. Their porous nature makes some MOFs among the most promising ways to store hydrogen.

Digging through the data

Researchers gathered information on all available MOFs, those previously constructed as well as those that remain hypothetical, into a database. The researchers then used high-throughput computer simulations to scour the resulting databank of nearly 500,000 MOFs for those having promising capacities.

They identified three candidates that could surpass previous records for hydrogen storage. The researchers then synthesized these materials and demonstrated their performance.

“We’re demonstrating more energy-dense storage than previously shown,” says Don Siegel, associate professor of mechanical engineering at the University of Michigan.

“You might describe it as more efficient—putting more energy into a smaller space and in a lighter package.”

The researchers dubbed the three MOFs SNU-70, UMCM-9, and PCN-610/NU-100. Each surpassed the performance of IRMOF-20, another MOF the team identified in 2017.

“These materials establish a new high-water mark for usable hydrogen capacities in MOFs,” the study says.

What’s holding hydrogen back?

Hydrogen fuel cells have long held promise as a no-emission power source for electric cars. They have, however, taken a backseat to lithium ion batteries, which you’ll find inside most of the portable electronic devices being produced today—from cell phones and tablets to digital cameras and electric vehicles.

Hydrogen fuel cell systems have several advantages over lithium ion batteries. The most abundant element in the universe, hydrogen is far more common than lithium, so there is little chance of there ever being a supply issue.

And a hydrogen fuel cell car can recharge at a station in a few minutes, about the same time it takes to fill a gas tank now. In contrast, full charge times for lithium battery electric vehicles are typically measured in hours.

There are drawbacks that have limited the auto industry’s embrace of hydrogen, however. For example, producing hydrogen is currently much more expensive than is extracting and refining petroleum.

Transportation of hydrogen fuel is another issue. As a gas, it’s difficult to move and store large quantities of hydrogen efficiently, raising questions of whether it needs to be moved in liquid form in semitrucks or shuttled through pipelines as a gas.

But the lure of what hydrogen could potentially mean for cars, and the environment, has kept major automakers like Ford, Hyundai, Toyota, Honda, and GM involved in its development.

Electric vehicle designers are constantly looking to decrease the size of a car’s power system as a means of increasing efficiency. By increasing the quantity of hydrogen that a MOF adsorbent can store, Siegel says, scientists can reduce the pressure needed to store it as well as its size.

“We want to eliminate the energy storage problem for hydrogen fuel cell vehicles. This shows we’re moving in that direction,” Siegel says.

The research appears in Nature Communications.

Source: University of Michigan