Purdue University chemical engineers have invented a method of storing hydrogen using ammonia borane and releasing it safely without the need for high-pressure tanks, only by using the fuel cell’s dissipated heat.
The U.S. Department of Energy has set a target of 5.5 weight percent hydrogen for hydrogen storage systems, and that means that the usable hydrogen should be at least 5.5% of the system’s total weight, when referring to future fuel cell-powered electric cars.
“This is the first process to provide exceptionally high hydrogen yield values at near the fuel-cell operating temperatures without using a catalyst, making it promising for hydrogen-powered vehicles,” says Arvind Varma, R. Games Slayter Distinguished Professor of Chemical Engineering and head of the School of Chemical Engineering. “We have a proof of concept.”
They used ammonia borane for the actual hydrogen storage, since this material encapsulates 19.6 percent hydrogen – very high, compared to other storage environments, like 5,000 psi pressurized tanks, which are both expensive and dangerous in case of accidents.
“The key is how to efficiently release the hydrogen from this compound, and that is what we have discovered,” Varma said. They researchers combined hydrolysis and thermolysis, two processes that would not normally be used in automobile applications, normally.
Through hydrolysis, ammonia borane is combined with water. A catalyst is then used to extract the hydrogen. In thermolysis, the material (ammonia borane) has to be heated to more than 170 degrees Celsius to release useful quantities of hydrogen. Though standard fuel cells work at 85 degrees Celsius, which is not enough to meet the perfect conditions for thermolysis, the Purdue researchers got 14% of the hydrogen extracted from ammonia borane, which is quite satisfying.
One fact worth noting is that the pressure they used in the vessel had been 200 pounds per square inch, far less than the 5,000 psi needed for high-pressure tanks. A maximum of 77 percent ammonia borane has been determined to be the ideal concentration in order to get the maximum hydrogen output.
By combining hydrolysis with thermolysis (hydrothermolysis), the researchers aim to build a hydrogen storage prototype that would propel a vehicle for 350 miles before needing to be refueled.