Fuel cell cars have come one step closer to practicality with researchers from Indiana’s Purdue University announcing a new process for the generation and storage of hydrogen. The process is called hydrothermolysis, and is a combination of hydrolysis and thermolysis – the two prevalent methods of hydrogen generation that some scientists consider impractical for use in automobiles. The new process utilizes powdered ammonia borane, a chemical that has one of the highest hydrogen yields of any solid substance.
Ammonia borane contains 19.6 percent hydrogen by weight, which means that when combined with water, a little of it can go a long way. To release that hydrogen using hydrolysis, a separate catalyst would need to be added, while in the case of thermolysis, the material would need to be heated to over 170C (330F). Hydrothermolysis, on the other hand, does the job without needing a catalyst, and at a temperature close to the regular running temperature of fuel cells.
What's more, the cells’ waste heat can be used to operate the hydrogen generation reactor, and that reactor must only maintain a pressure of 200psi, as opposed to the 5,000psi needed by current compressed hydrogen gas systems.
The researchers determined that a temperature of 85C (185F) and a concentration of 77 percent ammonia borane was ideal for maximum hydrogen yields. Under the best possible conditions, released hydrogen accounted for about 14 percent of the combined weight of the ammonia borane and water used in the process. This is reportedly much higher than hydrogen yields from other systems.
"This is important because the U.S. Department of Energy has set a 2015 target of 5.5 weight percent hydrogen for hydrogen storage systems, meaning available hydrogen should be at least 5.5 percent of a system's total weight," said Prof. Arvind Varma, head of Purdue’s School of Chemical Engineering. "If you're only yielding, say, 7 percent hydrogen from the material, you're not going to make this 5.5 percent requirement once you consider the combined weight of the entire system, which includes the reactor, tubing, the ammonia borane, water, valves and other required equipment."
The team is now looking at scaling up the size of their reactor, with the intention of being able to power a vehicle for 350 miles without refueling. That said, they believe that hydrothermolysis could additionally be used to recharge the batteries of small electronic devices - perhaps giving the HYDROFILL system a run for its money? They are also looking into ways of recycling the waste products back into ammonia borane.
The research was recently published in the AIChE Journal, published by the American Chemical Society.