Process may enable hydrogen-powered cars

A process for storing and generating hydrogen to run fuel cells in cars has been invented by chemical engineers at Purdue University in the US.

The process, named hydrothermolysis, uses a powdered chemical called ammonia borane, which has one of the highest hydrogen contents of all solid materials, said Arvind Varma, head of Purdue University’s School of Chemical Engineering.

‘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,’ he said. ‘We have a proof of concept.’

The process combines hydrolysis and thermolysis, two hydrogen-generating processes that are not practical by themselves for vehicle applications. 

Ammonia borane contains 19.6 per cent hydrogen, a high weight percentage that means a relatively small quantity and volume of the material is needed to store large amounts of hydrogen, Varma said.

‘The key is how to efficiently release the hydrogen from this compound and that is what we have discovered,’ he said.

Purdue has filed a patent application on the technology.

In hydrolysis, water is combined with ammonia borane and the process requires a catalyst to generate hydrogen while, in thermolysis, the material must be heated to more than 170°C, or more than 330°F, to release sufficient quantities of hydrogen.

However, fuel cells that will be used in cars operate at about 85°C (185°F). Hydrogen fuel cells generate electricity to run an electric motor.

The new process also promises to harness waste heat from fuel cells to operate the hydrogen-generation reactor.

Varma and his research team conducted experiments using a reactor vessel operating at the same temperature as fuel cells. The process requires maintaining the reactor at a pressure of less than 1.4MPa (200lb/in2) , far lower than the 34.5MPa (5,000lb/in2) required for current hydrogen-powered test vehicles that use compressed hydrogen gas stored in tanks.

In some experiments, the researchers used water containing a form of hydrogen called deuterium. Using water containing deuterium instead of hydrogen enabled the researchers to trace how much hydrogen is generated from the hydrolysis reaction and how much from the thermolysis reaction − details critical to understanding the process.

At the optimum conditions, hydrogen from the hydrothermolysis approach amounted to about 14 per cent of the total weight of the ammonia borane and water used in the process. This is significantly higher than the hydrogen yields from other experimental systems reported in the scientific literature, Varma said.

‘This is important because the US Department of Energy has set a 2015 target of 5.5 weight per cent hydrogen for hydrogen-storage systems, meaning available hydrogen should be at least 5.5 per cent of a system’s total weight,’ he said. ‘If you’re only yielding, say, seven per cent hydrogen from the material, you’re not going to make this 5.5 per cent 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 researchers determined that a concentration of 77 per cent ammonia borane is ideal for maximum hydrogen yield using the new process.