A chance discovery has paved the way for devices that can store and release hydrogen at the flick of a switch, bringing practical, fuel-cell cars a step closer.
The much-vaunted hydrogen economy, in which industrialised nations no longer need to depend on oil, will only emerge when three basic problems have been solved — the efficient production, distribution and storage of the gas itself.
The last of these three is perhaps the biggest barrier for the development of fuel-cell vehicles. Hydrogen is notoriously difficult to contain. Its molecules find their way through many materials and can cause embrittlement in the process. This is exacerbated when storage density is boosted by keeping it under pressure.
One alternative is to lock hydrogen molecules into the lattice structure of a metal hydride but this poses more problems. Hydrides only release the trapped gas at temperatures above 300ºC.
Another option is to use porous polymers to soak up hydrogen molecules like a sponge (The Engineer, 19 July). While this approach is still being perfected it has already been established that it will work only if cooled by liquid nitrogen to 77K and kept at low pressure of about five atmospheres.
So the invention at Bath University of a material that stores and releases hydrogen at ambient pressure and temperature opens exciting new avenues. When a charge of one volt is applied to the material, it releases hydrogen. When the power is switched off, it absorbs hydrogen.
‘Our new material works at room temperature and at atmospheric pressure at the flick of a switch,’ said Dr Andrew Weller from Bath’s chemistry department. He admits it is not the ultimate solution because it cannot store a lot of the elusive gas, although it can play a part in a larger system.
‘It’s made from rhodium, which is a heavy metal, so its weight to fuel ratio is low, 0.2 per cent, but it could certainly fill the time-lag between a driver putting their foot on the accelerator and a metal hydride fuel tank getting up to temperature,’ said Weller.
‘We are really very excited about the potential this technology offers because the mechanism of uptake and release is brand new. It’s a completely different way of looking at the problem.’
They have constructed an organo-metallic compound containing six rhodium atoms and 12 hydrogen atoms. They found that the material would absorb two molecules of hydrogen at room temperature and atmospheric pressure — and would release the molecules when a small electric current was applied to the material.
‘The new material absorbs the hydrogen into its structure and literally bristles with molecules of the gas. At the flick of a switch it rejects the hydrogen, allowing us to turn the supply of the gas on and off as we wish,’ said Weller. ‘The fact that we discovered the material by chance is a fantastic advertisement for the benefits of curiosity-driven research.’
The researchers made the discovery while investigating the effect that hydrogen has on metals. ‘It was serendipity really. We were looking to try to make a compound that modelled methane co-ordinating to a metal,’ said Weller. ‘We were trying to elucidate fundamental steps in that process and we stumbled across this rhodium molecule by chance.’
Now the team hopes to get funding to develop ways to print the material onto a substrate such as glass so it can be used in a prototype device. A fuel-cell car could have a unit containing many layers of the printed substrate, delivering a burst of hydrogen to the stack quickly before the main supply from the main store in the metal hydride or porous polymer comes on stream. Then it would absorb molecules ready for the next boost.
The research was initially funded by the Engineering and Physical Sciences Council. A further £500,000 grant to the department has enabled Weller and other researchers to buy two mass spectrometers that allow them to examine the molecular structure of the material.
With answers from the prototype, the researchers plan to move the chemistry away from rhodium and hope iron molecules may be created to do a similar job. Rhodium is one of the most expensive metals, costing up to six times the price of gold, weight for weight.
‘We’ve proved a completely new way of storing hydrogen but rhodium is too expensive for the method to be practical,’ admitted Weller. ‘So we’ll be looking to move towards iron or even towards ruthenium, which would be possible because it’s about 10 times cheaper than rhodium. But there’s a lot of chemistry to do first.’
Technology that stores and releases hydrogen could help speed the introduction of fuel-cell cars