MOF materials could make hydrogen fuel more practical

The US Department of Energy recently awarded Berkeley Laboratory a three-year, $2.1m (£1.3m) grant for the project, which will also include contributions from the National Institute of Standards and Technology (NIST) and General Motors (GM).

‘We’re working on materials called metal-organic frameworks to increase the capacity of hydrogen gas in a pressure cylinder, which would be the fuel tank,’ said Jeffrey Long, a Berkeley scientist who co-leads the project along with Berkeley chemist Martin Head-Gordon. ‘With these materials, we’re working on storing the hydrogen without the use of very high pressures, which will be safer, more efficient and without the significant compression energy losses.’

According to the Berkeley team, metal-organic frameworks (MOFs) are three-dimensional sponge-like framework structures that are composed primarily of carbon atoms and extremely lightweight.

‘What’s very special about these materials is that you can use synthetic chemistry to modify the surfaces within the materials and make it attractive for hydrogen to stick on the surface,’ Long explained.

Currently, vehicles using hydrogen fuel cells can achieve a range of close to 300 miles — but only if the hydrogen is stored at pressures of 600 to 700 bar, which is expensive and potentially unsafe. It is also energy intensive to pressurise the hydrogen.

To date, Long is said to have succeeded in more than doubling hydrogen capacity, but only at temperatures of around 77K, or -321°F.

‘It’s still very much basic research on how to create revolutionary new materials that would boost the capacity by a factor of four or five at room temperature,’ he said. ‘We have an idea of what kinds of frameworks we might make to do this.’

Long’s approach is to create frameworks with lightweight metal sites on the surface, making it attractive for hydrogen molecules to bind to the sites.

‘Our approach has been to make some of the first metal-organic frameworks that have exposed metal cations on the surface,’ he said. ‘Now we need to figure out ways of synthesising the materials so that, instead of one hydrogen molecule, we can get two, three or even four hydrogen molecules per metal site. Nobody’s done that.’

Head-Gordon, a computational chemist, will work on theoretical understanding of MOFs in order to predict their hydrogen-storage properties and then instruct Long’s team as to what kind of material to synthesise.

Scientists at GM will aid in providing accurate high-pressure measurements. The NIST scientist is an expert in neutron diffraction and neutron spectroscopy, which will allow Long and his team to pinpoint where exactly the hydrogen is going and to verify that it is binding to the metals.