Power pack

For hydrogen to prove itself as a versatile, practical and green fuel for vehicle and domestic use, scientists need to develop an affordable, compact and safe storage system.

One widely researched solid-storage method involves packing the hydrogen molecules into metal hydrides, with magnesium hydrides being a prime candidate. This gives a higher storage and energy density and is cheaper and safer than other methods, such as compressed gas or cryogenic liquid.

Hydrogen storage in metal hydrides occurs through a combination of physical and chemical absorption. Under the right conditions of high pressure and low temperature, the hydrogen binds itself into the nanostructure of the storage material. When heated under lower pressure, the process is reversed, with hydrogen released and the material returning to its original state.

The EPSRC has awarded the environmental sciences department at the University of East Anglia (UEA) a £208,000 grant to improve hydrogen storage in solid form using nanotechnology.

Dr Congxiao Shang, a lecturer in environmental sciences at the university, will be the principal investigator on the project, due to start in June. She has a PhD in hydrogen storage in magnesium hydride and has worked as a researcher for three and a half years on nanostructured hydrogen storage materials.



Need for the project

Explaining the need for the project, Shang said: ‘Theory states that magnesium hydrides can store 7.6 per cent by weight of hydrogen. In practice there are other limitations, meaning that in experimentation, only 6.7 per cent can be retrieved.

‘Our aim is to get towards the theoretical value by modification of the nanostructure of the metal hydrides.’

There are three main aspects to the research. The team will use novel synthesis methods to form a nanostructured magnesium hydride. They will then experiment with modifying the material, including mixing in catalysts and dopants (impurities that are added in small amounts to a pure substance to change its properties) to give better and faster sorption, desorption and increased capacity.

The third area of investigation is the possibility of removing impurities from hydrogen during its production.

‘The rapid sorption properties are important,’ said Shang, ‘as you don’t want to have to charge your car with hydrogen for two or three hours. Two or three minutes would be more acceptable.’

The project will employ a cryogenic milling procedure and a vortex-driven high-energy pulverisation process to make smaller, ultra-clean and high-surface-area nanostructures.

‘We will add doping agents and catalysts during the mechanical milling of the metal hydride to maximise the results, then work out the optimum composition of materials in the mix,’ said Shang.

Applying a catalytic coating not only makes full use of the catalysis function but also effectively stops powder growth, stabilising the hydrogen storage structure for improved cyclability. One catalyst the researchers will investigate is transitional metal carbides. The catalytic effect of them on hydrogen sorption has not been considered in the past, though it has been proven in other processes.

A doping agent can have a catalytic effect. ‘An example dopant for this project could be other metal hydrides, which have complementary properties to magnesium hydride and could optimise the hydrogen storage,’ said Shang.

The team will also investigate the tolerance of magnesium to impurities in hydrogen and the viability of using it as a down-stream storage and purification medium for an industrial hydrogen generation process.

‘Currently, one of the methods of hydrogen production is to reform fossil fuel,’ said Shang. ‘It is usually polluted with gases such as CO2 and CO. People normally use expensive membranes to purify the outlet gas. If metal hydrides can tolerate impurities, the system could be used for low-cost purification of the outlet stream.’



Collaboration

Three collaborative organisations will work alongside Shang on the project. The materials department at Queen Mary, University of London will provide facilities for the production of the nano-milled storage material.

Environmental sciences company Dynamic Environmental Solutions (DES) Enterprises UK is investigating the extraction of hydrogen from waste feedstock. The final result of the project could combine storage and purification with the extraction process. Technology transfer company Angle Technology will bring the resulting product to market.

Before work starts, Shang will recruit other scientists to form the research team.