Medium pores for maximum storage

University of Nottingham scientists have made a breakthrough in hydrogen storage which could help in the development of the next generation of environmentally friendly cars.

Hydrogen fuel needs to be safely stored in sufficient quantity for cars to cover a reasonable distance before their supplies must be replenished. One possible solution is to pack hydrogen into porous materials, which soak up the gas like a sponge.

Professor Martin Schröder and his colleagues, Professor Neil Champness and Dr Hubberstey from the School of Chemistry, with Dr Gavin Walker from the School of Mechanical, Materials and Manufacturing Engineering, have been investigating metal organic frameworks (MOFs). These are molecular scaffolding filled with tiny cylindrical pores that hydrogen gas can be forced into.

Professor Schröder said: ‘The idea up to this point has been to increase the pore volume, so as to fit in more gas.’

However, the study has quantified the amount of hydrogen that can be put into three MOFs made of identical material but with different pore sizes. Surprisingly, the study showed that the middle-sized pores could hold the highest density of hydrogen.

‘In a very small tube, the hydrogen gas molecules all see the wall and interact with it. But in a larger tube, the molecules see less of the wall and more of each other. That interaction is weaker, so they don’t pack together as closely,’ Schröder said.

The researchers concluded that there is an optimum pore size for any given material.

‘MOFs appear to be a viable alternative technology to other materials currently being investigated for hydrogen storage since they can show excellent reversible uptake-release characteristics and appropriate capacities,’ said Schröder.