Hydrogen purification method

© Northwestern University
Hydrogen (white) passes quickly through the pores of a new material developed at Northwestern, while CO2 (red and black) interacts with the walls, slowing it down

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Researchers have devised a new, far more efficient method of producing hydrogen cells.

There is no doubt that hydrogen powered vehicles are the way forward if the world is serious about reducing damage to the environment, but at present there are still huge barriers to overcome in order for us to make any significant improvement.

Hydrogen needs to be purified before it can be used as fuel for fuel cells in electric vehicles; this process is costly and not very clean, meaning the likelihood of the vehicles being put into mass production in the near future is slim.

Current methods of producing hydrogen first yield hydrogen combined with carbon dioxide, or hydrogen combined with carbon dioxide and methane.

The technology currently used for the next step - removing the hydrogen from such mixtures - separates the gas molecules based on their size, which is difficult to do.

Researchers at Chicago’s Northwestern University however, have developed a new, more efficient method of separating hydrogen from complex gas mixtures.

Chemist Mercouri G. Kanatzidis, and Postdoctoral Research Associate Gerasimos S. Armatas, both of Northwestern University, have developed a new class of porous materials, structured like a honeycomb, which they believe, exhibit the best selectivity in separating hydrogen from carbon dioxide and methane to date.

The researchers have, in essence, developed a completely new way of obtaining hydrogen from various mixtures, which is far more straightforward than conventional chemical processes.

Their new materials do not rely on size for separation but instead on polarisation - the interaction of the gas molecules with the walls of the material as the molecules move through the membrane.

Kanatzidis said, “A more selective process means fewer cycles to produce pure hydrogen, increasing efficiency. Our materials could be used very effectively as membranes for gas separation.”

Kanatzidis explained that he and his fellow researchers are taking advantage of ‘soft’ atoms, which form the membrane’s walls.

“These soft-wall atoms like to interact with other soft molecules passing by, slowing them down as they pass through the membrane.”

“Hydrogen, the smallest element, is a ‘hard’ molecule; it zips right through while softer molecules, like carbon dioxide and methane, take more time.”

Kanatzidis and Armatas tested their membrane on a complex mixture of four gases.

Hydrogen passed through first, followed in order by carbon monoxide, methane and carbon dioxide.

As the smallest and hardest molecule, hydrogen interacted the least with the membrane, and carbon dioxide, as the softest molecule of the four, interacted the most.

The new method brings hope that filters made by this new blueprint could one day be used to produce hydrogen for use in vehicles, and could also be installed at smokestacks used by coal and oil-powered electrical plants, in a bid to reuse as much energy as possible.

In a separate hydrogen-based announcement last week, researchers at the University of Nottingham in the UK, and General Motors in Michigan, USA, claimed they had developed a hydrogen fuel tank material that might one day replace the automobile petrol tank.

The need for effective hydrogen fuel tanks is another major stumbling block in the progression of a hydrogen economy; a litre of liquid hydrogen contains just a quarter of the energy of a litre of petrol.

The energy density can be increased if hydrogen gas is squeezed into a porous material able to hold hydrogen like a sponge does water, so far though, such materials have not been able to store enough energy to provide a realistic alternative to a car’s petrol tank.

The researchers however, have come up with a sponge-like material that can hold 10% of its own weight in hydrogen gas.

The material is a combination of copper atoms and organic molecules called a ‘metal-organic framework’.

With this setup, each copper atom is surrounded by a polyhedral ‘cage’ of the organic molecules.

The cages slot, or tessellate, together to form a highly porous material with exceptional hydrogen storage capacity.

The material can only achieve the (10%) figure at 77 times atmospheric pressure and -196˚c, which limits its real-world possibilities, but the researchers are confident that some further development will open up more opportunities.

Those working towards a hydrogen-fuelled world are continually met by barriers preventing them from progressing any further, but as fast as the barriers are arising, researchers and scientists are developing alternative routes, which are bringing us closer to a much greener future.

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