Does the answer to fuel cell research lie with H2 clathrates?

Hydrogen fuels are renewable and environmentally friendly, but a lack of practical methods of storage has restricted their use. Now scientists from the University of Chicago, US, have developed a new class of compounds that could prove to be a novel method for storing hydrogen, bypassing some of the current limitations.

The most popular ways of storing fuel hydrogen are as liquid hydrogen and compressed hydrogen gas. But these storage methods require the fuel to be kept at extremely low temperatures (20 K for liquid hydrogen) or high pressures (300 atmospheres for compressed hydrogen).

Wendy Mao and David (Ho-Kwang) Mao experimented with using ice cages to form crystalline molecular compounds of hydrogen under less stringent temperature and pressure conditions.

The father-and-daughter team synthesised hydrogen-water and hydrogen-methane compounds in a diamond-anvil cell, which researchers often use to simulate the high pressures found beneath the Earth's surface.

The most promising compound would appear to be a new hydrogen clathrate hydrate H ₂(H ₂O) ₂. This clathrate holds 50 g l ^-1 hydrogen by volume or 5.3 per cent, Wendy Mao reports.

The clathrate is synthesised at pressures of 20 000-30 000 atmospheres, but remains stable at atmospheric pressure and at moderately low temperatures (77 K). The clathrate contains a 'significant amount of hydrogen', which is released when the clathrate is warmed to 140 K, giving water as a byproduct.

'The hydrogen is retained by weak bonding to the ice host molecules under moderate synthesis pressure and temperature,' explains Wendy Mao. This bonding helps to stabilise molecular hydrogen, yet is sufficiently weak for easy release of the element when the temperature rises. Wendy Mao imagines that storing the compounds at 77K would be economically feasible because it could be done using liquid nitrogen, which is easy and cheap to make.

The Maos have applied for a patent on their hydrogen clathrate synthesis technique, but they still need to address the problem of how to make the clathrates in quantities sufficient to power a car. So far, they have only managed to make small amounts using diamond-anvil cells and gas pressure cells.

Maria Burke