Hong-Cai (Joe) Zhou at Miami University, Ohio, US, describes how metal-organic frameworks could play their part in the hydrogen economy
Hydrogen powered vehicles offer the prospect of an emission-free future for transport. Perfecting hydrogen fuel cells, which provide the energy to drive the car, is an intensively researched area. But it's important to remember that there is another major barrier to the practical fuel-cell vehicle - the storage of useful amounts of hydrogen on board.
For a vehicle to travel for 400-500 kilometres before needing to refuel, it is estimated that about five kilograms of hydrogen would be needed. But with a density of less than one tenth of a gram per litre at room temperature and atmospheric pressure, this would mean finding space for some 55,000 litres of hydrogen.
"Targets set by the US Department of Energy [for hydrogen storage]... cannot conceivably be met by either compression or liquefaction."
High-pressure compression or liquefaction could go some way to help, but these methods would be difficult to implement in a typical small personal vehicle. Compression of five kilograms of hydrogen to a reasonable volume, such as the typical 45 litre car fuel tank, would require dangerously high pressures of more than 1000 bar. And the tank itself would have to be quite heavy to withstand the pressure.
Liquefaction requires cooling to -252 °C, and even then the volume required would still be larger than typical car fuel tanks - not including all the refrigeration machinery and insulation required.
Targets set by the US Department of Energy say that, by 2015, we should be able to store 90 grams of hydrogen per kilogram of tank and 81 grams of hydrogen per litre. These targets cannot conceivably be met by either compression or liquefaction.
The porous structure of a metal-organic framework is ideal for storing hydrogen molecules
What options remain? One possibility is physisorption, in which hydrogen is adsorbed onto the interior surfaces of a porous material, and held there by relatively weak attractive forces. Candidate materials include carbon nanostructures (including nanotubes, fullerenes, and activated carbon), zeolites, and metal-organic frameworks (MOFs).
MOFs are hybrid organic-inorganic materials that contain metal atoms or clusters connected by organic linkers. Most have three-dimensional structures incorporating uniform pores and a network of channels. By varying the size of the organic linker, a wide range of pore sizes can be created. Hydrogen molecules can be adsorbed into the pores, where they weakly interact with the metal ions and the organic linkers.
At very low temperatures (about -196 °C) and atmospheric pressure, some MOFs can adsorb more than 25 grams of hydrogen per kilogram. Increasing the pressure can push that up as high as 60 or 70 grams per kilogram. But adsorption at room temperature, even at hydrogen pressures in excess of 50 bar, is rarely greater than 15 grams per kilogram. At these higher temperatures, thermal motions easily overcome the attraction between the hydrogen molecule and the MOF. A key measure of the magnitude of this attraction is the heat of adsorption - the higher the heat of adsorption, the greater the interaction between hydrogen molecules and the framework.
There are various ways of increasing the hydrogen uptake of these materials. Increasing the total amount of pore volume and/or surface area within the MOF is an obvious first step; however, several materials have been synthesised with over 80 per cent porosity (empty space) and much further increase seems unlikely. A better strategy is to increase the interaction between hydrogen molecules and the linker molecules, metal clusters, or both. This can be done by; changing chemical function of the organic linker, exposing uncoordinated or 'naked' metal atoms, and adjusting the pore size so that a single hydrogen molecule interacts with multiple parts of the framework at once. Each of these improvements has the potential to increase the attraction between hydrogen and the MOF, as measured by the heat of adsorption.
"A lot of materials have been synthesised and tested, but only a very few have been investigated in depth"
So far, in this emerging field, much of the work has been exploratory. A lot of materials have been synthesised and tested, but only a very few have been investigated in depth. Isolation of the several factors contributing to hydrogen adsorption has remained difficult. But, with increased effort comes increased understanding and a more systematic approach to these materials. The coming years are sure to see further development of novel MOFs with ever-greater uptake capacities - all in the hope of a more efficient, cleaner energy future.
Read the full feature article on 'Hydrogen storage in metal-organic frameworks' in the New Energy Materials themed issue (issue 30) of Journal of Materials Chemistry.