A world record for hydrogen storage capacity under normal operating conditions has been set using metal organic frameworks (MOFs) – porous materials with metal ions and bridging organic ligands.
Molecular hydrogen (H2) holds promise as a transport fuel and is already used in some vehicles and in forklifts. The only byproduct is water, so hydrogen offers zero emission transport. However, storing hydrogen is tricky.
While hydrogen boasts 2.6−3 times more energy per unit mass than gasoline, low temperatures and/or high pressures are needed to store it at sufficient density. Cryogenic storage demands the use of large, expensive, and well-insulated systems, while compression at high pressures is costly and requires expensive, bulky storage tanks.
Researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory in the US tested four known MOFs for hydrogen storage – two containing nickel and two containing cobalt as the coordinating metal. A MOF called Ni2(m-dobdc) showed the highest hydrogen-storage capacity: 11.9g of hydrogen per liter of MOF crystal at 25°C and 100 bar pressure (Chemistry of Materials, 2018, 30 (22), 8179).
‘The interest in MOFs comes from their very high surface area,’ says Andrew Burrows at the University of Bath. There are other porous materials, such as activated carbons, but MOFs offer crystallinity, which makes their structure repeatable and predictable.
The hydrogen molecule is challenging to store because it is so small. With only two electrons, it relies on relatively weak van der Waals interactions with the surface. Hydrogen has previously been stored in MOFs, but only in reasonable quantities at extremely low temperatures.
MOFs and the hydrogen interact so weakly at room temperature that it is difficult to store the gas at room temperature. This latest MOF paper shows how it’s possible to increase the strength of the interaction. ‘The empty metal sites in the framework provide a kind of anchor spot for the hydrogen molecules to attach to and so the strength of the interaction is higher,’ Burrows explains.
At ambient temperature and a much lower tank pressure than used in current hydrogen vehicles, the Ni2(m-dobdc) MOF’s sets a new record for the amount of hydrogen stored per unit volume, which is significantly greater than for compressed hydrogen gas, the researchers led by Jeffrey Long at Berkeley note.