Materials Case Study 2:
High-performance hydrogen storage

Hydrogen is a fantastic potential future fuel since it only produces water as waste, with no direct carbon emissions. Hydrogen-powered cars already exist, but the hydrogen in these cars is stored in high pressured cylinders. It would be much better if the hydrogen gas could be stored at lower, safer pressures within a container that held sufficient gas for the car to travel hundreds of miles. Scientists at the University of Nottingham are researching the preparation and characterisation of new porous materials called Metal Organic Frameworks (MOFs), which can store large amounts of hydrogen, (i.e. high volumes and masses of hydrogen) at relatively low pressure. You can then, in principle, place these materials in the back of your car, and you will have a lot of hydrogen fuel to power your car. Furthermore, it is safer than having a high pressure cylinder.

So how do you go about creating these metal-organic frameworks to store hydrogen at lower pressures?

Metal-organic frameworks are materials with metal ions bridged by organic molecules that can form internal porous structures (like sponges) with very small holes, cavities and channels within them. These are about a nanometre or so across, just about the right sized hole for storing other molecules. A hydrogen molecule, which is 2 hydrogen atoms joined together, is really rather small so you have to make a material with very small holes to catch the hydrogen. Our metal-organic frameworks have holes which are about the right size for trapping hydrogen gas.

How do you use Diamond Light Source?

If you are to understand why the material has the properties it has, you need to understand the internal structure and what these pores are actually like within the material. There aren’t many techniques which will tell you this and the one which is particularly critical is single crystal X-ray diffraction. A small crystal is prepared and you fire X-rays at it. The structure diffracts the X-rays and the structure of the material can then be worked out. This will tell you exactly where the atoms are and, in our case, exactly what the interior of the pore looks like, what the dimensions are and what the shape is. The advantage of the Diamond Light Source is that it has particularly high power X-rays, a much more intense X-ray beam, and this means that we can look at much smaller crystals. In the case of our materials, it is very common to get small crystals, and it is often not possible to make bigger crystals. As a result we can only get this structure by using the beam line at Diamond.

How are you looking to improve the efficiency of a metal organic framework to store hydrogen?

People thought a few years ago that making larger and larger pores was the way to go, but that turned out to be slightly misleading. This is because if the hydrogen molecules are too far away from the pore walls and are sitting in the middle of the pores, they just act like hydrogen gas. So what people are working towards now are cleverer shapes of pores and what they call ‘decorating’ the pores, that is, putting chemical groups on the inside that will actually hold on to the hydrogen more firmly.