A hole new approach

Jemma Rowlandson tells us how nanoporous materials are set to take on the world, and win

Here at the University of Bath we work on a very special class of material, known as nanoporous materials¹. These materials are deliberately full of holes, in fact we frequently try to put more in, and this makes them incredibly useful. A wide variety of materials are considered nanoporous, from your standard activated carbons to zeolites, metal organic frameworks and more recently even porous polymers. It is the presence of these nano-scale sized pores that makes these materials not only unique, but also potentially world-saving.

All these nanoporous materials work on a similar principle, called adsorption. Now this is a distinctly different process to absorption. Absorption is the process of taking molecules into a material, adsorption however is the process of molecules sticking to a surface is.

So how does this help us save the world? Well because molecules stick to the surface of nanoporous materials it means we now have the ability to separate and store them, opening up many useful applications. Carbon dioxide capture, water filtration, and hydrogen storage are just a few of the many applications we can use these materials in.

One application we research at Bath is nanoporous materials for hydrogen storage. Hydrogen has the potential to revolutionise our fuel economy as a clean, sustainable fuel. Theoretically you can produce hydrogen by splitting water, and then when it is burnt in an engine, or reacted in a fuel cell, only water is released out of the exhaust. One of the problems to this seemingly perfect solution however, is that at standard temperature and pressure, hydrogen is a gas. This makes hydrogen very difficult to store in a confined space, such as in a car. In industry the conventional approach to hydrogen storage is to compress it in a gas cylinder at 350 or 700 bar. This very energy-intensive solution has multiple problems associated; there are cost and safety concerns.

Research by the Chemical Engineering Department^2-4, at the University of Bath, is hoping to reduce the pressure of these gas cylinders by using nanoporous materials. If a nanoporous material is inserted into a gas cylinder then it will adsorb some of the hydrogen gas. Hydrogen molecules can pack together much more densely on the surface of these materials, in comparison to a standard cylinder; this means you can store the same amount of hydrogen but at a lower pressure. The amount of gas a material can adsorb, known as the adsorption isotherm, is measured experimentally and compared to the amount of gas a cylinder could store at the same temperature. Both experimental and modelling of these materials forms an important part of optimisation research. The experimental adsorption isotherms often take days to collect, and with thousands of possible materials we can use, it is far more efficient to model the performance of a material, and then choose the most-promising ones for experimental optimisation.

So the big question is, do these materials work? Yes they do…up to a point. Nanoporous materials work best at cryogenic temperatures (approximately -200°C), and this is due to the fundamental mechanism behind these materials, adsorption. Adsorption is a relatively weak force, molecules are attracted to the surface of the material by van der Waals interactions rather than a chemical bond. These weak interactions mean this process is fully reversible, which on one hand is very advantageous; we can remove the hydrogen just as easily as we put it in, simply by changing the pressure. The disadvantage is that as soon as your material heats up beyond cryogenic temperatures, the molecules adsorbed to your nanoporous material gain energy, and then simply leave the surface.

This causes a massive reduction in hydrogen storage capacity. Is this a problem? Only if you want a room temperature hydrogen storage system. Research is increasingly turning towards using adsorbents in cryogenic hydrogen storage systems. Although the development of novel porous materials with decent storage capacities at room temperature is also an important avenue of investigation.

Commercial hydrogen cars are starting to form part of our transport system, however the lack of a hydrogen network and the sheer cost of these cars has limited their availability. One of the most expensive components of the car is the fuel tank. Nanoporous materials could potentially be used to great effect in on-board cryogenic hydrogen storage systems, in order to decrease the costs of the fuel tank and the storage pressure. This is likely to become the next step in hydrogen storage technology, a room temperature solution however, is still under investigation.

References: 

1. L. Schlapbach and A. Züttel, Nature, 2001, 414, 353–358. 2. V. P. Ting, A. J. Ramirez-Cuesta, N. Bimbo, J. E. Sharpe, A. Noguera-Diaz, V. Presser, S. Rudic, and T. J. Mays, ACS Nano, 2015, 9, 8249–8254. 3. J. E. Sharpe, N. Bimbo, V. P. Ting, B. Rechain, E. Joubert, and T. J. Mays, Microporous Mesoporous Mater., 2015, 209, 135–140. 4. N. Bimbo, V. P. Ting, J. E. Sharpe, and T. J. Mays, Colloids Surfaces A Physicochem. Eng. Asp., 2013, 437, 113–119.

The author: 

Jemma Rowlandson is a PhD student at the Centre for Sustainable Chemical Technologies at the University of Bath. Her research focuses on the production of sustainable activated carbons from renewable, waste feedstocks.

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