Augsburg chemists present a new technology for compressing, storing and transporting highly volatile gases in porous frameworks/New prospects for gas-powered vehicles
Storage of highly volatile gases has always been a major technological challenge, not least for use in the automotive sector, for, for example, methane or hydrogen-powered vehicles. The storage materials known to date have insufficient cohesive force and/or loading capacity.
15 xenon atoms (blue spheres) in a nano-cavity in the material MFU-4, a compound developed by the Augsburg research team for gas storage by means of “kinetic trapping”.
© University of Augsburg
No convincing methods, moreover, have yet been found for loading and re-releasing the gas in normal conditions, without the need for the costly generation of high temperatures or pressure. A report, however, has now been published in the Journal of the American Chemical Society (J. Am. Chem. Soc.) on an innovative method by which gas molecules can be reliably “trapped” in nanoscopic cavities in a porous compound.
The report has been produced by a group of researchers working with Professor Dirk Volkmer at the Chair of Solid State and Materials Chemistry, University of Augsburg. They refer to the procedure they have developed, which is fundamentally different from traditional gas absorption methods, as “kinetic trapping”. The new host material MFU-4, on which this procedure is based, is characterised by high loading and storage capacity.
Porous gas storage materials
“Over the past decades, porous gas storage materials were investigated and developed with a view to finding the strongest possible interactions between absorbed gas molecules and the host material”, explains Volkmer. This, he continues, has led to a multiplicity of new frameworks, which can bind volatile gas molecules – potential energy sources such as hydrogen and methane, but also toxic gases such as carbon monoxide or hydrogen sulphide.
Low loading capacity in normal conditions due to insufficient cohesion
However, the loading capacity of these host materials, reflected, inter alia, in the weight ratio between the host material and the absorbed gas, tends to be very low, especially if the absorption takes place under normal conditions, i.e. at room temperature and atmospheric pressure.
Under these conditions, the gas molecules find very few places within traditional gas storage materials to which they can bind themselves sufficiently strongly. According to Volkmer, “Under normal conditions, to prevent a gas molecule detaching itself immediately again from the surface of the host material, you need binding interactions with energy levels of around 30 kilojoules per mole, i.e. at kJ/mol levels which only generate very weak chemical bonds.”
Traditional gas storage materials: not suitable for mobile applications
Although these required levels of around 30 kJ/mol seem low compared to the binding energy levels required for “proper” chemical binding – between, for example, the carbon atoms in an organic molecule – they are far higher than 10 kJ/mol, which is currently all that can be achieved between small, highly volatile gas molecules and current porous storage materials.
“This is not sufficient to compress hydrogen at room temperature and to bind it reliably to the carrier”, explains Volkmer. “Neither are these storage materials, therefore, suited to mobile applications, which would be extremely useful - for hydrogen or methane-powered vehicles, for example”. Despite numerous international research programmes aiming to develop sufficiently stable loadable carrier materials, the targets set by the US Department of Energy for technically feasible hydrogen storage systems have still not been met.
Problem: insufficient storage capacity despite high binding energy levels
“On the issue of sufficient binding energy”, reports Volkmer, “we ourselves thought we’d had a breakthrough four years ago, when, in the journal ‘Angewandte Chemie’ (‘Applied Chemistry’), we reported the successful development of a material which could bind hydrogen molecules up to 32 kJ/mol, thus setting a world record for porous materials." (Angew. Chem. Int. Ed. 2014, 53, 5832–5836; DOI: 10.1002/anie.201310004). This material, however, has far too low a loading capacity: there are too few places in its internal cavities to which the volatile hydrogen molecules can bind themselves sufficiently strongly, i.e. at the above-mentioned 32 kJ/mol. "This is annoying”, says Volkmer.
Totally new carrier material
He is all the more pleased, therefore, that four years later, he and his team have published a report in the "Journal of the American Chemical Society" on a framework which they have already shown to be highly effective in binding and storing a volatile and rare gas – xenon - in normal conditions. Using their new porous carrier compound MFU-4, they have been able to compress xenon to less than one percent of its original volume. The gas, moreover, remains stable in this state for many days after it has been successfully loaded.
Density at room temperature only otherwise achievable at –108 °C
“At room temperature, we were able to achieve a proportion of xenon, by weight, of up to 44.5%”, reports Dr Hana Bunzen, who performed many of the experimental gas absorption studies in Volkmer’s department. This corresponds to a density of the locked-in xenon of around 1.8 g/cm3, i.e. a value which is very close to the value of liquid xenon, i.e. when the gas is chilled to a temperature of below –108 °C.
Juxtaposition of "Nano gas bottles"
This high level of compression at room temperature is made possible by the unique structure of the storage material: it is made up of nano-sized cavities, linked to each other by very narrow channels. In order to achieve its high binding capacity, the diameter of these channels must be slightly narrower than the diameter of the gas molecules to be absorbed. The material, therefore, is like a series of miniature gas bottles lined up beside each other, linked together by “nano-valves”. “Each individual void in the compound”, explains Volkmer, “can only store up to 15 xenon atoms; however, since an almost infinite number of these small cavities can be linked together in a chain, unprecedented levels of gas storage density can be attained”.
High temperatures and pressure only required for loading and unloading
On loading, to enable the gas molecules to push their way through the nano-valves into the cavities, they must receive an input of activation energy, in the form of high temperatures and/or pressures. Once, however, its molecules have forced their way through the valves and are “trapped” in the voids, the gas can be safely and reliably stored, and transported, in a highly compressed state, without further energy costs – and without the use of awkward and heavy gas cylinder, which have been required and generally used up to now.
Dr German Sastre, a researcher from Valencia also working on the Augsburg study, has confirmed, using theoretical models, that provision of activation energy is also needed for the process of managed release of the gas atoms. Does this mean that even with “kinetic trapping”, expensive high temperatures and pressures are still needed?
"The key advantage of MFU-4", explains Volkmer, "is that - unlike with other frameworks – the energy required for unloading is not needed to break up the binding interactions between the gas molecules and the porous carrier. As the term “activation energy” suggests, it is only needed to give the gas atoms the momentum they require to push through the nano-valves into the material, and then back out again, not for the binding of the highly compressed gas molecules within the host.”
Electrical fields instead of high temperatures and pressure
This once again opens up the prospect of doing without expensive high temperature or pressure environments, not only for the stable storage of gas in the material, but even for loading and unloading. Last year, Volkmer and his team, together with colleagues from the Institute of Physical Chemistry and Electrochemistry of Leibniz University, Hannover (Prof Jürgen Caro) demonstrated in the magazine "Science" that porous frameworks alter their mechanical properties in electrical fields (Science 2017, 358,347-351. DOI: 10.1126/science.aal2456). This means that the activation energy required only for managed loading and unloading of the host material could possibly be obtained from electrical impulses.
May even be suitable for highly volatile hydrogen
Now that the new storage technology has been shown to work well for xenon, Volkmer is confident that “kinetic trapping” will make it possible to reliably and reversibly store and transport other highly volatile gases at room temperature and at maximum loading density. He is thinking, for example, of methane, the kinetic diameter of which is 380 picometres, only slightly smaller than that of xenon (396 picometres). Another real possibility, he believes, is hydrogen, which has a far smaller molecular diameter – only 289 picometres – and is therefore extremely volatile and known to be particularly difficult to compress and transport. Volkmer is sure that “kinetic trapping” in hydrogen tanks, designed for purpose, could also be of great interest to the automotive industry.
Prof. Dr. Dirk Volkmer
Chair of Solid State and Materials Chemistry
University of Augsburg
H. Bunzen, F. Kolbe, A. Kalytta-Mewes, G. Sastre, E. Brunner, and D. Volkmer, J. Achieving Large Volumetric Gas Storage Capacity in Metal–Organic Frameworks by Kinetic Trapping: A Case Study of Xenon Loading in MFU-4. J. Am. Chem. Soc. 2018, 140 (32), 10191–10197. DOI: 10.1021/jacs.8b04582
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