"Ceramic surfaces are more easily chemically influenced"
In the interview, Dr Alexandra Lieb, head of the interdisciplinary junior research group NEOTHERM at the Otto von Guericke University Magdeburg, highlights the characteristics that make metal-organic frameworks interesting for cooling and low-temperature storage, as opposed to zeolites.
forschung-energiespeicher.info: You use the adsorptive capacity of microporous materials to store heat. How would you explain this to a layperson?
Dr Alexandra Lieb: Sorptive heat storage systems use the movement energy of water molecules in humid air. When a molecule settles on a surface, its movement is stopped and this energy is released in the form of heat. To store heat, you simply have to reverse the process. So you add energy to a material in the form of heat and mobilise the adhering water molecules. This empties the surface and stores the heat energy.
If you preserve this state using a hermetically sealed container, you then have a heat storage system that saves energy for any length of time and without losses.
So storing means putting heat into the system and removing moisture. When you need heat once again, then add back the moisture and release the heat.
What characteristics should the materials used for sorption storage systems have?
Lieb: As large a surface as possible that can be covered with water molecules, but then again you cannot put a football field in a cellar. That means, you need microporous materials that have a very, very large surface area given a small volume.
Which materials do you work with?
Lieb: We work with two groups: zeolites and metal-organic frameworks. Both of these microporous material classes cover different temperature ranges and can be used to control different applications.
The first are zeolites, which in turn are comprised of substances with various chemical compositions. Conventional zeolites like silicon-aluminium-oxygen compounds are suitable for heat storage systems operating at higher temperatures, for example, for waste-heat utilisation in waste incinerators. Then there are the modified compounds, so-called AlPOs or aluminium phosphorus oxides. These, and especially SAPOs with doped silicon, are more suitable for adsorption chillers.
The second material group would be metal-organic frameworks, abbreviated as MOF.
Zeolites can be found in air-conditioning systems and heaters, or in the heat storage systems inside dishwashers. But thermal engineering applications of metal-organic compounds are still relatively unknown, right?
Lieb: Metal-organic frameworks have been known for quite some time, they have simply been ignored for a while. It was only in the last 15 to 20 years that they have been researched extensively. Now there are a large number of metal-organic frameworks, almost 70,000.
Why have MOFs been ignored for so long?
Lieb: The known and relatively inexpensive zeolites can be used very well in numerous applications, and application-oriented research was focussed in this direction. A major point of criticism levelled at MOFs was centred on the previously insufficient stability against higher temperatures or oxygen.
What is different now?
Lieb: There are MOFs that meet the necessary stability criteria. They had to be isolated first. This happened during the last few years. There are now a few outstanding MOF candidates that are very tough.
What exactly does that mean?
Lieb: With "very tough", I mean temperatures up to 200 or 250 degrees Celsius for a reasonable period of time. There are MOFs that can survive even higher temperatures. A second issue is that usable MOFs must tolerate moisture. Many MOFs don't. The first fairly known MOF material – MOF-5 – was not at all manageable in a humid environment. There were attempts to stabilise these MOFs. This is usually achieved via hydrophobisation so the material no longer absorbs any moisture. For obvious reasons, this isn't a viable option for sorptive applications. However, MOFs have also been found which can withstand both the relevant temperatures but also absorb moisture, release it again and do not degrade while doing so. There are three, four, five MOFs that are well-suited. All research groups working on applications with water-adsorption and MOFs are focussing on this category.
MOFs are suitable for lower tempteratures
Why should I even use MOFs in the first place if there are zeolites?
Lieb: Exactly those MOFs are interesting whose water-absorption characteristics are not possible with zeolite materials. And, of course, they have to be cheap and easy to manufacture. Take, for example, sorptive cooling. The state of the art includes SAPOs, that is zeolite materials. There are, however, a few MOFs, such as Al-fumarate and CAU-10, which have very promising adsorption isotherms. However, not only MOFs and zeolites are being discussed for such applications. There are research groups working with activated charcoals or mesoporous silicates. The future remains exciting.
At what temperatures do MOFs work better than zeolites?
Lieb: There is no clear limit to the temperature ranges. MOFs are generally better for lower temperatures, which excludes high temperature applications. The most important application is cooling. But MOFs can also be used to store low-temperature waste heat. It heavily depends on the intended use and on the triple point. A simple way to put it would be: zeolites for heat storage, MOFs for cooling.
From material to the coated part
You are not only researching improved sorption materials but are also developing methods to coat macroporous substrates with said materials?
Lieb: Yes, this has to do with the fact that powder is hard to use in devices. If, for example, a gas is to flow through the powder bed, there will be an enormous pressure loss. Valves get clogged up and the powder is discharged. So you have to do something about it. The powder is usually granulated, alternatively a substrate is coated with it.
You use open-cell structures such as metal foams as substrates?
Lieb: This not only has the advantage that the powder is fixated on a large surface, i.e. a lot of active material per volume is available. In addition to that, the selection of pore size and web thickness can optimise throughflow properties and heat transport. We use porous ceramics and porous metals.
Metals are known for their good thermal conductivity. What about ceramics?
Lieb: We mainly work with aluminium oxide ceramics and aluminium metal foams. Massive aluminium conducts heat about six times better than solid aluminium oxide. That is a huge difference. However, the achievable values approach each other when it comes to porous structures. At a comparable porosity, the conductivity of metal is about 4 watts per metre-kelvin and that of ceramic is 1.1 – a difference by a factor of 3.5.
We use ceramics because the surfaces are more easily chemically influenced. It permits different docking manoeuvres when coating with the active material. That had not been tested so far. In the meantime, we were able to coat ceramic foams with pore sizes of 0.9 to 2.4 mm successfully with different MOFs.
Informationen for scientific community and the public
You want to build a demonstrator to illustrate the functional principle?
Lieb: The demonstrator is supposed to demonstrate the heat transfer of our coated foam structures in a clear and understandable way. In addition, some MOFs exhibit an interesting colour shift that makes the phenomenon even more vivid.
A nice result of our research activities is that our research group NEOTHERM can present a few exhibits at the InnoTruck information and dialogue initiative organised by the German Federal Ministry of Education and Research. The truck is travelling through Germany for at least three years and offers guided tours for school classes.
Dr Steffen Beckert from the University of Leipzig and I are going to hold a workshop from 20 to 22 September, 2017, in Halle for the further scientific education of doctoral students, post-docs and other interested persons. The title of the workshop is "Porous materials for sorptive heat storages and refrigerating machines" and it is supported by the zeolites research group at DECHEMA with travel grants.