Cameron Kepert studied Chemistry at the University of Western Australia before undertaking a PhD at the Royal Institution of Great Britain/University of London on a Hackett Scholarship. In 1995, he took up a Junior Research Fellowship at the University of Oxford, where he commenced research into coordination framework materials. He was appointed to the University of Sydney in 1999 and currently holds the position of ARC Federation Fellow. He is the recipient of the Malcolm McIntosh Prize for Physical Scientist of the Year, the AAS Le Fèvre Memorial Prize, the RSNSW Edgeworth David Medal and the RACI Rennie Medal. He has published more than 70 papers in leading peer-reviewed journals on molecular materials.

Hydrogen storage in nanoporous materials
by Professor Cameron Kepert

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Thank you very much, Michael, firstly for the opportunity to come and participate in this very exciting day, but also for the scholarship that you awarded me those many years ago. I am very appreciative.

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The subject I am going to talk about today is hydrogen storage, and being one of the early speakers in this program I thought I would take the opportunity just to take a step back and talk about some of the things that we do know scientifically about hydrogen.

With hydrogen being the simplest and most common element in the universe, it is no surprise that it holds a very special place in chemistry. For example, it is the first molecule that we turn to in order to understand chemical bonding. Similarly, the reactivity of hydrogen is something used as a benchmark when comparing the reactivity of other compounds such as using qualities like electrochemical potentials.

People might think that everything should be known about hydrogen by now – it was first generated in the laboratory in the late 1600s – but in fact, this is not the case. Just in the last few years, there have been some very nice discoveries. For example, if very large pressures are applied to hydrogen gas, up to about 1.5 million atmospheres of pressure, it turns into a metal. Indeed this research experimentally confirmed what happens in the core of the planet Jupiter. In an even more recent example, that occurred just in the last few years, researchers have made the first ever measurements of electrical conductivity for the hydrogen molecule.

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One of the other properties known about hydrogen is that it has a very high energy density, on account of the fact that all of its electrons are valence electrons. So, as we have heard from Dr Crabtree and others, this means that hydrogen is potentially a very good fuel, for example in the hydrogen-oxygen fuel cell. However, the real problem here is, as we have heard, that the energy density of hydrogen in its normal state – that is, at atmospheric pressure and room temperature – is about 3,000 times lower than that of petrol. That is because it is a gas.

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Therefore if we want to use hydrogen as a fuel under normal conditions and avoid the rather impractical solution that can be seen here due to the problem of having extremely large tanks, then we need to find some really clever ways of compressing that hydrogen into a small volume with a small mass. This particular technical challenge is actually driving a lot of very interesting fundamental new science.

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Just by way of further background; and some of these concepts have already been presented by Dr Crabtree – one of the engineering solutions to this challenge is to turn the hydrogen into a liquid. Notable, this has quite a high energy cost associated with the process and temperatures must be lowered all the way down to 21 Kelvin to enable this to occur. Then there is the additional problem of boil-off of the liquid hydrogen which, depends on the amount of thermal insulation.

This is managed very well on the Space Shuttle where two-thirds of the main tank is full of liquid hydrogen. In addition, the Space Shuttle has to carry around an oxidising agent in the form of liquid oxygen that is located in the top one-third of this tank.

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It is not just the Space Shuttle that utilises this idea. In a very impressive feat of engineering, developed back in 1999, Munich Airport set up a robotic liquid helium refuelling station. This is used to refuel a number of cars and buses operating around the airport. Nevertheless, again as we have heard, one of the problems with liquid hydrogen is that you need very bulky, very highly insulated tanks with something like 70 layers of aluminium and fibreglass in order to get good enough insulation that will sufficiently minimise the boil-off of liquid hydrogen.

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A rather more practical solution is to pressurise the gas. For instance, where about 700 or 800 atmospheres of pressure can be achieved in state-of-the-art tanks, we can get about half the volume density present in liquid hydrogen. This is exactly the technology that has been used, for example, in the buses driving around Perth. (At the right of this slide you see one arriving, off the ship.) With this technology, there is currently about one-eighth the energy density of gasoline. Thus with contemporary fuel cells being typically about twice as efficient as combustion engines, operaters need hydrogen fuel tanks that are about four times larger than tanks for petrol to be used for similar purposes.

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This slide illustrates a plot that is similar to one seen in the previous talk. However it uses a log scale which makes things look a little bit better. Here the energy density, both gravimetrically and volumetrically, of gasoline and diesel can be seen. Then at this point in the diagram, the Department of Energy target that has been set to store hydrogen energy can be identified,The position of liquid hydrogen is  shown below this point. (These symbols take into account not only the liquid hydrogen itself but the tank that it sits in.) In this figure, it can be seen that about 90 per cent of the weight and about half of the volume is due to the tank. Furthermore, liquid hydrogen has a density of about 70 kilograms per cubic metre.

The position of compressed hydrogen on the diagram is quite a bit lower, but notably the disadvantage associated with the high energy cost of liquefaction and boil-off is absent.

There are a number of chemical compounds on this plot. In particular, the hydrides which I think Dr Evan Gray will cover later in the day. These have some quite appreciable uptakes of hydrogen gas, and these compounds are also able to release the hydrogen reversibly.

The better materials among the ones I am here to talk about today have actually only been developed in about the last two years. Some artistic licence must be permitted to enable an illustration of where some of the better materials may be located on this plot. Certainly since the publication of this plot two years ago, a large development in the so-called nanoporous materials has been witnessed. Some of these are showing quite a lot of promise in reversibly taking up substantial amounts of hydrogen gas.

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If the chemical approach rather than an engineering approach is favoured for use, there are two available options. Firstly, there is what is called ‘chemisorption’, and chemists would call this ‘chemical bonding’. This process is where the hydrogen is actually chemically bound to a storage material. For example, a metal hydride could be used, and this could be a covalent compound or it could be a fossil fuel or something like methane. It could even be water, in a process where a reactive metal could be added to generate hydrogen gas. In those sorts of compounds, the challenges really are in obtaining reversibility and getting the energetics just right so that enough hydrogen is released under favourable conditions.

The second approach is to move to what is now called physisorption – or, if you like, intermolecular forces. This is where the hydrogen actually remains as a hydrogen molecule but just binds quite weakly to the surface of a material. Here the reversibility is not such an issue. The real issues and challenges faced when using this approach is the need to maximise the surface area upon which hydrogen gas can bind and also the need  to maximise the strength of the physisorption interaction. As can be seen from the graph, it is a much weaker action. It is not a chemical bond and so typically quite low temperatures or quite high pressures are required to allow this process to happen.

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Some of the very first work done in this area occurred almost a decade ago now. At this time, there was a flurry of excitement, with a number of very high profile papers. In some of these papers, it was reported that more than 10 per cent per weight of hydrogen gas could be stored in carbon nanotubes. In reality, for that to have been true, there would have to be not just sufficient surface coverage but also a means of filling up all those tubes with hydrogen gas. There is just not enough surface area on the tubes to provide the type of coverage required. Thus now I think it is widely appreciated by just about everybody that those early papers were actually wrong, and contained erroneous results.

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More recently it has been more commonly accepted, I think, that the maximum amount of uptake attainable in carbon nanotubes is of the order of 1.5 to 2 per cent per weight. Furthermore, in carbon nanotubes with higher surface areas, where there is about 1,000 square metres per gram – if one gram of nanotubes are taken, cut open and then laid out on a flat area – then there will be enough graphite to actually cover something like a soccer field with graphite sheeting. According to theory then, about 20 micrograms per square metre is achievable. Thus it is possible to generate a nice linear relationship between the surface area and the amount of hydrogen that can be taken up. This amount is clearly far below those amounts required for vehicle use.

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Of course some higher surface areas with similar compounds can be produced. Indeed this was reported in a very exciting paper just last year. Researchers are now all waiting to find out just how well these materials will take up hydrogen since there is no report of this yet. As can be seen from this slide, by essentially punching holes in a graphite sheet and making these hexagonal covalent lattices, it is possible to generate materials with surface areas that are about 50 per cent larger than those of carbon nanotubes. To go out on a limb now, I could predict here that there might be 2 to 3 per cent hydrogen uptake in these covalent organic frameworks, based purely on their surface areas.

I think higher surface areas will be required if the goal is to achieve the 6.0 weight % that has been put forward as what is needed to build a vehicle. In terms of meeting these objectives, there is a need to move beyond two dimensions. This slide illustrates a two-dimensional sheet where the surface area is actually limited by the fact that those sheets stack on top of each other. Thus there is some loss of surface area due to this stacking. An attractive objective is then to build three-dimensional frameworks to achieve even higher surface areas.

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For instance, one material with a three-dimensional framework belongs to a very well known class of minerals - the zeolites. These have aluminium silicon atoms and oxygen atoms. Zeolites have fairly high mass densities within these frameworks, and as a result they have surface areas that are about half that of carbon nanotubes. So, unfortunately, in these materials typically only about 1 per cent uptake of hydrogen gas is observed.

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Just last year our group and that of Jeffrey Long, from Berkeley, reported on the use of Prussian blue materials. These use a more molecular approach to build porous materials. Shown here is a framework containing metal ions, with cyanide linkers between the metal ions. It can be seen that there is actually a very high surface area in this material that results from the cyanides being approachable from all directions.

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These materials show some relatively impressive uptakes of hydrogen, with the hydrogen molecules entering the holes within this framework. Note that 120 mls per gram equates to 1.0 weight %. From the results shown here it can be seen that uptake is greater than 1.0 per cent, but in fact some recent results have shown that in the more porous examples of these, about 4.0 weight % can be achieved under reasonably mild conditions, down at about 77K, which is the temperature of liquid nitrogen.

Perhaps more interestingly, with these frameworks the volume density of hydrogen is very high. It is more than half that of liquid hydrogen, because a lot of hydrogen is packed into a small space. Thus the level achieved is greater than the volumetric density present inside a hydrogen cylinder. So there is some interest in these methods.

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So there is really quite a good relationship between the surface area of a material and the amount of hydrogen that a material can take up. For instance, as this plot shows - low surface area materials like siliceous materials  (which do not take up very much hydrogen); zeolites; and a number of different carbon materials (including nanotubes, porous carbon, Prussian blue materials, and cyanide frameworks), sit fairly high. However, even higher surface areas are achievable using a new class of material that has really only been discovered in the last few years. Now metal and organic chemistry can be combined when forming new materials.

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To provide a simple example of how these new materials compare, the composition of Prussian blue, which has metal ions and cyanide linkers between those metal ions, can be seen in the context of how this metal-organic framework is modified by replacing the single metal ion (in this case) with a little cluster. Each one of these tetrahedra has a zinc atom in the centre that can be replaced by a cyanide bridge and a much larger linker, benzenedicarboxylate, or terephthalate. Thus it can be seen that in this way, a structure that is very similar to Prussian blue has been generated, but the framework has much larger holes and it also has a much higher surface area per mass.

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Just two years ago, Omar Yaghi, at Michigan University, showed that these materials have the predicted very high surface areas. He has made something like 200 different compounds utilising this little metal cluster, and a very large range of different organic bridges. This compound currently holds the record for maximal surface area of all his materials. He has since used a triangular bridge between these little metal clusters. Two years ago he showed that these materials take up reasonable amounts of hydrogen gas, up to about 1.5 per cent at one atmosphere and at the temperature of liquid nitrogen. As can be seen from the graph here, the gravimetric uptake curves are still rising.

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It was only a matter of weeks ago that Yaghi also reported that at higher pressures it was possible to achieve much, much higher loadings of hydrogen in these materials. So, for example, if up to 50 atmospheres of pressure and liquid nitrogen temperatures are acceptable, then it is actually possible, in the case of that last very high surface area material, to get up to about 7.5 weight % of hydrogen into the framework.

Again for these materials he believes that there is a very good relationship between the surface area and the amount of hydrogen that can be loaded onto the surface. So at this point, the size is getting close to the area of the MCG, in terms of the amount of surface that is possible in one gram of material.

Of course, it really is very early days – this was published just a matter of weeks ago – and it could be imagined that this might not be the best yet achievable. In fact, in these materials a cluster of four zinc atoms are located very close together to form a fairly dense unit. It might be a thought to find ways to remove this unit and replace it with something a little bit lighter. So I think there is some really exciting and challenging science ahead in this area.

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Now just to indulge in some of the work that we have done quite recently: using single metal sites and employing some lessons from architects who use triangular units to obtain strength, we have generated a number of frameworks containing octahedral units. The octahedral units link through the corners, known as vertices. Using this approach, we can generate materials in which 80 per cent of the volume is taken up just by, essentially, a vacuum. Some calculations, at least, have shown that these materials should have surface areas comparable to the high surface areas that have already been obtained in the materials reported by Yaghi. In these materials then there are very, very large pores, and the need for metal clusters, as in the other compounds that require these to obtain some stability, has been removed.

So watch this space. Hopefully, I will be able to report on some hydrogen storage in these types of materials in the near future.

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Another very intriguing feature of these molecular frameworks is that they show, in several cases, some structural flexibility. This slide illustrates an interesting compound that we have been looking at, as well as others. It has quite small pores within the structure which are able to take up hydrogen. However, in this case, it is actually not very much at a level of around 0.5 weight % of hydrogen. Though the interesting feature of the framework is the sorption and desorption isotherms, and more specifically, the fact that the conditions under which loading and release of hydrogen occur are different. So there is hysteresis. If, on the graph, the black triangles are followed on the way up, it can be seen that hydrogen enters the compound with increasing pressure. On the other hand, tracking this path on the way down again reveals that as the hydrogen pressure is reduced, it doess not follow the same path. This pattern has been tentatively attributed by Thomas to the fact that there is structural flexibility. Indeed, it has been likened a little bit to a cat flap where once the molecules go into the compound there is some rearrangement and then the molecules are then not able to come back out very easily.

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The other challenge is the need to maximise the energy of the interaction. This section will be kept brief because it is really even more speculative and more recent than the examples discussed already.

In terms of maximising the surface interaction energy, there are good precedents for this in the field of organometallic chemistry. For example, it is possible to have quadripolar or pi-type bonding interactions of hydrogen on metals.

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There have also been some examples with nickel phosphates. For example, the types of metal sites associated with these compounds has been attributed to some reasonably strong binding of hydrogen gas.

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There are some good prospects in the molecular area. The Prussian blue materials, for example, have a large concentration of metal sites within the lattice.

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Jeffrey Long, at Berkeley, has shown that alkali earth metals and alkali metals are attractive options. Some experiments have demonstrated that it is possible to get enthalpies for binding of hydrogen at levels that are something like 50 per cent or 100 per cent higher than that in nanotubes. However, by comparison, these interactions occur at higher energies of about 10 kJ per mol, compared to 5 kJ per mol.

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It is also possible to produce bare metal sites inside molecular frameworks. For example, in this interesting framework being studied in Sydney, there are copper or zinc atoms which have had water molecules removed from them. These compounds have very high surface areas; and this particular compound takes up about 4 weight % of hydrogen. Research is continuing towards revealing the as yet unknown role that these metal sites play in the uptake of hydrogen.

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Now to speculate briefly, and even further beyond that necessarily done already, it can be recognised that there is some prospect for combining the areas of chemisorption of hydrides with high surface area materials. Indeed this is actually already known for the hydride materials. For example, in the case of magnesium nickel hydride, this compound is not very good at storing hydrogen compared with the nanoporous version.

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In relation to the possibilities of improving the storage of hydrogen, one idea is to take a material such as borazine which can hold a very large amount of hydrogen (at about three times the gravimetric density) and place this material inside a zeolite. In this way, the thermodynamics and kinetics of the release of that hydrogen is altered. Normally, these reactions are not reversible, and so these compounds are not considered good hydrogen storage materials. However, there is potential, when such materials are embedded, thereby controlling the environment and allowing conditions suitable for reversible reactions.

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Now I would like to thank the members of my group, who have recently taken up the hydrogen storage challenge, and the ARC and AINSE for funding. Again I express my thanks to Michael for the invitation. Thank you.

Discussion

Philip Kuchel – Very nice, Cameron. I know about Prussian blue in the context of cellular reactions, and know it to be a redox-active compound. You didn’t talk about the fact that you could have a redox reaction which would disturb the whole structure. Also, you drew a lot of very rigid structures which didn’t include what must be high molecular mobility. So there will be entropic consequences for these supposedly reversible reactions. Surely that affects the efficiency of your battery?

Cameron Kepert – In relation to our storage materials, certainly entropy is the factor against which the battle occurs, and that is why hydrogen likes to be a gas under normal conditions. So the aim is really to try to get enthalpy on to become more favourable - to bind that hydrogen reasonably strongly, and to get the loading to occur under reasonable conditions.

I didn’t mention the redox area. There is certainly some potential for hydride formation inside one of these frameworks if there was redox activity on some of the metal sites in the framework. The difficulty there, I guess, is that you need to have enough of those metal sites in order to provide sufficient weight %. To think of many redox active metals, for every electron that is given up from the metal, there is only one hydrogen atom being bound. So there are going to be difficulties, I think, with gravimetric storage if we use a redox approach. However this is not to say that it shouldn’t be explored.

Robert Hunter, Sydney – You showed a situation which had some hysteresis, in which it would seem that that would be extremely useful because you could use a higher pressure to get the stuff in and then you wouldn’t need to hold it at high pressure to keep it in there. Does the hydrogen come out when you raise the temperature a bit, or what?

Cameron Kepert – It does come out, yes. That was published a couple of years ago. So the reaction is reversible, but occurs with hysteresis. The plots that I showed indeed indicated that for one of the compounds, it is necessary to go to quite low pressure to get the hydrogen out again; for the other compound, the hydrogen comes out reasonably easily but doesn’t follow the same pathway. So really there is quite a bit more work to be done in this area in order to understand what the mechanism is and exactly how it functions. I think there is still a degree of speculation as to whether this hysteresis arises from the flexibility of the lattice. Certainly, it is a reversible process.

Sue Serjeantson – Cameron, it was really lovely to see the logic that has gone into your thinking. I wanted to say that Robin Williams interviewed Omar Yaghi on the Science Show last Saturday. Omar glossed over the fact that he was working at 77K; I think that was a little factor that you might just have overlooked a bit too. However, he stressed that he had a reservoir of thousands and thousands of metal-organic compounds that were just waiting to be tested, and implied that it was just a matter of time before they came across the ‘gold medal standard’. Would you like to comment on that?

Cameron Kepert – Sure. Firstly, with the refinement of going down to low temperature, a number of car manufacturers I think are willing to entertain the idea of having liquid nitrogen DUERs that hold your physisorption hydrogen storage material, and in this case maintain that sort of temperature for storage. Of course, we would like to have a much stronger binding interaction so that ideally it becomes unnecessary to have to go right down to those temperatures. That is why we are looking at ways of maximising that physisorption interaction.

Omar has a remarkable array of compounds, including some that are, as the ones that I have shown, up to more than 5,000 square metres for one gram of surface area. He is, I think, pushing this very strongly and believes – as do many of us in this area – that these materials have enormous material. Certainly, purely on the time scale of their development (which is perhaps not a fair comparison, but they really have only been studied for the last two years) progress is appreciable and we are already seeing some excellent uptakes of hydrogen , though they are not yet competitive.

Michael Barber – Unfortunately, I think I had better close this session. You probably sense from this presentation that even if the hydrogen economy really is a myth, it is certainly stirring the chemists to produce some intriguing new materials.

Thank you to the speakers in this session this morning.

Symposium program

Other speakers

Dr John Wright
Setting the scene: What is the hydrogen economy?

Dr George Crabtree
The two hydrogen economies

Dr Sukhvinder Badwal
Fuel cells

Professor Andrew Dicks
Advanced nanomaterials for fuel cells

Dr Evan Gray
Hydrogen storage: status and prospects

Dr Ben Hankamer
Solar powered H₂ production from H₂O using engineered green algal cells

Dr Catherine Grégoire Padró
Production of hydrogen

Professor David Trimm
Catalysis and syngas for the production of hydrogen

Dr Wes Stein
Making hydrogen from the Sun

Professor Harry Watson
Hydrogen car prospects