Australasian Science Magazine

By Shane Huntington

Materials scientist Prof David Sholl explains how new hi-tech metal hydrides and metal-organic frameworks can be used to increase the efficiency of nuclear power stations and to capture carbon dioxide emissions in coal-fired power plants.

SHANE HUNTINGTON
I’m Dr Shane Huntington. Thanks for joining us. There's little doubt that the energy sources we currently rely on will be sticking around for decades to come despite their inefficiencies and environmental side effects. No amount of wishful thinking nor the gradual embrace of greener methods will rid the energy sector of the emission intensive process of burning coal. However we can work towards making the process less polluting. Similarly other energy sources such as nuclear will almost certainly be part of a complex solution to meet our future energy needs, but how can we take a process like burning a carbon rich material such as coal and transform it into a viable and clean way to produce energy for the future? Can we create a new generation of nuclear power plants that are genuinely safe and secure and what technologies are we currently missing that will allow us to meet these goals?
Today in Up Close we discuss some of the physical materials that have the potential to help resolve these issues. These materials don't exist naturally but must be carefully designed to have specific properties by teams of scientists and engineers. Our guest today, Professor David Sholl is one such material scientist who's working towards producing these new materials. Professor David Sholl is School Chair, the Michael Tennenbaum Family Chair and GRA eminent scholar in energy sustainability at the School of Chemical and Bio-molecular Engineering at Georgia Institute of Technology. Welcome to Up Close David.

DAVID SHOLL
Thank you for having me.

SHANE HUNTINGTON
Now let's talk firstly about issues around nuclear. Like it or not, we're going to have nuclear in the mix I think it's fair to say and you've been looking at some compounds called metal hydrides for use in these next generation nuclear power plants. Can you give us an idea of what these new power plants are going to be like, and how they differ from the current versions?

DAVID SHOLL
Yes, well one of the ideas with this next generation nuclear power plant is to try and develop the entire energy cycle so it's used more efficiently. And so one of the features of this proposed cycle, it's not something that's been built yet, is that there'd be a very high temperature gas stream that comes off part of the process, that you can then take that stream somewhere else and use the heat in some way. This is already done very commonly in the chemical process industry, people are very careful about recovering heat and using it efficiently. So the idea would be in this nuclear process that there'd be a gas stream, mainly helium, that would be at very, very high temperatures and so you could potentially recover heat from that.

SHANE HUNTINGTON
How is that different from the current nuclear reactors? I assume all of these in some way have to heat something to turn the turbine, so what's different?

DAVID SHOLL
They do. I mean, really this is - in some sense it's a difference of degree. This is just one part of the overall design and it's a way of recovering more energy in an efficient way from the overall process.

SHANE HUNTINGTON
The fuel types, are we still talking about uranium, the same sorts of fuels that we're using in existing systems?

DAVID SHOLL
Yes they are. I mean, I think there's some progress being made on slightly different ways of holding the uranium and so on to increase the safety, but fundamentally it's the same fuel.

SHANE HUNTINGTON
Now the metal hydrides that you're working on and designing, tell us how they fit into the system and what is a metal hydride?

DAVID SHOLL
What a metal hydride quite simply is a metal that's been exposed to hydrogen and then it forms a new chemical compound which is a combination of the metal and hydrogen. So the simplest example is magnesium hydride. So magnesium is a pretty common metal. If you heat it at high temperature in the presence of hydrogen it'll form this new compound, magnesium hydride, that's very, very stable. In fact you then have to heat that up to many hundreds of degrees before it will release the hydrogen.

SHANE HUNTINGTON
Why do we want to use these in nuclear reactors? I mean, what's the purpose of generating these materials?

DAVID SHOLL
In this particular application, the high temperature gas stream that I mentioned will be helium which is an inert gas, very safe, but just because of the way that the nuclear cycle works, almost inevitably, that stream will be contaminated with a very, very small amount of radioactive tritium. Tritium is the radioactive isotope of hydrogen. So as soon as you have that radioactive isotope in there, you don't want to use that gas stream in any way because you'll contaminate any material that it comes into contact with. So then the concept then is to put some kind of material in contact with that gas stream that will grab the tritium or the hydrogen out of it. So that's called a getter material. So the concept that we're looking at is using one of these metal hydrides, the idea basically is that you put the metal in contact with the stream at very high temperature, if we have something that really likes to bind hydrogen it will grab those tritium atoms or tritium molecules and that way we've purified the stream and we then can use it for heat exchange.

SHANE HUNTINGTON
Now just clarify for me, a metal hydride is a material where the metal has already somehow formed a new material with hydrogen. Why is it that it wants to grab more hydrogen, or in this case the tritium, a version of hydrogen?

DAVID SHOLL
So in this case the idea would be we would start with just the metal and so the reaction would be to take up the hydrogen from that gas stream and that the end product would be a metal hydride, in this case a radioactive metal hydride that we then have to dispose of in some appropriate way.

SHANE HUNTINGTON
You mentioned the high temperature, I assume high pressures as well, what sort of properties do these materials have to have to work in these environments, what are you trying to create?

DAVID SHOLL
The really critical thing is we're trying to find things that like to hold on to hydrogen up to very, very high temperatures. So we're talking about temperatures of up to 1000 degrees Celsius. And so there's very few materials that will hold on to hydrogen at those temperatures. Most materials will just decompose into their elemental states. So that really is the challenge that we've been tackling, is trying to look at the whole known universe, if you like, of metal hydrides and understand if there are materials that could capture the tritium at high enough temperatures.

SHANE HUNTINGTON
Now normally when we think of metals, we often have this image of a sort of bulk material like a metal bar or something but presumably you somehow have to increase the surface area of this material in some way in order to get a lot of collection. I mean, if it's just a small surface, it won't work for long. How do you go about that?

DAVID SHOLL
That's right, so probably the way this would work is that you'd have a fine metal powder and that you would pass the gas through that metal powder, but what you said is exactly right, that the higher the surface area is of that powder, then the more efficient the reaction will be for taking up that hydrogen.

SHANE HUNTINGTON
Now the use of metal hydrides is not new as I understand it, they're used in certain fuel cells and others, how do these versions differ from what's currently out there?

DAVID SHOLL
There's been a lot of work on metal hydrides because they hold actually very high densities of hydrogen. So a surprising fact about magnesium hydride for example is that the density of hydrogen in magnesium hydride is higher than liquid hydrogen. So if you want to store hydrogen on a fuel cell vehicle or perhaps a stationery power source, metal hydrides can be a very, very space effective way to do that and so there's been a huge amount of work looking at what I would call low temperature metal hydrides. So in that case you'd like to release the hydrogen at some low temperature.
The thing that's new in what we're doing is we're sort of turning that problem on its head because we want to find materials that hold the hydrogen extremely strongly.

SHANE HUNTINGTON
This is a scenario where it's presumably not acceptable for some of the tritium, this very radioactive material to get out of the system. So what sort of efficiencies do you have to achieve in order for these metal hydrides to be acceptable in order to remove the waste.

DAVID SHOLL
Yes, that's a good question. I think the sort of acceptable release levels haven't been established for this process but certainly the idea is that you'd like to get it to truly trace or almost undetectable limits so that you don't have to worry about that radioactive exposure.

SHANE HUNTINGTON
One of the big questions I suppose from the industry would be, does this affect the operation of the plant itself, the nuclear power plant? Does it reduce the efficiency or the energy that is converted into a usable form? Are there any side effects of using these materials in this way?

DAVID SHOLL
No there shouldn’t be, because the way that this process is envisioned, this gas stream sort of comes off in a secondary way from the process, so it's somewhat independent of the core part of the fuel cycle.

SHANE HUNTINGTON
I'm Shane Huntington and you're listening to Up Close. We're discussing energy storage and fuel cells with computational material scientist Professor David Sholl. Now David, you're also looking at another type of compound called a metal organic framework, which is apparently potentially useful in coal fired power stations to make them cleaner. Tell us about this compound, how is that different from the metal hydrides we've been discussing?

DAVID SHOLL
The metal hydride chemically is very simple, it's just a compound made of metal and hydrogen. A metal organic compound is a class of porous materials that are combinations of very small metal oxide clusters of just a few atoms and then organic molecules that link them together. And so these things actually make a porous structure that sort of alternate the metal and the organic part and these things can have incredibly high surface areas. So there's a lot of interest in using these things for capturing gases and capturing things in a chemically specific way.

SHANE HUNTINGTON
When you say incredibly high surface areas, can you give us a picture of what this means? I have this image of a very small molecule and spreading it out on a piece of paper, how much surface area can we actually generate using these fine structures?

DAVID SHOLL
Yes, the usual way to measure these things is in metres squared of surface area per gram of material. These materials routinely have thousands of metres squared per gram. So if you imagine taking a sample the size of a sugar cube, that will have the surface area the same as many, many football fields.

SHANE HUNTINGTON
We're talking about using these potentially in carbon intensive industries like the burning of coal, and here in Australia we're very good at burning coal, unfortunately. Presumably you want to capture the CO2, is that what these materials are designed to do or are they looking for something else?

DAVID SHOLL
The particular application that we're working on is to use these as components in membranes to capture CO2. A good way to think about this is that you would have the stack gas from your power plant flow through a membrane and the idea then is that the CO2 preferentially goes through the membrane, everything else from the gas doesn't go through the membrane. So we're not keeping the CO2 inside this metal organic framework but we're using it as a way to selectively pull that CO2 out of the stream so that we can then dispose of it in a useful way.

SHANE HUNTINGTON
So what's doing the filtering, are we basing this on size or is it chemical reaction that's occurring with the other materials? How do you get the CO2 through whilst keeping all the other components in?

DAVID SHOLL
So the main components of the stack gases are carbon dioxide and nitrogen. So those are very similar molecules in terms of size and chemistry actually, but in these membranes, what we try and take advantage of is there's a small size difference between CO2 and nitrogen, CO2 is slightly, slightly smaller, and there's also a chemical difference. So by looking at the different affinity of CO2 for these materials, that also preferentially will pull the CO2 through.

SHANE HUNTINGTON
Why do we care about removing the other materials, can we not just sequester the entire exhaust from these systems?

DAVID SHOLL
Of course in principle you could. Whenever you talk about sequestration really it comes down to cost and the volume of gas that's generated in a coal fired power plant is just truly mindboggling. So really to make the cost even conceivable for sequestration, we have to minimal the cost of the overall process.
So in a typical flue gas you have about 15 per cent CO2 and the balance is nitrogen. So if we didn't purify the CO2 we'd be sequestering something like five times as much gas and that really would just completely make the economics non-viable.

SHANE HUNTINGTON
Is this the sort of process that's currently being used anywhere in existing systems? Are we capturing some of the CO2 in another way at the moment or is this something that we just haven't managed to crack yet?

DAVID SHOLL
There certainly are applications where people are capturing CO2 on small scales, and so for example in treatment of what's called sour gas. So there are lots of gas fields around the world where natural gas comes out of the ground, it's contaminated with significant levels of CO2. And so chemical engineers have developed very efficient methods to get the CO2 out of those processes. There are also a variety of other industrial processes where CO2 is captured, just nothing at the scale of power generation.

SHANE HUNTINGTON
When you talk about the difference in size between CO2 and nitrogen, these molecules, many of our listeners would be struggling to picture the difference between these two. I mean, these are very small items. Are you able to construct these particular organic materials in such a way that you can get it down to the specificity of those two sizes, and if so, could you make these materials able to filter any sort of molecule?

DAVID SHOLL
That's certainly the dream with these metal organic frameworks. So the idea is that we can control the crystal structure of these materials to a very, very high degree and we really can create pause in these materials that can be controlled on the sub-nanometre level. And that means there really is the promise to be able to distinguish between molecules that are very, very similar in size.
Having said that, I think that's a dream that's shared by many people in the community that makes these materials and all of us are still working on really turning that into reality.

SHANE HUNTINGTON
In the lab how do you go about determining the size? I mean, this sound like something you would imagine mixing up in the bucket chemistry sense, what determines the size of the pause and the structures and the crystals you're talking about?

DAVID SHOLL
So when these materials crystallise they form into very, very ordered structures. My colleagues and other people who make these materials in the lab can then use things like x-ray crystallography to find exactly where all the atoms are in these materials. So that way we really can know what the structure is. That's actually really important for the work that I do using computers to model these materials, because we have to know where all the atoms are before we can say something useful about how the materials behave.

SHANE HUNTINGTON
You're listening to Up Close. I'm Shane Huntington and my guest today is material scientist David Sholl. We're talking about ways to produce cleaner energy. David in both of the cases we've talked about, the metal organic frameworks and the metal hydride materials, you are doing modelling to predict how to make these appropriate for their various applications. Talk us through what sort of modelling you do. How do you predict what these materials will look like? I would imagine when you model these things you have to do them on a large scale, because that's where their use would actually be.

DAVID SHOLL
Well we do two kinds of modelling. There's certainly the large scale, what I would call process modelling that you just alluded to. So that's the kind of modelling where you try and figure out what would be the economics of the overall process, how would you actually configure the process, what are the right gas flows and so on? What I spend most of my time doing though is materials modelling, what we're trying to do is predict the physical properties of very specific materials. And in some sense, what we're trying to do is to mimic what people do experimentally but to do it more efficiently and perhaps do it for materials that are difficult to access experimentally.

SHANE HUNTINGTON
We have a range of materials that we can readily produce. Why don't we just go and test those and see how we go? Why the modelling?

DAVID SHOLL
That's a terrific question. So as an example with these metal organic frameworks, there are at least 10,000 of these materials that are known. In order for us to test those in the membrane application that's of interest to me, you have to make a sufficient quantity of one of those materials, you have to decide how to make it into a membrane, which is a non-trivial task. You then need to do a test, which takes some time and some money and so the reality is that it's just not really feasible to systematically test all those thousands and thousands of materials.
This is a very, very common problem in engineering and material science and so there are many instances where we have these material selection problems. So really what I'm interested in doing is using computational methods to attack that material selection problem, not so that we can avoid doing all experiments but so that we can really accelerate the experimental process.

SHANE HUNTINGTON
The one thing that's been missing in a lot of this sort of work in decades gone by are the other elements that should go into the modelling around can we produce this much material sustainably, is it economically viable, what are the energy costs in producing it? Is that sort of information being put into the models that you're creating.

DAVID SHOLL
We tend to not put that in upfront. So we start off with really a very simple question, let's say for a particular physical process, of simply is there any material that will have the physical performance that we want? As soon as we've done that though, if there are viable lead candidates, then we go on to the kinds of questions that you just asked? What will it cost to produce this material? There are then all kinds of other questions that can't be addressed by a model but are incredibly important in an engineering sense, like how durable will the material be, how readily can we manufacture it, things like that.

SHANE HUNTINGTON
That brings me to my next question around the accuracy of the models you're doing and how well they actually give the sort of information you'd need. You mention that there are several levels of the modelling in the nano and micro scale, we have a very small quantity of the material and then there's the large quantities that would go into the actual power stations. How well does the model work in those two regimes, and do the results you come out with compare well with what you're seeing in the field?

DAVID SHOLL
This is really a key question in doing this kind of work, that to be valuable for material selection we have to have models that are truly predictive. We have to be able to trust these models without going and doing an experiment. We also have to be able to use the models without parameterising them to experimental data. So we have to use really very fundamental physical science's methods.
As one example, in looking at the metal hydrides we use quantum chemistry calculations. So these are high level computational methods that give very reliable results. We've spent years validating these methods and very, very carefully looking at the results of our calculations and comparing them with experimental data for known materials. So by doing that for appropriate properties, we can establish that these things truly are predictive. And we can then look at hundreds and hundreds of new materials and trust the results enough that we can make decisions based on those results.

SHANE HUNTINGTON
I think the metal hydrides example you gave earlier is a good one where when you are looking at low temperature environments and so forth, that's one thing, but you just mentioned quantum chemistry. We're looking at environments where the temperatures are quite extraordinary. Have you determined any aspects of these materials that were not known before as a result of looking in these new environments with more detail?
DAVID SHOLL
Yes, well we have to be quite careful, as you said, looking in these extreme environments. In our case we're interested both in the radioactive isotope of hydrogen and we're interested in very, very high temperatures. So you can imagine that those two things combined make it extremely difficult for anyone to do an experiment. We've done a series of studies to first of all determine how much does it matter that we're using that isotope rather than regular hydrogen. The answer fortunately for the predictive side of things is it doesn't make very much difference. That's also good news in experiments. It means that you can do experiments just with regular hydrogen and the information we get is useful
Certainly there are some cases where we look at the known information that is available regarding these materials and decide that it's either incomplete or perhaps even incorrect based on these calculations. These calculations are now sufficiently precise that we're able to go back and assess the experimental data in some sense.

SHANE HUNTINGTON
Most materials engineers would have to deal with various aspects of longevity around temperature, pressure, fatigue, but you're adding in radiation as an extra item which can be quite extraordinary in the damage it can produce, how are the materials coping with the tritium and the fact that they will be exposed over a very protracted period to that particular radiation?

DAVID SHOLL
That's something that our work doesn't directly address, so we're just trying to find materials that will grab the tritium. One of the nice things though in this particular application is the idea that we want to capture the tritium and then just take that material out. Now there are severe challenges around tritium decay in other nuclear applications and also infusion applications. So there's a significant field of work associated with what happens when tritium gets embedded inside a metal because when it radioactively decays it forms helium which can actually be quite damaging to the metal.
So that's something we don't have to deal with but it's a very real issue in other nuclear applications.

SHANE HUNTINGTON
We've talked about both coal fired power stations and nuclear power stations, I think it's fair to say that we'd like to get rid of or at least clear up coal fired power stations as quickly as possible and in a similar way we'd like potentially to utilise nuclear power as perhaps a stopgap measure in the near term, to reduce our carbon emissions. All of these things are fairly urgent. How far off do you think some of these materials are in terms of application to these two energy sectors?

DAVID SHOLL
It's a great question. I think the reality is that moving any technology from an idea in the lab into large scale application, if that can be done in 10 years, that's actually an amazing fast achievement. Really the timescale for implementing these things at scale is decades I think. One way to think about this is that in order to implement CO2 sequestration at a typical coal fired power plant, the capital cost just for the equipment to do that, even with today's technology, is going to be probably hundreds of millions of dollars, perhaps a billion dollars. That's a great deal of money.
Now of course we spend a great deal of money on power generation but I'm sure you can appreciate that if it's your job to decide to spend that $1billion, you want to be pretty darn sure that it's going to work properly. So you have to go through multiple stages of proof of concept and really run these processes for extended periods of time before you move to the next scale in each case.

SHANE HUNTINGTON
David you work in two areas that are arguably the most confidential in terms of energy in the world. How do you go about making the case to the general public for the work you're doing, given I suspect a large portion of our population would like to see both of them disappear completely, despite what we know as their absolute required base-load contribution at the moment that we just can't turn off?

DAVID SHOLL
I think the real challenge is to help people understand how vital energy is to the lifestyle that we have, certainly in the western world, and just to understand the timescales involved for making changes. I think that most people have never really thought about how much energy they use and where that energy comes from, anything that we can do to communicate that better to people I think will help.

SHANE HUNTINGTON
David when you're in your office, lab - I suspect more office…

DAVID SHOLL
Office, yes. [Laughs].

SHANE HUNTINGTON
… and you're looking at the modelling of these materials, I mean what would we see if we were sitting in there with you? Is this all numbers on a computer, is it visualisation? I mean, what sort of things do you actually do in terms of the modelling?

DAVID SHOLL
I think it'd look very unexciting actually. So most of these calculations are done at something like a supercomputer centre, which is not housed in our facility. So really what you'd see is you would see a PhD student or a post-doc sitting in an office looking at a computer and it'd be fairly indistinguishable from them reading their email perhaps. The interest comes in the long term application of these materials. Certainly when we have people visit our facility and ask for a tour, we don't take them to my part of the building. [Laughs].

SHANE HUNTINGTON
I know computing has increased in its capabilities remarkably over the last couple of decades. Is this the sort of work we could have done 10, 20 years ago, or is it just now with today's supercomputers and so forth that these - as you mentioned, quantum calculations and others are viable as a computational tool?

DAVID SHOLL
The advances in computer technology really have made this possible. We now routinely use a computer cluster that has about 1000 nodes. That's something that for most universities is a fairly normal piece of equipment. 10 years ago that really would have been unheard of. One of the things that's actually exciting working in this area is that we have continued change. I think in another five to 10 years what we now consider to be challenging calculations will become routine. I'm constantly sort of pushing my students to think about not just what can we do in six months, but really what can we do if we have 10 times more capability, because that will be coming.

SHANE HUNTINGTON
David thank you very much for being our guest on Up Close today.

DAVID SHOLL
Thank you.

SHANE HUNTINGTON
Professor David Sholl is the School Chair, the Michael Tennenbaum Family Chair and GRA eminent scholar in energy sustainability at the School of Chemical and Bio-molecular Engineering at Georgia Institute of Technology.