Andrew Dicks is an expert in industrial chemistry, particularly catalytic process development including coal gasification and energy systems analysis that stem from a career in the UK gas industry. He has worked with leading developers of fuel cell systems and particularly high temperature systems (moltencarbonate fuel cells and solid oxide fuel cells) in Europe, the USA and Canada. He helped draft the European Hydrogen and Fuel Cell Strategy, and has been a consultant for the UK, Swedish and Australian governments. He co-authored a widely acclaimed student textbook on fuel cells that was translated into Japanese last year. In recognition of his research achievements, he was awarded the Henry Jones Medal of the Institution of Gas Engineers. He serves on the Board of the Australian Institute of Energy and is Chairman of the World Hydrogen Energy Conference to be held in Brisbane in 2008.

Advanced nanomaterials for fuel cells
by Professor Andrew Dicks

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Thank you very much. Good morning, ladies and gentlemen.

First I should like to thank the Academy for giving me the opportunity to talk about advanced nanomaterials and fuel cells, and our work at the university.

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I would now like to begin by introducing ‘nanomaterials’ – a word which is getting more and more familiar – and then I will focus a little bit on fuel cells. It is nice that Sukhvinder and previous speakers have already covered most of what I want to say in terms of fuel cell construction and the fuel cell system. So I would like to look at the components, the types of fuel cells, and the technological issues that need to be addressed before these become commercial. I will then focus particularly on the materials side.

In terms of our research at the university, I am going to focus on solid oxide fuel cells and PEM – proton exchange membrane – fuel cells, and the related direct methanol fuel cells. I will then draw a few conclusions.

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Nanomaterials are very interesting. Most solid materials are essentially nanomaterials; since they have some structure at the nanoscale. However here we are interested in assemblies of molecules and molecular structures that are present at the size of 100 nanometres or less.

The materials are characterised at the nanoscale by surface effects such as van der Waals forces – long-range intermolecular forces, hydrogen bonding, covalent bonding within molecules, hydrophobicity, quantum mechanical tunnelling, and ionic conduction, which are all very important for fuel cells. Indeed, without ionic conduction we wouldn’t have the possibility of fuel cells. Finally, nanomaterials are characterised by potentially vast surface areas as compared with volumes, and this is important for catalysts.

It is possible to characterise nanomaterials at the appropriate scale by x-ray diffraction, by electron microscopy, by small angle x-ray scattering, and by small angle neutron scattering. In fact, the little diagram at the top right-hand corner of this slide was produced by ANSTO using some Nafion® that we supplied and analysed using small angle neutron scattering.

Probably one of the most important features is that nanomaterials can be built from the bottom up. This means that we can apply organic templates to actually create the molecular structures that we are interested in (a process known as self-assembly).

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Here is an example of a well-known material, a single-walled carbon nanotube that was produced by chemical vapour deposition a few years ago. This material is easily characterised with tube diameters of something between 300 and 500 nanometres, and lengths of several microns.

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You can also easily produce multi-walled carbon nanotubes by catalytic cracking or pyrolysis of hydrocarbons. These are again very well known and characterised.

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Here is an example of a nanomaterial which was prepared for a fuel cell. This is the anode catalyst in a PEM fuel cell. This is an electron micrograph showing small deposits of platinum catalyst on a high-area carbon material, and again the carbon was produced by pyrolysis of a hydrocarbon.

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Here is a more recently developed nanomaterial. This is a ceria-doped gadolinia material that is produced by a company in the United States specifically for solid oxide fuel cells. You can see the small molecular structure highlighted at the scale of below 5 nm.

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Here is a nano-composite electrolyte which we produced a few years ago at the university, and I will talk more about this later.

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I don’t have an animated diagram of a fuel cell construction, but just a few slides that illustrate the different components of a fuel cell. Sukhvinder Badwal has already highlighted some of these.

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The electrolyte, of course, is important, as are the electrodes – the anode and the cathode – and also the sealing components.

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Each of these can have specific functions which, when built together, produce a stack.

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An important feature of the stack is the manifolding of gases, so that it is possible to feed oxidant into one side of the fuel cell and fuel into the other.

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Sukhvinder has also mentioned that a fuel cell system is more than just the stack. So, at the university we are interested in materials which comprise not only the components of the fuel cell stack itself but also the fuel processor. Today, I am not going to talk at all about our work on fuel processor catalysts, because a lot of this subject (I hope) will be covered by David Trimm, later on. I am going to cover only our work on fuel cell components.

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Here we have a slide which, again, lists the main types of fuel cell. These range from solid polymer electrolyte fuel cells that are at the low temperature end of the spectrum, through the phosphoric acid and alkaline fuel cells, and then to molten carbonate and two types of solid oxide fuel cells. The more conventional high-temperature SOFC type uses an electrolyte of Yttria-stabilised zirconia, and, more recently a low temperature type has emerged that usesvarious classes of low-temperature electrolyte materials such as cerium oxide.

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Fuel cells now are commercially available, and if you have enough money you can buy, for example, a Ballard 1kW stack. Indeed, through collaboration with the ANU, we have bought one of these and it is sitting in a lab at the ANU, where I saw it yesterday.

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There are a number of technological issues with fuel cells and particularly the materials. Firstly, they are too expensive.  In fact, I have heard many presentations from fuel cell conferences saying that there are three issues with fuel cells: 'cost, cost and cost'.

For the PEM fuel cell, this issue is really related to the high platinum loading and cost of the membrane. The membranes in the low-temperature PEM fuel cells are certainly very expensive. Membranes for solid oxide fuel cells are also very expensive and fragile, because they are ceramic materials. Furthermore, each of these membranes has a poor lifetime, and Sukhvinder emphasised this point in his talk as well. Poor power density is also an issue for high-temperature fuel cells. When you see the size of the 1MW molten carbonate plant, you realise that there is a long way to go before these systems can compete with gas turbines, for example. In addition, there are a number of stack engineering issues, such as heat management, fuel processing, and hydrogen storage, which will be covered other presentations today.

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This slide lists most of the principal materials used in constructing the different fuel cell types. I would now just like to draw your attention to the materials used.

The proton exchange membrane consists of a perfluorinated polymer electrolyte – I will talk about this later – and the catalysts for the anode and the cathode are platinum or platinum-metal alloys, supported on carbon.

At the other extreme, the solid oxide consists conventionally of a ceramic electrolyte made of Yttria-stabilised zirconia, with a nickel cermet anode and another ceramic material as the cathode.

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Now I would like to talk about our work on solid oxide fuel cell materials. This is actually done in collaboration with Ceramic Fuel Cells, a CSIRO spin-off company which is currently based in Melbourne.

The materials consist of the previously mentioned zirconia electrolyte and, on either side of it, the catalyst layers (as I have just outlined earlier which comprise nickel cermet on the anode and LSGM ceramic on the cathode), and there is an interconnect or bipolar plate, if you like, between each of the cells, which contains a conducting layer.

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The stack design is actually very clever. It has an all-ceramic planar design and an electrolyte supports the different components of the cells. It operates at round about 850°C, and is able to convert natural gas or methane directly within the fuel cell. In this way, methane is directly converted into hydrogen on the nickel anode, and that of course gives particular advantages if you are going to use these for stationary power.
These systems are also modular - so they start with small stacks and then we can stack those together and build larger systems.

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We are interested in the particular components of the cell, and interested in how the cells operate in real life. The way that they are built up is to start with a tape cast electrolyte plate, onto which is screen-printed a thin layer of anode, and then a gas diffusion layer followed by a cathode and a cathode gas diffusion layer.

A point that you will notice on this slide is that at each stage there is a degree of firing. This is very expensive, and although it produces a very well-operating stack, we want to try and reduce the number of those steps, if we can.

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The first thing that we need to do is to understand how the materials end up in a working SOFC . One of the most powerful techniques that we use involves electron microscopy. This slide shows scanning electron microscope (SEM) pictures of the cell components at various stages of firing. As you can see, the grain size changes as we go through the steps of firing.

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Even more exciting is the fact that you can actually map the elements that occur in different phases within the fuel cell.

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Furthermore, and this is even more exciting, is that to do what some people thought was impossible! That is, to produce a very thin film of solid oxide material and use a transmission electron microscope. We have found that we can do that, and in the two panels on this slide a bright field image and a dark field are shown. On the right-hand side of each picture you have the non-porous, or dense, electrolyte, and on the left-hand side you see the more open pore structure of the anode, in this case.

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Not only that, but we can now also focus our electron beam to scan across grain boundaries. This is really quite exciting, because we can see where the different elements are. On the right-hand side of the slide we can see that at the grain boundaries (that are shown here on the anode) we have a high concentration of nickel and alumina.

So this is an example of the sort of work that we are doing on solid oxide fuel cells, to try and understand how these materials actually work in practice.

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The other element of our research is on PEM and direct methanol fuel cell-related technology. Now you have already seen this kind of diagram.

The reason we are interested in this area is that we can apply PEM fuel cells to portable automotive and electronic devices. Our goal is to produce materials and by this we mean not just to understand how materials work but to actually produce them. This will be achieved by starting with novel nanocomposites for reducing methanol crossover and increasing operating temperatures.

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So in order to understand what we are doing in this area of research, we need to go back a step and examine certain aspects of the chemistry of the membrane.

The electrolyte is based on the polymer polytetrafluoroethylene (PTFE), otherwise known as Teflon®, which we know is used in non-stick frypans and so on. It is a substance that repels water and is based on a polyethylene backbone.

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Now, some years ago DuPont created a material which they called Nafion®. This product has fluorinated side chains on which sulfonic acid groups are located. These sulfonic acid groups are hydrophilic, and they also weakly bind protons which enable the migration of protons to occur throughout the membrane.

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Another component that determines the way in which the polymer fuel cell works, we believe, involves clusters of water molecules. These are actually held within the case of the Nafion® membrane, and kept in place by the sulfonated groups. Now what happens is that the protons literally hop from one group of water molecules to the other, bringing water along with them. So one of the interesting features and challenges of the PEM fuel cell, is that there is a net transfer of water from one side of the fuel cell to the other. This needs to be managed.

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So this situation led our group to decide to try and reduce the crossover. However at this point I should mention that not only is there crossover of water, but if you try and use liquid methanol then this issue becomes a big challenge. This is because methanol dissolves in water, and if you are using it as a fuel you can find that a large amount of this methanol ends up on the other side of the fuel cell, in an unreacted form. So the objective in this case is to block the passage of this methanol.

Part of our research, as shown in this slide, was to generate some silica particles within the Nafion® structure. We achieved this by applying a simple sol-gel process that is very well known and uses tetraethylorthosilicate (TEOS), and specifically, hydrolysing this substance in situ. So as we move in this cycle between hydrolysis and drying, we can produce a number of different particles, at different particle sizes.

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This procedure worked fairly well, but unfortunately it reduced the proton conduction of the system. So we looked for a different approach, and using the same kind of idea – that is, a sol-gel route – we came across this material: 3-mercapto-propyl-trimethoxy-silane (MPTMS), which actually reacts to produce a material that has these sulfonated side groups attached to it. The theory was that not only would we block some of the sites within the Nafion®, but this material itself, would be a good proton conductor.

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Well, we produced some materials, and yes, by and large the proton conductivity did increase to a certain extent, at least at the high temperatures. At low temperatures, as you can see on the right-hand side of the graph, it reduced.

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But the great thing was that we did observe a significant reduction in methanol crossover. So this is one approach that we and others have now taken to reducing methanol crossover in direct methanol fuel cells – that is, by incorporating another component and making a nanocomposite.

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In our research, we also took this one stage further and incorporated polyaniline. Now, polyaniline can be incorporated quite easily into the Nafion® structure. This is shown here and  the results of some water uptake and methanol permeation experiments. You will see that the green line at the top of each one is in the Nafion®, and both the water uptake and methanol crossover increase, as you increase temperature - which is expected.

The nanocomposites, shown by the pink and blue lines in the middle, had slightly reduced methanol crossover. By comparison, the combination of polyaniline, silica and Nafion® produced the best results, particularly at high temperature.

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These results are shown again on this slide. What is believed to happen is that the polyaniline aligns with the Nafion® polymers and imparts some rigidity to the molecular structure. This is a positive feature because one of the problems with Nafion® is that it swells. Then as it swells, there is increased mobility of water and methanol. Notably, this swelling increases with increasing temperature. So this modification is perhaps one way of reducing methanol crossover for direct methanol fuel cells at higher temperatures.

So this modification of Nafion® has provided an example of the use of nanocomposites.

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Now let’s venture slightly further afield and look at functionalising other materials for membranes. Here we have examples of sulfanilic acid functionalisation of zirconium phosphate. Zirconium phosphate has been widely used as a proton conductor; its proton conducting characteristics have been known for some time. It is now possible to enhance these qualities by functionalising them with sulfanilic acid.

Returning to an earlier point in this talk, you may recall that I mentioned the whole business of templating nanomaterials? Well, our research functionalised material which we had actually templated to make a mesoporous material. This slide provides you with some idea of the results.

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Then we carried out the same kind of process with titanium phosphate, and achieved remarkable results. In fact, we have called this material Proton Plus and it has now been patented. When the proton productivity of this compound is compared with other materials using measurements taken by AC impedance  spectroscopy, then results indicate that it is better than most other materials that have been used for low-temperature fuel cells. So we are quite pleased with this advance.

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Further developments that have become really exciting for us now involve the idea of taking these functionalised nanomaterials to another plane. In this research in collaboration with ANU we are attempting to develop techniques for making fuel cell materials. This will involve making not only the membrane but also the catalyst in situ, in a plasma.

Essentially this means that we are attempting to adopt the type of techniques used in the semiconductor industry for making small fuel cells. At the moment, it is very early days; and there are no results that we can show from this project yet, however you should certainly ‘watch this space’.

In other developments, we are trying to plasma-deposit platinum metal catalysts on carbon, and generate carbon and platinum from single-source materials. This is not normally done because people normally start with a high-area carbon and simply use wet chemistry to deposit platinum. The problem with this process is that the platinum just goes through the whole of the structure and is not specifically located on the surface of the material, where you really need it to be located.

Another area that we are aiming to develop involves looking at the in-situ formation of polymers via plasma polymerisation process. This follows some work that has been carried out in France recently.

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Now we return to the subject of catalysts. In PEM and direct methanol fuel cells, the catalyst sits in a relatively thin layer of carbon, which can be anywhere up to two microns in thickness, and is usually set on a carbon paper or carbon cloth support, with – as you saw in earlier slides – small platinum particles that are typically around 5 nm in size.

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The results shown in this slide were obtained last year. They give us some hope that this process of preparing catalysts can in fact can be done. The data is from the deposition of platinum and carbon from a single-source material, and the great thing is that we can produce platinum not only on conventional-looking high-area carbon but also on carbon nanotubes as well. So this is the sort of area that we are working on now, and we hope that through developments in this area, we can significantly improve the materials of fuel cells.

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So to conclude:

• we believe that functional nanomaterials are an essential feature of fuel cells of all different types. Today, I have concentrated mostly on solid oxide and PEM; and shown that micro-analysis and other techniques can be used for increasing our understanding of fuel cell materials;

• new methods of preparation, such as sol-gel synthesis and, hopefully, plasma deposition may help to reduce the costs of fuel cells and increase the likelihood of their commercialisation.

Thank you.

Discussion

George Crabtree – I am curious: what do you think are the prospects for replacing Nafion® as a membrane? Is that getting a lot of attention, and likely to be successful?

Andrew Dicks – The short answer to that is - yes. There are a number of groups now working on non-fluorinated polymers, hydrocarbon polymers. PolyFuel®, in the United States, claims to have a hydrocarbon polymer which has a performance that is as good as that of Nafion®. However, there are also various other materials such as polyethylketone which are being used by some of the major developers. So yes, it has taken a long time, but people are moving away from Nafion®.

Gus Nossal – I was impressed by the images of your nanomaterials that were obtained by scanning electron microscopy. Down in Melbourne we are spending 207 million of the taxpayer’s dollars building a Synchrotron, and I am wondering whether the Synchrotron will be of any help in your research, particularly in imaging new nanomaterials.

Andrew Dicks – Yes, I am sure it will. I am sure that we need every technique that we can muster, to try and understand what is going on, particularly at the interface of these materials. At the moment, this seems to be where our understanding is really lacking.

Furthermore, I highlighted the solid oxide fuel cell. There is a lot that we still don’t understand about the solid oxide fuel cell, and how that three-phase boundary really works.

Les Field – Thank you very much. Please join with me in thanking Andrew for an excellent presentation on fuel cells.

Symposium program

Other speakers

Dr George Crabtree
The two hydrogen economies

Professor Cameron Kepert
Hydrogen storage in nanoporous materials

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