Max Lu is Chair of Nanotechnology and Director of the Nanomaterials Centre (Nanomac) at the University of Queensland. His research expertise lies in the field of molecular engineering and applications of nanomaterials such as nanoparticles, membranes and nanoporous materials for energy and environmental applications. He has co-authored over 250 scientific papers. He is a sought-after consultant by industries in the fields of porous materials, energy and environment. In 2002, he was appointed by the Australia Research Council as an expert advisory committee member for Engineering and Environmental Sciences. He is a Fellow of IChemE, and a Fellow of the Australian Academy of Technological Sciences and Engineering.

Nanomaterials for emerging energy and environmental technologies – current research and perspectives
by Professor Max Lu

I would like to share with you some of our work in the area of nanomaterials for energy and environmental applications. I am not a scientist. I am a chemical engineer by training, so our approach is a bit different, but nonetheless we look at the fundamental science that underpins the development of nanomaterials and try to make tailor-designed nanomaterials.

Scientists and engineers from various disciplines can all contribute to the development of nanoscience and nanotechnology from their respective backgrounds. Figure 1 illustrates the convergence of the basic sciences, and as a result we have the interdisciplinary areas such as bio-informatics, nano-biotechnology, nanomaterials and nanoelectronics. What we at the University of Queensland primarily focus on is the development of nanomaterials in the areas of energy and environment. We have heard about the impact that nanotechnology could have on various industries such as ITC, manufacturing, medicine, communications and health. My talk focus on applications in energy and environment.

Click on image for a larger version of figure 1

My background is in nanoporous materials, and we started to do synthesis of nanoparticles and nanofilms as well.

Nanoporous materials are a subset of porous materials, typically having a porosity larger than 0.4. The definition according to the International Union of Pure and Applied Chemistry (IUPAC) is that the micropores are basically smaller than 2 nanometres in diameter, the mesopores are 2 to 50 nanometres, and the macropores are larger than 50 nanometres. There are some typical examples such as zeolite and active carbons falling into the class of microporous materials. But in between are the mesopores, of 2-50 nanometres. So in line with one to 100 nanometres as the size definition of nanoparticles; I define nanoporous materials as those porous materials having the pore diameter from one to 100 nanometres (figure 2). There is a lot of examples in recent years: the mesoporous molecular sieves, the uniform pore structure such as the MS1 family of materials that were invented in the early ’90s by Mobil scientists.

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So what are the unique properties of those materials? Nanoporous materials have specifically a high surface to volume ratio, with a high surface area and large porosity, of course, and very ordered, uniform pore structure. They have very versatile and rich surface composition, surface properties, which can be used for functional applications such as catalysis, separation and sensing.

A lot of inorganic nanoporous materials are made of oxides. They are often non-toxic, inert, and chemically and thermally stable, although in certain applications the thermostability requirement is very stringent so you have to have a very highly thermostable catalyst.

Figure 3 shows how nanoporous materials are made: a typical example of the bottom-up approach to building the nanostructures, from molecular precursors. For example, this is a zeolite A cage. Those of you who are in the physical chemistry and the catalysis area probably know this example very well – you have a very ordered cavity which is templated by single organic molecule, solvated molecule or cation. From the tetrahedrons of aluminum silicate, they self-assemble into this cage. That is one example.   We also know that a lot of amphiphilic molecules such as surfactants and some polymers tend to form micelles like this, or reverse micelles in organic solvent and oil. These two approaches are widely used for forming a shell structure or nanotube structure, and nanoparticles, respectively.

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Another very widely used technique is sol gel processing. We know that sol gel has been used for forming ceramics for a long time. It is a wet chemistry approach; it is very simple and easy to scale up. What we have done in the last five or six years in this area is to develop a molecular sieve silica membrane. It is quite simple. Basically, you start with metallic oxide – for example, in this case it is gelled silica oxide – and go through the hydrolysis and alcohol condensation – the water condensation reactions – so you have a film (Figure 4). So in this case you have the uniform sol formed and dip the substrate into the sol, and you control the gelation and evaporation rates by controlling very carefully, using two-step catalysis, the rates of hydrolysis, and condensation, to drive the desirable amount of the Q 4 silicon groups.

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After heat treatment you have a film with a very ordered pore structure that is about 200 to 300 nanometres thick film, and that can be used for gas separation – for example, for hydrogen separation. The left graph in Figure 5 shows the results of hydrogen separation in pair with methane, and with CO₂ as well. You can see that two trends generally can be observed. First, the flux across the membrane increases as the temperature increases. This is a typically characteristic of the molecular sieving effect. It is not in the Knudsen diffusion regime; it is rather in the molecular sieving regime. Also, for one particular pair, for example, you have a quite significant separation factor that almost creates two orders difference in flux close to a separation factor of 100. That is clearly shown on the right-hand side graph in Figure 5. As the partial pressure increases, that separation factor also increases, which is quite expected. So for H₂/CH₄, H₂/CO₂, you can reach about 100 in the separation factor.

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We have a project to do with a larger scale test with Johnson Matthey Ltd in the UK. They have funded us on this project. We have had the first stage scale-up and now the second stage scale-up is being conducted. One of my students is there now.

Figure 6 shows a comparison of the small disc, the plates that we have in the lab: the flux of the hydrogen across the membrane, and this is a small tube, a large tube, and the improved large tube. So obviously the coating, the fabrication technology, when you scale up, becomes very important. You can see that when we did some surface treatment you can improve the flux.

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But then you look at the selectivity or separation factor of the hydrogen – this is H₂/CO – the particular application here is for the gas processing for hydrogen fuel cells; you want to remove the impurity CO to avoid the poisoning of the electrochemical catalyst. The small plate has a very high separation factor, whereas when we use the large tube, the separation factor actually deteriorates. It is not very good. That means that there are cracks or pinhole defects in the membrane when you coat it on the tube. We are currently resolving this issue with the second stage scale-up.

In terms of the technical performance of this membrane, Figure 7 compares the standard industrial membrane with palladium membranes – palladium dense-phase membrane, palladium/silver standard, and the very thin film. Of course, you have very high flux when you have a thin film. Our membrane, the MSS membrane, actually has a performance comparable with the industrial standard, which is typically the thick palladium/silver alloy membranes. This is tested at the Johnson Matthey facility.

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Now I move on to another example of nanomaterials application in the emerging energy area. As we know, the fuel cell has been heralded as a totally environmentally friendly technology. There is a lot of effort going on the world in developing fuel cells. One of the reasons is that it does not generate any pollution, it has very high energy storage in the fuel compared with all the best batteries that we have, such as are listed in Figure 8. Some of these are the batteries that are typically used in the current hybrid electric vehicles, and others are the new generation batteries in the mobile devices such as mobile phones. But when they are all compared with the hydrogen and the gasoline and methanol, we have I think about 100-factor less energy capacity. So that is why the fuel cell development has been very high on the agenda for the energy companies as well as the car manufacturers.

Click on image for a larger version of figure 8

What we have been doing in the last couple of years is to develop materials as key components for low temperature fuel cells that can be used for transport vehicles, for example in the PEM fuel cell – the polymer electrolyte membrane fuel cells – which is typically used for cars and mobile devices. This cartoon in Figure 9 shows the matrix of the Nafion polymer that is used in those fuel cells. That material was developed by DuPont first in the ’80s. Basically what it does is collect the water in clusters around the side chains of the sulfonate groups of this PVC type of polymer.   Then the protons, as part of the disassociation of the hydrogen on the electrode, hop through this to combine with oxygen at the cathode to form water.

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But currently this material is restricted to working well at around 80°C. So in order to have a very highly efficient and robust fuel cell you need high proton conductivity, which this polymer has, and good thermostability, mechanical and chemical stability. This polymer doesn’t have that good stability. Another limitation to the current fuel cell of this type is that the polymer has free volumes that allow the diffusion of molecules such as hydrogen and methanol to cross over, therefore reducing the efficiency. So that is a problem called the cross-over effect.

What we have been doing is to introduce nanoclusters or nanomaterials into the conducting polymers to increase the efficiency. Also introducing nanoparticles that are hydrophilic can help in retaining water, therefore making better water management in the fuel cell.

The micrographs in Figure 10, the transmission electron microscope (TEM) images, show a few different other nanoporous materials that we have made. One is aluminum phosphate, on the left, and zirconia oxide is on the right. They are typically proton or oxygen conductors – ionic conductors. They offer tremendous opportunity for new generation fuel cells such as the zirconia oxide with nanocrystalline domains. They can be very highly thermal stable. So in this case they can be used as electrode materials in solid oxide fuel cells, the high temperature fuel cells.

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Another example of the nanoporous materials that we have been working on the last five or six years is the MCM-41: mobile catalytic material No. 41. It is a typical example of the supermolecular templating. We call it the self-assembly of the supermolecular entities, such as the micelles formed from surfactants. Basically, with surfactants, when you increase the concentration beyond the critical micellisation concentration they form micelles, and then you have the silicates or oxide precursors; they condense, surrounding the micelles. With further condensation they self-organise into hexagonal arrays of the tubes. And upon the removal of the surfactant molecules by calcination or organic solvent extraction, you have very uniform channels of a few nanometres diameter.

The beauty of this technique is that you can make tailor-designed molecular sieves – mesoporous molecular sieves – by controlling the alkyl chain length, the number of carbons in the surfactants. Figure 11 shows the argon adsorption pore size; that is from the X-ray diffraction – these two can be correlated very well – and the wall thickness. So you can have a linear control of the parameters of the material developed.

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Figure 12 shows the X-ray diffraction (XRD) patterns. You can see the very nice low-range order, at the very low 2θ angles, a hexagonal structure showing the 100 peaks, and the adsorption isotherms also indicate this is mesoporous – a very uniform pore structure, because there is a sharp capillary condensation branch there. On the right hand side of Figure 12 is a TEM image showing the hexagonal arrays of the pores.

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What we have done in the last few years is that we have been tailoring the surface chemistry of these materials to be hydrophobic, so that you can use them as a adsorbent for adsorbing volatile organic compounds. In Figure 13 we used benzene as a model compound. The figure shows the comparison of the isotherms of various adsorbents from silicate-1 to zeolite Y, active carbon, and MCM-41. The saturation capacity, the adsorption capacity of MCM-41 at a high concentration is quite large. But the low concentration branch of the isotherm is not as good, because the isotherm is a type IV isotherm. It is not a Langmuir type of isotherm like active carbon has, that means that if you use this as adsorbent at a lower concentration, as typically is the case in industrial waste, a stream containing VOCs, it is not desirable.

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We did a surface functionalisation and developed a technique to modify the surface chemistry, using mesogroups to silylate the surface and also to engineer the pore entrance so that you have very high hydrophobicity of the modified MCM-41 compared with the others. As a result, you modify the isotherm. Figure 14 shows that there is still high capacity at a high concentration, but a low concentration has increased the capacity. So it has become a very effective adsorbent.

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My last example is that titanium nanocrystals have been widely used for environmental clean-ups – for self-cleaning surfaces, for example. Nanocrystals of titanium have very high valence band energy – Figure 15 shows that the valence band is high compared to others. This is in aqueous electrolyte. It is very robust in a range of pH and it is cheap and non-toxic. That is why there is a lot of interest in using these materials for clean-ups.

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Figure 16 shows a TEM image of the nanopowders we made. The powder has about 15 to 20 nanometres crystallised of anatase.

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We can use this property to degrade organic pollutants. The cartoon in Figure 17 shows when the photons adsorb, an electron-hole pair is generated. The hole is a very powerful oxidant to convert the hydroxyl groups into hydroxyl radicals, therefore oxidising any organic species like chloromethane.

Click on image for a larger version of figure 17

In conclusion, I have shown that nanostructured materials are enabling technologies that will impact on a lot of industries, particularly the environmental and energy industries. At the University of Queensland we have developed the techniques using a bottom-up approach to build nanoparticles, nanofilms and nanoporous materials and tried to apply them in fuel cells and hydrogen separation.

But I want to stress one point. By the very nature of nanotechnology, the interdisciplinary university/industrial collaboration becomes very, very important to the early adaptation and implementation of technology in various industries.

Questions/discussion

Question: I don’t know whether this is quite in your area, but you have got this factor of 100:1 between battery technology and the fuel cell technology. What does nanotechnology have to offer yet, in terms of the improvement of those existing battery technologies?

ML: That is a good question. We have a project in the battery area to use carbon nanotubes to enhance the Lithium ion adsorption - the uptake as well as the release kinetics. Also, the hydrogen storage is to do with that, and but this is still in its early days – we have had some encouraging results. But I think in terms of technology development the nanomaterials eventually will make a contribution, but the time scale is a bit long because those materials such as carbon nanotubes that people have been working on are very expensive. Of course, materials science will make a big contribution in the battery area.

Question: You have mentioned zeolites and their capacity for storing hydrogen. Are there any nanomaterials available for storing gaseous oxygen? Some of us believe in the oxygen economy, and not the hydrogen economy.

ML: Molecular sieves are widely used.

Question: For storage, not separation of oxygen.

ML: Okay, for storage of oxygen. Some of the ionic conductors can also be used for storage, but I am not familiar with that set of materials. I am sure there are people who are working on the storage of oxygen. I am not sure about the terminology, but I would like to hear any comments on the ‘oxygen economy’.

Question: Just a comment about the last speaker’s question: cerium dioxide obviously is a very good oxygen storage medium. The other thing about cerium dioxide is that it also is a hydrogen storage medium. It’s a bit like palladium metal: it can absorb hydrogen gas and release it under certain conditions. It is not very well known that it does that. So it can be both oxygen and hydrogen storage.