Nanotechnology is an emerging set of platform technologies which has potential to change many of our current industries, including energy. Nanotechnology is attracting significant public and private funding from all nations, particularly the United States, and many companies and industries are starting to look to the field to generate competitive advantages.

Three speakers presented different aspects of nanotechnology applications in the energy sector: Dr Peter Binks, CEO of Nanotechnology Victoria, Dr Rachel A Caruso, ARC Australian Research Fellow, School of Chemistry, University of Melbourne, and Dr Paul A Webley, Reader, Chemical Engineering Department, Monash University.

Dr Binks provided an overview under the title, “Nanotechnology and the role it may play in future energy programs”, Dr Caruso spoke on “Controlling structures on the nano-scale with application in photovoltaics”, and Dr Webley on “Design and development of carbon adsorbents for hydrogen storage”. This is a summary of their presentations.

NANOTECHNOLOGY IN ENERGY (Dr Binks)

Nano-science is the study of materials and events at 10-9 m. It is engineering at the molecular level. Nanotechnology is not an industry, nor will it produce new industries. However it provides new competitive dimensions to existing industries and activities, particularly energy. International energy initiatives based on nanotechnology may have implications for Australian programs.

Nano-scale particles have fundamentally different optical and reactive properties to larger particles. Recently identified nano-structures such as carbon nano-tubes have fundamentally different mechanical and electrical properties:
· diameter around 1 nanometre; length typically up to 50 microns
· density two times lighter than aluminium
· tensile strength 100 times that of steel
· electrical conductivity superior to copper
· thermal conductivity superior to diamond
· biocompatible.

Nano-scale particles provide a different proposition in biological systems to larger particles. Further, nanotechnology allows morphological control, dimensional control and interfacial control.

In 2004, governments, corporations and venture capitalists spent more than US$8.6 billion worldwide, and national and local governments across the world invested more than US$4.6 billion, on nanotechnology R&D. This is the last year that governments will outspend corporations on nanotechnology activity as the focus shifts from basic research to applications development. Approximately 1,500 companies worldwide have announced nanotechnology R&D plans. Eighty percent of them are start-ups, 670 of which are in the United States. A key driver is energy independence.

In the United States, activities related to energy include (key agencies in brackets):

· Energetic materials for propulsion, explosives (DOD)
· Catalysis, fuel cells, hydrogen (DOE)
· Advanced power systems (IA)
· Energy conversion and storage for space (NASA)
· Materials science and engineering (NSF)
· Manufacturing processes and equipment (NIST)
· Biomass conversion, hydrogen production, distributed power (USDA)

American nanotechnology companies are working on catalysts and photovoltaics. Nano-stellar is developing highly-efficient platinum nano-composite catalysts for automobile emission control, fuel cells and chemical industry applications. The next-generation technology will finally make solar power competitive. The new photovoltaics use tiny solar cells embedded in thin sheets of plastic to create an energy-producing material that is cheap, efficient, and versatile. Massachusetts-based Konarka expects to deliver its first commercial solar cells designed for use with consumer electronics like laptops, by the end of 2004.
 

Figure 1: Nanotechnology Australia: Capabilities & Commercial Potential (Invest Australia, Feb 2004)

In Australia, Sustainable Technologies International achieved the first commercial installation of dye-sensitised solar panels. The high efficiency was achieved via nano-sized powders used in the electrodes of the panel. The powders are lightly sintered to form a nano-network which is used as the charge collector. Ceramic Fuel Cells Limited is exploring nanotechnology in solid oxide fuel cells for application in the anode, cathode and electrolyte materials. This allows increased manufacturing control and higher surface areas that would enable higher power production.

There are opportunities to capture the benefits of nanotechnology in Australian energy through:

· Reduced reliance on fossil fuels and increased use of renewables, in particular solar energy and hydrogen
· Development of solutions tailored to Australia, e.g. distributed energy storage and production
· Growth of companies to manufacture components, catalysts and cells, based on new technologies
· Integration with specialist manufacturing industries, in particular medical and automotive.

PHOTOVOLTAICS (Dr Caruso)

Energy is an essential element for our livelihood and for the advancement of humankind. The majority of our energy is derived from fossil fuels, which we know to be a finite resource that has had a devastating effect on our environment. For some time now there has been a quest for cheap, reliable alternative energy sources which are sustainable. Renewable energy sources include wind turbines, hydropower, and solar power. This presentation focusses on photovoltaics, where light is used to produce electrical power.

The Solar Photovoltaic Roadmap (www.bcse.org.au), which was released in August 2004, demonstrates the potential Australia has to be a main player in the global arena of solar energy, due to Australia’s current standing in research, manufacture, marketing and development. If we were to cover an area about one-tenth the size of Australia with solar cells functioning at 10% efficiency, we could supply all of the world’s current energy requirements (electrical, transport and heating, see www.electrosolar.co.uk for calculations).

A number of “generations” of photovoltaic cells exist operating with different technologies and coming with varying degrees of efficiency and cost. Third generation cells have just started to enter the market and it is one of these, the dye-sensitized solar cell (DSSC) that relies on nanotechnology. The first manufacturing facility for the DSSC was Sustainable Technologies International in New South Wales which made these cells commercially available in 2003 (www.sta.com.au).

The DSSC originates from Professor Michael Grätzel’s laboratories at the Swiss Federal Institute of Technology (A low-cost, high efficiency solar cell based on dye-sensitized colloidal TiO2 films B. O’Regan and M. Grätzel Nature 353, 737-740 (1991)). The working mechanism is based on the principle of photosynthesis – where energy is provided to the plant by the absorption of light by the chlorophyll in a plant leaf converting CO2 and water to carbohydrates and oxygen. However, covering a large crystal of titanium dioxide with a layer of chlorophyll does not give efficient energy conversion. By decreasing the titanium dioxide crystal size to the nano-regime, a substantial increase in surface area is obtained giving a great increase in photovoltaic efficiency. The DSSC works as follows:

1) Light shining on the cell causes photo-excitation of the dye
2) The electron from the dye is injected into the TiO2
3) The electron flows through the porous TiO2 and along the conducting layer
4) Energy is harnessed from the electron flow
5) Electrons reduce the tri-iodide to iodide
6) Iodide undergoes oxidation at the dye, replacing the electron injected into the TiO2, thereby completing the cycle.
 

Figure 2: The dye-sensitized solar cell, after Michael Grätzel.

Nanotechnology can be applied to this cell design in an attempt to further enhance the efficiency of the cell. Firstly, nano-scale control of the morphology could increase the surface area further, resulting in increased dye adsorption, hence light absorption, and therefore more electron flow. Research is being conducted using a templating technique where a mould is utilized to structure the titanium dioxide that affords control of the porosity and final surface area of titanium dioxide. Secondly, nano-scale control of composition can change the crystal size of the TiO2 (again influencing the surface area) and decrease back flow of electrons that result in decreased efficiency. Again, templating techniques allow post-treatment steps to layer a second metal oxide on the titanium dioxide which can act as a barrier preventing back flow of the electrons. This work is being conducted in conjunction with the Nano-crystalline Dye-Sensitized Solar Cells Group at Monash University.

HYDROGEN STORAGE (Dr Webley)
Acknowledgements: PhD students Louis Chen and Yunxia Yang, post-doctoral student Ranjeet Singh, Chemical Engineering technical staff for analytical assistance, and School of Physics and Materials Engineering for microscopy.

Mobile applications of hydrogen rely on safe, reliable and cost-effective storage. A car would need six to eight kilograms of hydrogen for an internal combustion engine, or four kilograms for a fuel cell engine, to achieve a range of 400 kilometres. The United States Department of Energy has now prepared specifications for such a vessel. There are essentially three ways of meeting this specification: physical storage via compression or liquefaction, use of metal hydrides, and gas-to-solid physi-sorption.

A typical 200 bar steel tank would need some 500 litres volume for eight kilograms. Industry has also developed high-strength composite vessels. However, this method incurs a high energy demand for compression and there are potential safety issues. Liquid hydrogen can be stored in cryogenic tanks as 21.2 K and ambient pressure. Boil-off rates vary with size but are typically 1.5mass% per day. Liquefaction requires roughly half the energy produced in combustion.

Metal hydrides are formed when atomic hydrogen is chemically bonded to a solid and released by heat. The hydrogen is dissociated at the surface and forms a solid solution and hydride phase. Mg2NiH2 and LaNi5H6 alloys look very promising. However, elevated temperatures are required to release hydrogen. The United States Department of Transport has approved the Texaco Ovonic model hydride system for transport, but it takes some 20 minutes to reach 90% capacity.

Carbon-based absorbents are well known as one of the better absorbent groups for their ability to exist in a very fine powdered form with a highly porous structure, ranging in pore size from A to microns. The surface chemistry is readily modified to produce a range of intermolecular forces, and they are cheap! There are several forms of carbon suitable for physi-sorption: graphite, activated carbons, nano-structured carbons (nano-tubes, films, rods, horns, etc.) and fullerenes. This is the most active area for research and, in 2003, some 65 Japanese patents and 75 US patents were filed for carbon structures designed for hydrogen storage. Activated carbons are made from a variety of sources: coal, biomass, polymers, etc. The overlap of the force fields from opposite pore walls leads to strong adsorption. However, it is difficult to control pore structures and pore size distribution due to its intrinsic disorder characteristics.

Carbon nano-tubes were first discovered in 1991. There are two types of nano-tube: single-walled (SWNT) and multi-walled (MWNT).
 

Figure 3: SWNT, 50-60nm diameter (X.Chen et al, Int.J.Hydrogen Energy,2004)
 

Figure 4: Multi-walled carbon nano-tube microstructure

SWNT perform better then MWNT in adsorption. However, adsorption results are scattered and inconsistent because it is difficult to produce nano-tubes in high quality and quantity. Impurities such as other forms of carbon obscure the results, and adsorption experiments are difficult to perform correctly. Further progress will only be made with production of nano-tubes of high quality and quantity with standardised procedures. Graphite nano-fibres are produced by catalysed decomposition of carbon-containing compounds. They consist of graphitic platelets of mean diameter between 30 and 500Å which are arranged in either parallel, perpendicular or at an angle to the fibre axis. Hydrogen can “intercalate” within the graphite sheets.

Fullerenes are a new form of carbon with a closed-caged of pentagon and hexagon molecular structure. A high energy barrier prevents it from practical use for hydrogen adsorption. The adsorption results of various research groups are shown in Table 1.

Figure 5: Graphite nano-fibres
 

Table 1: Adsorption results of various research groups

Source: Darkrim, Malbrunot et al, 2002.

The question which needs to be addressed is whether one can engineer adsorbents with required surface area, chemistry, and pore sizes. Hydrogen diameter is 0.41 nm. In micro-pores with diameter 0.8 nm, two hydrogen molecules can fit with overlapping potential fields from walls. C-H Van der Waals forces are relatively weak. It needs some 19 KJ/mol to store hydrogen at room temperature.

Zeolites, commonly used for catalysis and gas separation, are micro-porous crystalline solids with well-defined Si/Al pore structures. The Adsorption Research Group at Monash University in the Chemical Engineering Department uses templating techniques, first forming the 1?m zeolite powder into pellets. The next step is to polymerise and pyrolise small monomers in the zeolite supercage to link the supercages in a carbon network, and then remove the template by leaching. Surface treatment of the resulting carbon network will further produce nano-pores. Surface doping of carbon can improve surface chemistry.

The synthesis has a number of advantages: it is reproducible, the pore structure is closely controllable, it is relatively cheap, the control of the pore structure is possible through the use of different templates and/or processing conditions, and post-processing to modify the surface can significantly alter properties. Further experimental design is required to optimise pore size and regulate post-synthesis activities to create ultra-micropore in the carbon framework, with post-syntheses doping of the surface to enhance adsorption energy.

The conclusions to date are that careful control of pores in carbon at the nano-scale is possible and the final material may be a good hydrogen storage material.