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The ‘green energy' revolution is gradually gathering pace, seeking to produce energy from renewable and low carbon footprint resources. In many of the future clean energy scenarios, nanomaterials are starting to deliver the critical solution to technical challenges.
February 11th, 2009
The NanoKTN and the 'Green Energy' Revolution
Dr Martin Kemp
Materials Theme Manager
The Nanotechnology Knowledge Transfer Network (NanoKTN) has a remit to facilitate commercial exploitation of nanotechnologies and has launched an industry focus group, Nano4Energy. This network will facilitate the development of UK research consortia supply chains, in order to achieve commercial exploitation. At the kick-off meeting in January this year, the Nano4Energy Steering Committee identified the elements of energy generation and conversion, storage and conservation. Four working groups will cover Built Environment; Mobile Power; Supply Chain; and Communication. These working groups will define activities to be addressed in these areas, in order to facilitate technology transfer and supply chain development.
The world is not short of ‘clean' or renewable energy. The sun alone delivers 219,000 billion kilowatt hours of energy every year: as much in one day as the global population uses in one year. However, solar, geothermal and wind energy currently supply only 1% of world consumption. Even Germany, the global leader in renewable energy, only generates 0.3% of national demand from renewables. There is also no shortage of good ideas - the proliferation in research on new energy nanomaterials is rather like the start of a marathon - thousands of new materials poised at the starting line, but just a few currently up and running as commercial products. However, many more are about to emerge.
At the forefront of solar energy harvesting, are panels based on crystalline silicon photovoltaic cells (PV's). Although the conversion efficiency of sunlight into electricity is relatively high (typically ~16%), manufacturing costs create an expensive product, which limits widespread use. Organic photovoltaic (OPVs) are alternative solar cell materials, and although they are generally of lower efficiency and reduced lifetime by comparison, they do offer a potentially high-volume manufacturing route, with the benefit of much lower costs. This more affordable option will open up the possibility of very large area panels.
As an example, solar panel manufacturer G24 Innovations, combines materials science and nanotechnology to create dye-sensitised thin film solar cells, which generate electricity in a process similar to photosynthesis. The company is using ‘Graetzel Cells' to manufacture highly portable power packs for recharging batteries and other low power applications. These solar cells are produced on a roll-to-roll production line, which reduces cost, and can be integrated into textiles and non-rigid formats - benefitting from ruggedness, low weight and portability (Fig A). Uniquely, the cells work in low light and indoor conditions, which significantly increases their usefulness.
Quantum dots are another photovoltaic technology with potential for low cost solar panels. Nanoco Technologies Ltd, a spin-out from the University of Manchester, has developed a method for the volume manufacturing of quantum dots. These 4-5 nm nanoparticles of Copper Indium Gallium Diselenide (CIGS), and Copper Indium Diselenide (CIS), absorb light up to 800nm in wavelength and are soluble across a range of solvents. These features provide the possibility for application by printing using inkjet or roll-to-roll techniques. After an annealing treatment, the layer can be converted into crystalline thin films suitable for solar cells (Fig B). Research on quantum dots has indicated that an efficiency of 19% is possible.
A hybrid organic-inorganic photovoltaic material is the focus for research at Surrey University under Professor Ravi Silva. Using the unique properties of carbon nanotubes, the team hopes to improve on the 5% power conversion efficiency barrier in state of the art organic photovoltiacs. The goal is to achieve the 10% efficiency target for these inexpensive solution processable ‘inorganics-in-organics' PVs, which is the market accepted minimum for widespread commercial use. This applied research project was initially funded by ESPRC and now has attracted the funding from energy company E.ON.
Amorphous thin film, á-silicon deposited onto glass, is widely available in applications such as solar powered calculators and battery chargers. However, the conventional processing route uses aggressive materials, and PlasmaQuest Ltd has developed an alternative, low temperature, plasma technique. This new low-temperature process is not only environmentally clean, but allows deposition onto plastic substrates. This would mean it would be possible to produce flexible low cost solar cells with a potential efficiency of 5-6%.
The ‘Hydrogen Economy', based on hydrogen fuel cells, which convert a fuel such as hydrogen into electricity and water, has received much attention, and a number of systems are now commercially available. The majority of hydrogen produced today (85%) is by steam reformation of natural gas, which can be contaminated by CO2, so expensive cleaning is required to avoid contamination of the fuel cell membrane.
The splitting of water into oxygen and hydrogen fuel is an attractive proposition and involves two half-reactions, at the anode and at the cathode. Directly splitting water into hydrogen and oxygen by sunlight (photolysis), is potentially a simple method for on-site production of pure hydrogen. US company, QuantumSphere Inc (QSI) has launched a nickel-iron nanoparticle catalyst to replace platinum cathode, normally used in acidified water electrolysers. Its material exploits the high surface area of nanoparticulates, claiming efficiency improvement of several per cent, which translates into an increase in hydrogen production of 60-200% for a fixed efficiency.
The anodic half reaction - the oxidation of water - is a major challenge currently attracting numerous research groups. Recently generating news coverage is the work carried out by Professor Daniel Nocera and Dr Matthew Kanan, of the Massachusetts Institute of Technology (MIT). The new catalyst comprises cobalt metal and a phosphate on an electrode and produces oxygen when a voltage is applied to the cell. An alternative approach by a collaboration between Monash University, CSIRO and Princeton University, led by Robin Brimblecombe, has developed a catalyst comprising 2nm clusters of iridium oxide, surrounded by 2nm clusters of orange-red dye molecules. The dye absorbs the more energetic blue light spectrum and splits water to form oxygen. This material, when impregnated into a TiO2 anode and combined with a platinum electrode in a salt water cell, generates 1.17V, and with an addition of 0.3V, produces hydrogen and oxygen. The efficiency of 0.3% compares favourably with the ‘benchmark of photosynthesis which is between 1-3%.
Titanium dioxide itself is an interesting material since it is photocatalytic and is the ‘building block' of a number of potential PV systems. Nottingham based manufacturer Promethean Particles specialise in manufacturing high quality nanoparticles with controlled composition, size and morphology.
Storage and transportation of hydrogen is another issue that is being tackled using nanomaterials. A consortium comprising Rutherford Appleton Laboratories and the Universities of Oxford and Birmingham has developed one example of many systems under development. The ultra-lightweight materials based on Li4BN3H10 and LiNH2 have been found to show good recharging properties and indicate a higher hydrogen storage density than found within liquid hydrogen.
An alternative route to storage and use of clean energy is based on developments in batteries and supercapacitors. Improved performance batteries offer an easy route to market, since hybrid and electric vehicles, for example, are already available. Supercapacitors offer a rapid energy storage device for storing vehicle braking energy, and transient high power output, so prolonging battery life.
Nanotecture Ltd, a spin-out from Southampton University, uses a liquid crystal templating technique to manufacture nanoporous materials for battery electrodes and supercapacitors. Depositing materials on the template produces pores of diameter ~3nm with a wall thickness of ~5nm. This structure can increase ionic mobility such as Lithium ion in Li-ion batteries. Controlling the pore diameter and wall thickness, through changing the liquid crystal surfactant, can optimise energy density. A 1.5V micro battery 0.3mm thick has been developed, and the technology has been shown to enable new high power applications such as camera flashes on mobile phones.
This article gives only a snapshot of the developments in nanomaterials for energy. The first Nano4Energy Conference will be held in Nottingham on 4th March 2009. Entitled ‘'Nano4Energy - Commercialistion of Nano-Enabled Clean Energy', the conference will feature a range of speakers including some of the technologies mentioned here, and will examine the funding and commercialisation of nano-enabled energy materials (contact