Next generation energy - nanotechnology's role
Providing low cost and low carbon emission energy is one aim of the Nanotechnology Knowledge Transfer Network (NanoKTN). Dr Martin Kemp, Materials Theme Manager, describes its developments.
The green energy revolution is gathering pace, seeking to produce energy from renewable and low carbon footprint resources, and nanomaterials are starting to provide solutions to technical challenges.
The Nanotechnology Knowledge Transfer Network (NanoKTN) was formed to facilitate commercial uses of nanotechnologies and has launched an industry focus group called Nano4Energy, which will assist technology transfer, supply chain development and the growth of UK research consortia.
In January 2009, the Nano4Energy Steering Committee identified four technical focus areas – energy generation and conversion, storage and conservation. Four working groups will cover the activity areas of built environment, mobile power, supply chain development and communication with stake-holders such as industry, government and the public.
There is no shortage of clean or renewable energy. The sun alone delivers 219,000bln 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 supply only one per cent of the world’s consumption. Even Germany, the global leader in renewable energy, only generates 0.3% of national demand from these sources. There are many good ideas and the proliferation of research into energy nanomaterials is widespread, creating thousands of new materials. Yet only a few are up and running as commercial products. The race is on to change that for good.
At the forefront of solar energy harvesting are panels based on crystalline silicon photovoltaic cells (PVs). Although the conversion efficiency of sunlight into electricity is relatively high at ~16%, manufacturing costs result in expensive products, limiting widespread use. Organic PVs (OPVs) are alternative solar cell materials, and, although they have a lower efficiency and reduced lifetime by comparison, they offer a potentially high-volume manufacturing route with the benefit of lower costs. This more affordable option opens up the possibility of very large area panels.
Organic PVs mimic photosynthesis, and early work by Michael Graetzel at EPFL Switzerland in the late 1980s examined chlorophyll on monolithic titanium dioxide, which is a wide bandgap semiconductor. By the use of a layer of nanoparticles of titania, Graetzel saw a marked improvement compared to monolithic, and this work led to the ‘Graetzel cell’, which comprised electrolyte, nano-titania and a stable ruthenium dye.
Now in the process of commercialisation by Australian company Dyesol, the dye sensitised cell (DSC) comprises an upper transparent conductor and lower conducting layer, sandwiching a tri-iodide redox electrolyte and nanoporous titania, which is photosensitised by a monolayer of dye. A facility is now being developed in North Wales to implement DSC onto steel building products in collaboration with Corus and the Welsh Assembly Government.
Solar heat is an alternative source of power which can be converted to electricity. Lithiated nanoparticle diamond is an emerging technology, currently under investigation by Dr Neil Fox at Bristol University. Based on thermionic emission from this material, solar energy will be concentrated and converted to electricity in a process that promises low costs and a conversion efficiency of over 40%. The technique will use commercial diamond dust as the raw material, and due to the low temperature for thermionic emission in diamond, can be used for solar thermal or waste heat as the energy source.
Quantum dots are another PV technology with potential for low cost solar panels. Nanoco Technologies Ltd, a spin-out from The University of Manchester, UK, has developed a method for volume manufacturing quantum dots. Particles four to five nanometres in size, consisting of copper indium gallium diselenide (CIGS) and copper indium diselenide (CIS), absorb light up to 800nm in wavelength and are soluble in 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. Research on quantum dots has indicated that an efficiency of 19% is possible.
Nottingham-based Promethian Particles has developed a novel continuous hydrothermal synthesis manufacturing method for nanoparticles such as TiO2, ZnO and indium tin oxide (ITO), which can be used in PVs. The process is scaleable and can produce controlled size composition and morphology of particles of interest to energy applications.
A hybrid organic-inorganic photovoltaic material is being researched at Surrey University, UK, under Professor Ravi Silva. Using carbon nanotubes, the team is attempting to improve on the five per cent power conversion efficiency barrier in OPVs. The goal is to achieve a 10% efficiency target for these inexpensive, processable ‘inorganics-in-organics’ PVs, which is the market minimum for widespread commercial use. This applied research project was initially funded by EPSRC and is now financed by German energy company E.ON.
Amorphous thin film, α-silicon deposited onto glass, is widely available for applications such as solar powered calculators and battery chargers. However, conventional processing uses aggressive materials, so an alternative, low temperature, plasma technique has been developed by PlasmaQuest Ltd in Hampshire, UK. This process is not only environmentally clean, but allows deposition onto plastic substrates. This means it would be possible to produce flexible low cost solar cells with a potential efficiency of five to six per cent.
The hydrogen economy, based on fuel cells, which converts hydrogen into electricity and water, has received much attention, and a number of systems are commercially available. The majority of hydrogen produced today (85%) is by steam reformation of natural gas, where expensive cleaning is required to avoid contamination of the fuel cell membrane by CO2.
The splitting of water into oxygen and hydrogen fuel is an attractive proposition and involves half-reactions at the anode and cathode. Directly splitting water into hydrogen and oxygen by sunlight (photolysis) is potentially a simple method for onsite production of pure hydrogen.
American company QuantumSphere Inc has launched a nickel-iron nanoparticle catalyst to replace platinum cathodes, normally used in acidified water electrolysers. This material uses the high surface area of nanoparticulates, claiming efficiency improvement of several per cent. This translates into an increase in hydrogen production of 60-200% for a fixed efficiency.
The anodic half reaction – oxidation of water – is a major challenge. Professor Daniel Nocera and Dr Matthew Kanan of MIT, USA, are researching this area. Their 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 comes from an Australian collaboration of Monash University and the Commonwealth Scientific and Industrial Research Organisation, and Princeton University in the USA, led by Robin Brimblecombe. The group has developed a catalyst comprising two nanometre clusters of iridium oxide, surrounded by two nanometre 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 titanium oxide 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 one and three per cent.
Storage and transportation of hydrogen is also being tackled using nanomaterials. A UK consortium from Rutherford Appleton Laboratories and the Universities of Oxford and Birmingham is developing one of many systems within the Engineering and Physical Sciences Research Council Supergen bioenergy project. The ultra-lightweight materials based on Li4BN3H10 and LiNH2 show good recharging properties and indicate a higher hydrogen storage density than liquid hydrogen.
Graphene, or ‘Buckypaper’, is a 2D, atomically thin sheet of sp2-bonded carbon atoms. Professor Andre Geim at The University of Manchester, UK, has investigated the interaction of graphene with hydrogen, which has led to a further discovery of graphane, in which each carbon atom has a hydrogen atom attached (see also Materials World, April 2007, p11). Graphane is an insulator, and offers a potential material for solid state storage of hydrogen with rapid charge/discharge performance.
Batteries and superconductors
Storage and use of clean energy comes from improvements in batteries and supercapacitors. Higher performance batteries offer an easy route to market, since hybrid and electric vehicles are already available. Supercapacitors offer a rapid energy storage device for vehicle braking energy, and transient high power output, which prolongs battery life.
In March, Gerbrand Ceder and Byoungwoo Kang of MIT, USA, reported experimental batteries that are able to charge and discharge in a few seconds.They used LiFePO4 to create a fast ion-conducting surface phase.
Supercapacitors operate like conventional capacitors and store electric charge, but instead of conductive plates separated by a dry separator, they use special electrodes and an electrolyte. The main advantage is rapid charging, and these are under scrutiny by Professor George Chen at the University of Nottingham, UK, for hybridising with batteries to produce a ‘supercabattery’.
A liquid crystal templating technique to manufacture nanoporous materials for battery electrodes and supercapacitors has been completed by Nanotecture Ltd, a spin-out from Southampton University, UK. Depositing materials on the template produces pores of around three nanometres in diameter with a wall thickness of around five nanometres. This structure can increase ionic mobility such as lithium ion in Li-ion batteries. Controlling the pore diameter and wall thickness, by changing the liquid crystal surfactant, can optimise energy density. A 1.5V micro-battery that is 0.3mm thick has been developed, and the technology enables new high power applications such as camera flashes on mobile phones.
Supercapacitor research has largely focused on high surface area activated carbon, metal oxides such as ruthenium dioxide (which is inherently expensive), and conducting polymers. Research at Surrey University by Professor Bob Slade is investigating lower cost alternatives such as nanoporous vanadium oxide and manganese oxide. The high surface area offered by nanomaterials greatly increases the storage capacity compared to monolithic materials.
Further information: NanoKTN