Enabling ‘greener’ energy - nanotechnology for power

Materials World magazine
,
1 Jun 2010

Dr Martin Kemp FIMMM, Theme Manager for Engineering Applications at the UK Nanotechnology Knowledge Transfer Network, provides an overview of nanotech applications in energy.

Renewable energy is now firmly on the political and industrial map as the momentum to reduce carbon emissions and increase energy harvesting from sustainable sources increases. The main processes in the ‘renewable energy cycle’ are illustrated below. Nanotechnology is increasingly influencing all these areas, with emphasis on energy harvesting, storage and in reducing energy loss.

Solar harvesting

Developments in photovoltaics (PVs) are challenging the dominance of devices based on silicon wafers which although having a high conversion efficiency, are costly and heavy (20kg/m2). Crystalline or amorphous thin film devices comprising photoactive nanomaterials deposited onto flexible substrates offer a high throughput, hence cost-effective, manufacturing route using techniques such as roll-to-roll printing. For example, The University of Manchester, UK, spin-out company Nanoco Technologies Ltd has developed a scaleable method to manufacture nanosized particles of copper indium diselenide, copper gallium diselenide and the mixed material copper indium gallium diselenide.

These are manufactured with an organic capping agent (ligand) which allows them to be deposited by inexpensive printing methods, rather than expensive vacuum methods, onto a substrate. Subsequent heating removes the ligand, leaving a dye layer that can absorb photons when deposited in a heterojunction structure, produces an electric current.

In a process more akin to photosynthesis, dye sensitised solar cells (DSSC), first developed by Professor Michael Grätzel of École Polytechnique Fédérale de Lausanne, Switzerland, use an organic ruthenium dye to capture the light energy. This is then transferred to nanoparticles of titanium oxide on a conductive base layer to generate a voltage.

Although DSSC PVs currently have efficiencies of around five per cent, they are potentially cheap to produce and can operate at lower light intensities than silicon devices. These devices can therefore operate longer during the day, an important factor for countries at higher latitudes such as the UK. Dye sensitised solar cell products for building integrated photovoltaics are being developed under collaboration between Dyesol UK and a major building materials producer, which will represent a significant new application.

Storage solutions

Renewable energy sources such as wind, tidal, wave and solar are cyclical or transient. The development of the ‘green economy’ is therefore reliant on a way to store energy. The trend towards hybrid and electric vehicles has also stimulated the demand for lower cost and higher performance mobile power storage.

Nanotechnology is already finding use in batteries and supercapacitors. The Wilson Center, Washington, USA, has produced a list of nano-enabled products. In 2008 this included nanostructured lithium titanate batteries, Energizer lithium batteries (incorporating nanoparticles to prevent reduction) and a De Walt/Black & Decker nano-phosphate 36V lithium ion battery. The application of nanomaterials in storage devices such as these will continue as they offer benefits due to their high surface area, unique stoichiometry and crystal structures, and enhanced properties generated through nanostructuring.

An example of the latter is the novel polymer system developed by Leeds Lithium Power, UK, comprising an interpenetrating network of rigid ‘gel’ and liquid ‘sol’ phases (see image, top, right). Because the liquid phase is continuous, this gel acts as a ‘solid electrolyte’ to replace the usual liquid electrolyte in a battery cell. The material offers advantages in safety of manufacture and use, plus flexibility and economy from manufacturing electrode assemblies in a continuous process. Application developments are under way, including a joint venture to manufacture rechargeable lithium batteries and collaborations to develop new powered ‘smart’ cards.

In situations where energy needs to be rapidly captured, or discharged, such as in vehicle regenerative braking, supercapacitors offer an elegant solution. University of Southampton, UK, spin-out Nanotecture has developed nanoporous materials by liquid crystal templating. This technique is based on surfactants that self-assemble to develop highly ordered micelles resulting in a liquid crystal structure. By controlling pore size and geometry, wall thickness and surface area, customised material properties can be achieved (see image, bottom, right). Nanotecture has developed a nanoporous nickel (II) hydroxide electrode, for example, which shows a unique combination of high specific power and specific energy between that of a battery and supercapacitor. This electrode can discharge 75% of its power in less than two seconds and is being applied to hybrid electric vehicles, renewable energy storage and infrastructure energy harvesting.

Nanocatalysts are being used in hydrolysis to split water, and nanostructured materials are being developed for solid state hydrogen storage. A UK Technology Strategy Board funded consortium, HYSTORM, is researching storage in nanostructured materials such as metal hydrides with the aim of finding a material that exhibits both high hydrogen absorption and ready desorption. These properties are normally mutually exclusive, so the materials development company, Ilika, Southampton, UK, is using high throughput techniques to generate a large number of samples which span wide compositional ranges in order to identify compositional ‘hot spots’. Absorption is measured using an optical transmission technique and desorption by quantitative temperature-programmed-desorption, which uses a mass spectrometer to detect hydrogen evolution as temperature is ramped up.

Loss reduction

Reducing energy wastage is a complementary approach to mitigating environmental impact. Insulation of buildings and special coatings on external glazing, can reduce heat loss and ingress which burdens air conditioning. Aerogels for example, comprise a nanostructured glass which exhibits a thermal conductivity (k value) of less than half that of mineral wool at ambient temperatures and are even more effective at higher temperatures up to 650oC.

Reduction of energy losses due to friction in machines and car engines can lessen energy and fuel consumption with resultant lower emissions. Ultra hard coatings such as nanodiamond from Diamond Hard Surfaces Ltd, Oxford, UK, or CrTiAlN from Teer Coatings Ltd in Droitwich, UK, are being applied to contact surfaces to good effect. Nanoadditives in engine oil are an alternative approach to reducing friction, and a European Commission funded project (AddNano) has recently commenced with UK partners Infineum UK Ltd (coordinator) and BHR Group. The project will investigate the friction reducing properties of inorganic fullerene-like dichalcogenide nanoadditives such as tungsten disulphide in engine oil. These closed polyhedra nanoparticles exhibit an ‘onion-shell’, structure which exhibit high lubricity.

This brief overview of the applications of nanotechnology to ‘green energy’ illustrates just some of the ways in which nanotechnology is providing benefits. Technology transfer is a key part of the process to commercialise these innovations, and the NanoKTN is driving forward the commercialisation agenda through the activities of the ‘Nano4Energy’ focus group.

Further information: Dr Martin Kemp