Nanostructures for energy applications

Nano-structured materials are attractive for some energy related applications because they can provide very high surface areas per unit mass, leading to high energy densities in various hydrogen or electrical storage applications.

Supercapacitors

A supercapacitor (electrochemical capacitor) stores electrical energy either in the form of ions at an electrode/electrolyte interface (electrical double-layer capacitor, EDLC) or by faradic redox reactions at the electrode (pseudo-capacitors). Both types offer high power density (rapid discharge), excellent reversibility, and long cycle life. Supercapacitors are finding increased use in hybrid power systems, for example, in providing peak load power in fuel cells/supercapacitor hybrids that dramatically extend fuel cell lifetimes. Supercapacitors usually use activated (meso-porous) graphite for their electrodes, but alternatives with higher power capability are being studied intensively, including entangled, meso-porous carbon nanotube (CNT) films – an application that makes use of the “natural” tendency of the CNTs to entangle and percolate current at low volume fractions.

We are fabricating comparatively large amounts of both multi-walled CNTs (by chemical vapour deposition) or single wall CNTs (by arc discharge) in-house (with Dr Nicole Grobert), purifying them, functionalizing their surface to improve their ion storage capability, and then processing them into large area films – or “buckypaper” - on a variety of flexible or stiff substrates. In some cases, other process steps can add nanoparticles to provide a superimposed pseudo-capacitance. Our goal is to demonstrate the potential benefits of this approach over existing materials at the laboratory scale, and also to ensure that we develop processing technologies that at all stages offer the potential for cost-effective scaling to the near-industrial, and then full industrial use. The ability to process and characterize fully these materials in-house is key to this strategy.

Read more about our work in: Spray deposition of steam treated and functionalized single and multi-walled carbon nanotube films for supercapacitors, X. Zhao, W. Wang, B.T. Chu, B. Ballesteros, W. Wang, C. Johnston, J.M. Sykes and P.S. Grant, Nanotechnology, 20 (2009), 065605, doi:10.1088/0957-4484/20/6/065605.

Novel high energy density, high reliability capacitors

Project Partners: IeMRC, MoD, Rolls-Royce, Norfolk Capacitors, Nanion, Scott Bader

Complex transportation systems and especially civil and military aircraft are seeking new low weight, high reliability and wide temperature capability power capacitors for use in "more electric" transmission and actuation systems. For example, the aerospace sector is seeking to reduce progressively the use of pneumatic and hydraulic actuation systems, along with the use of parasitic compressor air for various heating/air conditioning functions. Replacing these seperate systems with a single electrical system (driven by electricity generated from the aeroengine) is forecast to produce an overall thermodynamic efficiency gain (fuel, cost, emissions) in a "power optmised airframe". Considerable maintenance cost reduction, increased reliability/availability and weight reduction are also forecast.

However, current capacitor technology is becoming increasingly massive as electrical systems grow and powers increase, and lifetime and passive failure mode cannot always be predicted or ensured.

Polymer–ceramic nanocomposites have attracted interest for dielectric applications because they may combine the high dielectric constants of ceramic powders, such as ferroelectrics, with the low temperature processability, low dielectric loss and high dielectric strength of polymer matrices.

We are studying, in particular, the processing of polymer based nanoparticle-containing thin films for high voltage/power dielectric (capacitor) applications, manufactured in-house by both a novel spray deposition variant developed here, and by a modified web coating approach at the near industrial scale. Key challenges concern maintaining the optimum volume fraction of nanoparticles in the film while avoiding agglomeration and current leakage/percolation, and processing into meaningful large areas. While many other groups have produced nanocomposite films for microcapacitors with some excellent dielectric performance, we are focused on processes that offer potential for power capacitor industrial application.

Read more about our work:

Evolution of percolation properties in nanocomposite films during particle clustering, T.K.H. Starke, C. Johnston and P.S. Grant, Scripta Mat., 56 (2007), 425-428. doi:10.1016/j.scriptamat.2006.10.034.

Spray deposited fluoropolymer/multi-walled carbon nanotube composite films with high dielectric permittivity at low percolation threshold, X. Zhao, A.A. Koos, B.T.T. Chu, C. Johnston, N. Grobert, P.S. Grant, Carbon (2008), doi:10.1016/j.carbon.2008.10.042.

Spray deposition of polymer nanocomposite films for dielectric applications X. Zhao, C. Hinchliffe, C. Johnston, P.J. Dobson and P.S. Grant, Mat. Sci. Eng. B, 151 (2008), 140–145. doi:10.1016/j.mseb.2008.05.024.

Processing of Advanced Materials