layer capacitance at the electrode/ electrolyte interface as the principal charge-storage mechanism. As a result, ECs typically exhibit sloping charge- discharge profiles reminiscent of electrostatic capacitors (hence their designation ‘electrochemical capacitors’). Due to the combination of sloping


charge-discharge profiles and the lower charge-storage capacity of the double- layer mechanism relative to the redox processes in batteries, ECs have lower energy densities than batteries. As an example, the energy density of a typical carbon-carbon EC is 3-5 W h kg-1 compared with > 100 W h kg-1 for high-performance Li-ion batteries. Compared with advanced batteries,


the chief advantages of ECs are the rates at which their energy can be stored and released (charge-discharge response times are typically < 10 seconds for carbon-carbon ECs); long cycle


life


(oſten hundreds of thousands of cycles); and graceful fade characteristics. The sloping charge-discharge profiles of ECs


also provide an important benefit as an indicator of the state-of-charge of EC cells as they are electrochemically cycled. The term ‘electrochemical capacitor’


is used to describe a diverse array of energy storage devices that incorporate a variety of


active materials (high


surface area carbons, electroactive polymers and/or transition metal oxides), electrolytes (conventional aqueous and non-aqueous electrolytes, advanced polymer electrolytes or ionic liquids), cell configurations


(symmetric and


asymmetric) and electrode architectures. Because of this diversity in design and


cell chemistry, as a class of energy storage technologies ECs cover a broad region on the power-versus- energy density plane and bridge the critical performance gap existing between the high power densities offered by conventional capacitors and the high energy densities of batteries. Applications with challenging power


requirements have increased interest in ECs as viable energy storage solutions: either as stand-alone devices for high


power demands or, more oſten, as integral elements in hybrid systems that also include other energy storage/generation technologies (for example, batteries, fuel cells and combustion engines). Te fast charge-discharge characteristics


and long cycle life of ECs are particularly well suited for hybrid-electric power systems designed to capture energy from repetitive motion that would normally be wasted by conventional braking. EC development and commercialisation


is in its infancy compared with battery technology but the demonstrated capabilities are sufficient


to warrant


interest, the US Naval Research Laboratory reports. Continuing research worldwide has a particular emphasis on advances in nanoscience that will result in new high-performance electrode materials and electrolytes. A more detailed understanding of key


fundamental processes at the electrode/ electrolyte interface is also being pursued. Next-generation EC technologies based on new materials and discoveries should


NEW WEB-SITE www.halehamilton.com


Warship Technology January 2012


19


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