Unusual energy enhancement in carbon-based electrochemical capacitors

Electrochemical capacitors - also called supercapacitors - represent a relatively new electricity storage system applied for harvesting energy and delivering high power pulses for short periods. The main developed technology is based on charging an electrical double-layer (EDL) at the electrode-electrolyte interface of high surface area carbons. The main disadvantages of the latter are a relatively low energy density and safety issues related to the use of organic electrolytes. This paper describes alternative solutions where pseudocapacitive contributions play together with the EDL capacitance in a protic aqueous electrolyte, giving rise to a possible enhancement of energy density. It first reviews traditional solutions by using materials that are able to undergo fast faradic redox reactions, e.g. oxides and electrically conducting polymers (ECPs). Since the surface of materials essentially plays in this case, the realized capacitance values are much lower than theoretically expected; additionally the working voltage is generally lower than 1 V and the stability of some of these materials, e.g. ECPs, is relatively low. The realization of composites with carbon nanotubes which play as three-dimensional conductive supports contributes to the enhancement of performance of oxides and ECPs. A new source of pseudocapacitance involving a redox electrolyte is demonstrated by high surface area carbons. The latter is based on the interface formed by species with a large variety of oxidation states, e.g. iodine, bromine and vanadium, and gives rise to capacitance values as high as 250 F g(-1) at 1 A g(-1) for 1 mol L-1 KI aqueous solution. The last pseudocapacitive phenomenon involves electrochemical hydrogen storage in the negative electrode of carbon-carbon capacitors in neutral aqueous electrolytes, e.g. alkali sulphates. The voltage (and consequently the energy density) is considerably enhanced by comparison with systems in basic or acidic electrolytes owing to an important overpotential for di-hydrogen evolution at the negative electrode; besides a pseudocapacitive contribution related to hydrogen storage appears at the highest voltage values.