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Electrochemical Capacitors: When the Levee Breaks

2013; The Electrochemical Society of Japan; Volume: 81; Issue: 10 Linguagem: Inglês

10.5796/electrochemistry.81.773

2186-2451

Thierry Brousse,

Fuel Cells and Related Materials

For the last 5 years, Electrochemical Capacitors (ECs) have taken off in everyday's life applications.Although the use of supercapacitors has so far not been directly visible for consumers despite their successful implementation in aircrafts (Airbus A380), tramways and harbor cranes, recent examples such as the i-ELOOP• system in the new Mazda6 or the e-booster of PSA Peugeot Citroën C2 & C3 are directly impacting the life of thousands of drivers worldwide.Many more applications are envisioned in transportation (cranking of vehicles in cold climate), in buildings (energy saving in elevators), or in consumer electronics (portable tools, energy harvesting), as obvious maturity markers for a technology ready for the market.It looks like the levee breaks and a flood of innovations is submerging every field of today's industries.Microdevices are also driving much attention as a new market opportunity for ECs, with an increasing number of developments targeting the fabrication of thin or flexible ECs to power sensor network, RFID tags, MEMs, etc+As for many mature technologies, suppliers and customers continuously request for more to electrochemical capacitors in terms of energy density, power density, safety, cycle life, etc+ These requirements are obviously the targets of nowadays research projects.The main challenge is the increase in energy density without sacrificing the performance advantages of ECs over batteries, i.e. power capability, very fast charging rate, very long term cycling efficiency (>1,000,000 at room temperature).Energy density is a critical issue especially with regards to volume.The two main paths for achieving larger energy density are the use of electrodes with enhanced capacitance or /and the implementation of new electrolytes with larger electrochemical potential windows.The first path is addressed by considering new materials especially those based on pseudocapacitive compounds.Although the concept of pseudocapacitance is not fully understood, earlier works on ruthenium or manganese oxides are of precious help for new comers on the field to grab the basics of pseudocapacitance: fast and reversible surface redox reactions making the capacitive signature of electrodes resemble carbon ones but with much more capacitance than usual double layer capacitance electrodes.More or less attractive transition metal oxides and nitrides, conducting polymers and much more compounds are presented daily in the scientific literature.Designing carbons with optimized pore size distribution is an alternative for increasing the energy density: carbide derived carbons, carbon nano-onions, etc+ have been specifically tailored according to new electrolytes.Subsequently, the search for optimized electrolytes is also a major issue either for cell voltage enlarging, by ionic liquids for example, but also for improving the safety of ECs by the use of non flammable solvents and harmless salts.The safety aspect is also the trigger for the regain of interest in aqueous based devices.Even if some work based on micro-electrodes with very small amount of active materials let feature that tomorrows supercapacitors will store as much energy as a battery, real life considerations rather suggest the improvements will be incremental especially with regards to medium and large scale devices from 100 F up to 5000 F. Interestingly, all these exciting fields of research have also triggered the fundamental investigation of electrochemistry in ECs as well as attracted number of theoreticians who currently develop models (based on Molecular Dynamics, Monte Carlo or DFT) but also macroscopic approaches for understanding and predicting the properties of capacitive materials.Impressive studies are presented either in theoretical considerations or on the use of in-situ / in-operando even more sophisticated techniques in order to unveil the secret life of these amazing devices.As every blooming new field, electrochemical capacitors have their share of innovative concepts such as Li-ion capacitor, 4 Vclass aqueous hybrid electrochemical capacitor, redox flow capacitor, carbon electrodes functionalized with electroactive molecules, and much more to come.All these exciting studies are the cement that linked an emerging community of researchers all over the world, based on mutual respect and fruitful interactions.