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WIREs Energy Environ.
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Review of nanostructured carbon materials for electrochemical capacitor applications: advantages and limitations of activated carbon, carbide‐derived carbon, zeolite‐templated carbon, carbon aerogels, carbon nanotubes, onion‐like carbon, and graphene

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Electric double layer capacitors, also called supercapacitors, ultracapacitors, and electrochemical capacitors, are gaining increasing popularity in high power energy storage applications. Novel carbon materials with high surface area, high electrical conductivity, as well as a range of shapes, sizes and pore size distributions are being constantly developed and tested as potential supercapacitor electrodes. This article provides an overview of the electrochemical studies on activated carbon, carbide derived carbon, zeolite‐templated carbon, carbon aerogel, carbon nanotube, onion‐like carbon, and graphene. We discuss the key performance advantages and limitations of various nanostructured carbon materials and provide an overview of the current understanding of the structure–property relationships related to the transport and adsorption of electrolyte ions on their surfaces, specific and volumetric capacitance, self‐discharge, cycle life, electrolyte stability, and others. We discuss the impact of microstructural defects, pore size distribution, pore tortuosity, chemistry and functional groups on the carbon surface, nanoscale curvature, and carbon‐electrolyte interfacial energy. Finally, we review state‐of‐the art commercial large scale applications of supercapacitors, including their use in smart grids and distributed energy storage, hybrid electric and electric vehicles, energy efficient industrial equipment, ships, wind power stations, uninterruptible power supplies, power backup, and consumer devices. WIREs Energy Environ 2014, 3:424–473. doi: 10.1002/wene.102 This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Energy Infrastructure > Science and Materials Energy and Development > Science and Materials Energy Research & Innovation > Science and Materials
Schematic energy characteristics of various types of commercially available electrochemical capacitors in comparison with lead‐acid, nickel metal hydride and Li‐ion batteries as a simplified Ragone plot: (a) specific (mass‐normalized) power versus specific energy and (b) volumetric power versus energy density.
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Leakage current cause by carbon functionalization: (a) schematics showing the cyclic voltammetry diagrams of idealized (infinitely fast with stable electrolyte) electrochemical capacitors with no leakage current and with significant leakage current and the equivalent circuits for these two conditions; (b) pseudocapacitance of functionalized activated carbon versus a leakage conductance in different aqueous electrolytes. (Reproduced from Ref 323. Copyright 2013, Elsevier)
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Effect of pH of electrolyte on its electrochemical stability during the operation of a symmetrical EDLC: (a) cyclic voltammograms recorded at the rate of 2 mV/second and showing the potential stability window of EDLC electrodes with electrolytes composed of 6 M KOH, 1 M H2SO4 and 0.5M Na2SO4. Reproduced with permission from Ref 313. Copyright 2010, Elsevier; (b) cyclic voltammograms of symmetric EDLC with electrolytes composed of 1M alkali metal (Li, Na, K) sulfate solutions recorded at the rate of 1 mV/second. (Reproduced with permission from Ref 312. Copyright 2012, Royal Society of Chemistry)
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Effect of surfactants on the performance of a symmetrical EDLC: (a) capacity retention without and with surfactants at different current density, (b) cyclic voltammetry recorded at the rate of 10 mV/second with Triton X® surfactant added to 6 M KOH electrolyte in two different concentrations: below (black line) and above (red line) critical micellar concentrations. (Reproduced with permission from Ref 311. Copyright 2012, Elsevier)
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Effect of pore tortuosity on the rate of the ion transport in microporous carbons: (a) effect of the ZTC synthesis on the alignment of micropores originating from pore ordering in a zeolite template; (b) measured capacitance retention in the produced ZTC electrodes with increasing operating frequency. A major improvement in the frequency response of ZTC electrodes having more aligned and thus less tortuous pores is clearly seen. (Reproduced with permission from Ref 14. Copyright 2010, American Chemical Society)
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In situ neutron scattering experiments on AC electrodes immersed into H2O (a, c) and D2O (b, d)‐based electrolytes under an application of a potential between the working and counter electrodes: (a and b) SANS profiles normalized by the 0 V one, (b, d) relative changes in the intensity of the normalized SANS profiles. (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd)
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Effect of the average pore size on the specific capacitance normalized by SSA in porous carbons TEATFB‐based electrolyte: (a) carbon capacitance normalized by a BET specific surface area SBET (DFT specific surface area reveals weaker, but similar trends); Reproduced with permission from Ref. 110. Copyright 2006, The American Association for the Advancement of Science. (b) comparison of carbon capacitance normalized by a BET SSA and Sav = 0.33(SDubinin–Radushkevich + Sphenol adsorption + Ssubtracting pore effect). (Reproduced with permission from Ref 294. Copyright 2011, Elsevier)
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Comparison of EDLC charge storage in a large cylindrical mesopore and a small cylindrical micropore: (a) traditional view on Helmholtz double‐layer formation within a negatively charged mesopore with cations (green spheres) approaching the pore wall to form an electric double‐cylinder capacitor with radii b and a for the outer and inner cylinders and (b) a negatively charged micropore with cations lining up to form an electric wire‐in‐cylinder capacitor. Solvent molecules are shown in (a) as small light‐blue spheres. (Reproduced from Ref 283. Copyright 2008, John Wiley and Sons, Ltd)
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ALD deposition of vanadium oxide coating on a CNT paper: (a) average tube diameter changes with increasing the number of ALD cycles, showing linear growth dependence, (b) a typical micrograph of the VOx‐coated CNT paper sample, showing smooth and uniform coatings, (c and d) specific capacitance of the selected samples calculated from the charge–discharge tests measured as a function of current density, (e) an example of the charge–discharge test showing very small resistance drop at a very high current density. (Reproduced from Ref 19. Copyright 2012, Royal Society of Chemistry)
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Schematic of the formation of CDC with dual pore size distribution—ordered mesopores for rapid ion transport and rapid charging of EDLCs and small disordered micropores for high volumetric capacitance. (Reproduced with permission from Ref 127. Copyright 2010, American Chemical Society)
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Performance of some of the advanced high surface area carbons in EDLC applications: (a) specific capacitance of polymer‐derived activated carbons in an ionic liquid electrolyte at elevated temperature (Reproduced with permission from Ref 43. Copyright 2011, John Wiley and Sons, Ltd), (b) low temperature cyclic voltammetry of ZTC recorded at the rate of 1 mV/second in an organic electrolyte (Reproduced with permission from Ref 98. Copyright 2012, John Wiley and Sons, Ltd), (c) volumetric capacitance of micro‐EDLC with OLC electrodes in an organic electrolyte compared to that of typical electrolytic capacitor, EDLC and micro‐EDLC with AC electrode recorded at ultra‐high scan rates (Reproduced with permission from Ref 99. Copyright 2010, Nature), (d) room temperature cyclic voltammetry of activated graphene in an organic electrolyte. (Reproduced with permission from Ref 100. Copyright 2011, The American Association for the Advancement of Science)
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SEM micrographs of high surface area carbon materials: (a) AC fabrics (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd), (b) AC fibers (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd), (c) CNT fabric, (d and e) vertically aligned CNT forest (Reproduced with permission from Ref 18. Copyright 2013, John Wiley and Sons, Ltd), (f) randomly oriented CNTs within CNT paper mats. (Reproduced with permission from Ref 19. Copyright 2012, Royal Society of Chemistry)
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Electron microscopy images of high surface area carbon materials: (a) scanning electron microscopy (SEM) of micron‐scale micro‐/meso‐/macro‐porous CDC (Reproduced with permission from Ref 12. Copyright 2006, Elsevier), (b) SEM of mesoporous silica‐templated CDC (Reproduced with permission from Ref 13. Copyright 2011, John Wiley and Sons, Ltd), (c) SEM of AC particles, (d) SEM of ZTC (Reproduced with permission from Ref 14. Copyright 2010, American Chemical Society), (e) SEM of multilayer graphene flakes (Reproduced from Ref 15. Copyright 2011, Wiley‐VCH), (f) transmission electron microscopy (TEM) of carbon onions. (Reproduced with permission from Ref 16. Copyright 2007, Elsevier)
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Schematic energy diagrams of (a and b) regular and (c) doped EDLC electrodes as well as (d) electrodes in an asymmetric capacitor upon charging and discharging. In (b) doping of one of the electrodes compensates for the difference in the capacitance between positive and electrodes, thereby increasing the maximum operational voltage of an EDLC. In (d) a battery‐like graphitic negative electrode operates at the potential below the decomposition limit of the organic electrolyte. However, the layer of decomposed electrolyte (called SEI) is electrically isolative and is stable, which prevents further electrolyte decomposition. The area within the discharge curves (colored in slightly darker green or darker blue) is proportional to the energy stored in each device.
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Examples of the large‐volume applications of electrochemical capacitors: (a) an electric bus with a small distance range but fast (1 min) charging (Yaxing, China) (Reproduced from China Buses.com), (b) a hybrid energy‐efficient forklift (Still, Germany) (Reproduced with permission from Still GmbH), (c) off‐shore variable speed wind turbines with enhanced reliability and efficiency (Reproduced with permission, Copyright: F. Schmidt/Fotolia), (d, e) hybrid energy‐efficient automated stacking and harbor cranes (Gottwald, Germany) (Reproduced with permission of Gottwald Port Technology GmbH), (f) an electric ferry with rapid charging and low vibration operation (STX Europe, South Korea). (Reproduced with permission from STX Europe)
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