A Review for Aqueous Electrochemical Supercapacitors (PDF Download Available)
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48.34Jilin UniversityAbstractElectrochemical capacitor is the most promising energy storage device that can meet the demands of high power supply and long cycle life, however low energy density and high fabrication cost limit its further development. Researchers have paid more attention to the development of electrode material in the past, and very few people attach importance to the research of the electrolyte, especially the redox electrolyte, which is important for improving specific capacitance greatly. This paper presents a review of the research in not only electrode material but also redox aqueous electrolyte and together with an important part of supercapacitor device. The advantages and disadvantages for different electrode material and electrolyte are discussed. And the new trends in supercapacitor development are also summarized.Discover the world's research14+ million members100+ million publications700k+ research projectsFigures
REVIEWpublished: 08 May 2015doi: 10.3389/fenrg.Edited by:Peter G. Bruce,University of St. Andrews, UKReviewed by:Jinping Liu,Wuhan University of Technology,ChinaJie Xiao,Pacific Northwest NationalLaboratory, USAShichun Mu,Wuhan University of Technology,China*Correspondence:Weitao Zheng,Department of Material Science, Keylaboratory of Automobile Materials ofMOE, and State Key Laboratory ofSuperhard Materials, Jilin University,Changchun 130012, P. R. Chinawtzheng@Specialty section:This article was submitted to EnergyStorage, a section of the journalFrontiers in Energy ResearchReceived: 03 December 2014Accepted: 24 April 2015Published: 08 May 2015Citation:Zhao C and Zheng W (2015) A reviewfor aqueous electrochemicalsupercapacitors.Front. Energy Res. 3:23.doi: 10.3389/fenrg.A review for aqueouselectrochemical supercapacitorsCuimei Zhao 1and Weitao Zheng 2*1Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Ministry of Education, College ofChemistry, Jilin Normal University, Siping, China, 2Department of Materials Science, Key Laboratory of Mobile Materials,Ministry of Education and State Key Laboratory of Superhard Materials, Jilin University, Changchun, ChinaElectrochemical capacitor is the most promising energy-storage device that can meet thedemands of high-power supply however, low-energy density and high-fabrication cost limit its further development. Researchers have paid more attention to thedevelopment of electrode material in the past, and very few people attach importance tothe research of the electrolyte, especially the redox electrolyte, which is important forimproving specific capacitance greatly. This paper presents a review of the researchin not only electrode material but also redox aqueous electrolyte and together with animportant part of supercapacitor device. The advantages and disadvantages for differentelectrode material and electrolyte are discussed. And the new trends in supercapacitordevelopment are also summarized.Keywords: electrode, electrolyte, additive, supercapacitor, energy density, power density, cycling stabilityIntroductionEnergy is an important issue all over the world. The rapid deterioration of environment anddepletion of fossil fuels call not only for more clean and renewable energy sources but also moreadvanced energy conversion/storage systems. Research efforts have mostly focused on two typesof electrochemical devices: batteries and capacitors. As shown in Figure 1 (Simon and Gogotsi,2008), batteries suffer from low-power density although they have high-energy density, whereasconventional capacitors exhibit high power but low-energy density. Supercapacitor, also knownas an electrochemical capacitor (EC), has bridged the gap between battery and conventionalcapacitor, because of the great advantages including high power and energy supply, long cyclelife, flexible operating temperature, and environmental friendliness (Conway, 1999;Burke, 2000,2010;Frackowiak and Beguin, 2001;Pandolfo and Hollenkamp, 2006;Frackowiak, 2007;Zhangand Chen, 2008;Zhang and Zhao, 2009;Conte, 2010;Inagaki et al., 2010;Zhai et al., 2011;Chenet al., 2013a;Zhi et al., 2013). ECs are widely used in consumer electronics, hybrid vehicles, andindustrial power/energy managements. However, the disadvantages of low-energy density and high-fabrication cost for ECs have been identified as a major challenge for the capacitive storage science.To meet the energy demands for practical applications, advanced supercapacitors must be developedwith high-energy density without sacrificing the power density and cycle life. The energy density (E)can be obtained by the total capacitance (C) and the cell voltage (V) based on Eq. 1:E=1/2CV2(1)Therefore, there have already appeared two methods in increasing energy density. First,to improve the total capacitance, the most intensive approaches include the discovery ofadvanced electrode materials and the improved understanding of ion transport mechanismFrontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 231
Zhao and Zheng A review for aqueous electrochemical supercapacitorsFIGURE 1 |Specific power against specific energy, also called aRagone plot, for various electrical energy-storage devices. The shadedcurves were obtained from Simon and Gogotsi (2008).in small pores (Zhai et al., 2011); however, the improvement intotal capacitance is not significant only by electrode materials.Very recently, there have been a few reports that redox additivesare introduced into the conventional electrolyte, with additionalpseudocapacitive attribution of the additives between the elec-trode and electrolyte, the capacitance is obviously enhanced (Suet al., 2009;Roldán et al., 2011;Wu et al., 2012;Chen et al.,2013b;Senthilkumar et al., 2013). Second, to improve the cellvoltage, asymmetric supercapacitors combining electric double-layer anode and redox reaction cathode show promising capacitiveperformance.Electrode MaterialsIn principle, capacitive behavior can be classified into two types:the electrical double-layer capacitance (EDLC) arising from elec-trostatic attraction between electrolyte a thepseudocapacitance associated with fast and reversible faradic reac-tions of the active species on the surface of the electrode (Zhangand Zhao, 2009;Zhai et al., 2011). Therefore, according to thefundamental energy-storage mechanisms, the typical electrodematerial is EDLC material, such as various carbon materials. Theother is pseudocapacitance material based on transition-metaloxides/hydroxides and conducting polymer. Besides, MXenes andMOF material are the latest electrode material.Carbon MaterialsCarbon materials, corresponding to EDLC, are considered themost ideal materials for ECs for high-specific surface area, goodelectronic conductivity, excellent chemical stability, easy process-ing, lower cost, and wide operating temperature range (Zhanget al., 2009a). A variety of carbon materials are involved, suchas activated carbon (AC) (Ricketts and Ton-That, 2000;Pognonet al., 2012), carbon aerogels (Saliger et al., 1998;Wei et al., 2005),carbon nanotubes (CNTs), carbon nanofibres (An et al., 2001;Zhang et al., 2009b), and so on. Conventional AC, with a theoret-ical capacitance of 100 ~ 300 F g-1, exhibits an excellent cyclingstability in several electrolytes. Recently, graphene nanosheets,two-dimensional layers of sp2-bonded carbon, have been foundto be an ideal carbon electrode material for ECs (Stoller et al.,2008;Zhao et al., 2009a;Zhu et al., 2010;Le et al., 2011), withan ideal capacitance of 550 F g-1when the specific surface areaof 2675 m2g-1is fully used (Liu et al., 2010).A lot of research has been done for various carbon materials athome and abroad, but the study was only limited to improving thepore size distribution, surface area, and surface functional groupsof the carbon material. However, the specific capacitance cannotbe improved significantly because of double-layer capacitancemechanisms. At present, the combination of carbon material andpseudocapacitive material can bring synergistic effect, realizingthe overall system excellent electrochemical performance.Metal Oxides/HydroxidesBased on the faradic pseudocapacitive energy-storage mecha-nism, metal oxides/hydroxides, such as ruthenium, manganese,cobalt, nickel, and vanadium, can provide higher specific capac-itance than conventional carbon materials and better cyclingstability than polymer materials.Ruthenium oxide, with wide potential window, high-redoxreversibility and high conductivity shows remarkably high-energydensity, power density, and cycling stability (Sakiyama et al., 1993;Jia et al., 1996;Kim and Kim, 2006;Lee et al., 2010). For example,the capacitance can be as high as 1300 F g-1for nanotubulararrayed electrode of hydrous ruthenium oxide (Hu et al., 2006).However, the high cost for precious metal becomes a major barrierfor the commercialization. The present study mainly concentratedin two aspects: first, composite ruthenium oxide with carbon orcheap metal oxide material, to improve the utilization ratio of second, seek cheap metal oxide/hydroxide toreplace precious metals (Zheng and Jow, 1996;Fang et al., 2001;Hu and Huang, 2001;Jeong and Manthiram, 2001;Ke et al., 2005;Zhao et al., 2012a;Sellam and Hashmi, 2013). For example, Zhanget al. (2014) have reported a facile hydrothermal method withoutreducer to fabricate graphene-RuO2nanocomposites, achievinghigh-specific capacitance in alkaline/acidic/neutral electrolyte.Wang and Zhang (2004) have designed to load Ru1-yCryO2onthe TiO2nanotubes with a maximum specific capacitance of1272.5 F g-1obtained. Partially due to the discovery of advancedelectrode materials and new synthetic method, more and moreelectrode materials with high performance have been developed(Simon and Burke, 2008;Simon and Gogotsi, 2008). In recentyears, inexpensive pseudocapacitive active materials, such asCo3O4/Co(OH)2(Simon and Burke, 2008;Wang et al., 2009;Gaoet al., 2010;Xu et al., 2010;Yuan et al.,2010,2012;Anantharamuluet al., 2011;Asano et al., 2011;Xia et al., 2011a;Huang et al.,2012), NiO4/Ni(OH)2(Song and Gao, 2008;Yuan et al., 2009,Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 232
Zhao and Zheng A review for aqueous electrochemical supercapacitors2011;Zhang et al., 2010;Cao et al., 2011;Jiang et al., 2011;Xia et al.,2011b;Zhu et al., 2012), MnOx(Ragupathy et al., 2009;Yan et al.,2010a;Wang et al., 2011;Yu et al., 2011a,b;Lee et al., 2012a, 2013;Song et al., 2012;Si et al., 2013), Fe3O4(Obradovic et al., 2009;Zhao et al., 2009b;Lee et al., 2012b),and so on have attracted theattentions of the researchers. Chang et al. (2010) created nanos-tructure Co(OH)2electrode, exhibiting a high pseudocapacitanceof 2800 F g-1and an outstanding rate capability and cyclic stabil-ity. Wang et al. have reported a facile synthesis of nickel cobaltlayered double hydroxides (LDHs) on conducting Zn2SnO4byCVD and electrochemical deposition. This novel material demon-strates an outstanding electrochemical performance with a high-specific capacitance of 1805 F g-1at 0.5 A g-1, and an excellentrate performance of 1275 F g-1at 100 A g-1(Wang et al., 2012a).Zhao et al. (2011) have prepared layered Co(OH)2, showing ahigh-specific capacitance of 651 F g-1, but only 76% of initialcapacitance remains after 500 cycles at 50 mV s-1. With higherspecific capacitance, the cycling stability of the pseudocapaci-tive materials has been sacrificed. This drawback must be over-come (Simon and Gogotsi, 2008). To improve low conductivityand poor stability, such inexpensive pseudocapacitive materialsare often incorporated into highly conductive nanostructuredmaterials. Recently, composite electrode material has been stud-ied for ECs, showing good electrochemical performance thanthat of pure oxide/hydroxide. For example, a facile strategy isdesigned to deposit Co(OH)2nanoparticles on graphene sheets ina water-isopropyl alcohol system. The specific capacitance of thegrapheme/Co(OH)2nanocomposite reaches 972.5 Fg-1, leadingto a significant improvement compared to each individual coun-terpart (Chen et al., 2010). Yan et al. (2010b) have compositedgraphene and Co3O4, exhibiting a high cycle stability (95.6%specific capacitance is retained after 2000 cycles); however, thespecific capacitance is only 243.2 F g-1. Unfortunately, most ofthe reported oxide/graphene materials have been prepared bychemical precipitation method, there exist some problems, theparticles are agglomerated easily and a binder is often required,that could decrease the active surface area and increase the inter-nal resistance of the ECs. To overcome the disadvantages, we havefocused on binder-free Ni(OH)2/graphene/Ni foam (Wang et al.,2012b) or Co(OH)2/graphene/Ni foam (Zhao et al., 2012b) elec-trode prepared by a combination of PECVD and electrodepositionmethod, which could be explored as a promising material for ECsbecause of high-electrochemical activity and cycle stability. Fromthe discharge curves of Figures 2A,B, the Ni(OH)2/graphene/Nifoam electrode maintains 93.3% of the initial specific capacitanceafter 500 cycles at a high-current density of 60A g-1, while underthe same conditions only 54% specific capacitance retains for theNi(OH)2/Ni foam electrode, which shows the obvious improve-ment of cycling stability for the composite electrode (Wang et al.,2012b). In addition, a highly porous Co(OH)2/graphene film onNi foam has been obtained, as shown in Figure 3. The correspond-ing electrochemical measurement technology also confirmed thatthe introduction of graphene between Co(OH)2and Ni foamdemonstrates an obvious enhancement of electrochemical sta-bility compared with Co(OH)2/Ni foam (Zhao et al., 2012b).The excellent pseudocapacitive behavior can be explained to thefollowing (Wang et al., 2012b;Zhao et al., 2012b): (1) graphenecan provide high-electrical conductivity and high-specific surfaceFIGURE 2 |Discharge curves of (A) the Ni foam/graphitenanosheets/Ni(OH)2electrode at the current density of 60 A g-1for700 cycles and (B) the Ni foam/Ni(OH)2electrode at the same currentdensity for 500 cycles. The inset in (A) shows the charge–discharge curvesof the composite electrode. The curves were obtained from Wang et al.(2012b).area, allowing rapid and effective ion c (2) Ni(OH)2or Co(OH)2nanosheets with high-electrochemical activity aredirectly grown on graphene, with the covalent chemical bonding(C–O–Co or C–O–Ni) formed, favoring the electrochemical sta- (3) graphene has been directly deposited on the Ni-foamcollector, can avoid increasing the contact resistance between theelectrode and the collector. In order to further improve capacitiveproperties, the effects of crystallinity and electroconductivity ofcarbon nanomaterials on Co(OH)2are explored (Zhao et al.,2013a). As shown in Figure 4, the Co(OH)2nanosheet has beenelectrochemically deposited on graphene nanosheets (GNS) oramorphous carbon (APC). The electrochemical test results arelisted in Table 1, Co(OH)2/GNS exhibits a high cycling stabil-ity, which is due to that the GNS with high conductivity andexcellent flexibility can transfer smooth electrons and accom-modate the volume expansion/contraction for Co(OH)2in thecharge–discharge process. By contrast, Co(OH)2/APC shows ahigh-specific capacitance, which can be ascribed to the lower con-ductivity for APC, leading to the larger nanosheets of Co(OH)2,promoting the complete redox reaction of Co(OH)2(Zhao et al.,2013a). The carbonaceous supports can not only act as a templateto direct the growth of the as-deposited hydroxide but also affectthe conductivity of the whole electrode, which may provide aFrontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 233
Zhao and Zheng A review for aqueous electrochemical supercapacitorsFIGURE 3 |(A,B) SEM images for Co(OH)2/GNS on Ni foam and (C) TEM images for Co(OH)2/GNS. Obtained from Zhao et al. (2012b).technical reference for improvement in the pseudocapacitancebehaviors and controllable preparation of composite electrodematerial in the future. Besides hybrid metal oxide with carbonmaterial, a novel hybrid metal oxide core/shell nanowire arraysfor ECs has been constructed. For example, Liu et al. chooseMnO2and Co3O4as the shell and core materials, respectively.The hybrid nanowire array exhibit a high capacitance (480 F g-1at 2.67 A g-1) and good cycle performance (97.3% capacitanceretention after 5000 cycles), which is 4~ 10-fold increase respectto pristine Co3O4array. With this electrode design method, thefunctions of each pseudocapacitive constituent are effectively uti-lized, and realize a synergistic effect. Growing from the Co3O4nanowire scaffold directly, the surfaces of MnO2nanosheets arewell separated, making active material fully available to the Li+inthe electrolyte. As ultrathin MnO2nanosheets construct a highlyporous structure on Co3O4nanowire, the Co3O4core nanowirescan be accessed by OH-and initiate the redox reaction (Liuet al., 2011). The concept opens up the possibility of constructinghigh-performance pseudocapacitive materials without using anycarbon-based or conducting media.Conducting PolymersConducting polymers can provide capacitance behavior throughthe faradic process. When oxidation takes place, ions aretransferred to the polymer backbone, and when reductionoccurs, the ions are released from this backbone into theelectrolyte (Sharma and Bhatti, 2010). Conducting polymerswidely used in supercapacitor mainly contain polyaniline(PANI), polythiophene (PTh), polypyrrol (PPy), and theircorresponding derivatives, due to its advantageous propertiesof low cost, high-voltage window, and high-doping rate duringcharge–discharge process. However, high resistance and lowstability limit the wide application. Especially, swelling andshrinkage may occur during charge–discharge processes, leadingto mechanical degradation of the electrodes and fading of thecapacitive performance.Fabricating composite electrode materials to improve theconductivity and stability become the new development direc-tion. For example, a high-performance polyaniline electrode hasbeen prepared by electrochemical deposition on a porous carbonmonolith, showing the advancement of high-specific capacitance,excellent rate capability, and good cycling stability. A capacitancevalue as high as 2200 F g-1is obtained at a current density of0.67 A g-1, and the specific capacitance is still up to 1270 F g-1even at the high-current density of 66.7A g-1(Fan et al., 2007).Tao et al. (2014) have developed PPy/MnO2composite througha simple electrodeposition process. The highest areal capacitanceof the PPy/MnO2electrode is about 2.45 F cm-2at a currentdensity of 0.2 mA cm-2, and the capacity retention is 98.6% after1000 cycles.Latest Electrode MaterialsRecently, MXenes have been prepared combining hydrophilicsurfaces with good conductivities. MXenes are synthesized bythe extraction of the “A” layers from the layered carbides orcarbonitrides known as MAX phases. MXenes show promiseas electrode materials for Li-ion batteries and Li-ion cahowever, many are theoretically predicted (Khazaei et al., 2012;Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 234
Zhao and Zheng A review for aqueous electrochemical supercapacitorsFIGURE 4 |SEM images for (A) GNS, (B) APC, (C) Co(OH)2/GNS, and (D) Co(OH)2/APC before discharge/ (E) Co(OH)2/GNS and(F) Co(OH)2/APC after 2000 discharge/charge cycles. Obtained from Zhao et al. (2013a).Kurtoglu et al., 2012). Maria et al. have reported an intercalation-induced high capacitances of Ti3C2Txpaper electrodes in aqueouselectrolytes (KOH or NaOH), 350 F cm-1at 1 A g-1and 60% ratecapability from 2 to 100 mV s-1achieved (Lukatskaya et al., 2013).The Ti3C2Txmaterial opens the door for the use of MXenes inenergy-storage devices. As noted, the reported capacitances areprobably far from the maximum values possible for MXenes ingeneral. A variety of ions should be accommodated between theMXene layers.Compared to inorganic high-surface-area materials, MOFsoffer the theoretical advantage of 100% utilization and improvedaccessibility of the metal-cation centers due to their regular 3Ddispersion in an openstructure. For example, the introduction ofMOFs in EDLCs has been reported only recently (Diaz et al., 2012;Lee et al., 2012c). Lee et al. have synthesized a Co-based MOF fromcobalt nitrate and terephthalic acid, and the capacitive behaviorwas investigated in various 1M aqueous electrolytes (LiOH, KCl,LiCl, and KOH). Only in LiOH electrolyte, did the Co-based MOFshows an interesting pseudocapacitive behavior with a specificcapacitance of 150 ~ 200 F g-1and 98.5% capacitance retentionafter 1000 cycles. However, to date, it is difficult to find that MOFselectrode materials is clear advantaged in performance, durability,or cost compared to other materials. For real breakthroughs,MOFs will probably need to be designed and optimized, with con-trolling the MOF particle size, improving conductive character,and so on.ElectrolyteBesides two electrodes, electrolyte plays an important role incapacitor performance, such as ion supplementary, electric chargeconduction, and electrode particles adhesive. The requirementsfor an electrolyte in ES include wide voltage window, high-electrochemical stability, low resistivity, low toxicity, and so on.Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 235
Zhao and Zheng A review for aqueous electrochemical supercapacitorsTABLE 1 |Electrochemical properties of Co(OH)2/GNS, Co(OH)2/APC andCo(OH)2/GNS-APC.Active mass Co(OH)2/GNS Co(OH)2/APC Co(OH)2/GNS-APCCurrent density2 A g-1692.0 F g-11287.2 F g-1784.0 F g-14 A g-1653.1 F g-11194.9 F g-1754.9 F g-18 A g-1610.9 F g-11051.6 F g-1724.4 F g-116 A g-1567.3 F g-1904.7 F g-1692.4 F g-132 A g-1517.8 F g-1762.2 F g-1651.6 F g-1Maintained capacitance2 ~ 32 A g-174.8% 59.2% 83.1%capacitance after 2000 cpat 40 A g-195.7% 83.8% 93.2%Bold are the best pseudocapacitive properties including cycling stability, specific capac-itance, and rate capability among the three materials, further confirmed that differentcarbon nanomaterials have different influences on the electrochemical properties ofCo(OH)2.This table was obtained from Zhao et al. (2013a).The electrolyte used in ECs manly includes aqueous electrolyte,organic electrolyte.Aqueous ElectrolytesAs far as the electrolytes are considered, aqueous electrolytes (suchas KOH, H2SO4, Na2SO4aqueous solution, and so on) have theadvantages of high-ionic conductivity, low cost, non-flammability,non-corrosiveness, safety, and convenient assembly in air, com-pared to organic electrolytes, which are believed to be less con-ductive, expensive, usually flammable, and higher toxic. However,the potential window (~1.2 V) for aqueous electrolytes is far lowerthan that of organic electrolyte. According to Eq. 1, low-potentialwindow is the reason of low-energy density that limits its marketapplication. The introduction of redox active substance in aqueouselectrolytes can effectively enhance the capacitance via extra redoxreaction between the electrode and electrolyte, which is shown inthe part of symmetric supercapacitors.Organic ElectrolyteThe restriction of aqueous electrolytes is the limited operatingvoltage (about 1.2 V). By contrast, organic electrolytes can providea high-operating voltage up to 4V. High-potential window isa large advantage for organic electrolyte. Among organic elec-trolytes solvents, acetonitrile (AN) and propylene carbonate (PC)are the most common. Besides, organic salts such as tetraethylam-monium tetrafluoroborate and tetraethylphosphonium tetrafluo-roborate have also been used in ECs electrolytes. However, thereexist the following problems for organic electrolyte: first, highresistance limits the power density of ca second, high watercontent limits the working voltage of capacitor.SupercapacitorSymmetric SupercapacitorsSymmetric supercapacitors with positive and negative electrodematerial identical or similar can be divided into electric double-layer capacitor and faradic capacitors according to electrodematerial working mechanism. Symmetric supercapacitors aremainly carbon//carbon, Ru//Ru, and cheap metal symmetricsupercapacitor.Electric double-layer capacitor contains carbon material withhigh-surface area as the electrode material in aqueous or organicelectrolytes. Compared to organic electrolytes, aqueous electrolyteis believed to high-ionic conductivity, low cost, and convenientassembly of ECs in air. However, EDLCs only utilize the electro-static charge at the electrolyte–electrode surface to store electricenergy so that the capacitance is limited. Very recently, an effectiveapproach has been reported wherein redox additives are intro-duced into the conventional electrolyte to enhance the capacitancevia extra redox reaction between the electrode and electrolyte. Yuet al. (2012) have doped m-phenylenediamine into a conventionalKOH electrolyte for carbon-based ECs. The specific capacitanceof the supercapacitor based on the new electrolyte is 78.01F g-1,showing an increase of 114.16% over that of a supercapacitorbased on a conventional KOH electrolyte. Similarly, Roldán et al.(2011) have reported that via adding electrochemically activecompound quinone/hydroquinone (Q/HQ) into the H2SO4elec-trolyte, an ultrahigh specific capacitance of 5017F g-1achievedfor carbon-based EC. The significantly improved capacitanceattributes to that the faradaic reactions of the Q/HQ couples(HQ 2H+2e-←→ Q)contribute pseudocapacitance to the electrode.However, working condition is not easily maintained, leadingto a poor electrochemical stability. Senthilkumar et al. (2013)have improved the capacitance for AC materials through addingeither KI or KBr into 1M H2SO4or Na2SO4electrolyte. With animproved specific capacitance, there is also a problem with theelectrochemical stability for only relying on the pseudocapacitivecontribution from additive for EDLCs.Although more attention has been paid to investigating redoxelectrolyte for enhancing the EDLCs, no any efforts have beendone to improve pseudocapacitors. We have aimed at improv-ing the pseudocapacitive behavior of Co(OH)2/GNS electrodethrough adding K3Fe(CN)6into KOH electrolyte, in our CV testresult, two symmetric anodic/cathodic pairs are superimposedon a broad redox background, corresponding to the redox reac-tion of Co(OH)2electrode and K3Fe(CN)6electrolyte, respec-tively, indicating that reversible redox reactions of Co(OH)2solid electrode and K3Fe(CN)6liquid electrolyte occur simul-taneously and independently (Zhao et al., 2013b). The cyclingstability for Co(OH)2/GNS electrode either in KOH electrolyte orK3Fe(CN)6+KOH electrolyte is examined by charge–dischargetests for 2000 cycles at a high current density of 80A g-1. Bothsystems exhibit a high cycling stability, with retention of 94.4 and91.1% initial capacitance after 2000 cycles. This indicates that theaddition of K3Fe(CN)6into KOH electrolyte does not affect thestability of the electrode material. In addition, the system canbe charged quickly, and discharged slowly, which means that thepromise can be offered to realize a high-performance battery-typesupercapacitor.Although the cost is relatively high, Ru//Ru symmetric super-capacitor shows large specific capacitance, high potential win-dow, and excellent cycling stability. For example, Xia et al.(2014) have developed 1.8 V symmetric supercapacitor usingnanocrystalline Ru films as both negative and positive electrodes.The Ru//Ru symmetric supercapacitor exhibits a large arealFrontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 236
Zhao and Zheng A review for aqueous electrochemical supercapacitorsTABLE 2 |Electrochemical performance of the asymmetric supercapacitors based on conventional electrode materials.Electrode Electrolyte Capacitance(F g-1)Potentialwindow (V)Energy density(Wh kg-1)Power density(kW kg-1)Stability ReferenceNaMnO2//AC Na2SO438.9 1.9 19.5 0.13 97% (10,000 cp) Qu et al. (2009)K0.27MnO2//AC K2SO4 57.7 1.8 25.3 0.14 98% (10,000 cp) Qu et al. (2010)NiO//AC KOH 38 1.5 – – 50% (1000 cp) Wang et al. (2008)MnO2-AC//AC Na2SO433.2 2.0 18.2 10.1 92% (2500 cp) Gao et al. (2011)TABLE 3 |Electrochemical performance of the asymmetric supercapacitors based on new electrode materials.Electrode Electrolyte Capacitance(F g-1)Potentialwindow (V)Energy density(Wh kg-1)Power density(kW kg-1)Stability ReferencerGO//MnO2-rGO Na2SO459.9 1.6 21.2 0.82 89.4% (1000 cp) Wu et al. (2014)MnO2-NF//G Na2SO4– – 30.2 14.5 83.4% (5000 cp) Gao et al. (2012)NiCo2O4-Co0.33Ni0.67(OH)2//CMK-3KOH 87.9 1.6 31.2 396 82% (3000 cp) Xu et al. (2014)MnO2–NPG//PPy–NPG LiClO4193 1.8 86 25 85% (2000 cp) Hou et al. (2014)Co(OH)2//VN KOH 62.4 1.6 22 19.5 80% (4000 cp) Wang et al. (2014a)NiO-GF//HPNCNTs KOH 116 1.4 32 0.7 94% (2000 cp) Wang et al. (2014b)CoO-PPy//AC NaOH – 1.6 43.5 5.5 91.5% (20,000 cp) Zhou et al., 2013NiCo2O4-rGO/AC KOH 99.4 1.3 23.3 0.32 93% (2500 cp) Wang et al. (2012c)capacitance (68 mF cm-2), high cycling stability (no capacitanceloss after 2000 charge–discharge cycles), and good rate capability.Similarly, a novel symmetric RuO2/RuO2supercapacitor with ahigh-operating voltage of 1.6V is also built using the nanocrys-talline hydrous RuO2electrode, exhibiting an energy density of18.77 Wh kg-1at a power density of 500 W kg-1(Xia et al., 2012).For other inexpensive transition-metal oxides symmetriccapacitors, working potential window are relatively low (nomore than 1 V), leading to relatively low-energy density. Theresearchers try to improve working potential window for thecheap metal oxides symmetric supercapacitor, but the effect isnot obvious. For example, Reddy et al. (2009) have synthesizedAu–MnO2/CNT hybrid coaxial nanotube arrays and preparedMnO2//MnO2symmetric capacitor. CNTs serve as an additivefor improving the electrical conductivity of the manganese oxide,leading high-specific capacitance, power density, and cycling sta- however, the potential window is only 0.7 V. Lu et al.(2012) have synthesized nickel–cobalt oxide (NCO) nanosheetson FTO substrates and prepared a symmetric supercapacitorbased on two NCO electrodes, exhibiting a high-specific capac-itance of 89.2 F g-1at 0.17 A g-1, but a low-potential windowof 0.5 V.Asymmetric SupercapacitorsAs an improvement to symmetric supercapacitors, asymmetricsupercapacitors combining electric double-layer anode (carbonmaterials) and redox reaction cathode (such as metal hydrox-ide/oxide materials, Li-ion battery materials, composite electrodematerial) show promising capability to enhance the energy den-sity, that is because asymmetric supercapacitors can make fulluse of the different potential windows of the two electrodes toincrease the operation voltage and enhanced specific capacitancein the cell system. More recently, to obtain higher energy density,considerable research efforts have been devoted to the various ofasymmetric capacitor systems, mainly including AC//LiMn2O4(Qu et al.,2009,2010), AC//NiO (Wang et al., 2008), AC//MnO2(Gao et al., 2010), and so on. Table 2 summarizes the electrochem-ical performances of these conventional asymmetric supercapaci-tors. For these asymmetric supercapacitors, AC is commonly usedas a typical negative electrode material, metal oxides are employedas the positive electrode material. The poor specific capacitanceof AC and low stability of metal oxides/hydroxides would alsogreatly restrict the supercapacitor performance, resulting rela-tively low-energy density (10 ~ 30 Wh kg-1), power density, andcycling stability. Recently, partially due to the discovery of newelectrode materials and new synthesis method, advanced superca-pacitors with high performance have been developed. As shown inTable 3, asymmetric supercapacitors with high-performance elec-trode materials as cathode and anode materials show significantsuperiority in energy density (~90 Wh kg-1), power density, andcycling stability.Trends in ESElectrochemical capacitors with high-power density and cyclingperformance have bridged the gap between conventional capaci-tors and batteries. However, one of the key challenges for ES is thelimited energy density. To overcome this challenge, ECs researchshould focus on improving the specific capacitance and widenpotential window.(1) Improve the specific capacitance. One common strategy isto design and synthesis new materials, which must satisfythe following conditions: first, good conductivity for efficient second, high-specific surface area for more third, good mechanical and electrochemical sta-bility for good last, favorable pores-izedistribution for high-rate ions diffusion. The ECs electrodematerial exploration directions are composite materials andnanomaterials, with coating active materials with conductiveFrontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 237
Zhao and Zheng A review for aqueous electrochemical supercapacitorscarbon or constructing a novel hybrid metal oxide nano-structure electrode for ECs. Nanocomposite can real-ize synergistic effect, facilitating electron and protonconduction, enhancing specific surface area, expandingactive sites, inducing porosity, protecting active materialsfrom mechanical degradation, improving cycling stability.Another effective one is to pursue new electrolyte withhigh-electrochemical activity and reversibility, contributingadditional pseudocapacitance in specific potential window,and the problem of electrochemical stability for thecomposite electrode system should to be paid more attentionfurther.(2) Widen the potential window. The strategy of developing andmatching asymmetric capacitors can make use of the differ-ent potential windows of the two electrodes to increase theoperation voltage in the cell system. However, the matchingproblem appears between positive and negative electrode. Ifthe electrodes are matched well, the overall performance ofthe supercapacitor will be improved. Otherwise, its perfor-mance may drop or the supercapacitor may be damaged.Solid-state supercapacitors (SSCs) are emerging as energy-storage devices due to excellent stability, light weight, and easy tohandle, however, suffer from poor rate capability due to limitedion-diffusion rate in solid-state electrolytes, large resistance of theelectrode material. Asymmetric capacitors with redox electrolytecan exhibits large specific capacitance and wide potential window,resulting in an enhanced energy density. The supercapacitor canbe regarded as a novel hybrid supercapacitor application thatcombines two energy-storage processes: the double-layer capaci-tance or faradaic pseudocapacitance characteristic from electrodematerials and the faradaic pseudocapacitance from electrolyte.It is important to design high-performance electrode materials,choose the suitable electrolyte, solve well matching problem. Withthe high resistance and low charge–discharge stability resolved,such asymmetric supercapacitor is expected to be a highly promis-ing candidate for application in high-performance energy-storagesystems.AcknowledgmentsThe support from National Natural Science Foundation of China(Grant Nos.
and ), the special Ph.D. program(Grant No. ) from MOE, Major science and tech-nology project of Jilin Province (Grant No. 11ZDGG010), NSFof China (grant no. ), program for Changjiang Scholarsand Innovative Research Team in University, the “211” and “985”project of Jilin University, China, is highly appreciated.ReferencesAn, K. H., Kim, W. S., Park, Y. S., Choi, Y. C., Lee, S. M., Chung, D. C., et al. (2001).Supercapacitors using single walled carbon nanotube electrodes. Adv. Mater.13, 497–500. doi:10.95(:7& 497::AID-ADMA497& 3.0.CO;2-HAnantharamulu, N., Rao, K. K., Rambabu, G., Kumar, B. V., Radha, V., and Vithal,M. (2011). A wide-ranging review on Nasicon type materials. J. Mater. Sci. 46,. doi:10.-011- 5302-5Asano, Y., Komatsu, T., Murashiro, K., and Hoshino, K. (2011). Capacitance studiesof cobalt compound nanowires prepared via electrodeposition. J. Power Sources196, . doi:10.1016/j.jpowsour.Burke, A. (2000). Ultracapacitors: why, how, and where is the technology. J. PowerSources 91, 37–50. doi:10.-85-7Burke, A. (2010). Ultracapacitor technologiesand application in hybrid and electricvehicles. Int. J. Energ. Res. 34, 133–151. doi:10.1002/er.1654Cao, C. Y., Guo, W., Cui, Z. M., Song, W. G., and Cai, W. (2011). Microwaveassisted gas/liquid interfacial synthesis of flowerlike NiO hollow nanosphereprecursors and their application as supercapacitor electrodes. J. Mater. Chem.21, . doi:10.749DChang, J. K., Wu, C. M., and Sun, I. W. (2010). Nano-architectured Co(OH)2electrodes constructed using an easily-manipulated electrochemical protocolforhigh-performance energy storage applications. J. Mater. Chem. 20, .doi:10.fChen, L. F., Huang, Z. H., Liang, H. W., Yao, W. T., Yu, Z. Y., and Yu, S. H.(2013a). Flexible all-solid-state high-power supercapacitor fabricatedwith nitrogen-doped carbon nanofiber electrodematerial derivedfrom bacterial cellulose. Energ. Environ. Sci. 6, . doi:10.1039/C3EE42366BChen, W., Rakhi, R. B., and Alshareef, H. N. (2013b). Capacitance enhance-ment of polyaniline coated curved-graphene supercapacitors in a redox-activeelectrolyte. Nanoscale 5, . doi:10.773aChen, S., Zhu, J., and Wang, X. (2010). One-step synthesis of graphene-cobalthydroxide nanocomposites and their electrochemical properties. J. Phys. Chem.C114, 1. doi:10.1021/jp1048474Conte, M. (2010). Supercapacitors technical requirements for new applications.FuelCell 10, 806–818. doi:10.1002/fuce.Conway, B. E. (1999). Electrochemical Supercapacitors: Scientific Fundamentals andTechnological Applications. New York,NY: Kluwer Academic Publishers,PlenumPress.Diaz, R., Orcajo, M. G., Botas, J. A., Calleja, G., and Palma, J. (2012). Co8-MOF-5as electrode for supercapacitors. Mater. Lett. 68, 126–128. doi:10.1016/j.matlet.Fan, L. Z., Hu, Y. S., Maier, J., Adelhelm, P., Smarsly, B., and Antonietti, M. (2007).High electroactivity of polyaniline in supercapacitors by using a hierarchicallyporous carbon monolith as a support. Adv. Funct. Mater. 17, . doi:10.1002/adfm.Fang, Q. L., Evans, D. A., Roberson, S. L., and Zheng, J. P. (2001). Ruthenium oxidefilm electrodes prepared at low temperatures for electrochemical capacitors.J. Electrochem. Soc. 148, A833–A837. doi:10.9739Frackowiak, E. (2007). Carbon materials for supercapacitor application. Phys.Chem. Chem. Phys. 9, . doi:10.MFrackowiak, E., and Beguin, F. (2001). Carbon materials for the electrochemi-cal storage of energy in capacitors. Carbon N. Y. 39, 937–950. doi:10.1016/S)00183- 4Gao, H., Xiao, F., Ching, C. B., and Duan, H. (2012). High-performance asymmetricsupercapacitor based on graphene hydrogel and nanostructuredMnO2.ACSAppl. Mater. Interfaces 4, . doi:10.1021/am300455dGao, P. C., Lu, A. H., and Li, W. C. (2011). Dual functions of activated carbon in apositive electrode for MnO2-based hybrid supercapacitor. J. Power Sources 196,. doi:10.1016/j.jpowsour.Gao, Y., Chen, S., Cao, D., Wang, G., and Yin, J. (2010). Electrochemical capacitanceof Co3O4nanowire arrays supported on nickel foam. J. Power Sources 195,. doi:10.1016/j.jpowsour.Hou, Y., Chen, L., Liu, P., Kang, J., Fujita, T., and Chen, M. (2014). Nanoporous-metal based flexible asymmetric pseudocapacitors. J. Mater. Chem. A2,1. doi:10.969jHu, C. C., Chang, K. H., Lin, M. C., and Wu, Y. T. (2006). Design and tailoringof the nanotubular arrayed architecture of hydrous RuO2for next generationsupercapacitors. Nano Lett. 6, . doi:10.1021/nl061576aHu, C. C., and Huang, Y. H. (2001). Effects of preparation variables on thedeposition rate and physicochemical properties of hydrous ruthenium oxidefor electrochemical capacitors. Electrochim. Acta 46, . doi:10.1016/S)00543-6Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 238
Zhao and Zheng A review for aqueous electrochemical supercapacitorsHuang, S., Zhu, G. N., Zhang, C., Tjiu, W. W., Xia, Y. Y., and Liu, T. (2012). Immo-bilization of Co-Al layered double hydroxides on graphene oxide nanosheets:growth mechanism and supercapacitor studies. ACS Appl. Mater. Interfaces 4,. doi:10.1021/am300247xInagaki, M., Konno, H., and Tanaike, O. (2010). Carbon materials forelectrochemical capacitors. J. Power Sources 195, . doi:10.1016/j.jpowsour.Jeong, Y. U., and Manthiram, A. (2001). Amorphous tungsten oxide/rutheniumoxide composites for electrochemical capacitors. J. Electrochem. Soc. 148,A189–A193. doi:10.1002/chin.Jia, Q. X., Song, S. G., Wu, X. D., Cho, J. H., Foltyn, S. R., Findikoglu, A. T., et al.(1996). Epitaxial growth of highly conductive RuO2thin films on (100) Si. Appl.Phys. Lett. 68, . doi:10.715Jiang, H., Zhao, T., Li, C., and Ma, J. (2011). Hierarchical self-assembly of ultrathinnickel hydroxide nanoflakes for high-performance supercapacitors. J. Mater.Chem. 21, . doi:10.830JKe, Y. F., Tsai, D. S., and Huang, Y. S. (2005). Electrochemical capacitors of RuO2nanophase grown on LiNbO3 (100) and sapphire (0001) substrates. J. Mater.Chem. 15, . doi:10.CKhazaei, M., Arai, M., Sasaki, T., Chung, C. Y., Venkataramanan, N. S., Estili,M., et al. (2012). Novel electronic and magnetic properties of two-dimensionaltransition metal carbides and nitrides. Adv. Funct. Mater. 23, . doi:10.1002/adfm.Kim, I. H., and Kim, K. B. (2006). Electrochemical characterization of hydrousruthenium oxide thin-film electrodes for electrochemical capacitor applications.J. Electrochem. Soc. 153, A383–A389. doi:10.7406Kurtoglu, M., Naguib, M., Gogotsi, Y., and Barsoum, M. W. (2012). First principlesstudy of two-dimensional early transition metal carbides. MRS Commun. 2,133–137. doi:10.1557/mrc.2012.25Le, L. T., Ervin, M. H., Qiu, H., Fuchs, B. E., and Lee, W. Y. (2011). Graphenesupercapacitor electrodes fabricated by inkjet printing and thermal reduction ofgraphene oxide. Electrochem. commun. 13, 355–358. doi:10.1016/j.elecom.2011.01.023Lee, H., Cho, M. S., Kim, I. H., Nam, J. D., and Lee, Y. (2010). RuOx/polypyrrolenanocomposite electrode for electrochemical capacitors. Synth. Met. 160,. doi:10.1016/j.synthmet.Lee, J. W., Hall, A. S., Kim, J. D., and Mallouk, T. E. (2012a). A facile and template-free hydrothermal synthesis of Mn3O4nanorods on graphene sheets for super-capacitor electrodes with long cycle stability. Chem. Mater. 24, .doi:10.1021/cm203697Lee, K. K., Deng, S., Fan, H. M., Mhaisalkar, S., Tan, H. R., Tok, E. S., et al.(2012b). a-Fe2O3nanotubes-reduced graphene oxide composites as synergis-tic electrochemical capacitor materials. Nanoscale 4, . doi:10.1039/c2nr11902aLee, D. Y., Yoon, S. J., Shrestha, N. K., Lee, S. H., Ahn, H., and Han, S. H.(2012c). Unusual energy storage and charge retention in co-based metal-organic-frameworks. Microporous Mesoporous Mater. 153, 163–165. doi:10.1016/j.micromeso.Lee, S. H., Lee, H., Cho, M. S., Nam, J. D., and Lee, Y. (2013). Morphology andcomposition control of manganese oxide by the pulse reverse electrodeposi-tion technique for high performance supercapacitors. J. Mater. Chem. A 1,1. doi:10.828HLiu, C. G., Yu, Z. N., Neff, D., Zhamu, A., and Jang, B. Z. (2010). Graphene-basedsupercapacitor with an ultrahigh energy density. Nano Lett. 10, .doi:10.1021/nl102661qLiu, J., Jiang, J., Cheng, C., Li, H., Zhang, J., Gong, H., et al. (2011). Co3O4Nanowire@MnO2ultrathin nanosheet core/shell arrays: a new class of high-performance pseudocapacitive materials. Adv. Mater. 23, . doi:10.1002/adma.Lu, X., Huang, X., Xie, S., Zhai, T., Wang, C., Zhang, P., et al. (2012). Controllablesynthesis of porous nickel-cobalt oxide nanosheets for supercapacitors. J. Mater.Chem. 22, 1. doi:10.927KLukatskaya, M. R., Mashtalir, O., Ren, C. E., Dall’Agnese, Y., Rozier, P., Taberna,P. L., et al. (2013). Cation intercalation and high volumetric capacitance oftwo-dimensional titaniumcarbide. Science 341, . doi:10.1126/science.1241488Obradovic, M. D., Tripkovic, A. V., and Gojkovic, S. L. (2009). The origin of highactivity of Pt-Au surfaces in the formic acid oxidation. Electrochim. Acta 55,204–209. doi:10.1016/j.electacta.Pandolfo, A. G., and Hollenkamp, A. F. (2006). Carbon properties and their rolein supercapacitors. J.Power Sources 157, 11–27. doi:10.1016/j.jpowsour.2006.02.065Pognon, G., Cougnon, C., Mayilukila, D., and Bélanger, D. (2012). Catechol mod-ified activated carbon prepared by the diazonium chemistry for applicationas active electrode material in electrochemical capacitor. ACS Appl. Mater.Interfaces 4, . doi:10.1021/am301284nQu, Q., Li, L., Tian, S., Guo, W., Wu, Y., and Holze, R. (2010). Acheap asymmetric supercapacitor with high energy at high power:activated carbon//K0.27MnO2·0.6H2O. J. Power Sources 195, .doi:10.1016/j.jpowsour.Qu, Q. T., Shi, Y., Tian, S., Chen, Y. H., Wu, Y. P., and Holze, R. (2009). A newcheap asymmetric aqueous supercapacitor:ac tivatedcarbon//NaMnO2.J. PowerSources 194, . doi:10.1016/j.jpowsour.Ragupathy, P., Park, D. H., Campet, G., Vasan, H. N., Hwang, S. J., Choy, J. H.,et al. (2009). Remarkable capacity retention of nanostructured manganese oxideupon cycling as an electrode material for supercapacitor. J. Phys. Chem. C 113,. doi:10.1021/jp811407qReddy, A. L. M., Shaijumon, M. M., Gowda, S. R., and Ajayan, P. M. (2009).Multisegmented Au-MnO2/carbon nanotube hybrid coaxial arrays for high-power supercapacitor applications. J. Phys. Chem. C 114, 658–663. doi:10.1021/jp908739qRicketts, B. W., and Ton-That, C. (2000). Self-discharge of carbon-based super-capacitors with organic electrolytes. J. Power Sources 89, 64–69. doi:10.1016/S)00387-6Roldán, S., Granda, M., Menéndez, R., Santamaría, R., and Blanco, C. (2011).Mechanisms of energy storage in carbon-based supercapacitors modified witha quinoid redox-active electrolyte. J. Phys. Chem. C 115, 1. doi:10.1021/jp205100vSakiyama, K., Onishi, S., Ishihara, K., Orita, K., Kijiyama, T., Hosoda, N., et al.(1993). Deposition and properties of reactively sputtered ruthenium dioxidefilms. J. Electrochem. Soc. 140, 834–839. doi:10.1002/chin.Saliger, R., Fischer, U., Herta, C., and Fricke, J. (1998). High surface area car-bon aerogels for supercapacitors. J. Non Cryst. Solids 225, 81–85. doi:10.1016/S)00104-5Sellam, A., and Hashmi, S. A. (2013). High rate performance of flexible pseu-docapacitors fabricated using ionic-liquid-based proton conducting polymerelectrolyte with poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) andits hydrous rutheniumoxidecomposite electrodes. ACS Appl. Mater. Interfaces 5,. doi:10.1021/am4005557Senthilkumar, S. T., Selvan, R. K., Lee, Y. S., and Melo, J. S. (2013). Electricdouble layer capacitorand its improved specific capacitance using redox additiveelectrolyte. J. Mater. Chem. A 1, . doi:10.210hSharma, P., and Bhatti, T. S. (2010). A review on electrochemical double-layercapacitors. Energ. Convers. Manag. 51, . doi:10.1016/j.enconman.Si, W. P., Yan, C. L., Chen, Y., Oswald, S., Han, L. Y., and Schmidt, O. G. (2013). Onchip, all solid-state and flexible micro-supercapacitors with high performancebased on MnOx/Au multilayers. Energ. Environ. Sci. 6, . doi:10.1039/C3EE41286ESimon, P., and Burke, A. (2008). Nanostructured carbons: double-layer capacitanceand more. Electrochem. Soc. Interface 17, 38–43.Simon, P., and Gogotsi, Y. (2008). Materials for electrochemical capacitors. Nat.Mater. 7, 845–854. doi:10.1038/nmat2297Song, M. K., Cheng, S., Chen, H., Qin, W., Nam, K. W., Xu, S., et al. (2012). Anoma-lous pseudocapacitive behavior of a nanostructured, mixed-valent manganeseoxide film for electrical energy storage. Nano Lett. 12, . doi:10.1021/nl300984ySong, X., and Gao, L. (2008). Facile synthesis and hierarchical assembly of hollownickel oxide architectures bearing enhanced photocatalytic properties. J. Phys.Chem. C 112, 1. doi:10.1021/jp804921gStoller, M. D., Park, S., Zhu, Y., An, J., and Ruoff, R. S. (2008). Graphene-basedultracapacitors. Nano Lett. 8, . doi:10.1021/nl802558ySu, L. H., Zhang, X. G., Mi, C. H., Gao, B., and Liu, Y. (2009). Improvementof the capacitive performances for Co-Al layered double hydroxide by addinghexacyanoferrate into the electrolyte. Phys. Chem. Chem. Phys. 11, .doi:10.aTao, J., Liu, N., Li, L., Su, J., and Gao, Y. (2014). Hierarchical nanostruc-tures of polypyrrole@MnO2composite electrodes for high performanceFrontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 239
Zhao and Zheng A review for aqueous electrochemical supercapacitorssolid-state asymmetric supercapacitors. Nanoscale 6, . doi:10.1039/C3NR05845JWang, D. W., Li, F., and Cheng, H. M. (2008). Hierarchical porous nickel oxide andcarbon as electrode materials for asymmetric supercapacitor. J. Power Sources185, . doi:10.1016/j.jpowsour.Wang, G., Shen, X., Horvat, J., Wang, B., Liu, H., Wexler, D., et al. (2009).Hydrothermal synthesis and optical, magnetic, and supercapacitance propertiesof nanoporous cobalt oxide nanorods. J. Phys. Chem. C 113, . doi:10.1021/jp8106149Wang, H. Q., Yang, G., Li, Q. Y., Zhong, X. X., Wang, F. P., Li, Z. S., et al. (2011).Porous nano-MnO2: large scale synthesis via a facile quick-redox procedureand application in a supercapacitor. New J. Chem. 35, 469–475. doi:10.1039/C0NJ00712AWang, R., Yan, X., Lang, J., Zheng, Z., and Zhang, P. (2014a). A hybrid superca-pacitor based on flower-like Co(OH)2and urchin-like VN electrode materials.J. Mater. Chem. A 2, 1. doi:10.296hWang, H., Yi, H., Chen, X., and Wang, X. (2014b). Asymmetric supercapaci-tors based on nano-architectured nickel oxide/graphene foam and hierarchicalporous nitrogen-doped carbon nanotubes with ultrahigh-rate performance.J. Mater. Chem. A 2, . doi:10.046AWang, X., Sumboja, A., Lin, M., Yan, J., and Lee, P. S. (2012a). Enhancing electro-chemical reaction sites in nickel-cobalt layered double hydroxides on zinc tinoxide nanowires: a hybrid material for an asymmetric supercapacitor device.Nanoscale 4, . doi:10.590dWang, X., Wang, Y. Y., Zhao, C. M., Zhao, Y. X., Yan, B. Y., and Zheng, W. T.(2012b). ElectrodepositedNi(OH)2nanoflakes on graphite nanosheets preparedby plasma-enhanced chemical vapor deposition for supercapacitor electrode.New J. Chem. 36, . doi:10.308KWang, X., Liu, W. S., Lu, X., and Lee, P. S. (2012c). Dodecyl sulfate-induced fastfaradic process in nickel cobaltoxide–reduced graphite oxide composite materialand its application for asymmetric supercapacitor device. J. Mater. Chem. 22,2. doi:10.307EWang, Y., and Zhang, X. (2004). Preparation and electrochemical capacitance ofRuO2/TiO2nanotubes composites. Electrochim. Acta 49, . doi:10.1016/j.electacta.Wei, Y. Z., Fang, B., Iwasa, S., and Kumagai, M. (2005). A novel electrode materialfor electric double-layer capacitors. J. Power Sources 141, 386–391. doi:10.1016/j.jpowsour.Wu, J. H., Yu, H. J., Fan, L. Q., Luo, G. G., Lin, J. M., and Huang, M. L. (2012).A simple and high-effective electrolyte mediated with p-phenylenediamine forsupercapacitor. J. Mater. Chem. 22, 1. doi:10.856DWu, S., Chen, W., and Yan, L. (2014). Fabrication of a 3D MnO2/graphene hydro-gel for high-performance asymmetric supercapacitors. J. Mater. Chem. A 2,. doi:10.387BXia, H., Li, B., and Lu, L. (2014). 1.8 V symmetric supercapacitors developed usingnanocrystalline Ru films as electrodes. RSC Adv. 4, 1. doi:10.1039/C3RA47396AXia, H., Meng, Y. S., Yuan, G., Cui, C., and Lu, L. (2012). A symmetric RuO2/RuO2supercapacitor operating at 1.6 V by using a neutral aqueous electrolyte. Elec-trochem. Solid State Lett. 15, A60–A63. doi:10.204eslXia, X. H., Tu, J. P., Wang, X. L., Gu, C. D., and Zhao, X. B. (2011a). MesoporousCo3O4monolayer hollow-sphere array as electrochemical pseudocapacitormaterial. Chem. Commun. 47, . doi:10.281cXia, X., Tu, J., Mai, Y., Chen, R., Wang, X., Gu, C., et al. (2011b). Graphenesheet/porous NiO hybrid film for supercapacitor applications. Chemistry 17,1. doi:10.1002/chem.Xu, J.,Gao, L., Cao, J., Wang, W., andChen, Z. (2010). Preparation and electrochem-ical capacitance of cobalt oxide (Co3O4) nanotubes as supercapacitor material.Electrochim. Acta 56, 732–736. doi:10.1016/j.electacta.Xu, K., Zou, R., Li, W., Liu, Q., Liu, X., An, L., et al. (2014). Design and synthesis of3DinterconnectedmesoporousNiCo2O4@CoxNi1-x (OH)2core-shell nanosheetarrays with large areal capacitance and high rate performance for supercapac-itors. J. Mater. Chem. A 2, 1. doi:10.489HYan, J., Fan, Z., Wei, T., Qian, W., Zhang, M., and Wei, F. (2010a). Fast and reversiblesurface redox reaction of graphene-MnO2composites as supercapacitorelectrodes. Carbon N. Y. 48, . doi:10.1016/j.carbon.2010.06.047Yan, J., Wei, T., Qiao, W. M., Shao, B., Zhao, Q. K., Zhang, L. J., et al. (2010b).Rapid microwave-assisted synthesis of graphene nanosheet/Co3O4compositefor supercapacitors. Electrochim. Acta 55, . doi:10.1016/j.electacta.Yu, G., Hu, L., Vosgueritchian, M., Wang, H., Xie, X., McDonough, J. R., et al.(2011a). Solution-processed graphene/MnO2nanostructured textiles for high-performance electrochemical capacitors. Nano Lett. 11, . doi:10.1021/nl2013828Yu, G., Hu, L., Liu, N., Wang, H., Vosgueritchian, M., Yang, Y., et al. (2011b).Enhancing the supercapacitor performance of graphene/MnO2nanostructuredelectrodes by conductive wrapping. Nano Lett. 11, . doi:10.1021/nl2026635Yu, H., Fan, L., Wu, J., Lin, Y., Huang, M., Lin, J., et al. (2012). Redox-activealkaline electrolyte for carbon-based supercapacitor with pseudocapacitiveperformance and excellent cyclability. RSC Adv. 2, . doi:10.1039/C2RA20503CYuan, C., Yang, L., Hou, L., Shen, L., Zhang, X., and David Lou, X. W. (2012).Growth of ultrathin mesoporous Co3O4nanosheet arrays on Ni foam forhigh-performance electrochemical capacitors. Energ.Environ. Sci. 5, .doi:10.745GYuan, C., Zhang, X., Hou, L., Shen, L., Li, D., Zhang, F., et al. (2010). Lysine-assistedhydrothermal synthesis of urchin-like ordered arrays of mesoporous Co(OH)2nanowires and their application in electrochemical capacitors. J. Mater. Chem.20, 1. doi:10.174AYuan, C., Zhang, X., Su, L., Gao, B., and Shen, L. (2009). Facile synthesis and self-assembly of hierarchical porous NiO nano/micro spherical superstructures forhigh performance supercapacitors. J. Mater. Chem. 19, . doi:10.1039/B902221JYuan, Y. F., Xia, X. H., Wu, J. B., Yang, J. L., Chen, Y. B., and Guo, S. Y. (2011). Nickelfoam-supported porous Ni(OH)2/NiOOH composite film as advanced pseu-docapacitor material. Electrochim. Acta 56, . doi:10.1016/j.electacta.Zhai, Y., Dou, Y., Zhao, D., Fulvio, P. F., Mayes, R. T., and Dai, S. (2011). Carbonmaterials for chemical capacitive energy storage. Adv. Mater. 23, .doi:10.1002/adma.Zhang, C., Zhou, H., Yu, X., Shan, D., Ye, T., Huang, Z., et al. (2014). Synthe-sis of RuO2decorated quasi graphene nanosheets and their application insupercapacitors. RSC Adv. 4, 1. doi:10.641CZhang, L. L., and Zhao, X. S. (2009). Carbon-based materials as supercapacitorelectrodes. Chem. Soc. Rev. 38, . doi:10.jZhang, S. W., and Chen, G. Z. (2008). Manganese oxide based materials forsupercapacitors. Energ. Mater. 3, 186–200. doi:10.09X427940Zhang, X., Shi, W., Zhu, J., Zhao, W., Ma, J., Mhaisalkar, S., et al. (2010). Syn-thesis of porous NiO nanocrystals with controllable surface area and theirapplication as supercapacitor electrodes. Nano Res. 3, 643–652. doi:10.1007/s- 0024-6Zhang, Y., Feng, H., Wu, X., Wang, L., Zhang, A., Xia, T., et al. (2009a). Progressof electrochemical capacitor electrode materials: a review. J. Hydrogen Energ. 34,. doi:10.1016/j.ijhydene.Zhang, H., Cao, G., and Yang, Y. (2009b). Carbon nanotube arrays and their com-posites for electrochemical capacitors and lithium-ion batteries. Energ. Environ.Sci. 2, 932–943. doi:10.kZhao, C. M., Wang, X., Wang, S. M., Wang, H. X., Yang, Y. C., and Zheng,W. T. (2013a). Pseudocapacitive properties of cobalt hydroxide electrodepositedon Ni-foam-supported carbon nanomaterial. Mater. Res. Bull. 48, .doi:10.1016/j.materresbull.Zhao, C. M., Zheng, W. T., Wang, X., Zhang, H. B., Cui, X. Q., and Wang, H. X.(2013b). Ultrahigh capacitive performance from both Co(OH)2/graphene elec-trode and K3Fe(CN)6electrolyte. Sci. Rep. 3, . doi:10.1038/srep02986Zhao, D., Guo, X., Gao, Y., and Gao, F. (2012a). An electrochemical capaci-tor electrode based on porous carbon spheres hybrided with polyaniline andnanoscale ruthenium oxide. ACS Appl. Mater. Interfaces 4, . doi:10.1021/am301484sZhao, C. M., Wang, X., Wang, S. M., Wang, Y. Y., Zhao, Y. X., and Zheng,W. T. (2012b). Synthesis of Co(OH)2/graphene/Ni foam nano-electrodeswith excellent pseudocapacitive behavior and high cycling stability forsupercapacitors. Int. J. Hydrogen Energy 37, 1. doi:10.1016/j.ijhydene.Zhao, T., Jiang, H., and Ma, J. (2011). Surfactant-assisted electrochemical depositionof a-cobalt hydroxide for supercapacitors. J. Power Sources 196, 860–864. doi:10.1016/j.jpowsour.Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 2310
Zhao and Zheng A review for aqueous electrochemical supercapacitorsZhao, X., Tian, H., Zhu, M., Tian, K., Wang, J. J., Kang, F., et al. (2009a). Carbonnanosheets as the electrode material in supercapacitors. J. Power Sources 194,. doi:10.1016/j.jpowsour.Zhao, X., Johnston,C., and Grant, P. S. (2009b). A novel hybrid supercapacitorwith acarbon nanotube cathode and an iron oxide/carbon nanotube composite anode.J. Mater. Chem. 19, . doi:10.AZheng, J. P., and Jow, T. R. (1996). High energy and high power density electro-chemical capacitors. J. Power Sources 62, 155–159. doi:10.-7753(96)02424-XZhi, M., Xiang, C., Li, J., Li, M., and Wu, N. (2013). Nanostructured carbon-metaloxide composite electrodes for supercapacitors: a review. Nanoscale 5, 72–88.doi:10.040aZhou, C., Zhang, Y., Li, Y., and Liu, J. (2013). Construction of high-capacitance 3DCoO@polypyrrole nanowire array electrode for aqueous asymmetric superca-pacitor. Nano Lett. 13, . doi:10.1021/nl400378jZhu, X., Dai, H., Hu, J., Ding, L., and Jiang, L. (2012). Reduced grapheneoxide-nickel oxide composite as high performance electrode materialsfor supercapacitors. J. Power Sources 203, 243–249. doi:10.1016/j.jpowsour.Zhu, Y., Murali, S., Stoller, M. D., Velamakanni, A., Piner, R. D., and Ruoff,R. S. (2010). Microwave assisted exfoliation and reduction of graphiteoxide for ultracapacitors. Carbon N. Y. 48, . doi:10.1016/j.carbon.Conflict of Interest Statement: The authors declare that the research was con-ducted in the absence of any commercial or financial relationships that could beconstrued as a potential conflict of interest.Copyright (C) 2015 Zhao and Zheng.This is an open-access article distributedunder theterms of the Creative Commons Attribution License (CC BY). The use, distribution orreproduction in other forums is permitted, provided the original author(s) or licensorare credited and that the original publication in this journal is cited,in accordance withaccepted academic practice. No use, distribution or reproduction is permitted whichdoes not comply with these terms.Frontiers in Energy Research | www.frontiersin.org May 2015 | Volume 3 | Article 2311
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