Flexible energy storage devices based on nanocomposite paper Victor L. Pushparaj*, Manikoth M. Shaijumon*, Ashavani Kumar*, Saravanababu Murugesan, Lijie Ci*, Robert vajtai't Robert J Linhardt, Omkaram Nalamasu*, and Pulickel M. ajayan*+s ering andChemical and Biological Engineering, and Center for Biotechnology and Interdisciplinary Studies, sselaer Nanotechnology Center; Rensselaer Polytechnic Institute, NY12180 Communicated by Mildred S Dresselhaus, Massachusetts Institute of Technology, Cambridge, MA, July 11, 2007(received for review February 23, 2007) There is strong recent interest in ultrathin, flexible, safe energy nonaqueous), limiting high-power capability and packaging designs storage devices to meet the various design and power needs of is the other important factor in supercapacitors and batteries(8, 9) modern gadgets. To build such fully flexible and robust electr If integrated structures containing the three essential componen chemical devices, multiple components with specific electrochem-(electrodes, spacer, and electrolyte) of the electrochemical device ical and interfacial properties need to be integrated into single can be made mechanically flexible, it would enable these to be units. Here we show that these basic components, the electrode, embedded into various functional devices in a wide range of parator, and electrolyte, can all be integrated into single con- innovative products such as smart cards, displays, and implantable tiguous nanocomposite units that can serve as building blocks for medical devices. Previous of flexible energy-storage devices a variety of thin mechanically flexible energy storage devices. (1)have been based on separated thin-electrode and spacer layers Nanoporous cellulose paper embedded with aligned carbon nano- proving less-than-optimum in performance and handling because tube electrode and electrolyte constitutes the basic unit. The units of the existence of multiple interfaces between the layers. Here we and dual-storage battery-in-supercapacitor devices. The thin free- integrated nance fabrication of electrode-spacer-electrolyte are used to build various flexible supercapacitor, battery hybrid, demonstrate osite units to build a variety of thin flexible tanding nanocomposite paper devices offer complete mechanical energy-storage devices. We combine two essential materials, cel- supercapacitors operate with lulose and carbon nanotubes( CNTs), that fit the characteristics of electrolytes including aqueous solvents, room temperature ionic spacer and electrode and provide inherent flexibility as well as liquids, and bioelectrolytes and over record temperature ranges. porosity to the system. Cellulose, the main constituent of paper and These easy-to-assemble integrated nanocomposite energy-storage a inexpensive insulating separator structure with excellent biocom- systems could provide unprecedented design ingenuity for a va- patibility, can be made with adjustable porosity. CNTs, a structure riety of devices operating over a wide range of temperature and with extreme flexibility, have already been widely used as electrodes in electrochemical devices(10-16). The major challenge in fabri- cating CNT-integrated cellulose composites is the insolubility of batteries carbon nanotubes supercapacitor cellulose in most common solvents. This issue is solved here by using a room temperature ionic liquid(RTIL)(17)1-buty here has been recent interest in flexible safe energy devices, methylimidazolium chloride([bmImI()(17), which dissolves up based on supercapacitors and batteries, to meet the various to 25%(wt/wt)of unmodified cellulose by using microwave irra- requirements of modern gadgets(1-3). Electrochemical energy diation(18). Interestingly, the ionic nature of RTIL(19)permits it can be stored in two fundamentally different ways. In a battery, be used as an electrolyte in supercapacitors(20), allowing the the charge storage is achieved by electron transfer that produces assembly of all three components(Fig. 1) via a simple scalable a redox on in the electroactive materials (3). In an electric process. louble-layer capacitor, namely the supercapacitor, the charge Uniform films of vertically aligned thin-walled multiwalled torage process is non Faradic, that is, ideally no electron transfer nanotubes(MWNT) are grown on silicon substrates by using a kes place across the electrode interface, and the storage of thermal-chemical vapor-deposition method (supporting infor electric charge and energy is electrostatic. Because the charging mation (SI)). Unmodified plant cellulose dissolved in RTIL and discharging of such supercapacitors involve no chemical (lbmIm cip(17)is infiltrated into the MWNT to form a phase and composition changes, such capacitors have a high uniform film of cellulose and [bmIm(cl], embedding the degree of cyclability. However, in certain supercapacitors based MWNT(Fig. la). After solidification on dry ice, this nanocom on pseudocapacitance, the essential process can be Faradic, posite is immersed in ethanol to partially or completely extract similar to that in a battery. However, an essential fundamental excess rTiL and dried in vacuo to remove residual ethanol. The the chemical and associated electrode potentials are a continu- building unit in our devices, is peeled from the substrate for use demand for efficient power devices to meet the high-power and lergy applications, there seems to be the possibility of an ideal compromise, which combines some of the storage capabilities of uthor contributions: V LP, M. MS, AK,RJ.L O N, and P. M. A designed research: V L.P. batteries and some of the power-discharge characteristics of ca agents/analytic tools, V L.., M.M. R. L, O N, and P M.A. analyzed data; and V.L.P. actors in devices capable of storing useful quantities of elec- MMS, and P.M.A. wrote the pape tricity that can be discharged very quickly. We address here this The authors declare no conflict of interest. need to develop new integrated hybrid devices with adaptability Abbreviations: CNT, carbon nanotube; [bmlmIICIL 1-butyl, 3-methylimidazolium chloride: in various thin-film as well as bulk applications by using engi- MwNT, multiwalled nanotubes; RTIL, room temperature ionic liqt peered electrode nanostructures STo whom correspondence should be addressed. E-mail: ajayan Grpi edu Theperformancecharacteristicsofenergydevicesarefunda-Thisarticlecontainssupportinginformationonlineatwww.pnas.org/cgilcontentfull entally determined by the structural and electrochemical pre 0706508104Dc1 erties of electrode materials (4-7). Electrolyte choice(aqueous e 2007 by The National Academy of Sciences of the USA 13574-13577|PNAs| August21.2007|vol.104|no.34 www.pnas.org/cgi/doi/10.1073/pnas.0706508104Flexible energy storage devices based on nanocomposite paper Victor L. Pushparaj*, Manikoth M. Shaijumon*, Ashavani Kumar*, Saravanababu Murugesan†, Lijie Ci*, Robert Vajtai‡, Robert J. Linhardt†, Omkaram Nalamasu*, and Pulickel M. Ajayan*‡§ Departments of *Materials Science and Engineering and †Chemical and Biological Engineering, and Center for Biotechnology and Interdisciplinary Studies, ‡Rensselaer Nanotechnology Center; Rensselaer Polytechnic Institute, Troy, NY 12180 Communicated by Mildred S. Dresselhaus, Massachusetts Institute of Technology, Cambridge, MA, July 11, 2007 (received for review February 23, 2007) There is strong recent interest in ultrathin, flexible, safe energy storage devices to meet the various design and power needs of modern gadgets. To build such fully flexible and robust electro￾chemical devices, multiple components with specific electrochem￾ical and interfacial properties need to be integrated into single units. Here we show that these basic components, the electrode, separator, and electrolyte, can all be integrated into single con￾tiguous nanocomposite units that can serve as building blocks for a variety of thin mechanically flexible energy storage devices. Nanoporous cellulose paper embedded with aligned carbon nano￾tube electrode and electrolyte constitutes the basic unit. The units are used to build various flexible supercapacitor, battery, hybrid, and dual-storage battery-in-supercapacitor devices. The thin free￾standing nanocomposite paper devices offer complete mechanical flexibility during operation. The supercapacitors operate with electrolytes including aqueous solvents, room temperature ionic liquids, and bioelectrolytes and over record temperature ranges. These easy-to-assemble integrated nanocomposite energy-storage systems could provide unprecedented design ingenuity for a va￾riety of devices operating over a wide range of temperature and environmental conditions. batteries
carbon nanotubes
supercapacitor There has been recent interest in flexible safe energy devices, based on supercapacitors and batteries, to meet the various requirements of modern gadgets (1–3). Electrochemical energy can be stored in two fundamentally different ways. In a battery, the charge storage is achieved by electron transfer that produces a redox reaction in the electroactive materials (3). In an electric double-layer capacitor, namely the supercapacitor, the charge￾storage process is nonFaradic, that is, ideally no electron transfer takes place across the electrode interface, and the storage of electric charge and energy is electrostatic. Because the charging and discharging of such supercapacitors involve no chemical phase and composition changes, such capacitors have a high degree of cyclability. However, in certain supercapacitors based on pseudocapacitance, the essential process can be Faradic, similar to that in a battery. However, an essential fundamental difference from battery behavior arises because, in such systems, the chemical and associated electrode potentials are a continu￾ous function of degree of charge, unlike the thermodynamic behavior of single-phase battery reactants (3). Now, with the demand for efficient power devices to meet the high-power and -energy applications, there seems to be the possibility of an ideal compromise, which combines some of the storage capabilities of batteries and some of the power-discharge characteristics of ca￾pacitors in devices capable of storing useful quantities of elec￾tricity that can be discharged very quickly. We address here this need to develop new integrated hybrid devices with adaptability in various thin-film as well as bulk applications by using engi￾neered electrode nanostructures. The performance characteristics of energy devices are funda￾mentally determined by the structural and electrochemical prop￾erties of electrode materials (4–7). Electrolyte choice (aqueous vs. nonaqueous), limiting high-power capability and packaging designs, is the other important factor in supercapacitors and batteries (8, 9). If integrated structures containing the three essential components (electrodes, spacer, and electrolyte) of the electrochemical device can be made mechanically flexible, it would enable these to be embedded into various functional devices in a wide range of innovative products such as smart cards, displays, and implantable medical devices. Previous designs of flexible energy-storage devices (1) have been based on separated thin-electrode and spacer layers, proving less-than-optimum in performance and handling because of the existence of multiple interfaces between the layers. Here we demonstrate the fabrication of electrode-spacer-electrolyte￾integrated nanocomposite units to build a variety of thin flexible energy-storage devices. We combine two essential materials, cel￾lulose and carbon nanotubes (CNTs), that fit the characteristics of spacer and electrode and provide inherent flexibility as well as porosity to the system. Cellulose, the main constituent of paper and a inexpensive insulating separator structure with excellent biocom￾patibility, can be made with adjustable porosity. CNTs, a structure with extreme flexibility, have already been widely used as electrodes in electrochemical devices (10–16). The major challenge in fabri￾cating CNT-integrated cellulose composites is the insolubility of cellulose in most common solvents. This issue is solved here by using a room temperature ionic liquid (RTIL) (17) 1-butyl,3- methylimidazolium chloride ([bmIm][Cl]) (17), which dissolves up to 25% (wt/wt) of unmodified cellulose by using microwave irra￾diation (18). Interestingly, the ionic nature of RTIL (19) permits it be used as an electrolyte in supercapacitors (20), allowing the assembly of all three components (Fig. 1) via a simple scalable process. Uniform films of vertically aligned thin-walled multiwalled nanotubes (MWNT) are grown on silicon substrates by using a thermal-chemical vapor-deposition method [supporting infor￾mation (SI)]. Unmodified plant cellulose dissolved in RTIL ([bmIm][Cl]) (17) is infiltrated into the MWNT to form a uniform film of cellulose and [bmIm][Cl], embedding the MWNT (Fig. 1a). After solidification on dry ice, this nanocom￾posite is immersed in ethanol to partially or completely extract excess RTIL and dried in vacuo to remove residual ethanol. The resulting nanocomposite paper (Fig. 1b), which forms the basic building unit in our devices, is peeled from the substrate for use as the supercapacitor. The excellent mechanical flexibility of the nanocomposite paper (CNT cellulose–RTIL) is shown in Fig. 1b. Author contributions: V.L.P., M.M.S., A.K., R.J.L., O.N., and P.M.A. designed research; V.L.P., M.M.S., A.K., and S.M. performed research; V.L.P., A.K., S.M., L.C., and R.V. contributed new reagents/analytic tools; V.L.P., M.M.S., R.J.L., O.N., and P.M.A. analyzed data; and V.L.P., M.M.S., and P.M.A. wrote the paper. The authors declare no conflict of interest. Abbreviations: CNT, carbon nanotube; [bmIm][Cl], 1-butyl,3-methylimidazolium chloride; MWNT, multiwalled nanotubes; RTIL, room temperature ionic liquid. §To whom correspondence should be addressed. E-mail: ajayan@rpi.edu. This article contains supporting information online at www.pnas.org/cgi/content/full/ 0706508104/DC1. © 2007 by The National Academy of Sciences of the USA 13574–13577
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August 21, 2007
vol. 104
no. 34 www.pnas.orgcgidoi10.1073pnas.0706508104