Nanostructured Paper For Xible E fl Energy And Electronic .
Nanostructured paper for flexibleenergy and electronic devicesGuangyuan Zheng, Yi Cui, Erdem Karabulut, Lars Wågberg,Hongli Zhu, and Liangbing HuCellulose is one of the most abundant organic materials on earth, and cellulose paper isubiquitous in our daily life. Re-engineering cellulose fibers at the nanoscale will allow thisrenewable material to be applied to advanced energy storage systems and optoelectronicdevices. In this article, we examine the recent development of nanofibrillated cellulose anddiscuss how the integration of other nanomaterials leads to a wide range of applications. Theunique properties of nanofibrillated cellulose enable multi-scale structuring of the functionalcomposites, which can be tailored to develop new concepts of energy and electronic devices.Tapping into the nanostructured materials offered by nature can offer many opportunities thatwill take nanotechnology research to a new level.IntroductionFor thousands of years, cellulose paper has been a majormedium for displaying and transmitting information in manyparts of the world. Its chemical and mechanical stability underatmospheric conditions and ability to absorb ink readily remainunmatched by other materials used in large abundance. Cellulose,the major component of paper, can be obtained from plants andrepresents one of the most abundant organic materials on earth.In the past decade, research on nanostructures of cellulose has increased dramatically due to the potential applicationsin electronics, biosensors, and energy storage devices. 1–4Large-scale, energy-efficient production of nanofibrillatedcellulose (NFC) has recently become possible by employingvarious physical, chemical, and enzymatic pretreatment methodsbefore the homogenization step.1 In parallel, the development ofnanostructured inorganic materials in the form of nanocrystals,nanowires, and nanotubes provides a list of functional inks forintegration into paper.5–9Cellulose by itself is usually limited in functionalities. However, the three-dimensional (3D) hierarchical structures formedby cellulose fibers at different length scales, combined with theability to accommodate other functional materials, open upmany opportunities for applications in electrical, electrochemical,and optical devices.The focus of this article is to present recent progress in thedevelopment of energy and electronics devices fabricated usingwood fiber cellulose as the building block in conjunction withother nanomaterials. The first part of this article will focus onthe hierarchical structure of wood cellulose, as well as the fabrication and properties of paper. In particular, regular cellulosefibers with a diameter of 20 μm and nanocellulose fibers with adiameter of 20 nm will be discussed in detail. The second partwill focus on the recent development of conductive paper forenergy devices, particularly for ultracapacitors and batteries.The last part will focus on the development of transparentnanocellulose paper and its potential applications in electronicsand optoelectronic devices.Cellulose: The building blockThe cell wall of wood has a fascinating 3D hierarchical structuredesigned to maximize the stability and durability of the trees.The wood fiber is made up of crystalline cellulose nanofibrils(around 40 wt% of the wood), random amorphous hemicellulose (around 25 wt% of the wood), and organic “glue” lignin(around 30 wt% of the wood) that cross-link different polysaccharide in wood to form a strong and durable structure.10 Atthe molecular scale, the cellulose polymer molecules have alinear chain structure consisting of glucose repeating units withGuangyuan Zheng, Department of Chemical Engineering, Stanford University; gyzheng@stanford.eduYi Cui, Department of Materials Science and Engineering, Stanford University; yicui@stanford.eduErdem Karabulut, KTH Royal Institute of Technology, Sweden; kerdem@kth.seLars Wågberg, KTH Royal Institute of Technology, Sweden; wagberg@kth.seHongli Zhu, Department of Materials Science and Engineering, University of Maryland; hongli@umd.eduLiangbing Hu, Department of Materials Science and Engineering, University of Maryland; binghu@umd.eduDOI: 10.1557/mrs.2013.59320MRS BULLETIN VOLUME 38 APRIL 2013 www.mrs.org/bulletin 2013 Materials Research Society
NANOSTRUCTURED PAPER FOR FLEXIBLE ENERGY AND ELECTRONIC DEVICESmany hydroxyl groups. They pack into cellulose crystals withdimensions of a few Å, which in turn are organized into nanofibrils with a diameter of around 4 nm and a length over 1 μm.11These nanofibrils aggregate into bundles with cross-sectionaldimensions of around 20 nm 20 nm2, which further combineto form a large cylindrical wood fiber with a length of 1 3 mm,a diameter of 20 50 μm, and a fiber wall thickness of about4 μm (Figure 1a). In its natural state, the delignified fiber wallhas a specific surface area of around 100 m2/g. The fiber wallcollapses during drying, and the specific surface area decreasesto around 1 m2/g. The rich structural motifs of wood fibercellulose at different scales make them attractive for 3D structuralmanipulation.To design the next-generation high-performance fibril-basedpaper, regular fibers with 20 μm diameter need to be disintegratedinto NFC (Figure 1b). Turbak et al. first reported the productionof NFC, with a diameter of 2–3 nm and a length of 1–2 μm, byusing high-pressure mechanical disintegration of wood pulp.12Excess energy consumption in the homogenization processwas one of the major drawbacks that limited practical applications of this material. It was later shown that the introductionof charged functional groups by carboxymethylation priorto mechanical disintegration enhanced the swelling of thefiber wall, and hence decreased the energy consumption ofthe fi ber disintegration process. 13 The new approach candecrease energy consumption by approximately ten timeswhen comparing to the traditional methods that do notemploy the pretreatment.1,13The final product after the delamination process is a gel-likeNFC dispersion in water (Figure 1c). Depending on the chargedensity and the concentration, the transparency of the gel canbe controlled. The viscous NFC gel can be diluted further,and the remaining fibril aggregates removed by sonication andcentrifugation. It has been shown that the colloidal stability ofthe charged nanofibril dispersion is significantly dependenton the pH and salt concentration. The fibrils form gels at lowpH and high salt concentration, provided that they are abovea critical concentration ( 1g/L).14 Isogai and co-workers haveFigure 1. Structure of cellulose fiber. (a) Schematic to show the hierarchical structure of cellulose from the wood cell wall to microfibrillatedfiber to nanofibrillated fiber to a cellulose molecule. Image courtesy of Mark Harrington. 1996 University of Canterbury. (b) Nanocellulosefibrils (with a square cross-section of around 20 20 nm2) within the open wet fiber wall, where the lamellar organization of fibrils in the fiberwall is obvious.1 Reprinted with permission from Reference 1. 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Photograph ofnanofibrillated cellulose (NFC) gel at 2 wt% in water.MRS BULLETIN VOLUME 38 APRIL 2013 www.mrs.org/bulletin321
NANOSTRUCTURED PAPER FOR FLEXIBLE ENERGY AND ELECTRONIC DEVICESreported another way of introducing negative charges to thecellulose nanofibrils, where the primary hydroxy groups onthe C6 carbons of the cellulose molecules are converted to theircarboxylic form by TEMPO (2,2,6,6-tetramethylpiperidine1-oxyl)-mediated oxidation.15,16 This modification is a sitespecific method in which the oxidation mostly involves C6 carbonatoms.Energy storage with conductive paperThe high conductivity was attributed to the strong solventabsorption properties of the porous paper structure and theconformal coating of flexible CNTs on the cellulose fibersto form continuous electrical conduction pathways. Thisfabrication method is also applicable to coating nanowiressuch as silver ink onto cellulose paper. Filtration methodswere used to deposit graphene on filter paper to produceconductive paper for ultracapacitors.8Conductive paper made from cellulose fibers and CNTsdemonstrates excellent mechanical properties. In the case ofCNT-coated photocopy paper, the sheet resistance increasedonly slightly ( 5%) after the conductive paper was bentto a 2 mm radius 100 times. In contrast, conductive paperfabricated with a metal evaporation coating does not withstand bending very well, and the sheet resistance in this caseincreased by 50% after three bending cycles to a radius of2 mm.6 The graphene cellulose paper was shown to withstand high tensile stress up to 8.67 MPa.8 The increase inresistance was relatively small ( 5%) when the change instrain was around 2%.The 3D hierarchical structure of cellulose paper is veryinteresting for an energy storage system that involves liquidelectrolytes, since the interconnected porosity allows fastaccess of ionic species to the electrode surfaces. In order torender electrical conductivity in cellulose paper, conductingmaterials such as conductive metal oxide,17 graphene,8 carbonnanotubes (CNTs),6 metal nanowires,6 and conductive polymers18can be integrated (Figure 2). The conductive materials can beintroduced into paper at different length scales, from molecularmixing with a cellulose polymer to surface coating on photocopypaper.CNTs are particularly versatile in binding withcellulose. Ajayan and co-workers developeda method to dissolve unmodifi ed cellulosefi bers in a room-temperature ionic liquid,1-buty1,3-methylimidazolium chloride([bmIm][Cl]). The cellulose solution was thencoated onto vertically grown CNTs to formthe conductive paper, which can be used as anelectrode for supercapacitors and lithium-ionbatteries (Figure 3a).19At the nanofibrils scale, the similarity indimensions of the nanocellulose fibers and CNTsallows uniform mixing of the two materials,resulting in a highly conductive porous compositesuitable for high surface area electrodes(Figure 3b).20 Conductive polymers are alsowidely used to coat nanocellulose fibers. Polypyrrole has been found to wet cellulose very well,and polymerization of pyrroles on the surfaceof cellulose results in conformal coatings ofpolypyrrole on the nanocellulose fibers.21 Coatingof cellulose fibers aerogel was demonstrated byusing a polyaniline-dodecyl benzene sulfonicacid doped solution in toluene.22 The use of anorganic solvent allows the high porosity of theaerogel to be preserved.Alternatively, simple Meyer-rod coating ofCNT ink onto commercially available photoFigure 2. Diagram showing conductive materials, such as (a) tin doped indium oxidecopy paper was shown to be highly effective in(ITO). Reprinted with permission from Reference 17. 2013 Royal Society of Chemistry.producing conductive paper with a sheet resis(b) Graphene. Reprinted with permission from Reference 8. 2011 Wiley-VCH Verlag6tance of around 10 ohm/sq (Figure 3c). (TheGmbH & Co. KGaA, Weinheim. (c) Carbon nanotubes and (d) silver nanowires. Reprintedwith permission from Reference 6. 2009 National Academy of Sciences. (e) ConductiveMeyer-rod is a stainless steel rod that is woundpolymers, reprinted with permission from Reference 18. 2009 American Chemicaltightly with wires of a certain diameter. TheSociety, can be integrated into cellulose at different scales (f), ranging from the molecularrod is usually used for conformal coating oflevel to nanofibrils, and the microfiber level.solution-based materials on a flat substrate.)322MRS BULLETIN VOLUME 38 APRIL 2013 www.mrs.org/bulletin
NANOSTRUCTURED PAPER FOR FLEXIBLE ENERGY AND ELECTRONIC DEVICESFigure 3. Fabrication of the nanocomposite paper for supercapacitor and battery applications. (a) Room-temperature ionic liquid (RTIL)derived cellulose/carbon nanotubes (CNTs) composite with the cross-sectional SEM image showing multi-walled nanotubes (MWNTs)protruding from the thin films. (Scale bar is 2 μm.) The schematic displays the partial exposure of MWNT (top) and the structures of theultracapacitor and lithium-ion batteries fabricated using the conductive paper (bottom). (Reprinted with permission from Reference 19. 2007 National Academy of Sciences.) (b) Nanofibrils cellulose/CNT thin film derived from filtration of the solution mixture (top) and aftercoating of silicon by chemical vapor deposition (bottom). (Reprinted with permission from Reference 20. 2013 Elsevier.) (c) Meyer-rodcoating of CNTs solution onto photocopy paper and the corresponding SEM image showing the micron-sized fibers (bottom). (Reprintedwith permission from Reference 6. 2009 National Academy of Sciences.)Due to the hydrophilicity of cellulose fibers, conductivepaper has been widely used as electrodes for ultracapacitorswith aqueous electrolytes. Unmodified CNTs and grapheneare hydrophobic, and their applications in ultracapacitorshave been hindered by poor electrolyte wetting. By usingcellulose as a substrate for the carbon matrix, aqueous electrolytecan be readily absorbed into the electrode, providing intimatecontact between the carbon electrode and electrolyte.23 Forlithium-ion battery applications, the use of conductive cellulosepaper offers new opportunities in fabricating highly flexibleelectrodes and battery systems.The unique structure of conductive cellulose paper madefrom ionic liquid and CNTs (Figure 3a) allows the working electrode (CNTs) and separator (excess cellulose) to becombined in a single sheet of paper. A simple symmetricalultracapacitor was built by combining two such electrodesand using KOH as an electrolyte. For fabricating lithium-ionbatteries, lithium metal is evaporated onto the excess celluloseside, and the CNTs are used as the working electrode. Theas-fabricated flexible lithium-ion battery exhibited a specificcapacity of around 110 mAh/g.Mixing CNTs directly with NFC allows for better tuningof the 3D structure of the composite. Upon freeze-drying themixture, a highly conductive cellulose aerogel can be fabricated(Figure 3b). Further deposition of silicon onto the nanofibrilcellulose using plasma-enhanced chemical vapor depositioncan produce a highly flexible silicon electrode, which hasstable cycling at around 1500 mAh/g for more than 100 cycles.NaCl-based ion exchange batteries were demonstrated usingpolypyrrole coated nanocellulose fibers as electrodes. Sincepolypyrrole can absorb and expel chloride ions during theoxidation/reduction process, the reversible process is utilizedto store energy. The batteries have a reasonable capacity ofaround 25–33 mAh/g.18Coating CNTs directly on commercial photocopy paperoffers a simpler approach for producing a highly conductivepaper substrate. The high porosity of the paper allows rapidabsorption of the conductive inks. Conductive paper coatedwith Li4Ti5O12 and LiMn2O4 demonstrated stable performancefor more than 50 cycles. Although full cell performance stillneeds to be improved, the paper-based electrodes provide aunique approach for fabricating energy storage devices withhigh mechanical flexibility. The CNT coated photocopy paperalso showed excellent performance as an ultracapacitor electrode (Figure 4a), with a specific capacitance of 200 F/g andstable cycling of over 40,000 cycles (Figure 4b). The graphenecellulose paper electrode also demonstrated a high specificcapacitance of 120 F/g, and retained 99% capacitance over5000 cycles.8 In another simple approach, ultracapacitors werefabricated by a simple pencil drawing of graphite onto bothsides of cellulose paper. In an aqueous electrolyte, the deviceshowed good areal capacitance of around 2.3 mF cm–2 and upMRS BULLETIN VOLUME 38 APRIL 2013 www.mrs.org/bulletin323
NANOSTRUCTURED PAPER FOR FLEXIBLE ENERGY AND ELECTRONIC DEVICES(Figure 5d).29 The optical transmittance of nanopaper can be tailored by varying the diametersof the NFC nanofibers.30 Meanwhile, nanocellulose paper is generally as lightweight as regularpaper but with a much higher Young’s modulus.Figure 6 shows an Ashby plot of specific modulusand strength.2 Since nanocellulose paper has nobinder or any other additives, it belongs to the “D”zone of the CN neat film, where the specific stiffness and strength can go up to 20 GPa (g cm–3)–1and 300 MPa (g cm–3)–1, respectively. Sehaquiet al. showed that the elastic modulus of nanocellulose paper can reach 33 GPa after fiberalignment.28The excellent optical transmittance andmechanical strength of nanopaper, together withthe roll-to-roll printing processes, will allow thenext-generation flexible electronics and optoelectronics devices to be integrated on the renewablematerial. Electronic and optoelectronic devicesbased on transparent nanocellulose paper andprinting techniques have been recently demFigure 4. (a) Galvanostatic charging/discharging curves of carbon nanotube (CNT)/paperonstrated. Figure 5c shows a picture of a fullyultracapacitor with organic electrolytes and sulfuric acid. (b) Capacitance retention of thetransparent organic field-effect transistor fabriultracapacitor in different electrolytes. (c) Galvanostatic charging/discharging of LiMn2O4and Li4Ti5O12 electrodes using the photocopy paper/CNT composite as a current collector.cated on nanocellulose paper.29 The transmittance(d) Cycling performance of the LiMn2O4 and Li4Ti5O12 electrodes. (Reprinted with permissionof the device is 83.5% at 550 nm (Figure 5d).from Reference 6. 2009 National Academy of Sciences.)The transistor also shows excellent mechanicalproperties, with only around a 10% decrease inmobility when the device was bent in directionsto 15,000 cycles of charge/discharge, with more than 90%parallel or vertical to the conduction channel. Organic photovoltacapacitance retention.23 The relatively good electrochemicalics on transparent nanocellulose paper has also been demonstratedstability of paper, combined with the low cost and good(Figure 5b).17 Other important devices, such as integrated transparent sensors and 3D microfluidic devices, may also be fabricatedcompatibility with other nanomaterials, provide new opportuon nanocellulose paper. As a potential replacement for plasticnities in applying this renewable material to advanced energysubstrates, nanocellulose paper holds great promise for fabricatstorage systems.ing fully integrated flexible electronics and displays with uniqueTransparent nanocellulose paper for electronicproperties, and at the same time is compatible with high throughputand optoelectronic devicesprocesses such as roll-to-roll printing.3The use of nanocellulose paper as a “green” substrate forConclusionelectronic and optoelectronic devices has attracted broadIn this article, we have reviewed recent progress in the applicationattention.24–26 Commercial paper has a relatively rough surfaceand weak mechanical properties, which can be problematic forof nanocellulose paper for flexible energy storage and electronicelectronic device fabrication. The porosity and thus the refracdevices. Broadly speaking, there are three major building blockstive index variation inside paper will cause significant lightthat can be derived from cellulose: regular fiber with an averagescattering, rendering the paper substrate opaque. Re-engineeringdiameter of 20 μm, nanocellulose fibers with a diameter ofpaper substrates using NFC as the building blocks can address 20 nm, and molecular scales of cellulose molecules.the previously mentioned problems. To prepare nanocelluloseConductive paper made from cellulose fibers of 20 μm inpaper from NFC, a simple vacuum filtration method can bediameter can function as a new type of current collector that enablesused, followed by oven drying, pressing, or freeze-drying.27 Thehigh-performance paper ultracapacitors and Li-ion batteries.nanocellulose paper fabricated is smooth, flexible, transparent,Another emerging area is the development of nanocellulosemechanically strong, and has an extremely low coefficient ofpaper, which is made of nanofibrillated cellulos
energy and electronic devices Guangyuan Zheng , Yi Cui , Erdem Karabulut , Lars Wågberg , Hongli Zhu , and Liangbing Hu Cellulose is one of the most abundant organic materials on earth, and cellulose paper is ubiquitous in our daily life. Re-engineering cellulose fi bers at the nanoscale will allow this
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