Flexible supercapacitors based on paper substrates: a new paradigm for low-cost energy storage.

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Flexible supercapacitors based on paper substrates: a new paradigm for low-cost energy storage Yi-Zhou Zhang,a Yang Wang,a Tao Cheng,a Wen-Yong Lai,*ac Huan Pang*b and Wei Huang*ac Paper-based supercapacitors (SCs), a novel and interesting group of flexible energy storage devices, are attracting more and more attention from both industry and academia. Cellulose papers with a unique porous bulk structure and rough and absorptive surface properties enable the construction of paperbased SCs with a reasonably good performance at a low price. The inexpensive and environmentally

Received 25th February 2015

friendly nature of paper as well as simple fabrication techniques make paper-based SCs promising

DOI: 10.1039/c5cs00174a

candidates for the future ‘green’ and ‘once-use-and-throw-away’ electronics. This review introduces the design, fabrication and applications of paper-based SCs, giving a comprehensive coverage of this

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interesting field. Challenges and future perspectives are also discussed.

1. Introduction Efficient energy production and storage in an eco-friendly and sustainable way has become a global objective.1 Novel energy storage methods are especially needed for the utilization of

a

Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: [email protected], [email protected] b College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, China. E-mail: [email protected] c Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China

Yi-Zhou Zhang

Yi-Zhou Zhang received his BSc from Department of Chemistry and Chemical Engineering of Nanjing University in 2010, after which he has been studying as a PhD candidate under Professor Wei Huang’s and Professor WenYong Lai’s supervision. His current research focuses on flexible and stretchable energy storage devices with a special focus on supercapacitors. He is also interested in using inkjet printing as a fabrication tool for electronics.

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energy from renewable sources such as wind and solar where irregular electricity generation (great fluctuation and low efficiency) pose a great challenge for conventional electrical energy storage systems.2 A variety of energy storage technologies have been developed to meet this challenge, including mechanical, physical, thermal, chemical and electrochemical energy storage systems.3 Among these, batteries, (electrolytic) capacitors and supercapacitors, as three main groups of electrochemical energy storage devices, show great promise.4–7 SCs (also known as electrochemical capacitors or ultracapacitors) hold an important and unique position that bridges the gap between conventional capacitors and batteries in terms of power density and energy density.8–11 The specific energy density of SCs is several orders of magnitude higher than that of conventional electrolytic capacitors. Although SCs possess a relatively low energy density with respect to batteries

Yang Wang is now a graduate student under Professor Wei Huang’s supervision in the Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, China. His research mainly focuses on using inkjet printing to fabricate electronics especially paper-based microfluidic devices.

Yang Wang

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(e.g., lithium-ion batteries), SCs can store and deliver a large amount of charge in a very short period of time (in seconds), thus providing higher power density than batteries. SCs are widely applied in a variety of applications, including back-up power systems, load-leveling applications, electric vehicles and industrial energy management systems.12–16 Depending on the charge–discharge mechanism, SCs can be divided into electrical double layer capacitors (EDLCs), where electrochemical energy is stored by ion adsorbing–dislodging (more like ideal capacitors), and pseudocapacitors, where energy is stored by fast surface redox reactions (more like batteries).17,18 EDLCs mainly utilize carbon materials (activated carbon, carbon nanotubes, graphene) as the electrode materials, while pseudocapacitors mostly use transition metal oxides and conducting polymers.

Because of the different charge–discharge mechanism, EDLCs show high cycling stability and high power density but low capacitance and low energy density, whereas pseudocapacitors manifest the opposite performance. Hybrid SCs employ one EDLC type electrode and one pseudocapacitive electrode, combining the charge storage mechanism of both, so they are promising for demonstrating a high energy density, high power density as well as an excellent cycling stability.19 Portable and flexible electronics require a new paradigm of energy storage systems that suits the requirement for flexibility. Flexible electronics is expected to bring us a variety of fancy applications: bendable cell phone, implantable heart sensor, flexible OLED (organic light emitting diode) display, etc.20 A flexible electronic device can still operate when bent, folded,

Tao Cheng is now a PhD candidate under Professor Wei Huang’s and Professor Wen-Yong Lai’s supervision in the Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications. His current research focuses on flexible and stretchable thin film electrodes for organic optoelectronic devices and energy storage devices like supercapacitors.

Wen-Yong Lai is a full professor at the Nanjing University of Posts and Telecommunications. He received his PhD from Fudan University in 2007. He then joined the Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications. His research mainly focuses on the design, synthesis, and application of Wen-Yong Lai organic and polymer optoelectronic materials for organic/plastic electronics. He is also interested in the exploration of novel materials and processes for printed electronics.

Huan Pang received his PhD degree from Nanjing University in 2011. He then founded his research group in Anyang Normal University where he was appointed as a distinguished professor in 2013. He is also a distinguished professor of Yangzhou University. His research interests include the development of inorganic semiconductors nanostructures and their applications in flexible electronics with a focus on energy devices.

Wei Huang received his PhD degree from Peking University in 1992. In 1993, he began his postdoctoral research with the Department of Chemistry under the supervision of Prof. HUANG Hsing Hua in the National University of Singapore (NUS) and then taught at NUS. In the mean time, he took part in the foundation of the Institute of Materials and Engineering (IMRE), Singapore. In 2002, he moved to Wei Huang Fudan University where he founded the Institute of Advanced Materials (IAM). In 2006, he was appointed as the Deputy President of the Nanjing University of Posts and Telecommunications (NUPT). He was elected as Academician of the Chinese Academy of Sciences in 2011. In 2012, he assumed his duty as the President of the Nanjing Tech University (NanjingTech). His research interests include organic/plastic/flexible (opto)electronics, nanomaterials and nanotechnology.

Tao Cheng

Huan Pang

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compressed, or even stretched. The power source suitable for flexible electronics, accordingly, must be able to deliver a high enough power density and energy density under the above mentioned conditions. Due to the long lifespan compared to batteries and the high power performance, flexible SCs are especially important and thus receive a great deal of attention from both fundamental studies and technological applications.21–28 However, it remains a great challenge to fabricate flexible SCs that are inexpensive, flexible, light-weight and environmentally friendly. As with conventional SCs, further optimization of flexible SCs is mainly oriented toward improving the device’s energy density through the design of new electrode materials,29,30 new electrolytes,31–34 and novel device architectures.35–37 Unique to flexible SCs, it is also crucial to search for and modify flexible substrates onto which flexible SCs can be built and integrated with energy generation devices or power outlets. It is a rather interesting and imaginative idea that paper, a commodity ubiquitous in everyday life, instead of being the embodiment of human knowledge, can be exploited for future electronics in that it can be utilized as the substrate to build flexible SCs. The advantages are obvious: paper is truly a lowcost substrate compared with other flexible substrates such as PET (polyethylene terephthalate); paper is highly flexible as well as bendable; the process of making and disposing of paper is environmental-friendly; paper-based energy storage devices are necessary for all-paper electronics to operate.38–42 The inexpensive and environmental-friendly nature of paper enable some novel and interesting applications such as once-use-and-throwaway glucose sensors. Combined with the power of ‘printed electronics’, paper electronics seems promising as a forerunner of future electronics, in the sense that functional electronic devices are made in the way that people commonly think of as drawing pictures or printing newspapers.43–47 More importantly, to fabricate flexible SCs on paper represents the power of using logic to turn the negative into the positive. As paper is usually composed of cellulose fibers with a 3-dimensional hierarchical arrangement, the paper surface is not only very rough but also highly porous in comparison to common flexible substrates. The surface properties of papers may not be suitable for common electronic components such as conductive lines, diodes, capacitors, etc., but the porous and absorptive nature of paper is actually advantageous for some applications, such as for many energy devices in which large surface roughness is preferred. Especially, people can make full use of the nature of paper when the cellulose paper structure soaked with active materials is used in electrochemical energy storage devices such as SCs. Paper-based SCs are based on the composite of conductive active materials with cellulose fibers, which comprise repeating cellobiose units with a high aspect ratio and the ability to form strong inter- and intra-molecular hydrogen bonds resulting in the fibers with hydroxyl-functionalized surfaces. These properties make cellulose fibers perfect structural units in conjunction with common conductive active materials such as carbon nanotubes (CNTs),38 graphene,48 polypyrroles (PPy),49 and polyanilines (PANI).50 Furthermore, cellulose fiber networks in a piece of paper can also provide pathways for ion transport.51


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It is worth noting that many so-called paper SCs in the literature actually do not involve paper at all in its literal sense; these so-called paper SCs are actually composed of freestanding electrodes that are made into a paper-like shape with no cellulose involved. These free-standing paper-like SCs will not be discussed in this review. One would refer to some reviews on this topic.10,52 Although there are some excellent reviews in the field of flexible SCs, covering topics like all-solid-state flexible SCs,17,53 fiber SCs,36 and flexible SCs from various electrode materials’ perspectives,8,9,11,52,54–58 flexible SCs based on paper have seldom been touched upon.59,60 A review that covers all aspects of paper-based SCs is thus highly desirable to build a comprehensive overview and highlight the importance of this exciting direction. In this context, this review will first introduce the basics of paper SCs, i.e., the device structure and characterization methods of the electrochemical performance. Then, methods for fabricating paper SCs will be introduced, and recent developments of paper SCs will be discussed; lastly some concluding remarks will be presented to outline the challenges and future research directions in this exciting field.

2. Performance evaluation for paper supercapacitors The performance of a supercapacitor is usually reported in terms of their specific capacitance, energy density (energy stored per unit mass/length/area/volume) and power density (power unleashed per unit mass/length/area/volume). Although depending on the specific application and measurement technique, these parameters can be expressed in terms of gravimetric, volumetric and areal figures, for paper-based SCs, the specific capacitance, energy density and power density are usually expressed in areal terms. Electrode materials are often studied in a three-electrode electrochemical cell before being assembled into a two-electrode cell.61–63 The capacitance (C) and specific (areal) capacitance (CA) of each electrode in the three-electrode configuration are calculated from the cyclic voltammetric curves (CV) curves at different scan rates using the formula (1) and (2). Ð C = ( idV)/v(DV) (1) CA = C/A

(2)

where i (in amps, A) is the response current, V is the potential scan rate (V s 1), and DV is the applied potential region (in volts, V) and A refers to the area (cm2) of the electrode. For an assembled SC device, the capacitance (C) and areal capacitance (CA) of each device in the two-electrode configuration are calculated from the galvanostatic charge–discharge curves using formula (3) and (4) C = i/( dV/dt)

(3)

CA = C/A

(4)

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where i (in amps, A) is the applied current, dV/dt is the slope of the discharge curve (in volts per second, V s 1), and A is the areal (cm2) of the device. The areal energy density (E) in W h cm 2 and power density (P) in W cm 2 derived from galvanostatic charge–discharge curves are calculated from formula (5) and (6), respectively.


E = C(DE)2/(2A 3600)

(5)

where DE is the operating voltage window (measured in volts and obtained from the discharge curve excluding the voltage drop) and A refers to the total areal of the device. P = DE2/(4RA)

(6)

where R (=Vdrop/2i) is the internal resistance of the device that is estimated from the voltage drop (Vdrop) at the beginning of the discharge curve. One major problem when dealing with paper SCs and flexible SCs in general is the lack of standardized methods to evaluate and compare the device performance. Consequently, different reports are hard to compare and some results are misleading. For example, some reports use the mass of the active materials without the weight of the binder and carbon conductive additives instead of the whole electrode mass, and there is a great variation between different reports on the choice of length/area/volume of the electrode. It is recommended to calculate energy densities and power densities based on the total mass/ area/volume of assembled SCs.53 It is a good practice to provide detailed information on the device, including the mass of the electrode, and the mass/thickness/area of the assembled devices. For paper-based SCs, the gravimetric figures are sometimes not so meaningful, whereas areal and volumetric figures are more useful. As discussed by Gogotsi and Simon,8 the gravimetric performance of the device is dependent on the thickness and density of the electrodes as well as the weight of the other components, which results in unreliable comparison of different SCs. Detailed

discussion on the evaluation methods of SCs can be found in books and reviews.8,55,64–66

3. Device structure of paper-based supercapacitors The main components of a SC are the electrodes, electrolyte, current collectors and sometimes the separator. As with many solid-state flexible SCs, the polymer gel electrolyte film often acts as the separator. In the case of paper-based SCs, the electrodes are often conductive enough to take the role of the current collector as well. In this kind of SCs, the two main device configurations are the ‘in-plane’ device and the ‘sandwiched’ device. The corresponding device structures and mechanism are illustrated in Fig. 1. The appropriate choice of paper is the first issue to consider in fabricating paper-based SCs. Properties such as porosity, surface roughness, thickness, mechanical robustness, impurities, environmental stability, and cost can all have an impact on the performance and scale-up potential of paper-based SCs. It is important to keep the device structure and the specific materials in mind when choosing paper, as there are more than hundreds of kinds of paper commercially available. It can be a timeconsuming yet worthy endeavor. Instead of choosing from the commercial stock, it is also a promising approach to utilize ‘paper-making’ techniques to make suitable paper with desired properties as shown by Shen et al.67 They took advantage of a special type of paper-bacterial cellulose (BC), which is an interesting eco-friendly biomaterial composed of ribbon-shaped ultrafine nanofibers with width less than 100 nm. Through a process much like paper-recycling, a unique kind of paper was invented based on the ultrafine nanofibers from BC and added conductive materials-PPy and multi-walled carbon nanotubes (MWCNTs). Typical cellulose fibers and paper substrates coated with conductive materials are shown in Fig. 2.

Fig. 1 (a) Schematic diagrams of paper-based SCs with a conventional sandwiched architecture and a novel in-plane architecture. (b) Diagrams showing the ion movement involved in the operation of a ‘sandwiched’ (up) and an ‘in-plane’ (down) structured SC, respectively. Graphene sheets are used as active electrode materials, increased ability of the electrolyte ions to diffuse in between the graphene sheets are illustrated, resulting in a better electrochemical performance. Reproduced with permission from ref. 89. Copyright 2011, American Chemical Society.

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Fig. 2 SEM images of the (a) cross section of a cellulose fiber. (b) SEM image of the framed area in (a). Reproduced with permission from ref. 51. Copyright 2013, American Chemical Society. SEM images of (c) surface morphology of Xerox paper, (d) conformal CNT coating along fibers in Xerox paper. Reproduced with permission from ref. 38. Copyright 2009, National Academy of Sciences.

Current collectors support electrodes and provide electrical conductive pathway for active materials. Traditionally, metal films such as Cu and Al are used for this purpose, however, they account for about 10–20% of the total weight of a device, thus lowering its gravimetric energy density. In addition, they also account for a substantial amount of the device’s cost. Besides, metal films are not suited for flexible devices due to mechanical constraints. Some paper-based SCs adopt easily evaporated or sputtered metals such as Au as the current collector,68 whereas most paper-based SCs adopt non-metallic conductive materials as the current collector.69–73 Carbon-based conductive materials like graphene and CNTs, due to the good conductivity and excellent affinity with the cellulose fibers, are particularly important for paper-based SCs. Arguably the most effective approach of increasing the SC performance is by choosing the appropriate electrode materials. Despite the high price, most paper-based SCs use carbon nanomaterials like CNTs and graphene, however commercialization efforts will reduce the cost significantly in the near future as is shown for other ‘star’ materials such as C60. Compared with carbon materials, other materials, albeit promising, are in need of exploitation; these materials include at least metal nanostructures, conductive polymers and composites of above mentioned materials.52,56 Ideally an electrode material is supposed to have a high capacitance and a wide potential window. To be more specific, a successful (nano-structured) SC electrode material should have a low electrical resistance that is required for efficient charge transport; good electrochemical and mechanical stability for good cycling stability; a high specific surface area; and an optimized pore-size distribution that facilitates


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ion diffusion.9 Composites of different materials are promising as electrode materials because individual substances in the composites usually have a synergistic effect through enhancing specific surface area, optimizing porosity, extending the potential window, etc. Binders are sometimes needed as part of electrodes in the field of flexible SCs.74 Binders are normally insulating and electrochemically inactive, and their presence will enhance the mechanical connections between active materials and between the electrode and the current collector.75 However, the enhancement is achieved at the expense of decreased conductivity, decreased ion transportation and increased electrode polarization. It is necessary to eliminate binders from the electrode fabrication process if possible, and most paper-based SCs avoid the use of binders in the electrode making. The electrolyte is also an important component of SCs, especially for paper-based flexible SCs. A good electrolyte should be ionically conductive, electronically insulating and electrochemically stable in the potential window where the device is operated. Traditional liquid electrolytes based on aqueous and organic solutions of salts or ionic liquids are extensively tested and researched as electrolytes for SCs, but nearly all flexible SCs adopt solid electrolytes. Among these, polymer electrolytes are the most widely studied,76 and polymer electrolytes are ideal candidates for flexible solid supercapacitors: polymeric solid electrolytes are flexible, and can be easily fabricated as thin and large-area membranes. Specifically, gel polymer electrolytes (GPEs), which exhibit liquid-like ionic conductivity while maintaining the dimensional stability of a solid system, are the most popular candidates for paper-based SCs and flexible SCs in general.77 GPEs can not only mitigate the problem imparted by the leakage or evaporation of the traditional liquid electrolytes, but also exhibit high flexibility with little performance decay under strain or tension, thus benefitting the long term stability of the paper-based SC devices. GPEs are also easily fabricated as thin and large area membranes, suitable for the large-scale fabrication of paper SCs. GPEs typically consist of a polymeric framework as the host, an organic/aqueous solvent as the plasticizer (when water is used as the plasticizer, the GPE is called hydrogel polymer electrolyte), and a supporting electrolytic salt (ionic dopant). Most polymer electrolytes conduct via the movement of protons, lithium ions,78 hydroxide ions,79 or the ionic species in ionic liquids.80 Accordingly, there are four categories of gel polymer electrolytes: (1) lithium ion gel polymer electrolytes, (2) proton conducting gel polymer electrolytes, (3) alkaline gel polymer electrolytes, (4) other ion gel polymer electrolytes.53 Among the various polymer hosts, polyvinyl alcohol (PVA) is the most examined because of its low cost, good electrochemical stability, good mechanical properties and non-toxic nature. Paper SCs so far almost inevitably adopt PVA-based hydrogel polymer electrolytes. Aqueous acid (sulfuric acid, phosphate acid, etc.) or base (KOH, NaOH, etc.) are used as the ionic dopant. The long term stability and ionic conductivity of GPEs can be improved via the cross-linking approach81 and the additive-adding approach.82 The ionic conductivity can also be influenced by the concentration of the ionic dopant.83–85 The study on the electrolyte is lacking in the field of paper-based SCs, although it is an effective

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approach one can take to improve device performance. Detailed discussions on (gel) polymer electrolytes can be found in recent review articles.53,77,86 Lastly, it is necessary to take a holographic view to produce paper-based SCs with better performance. One should not only focus on single components like electrodes but also take the interactions between all components and the possible synergistic effects into consideration. Because improving the properties of isolated components is not necessarily sufficient to enhance the properties of the SCs, for example, a matching electrolyte in terms of the solvated ion size compared with the pore-size distribution of the electrode can significantly increase the electrochemical performance of a given electrode.64 Thus it is important to choose the appropriate ionic conduction mechanism of the electrolyte with the chemistry of the electrodes in mind when designing a SC device.

4. Methods for fabricating paperbased supercapacitors As the most important advantage of using paper to make SCs is the cheap price and large availability of paper, the ideal fabrication method should not only be able to achieve outstanding performance in devices, but also lower the cost of SC fabrication by utilizing existing materials and simple manufacturing infrastructure and procedures. Thus, the traditional vacuum evaporation as a deposition method, albeit being the method for making the first electronic device on paper in the late 1960s,87 can not be the first option of large scale fabrication of paper SCs. In contrast, the mainstream methods are solution-based deposition methods such as printing. We even see examples of making paper-based SCs using ink pen and brushes on common office paper,38 which might provide us with an ultimate simple version of DIY (do it yourself) electronics.

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The first step of the fabrication of paper-based SCs is to make the cellulose network conductive. Regarding this, there are three main approaches: (1) coating a conductive layer on the surface of paper; (2) using methods such as ‘soak and dry’ to ensure the conductive materials permeate the cellulose network; (3) using ‘paper-making’ techniques to allow molecularlevel mixing of the conductive materials and cellulose fibers. In summary, the three approaches are schematically illustrated in Fig. 3. Before fabricating electrodes, it is often necessary to make pre-treatments on the paper. Typical pre-treatments were shown by Chen et al.88 A common A4 printing paper was immersed into 0.3 M HCl aqueous solution for about 10 min, then washed with deionized water thoroughly to remove any foaming composition such as carbonates and placed in a fume hood at room temperature until fully dried. The A4 printing paper after HCl treatment suffered some loss in thickness which may be attributed to the decomposition of carbonates. In terms of assembly of a whole device, so far, nearly all paper SCs have adopted a gel polymer electrolyte system (PVA/H3PO4 and PVA/KOH as typical examples). The whole device mostly adopts a conventional sandwiched structure with the gel electrolyte film sandwiched in between two electrodes (either two identical electrodes or two different electrodes in which case the device is called asymmetric), whereas some adopt an in-plane configuration as in Ajayan’s work.89 This fabrication method section will focus on the fabrication of electrodes (sometimes also as current collectors). Some illustrations can be found in Fig. 4. 4.1

Pencil drawing

Pencil is one of the common writing tools utilized for fabricating paper-based SCs, besides other writing tools such as a pen or a Chinese writing brush.38 As hybrids of graphite and clay (mainly SiO2), pencil rods show relatively high conductivity, and have been applied to make electrodes in devices including Li–air

Fig. 3 Schematic illustration of three manufacturing approaches for making paper-based SCs (a) schematic diagram of the fabrication of G/PANI–Paper. Reproduced with permission from ref. 88. Copyright 2013, Elsevier. (b) Schematic of the fabrication process of graphene–cellulose paper (GCP) membrane by vacuum filtration. Reproduced with permission from ref. 48. Copyright 2011, Wiley-VCH Verlag GmbH & Co. (c) Diagram for the preparation of conductive cellulose films by in situ polymerization of aniline in the presence of cellulose scaffolds. Reproduced with permission from ref. 97. Copyright 2013, Royal Society of Chemistry.

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Fig. 4 (a) Fabrication procedure for PANI/Au/paper structures using an evaporation–electrodeposition method. Reproduced with permission from ref. 68. Copyright 2012, Wiley-VCH Verlag GmbH & Co. (b) Schematic diagram of drawing of a conductive electrode on cellulose paper using a graphite pencil. Reproduced with permission from ref. 95. Copyright 2011, Royal Society of Chemistry. (c) Direct writing of CNT ink on the paper with Chinese calligraphy. Reproduced with permission from ref. 38. Copyright 2009, National Academy of Sciences. (d) Schematic of the fabrication process of BC-MWCNTs-PANI paper electrodes using vacuum filtration and electrodeposition. Reproduced with permission from ref. 69. Copyright 2012, WileyVCH Verlag GmbH & Co. (e) Meyer rod coating of CNT ink on commercial Xerox paper. Reproduced with permission from ref. 38. Copyright 2009, National Academy of Sciences. (f) A photo of ink-jet printed SC on Xerox paper. Reproduced with permission from ref. 39. Copyright 2010, American Institute of Physics.

battery,90 and zinc oxide ultraviolet sensor;91 the active components in piezoresistive sensors;92 recently, even pencil drawn functional electronic circuits have been made.93 Pencil traces can be regarded as conductive thin films consisting of percolated graphite particle networks on paper. Based on the hardness of these leads, pencils are classified on a scale from 9B to 9H, meaning different relative fractions of graphite in pencil leads. One can tune the conductivity of pencil traces by changing the type of pencils, the force, and repeating times one applies when ‘writing’ with a pencil. Paper is another parameter that can be adjusted to change the device properties. As with pencils, papers have also been graded based on their end use, manufacturing processes and raw materials. There are now hundreds of different types of paper available to us. For instance, commonly used paper substrates in paper electronics are Xerox paper, filter paper, weighing paper and glossy paper, which have varied roughness. Nevertheless, glossy paper cannot support the pencil-trace as there is no abrasion effect from the smooth surface. Normally, paper surfaces with a roughness of 1–5 mm are required to make a pencil-trace, onto which the graphitic deposits are well connected in order to obtain conducting graphitic tracks. Particularly, the device performance can be optimized by tuning paper properties such as the surface roughness, porosity and coatings. Thus, this simple yet elegant deposition technique can be extended to fabricate low cost conductors for circuits,94 electrodes for energy storage devices90,95 and other electronic devices.91,93,96 From an educational perspective, the combination of pencil and paper is great for science demonstration for kids. 4.2

Chemical and physical deposition

One obvious method to make paper conductive is to deposit a thin layer of metal on its surface. Despite the high cost and


inconvenience, chemical and physical deposition methods such as vacuum evaporation and sputtering are still the most reliable method to deposit a thin layer of metal on paper. As an example shown by Wang et al.,68 a thin film of gold (80 nm) was deposited on paper by an e-beam evaporation system onto which PANI nanowire would be electrochemically deposited. The as-fabricated paper was lightweight and highly flexible, exhibiting electrical conductivity of about 7 W sq 1. Furthermore, the Au/paper structure could preserve the porous structure of paper. Generally, due to the large surface roughness of paper, the metal layer needs to be thick enough (50 nm) to obtain decent conductance. When patterning is needed, evaporation methods are reliable as shown in the work of Lee et al.97 Prior to the deposition of metal, parylene was first thermally evaporated onto the paper for waterproofing and insulation purposes, then 200 nm gold patterns were directly thermal evaporated onto the parylene passivated paper using different hard masks. Different interdigital electrode patterns were easily obtained with excellent reproducibility for further study of the electrode pattern design on the performance of the micro in-plane SC. Chemical vapor deposition (CVD) is an effective way of depositing carbon materials like CNTs, graphene, and their composites. Superior to solution-based methods of depositing such materials, the CVD self-assembly of the flexible CNT/graphene film avoids the use of surfactants which is beneficial to the increase of electrochemical performance of the SC device.98 4.3

Dip coating and (vacuum) filtration

Most paper based SCs are fabricated via simple solution-based processes such as dip coating and vacuum filtration. However, many of these processes involve environmentally unfriendly chemicals and elaborate procedures. Vacuum filtration is usually

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the first step to make the paper conductive with CNTs or graphene, then the conductive paper after processes such as electrodeposition69 can be coated with pseudocapacitive materials and used as electrodes for paper SCs. As an example, Shen et al. reported paper-based electrodes based on PANI-coated BC paper for SCs.69 The fabrication process of BC-MWCNTs-PANI paper electrodes is shown in Fig. 4d. BC paper was first fabricated from its suspension via vacuum filtration technique. Then the same method was applied to deposit a MWCNTs layer onto BC paper. PANI was coated onto MWCNTs surface through electrochemical polymerization.

into highly conductive paper with a low sheet resistance of around 10 O sq 1 based on which a paper-based flexible SC was fabricated. The low sheet resistance of conductive paper by Meyer rod coating was mainly attributed to the conformal coating of CNTs on the fiber structure of paper as shown in Fig. 4e. Sometimes, when patterning is needed, similar CNT/ graphene inks can also be ink-jet printed on paper substrates in patterns, significant for the fabrication of micro-SCs which usually need interdigital finger electrodes.53 Direct printing with an ink-jet printer is a material-saving, high-speed, and low cost process.101

4.4

4.5

Printing

To take full advantage of paper as an excellent printing substrate, printing methods are preferred in the fabrication of paper-based SCs. A variety of nanoscale inks have been successfully printed onto paper including conductors (silver nanoparticles, CNTs, graphene, etc.), semiconductors, and insulators. Common printing methods include inkjet printing, screen printing, gravure printing, etc.99 Given the traditional use for paper, some unorthodox solution based deposition methods like Chinese calligraphy using brushes (Fig. 4c) and pen writing were also demonstrated as examples.38 Printing methods such as inkjet printing (Fig. 4f) and Meyer rod coating (Fig. 4e) are potentially scalable methods. Two important aspects are ink preparation and film formation. Preparation of stable ink with the right rheological properties is critical and sometimes difficult, and it is crucial to develop proper ink with the suitable rheology (viscosity and surface tension) to be printable and to obtain smooth patterns.100 Inks based on single-wall-carbon-nanotubes (SWCNTs) and graphene are two common materials developed for paper-based SCs. Paper is a perfect substrate for printing methods to show their power. Usually for printing, after deposition on substrates, complex post-treatments are sometimes demanded such as high temperature annealing or washing to remove the surfactants and improve conductivity. Moreover, the surface treatment of the substrate is also generally needed to facilitate uniform film adhesion. However, for paper-based SC applications, due to the porous nature, the surface treatment is usually not needed, since the porous structure of paper also causes a smaller contact angle of inks on paper than that on a plastic substrate. Thus, it is generally not imperative to pay attention to the surface tension matching problem between ink and the paper surface, as the coating and drying is almost instant owing to the porous structure of paper, which can absorb the solvent quickly. Capillary forces also facilitate the large area contact between SWCNTs/graphene and cellulous fibers, thus favoring a better electrochemical performance than otherwise. Since continuous films are normally needed for use as current collectors and/or electrodes for SCs, large scale coating methods are usually applied. For example, a scalable Meyer rod coating method was adopted to fabricate CNTs or Ag NWs continuous films.38 The SWCNT ink was applied to the paper surface, and a Meyer rod, which provided the thickness control, was rolled over the ink. Instantly, the paper was transformed

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Paper-making technique

It is also an effective approach to make cellulose/conductive material composite paper starting with cellulose pulp instead of a piece of already-made paper. As an example, Su et al. made a piece of composite paper composed of cellulose fibres coated with graphene nanosheet (GNS) by dispersing chemically synthesized GNS into a cellulose pulp, followed by infiltration.102 The GNS formed a continuous network through a bridge of interconnected cellulose fibres at small GNS loadings. The GNS/ cellulose paper was as flexible and mechanically tough as the pure cellulose paper. The composite paper was used as an electrode for flexible EDLC type SCs. The paper-making process with a better coating efficiency is attractive for the development of high performance paper for electrical, electrochemical and multifunctional applications.103–105

5. Recent developments of paper-based supercapacitors As with other kinds of flexible SCs, the electrode is the key to improving the performance of paper-based SCs. Unique to paper-based SCs is the way the cellulose fiber network interacts with conductive materials to form a conductive cellulose-based electrode (sometimes also as a current collector), based on which the whole paper-based SC is built as mentioned in the Device structure section. There are three main kinds of methods to achieve this, thus the development of paper-based SCs is first discussed in terms of these three methods: surface coating, bulk mixing, and molecular mixing. 5.1

Surface coating

In many cases, it is sufficient to make only the surface of papers conductive, while the bulk of cellulose fibers acts more like a mechanically flexible substrate than a conductive network. Commonly used conductive materials for this purpose are mainly metal thin films and carbon-based materials such as CNTs and graphite. CNTs have attracted increasing interest for SC applications due to their excellent electrical conductivity, unique pore structure, and exceptional mechanical, chemical and thermal stability.106,107 For this purpose, CNTs can be coated directly onto cellulose paper as both the electrode and current collector. As active materials in electrodes, CNTs not only are highly conductive and flexible, but also can increase


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the effective surface area in the films. Compared with metallic current collectors such as Cu or Al foil which are heavy and have smooth surface with a weak adhesion to active materials, the CNT-based current collectors/electrodes are much lighter and rougher on the surface and thus can reduce weight and facilitate interface contact. Furthermore, the interconnected porous channels in the CNT film facilitate ion transportation; the chemical inertness of CNTs ensures the stability of CNT current collectors during electrochemical reactions. Cui et al. at Standford University reported an integrated structure, in which the electrodes and separator are integrated on a single sheet of paper.39 This integrated structure allows for the use of high-speed printing methods, which can lead to low cost, lightweight paper-based energy storage devices for disposable paper electronics. SWCNTs as both the electrode and the current collector were integrated into a single sheet of commercial printing paper. However, the device would become shortened if SWCNTs were to be directly printed onto the paper substrates, because the micro-sized pores in paper allowed the conductive SWCNTs to penetrate. To solve this problem, the authors came up with a novel solution which held promise to be a general solution: the paper substrates were coated with polyvinylidene fluoride (PVDF) to be impermeable to SWCNTs as shown in Fig. 5a. Fig. 5b shows the SC device structure where electrodes and separators were integrated into a single sheet of printer paper. Fig. 5c shows a printed SC which used newspaper as both the substrate and the separator, and the thickness of the entire device was around 30 mm. Electrochemical testing of the SC in organic electrolyte (LiPF6 in ethylene carbonate/ diethylene carbonate) showed the specific capacitance of 33 F g 1 at a specific power of 250 000 W kg 1 (Fig. 5d). Most work has utilized solution-based processes to fabricate the electrode, which involve dispersing the active materials in a special solvent, followed by forming the electrode on paper substrates. The use of environmentally unfriendly chemicals

Fig. 5 (a) Schematic of Xerox brand printer paper treatment with PVDF. (b) Paper SC structure with SWCNTs film printed on both sides of the treated Xerox paper. (c) A printed SC on a 30 mm thick newspaper substrate/separator membrane. (d) Specific capacitance at different current densities. Reproduced with permission from ref. 39. Copyright 2010, American Institute of Physics.


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Fig. 6 (a) Schematic diagram of the conductive electrode on cellulose paper by simple pencil-drawing. (b) Digital camera image of the conductive paper electrode by drawing. (c) Resistance measurement of the graphitic coating on paper (width of the graphite stripe is 2.0 cm) over different lengths. (d) Schematic diagram of the paper SCs. Reproduced with permission from ref. 95. Copyright 2011, Royal Society of Chemistry.

and the requirements of post-treatments pose challenges for the wide applications of these techniques. Cui et al.95 reported a novel and interesting approach to fabricate supercapacitors by drawing with a graphite rod on a standard printing paper (Fig. 6a), as graphite itself is an excellent electrode material for SCs, and paper surface is rough enough that pencil drawing can leave a large amount of graphite coating. For accurate characterizations, ruler-guided drawing in orthogonal directions was repeated several times to form a strip of uniform graphite coating. The graphite trace is very stable and flexible even upon bending down to a radius of 2 mm as shown in Fig. 6b. The resistance of the conductive strip showed a linear relationship with length, indicating a relatively uniform coating of graphite on the paper surface (Fig. 6c). The SC was assembled by two conductive paper electrodes sandwiching a separator (Xerox paper) as shown in Fig. 6d. Electrochemical testing of SCs showed a high areal capacitance of 2.3 mF cm 2 and excellent long cycling performance. This solvent-free method demonstrated was highly scalable and could be a viable alternative approach for making low cost and environmental-friendly energy storage devices. Graphite by itself does not result in high electrochemical performance. A further step from pencil-drawing was reported by Chen et al.88 The device was fabricated by first drawing a uniform graphite layer on ordinary printing paper by pencil to make it conductive. Then PANI nanowire networks were synthesized by oxidation of aniline via electrochemical deposition to enhance the conductivity and electrochemical activity of the electrode. Electrochemical performance of the hybrid electrode in a three-electrode configuration in H2SO4 solution showed a high areal capacitance of 355.6 mF cm 2 at a current density of 0.5 mA cm 2. The solid-state SCs assembled by two graphite/ PANI–paper electrodes sandwiching PVA/H2SO4 solid-state

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electrolyte manifested a high energy density of 0.32 mW h cm 3 at a power density of 0.054 W cm 3 and retained 83% of its initial capacitance after 10 000 cycles. One advantage of graphite as the current collector on paper is the good adhesion property between graphite and cellulose, which is a common problem faced by paper-based SCs with metal films as the current collector. In order to overcome the weak adhesion between metal films and paper substrates, Wang et al. reported using a polymeric layer as a binder between the current collector and paper in their work on paper-based all-solid-state PANI based flexible SCs where common printing paper was used as substrates.68 Firstly, a thin layer of PVA film was coated on a piece of a common printing paper, then an 80 nm thick Au film was evaporated onto the pre-treated paper. Lastly, PANI networks were electrochemically deposited on the surface of Au films. Fig. 7a illustrates the structure of the fabricated solid-state SCs which were assembled by two PANI/Au paper electrodes sandwiching a layer of PVA/ H3PO4 gel electrolyte. Fig. 7b shows CV scans of the solid-state SC which exhibited a rectangular shape at different scan rates in the range of 5 to 100 mV s 1. Galvanostatic charge–discharge curves of the solid-state SC are shown in Fig. 7c. Moreover, the solid-state SC was highly flexible (Fig. 7a) and could undergo severe bending without obvious capacitive performance changes (Fig. 7d). It is worthwhile to note that the solid-state SC showed a

Fig. 7 (a) The structure of solid-state SCs (upper) and photographs of the SC (down). (b) CV and (c) galvanostatic charge–discharge curves for a solid-state SC. (d) CV scans of the all-solid-state SC at different curvatures. (e) Areal capacitance versus discharge current for the all-solid-state SC with PANI networks deposited at with 5 min. (f) Cycle performance and coulombic efficiency of the all-solid-state SC. Reproduced with permission from ref. 68. Copyright 2012, Wiley-VCH Verlag GmbH & Co.

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Fig. 8 (a) Schematic illustration for the fabrication of graphite/ Ni/Co2NiO4–CP electrode. (b) Schematic illustration for the assembled graphite/Ni/Co2NiO4–CP//graphite/Ni/AC–CP ATFSCs. (c) The flexible assembled ATFSCs at a normal (flat) state. (d) CVs of the ATFSC device at different scan rates. (e) Galvanostatic charge–discharge curves of ATFSC at different current densities. Reproduced with permission from ref. 70. Copyright 2014, Wiley-VCH Verlag GmbH & Co.

high areal capacitance of about 50 mF cm 2 with respect to the discharge current increasing from 0.1 to 2 mA cm 2 (0.2 to 4.3 A g 1) and a power density of around 3 W cm 3 at an energy density of around 0.01 W h cm 3 as shown in Fig. 7e. Furthermore, the fabricated solid-state SC showed excellent stability after 10 000 charge–discharge cycles at 1 mA cm 2, and Coulombic efficiency remained almost 100% (Fig. 7f). Li et al. utilized the simple drawing method to deposit a graphite layer on cellulose paper as the substrate for electrodeposition as shown in Fig. 8. An asymmetric paper SC with graphite/Ni/Co2NiO4–cellulose paper (CP) as the positive electrode and the graphite/Ni/active carbon (AC)–CP as the negative electrode sandwiching PVA/KOH gel electrolyte was constructed.70 The electrochemical properties of graphite/Ni/Co2NiO4–CP and graphite/Ni/AC–CP electrodes were measured by a three-electrode test system in KOH solution. The graphite/Ni/Co2NiO4–CP electrode showed a high areal capacitance of 734 mF cm 2 at 5 mV s 1 and excellent cycling performance with only 2.4% decrease after 15 000 cycles. The graphite/Ni/AC–CP electrode also exhibited superior areal capacitance of 180 mF cm 2 at 5 mV s 1 and excellent cycling performance with only 2% decrease after 15 000 cycles. The two paper-based electrodes were used to assemble an asymmetrical thin film supercapacitor (ATFSC), and the detailed structure can be found in Fig. 8a–c. The assembled flexible asymmetrical thin film SCs exhibited a superior volumetric specific capacitance of 7.6 F cm 3 at 5 mV s 1, a high volumetric energy density of 2.48 mW h cm 3 at 4.0 mA cm 2, a high volumetric power density of 0.79 W cm 3 at 20 mA cm 2, and excellent cycle stability (less than 4% specific capacitance loss after 20 000 cycles). The CV and galvanostatic charge–discharge curves are shown in Fig. 8d and e. It is worth


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noting that the CV curves of paper-based SCs usually deviate from the ideal rectangle, (Fig. 7(d), 8(b) and 11(d)) mainly due to the relatively large inner resistance of the paper-based SCs. This is a common phenomenon among flexible SCs. The fabrication of asymmetric paper SCs via a simple pencil drawing method proves the power and the great potential of this method for making paper-based SCs.


5.2

Bulk mixing

Surface coating provides a simple method to utilize the surface and mechanical properties of cellulose networks. A more common approach to fabricate a conductive active material– cellulose composite is to use a simple ‘soak and dry’ or ‘filtration-dry’ method to disperse conductive materials in the bulk of the cellulose network. The high conductivity of the thusprepared composites can be attributed to the strong solvent absorption properties of the porous paper structure and the conformal coating of conductive materials on the cellulose fibers to form continuous electrical conduction pathways. This fabrication method is applicable to many conductive materials including CNTs, graphene, and conductive polymers. For example, Cheng et al. used a vacuum filtration method to deposit graphene on a filter paper to produce conductive paper electrodes for SCs.48 Conductive paper made in this way showed excellent mechanical properties. For example, the sheet resistance increased by only 5% after the conductive paper was bent to a 4 mm radius 1000 times. To further illustrate the bulk mixing approach, Cui et al.38 used a simple solution process of Meyer rod coating to coat CNTs on commercially available paper to get CNT–paper composites. CNTs were first dispersed in deionized water assisted by surfactants to form the CNT ink. Then the CNT ink was coated on a piece of commercial Xerox paper by a simple and scalable Meyer rod coating method. After CNT coating, the paper became highly conductive with a sheet resistance of around 10 O sq 1 (Fig. 9a). The Scotch tape test implied that the CNT coating was very stable, and the sheet resistance remained constant at around 10 O sq 1 (Fig. 9b). The conductive paper also showed excellent mechanical properties. The conductive paper could be bent down to a radius from 2 mm to 10 mm without any obvious conductivity changes as shown in Fig. 9c. The strong binding between CNTs and paper was attributed to the large capillary force, maximized contact area and Van der Waals force. With the high conductivity and the large surface area, the conductive paper was studied in SC applications as electrodes and current collectors with both aqueous and organic electrolytes (Fig. 9d). A high specific capacitance of 200 F g 1 was achieved with a CNT loading of 72 mg cm 2 with H2SO4 as the electrolyte. The electrodes were also tested for high-current operations. Even at 40 A g 1, specific capacitances larger than 70 F g 1 were maintained in both aqueous and organic electrolytes. Comparing this example with the other work by the same group using the surface coating method, the difference is easy to see: in this work, CNTs were coated separately onto different anode and cathode substrates and were assembled together with a separator, whereas in the other work, electrodes and the separator were integrated on a single piece of paper.39


Fig. 9 (a) Conductive Xerox paper after CNT coating. (b) Film adhesion test with Scotch tape. (c) Sheet resistance changes after bending the conductive paper into different radii. (d) Schematic illustration of all-paper SCs based on CNT conductive paper. Zoomed-in schematic illustrates that ion accessibility is enhanced by the strong solvent absorption. Reproduced with permission from ref. 38. Copyright 2009, National Academy of Sciences.

Compared with carbon-based materials, pseudocapacitive materials can achieve much higher capacitance and energy density by introducing reversible redox Faradaic reactions upon charging and discharging.108 Carbon materials are often combined with pseudocapacitive materials for SC application to achieve improved energy and power performance. Common pseudocapacitive materials include transition metal oxides such as MnO2, CoO, NiO,109 and electrical conducting polymers like PANI,69 PPy,110 and poly(3,4-ethylenedioxy-thiophene) (PEDOT).111 Recently, Hu et al. prepared hierarchical hybrid SC electrodes based on cellulose paper/CNTs/MnO2/CNTs electrodes by dipping and electrodeposition as illustrated in Fig. 10.51 Because of the water-swelling effect of the cellulose fibers that could absorb electrolyte, and the mesoporous internal structure of the fibers that could provide channels for ions to diffuse to the electrode materials (Fig. 10d), the electrochemical performance of the paper/CNTs/MnO2/CNTs electrode, which was studied in a standard three-electrode system in Na2SO4 solution, achieved a capacitance as high as 327 F g 1 at the scan rate of 10 mV s 1, demonstrating great promise for SC application. PANI as a major group of conducting polymers has been widely investigated as pseudocapacitive materials for paperbased SCs. For instance, Shen et al. reported a flexible, lightweight paper electrode, which was MWCNTs-PANI thin films coated on BC as current collectors and active materials.69 PVA/ H2SO4 was used as polymer gel electrolyte to fabricate an all-solid-state SC based on two pieces of BC-MWCNTs-PANI hybrid electrodes. The resultant flexible BC-MWCNTs-PANI hybrid electrode can reach a specific capacitance as high as 656 F g 1 at a discharge current density of 1 A g 1. Moreover, the hybrid electrode showed remarkable cycling stability with capacitance degradation of less than 0.5% after 1000 charge/ discharge cycles at a current density of 10 A g 1. Instead of coating conductive polymers onto the conductive paper, Zhou et al.110 reported a simple ‘soak and polymerization’

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Fig. 11 (a) Schematic diagram of the fabrication of PPy-coated paper. (b) Cross-section SEM image of the PPy–paper composite. The inset shows an enlarged PPy-coated cellulose fiber. (c) PPy-coated paper passes the tape test. (d) Cyclic voltammetry curves for the solid-state SC with PPy/paper electrodes. The inset shows the schematic diagram of solid-state SCs. Reproduced with permission from ref. 110. Copyright 2013, Royal Society of Chemistry. Fig. 10 Schematic illustration of the electrode synthesis process. (a) One single cellulose fiber was used to illustrate the process. (b) CNT dip-coating. (c) Electrodeposition of MnO2. (d) Second CNT dip-coating. (e) Magnification of the square area highlighted in (d) to illustrate the dual electron charge transfer and ion diffusion paths in the paper/CNTs/MnO2/CNTs electrode. Reproduced with permission from ref. 51. Copyright 2013, American Chemical Society.

method to fabricate PPy-coated paper. PPy was directly coated onto cellulose networks and served as both electrodes and current collectors to assemble flexible solid-state SCs with high conductivity and high specific capacitance.112,113 The detailed preparation process was as follows: a piece of common printing paper was soaked in the pyrrole monomer, then transferred into ferric chloride solution with hydrochloric acid and thus obtained the PPy-coated paper (Fig. 11a). The cellulose fibers on the surface and in the bulk of the paper have been coated with PPy uniformly by polymerization of pyrrole monomers. The as-fabricated porous, flexible, and conductive paper had a lower sheet resistance of 4.5 O sq 1 than Au/paper (7 O sq 1)68 mainly due to the efficiency of the bulk mixing method to make a cellulose-based conductive network in Fig. 11b, indicating the strong adhesion between PPy and the cellulose network. Moreover, no obvious PPy could be observed on tape after tape tests showing the excellent mechanical stability (Fig. 11c). Symmetrical solid-state SCs were fabricated by sandwiching PVA/H2SO4 electrolyte between two identical PPy/paper electrodes and the SC showed an areal capacitance of 0.42 F cm 2 at a discharge current of 1 mA cm 2 (Fig. 11d), which corresponded to a high energy density of 1 mW h cm 3 at a power density of 0.27 W cm 3 normalized to the volume of the whole cell (electrode, electrolyte, and separator). Despite the low specific capacitance (a theoretical capacitance of 210 F g 1) when compared with other conducting polymers such as PANI (750 F g 1) and PPy (620 F g 1), much effort has been made to optimize the specific capacitance of PEDOT. For instance, Alshareef et al. demonstrated that a

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nucleation seed layer of PEDOT:poly(styrenesulfonate) (PSS) used in electrochemical growth of the conducting polymer PEDOT has a significant impact on the performance of PEDOT/ paper SCs.111 Common printing paper was used as the substrate for drop casting PEDOT:PSS followed by acid treatments, and then PEDOT was electrochemically deposited on PEDOT:PSS/ paper substrates. The optimized PEDOT/PEDOT:PSS/paper electrodes were employed to fabricate solid-state SCs which showed a cell capacitance of 32 mF cm 2 with the PVA/H2SO4 gel electrolyte and 11 mF cm 2 with an ion gel electrolyte. The capacitance showed 20% degradation over 10 000 cycles. 5.3

Molecular mixing

At the individual fibril scale, the similarity in dimensions and aspect ratio of the cellulose fibers and CNTs or chains of conductive polymers allows uniform mixing of the two materials, resulting in a highly conductive porous composite suitable for high surface area electrodes.114 The specific methods often appear in the form of ‘paper making’. As a typical example, an electrically conductive BC/PPy/MWCNTs nanofiber composite paper was prepared by filtering a mixed BC/PPy/MWCNTs pulp and vacuum drying.115 Due to an interpenetrative network structure formed among BC/PPy nanofibres/MWCNTs, the BC/PPy/MWCNTs hybrid films exhibited highly conductive properties with a sheet resistance of 4.37 O sq 1, which was highly promising for paper based SCs. In another example, Li et al. prepared conductive cellulose composite films by in situ polymerization of aniline monomers in the cellulose scaffolds.103 The same group had been devoted themselves to research on cellulose dissolving using alkaline and urea based aqueous solvents and the construction of functional cellulose-based materials from the developed solvent.116,117 The regenerated cellulose films prepared from LiOH/urea or


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NaOH/urea aqueous solution had porous structures that could be used as scaffolds for the synthesis of inorganic nanoparticles118 or curable organic prepolymers119 in situ for the construction of functional cellulose-based materials. In this particular case of PANI, the effects of reaction time, dopants and the concentration of aniline monomers on the structures and properties of the composite films were investigated. Structurally defined PANI– cellulose composite films with a PANI content of about 24.6% exhibited comparable electrical conductivity as pure PANI films. The excellent conductivity was attributed to the effect from the porous cellulose scaffold. The PANI/cellulose films integrated the merits of cellulose and conductive PANI. The composite films were foldable and applied as flexible electrode materials for paper-based SCs. The electrochemical properties of the composite film were studied in a three-electrode system showing a specific capacitance as high as 160 F g 1 at a current density of 0.5 A g 1, which maintained at least 81% of the initial value after 1000 cycles at a current density of 0.5 A g 1. Therefore, this approach sheds light on the production of environmentalfriendly and biocompatible energy devices. Nyholm et al. reported a facile in situ polymerization method to prepare PPy@nanocellulose reinforced 3D PPy composites.105 The synthesis of the PPy@nanocellulose–3D PPy composites was illustrated in Fig. 12. The conductive and electro-active 3D network, fabricated via the polymerization of Py in the presence of phytic acid, was tuned to contain pores with a size of 20 to 50 nm with the larger pores facilitating the diffusion of electrolyte ions, and smaller pores accommodating the PPy volume changes during the redox processes. The facile PPy coating onto the nanocellulose fibers can be explained by the following reasons: (1) the cellulose fibers are well wetted by PPy; (2) both cellulose and PPy have the ability to form hydrogen bonds via OH- and NH-groups; and (3) the phytic acid can serve as a counter ion for two separate PPy chains, acting as a crosslinking agent. As a result, a porous and conducting PPy network reinforced with the nanocellulose fibers was readily obtained. The inclusion of nanocellulose fibers considerably improved mechanical properties compared to those of the pristine 3D PPy materials. A symmetrical device with a piece of ordinary filter paper as a separator between two rectangular pieces of the

Fig. 12 Schematic illustration of the synthesis of the PPy@nanocellulose reinforced 3D PPy electrodes. Reproduced with permission from ref. 105. Copyright 2014, Royal Society of Chemistry.


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composite PPy-based paper-like electrode was also made, exhibiting specific capacitance of 5.5 F cm 2 at a current density of 2 mA cm 2 in aqueous NaCl solution electrolyte. 5.4

Paper-based in-plane supercapacitors

Micro-scale power sources are in urgent need with the development of small-scale and portable electronics.35 Being able to be effectively self-powered for a long time is the requirement for many devices such as implantable medical devices. For this sort of applications, SCs prevail over the presently prevalent batteries due to a longer lifetime and higher power density. Micro SCs usually follow one of the two device architectures, either sandwiched or in-plane. As a matter of fact, most paperbased SCs discussed in this review fall in the realm of micro SCs, and most paper-based SCs adopt a sandwiched device architecture with two paper-based electrodes (identical in the case of symmetrical and different in the case of asymmetrical) sandwiching a gel electrolyte layer acting as the electrolyte as well as the separator. However, we have already seen some in-plane structured devices showing their promise on paper substrates, maybe due to the fact that paper reminds people of easy patterning, which is crucial for fabricating the interdigital electrodes often needed for in-plane (micro) SCs. Micro in-plane paper SCs on the basis of interdigital electrodes have many advantages over conventional sandwiched structured counterparts, such as improved rate performance due to enhanced ion transport and easy integration with other in-plane devices,53 as shown both on rigid substrates120 and on flexible substrates such as PET.121 So far, reports about micro in-plane paper SCs based on interdigital electrodes have been scarce, not to mention detailed studies. Lee et al. presented the first report on the design and fabrication of a high performance all solid-state flexible microSC on paper.97 In order to overcome the high surface roughness of paper which was a major concern for metallic interconnections and functional device fabrication, parylene was thermally evaporated onto a piece of commercially available photographic paper for waterproofing and insulation purposes. Gold interdigital electrode patterns were then directly thermal-evaporated onto the parylene treated paper using masks. Three dimensional interconnected coral-like PANI–MnOx composite materials were electrochemically deposited on interdigital-finger shaped gold electrodes. The influence of interdigital-finger electrode design on the electrochemical performance was studied and the high aspect ratio and small inter-electrode gap design was found to be crucial in improving the overall device performance. It was surmised that different designs of finger electrodes would change mass diffusion around the patterned gold electrodes during electrochemical deposition. It was found that the high aspect ratio of the interdigital finger electrode pattern led to a higher mass loading under the same electrochemical deposition conditions. A symmetrical SC based on the PANI–MnOx electrodes was studied with PVA/H3PO4 gel electrolyte. As shown in Fig. 13a, the CV curves showed typical rectangular shapes at different scan rates, indicating typical capacitor performance with good rate capability. In Fig. 13b, the charge–discharge

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Fig. 13 (a) CV curves of a sample tested in a gel electrolyte at different scan rates. (b) Charge–discharge curves of the sample tested in the gel electrolyte at different current densities. (c) Cycling test of the sample tested at 0.5 mA cm 2 in the gel electrolyte. (d) Ragone plot of the sample in the gel electrolyte. Reproduced with permission from ref. 97. Copyright 2013, Royal Society of Chemistry.

curves showed a symmetric triangular shape with good capacitor behavior. The specific areal capacitances calculated from the charge–discharge tests were 94.73 mF cm 2 at 0.1 mA cm 2 and maintained at 85.7% after 2000 charge–discharge cycles, suggesting a good cycling stability (Fig. 13c). As shown in Fig. 13d, the symmetrical SC demonstrated a high energy density of 6.3 mW h cm 2 at a power density of 35 mW cm 2. Chen et al. adopted the pencil-drawing method to fabricate planar SCs based on interdigital graphite patterns on paper (Fig. 14a).88 Then PANI networks were electrodeposited on the interdigital electrodes. As can be seen in the inset of Fig. 14a, the as-fabricated planar G/PANI–paper electrodes were flexible and could be used in portable and wearable electronics. Solidstate planar SC was fabricated by coating solid-state PVA/H2SO4 electrolyte onto the surface of the interdigital electrodes and the gaps between them. The charge–discharge curve in Fig. 14b displayed good capacitive behavior. There are generally two approaches to fabricate interdigital electrodes. In the first approach, interdigital current collectors are fabricated where active materials are then deposited. In the second approach, a patterning step is performed on a thin film of active materials to form the interdigital electrodes. The second approach was taken by Cheng et al.48 on paper by patterning the active GCP membrane (GCP material is a sheet of filter paper in which GNSs are strongly bound to the cellulose fibers and fill the pores) into two interdigital GCP electrodes, combined with PVA/H2SO4 gel electrolyte, a flexible interdigital micro-SC was fabricated, the capacitance per geometric area (7.6 mF cm 2) was lower than that of the flexible laminated SC based on the same active material, which might result from a larger internal resistance and longer ion diffusion path in the interdigital device. Despite the low capacitance, this in-plane

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Fig. 14 (a) Optical picture of pencil-drawing interdigital electrodes. The insert shows the flexible planar interdigital electrode deposited PANI on the interdigital substrate. (b) Charge and discharge curves of the planar interdigital SC with G/PANI–paper electrodes at different currents. Reproduced with permission from ref. 88. Copyright 2013, Elsevier. (c) Optical photograph of a planar structured solid-state SC with PPy-coated paper electrodes. The corresponding schematic diagram is plotted below. (d) Galvanostatic charge–discharge curves for the planar SC with different device configuration at a fixed current of 1 mA. Reproduced with permission from ref. 110. Copyright 2013, Royal Society of Chemistry.

SC was very thin, which was desirable for ultrathin electronic devices. Zhou et al. fabricated two in-plane SCs on one piece of PPy-coated paper in order to demonstrate the feasibility of integrating in-plane paper-based flexible SCs.110 As shown in Fig. 14c, the two SCs (devices A and B) adopted an in-plane structure, each with 2 pairs of interdigital electrodes. The whole device was coated with solid-state PVA/H3PO4 electrolytes except for the margin area acting as the electrical contact. The two SCs were separated by terminals A2 and B2 as shown in Fig. 14c. The galvanostatic charge–discharge curves of Devices A and B in Fig. 14d show that the obtained micro in-plane SCs roughly obeyed the basic rule of series and parallel connections. (A and B were connected in series when terminal A1 and B2 were selected to discharge; in parallel when terminal A1 and B2 were connected as one pole while A2 and B1 as the other pole). Furthermore, in terms of capacitance, the calculated capacitance of devices A and B was 401.2 and 434.2 mF, respectively, corresponding to 208.5 and 835.4 mF for the series and parallel combinations, whereas the series and parallel calculated from the galvanostatic charge–discharge curves were 156.5 and 1063.4 mF, respectively. 5.5 Application and integration efforts for paper-based supercapacitors Potential applications for paper-based SCs are mainly energy storage units for paper-based systems. For example, to store electrical power extracted from the environment by energy


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Fig. 15 (a) Galvanostatic charge–discharge curves of two individual SC devices and the device in series or in parallel based on them at a fixed current of 5 mA. (b) Optical image of a red LED lighted by three charged SCs connected in series. Reproduced with permission from ref. 67. Copyright 2014, Elsevier. (c) Schematic diagram of the self-powered nanosystems. (d) Charging curve for six all-solid-state SCs connected in series charged by a piezoelectric generator. The inset shows an optical image of a blue LED lighted by charged SCs connected in series. (e) Intensity–time curves of a strain sensor driven by direct current voltage (gray curve) and SCs (black curve) for comparison. Reproduced with permission from ref. 68. Copyright 2012, Wiley-VCH Verlag GmbH & Co. (f) Mechanism of the m-PADs based on a paper SC. (g) Photograph of the m-PADs based on a paper SC. (h) Structure (cross section) of the paper SC for the m-PADs. Reproduced with permission from ref. 130. Copyright 2013, American Chemical Society.

harvesters,122 or to power outlets such as radio frequency identification (RFID) tags.123 If paper-based SCs were to be useful in practice, they should be able to connect in series or parallel to result in a certain working potential window and energy storage capacity for a certain application. In-plane SCs have been shown to follow this connection rule in the previous section. The sandwiched structured paper-based SCs were studied by Shen et al.67 Fig. 15a shows the galvanostatic charge–discharge curves for individual devices (device A and B) and the two devices connected in series and parallel at a current density of 5 mA cm 2. The capacitance of device A and B was 409 mF and 499 mF, respectively. Accordingly, the capacitance of their combination in series and parallel should be 225 mF and 908 mF, which is in close agreement with the calculated values from the curves of 243 mF and 1006 mF, respectively. This indicated that the paper SC obeys the basic theorem of series and parallel connections of capacitors. Three SCs were assembled in series and could be charged to 2.4 V and easily light up a red LED (light emitting diode) (Fig. 15b). Nam et al.124 demonstrated an all-solid-state, origami-based and stretchable SC system with integrated series circuit analogues on paper. This new energy system comprises periodically isolated electrodes (IEs) and sectionalized ion transferring paper (SITP), which are key factors for the densely packed series


circuit analogues in a single system. The system exhibited a linear relationship between the potential window and the number of IEs, and increased energy and power densities could be achieved simultaneously. The unique origami concept-based system enabled high flexibility and stretchability, and a threeIEs sample septuple-folded in a zigzag formation was fabricated as a concept device and evaluated for electrochemical characteristics. This system showed similar CV curves in the compressive (60%), planar and tensile deformation (30%) conditions with the specific capacitance of 0.94, 0.98 and 0.93 mF cm 2 for each deformation condition, respectively, showing the highly foldable nature of this design, thus promising for the nextgeneration portable consumer electronics. The unique folding characteristics of the origami design were further proved by simulations based on ab initio calculations and the finiteelement method. Generally speaking, one of the ideal applications of paperbased SCs and flexible SCs is when a sustainable and renewable energy source acts as the energy source, a self-powered electronic system can be formed with SCs acting as the bridge between the energy source and energy outlet.125 A model system was built by Wang et al. to test the feasibility of such an idea.68 The system is shown in Fig. 15c. A piezoelectric generator was used to harvest energy by a resonator, the alternating current from the generator was rectified to direct current by a bridge

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rectifier, and thus the SCs could be charged continually. The charging curve of six paper-based SCs connected in series is shown in Fig. 15d. The SCs were used to power a strain sensor, thus the sensor could be operated without a battery. The current–time (I–t) curve of the system was measured under cyclic straining at a frequency of 2 Hz (Fig. 15e). The current reached the same value in each cycle and fully recovered the zero strain state, indicating the stability of the output voltage of SCs. This demonstration manifested the potential of paperbased SCs for self-powered systems such as a mobile sensor. Paper-based SCs can also be combined with microfluidic paper-based analytical devices (m-PADs). Microfluidic paper-based analytical devices (m-PADs) as a concept was first proposed by Whitesides et al.126 and has been widely used for health monitoring, molecular analysis, and environmental detection in resource-limited and remote regions due to its advantages of being simple, portable, disposable, and inexpensive. Photoelectrochemical (PEC) method which has the advantages of both optical and electrochemical method is a promising analytical method that can be utilized for m-PADs.127 However, conventional PEC method required lock-in amplifier128 or electrochemical workstation129 to measure low photocurrents thus departing from the portable and simple virtues of m-PADs. To meet this challenge, Huang et al. came up with a novel device which integrated all-solid-state paper SCs into the PEC m-PADs as an effective electrical energy storage unit to collect and store the photocurrents produced by the PEC m-PADs.130 The mechanism and device structures are shown in Fig. 15f–h. As shown in Fig. 15f, the photocurrents generated by CdS nanoparticles under an internal light source were collected and stored in all-solid-state paper based SCs. The stored electrical energy could be released instantaneously through a simple, portable, and low-cost digital multimeter to obtain an amplified current which was higher than the direct photocurrent measurement, thus improving sensing sensitivity.

6. Conclusions Research of paper-based SCs is in its infancy compared with other kinds of flexible SCs and other paper-based electronic devices such as paper batteries. This review tries to cover the current status of design, fabrication and application of paper-based SCs. In this section, the major challenges are identified and future trends are discussed. The structural design plays an important role in the performance enhancement of paper SCs and flexible SCs in general. Most paper-based SCs utilize a traditional sandwiched structure, while this device configuration can not compete with the novel configuration based on the in-plane interdigital electrodes in achieving high power and energy density, especially when 2D materials such as graphene are used as electrode materials due to a decreased ion transport resistance and increased accessibility of electrode materials. The latter in-plane configuration also has the advantage of ease of fabrication in the context of paper-based SCs where printing is the mainstream fabrication technique and patterning is easily achievable via printing.

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Most paper-based SCs adopt a symmetrical configuration with two identical electrodes, however, the potential window of a symmetrical SC is rather limited, thus leading to a limited energy density. One solution is to develop asymmetric paper SCs which utilize an EDLC electrode as the power source and a pseudocapacitive electrode as the energy source. Nevertheless, this device configuration is relatively lacking in the field of paper-based SCs. Nearly all solid-state paper SCs so far use PVA based GPEs, the current GPEs are limited in ionic conductivity and the potential window. They are also time-consuming and difficult to prepare and to solidify. Further, it is hard to make GPEs into thin films, and the resulting large inner resistance is not beneficial for the electrochemical performance of paper SCs. Improvement of existing GPEs and development of novel solid electrolytes for paper-based SC application are in urgent need. To use a liquid electrolyte with appropriate leakage-free encapsulation techniques is also promising due to the much larger ionic conductivity, which is especially attractive in the case of in-plane paper SCs. However, this approach has not been taken yet. As with many kinds of flexible SCs, a major challenge in the development of paper SCs is that there is a lack of standardized methods to fabricate devices and to evaluate the performance of paper SCs. Fundamental understanding is also needed in both the device structure and the working mechanism of paperbased SCs. Questions such as the interaction between electrode materials and the cellulose fibers, the transport of ions in the paper electrodes have seldom been asked by far. Further development of paper SCs also relies on the integration of the SC device with other devices on a single piece of paper such as with an energy generator or a power outlet into multi-functional or self-powered systems. Thus research on paper based SCs should concentrate more on utilizing solution-based cheap manufacturing methods that can be easily scaled up such as a simple ‘soak and try’ method or printing methods such as screen printing and inkjet printing. Industrial efforts are mainly relied upon to lead the charge on this front. So far, research is focused on individual devices, however, a system-level view should be taken, after all, the potential of the paper SC mainly lies in it being the energy storage unit of a function paper-based system. The ideal application seems to be one-use-and-throw-away functional devices on paper that will enable, for example, cheap blood test paper chips for the area in desperate need of emergent medical care. There are a number of companies focusing on the commercialization of paper-based devices, however, in terms of energy storage units, more attention has been paid to batteries rather than SCs. This represents more an opportunity than a problem, as we see in this review, paper SCs are developing so fast that it is a worthy goal to develop a functional paper-based useful system based on paper SCs as the energy storage units for both industry and academia. Efforts from both sides will soon produce products far beyond what has been demonstrated in this review, revolutionizing the fields such as flexible energy storage and paper electronics.


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Abbreviations m-PADs AC ATFSC BC CNTs CP CV CVD DIY EDLCs GCP GNS GPEs IEs LED MWCNTs OLED PANI PEC PEDOT:PSS PET PPy PVA PVDF RFID SC SITP SWCNTs

Microfluidic paper-based analytical devices Active carbon Asymmetrical thin film supercapacitor Bacterial cellulose Carbon nanotubes Cellulose paper Cyclic voltammetric Chemical vapor deposition Do it yourself Electrical double layer capacitors Graphene–cellulose paper Graphene nanosheet Gel polymer electrolytes Isolated electrodes Light emitting diode Multi-walled carbon nanotubes Organic light emitting diode Polyanilines Photoelectrochemical Poly(3,4-ethylenedioxy-thiophene): poly(styrenesulfonate) Polyethylene terephthalate Polypyrroles Polyvinyl alcohol Polyvinylidene fluoride Radio frequency identification Supercapacitor Sectionalized ion transferring paper Single-wall-carbon-nanotubes

Acknowledgements We acknowledge financial support from the National Key Basic Research Program of China (973 Program, 2014CB648300), the Program for New Century Excellent Talents in University (grant no. NCET-13-0645, and NCET-13-0872), the National Natural Science Foundation of China (21422402, 21201010, 20904024, 51173081, 61136003, and U1304504), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN004), the Science& & Technology Foundation of Henan Province (14B150001), the Natural Science Foundation of Jiangsu Province (BK20140060, BK20130037, BM2012010), Specialized Research Fund for the Doctoral Program of Higher Education (20133223110008), the Ministry of Education of China (IRT1148), the Program for Graduate Students Research and Innovation of Jiangsu Province (CXZZ12-0454), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Six Talent Plan (2012XCL035) and Qing Lan Project of Jiangsu Province.

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Lactate Sensors on Flexible Substrates.

MoS2-Based Nanocomposites for Electrochemical Energy Storage.

Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage.

Flexible energy-storage devices: design consideration and recent progress.

Paper chase: a computer-based reprint storage and retrieval system.

A Bamboo-Inspired Nanostructure Design for Flexible, Foldable, and Twistable Energy Storage Devices.

All-solid-state flexible supercapacitors based on highly dispersed polypyrrole nanowire and reduced graphene oxide composites.

Highly flexible and adaptable, all-solid-state supercapacitors based on graphene woven-fabric film electrodes.

Au@MnO2 core-shell nanomesh electrodes for transparent flexible supercapacitors.

Selective wetting-induced micro-electrode patterning for flexible micro-supercapacitors.

Stretchable and semitransparent conductive hybrid hydrogels for flexible supercapacitors.

Nanostructured graphene composite papers for highly flexible and foldable supercapacitors.

Printed all-solid flexible microsupercapacitors: towards the general route for high energy storage devices.