Nanocellulose based functional materials for supercapacitor applications

Environmental protection and renewable sources for energy conversion and storage remain an important topic nowadays. Major challenges of the 21st century that mankind has to face are certainly energy supply, its storage and conversion in a way that essentially protect the environment. In this scenario the nanocellulose has come up as a sustainable and promising nanomaterial with its unique structure and remarkable properties, such as high specific modulus, excellent stability in most solvents, low toxicity and natural abundance. Its ecofriendly nature, low cost, easy availability and simple synthesis techniques render the nanocellulose as a promising candidate for the fabrication of green renewable energy storage devices. Herein, we present a comprehensive review of the current research activities that focus on the development of nanocellulose materials for energy storage applications, particularly on supercapacitors. To begin with, we give a brief introduction on the necessity of ecofriendly approaches towards the development of supercapacitors that make use of nanocellulose. We then focus on various investigations that have been carried out to fabricate supercapacitors based on nanocellulose or its composites. Finally, we describe our outlook on several issues that warrant further investigations in this topic with immense potentials.

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ls an 8912 , Ind , Ind 30 April 2019 Accepted 3 June 2019 Available online 12 June 2019 Keywords: Nanocellulose Supercapacitor topic nowadays. Major challenges of the 21 century that mankind has to face are certainly energy In recent time we have seen considerable improvements in the environmental concerns due to the generation of electronic and plastic wastes [1]. The extraordinary growth in portable electronic batteries, super capacitors, or fuel cells [2,3]. Super capacitors, also itors, have a high interesting prop- g the gap between rrently, polymeric ained wide atten- y, lightweight, and tive polymers due tter and safer re- des [6]. Since the able there must be extraordinary and viable efforts for the devel- opment of low-cost, lightweight, environment-friendly, high- performance super capacitors and batteries [7]. Among bio- polymers, natural polysaccharides are being developed as sub- stitutes to synthetic materials for electrochemical devices [4]. Nanocellulose, is a structural polysaccharide that has gained much attentions nowadays due to its renewability, inherent biocompati- bility and biodegradability, cost-effectiveness, natural abundance * Corresponding author. E-mail addresses: jasminejose764@gmail.com (J. Jose), vinoythoma@gmail.com (V. Thomas), vrindavinod.kk@gmail.com (V. Vinod), raniabraham@gmail.com (R. Abraham), susanmi@gmail.com (S. Abraham). Contents lists availab Journal of Science: Advance journal homepage: www.el Journal of Science: Advanced Materials and Devices 4 (2019) 333e340Peer review under responsibility of Vietnam National University, Hanoi.tionary changes to our society and along with this arises serious future generation electronics industry needs to be highly sustain-living standard which doubtlessly could be attributed to the inno- vation and development of more viable products through more efficient processes. To keep this process moving we need to make use of resources that are renewable and sustainable. Moreover, the technologies have reached a new era of information technology and material innovation is experiencing an unpredictable pace of pro- gressing. Numerous types of sensors, electronic devices and so on are built everyday using advanced ceramics, metals, and plastics. The huge advancement of today's technology is bringing revolu- termed as electrochemical double-layer capac capacity with a fast charge/discharge rate. These erties havemade super capacitors capable of fillin batteries and conventional capacitors [4]. Cu materials for super capacitor applications have g tions because of their properties, such as flexibilit stable cycling performance [5]. Further, redox ac to their recyclability and sustainability are be placements for heavy metals in battery electroresearchers in developing versatile energy storage devices, such as 1. Introductionhttps://doi.org/10.1016/j.jsamd.2019.06.003 2468-2179/© 2019 The Authors. Publishing services b ( has come up as a sustainable and promising nanomaterial with its unique structure and remarkable properties, such as high specific modulus, excellent stability in most solvents, low toxicity and natural abundance. Its ecofriendly nature, low cost, easy availability and simple synthesis techniques render the nanocellulose as a promising candidate for the fabrication of green renewable energy storage devices. Herein, we present a comprehensive review of the current research activities that focus on the development of nanocellulose materials for energy storage applications, particularly on supercapacitors. To begin with, we give a brief introduction on the necessity of ecofriendly approaches towards the development of supercapacitors that make use of nanocellulose. We then focus on various investigations that have been carried out to fabricate supercapacitors based on nanocellulose or its composites. Finally, we describe our outlook on several issues that warrant further investigations in this topic with immense potentials. © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license ( systems during the past few decades have gained the attention ofReceived 8 January 2019 Received in revised form supply, its storage and conversion in a way that essentially protect the environment. In this scenario theArticle history: Environmental protection and renewable sources for energy conversion and storage remain an important stReview Article Nanocellulose based functional materia applications Jasmine Jose a, Vinoy Thomas a, *, Vrinda Vinod a, R a Centre for Functional Materials, Christian College, Chengannur University of Kerala, 6 b Department of Chemistry, Christian College, Chengannur University of Kerala, 689122 c Department of Economics, Christian College, Chengannur University of Kerala, 689122 a r t i c l e i n f o a b s t r a c ty Elsevier B.V. on behalf of Vietnamfor supercapacitor i Abraham b, Susan Abraham c 2, India ia ia le at ScienceDirect d Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license and eco-friendly nature [8]. Cellulose is an odourless, biodegrad- able, hydrophilic, water insoluble material which can be obtained from plants, animals, or bacteria, and the term ‘nanocellulose’ usually refers to the cellulosic extracts or processed materials, having nano-scale structural dimensions. Nanocellulose can be classified into three types: (i) cellulose nanocrystals (CNCs)/nano- crystalline cellulose (NCC) and cellulose nanowhiskers (CNWs), (ii) cellulose offers a unique combination of properties including flex- films, can be synthesized by combining with nanocelluloses [25]. This type of porous structures reveal a great potential as substrates g1] Young's Modulus [GPa] Average size Diameter length 20e60 10 mm >10 mm 50e160 10e80 nm <10 mm J. Jose et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 333e340334ible surface chemistry, transparency, low thermal expansion, high elasticity, anisotropy and the ability to bind to other conductive materials, enabling extensive application in flexible energy-storage devices [11,12]. Extraction of nanocelluloses can be done through chemical treatment, mechanical processing, and enzymatic treatments. The different methods of preparation are shown schematically in Fig. 2. Integrating nanocelluloses into energy-related devices would definitely inspirate research in the area of eco-friendly materials and we find it very promising to address the important environ- mental concerns. In addition, a cellulose expresses itself as a low cost material with large-scale promises [17]. With the cellulose- related materials gaining more and more popularity, a number of good review articles have appeared in the area of the synthesis of nanocelluloses and their properties. Several reviews published in the past few years have also focused on the application of cellulose- based energy related devices, such as batteries, sensors, and other electronic systems [18]. Current focus, however, seems to be in the field associated with cellulose-based sustainable supercapacitors which is fast growing with a very high progress rate. Although the interest for designing cellulose based devices for energy storage applications grew fast in the recent past according to Nyholm et al. [19] it is quite interesting to note (Fig. 3) that investigations in the field have hardly increased since 2012. In this review, we essentially focus on the potential applications of nanocelluloses in energy-related areas, and particularly, in the field of supercapacitors. Our investigations in the area were concentrated in the synthesis of nanocelluloses and the experi- mental evaluation of their thermo-optic properties using the thermal lensing technique. We were able to develop an UV pro- tection system using nanocellulose/PVA composites. With the current experimental background and information we plan to expand our area of research and focus on the possible applications of nanocelluloses in the field of energy-related devices, especially in the development of supercapacitors. 2. Cellulose-based functional materials Cellulose is one of the most abundant natural organic polymers on the Earth and is usually extracted from plants, agricultural by- products, woods etc. [4]. The properties of a cellulose and cellulose derived materials can be enhanced by functionalizing it with Table 1 Primary characteristics of cellulose types [13e15]. Material Crystallinity Surface area [m2 Cellulose micro fibres (CMF) <60% <1 Cellulose nano fibres (CNF) 50% z 90% z100cellulose nanofibrils (CNFs), also referred to as nano-fibrillated cellulose (NFC), and (iii) bacterial cellulose (BC) (Table 1) [9]. Chemically, a cellulose consists of linear chains of repeating b-D- glucopyranose units, covalently connected through b-1, 4 glycosidic bonds (Fig.1) [10]. A large number of hydrogen bonds exist intra and inter molecularly and yield different cellulosic structures. Nano-Cellulose nano crystals (CNC) z90% z200suitable materials [20] or through structural modifications for specific applications. Cellulose nanofibres (CNF) (<100 nm width) were synthesized very recently in our laboratory using raw cotton as the source material through green techniques (Fig. 4). In-depth investigations were carried out to evaluate their thermo-optical properties and ultraviolet blocking capabilities. Ultra-sensitive dual beam mode matched thermal lensing technique was used for the measurement of thermo- optical properties (Fig. 5). The low ther- mal diffusivity (2.61  10 8 m2/s) and thermal conductivity (0.108 W/mK) value of the system indicated its thermal insulation behavior. Further, highly transparent plasmon-enhanced ultravio- let radiation blocking CNF-PVA films (Fig. 6) for environmental applications were developed and we found that the UV blocking capability of the synthesized films is almost the same as that of commercial films available in the market. With our current interest on the development of functionalized nanocellulose based systems for energy storage applications, the development of functionalized nanocellulose based aerogels for supercapacitor applications is underway in our lab. This review therefore has been carried out to bring a fresh picture of nanocellulose based supercapacitors, which we believe, is highly significant in the present scenario. 3. Cellulose based supercapacitors Supercapacitors, also called ultracapacitors or electrochemical capacitors, offer a promising approach to meet the growing power demands owing to their high power density, superior rate capa- bility, quick charging/discharging rate, long cycle life, simple prin- ciples, fast dynamics of charge propagation and low maintenance cost [21]. These supercapacitors are classified into electrical double layer capacitors, which store energy through electrostatically accumulated charges at the electrode/electrolyte interface and pseudo capacitors based on fast redox reactions at the electrodes for huge pseudo capacitance [22]. There are also hybrid super- capacitors comprising of both types of electrodes thereby combining both double layer capacitive and pseudocapacitive charging and discharging schemes. Such supercapacitors have high energy density, high power density and high cycling stability. With these developments the supercapacitors are proficient to bridge the gap between the batteries/fuel cells (with high energy density) and the conventional capacitors (with high power density) [23,24]. For electro-chemical double layer supercapacitors a large specific sur- face area is favorable as it delivers more space for the adsorption of the electrolyte ions. Highly porous materials that possess high mechanical strength and large surface area, such as aerogels and Fig. 1. Structure of celluloses.50e140 5e30 nm z100 nm Fig. 2. Various preparation methods of nanocelluloses [adapted from reference [16]]. Fig. 3. Scopus database on the research output in form of research articles using celluloses and supercapacitors as keywords. Fig. 4. HR-TEM image of CNF prepared from cotton. J. Jose et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 333e340 335 ed MFig. 5. Thermal lensing plot of CNF. J. Jose et al. / Journal of Science: Advanc336for flexible supercapacitors [29] and their efficiencies modified by doping with carbon conductive materials like CNTs [26,27] and graphene oxides (GO) [28]. Though cellulose-based materials are environmental friendly the chemical processes involved in its preparation indicates a threat to the ecology. Therefore, while designing the green energy storage devices, one has to focus more on reducing the effect of chemical treatments. Koga and co-workers successfully converted recycled waste pulp-fibres and single-layer GO sheets into a cellulose paper/ Reduced Graphene oxide (RGO) composite using a blend of papermaking process and flash reduction techniques (Fig. 7). The room temperature, additive free, and millisecond time scale reduction of GO was achieved in the composite by irradiating with high intensity pulsed light. This cellulose paper/RGO composite electrode gave a high specific capacitance of 177 F/g. It was suc- cessfully applied for an all-paper based flexible supercapacitor that provided a capacitance of 212 F/g [30]. The extraction of cellulose fibres from waste paper, and subse- quent application in supercapacitor electrodes is proposed as a cost effective route for developing energy storage devices with improved performance [32]. Habio Su et al. demonstrated an all solid state symmetric flexible supercapacitor based on the office waste paper fibers/reduced graphene oxide/manganese dioxide (PF/RGO/MnO2) which acts as both the positive and negative electrodes. Flexible PF/RGO/MnO2 electrodes with a good physical flexibility and excellent mechanical strength were fabricated via a simple solution phase assembly and vacuum filtration method without using any binding agents. Moreover, owing to the advan- tages of large surface area and microfibers present in paper fiber, the PF based hybrid flexible electrodes show a high specific capacitance of 410 F,g 1 at 0.8 A,g 1 and retain 93% of the Fig. 6. UV/Visible spectrum showing transparency of the PVA/CNF/AgNP film.capacitance over 5000 cycles, and display an outstanding electro- chemical performance. In addition, the assembled solid-state symmetric supercapacitors exhibit a high energy density (19.6 Whkg 1 at 400 Wkg 1) and an excellent cycling stability of 85.3% retention even after 2000 folding and bending cycles. These investigations, thus, demonstrate a renewable method of turning waste into wealth and deliver a new method to fabricate the sus- tainable and self-supporting paper-based supercapacitor for the application in flexible energy storage devices [33]. In a pioneering work, Weng and his co-workers [34] reported a simple and scalable method to fabricate graphene-cellulose paper (GCP) membranes that act as freestanding and binder-free elec- trodes for flexible supercapacitors. The electrical conductivity of the GCP membrane shows great stability with a reduction of only 6% after being bent 1000 times. This flexible GCP electrode has a high capacitance per geometric area of 81 mFcm 2, which is equivalent to a gravimetric capacitance of 120 Fg 1 of graphene, and retains >99% of the capacitance over 5000 cycles. Under highly flexible conditions, the supercapacitors exhibit a high capacitance per geometric area of 46mFcm 2 for the complete device. All these results indicate that polymer supercapacitors made using GCP membranes are versatile. The fabrication of supercapacitors and lithium ion batteries using commercial paper sheets that were coated with aqueous Carbon Nanotube (CNT) ink using sodium dodecyl benzene sulfo- nate (SDBS) as surfactant was introduced by Cui and team [35]. The process was based on a simple Meyer rod coating process in line with the main principles of green chemistry. The method implied ways to reduce the consumption of organic solvents, usage of low energy intake processes avoiding high temperature or high pres- sure since paper absorbs water easily and binds to CNTs. The resulting conductive paper exhibited flexibility and good mechan- ical strength as well. Supercapacitors based on CNT conductive paper also displayed a specific capacitance of 200 Fg1 and the stable cycling life over 40000 cycles. Later, the same team coated single-walled CNTs (SWCNTs) onto cotton fibres and attained a highly electrically conductive interconnecting network [36]. Supercapacitors made with such electrodes had excellent cycling performance (good capacity retention after 35000 cycles) and a specific capacitance of 70e80 Fg1. Due to these properties, the combination of the cotton/SWCNT electrodes into the wearable electronics was proposed widely. Kang et al. developed a solid state flexible supercapacitor by coating CNT onto office papers using the drop-dry method using gel electrolytes with ionic liquid (i.e. fumed silica nanopowders were mixed with ionic liquid, 1-ethyl-3- methylimidazolium). Even though the supercapacitor exhibited excellent stability, flexibility and a specific capacitance of about 135 Fg1, the approach was a pull out from green chemistry prin- ciples as the use of ionic liquids is debatable due to their unsafe synthesis [37]. Later, a simple route to obtain three dimensional, lightweight, hybrid aerogels possessing excellent capacitance retention at charge/discharge rates required for a flexible energy storage device was proposed by Xuan Yang et al. in their work. They demonstrated the use of CNC aerogels as universal substrates by incorporating polypyrrole nanofibers (PPy-NF), polypyrrole-coated carbon nanotubes (PPy-CNT), and spherical manganese dioxide nanoparticles (MnO2-NP), during the aerogel assembly [38]. Lee et al. demonstrated a new green method of cellulose acti- vation using coffee. In their work they soaked a piece of paper in the espresso coffee which acted as a natural activating agent followed by a pyrolysis to give paper derived carbons (EKACs). Potassium ions being the core ingredients in the espresso coffee play an important role in augmenting the pyrolysis kinetics and achieving a porous structure and a specific capacitance of 131 Fg1 at a scan aterials and Devices 4 (2019) 333e340rate of 1.0 mVs1 was obtained. All the flexible paper super- lose ed Materials and Devices 4 (2019) 333e340 337capacitors were fabricated by assembling the EKAC/CNT mixture embedded paper towel as the electrode, the PVA/KOH mixture paper towel as the electrolyte and polydimethylsiloxane infiltrated paper towel as the packing substance. This coffee mediated acti- Fig. 7. Preparation of the composite derived from single layer GO and recycled cellu reference [31]. J. Jose et al. / Journal of Science: Advancvation of cellulose and the resultant super-capacitor provide a new material and an environmental friendly power source [39]. Conductive polymers are usually the polymers with highly p- conjugated polymeric chains and among these polymers the most widely used ones include polypyrrole (PPy, 0.3e100 Scm1), poly- aniline (PANI, 0.01e5 Scm1), polyacetylene (PA, 3e1000 Scm1), polythiophene (PTh, 2e150 Scm1), poly (phenylenevinylene) (PPV, 103- 100 Scm1) and their derivatives [40]. These conductive polymers have been widely studied for use in sensors, electro- chemical capacitors, fuel cell electrodes, batteries, memory devices, and field emission devices [41]. In recent years, significant research efforts