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Conductive nanofinishes for textiles 17.1 INTRODUCTION As common textiles consist of polymers with surface resistivity of greater than 1010 Ω/square, accumulation of static charges results in undesirable clinging causing problems during textile manufacturing and/or consumer wear and safety issues. When we think of “conductive textiles,” we may first come up with the process of controlling static charge on a nonconducting textile to transfer the generated charge to the ground. This is generally known as antistatic finishing and has gained more attraction after the introduction of synthetic fibers to provide a level of conductivity for the dissipation of the static charges. During the 19th century, many research studies dealt with the introduction and development of antistatic finishes, which are mainly ionic surfactants such as salts of phosphate esters and quaternary amines, organic salts, glycols, and polyethylene glycols (Seyam et al., 2015). Apart from antistatic finishing, which produces relatively low conductive textiles for static dissipation, the concept of conductive textile has a long recorded history dating back to the 1980s when metal-coated fabrics formed by vapor deposition or electroless coating, fibers were filled with carbon or copper sulfide, and pyrrole or aniline were polymerized on textile surface to impart various degrees of conductivity. Gregorgy et al. (1989) reported an industrially feasible process to deposit a film of pyrrole or aniline conductive polymers onto individual fibers of any textile substrate. In spite of the historical background, the term conductive textile has become a hot topic only after the introduction of smart textiles. Fabric-based circuits and circuit boards, wearable electronics, fabric-based antennas, sensors and actuators, health monitoring, electrode surfaces in energy conversion and storage systems, textile batteries and solar cells, and smart sportswear are some of the recently developed functionalities provided by electronic textiles. Electronic smart sportswear with the ability to monitor athletes’ physical health during sport activities, protecting athletes especially in high-risk sports such as mountaineering, and monitoring the biological and physiological body Nanofinishing of Textile Materials https://doi.org/10.1016/B978-0-08-101214-7.00017-0

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changes and vital body signs provide enhanced sport performance (Harifi and Montazer, 2017). Application of conductive textile-based sensors in medical, long-term monitoring, remote health monitoring, and home health care has attracted researchers, which are capable of evaluating wearer’s physical and vital signs such as heart rate, temperature, and caloric consumption (Berzowska, 2005; Paradiso et al., 2005). Producing metal fibers from stainless steel, aluminum, copper, or nickel is one of the simple approaches for making conductive fibers, however suffering from heavy weight and low flexibility. Wrapping a metallic ribbon or foil such as copper, silver, or gold around a high strength fiber in a helix shape is another method to surpass the inflexibility of metal fibers through the helix arrangement. However, the conductivity is not continuous in these fibers due to the fragile nature of the metal foil. Another approach is adding conductive particles such as silver or carbon nanotubes (CNTs) to produce conductive textiles benefiting from low cost. With the advent of nanotechnology, one-dimensional (1D) nanostructures possessing small size and considerable elongation enables electrical carriers to move effectively along a controlled direction, making them suitable for application in electronic systems. In this regard, the development of nanofibers from conductive polymers, CNTs and carbon nanofibers has gained considerable attention. Nanocoating through different methods such as vapor deposition, electrochemical deposition, electroless deposition, pulsed laser deposition, and molecular self-assembly has been also widely investigated to impart conductivity into textiles. Printing techniques, namely screen and inkjet printing have been also developed to integrate conductive materials on textile substrates. After a brief overview of these promising nanotechnologies used for producing conductive textiles, specific information is provided concerning more common compounds with electrical conductivity properties. This will be completed with an insight into new applications of electrotextiles. We will describe the most recent improvements in this area to provide an updated overview.

17.2 CONDUCTIVE NANOFIBERS AND NANOCOMPOSITES Conductive nanofibers can be fabricated from electrically conductive polymers or composites, carbon nanofibers, CNTs, and other conductive nanoparticles. Among numerous conventional methods reported for nanofiber formation such as drawing, template synthesis, self-assembly, and phase

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Electrospun nanofibers

Needle syringe

High voltage

Fig. 17.1 Electrospinning design.

separation, electrospinning is more advantageous due to control over fiber diameter, morphology, and porosity (Feng et al., 2014). Electrospinning is based on the use of electric charge to convert polymer solution into nanofibers using the setup schematically shown in Fig. 17.1. In a typical procedure, an electrostatic force is applied between a syringe filled with a polymer solution and a collector at a fixed distance from the needle. The final morphological and microstructure properties of the electrospun nanofibers are strongly dependent on various factors such as molecular weight, viscosity and conductivity of the applied polymer, flow rate, applied potential, and needle-to-collector distance along with temperature and humidity of the applied conditions (Gaminian and Montazer, 2017).

17.3 NANOMETAL COATINGS One of the increasingly studied areas for producing conductive textiles is surface coating with metals, which can be done by vapor deposition and electroless plating methods. Sputtering and chemical vapor deposition (CVD) processes require high-temperature treatments, which are not desirable for textiles, and physical vapor deposition (PVD) processes need high vacuum and huge energy supply systems, which are very expensive. Among the different processes for metallic coating of textiles, chemical electroless plating is a useful process that has been used for a long time and has attracted much attention because of low cost and uniform metallic plating. Electroless

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plating of metals is a nonelectrolytic method based on catalytic redox reaction between metal ions and dissolved reducing agent that can be used on fibers, yarns, and fabrics to produce uniform conductive coating on the surface of substrate (Jiang and Guo, 2011). In electroless plating, a fiber surface is usually deposited with a catalytically active substance such as palladium, tin, silver, or gold, followed by metallizing through immersing in a chemical metallizing solution (Cho et al., 2007). The metals are mainly Cu, Ni, and Ag. Chemical composition, pH, temperature, plating duration, and surface area of the substrate are important factors affecting the metal coating (Yuen et al., 2007). In spite of uniform coating of textiles by electroless plating, it suffers from poor wear resistance through peeling off the coating. Thus, depending on the type of substrate to be electroless metal plated, prior surface modification such as plasma may be also required to enhance the adhesion of the metal coating (Yuen et al., 2007). Introducing organic functional groups such as amino, carboxyl, thiol, and silane groups on the polymer surface prior to metal ion adsorption is required to enhance the stability of metal layer (Montazer and Allahyarzadeh, 2013).

17.4 SCREEN AND INK-JET PRINTING Printing is a flexible technique to impart electric pattern on textile substrates to produce textronic systems. Textronic textiles are portable electronics based on textiles embedded with electronic and computational systems such as integrated MP3 players, textile keyboards, and integrated television control and antennas. While spray or Meyer rod coating methods can be used to spread conductive materials on textile substrates, control over the geometry and position of the conductive layers on the surface can be achieved by printing methods (Chen et al., 2010). This includes screen printing in which dispersion of conductive ink such as gold, silver, copper, or carbon in organic or inorganic solvents along with resins are applied into textiles. In a recent study, screen printing method was used to pattern graphene on woven cotton fabrics, using electrochemical reduction method as shown schematically in Fig. 17.2 (Abdelkader et al., 2017). The printed pattern was mechanically stable due to the interaction between the hydroxyl groups of graphene and cotton fibers. Applicability of screen printing method is restricted because of the discontinuous lines due to the structure of the screen, low printing resolution, and poor layer thickness control (Bidoki, 2005; Stempien et al., 2017). As an alternative, inkjet printing with no requirement to contact with the substrate

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Fig. 17.2 Screen printing of graphene on cotton fabric proposed by Abdelkader et al. (2017).

has been used to produce low cost and high-speed patterning on textiles. Metal nanoparticles, such as silver and copper, conductive polymers, CNTs, and graphene have been adopted in printing devices. Bidoki (2005) printed silver patterns on cotton fabrics using the reactive inkjet printing, with two separate cartridges of aqueous solutions of silver nitrate and ascorbic acid printed sequentially. Polyaniline (PANI) and polypyrrole (PPy) conductive layers were printed on textiles, including cotton, wool, wool/cotton, polyester, and cotton/polyester using chemical oxidation (Stempien et al., 2015). The key factor in inkjet printing method is the suitable viscosity and surface tension of the applied ink. Also dissolved or fine dispersion of the conductive materials is required to be prepared in order not to clog the nozzle (Stempien et al., 2015; Walker and Lewis, 2012). Another problem is the penetration of the ink into the textile structure during printing, which can be overcome by introducing interface layers on textile substrates. Chauraya et al. (2013) and Whittow et al. (2014) used commercial silver nanoparticles and produced wearable microstrip patch antennas on polyester/cotton fabrics previously screen printed by ultraviolet curable interface layer. This reduced the surface roughness of the textile and facilitated the printing of a continuous conducting surface. Difficulty in achieving high conductive lines with thin layers and the need for high-temperature sintering of some inks such as silver are some of the other obstacles involved in this method. Photonic and IR sintering have been proposed to eliminate the need for high-temperature curing step (Perelaer et al., 2012; Gaspar et al., 2016).

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Modified Tollens’ process was used to produce water-soluble silver ink for the inkjet printing sintering at low temperature (Stempien et al., 2015). In a very recent study, silver acetate is mixed with ammonium hydroxide to form silver complex, eliminating the need for dispersing silver nanoparticles. Final evaporation of the complexing ligands at 50°C was done to produce silver metals (Kastner et al., 2017).

17.5 CARBON NANOFIBERS It has been historically known that carbon materials are suitable to be used in electronic devices due to their high conductivity and corrosion resistance. It was more than 140 years ago when Thomas Edison used carbon fibers in light bulb (Feng et al., 2014). Since that time, there has been an increased attention to the area of carbon fibers. High porosity, low density, high surface area, significant fatigue, corrosion resistance properties, very good mechanical, thermal, and electrical properties make carbon nanofibers good candidates substitution for conventional carbon-based fibers in many application areas such as electrodes in supercapacitors. Carbon nanofibers with diameters of 50–200 nm can be produced through melt spinning, catalytic thermal CVD growth, and electrospinning. Catalytic thermal CVD growth is an approach based on the use of metals or alloys such as iron, cobalt, nickel, chromium, and vanadium to dissolve carbon to form metal carbide (Feng et al., 2014). This method leads to formation of two types of cup-stacked and platelet carbon nanofibers. Electrospinning on the other hand is the most versatile technique of forming carbon nanofibers, which is based on using polymer solutions as precursors (Feng et al., 2014). Polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyimides, polybenzimidazole, polyvinylidene fluoride, polyurethane, pitch, phenolic resin, and lignin are the widely studied polymers for electrospinning of carbon nanofibers. Final heating treatment is required to carbonize the polymer nanofibers to form carbon nanofibers. Pressure and temperature are important parameters affecting the morphology, purity, crystallinity, diameters, and porosity of the final product. The carbon nanofibers formed via electrospinning are in the form of web or mat that are suitable candidates for application in electrical devices, sensors, electromagnetic shielding, or electrodes for batteries or supercapacitors (Feng et al., 2014). A comprehensive review on the electrospun carbon nanofibers is published by Inagaki et al. (2012).

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Surface modification and functionalization of carbon nanofibers by treatments such as activation, acid, plasma, and fluorination have been also reported in literatures to enhance the electrical properties (Mirzaei et al., 2015). Blended polymer electrospinning has been also developed to surpass the limitations involved in using one polymer as a precursor such as low carbon yield or poor spinability. For instance, ultra-thin carbonized fibers were prepared from the two-phase solutions of flexible polymer PAN and rigid polymer polyamic acid (Cho et al., 2007). Investigations have been also carried out on the formation of composites between carbon nanofibers and metal nanoparticles such as silver to enhance the conductivity (Cauchy et al., 2017). A recent progress in electrospinning of carbon fibers is based on electrospinning polymer/sol-gel systems to produce metal oxide carbon nanocomposite fibers, benefiting from porous structures with high surface area. This provides the opportunity to produce porous carbon nanofibers by selectively etching metal ions and remaining carbide-derived carbon nanofibers (Atchison et al., 2015). This is based on the use of a sol gel solution of metal oxide precursors such as titanium butoxide, zirconium(IV) acetylacetonate, or niobium n-butoxide mixed with a carrier polymer namely cellulose acetate or polyvinylpyrrolidone to produce nanometal oxide/carbon fibers. This is generally followed by carbothermal reduction at high temperatures (1300–1700°C), forming metal carbide/carbon nanocomposite fibers. High surface area and porous structure of such nanocomposite fibers makes them good candidates to produce carbon electrodes for electrochemical applications (Atchison et al., 2015). Introducing carbon materials such as CNT, graphene, and carbon black to the preparation procedure of carbon nanofibers is another successful approach for enhancing the electrical conductivity of the final product with a main drawback of poor dispersion of the additives into the carbon fibers. In a recent study, fluorine modification of multiwall CNTs was effective to increase the affinity on interface between two carbon materials through introduction of hydrophobic functional groups, forming electrospun carbon nanofibers with improved conductivity (Sun Im et al., 2009). Embedding graphene nanosheets as 2D sheets composed of sp2 carbon atoms in a honeycomb crystal lattice into the precursor polymer matrix of carbon nanofiber formation has been also widely considered to enhance the electrical properties of carbon nanofibers due to the unique characteristics of graphene (Moayeri and Ajji, 2017). Most of the current research

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is devoted to the development of methods for the well dispersion of graphene in electrospinning solution reducing tendency to agglomeration. The methods, including the noncovalent functionalization of graphene, have been regarded as the best solution for the solubility challenge of graphene. Based on the recent study carried out in a center of applied research on polymers and composites of Canada, the core-shell electrospun carbon nanofibers embedded with various amounts of noncovalently functionalized graphene were prepared using PAN and PVP in DMF. Current focus of scientists on more environment-friendly methods resulted in development of green routes for carbon nanofibers production, free from releasing highly toxic gases during the carbonization with no need for use of toxic solvents such as dimethyl formamide and dimethyl acetamide to dissolve the precursors (Chakravarty et al., 2017). In this concern, cellulose has been applied as a biopolymer for preparation of carbon nanofibers, although the preparation step involved the use of a mixture of acetone and dimethyl acetamide as solvents, not making the process a 100% green route. Another environment-friendly approach was the use of silk cocoon to electrospun carbon fibers; however, it was unsuccessful to produce fibers in nanometer diameter (Liang et al., 2013). Water solubility of sucrose makes it a good natural biopolymer for the formation of carbon nanofibers by electrospinning. Carbon nanofibers with good electrical conductivity were electrospun from sucrose, followed by carbonizing in reducing atmosphere at high temperature. Further improvement in electrical conductivity was reported to be achieved by in situ doping of carbon nanofibers with silver nanoparticles (Chakravarty et al., 2017).

17.6 CARBON NANOTUBES CNTs, which are mainly grouped into single-walled nanotubes (SWNTs) with a single graphite sheet wrapped into a cylindrical tube, and multiwalled nanotubes (MWNTs) with an array of nanotubes that are concentrically nested similar to the rings of a tree trunk, are famous for their unique mechanical, optical, chemical stability, and electrical and thermal conductivity. These are making them good option to incorporate with polymers for enhancing electrical, mechanical, and physical features (Saeed et al., 2006). CNTs have high electrical conductivity more than 106 S/cm higher than conductive polymers and current-carrying capacity of 109 A/cm2, which is even higher than metals. The incorporation of CNTs into textile substrates will result in unique properties. In spite of the mentioned advantages, uniform dispersion and alignment of CNTs is a major obstacle to the application

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(Saeed et al., 2006) that could be overcome by sonication, functionalization of CNTs, and in situ polymerization. It is expected that due to the sink flow and high extension of the electrospun jet, alignment of CNTs can take place during electrospinning. SWNTs and MWNTs have been incorporated into several polymer matrices such as polyethylene oxide, PVA, epoxy resin, polycarbonate, polyvinylpyrrolidone, polystyrene, and polyurethane to produce electrospun nanocomposite fibers (Zhang et al., 2011). In addition to nanocomposite fibers, incorporation of CNTs into textiles through immersion into CNT dispersions provides a simple way to fabricate conductive textiles (Kang et al., 2006). Successful result of this method was also guaranteed by preparing the homogeneous dispersion of CNTs. In this regard, DC electric field, ultrasonic, and surfactants were some methods to improve the dispersion of CNTs in liquid. Various surfactants, polymers, enzyme, and natural macromolecules have been used as dispersing agents (Kang et al., 2006). For instance, incorporation of CNTs into textiles has been reported through conventional dyeing method, in which the fabric was immersed into an aqueous PANI-SWNT dispersion and dyed for 4 h at 90°C. Dyeing dispersions contained fully sulfonated PANI and CNTs. The authors claimed that the conductivity of CNT containing textiles increased by four times compared with samples only dyed in presence of sulfonated PANI aqueous dispersions (Panhuis et al., 2007). Our research group also developed a successful method for producing conductive wool fabrics using multiwall CNT and carboxylated CNTs. Prior to CNT application, wool surface was activated by three different methods enhancing the CNTs adsorption. These include the oxidation of wool with potassium permanganate creating more carbonyl groups, wool biofinishing with protease hydrolyzing the wool peptide bonds forming wool surface with more amine-carboxyl groups, and wool nanofinishing with TiO2 nanoparticles causing photoinduced wettability. Among these methods, activating wool surface by protease was more successful to improve CNT adsorption creating enhanced electrical and mechanical properties (Nafeie et al., 2016). In the proposed research, anionic and cationic surfactants, namely, sodium dodecyl sulfate and cetyltrimethylammonium bromide were used to provide the best conditions for dispersing the nanotubes (Nafeie et al., 2016). Recently, a surface microdissolution process was proposed as a method of embedding carboxylated multiwall CNT on cotton fabric (Li et al., 2016). This involves the treatment of fabrics with NaOH/urea aqueous system at low temperature, allowing the incorporation of CNTs into the surface layer of the fabrics via a surface microdissolution technique.

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17.7 GRAPHENE Graphene, a 2D nanomaterial with single atomic layer of carbon structure, has a reputation for unique properties such as compatibility, very high theoretically specific surface area of 2630 m2 g 1, high transparency, light weight, high surface area, high mechanical properties, excellent thermal (5000 W/mK) and electrical (108 S/m) conductivities (Babaahmadi and Montazer, 2016). There are two approaches to produce graphene sheets, namely, topdown and bottom-up. In the top-down approach, graphene sheets are prepared by mechanical or chemical exfoliation of graphite power or splitting of CNTs, while bottom-up procedures include total organic synthesis or CVD using small molecular precursors (Chen et al., 2016). Graphene oxide (GO) is an important precursor to make large quantities of graphene-like materials, commonly named as reduced GO (rGO) or chemically converted graphene. Several reduction methods have been introduced to produce rGO, which is widely applied on different substrates. These reduction methods include thermal, microwave, photo, chemical, and solvothermal techniques (Babaahmadi and Montazer, 2016). The widely studied method for preparation of GO was proposed by Hummers (Babaahmadi and Montazer, 2016). In specific approach, graphite and sodium nitrate are mixed with H2SO4, following by KMnO4 addition to the mixture and increasing the temperature to 98°C. Final step is the addition of hydrogen peroxide to the cooled mixture. In most cases, the prepared GO dispersions are used for application on textile substrates, which can be accomplished using exhaustion, pad-dry-cure, or coating methods. In addition to various studies reporting the addition of graphene into polymer matrix for electrospinning, textile coating with graphene has been also examined. A direct coating of graphene nanoribbon prepared from CNT on cotton fabric reported resulting in flexible and conductive textiles (Gan et al., 2015). Several researchers have reported cotton/graphene fabrics as electrodes in supercapacitors. More prominent properties will be achieved when combination of graphene and fillers such as conductive polymers, metals, and metal oxides is applied. Graphene/PPy, graphene/silver, graphene/TiO2 are some of the composite graphene-based materials applied on different textile substrates (Wang et al., 2017). As incomplete reduction of GO will result in oxygen containing functional groups causing defects decreasing the electrical conductivity of rGO, metals such as silver have been added to the production procedure for enhancing the electrical

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conductivity. Ag/rGO composites were prepared on polyester fabric indicating improved electrical conductivity (Babaahmadi et al., 2017). Polyester fabric previously activated by NaOH or ethylenediamine was treated with GO through exhaustion method, followed by addition of silver nitrate, which was reduced by ascorbic acid and ammonia to produce Ag/rGO on the fabric surface (Moazami et al., 2016). In a recent study, conductive polyester fabrics were prepared by dip coating of samples in GO and Ag+/ GO solutions, following by in situ synthesis of rGO and Ag/rGO nanostructures on the surfaces of polyester fabric using hydrazine as reducing agent. Samples were post-treated during thermal annealing to improve the conductivity. This was due to the sintering effect between Ag nanoparticles and rGO sheets. The applied method also resulted in durable properties arising from the interfusing of rGOs, silver nanoparticles, and polyester, due to the chain flexibility of polyester in the temperatures above Tg (Babaahmadi et al., 2017). Lately, microwave reduction has been used to coat Ag/rGO on polyester fabric previously treated with dopamine to increase the attachment of the coating on the surface (Wang et al., 2017). Deposition of graphene/ TiO2 nanocomposite on cotton fabric was reported by reduction of GO using TiCl3 as reducing agent and precursor in the presence of polyvinylpyrrolidone (Karimi et al., 2014). Incorporation of MnO2 into graphene-coated polyester fabric was also beneficial for enhanced electrochemical performance (Guo et al., 2016). Further, rGO/SnO2 nanocomposite has been in situ synthesized on polyester fabric producing electroconductive properties (Babaahmadi and Montazer, 2016).

17.8 INTRINSICALLY CONDUCTING POLYMERS Intrinsically conducting polymers (ICPs) such as PANI, PPy, and poly(3,4-ethylenedioxythiophene) as one of the polythiophene derivatives, are organic materials with electrical properties similar to semiconductors and metals, benefiting from light weight, low cost, and flexibility. The conjugated chemical bonds of ICPs under certain conditions (doping) ensure the electron conductivity of the polymers. All conjugated polymers with alternating single and double bonds in the polymer chain are not conductive unless they undergo structural modification process through p-doping (oxidation with halogen) or n-doping (reduction with alkali metal) (Wang et al., 2010). These polymers are inherently electroconductive compared with extrinsically conductive polymers in which an insulating polymer matrix is

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blended with conductive fillers such as carbon black, CNTs, and metals (Gao and Cranston, 2008). In some cases, ICPs work as fillers to produce extrinsically conductive polymers. ICPs are good alternatives to produce conductive nanofibers simultaneously benefiting from the physical and chemical properties of organic polymers and the electrical characteristics of metals. The electrical functionality of the conductive polymers can be improved by addition of a component mainly metal nanoparticles such as gold, silver, and copper to form nanocomposites, with the advantage of providing some other beneficial features arising from the particles. Synthesis of metal nanoparticles can be carried out using in situ and ex situ methods. Through in situ method, nanoparticles are synthesized and grown inside the polymer matrix, although uniform distribution of the particles as fillers is not possible causing detrimental effect on the final electrical properties of the composites (Lu et al., 2007). On the other hand, the ex situ route is the presynthesis of nanoparticles that are incorporated into the polymer matrix, benefiting from the control over the distribution of the component enhancing the electrical properties. The most challenging obstacle is the preparation of stable nanoparticle colloidal systems, which is resolved by the synthesis of capped metal nanoparticles using organic compounds as stabilizers (Lu et al., 2007). On the other hand, coating of textiles with ICP films is as an easy approach to produce conductive textiles although suffering from poor mechanical properties. Researches indicated that the best method of applying conductive polymers to textiles is in situ polymerization through chemical, electrochemical, and vapor phase polymerization processes (Bhat et al., 2004). In situ polymerization can be carried out in two different ways: 1. Fabric impregnation into a bath containing all the reagents including oxidant, monomer and dopant, following by padding and heating. The chemical polymerization occurs in the bulk of the solution, and the resulting polymers either deposit spontaneously on the surface of the immersed fabric, or precipitate as insoluble solids. 2. Two-step process in which fabric first treated with monomer or dopant/ oxidant and then added to a bath containing oxidant/dopant or monomer. Polymerization occurs at a surface and inside the fiber structure rather than in the solution. Composite of conducting polymers with silver has been also prepared by the oxidation of respective monomers with silver salts. For instance, cotton fabric was coated with PANI or PPy, while silver nanoparticles were deposited from silver nitrate solutions by the reduction ability of the applied conducting polymers (Bhat et al., 2004).

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Conductive polymer-coated textiles can be also prepared via CVD method in which after fabric impregnation into oxidant and doping agent bath, polymer vapor is applied. This has the advantages of dopant, oxidant, and untreated monomers recovery and uniform coating. These methods have been widely applied for producing conductive cotton, nylon, polyester, viscose, silk, and lyocell fabrics using PPy or PANI (Dall’Acquaa et al., 2004). Fiber surface morphology and reactivity toward the polymers are important factors affecting the coating performance and conductivity. In this regard, substrates such as wool possess functional groups enhancing the coating adhesion on the surface while polyester needs activation such as alkaline hydrolysis prior to polymer coating (Jassal and Agrawal, 2013). PANI conductive nylon fabrics were also developed via layer-by-layer assembly method in which the nylon fabric was repeatedly dipped into a solution of sodium salt of poly(styrene sulfonate) (PSS), rinsed, and immersed into PANI solution (He et al., 2013). This method was based on the consequent deposition of polyanion (PSS) and polycation (aniline cation) on the surface until the fabrication of the desirable film on the surface (He et al., 2013). Another method to produce conductive textiles is preparation of a coating paste of conductive polymer and binder for applying on the textile fabric (Teli et al., 2014). In a recent study, PANi was directly deposited on textile substrates through in situ method, and the surface and bulk conductivity of the treated textiles were compared with samples treated with emulsion of binder/PANi through pad-dry-cure method. In both cases, PANi was synthesized by treating aniline with hydrochloric acid (HCl), hydrobromic acid (HBr), and hydrofluoric acid (HF), followed by polymerization with ammonium persulfate as initiator (Teli et al., 2014). While binder was favorable for increased surface conductivity, it had detrimental influence on the bulk conductivity of the treated fabrics. This is due to the contribution of binder as a copolymer in uniform film formation on the surface and disability to penetrate into the interstices of the yarn structure (Teli et al., 2014).

17.9 ELECTROTEXTILES FOR ENERGY STORAGE AND CONVERSION Energy storage and conversion technologies, including Li-ion batteries, supercapacitors, solar cells, and fuel cells, are among the hot research topics owing to the importance of energy, economy, and environmental issues. Although microbatteries have been developed in the recent years, they suffer from drawbacks, including limited lifetime and low-power density

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limiting their application areas due to economic and environmental challenges. Supercapacitors, as an alternative to batteries, have been developed with superior properties to rechargeable batteries and capacitors. Supercapacitors, also called electrochemical capacitors, are electrochemical energy storage devices with high-power densities capable of fully charge and discharge in seconds. In spite of lower energy density of supercapacitors than that of batteries, their high-speed delivery of stored energy and long working time without losing capacitance, make them superior to batteries. Supercapacitors consist of current collector, electrodes, electrolyte, and a separator. Separator is generally a plastic, paper, or insulated textile providing electronic insulation between the electrodes, while providing ionic conduction. There are various types of electrolytes such as aqueous, organic, redoxtype, and solid or semisolid electrolytes (Zhong et al., 2015). The size of electrolyte ions and pore structures of electrodes should be compatible with each other. High electrical conductivity, high specific surface area and porous structure, high corrosion and temperature resistance, and feasible processability are the most important features of a suitable electrode for a supercapacitor (Chen et al., 2017). In addition to these features, flexibility and stable electrochemical performance under mechanical bending and twisting conditions should be also considered in electrodes for supercapacitors. Unique properties of textile electrodes such as low cost, light weight, and high flexibility make them good alternatives in supercapacitor electrodes (Jin et al., 2017). There have been vast number of studies on the graphene, CNT, carbon and metal fiber-based supercapacitor electrodes, which can be woven into energy storage textiles through complex processes (Gulzar et al., 2016). However, applying various nanofinishing methods to produce conductive textiles for energy storage devices has gained more attention in recent years, as intrinsic roughness of textile substrates is beneficial for providing large surface area enlarging the interface between electrodes and the electrolyte. In this regard, a pioneering step was made by Hu et al. (2010) coating cotton and polyester fabrics with CWCNT and using them as charge storage and current collector in a supercapacitor with LiF6 as electrolyte. Their approach was successful to store 140 and 80 F g 1 energy per mass after 3000 cycles for polyester and cotton-treated fabrics. Supercapacitors can be symmetric, in which both the electrodes are made from the same materials, while asymmetric supercapacitors consist of two types of electrode materials with different working electrochemical potential. In this approach, one of the electrodes is usually made of high-power density,

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but low-energy density materials such as carbon-based electrodes, while the other one possesses high-energy density but low-power density such as conducting polymers and/or transition metal compounds. Thus, asymmetric supercapacitors are advantageous to conventional symmetric systems (He and Chen, 2015). For instance, an asymmetric electrochemical supercapacitor based on GO/MnO2 polyester fabric as positive electrode and CNT-treated polyester as negative electrode in aqueous Na2SO4 electrolyte was prepared and showed capacitance of 315 F g 1 with high power and energy density (Hu et al., 2010). Many successful research studies introduced natural or synthetic textile substrates as supercapacitor electrodes using direct deposition of PPy nanoparticle layers on the fiber surface (Firoz Babu and Anbu Kulandainathan, 2011) or thermal reduction of GO and chemical polymerization of pyrrole in the presence of ferric chloride to prepare PPy-RGO-fabric electrode (Wang et al., 2017) with high specific capacitance in range of 200–400 F g 1. However, poor cycling stability owing to the continuous volume change during repeated charge-discharge processes is still a major challenge for practical application of conducting polymers in textile supercapacitors (Jin et al., 2017). One of the recent approaches to overcome such a challenge is the preparation of hierarchically porous electrode that exposes to more active surface area for charge storage, shortens the electrolyte ion diffusion distance, increases the electron transportation rate, and accommodates the volume change of conducting polymers (Jin et al., 2017). In this regard, conductive PANI/CNTs/graphene/polyester composite fabric was successfully prepared as a textile electrode with high electrochemical performance and good stability with capacitance retention of 76% after 3000 cycles. Unique properties of the proposed nanocomposite are related to the 3D hierarchically porous structure constructed by CNTs and graphene layers on the fiber surface generating a conductive network with rapid electron transportation (Jin et al., 2017). Another way to facilitate the electrolyte diffusion into electrode materials, a key factor in designing supercapacitor electrodes, is use of carbon cloth as a substrate with 2D structure and high surface area. For instance, two carbon cloths were treated with nitrogen-doped graphene/polyacrylic acid/PANi and used as electrodes in H2SO4/PVA electrolyte as a supercapacitor with capacitance of 521 F g 1 maintaining 80% durability after 2000 cycles (Wang et al., 2017). Carbon cotton mat, which is prepared by thermal annealing of natural cotton mat, has been also used as electrode in presence of Na2SO4 as electrolyte and cotton mat as a separator to

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produce supercapacitor with high flexibility tolerating the conductivity after 180 degrees folding (Xue et al., 2013). Guo et al. (2016) conducted a research in which conductive rGo/polyester composite fabrics were treated with MnO2 nanorods and sheets. The ultrathin sheet layers of MnO2 formed hierarchically pore structure on the surface resulting in a better diffusion of the electrolytic ions providing a capacitance of 265.8 F g 1 with 87% durability after 1000 cycles. Textile-based supercapacitors are based on sandwich designs (Fig. 17.3), in which electrolyte is sandwiched between two pieces of conductive textiles as electrodes. In most cases, polymer-gelled electrolytes are used as solid-state electrolytes acting also as separator (Guo et al., 2016). There are also fiber-shaped electrodes with a core-shell structure (Jiang et al., 2017). On the other hand, the planar configuration is more compatible with miniaturization and wearable devices for real-time measurements. This has been recently applied to produce microsupercapacitors mainly based on carbon/graphene-based electrochemical double-layer capacitors and the metal oxides-based pseudocapacitors. In this configuration, arrays of in-plane microelectrode fingers with microscale sizes at two dimensions are applied on textile substrates. Photolithography, plasma etching, laser scribing, and ink-jet printing are the main methods used to produce in-plane microsupercapacitors. More information on these methods can be found in review paper by Qi et al. (2016). In this regard, laser scribing is a superior method with potential to convert graphite oxide to graphene due to the photothermal effect, making the direct patterning of any graphite oxide surface into rGO structures in different geometries (Qi et al., 2016). In a very recent study, polyester-based solid-state microsupercapacitor is prepared via laser scribing of GO coatings using glutaraldehyde as crosslinker and PVA-gel electrolyte (Wang et al., 2017).

Electrodes

Electrolyte

Fig. 17.3 Different designs of supercapacitors.

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Porous NiFe2O4 nanofibers were prepared by electrospinning and directly fabricated to produce in-plane patterns by photolithography on polyester fabrics. The prepared microsupercapacitor was stable to convex and concave bending due to the in-plane configuration (Li et al., 2016). Metal organic frameworks (MOFs) are a class of new emerging materials with outstanding crystallinity, porosity, uniform spatial dispersion of components and tunable pore size properties making them suitable to be used in gas storage, photocatalytic solar fuel production, sensors, supercapacitors, fuel cells, solar cells and batteries (Wang et al., 2017). Their unique high surface area and porosity provides the opportunity to use in energy-related applications due to the fast mass and electron transportation required in energy storage and conversion areas. Fuel cells are energy storage and conversion devices work based on conversion of fuels such as hydrogen and methanol to electricity. The potential of MOFs and their composites to be used as electrolyte with high proton conductivity and electrode catalysts in fuel cells has been approved. Although high porosity of MOFs provides the good opportunities for fast diffusion of electrolyte ions in supercapacitor applications, MOFs suffer from low specific capacitance due to low conductivity. Thus, application of MOFs in supercapacitors requires the incorporation of other conductive materials such as graphene and conductive polymers with MOFs to enhance the conductivity. The potential of MOFs and their composites as electrode and electrolyte materials with permanent pores for Li+ ions storage and migration in lithium-based batteries has been also concerned in recent years (Wang et al., 2017). General structure of MOFs contains metal ions or cluster of metal ions as nodes (inorganic secondary building units) three-dimensionally binding to rigid single or multiple organic linkers by electrostatic attraction and coordinative metal–ligand interactions as shown in Fig. 17.4. The interesting potential of MOFs is various compositions of metal ions and organic linkers that can be used in synthesis of almost 2000 types of MOFs (Dhakshinamoorthy et al., 2016). Attachment of MOFs on substrates has been also developed in recent years by several methods of direct (hydrothermal) growth, seeded growth, dip-coating, layer-by-layer growth, electrochemical deposition, and spray coating (Zhuang et al., 2013). Incorporation of MOFs and their composites in textile substrates is in the first steps and only few studies have been reported on the specific application of MOFs on textiles. Some of the MOFs currently applied to textile substrates are summarized in Table 17.1. Pattern deposition of MOFs on textile substrates using ink-jet printing (Zhuang et al., 2013), immobilization of

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Metal ion or node

Organic linker

Fig. 17.4 General building blocks of MOFs. Table 17.1 Common MOFs applied on textile substrates MOFs Metal ion Organic linker

HKUST-1 MIL-100(Fe) MIL-101(Cr) MIL-125 UN-100 MOF(808) UiO-66

Cu(III) Fe(III) Cr(III) Ti(IV) Zr(VI) Zr(VI) Zr(VI)

btc (1,3,5-benzenetricarboxylate) btc (1,3,5-benzenetricarboxylate) 1,4-Benzodicarboxylic acid 1,4-Benzodicarboxylic acid 1,3,6,8-Tetrakis(p-benzoate)pyrene (TBAPy) btc (1,3,5-Benzenetricarboxylate) 1,4-Benzodicarboxylic acid

MOFs in fibers using electrospinning method (Rose et al., 2011), direct in situ growth of MOFs on carboxymethylated cotton fabrics (Pinto et al., 2012; Rodrıguez et al., 2014), radiation-induced graft polymerization, and self-assembling method are among the research studies (Yu et al., 2016). Rather than coating methods, which provide physical attachment of MOFs on substrates, the stable immobilization of MOFs on textile substrates is not very easy due to the steric hindrance. Thus, generally a linker is required to bridge between the MOF particles and the textile (Yu et al., 2016). Considering the promising properties of MOFs and the recent progress in use of MOFs and their composites for energy applications, incorporation of MOFs in textiles producing textile-bead energy storage and conversion devices will be definitely the subject of future studies.

17.10 CONCLUSION Since the introduction of antistatic finishing agents for dissipation of the static charges, revolution has occurred in smart conductive textiles brought about by nanotechnology. Metal fibers, yarns, and woven fabrics were

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effectively substituted by nanofibers developed from carbon-based materials, conductive polymers, and efficient nanocoating methods providing flexible conductive textiles adaptable to wearable electronics and sensors. Reviewing the literature showed the continuation of this trend toward introduction of new materials with high surface area and porosity such as MOFs to reach the long-term target of integrated textile-based electronics.

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FURTHER READING Mara´kova´a, N., Humpolı´cˇek, P., Kasˇpa´rkova´, V., Capa´kova´a, Z., 2017. Microelectron. Eng. 176, 84–88. Martinkova´b, L., Boberc, P., Trchova´c, M., Stej, J., 2017. Antimicrobial activity and cytotoxicity of cotton fabric coated with conducting polymers, polyaniline or polypyrrole, and with deposited silver nanoparticles. Appl. Surf. Sci. 396, 169–176. Ren, J., Bai, W., Guan, G., Zhang, Y., Peng, H., 2013. Flexible and weaveable capacitor wire based on a carbon nanocomposite fiber. Adv. Mater. 25, 5965–5970.