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Contents lists available at ScienceDirect

Nano Energyjournal homepage: www.elsevier.com

Human-motion interactive energy harvester based on polyaniline functionalized textilefibers following metal/polymer mechano-responsive charge transfer mechanismSumita Goswami , Andreia dos Santos , Suman Nandy ∗, Rui Igreja , Pedro Barquinha , Rodrigo Martins ,Elvira Fortunato ∗∗

i3N/CENIMAT, Department of Materials Science, Faculty of Science and Technology, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516, Caparica, Portugal

A R T I C L E I N F O

Keywords:Energy harvesterPolyanilineWearable electronicsCharge-transfer mechanismAtomic force microscopyConjugated polymers

A B S T R A C T

Our experimental outturn opens up a new vision by proposing mechano-responsive charge transfer mechanism(MRCTM) to π-conjugated polymers in the field of human-motion interactive energy harvester. Doped polyani-line (d-PANi) has been used to functionalize conducting textile fibers (f-CTFs) and integrated with our proposeddesign for wearable power plant. Each f-CTF generates current by patting, bending, or even soft touching. Lo-calized force deformation at the metal/polymeric interface layer with direct visualization of charge distributionpattern has been extensively studied by atomic force microscopy. The integrated arrays of f-CTFs produce a peakpower-density of ∼0.6Wm−2 with output current-density of ∼22mAm−2 and can power at least 10 white LEDsof 2.5W. The procured energy from f-CTFs is capable of charging a commercial 10μF capacitor to 3V in 80s andpowering portable electronic devices. The prototype energy harvester stably shows the same performance aftermore than 100 thousand times of patting, bending or twisting.

1. Introduction

In 21st century, technology is emerging more stylish and smarter,with the new generation paying more for trendy and fashionable wear-able gadgets, whereas essential sectors like health and defence de-manding for ultra-advanced, flexible smart technology for more pros-perity [1–7]. Wearables are emerging as part of a growing trend tomove data analysis and communication from the electrical device di-rectly to the body. More than ever, technologists are using a combi-nation of sensors, machine learning and big data analysis to provideconsumers more data about their bodies and lives [8–10]. This emerg-ing field of technology will have a dramatic impact on human-ma-chine interaction. Compared with the rapid growth of wearable technol-ogy like fitness tracker, smartwatch, GPS tracker and other categories,smart clothing represents less than 1% of the global wearables market[1,11,12]. According to a recent survey of 2407 consumers in devel-oped and emerging markets of wearable technologies, public awarenessis at a minimum level regarding smart clothing and e-textile products[11]. Therefore, recent research trend in flexible electronics demandsan increasing popularity in “Smart e-clothing”, prospecting a larger

global market in the near future. Moreover, the tremendous develop-ment of portable electronics urgently demands for sustainable, smartwearable energy sources that can spontaneously provide electricalpower to the daily used smart electrical products. This is now one ofthe biggest technical challenges also. Looking at the lack of expansionand adoption in this category, the vision of our scientific outturn is to-wards increasing competitiveness in the field of smart fiber-based inno-vative technical textiles, mainly for energy applications - answer of al-ternative sustainable energy power plant! Our newly prototyped designof human-motion interactive energy harvester based on polymer func-tionalized free-standing textile fibers will bring to bear a vast variety offields, from direct weaving in clothing system to inserting into differ-ent kind of daily usage articles like sofa, bed, car seat, chair, carpet etc.Considering existing huge garment industries, this human-motion inter-active textile power harvester will start a new era, which can also pro-mote a new business strategy in other essential industry sectors, such ashealth care and defence.

During the last decade, some research groups have reported onmechano-responsive energy harvesters, mostly based on either tribo-electric [13–18] or piezoelectric [19–24] platform. Among them, veryfew reports are solely on textile fiber based energy harvesting system.

∗ Corresponding author.∗∗ Corresponding author.

Email addresses: s.nandy@fct.unl.pt, snandy_ju@yahoo.co.in (S. Nandy); emf@fct.unl.pt (E. Fortunato)

https://doi.org/10.1016/j.nanoen.2019.04.012Received 3 January 2019; Received in revised form 10 March 2019; Accepted 3 April 2019Available online xxx2211-2855/ © 2019.

Full paper

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Triboelectricity is a phenomenon of contact electrification mechanismwhere two different kinds of material become electrically oppositecharged if rubbed against each other. Static polarized charges arise onthe materials surfaces due to the friction, and their separation by theback electrode (charges can flow through the back electrode of the ma-terial to the external circuit) creates a potential difference which furthergenerates current. Usually in this mode, dielectric to dielectric and con-ductor to dielectric types of different materials are being used [25–27].But herein, we report the noble observation of mechanical force stimu-lated energy harvesting from conjugated polymer, applied to free-stand-ing textile fibres for human-motion induced wearable power energysource. In 2016, initially we have reported polyaniline as a stress-in-duced energy harvester [28]. These π-conjugated polymers are fascinat-ing materials with a unique combination of hard (metal like) and soft(organic like) matter which can be exerted to convert mechanical-stim-uli to electrical potential energy [29–31]. It is not surprising that stim-uli-responsive polymeric materials that exhibit a desired output (the re-sponse) upon being subjected to a specific input (the stimuli such aschemicals, heat, light, electricity, stress etc.) are evolving nowadays intoan attractive class of smart materials which can be designed for differentapplication fields ranging from camouflage systems to artificial muscles,human-motion interactive motor and drug delivery [32–34]. However,comparatively few polymers have been developed to respond in a usefulway to mechanical stress. In that context, we consider this input-out-put relationship as an energy transduction process through convertingmechanical energy to electrical energy. The core idea behind this is toinitiate a mechano-responsive charge transfer mechanism (MRCTM) andenergy-transfer process in a π-conjugated polymer at the organic-metalinterface layer [28]. The common building blocks of π-conjugated poly-mers are mainly carbon atoms, schemed with alternating single anddouble bonds. The conjugation is a result of sp2pz hybridized carbonatoms, yielding three covalent s-bonds (i.e. localized σ-bonds) that formstrong chemical bonds. The remaining pz orbital is free to overlap withthe corresponding pz orbital of an adjacent carbon (i.e. π-bond), formallyleading to one unpaired electron per carbon that is delocalized over themolecule or, in the case of polymers, that allows carrier mobilities overlarge segments of the polymeric chain. The HOMO (valence band) andLUMO (conduction band) of π-conjugated polymers are generally de-rived from occupied π-bonding orbitals and unoccupied π*-antibondingorbitals, respectively. Polyaniline (PANi) [35,36] encompasses a fam-ily of conjugated polymers where the nitrogen atoms connect six-mem-bered carbon rings of benzenoid or quinoid character. However, conju-gation is not sufficient to make it conductive until mobile charge carri-ers are introduced into the π-electronic system through doping. With pri-mary and secondary doping, PANi shows an interesting feature [37,38]with plenty of mobile charge carriers, which can be tunable for desiredelectro-mechanics [39–41].

Very recent a few works have been reported on PANi basedmechano-responsive energy harvesters [42,43] in which the authorshave coated the 2D substrates like carbon mat or cotton piece withpolyaniline. Moreover, those groups have obtained their best results fordevices with additional triboelectric layers of PVDF or PTFE. But in thepresent report we have functionalized each individual textile fiber withd-PANi and designed them for harvesting mechano-responsive energyusing purely electrode/d-PANi/electrode system. Our device is basicallyworking on free standing d-PANi functionalized fibers (which can beconsidered as a 3D system) and the electrode was woven surroundingeach functionalized fiber. This unique design of our system practicallyhas the huge opportunity to increase the pressure contact area with in-creasing efficiency and can be further adopted easily into any form ofgarments or incorporated into any flexible/wearable systems.

2. Results and discussions

Herein, we have functionalized conductive fibers (commercial sil-ver coated thread with 400μm diameter) by hydrochloric acid dopedpolyaniline (d-PANi) nanostructure. Fig. 1a shows the field emissionscanning electron microscopic (FESEM, Carl Zeiss Auriga Crossbeammicroscope) image of a d-PANi functionalized conducting textile fiber(f-CTF). Center inset of Fig. 1a shows the zoom in on d-PANi layer andthe upper inset image shows the normal view of an f-CTF. The corre-sponding EDS elemental mapping (using Oxford XMax 150) of uncoatedCTF and f-CTF is shown in Fig. S2 and Fig. S3 respectively (support-ing information). The thickness of the d-PANi layer is approximately400–450nm over 400-μm diameter conductive fiber, which is uniformlycoated with our optimized wet chemical method. Details of synthesishave been described in experimental section. The cross-sectional FESEMimage of f-CTF has been shown in Fig. S4 in the supporting information.

2.1. Investigation on localized mechano-responsive charge transfermechanism (MRCTM)

Before exhibiting the mainstream approach of measuring hu-man-motion interactive current harvesting from f-CTFs, we have firstexplored the localized charge transfer mechanism at the metal-conju-gated polymer (i.e. d-PANi) layers based on free-standing textile fiber byatomic force microscopy (AFM) technique. The most unavoidable pri-mary requirement to any device performance depends on the interfacedynamics of the materials which is very relevant given that a new un-expected phenomenon may occur and may dramatically change materi

Fig. 1. Localized study of mechano-responsive charge transfer mechanism (MRCTM) ond-PANi functionalized textile fiber (f-CTF). (a) FESEM image of d-PANi functionalizedlayer on a textile fiber. Center inset shows the higher magnification of d-PANi nanostruc-ture and top inset shows the normal view of f-CTF. (b) Schematic illustration of AFM basedcharacterization on MRCTM. (c) Output current response from d-PANi surface for typicalthree different contacting forces of 8, 12 and 16nN delivered by AFM probe. Each apply-ing force conferring for 1s which shows that the output current increases with increasingapplying forces. (d) Schematic illustration (not in scale) on probable systematic investiga-tion on MRCTM: d-PANi layers arranged with uniform molecular networks at undisturbedcondition→localized stress delivered by the top electrode to the d-PANi layer→ molecularnetworks contracted→generating Fermi level pinning due to increasing localized chargecarrier density→ initiating a charge transfer mechanism from nearest energy level of metalelectrode (SDCC electrode)→yielding current generation→stress releasing→ causing mole-cular networks stretching→creating decrease in localized charge density→Fermi level de-pinning→charge carriers tunnels from nearest polymeric energy level or back electrodes.

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als properties and result in new useful functionalities such as elec-tronic memory effects [44] etc. Henceforth, as many nanomaterials pro-cure their applications in nanoengineering, characterizations of local-ized electronic transport mechanism through the interface of materialare not only pursuit in fundamental science but also of widespread prac-tical need.

In this work, we have used conductive AFM (c-AFM) [34,39] andelectrostatic force microscopy (EFM) [45,46] technique for a detailedinvestigation on nanoscale localized probe induced mechano-respon-sive effect through direct visualization of electrical mapping of d-PANilayer. Metal coated AFM nanoprobe was used to impart localized stresson the d-PANi layer deposited on conductive textile thread, as shownwith schematic in Fig. 1b. Fig. 1c exhibits a systematic investigationof mechano-responsive current output for three different forces appliedthrough the AFM probe in contact mode without applying any electri-cal bias, with a retaining time of 1 s for each. The electrical responseis increasing according with the increment of applied force. This naturecan be exerted as the pumping of mobile charge carriers from d-PANi tonearest energy level of stress delivered metallic electrode. The followingis our phenomenology that can be explained by charge transfer mech-anism in the conjugated polymer system, dealing with the mechano-re-sponsive effect of output current which is illustrated as MRCTM in Fig.1d.

In MRCTM, actually two electrodes act differently during stressingand stress-releasing, depending on the metal/polymer interface layerformation. The “back electrode” act as a static electrode where thed-PANi layer is deposited, and in this case the metal/polymer interfaceis formed chemically. The other electrode (i.e. top electrode) acts as astress-deliverer, charge-collector (SDCC) electrode, for which a metal/polymer interface is transiently created upon the application of a stressby the electrode to the d-PANi layer. When the mechanical force is ex-erted into the polymeric surface through any chosen point by the SDCCelectrode, the molecular structure at that point experiences a mechan-ical deformation, which leads to a localized squeezing between molec-ular chains (schematic as shown in stress-developed condition). As westated initially that the d-PANi exhibits a metallic character (degeneratemedium with the Fermi energy level in delocalized states due to disor-der), typically the mobile ions (charge carriers arise due to π-π∗ anti-bonding) become over crowded in that deformed area, which will pos-sibly give rise a localized Fermi energy level pinning. Therefore, the lo-calized mechanical stress creates a tiny non-uniform energy state, whichis further initiating a charge transfer mechanism between the SDCCelectrode and the d-PANi layer. Now, it is interesting that the mobilecharges from the Fermi level in d-PANi system jump into the nearestavailable energy level, i.e the energy level in the SDCC electrode, notto the back electrode which is far away from the stressed area. There-fore, during stress the SDCC electrode, which will embed a stress onthe d-PANi layer, will actively take part in the charge transfer mecha-nism through the metal/polymer transitory interface layer. This natureis quite equivalent to the energy pumping by mechanical stimuli whichgenerates a flow of electrons through the polymer/metal interface layer,therefore giving rise to a current. Now, when the stress is released,the squeezed polymeric chain becomes unfettered again, causing local-ized Fermi level depinning with scarcity of charge carriers (schemed asshown in stress-released condition), and creating a localized nonuniformelectrostatic field. The scarcity of charge carriers in the stressed region isfurther balanced by the energy transfer from the nearest energy level be-tween the d-PANi layers from neighbour polymeric chains through hop-ing conduction mechanism. Additionally, some of the charges are alsotransferred from the back electrodes on which the d-PANi is functional-ized.

EFM technique was commenced further for visualization ofmechano-responsive charge distribution pattern in the d-PANi layerthrough direct imaging of localized electric field gradient before and

after stress developing on the sample surface. EFM has emerged as apowerful tool for exploring the nanoscale charge propagation dynam-ics through electrostatic mapping of the materials surface [45,46]. Aconstant dc bias has been applied between probe and samples surface,causing an EFM phase shift in the oscillation resonant frequency of thecantilever to monitor variations in the electrostatic field between sam-ple and probe, corresponding to the different regions of the sample.Fig. 2a and b represent the mapping of localized charge distributionin the same region of d-PANi layer before and after applying the me-chanical stress, respectively. The first image was scanned with tip lift-offheight of 50nm above the filament where the electrostatic field wasacted using probe bias VProbe =3V. In the next step, the mechanicalstress was applied through the probe in the middle region (1×1 μm2)of the previous scanned area in contact mode. After that, again an EFMscanning was performed in the same region, keeping all the measuringdata unaltered. Different colour contrast in the images which is basi-cally a representation of EFM phase shift, can be attributed to dissimi-larity of localized charge distribution pattern in the d-PANi layer. Fig.2c and d shows the lateral and vertical line profiles of correspondingEFM mapping images. A gradual change in the surface charge distribu-tion phase shift (ΔФ) is observed and is maximized in the middle of the

Fig. 2. EFM and c-AFM study through direct visualization electrical image based onmechano-responsive effect. Visualization of localized electrostatic charge distribution im-age of d-PANi layer (a) before and (b) after stress has been applied through AFMnanoprobe in contact mode. Stress has been applied in the 1×1 μm2 scanning area, mid-dle of the 3.5×3.5μm2 of EFM images. (c) and (d) show the vertical (black dotted linesin EFM images) and lateral (white dotted lines in EFM images) line profile spectra of bothelectrostatic charge distribution images. A prominent phase shift (ΔФ) has been observedbefore and after stresses which indicates an existence of mechano-responsive charge trans-fer mechanism. (e) Shows the scanning image of mechano-responsive electrical responseeffected by different external applied biased from 0 to 225mV in steps of 25mV. As themechano-responsive output current is in negative polarity, a positive voltage is applied tooppose that current generation. (f) Corresponding plot of average electrical response withdifferent bias. It has been noticed (pointed by dotted circle) that after applying 75mV, theoutput current changes its polarity from negative to positive, indicating that mechano-re-sponsive current harvesting is suppressed by the external voltage generating current.

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stressed region, as a consequence of the charge flow from theun-stressed area of d-PANi layers to the stressed area after stress-releas-ing. Hence, this type of phase shift pattern in EFM mapping corroboratesour MRCTM scheme.

To further demonstrate the visualization of the output current fromd-PANi layer, we have also performed current mapping in contact modewith constant force embedding by the AFM probe but with different ap-plied bias from 0 to 225mV (Fig. 2e). As we have already noticed be-fore, d-PANi surface generates negative current when the AFM probescans its surface with force even without applying any external biasvoltage (please see Fig. 1c). Under a positive bias, a prominent func-tional change in output current from negative to positive is observed.Due to the probe induced localized stress, mechano-responsive currentharvesting from the d-PANi surfaces is negative, which is an evidencefor yielding of negative voltage. To counteract the effect of mechano-re-sponsive current harvesting, a gradual increment of positive externalbias through the AFM probe has been introduced. Fig. 2f shows the av-erage of output current for 0–225mV in steps of 25mV. It can be ob-served that the output current is negative until applying the positivebias of 75mV, after which it is noticed that the output current is negli-gible (shown by the dotted circle), indicating that the mechano-respon-sive generating voltage is completely neutralized by the opposite ex-ternal field (which is approximately 75mV). Above this 75mV exter-nal bias, the output current started to be positive, indicating an influ-ence of external bias over the mechano-responsive current harvesting.This is a localized charge transportation dynamic studied by the con

ductive AFM mechanism, to correlate the mechano-responsive voltagewith the external applied voltage.

2.2. Human-motion interactive textile energy generatrix (HiTEX) device

To investigate the feasibility of d-PANi functionalized textile fibersfor the application of smart wearable energy generator from human mo-tion, we have integrated arrays of f-CTFs (each with 9cm length) in par-allel, after a sinuous stream weaving with another conducting textilefiber (with lesser diameter ∼150μm) to form a HiTEX, shown in Fig. 3b.The style of weaving is shown by the schematic diagram in Fig. 3a. TheFESEM image shows the small portion of integrated f-CTF from HiTEX.There are two functional electrodes which are particularly designed fordifferent purposes in HiTEX. The first electrode is the thinner conduc-tive textile fiber (150μm diameter) which was used to wrap the d-PANifunctionalized fiber, acting as stress-deliverer; charge-collector (SDCC)electrode. The second electrode is the metallic layer of 400μm diame-ter textile fiber, which was used for back electrode substrate, where thed-PANi has been functionalized, usually acting as ground electrode. Wehave discussed earlier that most probably this electrode acted as sup-ply chain of mobile charge carriers, which are initiated to transfer fromd-PANi layer to SDCC electrode.

On the human-motion interactive application platform, we haverecorded open-circuit output voltage (Voc) and short-circuit output cur-rent (Isc) from our HiTEX device under stress induced/released se-quences, generated by hand patting. Fig. 3c and d shows the outputperformance of HiTEX which has been determined by measuring single

Fig. 3. Human-motion interactive textile energy generatrix (HiTEX). (a) Shows the schematic with FESEM image of the knitted f-CTFs. (b) HiTEX designed by 16 f-CTFs, each with 9cmlength. The f-CTFs were covered with thin PDMS layers. (c) and (d) show the single period for motion-induced effect on open-circuit voltage (Voc) and short-circuit current (Isc) respectively.There are two states in output signals. One forward signal which is due to charge transfer at the stress-induced condition. Another is the reverse signal, which is due to stress-releasedcondition. (e) and (f) show the Voc and Isc for different numbers of f-CTFs from 1, 2, 8 and 16, each f-CTF with 9cm length. Inset shows the nature of increment of average Voc and Isc withthe number of fibers integrated into HiTEX.

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period of Voc and Isc. A forward output signal (Voc or Isc) is generatedupon stressing the f-CTFs. This spontaneous forward voltage or conse-quently current has been produced due to mechanical stress impelled onthe metal/d-PANi interface layer which will be commencing a chargetransfer mechanism through the interface. As previously explained, thisfeature can be conceptualized by localized Fermi level pinning chargetransfer initiator. In contrast, upon the releasing of mechanical stress, areverse output signal is produced. This phenomenon can be explainedby localized Fermi level depinning stage. As discussed before (Fig. 1d),after the releasing of mechanical stress, localized area in d-PANi layersbecame extended, creating localized Fermi level depinning, by scarcityof charge carriers, which are already transferred at the stress-inducedperiod. Therefore, to get electrostatic stability, the charge will flow tothe ruptured polymeric area by the back electrode or nearby polymericenergy level through tunnelling process. As the flow of this charge car-rier due to scarcity is in opposite direction, it generates a voltage or acurrent in opposite polarity but, as expected, with a lower value. Also,this feature is much slower with irregular and less uniform value thanthe forward output signals.

Given that, HiTEX will be activated by body motion, each en-ergy-harvesting system (integrated with 1, 2, 8, or 16 f-CTFs, coveredwithin thin PDMS layers as shown in Fig. 3b) was tested by repeatedpatting (with an approximately constant force) to estimate the Voc andIsc (Fig. 3e and f). To measure the Voc, each system was directly con-nected to an oscilloscope, while the Isc measurement required each sys-tem to be firstly connected to a current-to-voltage converter. The es-timation of instantaneous power output (discussed later) was done byconnecting each system to different loads and monitoring the Voc uponpatting. An increment nature of average output voltage/current (Voc andIsc) is observed with the increasing number of fibers (f-CTFs) integratedinto HiTEX. The trend of increment flows the y=axb relation, where “a”and “b” depend respectively on length and diameter of the fibers. Insetof Fig. 3e and f depicts this nature of Voc and Isc, respectively. The aver-age recorded output signals were ∼116V and 18μA for 16 f-CTFs inte-grated into HiTEX. The effective working area (area under the knittingpoints) responding in stress-induced/released power harvesting processwas estimated by optical stereo microscope (Leica M80) measurements.The output current density (Jsc) is approximately 22.5mAm−2. This

value is quite high and promising compared with some recent triboelec-tric and piezoelectric based energy harvesters. Some important resultsare revisited for comparison in Table 1 based on literature review.

Real time videos of output voltage for the HiTEX, responding to dif-ferent forces of patting, bending and even to soft touch are shown (Sup-porting Video SV1, SV2, SV3, and SV4). Fig. 4a demonstrates that withthe motion induced by patting on HiTEX, the MRCTM charge density in-creases from 0 to 260μCm−2 in 30 s against the commercial capacitor of10μF. Therefore, the amount of spontaneous charge transferred duringthe operation of MRCTM is approximately 20μCm−2. The instantaneouspower output (W = I2∙R) was calculated from the output voltage againstdifferent loads. Fig. 4b shows the variation in peak current density andpower density versus external loading resistance of the HiTEX based onMRCTM. The current density decreases with the increased external loadfrom 1kΩ to 10MΩ, and the peak power density reaches 0.6Wm−2.

Supplementary data related to this article can be found online athttps://doi.org/10.1016/j.nanoen.2019.04.012.

For realizing the practical application of MRCTM based power har-vester, we have used a rectifier circuit to have a dc output and chargeda commercial 10μF capacitor, as shown in Fig. 4c. With simple pat-ting by hand, the power delivered from our HiTEX can charge the 10μFcommercial capacitor to 3V just in 80s (please see the supporting in-formation in Fig. S2 and Supporting Video SV6). This data confirmsthe potential applicability of the newcomer d-PANi functionalized Hi-TEX because it is very much comparable to some recent publication byrenowned Z.L. Wang group for their hybrid power textile [54] whichcan charge a 2mF commercial capacitor up to 2V in 1min under am-bient sunlight in the presence of mechanical excitations. As a next step,after charging the capacitor up to 3V, it was discharged through 5 whiteLEDs in saturated power. Those LEDs have enough power to light upour university logo in a dark room, as shown in Fig. 4d. Ten com-mercial white LEDs with 2.5W were also powered (real-time data cap-tured video is shown in the supporting information, Supporting VideoSV5) by the instantaneous patting on HiTEX, without the need for therectifier circuit and the commercial capacitor. Moreover, the procuredenergy from HiTEX was used to power commercial portable deviceslike LCD digital temperature-humidity-time display meter, digital stop

Table 1The output current, voltage and power density are presented in the table below, just to give a scenario of our standpoint with respect to the literature where very few groups have reportedon directly textile-based energy harvesting systems (based on the piezoelectric and tribo-electric behaviour of the active materials).

No. Key materials used for textile-based energy harvestersHuman-motion interactive energyharvester VOut IOut

Maximum powerdensity Year of Publication

1. ZnO nanowire arrays Piezoelectric 3mV 4nA 16mW/m2 2008 [19]2. Lead zirconate titanate (PZT) textile composed of aligned

parallel nanowiresPiezoelectric 6V 45nA 200 μw/cm3 2012 [47]

3. PVDF–NaNbO3 nanofiber nonwoven fabric Piezoelectric 3.4V 4.4μA … 2013 [48]4. Polyester-Nylon Triboelectric 90V 1μA … 2014 [49]5. Parylene-Ni based cloth Triboelectric 50V 4μA 393.7 mW/m 2 2015 [50]6. Al nanoparticles-PDMS Triboelectric 368V 78μA 33.6mW/cm2 2015 [51]7. ZnO nanorod arrays on a Ag-coated textile template Triboelectric 170V 120μA … 2015 [52]∗8. PDMS-covered Cu-coated ethylene vinyl acetate (EVA) tube Triboelectric 12.6V 0.91μA … 2016 [53]∗9. Polytetrafluoroethylene (PTFE) stripes Triboelectric ∼90V ∼75μA .. 2016 [54]10. Silicone rubber Triboelectric 150V 2.9μA 85mW/m2 2017 [17]11. Silicone rubber Triboelectric 540V 140μA 0.892mW/cm2 2017 [55]12. Foam Triboelectric 70V … 46.6μW/cm2 2017 [56]13. Silicone rubber Triboelectric 200V 200μA …. 2017 [57]14. PVDF Piezoelectric 3.5V … 1.9μW/cm3 2018 [58]#15. Polyaniline coated on carbon mat and PVDF combined

systemTriboelectric 95V 180μA … 2018 [42]

#16. Polyaniline coated torn apron piece and PTFE (layereddevice)

Triboelectric 350V 45μA 11.25Wm−2 2019 [43]

17. Polyaniline functionalized free-standing textile fiber MRCTM 116V 22.5mA/m2

0.6W/m2 2019 (Our presentreport)

*We have also included here the results of Z.L. Wang group published in 2016 regarding hybrid power textiles, but only mentioned the human motion induced contribution from them(as they have also solar cell and supercapacitor devices in these fibers). # They have used 2D substrates (carbon mat or cotton piece) to coat polyaniline, also for device structure anothertriboelectric counterpart like PVDF or PTFE have been used.

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Fig. 4. Application of HiTEX to electronic devices. (a) Measurement of spontaneous charge flow from HiTEX under motion-induced condition. (b) Current density and power density withvarious loads. (c) Schematic of rectifier circuit. (d) Procured energy from HiTEX has been used to power portable electronics through discharging a 47μF commercial capacitor. Snap-shotof the serial connections of 5 white LEDs powered by HiTEX; charge stored in 10μF capacitor up to 3V and discharged through these LEDs. (e), (f) Test results for stability check of theenergy harvester system for average (e) output voltage and (f) output current with number of patting.

watch etc (please see Fig. 4d; corresponding real time video is shown inSupporting Video SV7). We have also tested the robustness and durabil-ity of the functionalized fibers through different kind of bending testsand huge number of patting over six months. Its energy harvesting prop-erties remain essentially unchanged, as exemplified in Fig. 4 for a rep-resentative sample. Fig. 4e and f shows the average output voltage andcurrent after completing 5000 numbers of patting as each interval. Asthe patting on the devices is not applied through a machine, we havecounted it for several months. Each time after completing 5000 times ofpatting we took an average of 10 consecutive output current and volt-age data and noted it. The data as shown in the figure is an approximateaverage value. Even after patting more than 100 thousand times, HiTEXhas shown negligible changes in its electrical performances. Hence, ournewly designed HiTEX based on polyaniline as a conjugated polymer issustainable enough for practical applications.

Supplementary data related to this article can be found online athttps://doi.org/10.1016/j.nanoen.2019.04.012.

3. Conclusion

We have shown for the first time human-motion interactive powerharvesting from single textile fiber, where d-PANi as a conjugate poly-mer has been used as the active layer. A simple metal/polymer (semi-conductor in nature)/metal structure has been deployed to the way ofmotion induced power harvester device through MRCTM. Direct visu-alization of emerging current and electrostatic field distribution patternover the d-PANi functionalized fibers have been demonstrated by AFM,which suggests that the charge transfer mechanism has been incited atthe metal/polymer interface layer due to applied stress induction. By

introducing the MRCTM model for conjugated polymer, this report hasexhibited super-flexible, robust, light weight, low-cost sustainable en-ergy wearable modules, which is strong enough to take a challenge ofnext-generation wearable energy plant. As a very initial candidature inthe field of reported textile oriented energy harvester platform, we havereported 0.6Wm−2 impulsive power density. Each single functionalizedfiber is a generatrix of sustainable energy. Our prototype textile basedenergy harvester device responds stably over 100 thousand times of pat-ting, bending for six months without any degradation. Energy harvest-ing based on mechano-responsive platform, our recent scientific outturnhave opened up a new route using π-conjugated polymeric materials.

4. Experimental methods

4.1. Materials

Commercially available silver coated conductive textile fibers, CTF(Diameter 400μm, Statex, Germany), were used as the core fiber ma-terial for further coating process with doped polyaniline (d-PANi). Ani-line from Sigma-Aldrich has been used as the monomer to form PANithrough a simple and cost-effective wet chemical oxidation polymer-ization route. Each time it was distilled under vacuum, prior to usein the polymerization process. Ammonium persulfate (APS; 99.99%,Aldrich) was the oxidant used during the reaction. Besides, hydrochloricacid (HCl) and D-10-camphorsulfonic acid (CSA, Aldrich) were used asdopant and surface functionalization element, respectively, for the cur-rent fibers coating procedure. All of these chemicals were used as re-ceived without any prior purification process. Deionized water (DI wa

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ter) was used throughout different steps of the experiment. Methanol(Merck) was used during the final washing of coated fiber products.

4.2. Coating with d-PANi

The commercial fibers were at first cleaned with dip immersion inethanol for 15min, followed by washing with DI water several times.After that they were set inside the reaction vessel having one end cov-ered with kapton (to avoid polymer coating on that end and also tohave access to the metal core contact). Initially, the fibers used were of9cm in length; but the process is now adapted for longer fiber as well.Meanwhile, the solutions for the reaction were in making. Aniline andCSA were added in 4:1M ratio to 20mL DI water to make a colour-less aqueous solution of Aniline-CSA complex and left for cooling in anice bath (0 °C). In a separate container, the oxidant APS was dissolvedin 10mL DI water, followed by mixing of 100μL of HCl into it. Thiswas also set for pre-cooling process in the ice bath, before staring themain polymerization. Aniline and APS were used in a 1:1M ratio forthe optimized test. This mixture was then added dropwise to the Ani-line-CSA complex solution, while vigorously shaking the latter. The finalmixture was immediately transferred to the reaction vessel (already setwith fibers at 0 °C in an ice bath) to continue the polymerization processwithout any further agitation for the next 1h. After 1h, the fibers weretaken out from the solution and thoroughly washed with DI water forseveral times to remove residuals. Also, with the final rinsing of fiberswith methanol for three times, the removal of any excess monomer oroligomers was ensured. After this chemical deposition, a uniform darkgreen coating of d-PANi was obtained throughout the surface of thefibers. Finally, compressed air was passed smoothly over the surface ofcoated fibers for few minutes and then these functionalized conductingtextile fiber (f-CTF) were stored in ambient environment for a few daysbefore making the energy harvester system.

4.3. Fabrication of HiTEX

Each f-CTF was wrapped up with different number of windings withanother commercial silver-coated conductive textile fiber (150μm diam-eter) in a manner so that, the uncoated metallic end of the thicker fiber(the one end of the f-CTF which was left covered with kapton duringpolymer coating to further access its conductive core) was used as oneelectrode to reach the polymeric part, and the other electrode was ob-tained from taking contact to the thinner metal coated fiber on the otherside. The as-woven system was protected from usual mishandling witha thin polydimethylsiloxane (PDMS, purchased from Sylgard 184) cov-ering (elastomer to curing agent ratio of 10:1, curing at 65 °C). The con-tacts were taken outside this PDMS cover and connected through coppertape (3M) to the electrical measurement systems. Different number ofwindings and fibers have been systematically covered by the PDMS sys-tem according to the measurement requirements.

4.4. AFM analyses

AFM measurements in electrical mode were done by Asylum Re-search MFP-3D Stand-alone system using commercial Pt Ir tip coatedprobes. The electrical data recorded under the different applied forceswas measured with standard calibration. Measured data was analysedwith the Asylum Research tools developed within the IGOR Pro 6.22Adata analysis software.

Author contributions

S.G and S.N designed the experiments and wrote the manuscript.S.G developed the materials and functionalized the textile fibers. A.S,S.N and S.G performed the electrical experiments. S.N performed the

AFM experiments. All the authors discussed the results, commented andreviewed the manuscript throughly.

Acknowledgements

S. Goswami and S. Nandy acknowledge funding from EuropeanCommunity H2020 program under grant agreement No. 685758 (pro-ject 1D-Neon). Also, this work was partially financed by FEDER fundsthrough the COMPETE 2020 Programme and National Funds throughFCT under the project UID/CTM/50025/2013. Andreia dos Santos ac-knowledges the support from FCT and MIT-Portugal through the schol-arship PD/BD/105876/2014. The authors want to give special thanksto Ricardo Ferreira, Daniela Nunes and Ana Marques for their technicalsupports.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoen.2019.04.012.

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