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Contents lists available at ScienceDirectNano Energy
journal homepage: www.elsevier.comHuman-motion interactive energy harvester based on polyaniline functionalized textile
fibers following metal/polymer mechano-responsive charge transfer mechanism
Sumita 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 harvester Polyaniline Wearable electronics Charge-transfer mechanism Atomic force microscopy Conjugated 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 proposed design 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 distribution pattern has been extensively studied by atomic force microscopy. The integrated arrays of f-CTFs produce a peak power-density of ∼0.6Wm−2with output current-density of ∼22mAm−2and can power at least 10 white LEDs
of 2.5W. The procured energy from f-CTFs is capable of charging a commercial 10μF capacitor to 3V in 80s and powering portable electronic devices. The prototype energy harvester stably shows the same performance after more 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 to move 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 provide consumers 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 awareness is at a minimum level regarding smart clothing and e-textile products [11]. Therefore, recent research trend in flexible electronics demands an 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, smart wearable energy sources that can spontaneously provide electrical power to the daily used smart electrical products. This is now one of the biggest technical challenges also. Looking at the lack of expansion and 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 design of human-motion interactive energy harvester based on polymer func-tionalized free-standing textile fibers will bring to bear a vast variety of fields, 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 as health care and defence.
During the last decade, some research groups have reported on mechano-responsive energy harvesters, mostly based on either tribo-electric [13–18] or piezotribo-electric [19–24] platform. Among them, very few 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.2019.04.012
Received 3 January 2019; Received in revised form 10 March 2019; Accepted 3 April 2019
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Triboelectricity is a phenomenon of contact electrification mechanismwhere two different kinds of material become electrically opposite charged if rubbed against each other. Static polarized charges arise on the materials surfaces due to the friction, and their separation by the back electrode (charges can flow through the back electrode of the ma-terial to the external circuit) creates a potential difference which further generates 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 energy source. 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-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 as chemicals, heat, light, electricity, stress etc.) are evolving nowadays into an attractive class of smart materials which can be designed for different application 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 useful way to mechanical stress. In that context, we consider this input-out-put relationship as an energy transduction process through converting mechanical energy to electrical energy. The core idea behind this is to initiate a mechano-responsive charge transfer mechanism (MRCTM) and energy-transfer process in a π-conjugated polymer at the organic-metal interface layer [28]. The common building blocks of π-conjugated poly-mers are mainly carbon atoms, schemed with alternating single and double bonds. The conjugation is a result of sp2p
zhybridized carbon atoms, yielding three covalent s-bonds (i.e. localized σ-bonds) that form strong chemical bonds. The remaining pzorbital is free to overlap with the corresponding pzorbital of an adjacent carbon (i.e. π-bond), formally leading to one unpaired electron per carbon that is delocalized over the molecule or, in the case of polymers, that allows carrier mobilities over large segments of the polymeric chain. The HOMO (valence band) and LUMO (conduction band) of π-conjugated polymers are generally de-rived from occupied π-bonding orbitals and unoccupied π*-antibonding orbitals, 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 desired electro-mechanics [39–41].
Very recent a few works have been reported on PANi based mechano-responsive energy harvesters [42,43] in which the authors have coated the 2D substrates like carbon mat or cotton piece with polyaniline. Moreover, those groups have obtained their best results for devices with additional triboelectric layers of PVDF or PTFE. But in the present report we have functionalized each individual textile fiber with
d-PANi and designed them for harvesting mechano-responsive energy
using purely electrode/d-PANi/electrode system. Our device is basically working on free standing d-PANi functionalized fibers (which can be considered as a 3D system) and the electrode was woven surrounding each functionalized fiber. This unique design of our system practically has the huge opportunity to increase the pressure contact area with in-creasing efficiency and can be further adopted easily into any form of garments 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 doped polyaniline (d-PANi) nanostructure. Fig. 1a shows the field emission scanning electron microscopic (FESEM, Carl Zeiss Auriga Crossbeam microscope) image of a d-PANi functionalized conducting textile fiber (f-CTF). Center inset of Fig. 1a shows the zoom in on d-PANi layer and the upper inset image shows the normal view of an f-CTF. The corre-sponding EDS elemental mapping (using Oxford XMax 150) of uncoated CTF and f-CTF is shown in Fig. S2 and Fig. S3 respectively (support-ing information). The thickness of the d-PANi layer is approximately 400–450nm over 400-μm diameter conductive fiber, which is uniformly coated with our optimized wet chemical method. Details of synthesis have been described in experimental section. The cross-sectional FESEM image of f-CTF has been shown in Fig. S4 in the supporting information.
2.1. Investigation on localized mechano-responsive charge transfer mechanism (MRCTM)
Before exhibiting the mainstream approach of measuring hu-man-motion interactive current harvesting from f-CTFs, we have first explored the localized charge transfer mechanism at the metal-conju-gated polymer (i.e. d-PANi) layers based on free-standing textile fiber by atomic force microscopy (AFM) technique. The most unavoidable pri-mary requirement to any device performance depends on the interface dynamics 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) on d-PANi functionalized textile fiber (f-CTF). (a) FESEM image of d-PANi functionalized
layer 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 based characterization on MRCTM. (c) Output current response from d-PANi surface for typical three different contacting forces of 8, 12 and 16nN delivered by AFM probe. Each apply-ing force conferrapply-ing for 1s which shows that the output current increases with increasapply-ing applying forces. (d) Schematic illustration (not in scale) on probable systematic investiga-tion on MRCTM: d-PANi layers arranged with uniform molecular networks at undisturbed condition→localized stress delivered by the top electrode to the d-PANi layer→ molecular networks contracted→generating Fermi level pinning due to increasing localized charge carrier density→ initiating a charge transfer mechanism from nearest energy level of metal electrode (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 aselec-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 material are 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] and electrostatic force microscopy (EFM) [45,46] technique for a detailed investigation on nanoscale localized probe induced mechano-respon-sive effect through direct visualization of electrical mapping of d-PANi layer. Metal coated AFM nanoprobe was used to impart localized stress on the d-PANi layer deposited on conductive textile thread, as shown with schematic in Fig. 1b. Fig. 1c exhibits a systematic investigation of mechano-responsive current output for three different forces applied through the AFM probe in contact mode without applying any electri-cal bias, with a retaining time of 1s for each. The electrielectri-cal response is increasing according with the increment of applied force. This nature can be exerted as the pumping of mobile charge carriers from d-PANi to nearest energy level of stress delivered metallic electrode. The following is 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 stressing and stress-releasing, depending on the metal/polymer interface layer formation. The “back electrode” act as a static electrode where the
d-PANi layer is deposited, and in this case the metal/polymer interface
is formed chemically. The other electrode (i.e. top electrode) acts as a stress-deliverer, charge-collector (SDCC) electrode, for which a metal/ polymer interface is transiently created upon the application of a stress by the electrode to the d-PANi layer. When the mechanical force is ex-erted into the polymeric surface through any chosen point by the SDCC electrode, 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 we stated initially that the d-PANi exhibits a metallic character (degenerate medium 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, which is further initiating a charge transfer mechanism between the SDCC electrode and the d-PANi layer. Now, it is interesting that the mobile charges from the Fermi level in d-PANi system jump into the nearest available energy level, i.e the energy level in the SDCC electrode, not to the back electrode which is far away from the stressed area. There-fore, during stress the SDCC electrode, which will embed a stress on the d-PANi layer, will actively take part in the charge transfer mecha-nism through the metal/polymer transitory interface layer. This nature is quite equivalent to the energy pumping by mechanical stimuli which generates 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 as shown in stress-released condition), and creating a localized nonuniform electrostatic field. The scarcity of charge carriers in the stressed region is further 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 also transferred from the back electrodes on which the d-PANi is functional-ized.
EFM technique was commenced further for visualization of mechano-responsive charge distribution pattern in the d-PANi layer through direct imaging of localized electric field gradient before and
after stress developing on the sample surface. EFM has emerged as a powerful tool for exploring the nanoscale charge propagation dynam-ics through electrostatic mapping of the materials surface [45,46]. A constant dc bias has been applied between probe and samples surface, causing an EFM phase shift in the oscillation resonant frequency of the cantilever to monitor variations in the electrostatic field between sam-ple and probe, corresponding to the different regions of the samsam-ple. Fig. 2a and b represent the mapping of localized charge distribution in the same region of d-PANi layer before and after applying the me-chanical stress, respectively. The first image was scanned with tip lift-off height of 50nm above the filament where the electrostatic field was acted using probe bias VProbe=3V. In the next step, the mechanical stress was applied through the probe in the middle region (1×1 μm2) of the previous scanned area in contact mode. After that, again an EFM scanning was performed in the same region, keeping all the measuring data 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 corresponding EFM 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 on mechano-responsive effect. Visualization of localized electrostatic charge distribution im-age of d-PANi layer (a) before and (b) after stress has been applied through AFM nanoprobe in contact mode. Stress has been applied in the 1×1 μm2scanning area,
mid-dle of the 3.5×3.5μm2of EFM images. (c) and (d) show the vertical (black dotted lines
in EFM images) and lateral (white dotted lines in EFM images) line profile spectra of both electrostatic charge distribution images. A prominent phase shift (ΔФ) has been observed before and after stresses which indicates an existence of mechano-responsive charge trans-fer mechanism. (e) Shows the scanning image of mechano-responsive electrical response effected by different external applied biased from 0 to 225mV in steps of 25mV. As the mechano-responsive output current is in negative polarity, a positive voltage is applied to oppose that current generation. (f) Corresponding plot of average electrical response with different bias. It has been noticed (pointed by dotted circle) that after applying 75mV, the output 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 corroborates our MRCTM scheme.
To further demonstrate the visualization of the output current from
d-PANi layer, we have also performed current mapping in contact mode
with 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 probe scans its surface with force even without applying any external bias voltage (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 current harvesting from the d-PANi surfaces is negative, which is an evidence for yielding of negative voltage. To counteract the effect of mechano-re-sponsive current harvesting, a gradual increment of positive external bias 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 positive bias 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 voltage with the external applied voltage.
2.2. Human-motion interactive textile energy generatrix (HiTEX) device
To investigate the feasibility of d-PANi functionalized textile fibers for 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 textile fiber (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. The FESEM image shows the small portion of integrated f-CTF from HiTEX. There are two functional electrodes which are particularly designed for different purposes in HiTEX. The first electrode is the thinner conduc-tive textile fiber (150μm diameter) which was used to wrap the d-PANi functionalized 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 the
d-PANi has been functionalized, usually acting as ground electrode. We
have discussed earlier that most probably this electrode acted as sup-ply chain of mobile charge carriers, which are initiated to transfer from
d-PANi layer to SDCC electrode.
On the human-motion interactive application platform, we have recorded 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 output performance 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 9cm length. 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-released condition. (e) and (f) show the Vocand Iscfor 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 Vocand Iscwith
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period of Vocand Isc. A forward output signal (Vocor Isc) is generatedupon stressing the f-CTFs. This spontaneous forward voltage or conse-quently current has been produced due to mechanical stress impelled on the metal/d-PANi interface layer which will be commencing a charge transfer mechanism through the interface. As previously explained, this feature can be conceptualized by localized Fermi level pinning charge transfer initiator. In contrast, upon the releasing of mechanical stress, a reverse output signal is produced. This phenomenon can be explained by localized Fermi level depinning stage. As discussed before (Fig. 1d), after the releasing of mechanical stress, localized area in d-PANi layers became extended, creating localized Fermi level depinning, by scarcity of charge carriers, which are already transferred at the stress-induced period. Therefore, to get electrostatic stability, the charge will flow to the ruptured polymeric area by the back electrode or nearby polymeric energy level through tunnelling process. As the flow of this charge car-rier due to scarcity is in opposite direction, it generates a voltage or a current in opposite polarity but, as expected, with a lower value. Also, this feature is much slower with irregular and less uniform value than the 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, covered within thin PDMS layers as shown in Fig. 3b) was tested by repeated patting (with an approximately constant force) to estimate the Vocand
Isc(Fig. 3e and f). To measure the Voc, each system was directly con-nected to an oscilloscope, while the Iscmeasurement 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 by connecting each system to different loads and monitoring the Vocupon patting. An increment nature of average output voltage/current (Vocand
Isc) is observed with the increasing number of fibers (f-CTFs) integrated into HiTEX. The trend of increment flows the y=axbrelation, where “a” and “b” depend respectively on length and diameter of the fibers. Inset of Fig. 3e and f depicts this nature of Vocand 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 knitting points) responding in stress-induced/released power harvesting process was 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 piezoelectriboelec-tric based energy harvesters. Some important results are 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 with the motion induced by patting on HiTEX, the MRCTM charge density in-creases from 0 to 260μCm−2in 30s against the commercial capacitor of 10μF. Therefore, the amount of spontaneous charge transferred during the operation of MRCTM is approximately 20μCm−2. The instantaneous power output (W = I2∙R) was calculated from the output voltage against different loads. Fig. 4b shows the variation in peak current density and power density versus external loading resistance of the HiTEX based on MRCTM. The current density decreases with the increased external load from 1kΩ to 10MΩ, and the peak power density reaches 0.6Wm−2.
Supplementary data related to this article can be found online at https://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 charged a commercial 10μF capacitor, as shown in Fig. 4c. With simple pat-ting by hand, the power delivered from our HiTEX can charge the 10μF commercial capacitor to 3V just in 80s (please see the supporting in-formation in Fig. S2 and Supporting Video SV6). This data confirms the potential applicability of the newcomer d-PANi functionalized Hi-TEX because it is very much comparable to some recent publication by renowned Z.L. Wang group for their hybrid power textile [54] which can 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 white LEDs in saturated power. Those LEDs have enough power to light up our 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 Video SV5) by the instantaneous patting on HiTEX, without the need for the rectifier circuit and the commercial capacitor. Moreover, the procured energy from HiTEX was used to power commercial portable devices like LCD digital temperature-humidity-time display meter, digital stop Table 1
The 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 reported on 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 harvesters Human-motion interactive energyharvester VOut IOut
Maximum power
density Year of Publication
1. ZnO nanowire arrays Piezoelectric 3mV 4nA 16mW/m2 2008 [19]
2. Lead zirconate titanate (PZT) textile composed of aligned
parallel nanowires Piezoelectric 6V 45nA 200 μw/cm
3 2012 [47]
3. PVDF–NaNbO3nanofiber 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/m2 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
system Triboelectric 95V 180μA … 2018 [42]
#16. Polyaniline coated torn apron piece and PTFE (layered
device) Triboelectric 350V 45μA 11.25Wm
−2 2019 [43]
17. Polyaniline functionalized free-standing textile fiber MRCTM 116V 22.5
mA/m2 0.6W/m
2 2019 (Our present
report)
*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 another triboelectric 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 with various 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-shot of 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 the energy 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 in Supporting Video SV7). We have also tested the robustness and durabil-ity of the functionalized fibers through different kind of bending tests and 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 and current after completing 5000 numbers of patting as each interval. As the patting on the devices is not applied through a machine, we have counted it for several months. Each time after completing 5000 times of patting 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 approximate average value. Even after patting more than 100 thousand times, HiTEX has shown negligible changes in its electrical performances. Hence, our newly designed HiTEX based on polyaniline as a conjugated polymer is sustainable enough for practical applications.
Supplementary data related to this article can be found online at https://doi.org/10.1016/j.nanoen.2019.04.012.
3. Conclusion
We have shown for the first time human-motion interactive power harvesting from single textile fiber, where d-PANi as a conjugate poly-mer has been used as the active layer. A simple metal/polypoly-mer (semi-conductor in nature)/metal structure has been deployed to the way of motion induced power harvester device through MRCTM. Direct visu-alization of emerging current and electrostatic field distribution pattern over the d-PANi functionalized fibers have been demonstrated by AFM, which suggests that the charge transfer mechanism has been incited at the metal/polymer interface layer due to applied stress induction. By
introducing the MRCTM model for conjugated polymer, this report has exhibited super-flexible, robust, light weight, low-cost sustainable en-ergy wearable modules, which is strong enough to take a challenge of next-generation wearable energy plant. As a very initial candidature in the field of reported textile oriented energy harvester platform, we have reported 0.6Wm−2impulsive power density. Each single functionalized fiber is a generatrix of sustainable energy. Our prototype textile based energy 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 outturn have 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 PANi through a simple and cost-effective wet chemical oxidation polymer-ization route. Each time it was distilled under vacuum, prior to use in the polymerization process. Ammonium persulfate (APS; 99.99%, Aldrich) was the oxidant used during the reaction. Besides, hydrochloric acid (HCl) and D-10-camphorsulfonic acid (CSA, Aldrich) were used as dopant 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 in ethanol 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 to have access to the metal core contact). Initially, the fibers used were of 9cm in length; but the process is now adapted for longer fiber as well. Meanwhile, the solutions for the reaction were in making. Aniline and CSA 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 an ice bath (0°C). In a separate container, the oxidant APS was dissolved in 10mL DI water, followed by mixing of 100μL of HCl into it. This was also set for pre-cooling process in the ice bath, before staring the main polymerization. Aniline and APS were used in a 1:1M ratio for the optimized test. This mixture was then added dropwise to the Ani-line-CSA complex solution, while vigorously shaking the latter. The final mixture was immediately transferred to the reaction vessel (already set with fibers at 0°C in an ice bath) to continue the polymerization process without any further agitation for the next 1h. After 1h, the fibers were taken out from the solution and thoroughly washed with DI water for several times to remove residuals. Also, with the final rinsing of fibers with methanol for three times, the removal of any excess monomer or oligomers was ensured. After this chemical deposition, a uniform dark green coating of d-PANi was obtained throughout the surface of the fibers. Finally, compressed air was passed smoothly over the surface of coated fibers for few minutes and then these functionalized conducting textile fiber (f-CTF) were stored in ambient environment for a few days before making the energy harvester system.
4.3. Fabrication of HiTEX
Each f-CTF was wrapped up with different number of windings with another 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 during polymer coating to further access its conductive core) was used as one electrode to reach the polymeric part, and the other electrode was ob-tained from taking contact to the thinner metal coated fiber on the other side. The as-woven system was protected from usual mishandling with a 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 copper tape (3M) to the electrical measurement systems. Different number of windings 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 coated probes. The electrical data recorded under the different applied forces was measured with standard calibration. Measured data was analysed with the Asylum Research tools developed within the IGOR Pro 6.22A data 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 and reviewed the manuscript throughly.
Acknowledgements
S. Goswami and S. Nandy acknowledge funding from European Community H2020 program under grant agreement No. 685758 (pro-ject 1D-Neon). Also, this work was partially financed by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT 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 thanks to Ricardo Ferreira, Daniela Nunes and Ana Marques for their technical supports.
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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