Triboelectric Nanogenerator Driven Self-Charging and Self-Healing

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Triboelectric Nanogenerator Driven Self-Charging and Self-Healing Flexible Asymmetric Supercapacitor Power Cell for Direct Power Generation Anirban Maitra, Sarbaranjan Paria,† Sumanta Kumar Karan,† Ranadip Bera, Aswini Bera, Amit Kumar Das, Suman Kumar Si, Lopamudra Halder, Anurima De, and Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India

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S Supporting Information *

ABSTRACT: The expeditious growth of portable electronics has endorsed the researchers to develop self-powered devices that synchronically harvest and store energy. However, it is quite challenging to integrate two distinct phenomena in a single portable device. Here, we emphasize the fabrication of a triboelectric driven self-charging and self-healing asymmetric supercapacitor (SCSHASC) power cell composed of magnetic cobalt ferrite grown on a stainless steel (SS) fabric (CoFe2O4@SS) as positive and iron oxides decorated reduced graphene oxide grown on a SS fabric (Fe-RGO@SS) as negative electrodes separated by a KOH-soaked self-healing polymer hydrogel electrolyte membrane. The membrane contains Fe3+ cross-linked polyacrylic acid, whereas selfhealing carboxylated polyurethane was utilized for encapsulation. SS fabric and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/SS-impregnated micropatterned PDMS composite film-strip were employed as positive and negative triboelectric friction layers, respectively. During mechanical deformation, the SCSHASC harvests electrical energy and subsequently stores it as electrochemical energy for sustainable power supply. The sandwich-type SCSHASC power cell (a supercapacitor unit sandwiched between two parallelly connected high-performance triboelectric nanogenerators) was charged up to ∼1.6 V within ∼31 min under periodic compression/stress (F ≈ 17.6 N, f ≈ 3.80 Hz). Furthermore, the SCSHASC# (with two supercapacitor units in series) can instantly power-up several portable electronic appliances on periodic compression and release. Thus, the SCSHASC with unique design will be extremely beneficial for self-powered electronics. KEYWORDS: self-powered, self-charging, self-healing, asymmetric supercapacitor, sandwich-type, triboelectric nanogenerators the active surface of the triboelectric friction layers,21−23 surface modification of friction layers,24 and inclusion of certain dielectric fillers inside the friction layers25 play crucial roles. Conversely, Li-ion batteries26−29 and supercapacitors29−33 are widely employed as energy storage devices where electricity is converted into electrochemical energy for sustainable power supply. Asymmetric supercapacitors (ASCs), one of the auspicious members in the supercapacitor family with immense power densities, wide working potential range, and superior cyclic stability, are of great demand in portable electronics.34−36 Also, utilizing high-performance binder-free component electrodes by growing the electroactive materials directly on conductive current collector substrates essentially improves the overall electrical conductivity.37,38 To mitigate the requirements for flexible electronics, solid-state symmetric and ASCs are employed by sandwiching two flexible

1. INTRODUCTION Cumulative demand for smart portable electronics in the global market has drawn significant research interest in the arena of energy harvesting and storage devices.1−4 Typically, the mechanisms of energy harvesting and storage are completely discrete.5 Hence, the concept of integration of two such distinct mechanisms in a single portable device is quite new and challenging. As for energy harvesting and conversion, several harvesting devices have been employed to harvest electricity directly from ambient energies, that is, solar, mechanical, and thermal energies.6−9 Triboelectric and piezoelectric nanogenerators have widely been explored as energy harvesting devices for scavenging mechanical energy.10−15 Triboelectric nanogenerators (TENGs) can scavenge numerous forms of mechanical energies by the coupling effect of contact electrification and electrostatic induction.16−19 The prime benefits of utilizing TENGs are superior output performances, extreme durability, inexpensive, and relatively simple design.20 Amongst several approaches to enhance the output of the TENGs, instigation of micro- or nanopatterns on © XXXX American Chemical Society

Received: October 31, 2018 Accepted: January 8, 2019 Published: January 8, 2019 A

DOI: 10.1021/acsami.8b19044 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

self-healing polymer hydrogel electrolyte membrane composed of Fe3+ cross-linked polyacrylic acid (Fe3+-PAA). The reason behind choosing spinel CoFe2O4 as a positive electrode material is the presence of various oxidation states, improved magnetic properties, excellent electrochemical behavior, and cyclic stability.58 Likewise, Fe-RGO (negative electrode) reveals outstanding power density, stability, and magnetic behavior owing to the existence of iron oxides (solely Fe3O4 with traces of Fe2O3) in RGO. Later, the entire ASC was encapsulated by self-healing carboxylated polyurethane (CPU). The SHASC exhibits superior electrochemical performance, cyclic stability, flexibility, self-healing features, and wide operating voltage window (0−1.6 V). Finally, our SCSHASC power cell instantly converts the applied mechanical energy into electrical energy upon periodic compression and consequently stores it as electrochemical energy for sustainable power generation. Therefore, it reveals dual way charging features, that is, both electrically and mechanically. The SCSHASC can be charged up to ∼1.6 V within ∼31 min under periodic compression (F ≈ 17.6 N, f ≈ 3.80 Hz). It can be charged by casual body/limb movements (bio-mechanical motions). Furthermore, the SCSHASC# power cell (with two series connected SHASCs sandwiched between two parallelly connected HPTENGs) can instantly power-up portable electronic gadgets upon repetitive compression which is indeed promising for the incessant operation of daily utilized electronics.

supercapacitor electrodes in the presence of a solid-state gel electrolyte.39−41 In contrast to liquid electrolytes, solid-state gel electrolytes elude the issues of electrolyte leakage and short circuit during eternal operations.20,31 To integrate the mechanism of energy harvesting and storage, various self-powered systems16,42−51 have been implemented by scavenging ambient mechanical/bio-mechanical energy to sustainably power-up portable electronics. Currently, researchers are aiming to design flexible and wearable self-powered devices with improved power density and efficiency by efficaciously integrating flexible energy harvesters with flexible supercapacitors.52 Recently, Wang et al. fabricated a self-charging power unit by integrating stacked TENGs with a supercapacitor unit.53 The device can efficiently convert mechanical energy directly to electrochemical energy. Wen and Sun et al. designed an ultralight and flexible selfcharging power unit to scavenge mechanical energy.43 Zhang et al. invented a self-charging power system by integrating flexible supercapacitor and TENG.20 Thus, fabrication of exclusive self-charging power devices has lot of opportunities for advancement. Severe damages during repeated mechanical deformations and electrochemical cycling are the major concern for flexible and wearable self-charging power devices. Particularly, the supercapacitor components permanently fail and reduce the entire device performance over time. This inevitable failure ultimately causes malfunction and diminishes the lifespan of the self-charging power device.54 To evade these limitations, it is worthy to utilize component electrodes and separators that are capable of self-healing or restoring their electrical and mechanical properties inherently after severe damage.55 Nowadays, supercapacitors comprising self-healable polymerbased separators,3 encapsulators56 for easy restoration of mechanical properties, and magnetically healable constituent electrodes for the restoration of electrical properties even after several damages are quite significant and promising in flexible and wearable electronics.55 Herein, we demonstrate the facile fabrication of a triboelectric driven self-charging and self-healing ASC (SCSHASC) power cell by judicious integration of a SHASC sandwiched between two parallelly connected high-performance TENGs (HPTENGs). A polished stainless steel (SS) fabric and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFHFP)/SS-impregnated micropatterned polydimethylsiloxane (PDMS) composite film-strip have been employed as positive and negative triboelectric friction layers, respectively, in the HPTENGs. The presence of PVDF-HFP inside the friction layer eventually increases the dielectric constant and surface charge density (SCD) of the composite film, whereas the micropatterned contact surface of the film-strip provides a higher exposed surface area for better friction. Besides, extremely flexible and robust PDMS provides superior output performance even under small extent of deformations.57 As a result, single HPTENG reveals an output voltage (VO) of ∼274.3 V and a short-circuit current (ISC) of ∼29.0 μA under periodic compression (F ≈ 17.6 N, f ≈ 3.80 Hz) which are substantially higher than pure PDMS−SS-based TENG (VO ≈ 152.3 V; ISC ≈ 8.9 μA at F ≈ 17.6 N, f ≈ 3.80 Hz). On the other hand, the SHASC unit composed of magnetic cobalt ferrite electrochemically deposited on a polished SS fabric (CoFe2O4@SS) as positive and magnetic iron oxides decorated reduced graphene oxide (RGO) grown on a SS fabric (FeRGO@SS) as negative electrodes, separated by a KOH-soaked

2. EXPERIMENTAL SECTION 2.1. Fabrication of PVDF-HFP Flakes (Step-I). PVDF-HFP flakes were prepared through a low-cost nonsolvent-induced phase separation method. At first, 1 g of PVDF-HFP was dissolved in 20 mL of DMF and the solution was kept under constant stirring condition for 3 h at a temperature of ∼70 °C. Then, the PVDF-HFP solution was taken in a syringe (needle diam ≈ 0.04 mm) and gently sprayed over ∼15 mL water surface kept in a Petri dish (diam ≈ 10 cm). During spraying, the PVDF-HFP solution rapidly transforms into a thin sponge-type film owing to the solvent exchange process in between DMF and water. Finally, the sponge-type PVDF-HFP film was isolated from the Petri dish and dried at ∼50 °C for 5 days in a hot air oven. The sponge-type film thus obtained was cut into small flakes (area ≈ 0.2 × 0.2 cm2) for the fabrication of composite film.59 2.2. Preparation of (PVDF-HFP)/SS-Impregnated Micropatterned PDMS Composite Film-Strip and the Fabrication of HPTENG (Step-II). A cigarette wrapper (6 × 4 cm2) was adhered inside the cavity (micropatterned surface/Al-side of the cigarette wrapper was exposed) of an aluminum mold with an inner volume (mold cavity) of ≈6 × 4 × 0.2 cm3. The as-prepared PVDF-HFP flakes (∼3 wt %) was then uniformly embedded above the patterned surface inside the mold cavity followed by insertion of a polished SS fabric (area ≤6 × 4 cm2) just above the PVDF-HFP flakes. A thin SS wire was connected with SS electrode by soldering. Afterward, a uniform and transparent mixture of PDMS elastomer and its crosslinker (mixed at 10:1 ratio followed by vacuum-degassing) was gently poured inside the mold cavity under static condition to fill-up the entire mold cavity (including PVDF-HFP and SS electrode) with the PDMS mixture, followed by curing in a vacuum oven at 90 °C for 1 h. Finally, the cured PDMS-based composite film (6 × 4 × 0.2 cm3) was peeled-off from the mold. It is noteworthy that the micropattern of the cigarette wrapper (Al-side) was replicated on the active surface of the film. This film was referred to as PDMS/PVDF-HFP/SS and employed as a negative triboelectric friction layer in the HPTENG. On the other hand, a polished SS fabric with an area of 6 × 4 cm2 was directly employed as both the electrode and positive triboelectric friction layer in the HPTENG connected with a SS wire by soldering. Prior to assembling the two friction layers, the PDMS/PVDF-HFP/ SS composite film was bent to an arc shape and attached with a bent B


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Figure 1. Schematic for the fabrication of (a) PVDF-HFP flakes (step-I), (b) HPTENG (step-II), (c) CoFe2O4@SS and Fe-RGO@SS component electrodes and assembling SCSHASC device (step-III) for several applications. transparent polypropylene (PP) sheet (6 × 4 cm2) using an adhesive. This arc-shaped structure enhances the performance of the HPTENG. Likewise, the SS fabric electrode was attached with a flat PP sheet using an adhesive. Finally, the HPTENG device was assembled by merely joining the two PP sheets pre-attached with the respective friction layers and electrodes. The active area of the HPTENG was 6 × 4 cm2, whereas the gap between the two triboelectric friction layers was 0.6 cm. It is to be stated that each polished SS was washed with deionized water and ethanol several times using ultrasonication (model: OSCAR: PR-250, frequency ≈ 25 kHz, probe tip diameter = 6 mm and power = 250 W), before assimilation. For comparison, similar PDMS-based arc-shaped TENG has been assembled without employing the PVDF-HFP flakes. 2.3. Fabrication of the SHASC and SCSHASC Device (StepIII). A quasi-solid-state flexible and self-healable ASC (SHASC) was fabricated by employing magnetic CoFe2O4@SS (2 × 2 cm2) as positive and magnetic Fe-RGO@SS (2 × 2 cm2) as negative electrodes separated by a KOH-soaked Fe3+-PAA-based self-healing hydrogel electrolyte membrane. The comprehensive preparation of CoFe2O4@SS, Fe-RGO@SS, and Fe3+-PAA/KOH hydrogel electrolyte separator is illustrated in sections B1 and B2, consequently in the Supporting Information. The optimized mass ratio (m+/m−) for the positive and negative constituent electrodes was ∼0.6. Finally, after assembling, the entire system was encapsulated by self-healing CPU for protecting the device from abrupt mechanical impacts/shocks and offers good aesthetic appeal. The total thickens of the SHASC was ∼0.5 cm.

The SCSHASC device/power cell was fabricated by assembling a single SHASC unit sandwiched between two parallelly connected HPTENGs. During assembling, the top and bottom surfaces of the asfabricated SHASC unit were adhered to two HPTENGs, where the PP sheet protected flat SS electrode sides of the two HPTENGs adhered to both the surfaces of the SHASC unit. It is noteworthy that the presence of two HPTENGs ultimately improves the triboelectric charging efficiency, whereas the presence of both CPU and PP layers excludes the possibility of signal interference amongst the SHASC and HPTENGs.20 The complete circuit was made by connecting the two discrete units through a full-wave bridge rectifier. Finally, the entire device was wrapped by a thin layer of PP tape (0.05 mm thick) without disturbing the actual dimensions of the device. The tape shields the device from malfunctions during abrupt mechanical impacts and abrasions and provides good aesthetic appeal. The dimension of the whole power cell measured under fully compressed state was ∼6 cm × 4 cm × 0.7 cm. Additionally, another power cell (SCSHASC#) was assembled by efficaciously integrating two SHASC units (connected in series) sandwiched between two parallelly connected HPTENGs to power-up portable electronics. The stepwise preparation/fabrication of the HPTENG unit, SHASC unit, and SCSHASC device is schematically illustrated in Figure 1.

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analyses were employed to determine the C


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chemical composition of as-deposited Fe-RGO have further been explored by employing XPS. The survey scan profile of Fe-RGO (Figure 2c) reveals the co-existence of C 1s (∼290.9 eV), Fe 2p (∼710.4 and 724.8 eV), Fe 3p (∼55.1 eV), and O 1s (537.6 eV). The narrow scan profiles of Fe-RGO (as revealed in Figure S1a−c, Supporting Information) affirm the existence of both Fe2+/Fe3+ and O2− ions. Thus, the existence of mixed iron oxides (solely Fe3O4 with trace of Fe2O3) in FeRGO is evident. Additionally, the weigh amount of RGO and iron oxides in Fe-RGO has been estimated by employing thermogravimetric analysis (as displayed in Figure S1d, Supporting Information). It has been perceived that weigh amount of RGO and iron oxides in Fe-RGO (initial wt = 9.758 mg) was 4.6834 mg (48%) and 5.0741 mg (52%), respectively. Comprehensive surface morphologies/microstructures of the SHASC electrodes and the HPTENG components were investigated by field emission scanning electron microscopy (FE-SEM). Figure 2d−i represents the FE-SEM images of the SHASC component electrodes. As displayed in Figure 2d, wrinkled nanoflakes of CoFe2 alloy electrodeposited on SS (before annealing). The edge thickness of the nanoflakes is ∼20−30 nm. The FE-SEM images of CoFe2O4@SS after annealing are displayed in Figure 2e−g at different magnifications. It can be perceived from Figure 2e (a lowmagnification FE-SEM image) that uniform layers of CoFe2O4 were grown on the SS fabric substrate after annealing. The overall diameter of a single SS fabric yarn becomes ∼80 μm after complete wrapping by annealed CoFe2O4. Figure 2f,g represents the FE-SEM images of CoFe2O4 (after annealing) at different magnifications. It has been observed that several interconnected microflowers of CoFe2O4 grown on SS and eventually the microflowers are congregated together to form an array. The wrinkled nanoflakes of CoFe2 alloy grown on SS fabric (after electrodeposition) were converted to microflower arrays after annealing. The high-magnification FE-SEM image (Figure 2g) reveals that the microflowers of CoFe2O4 consist of several ultrathin nanopetals with a thickness of ∼12−14 nm and are assembled to form large number of cavities with different geometries. This unique interconnected architecture eventually provides a large surface area for sufficient electrode−electrolyte interactions and boosts the ion diffusion kinetics.64 The FE-SEM images of Fe-RGO deposited on a SS fabric are portrayed in Figure 2h,i where typical crumpled flake-like architecture of RGO decorated with iron oxides is found to be deposited on the SS surface. The thickness of a single RGO nanoflake is ∼16−20 nm, and they are often stacked together to construct an assembly with an average thickness of ∼130 nm. This stacked architecture of RGO affords enriched electrode−electrolyte interactions (i.e., adequate contact in-between RGO and the electrolytic ions or adsorption of K+ ions on the surface of RGO), whereas the presence of iron oxides (decorated on the surface of RGO) concurrently provides excellent magnetic property and pseudocapacitive behavior. Enriched electrode−electrolyte interactions also imply enhanced diffusion of the ions within the interior portions of the stacked microstructure of RGO in Fe-RGO. The porous and stacked flakes of RGO act like electrolytic ion reservoir. During electrolysis, the electrolyte penetrates in the void and contours of the microstructure. Existing iron oxides on the RGO surface eventually restricts the coalescence of RGO stacked flakes during electrolysis. Figure 2j,k displays the FE-SEM images of PVDF-HFP flake (HPTENG component) surface acquired at different magni-

crystallographic/structural features of the electroactive materials. The XRD patterns of CoFe2O4@SS, Fe-RGO@SS, and blank SS fabric substrate are shown in Figure 2a. As evident,

Figure 2. (a) XRD patterns of as-prepared CoFe2O4@SS, Fe-RGO@ SS, and SS fabric electrodes. (b) Relevant JCPDS patterns. (c) XPS survey scan spectrum of Fe-RGO. (d−l) FE-SEM images: (d) CoFe2 alloy@SS before annealing, (e−g) CoFe2O4@SS after annealing at different magnifications, (h, i) Fe-RGO@SS at different magnifications, (j, k) PVDF-HFP surface at different magnifications, (l) typical micropatterns replicated on the friction surface of the PDMS-based triboelectric film-strip.

both CoFe2O4@SS (red profile) and Fe-RGO@SS (blue profile) exhibit three intense diffraction peaks at 2θ ≈ 43.5°, 51.0°, and 74.5° owing to the presence of (111), (200), and (220) crystal planes of SS (as supported by the black profile), respectively.60,61 In addition, the XRD pattern of the CoFe2O4@SS electrode exhibits several moderate to high intensity peaks centered at 2θ ≈ 18.2°, 30.0°, 35.5°, 37.1°, 43.5°, 53.8°, 57.1°, 62.6°, 65.6°, 69.5°, 74.5°, 75.5°, and 78.7° that correspond to (111), (220), (311), (222), (400), (422), (511), (440), (531), (620), (533), (622), and (444) crystal planes of spinel cobalt ferrite, respectively.62,63 The acquired pattern is well matched with JCPDS card no. 003-0864. The high intense peaks suggest crystalline nature of the as-prepared CoFe2O4 grown on the SS fabric substrate. In contrast, the diffraction pattern of Fe-RGO@SS reveals several peaks centered at 2θ ≈ 18.5°, 30.1°, 30.2°, 35.2°, 35.5°, 43.7°, 56.9°, 57.4°, 63.1°, and 74.5° which are indexed to (011), (112), (200), (121), (103), (004), (231), (321), (224), and (143) planes of Fe3O4, respectively [JCPDS card no. 0751609]. Additionally, it also displays certain characteristic signatures of Fe2O3 at 2θ ≈ 15.0°, 18.5°, and 23.9° indexed to (103), (113), and (203) crystal planes [JCPDS card no. 025-1402], respectively. The intensity of the characteristic peaks for RGO (002) and (100) crystal planes at 2θ ≈ 25.6° and 41.4° is not prominent owing to the existence of high intensity peaks of Fe3O4 which tends to overwhelm the peaks of RGO. The corresponding JCPDS patterns relevant to the electroactive materials are sequentially displayed in Figure 2b. Moreover, the existence of different oxidation states and the D


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Figure 3. CV profiles of the SHASC: (a) under three different potential windows at 2 mV/s, (b) at several scan rates, (c) at initial condition, under bending mode, twisted mode, heel pressing, and finger imparting conditions at 2 mV/s (inset depicts heel pressing and twisting on the SHASC), (d) initially, under hand/elbow bending, shoe sole pressing conditions, and after 8000 GCD cycles at 2 mV/s (inset represents hand/elbow bending and shoe sole pressing), (e) at different bending angles (0°, 30°, 45°, 90°, and 135°) at 2 mV/s (inset represents the photo of bent SHASC). (f) GCD profiles at different current densities. (g) Variation of Csp with current densities. (Inset denotes the GCD curves after 1000, 5000, and 8000 consecutive GCD cycles.) (h) Variation of Csp with current densities at different conditions, for example, finger imparting, twisting, heel pressing, and bending at different angles. (Inset shows finger imparting and twisting of SHASC.) (i) % Csp retention vs bend times at different bending angles. (Inset depicts the GCD curves of SHASC at several bending angles.)

fications. As can be seen, the surface of PVDF-HFP flakes is fairly rough/uneven and consists of several entangled nanofibers with a diam ≈ 150 nm. This distinctive surface architecture usually provides exposed surface area and improved pressure sensitivity which eventually helps to generate more charges on the surface of the negative triboelectric layer. Figure 2l depicts the FE-SEM image of the micropatterns replicated on the friction surface of PDMSbased triboelectric film-strip (HPTENG component). Introduction of such micropatterns significantly enhances the effective contact in between the two friction layers and hence the triboelectric SCD. The co-existence of Co, Fe, and O in CoFe2O4 and C, Fe, and O in Fe-RGO has further been confirmed from the acquired selected-area elemental mappings and energydispersive X-ray line spectrum of the electroactive materials (Figure S2). The microstructures of the component electrodes acquired after 8000 repetitive galvanostatic charge−discharge (GCD) cycles and after periodic deformations (Figure S3) reveal outstanding electrochemical and structural stability/ durability under working conditions. Additionally, the Fouriertransformed infrared (FTIR) and Raman spectroscopic analysis of the electroactive materials have been illustrated in section H, Supporting Information. Complete electrochemical analysis such as cyclic voltammetry (CV), GCD, cyclic stability, and electrochemical impedance spectroscopic (EIS) analysis of CoFe2O4@SS within 0−0.6 V (as illustrated in section I, Supporting Information) and Fe-RGO@SS within −1 to 0 V potential window (as illustrated in section J, Supporting Information) was performed using a threeelectrode cell arrangement with 1 M KOH electrolyte. As

can be realized, both the electrodes reveal outstanding electrochemical performances. It is noteworthy that introducing trace amount of silver nanoparticles eventually improves the conductivity and charge transport property of CoFe2O4@ SS (as exposed in Figure S5), whereas the existence of iron oxides in Fe-RGO enhances the rate capability (Figure S6). Besides, the operating voltage window of the SHASC has been estimated from the CV (at 2 mV/s) and GCD (at 1 A/g) profiles of the positive (CoFe2O4@SS) and negative (FeRGO@SS) electrodes (Figure S7). As observed, the SHASC can be operated within 0−1.6 V wide potential window. The comprehensive electrochemical performance of the flexible quasi-solid-state SHASC has been demonstrated through CV and GCD analyses at several conditions. Prior to the electrochemical characterizations, the SHASC was subjected to GCD (at 3 A/g) for ∼100 cycles till a stable specific capacitance (Csp) is achieved. It is to be stated that the charge balance amongst the two component electrodes obeys the relation: q+ = q−, where q+ and q− denote the amount of charge stored in the positive and negative electrodes, respectively. Likewise, the quantity of charge (q) stored and the optimum mass ratio (m+/m−) of the two respective electroactive materials have been estimated using eqs E1 and E2, in Supporting Information, and the optimized mass ratio was ∼0.6. (The mass of CoFe2O4 and Fe-RGO on SS was 0.0157 and 0.0262 g, respectively.) The CV profiles of the SHASC (Figure 3a) under several working potential window at a scan rate of 2 mV/s clearly indicate that the device can be utilized within a wide voltage range (0−1.6 V) without any deterioration of its charge storage and capacitive behavior.65 The CV curves of the SHASC with variable scan rates of 150, E


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Figure 4. (a) Variation of CA with current densities of SHASC. (Inset shows the light-weight of the device.) (b) Nyquist plot of the SHASC: initially, after 1000 and 8000 GCD cycles. (Inset represents the fitted circuit diagram.) (c) Csp retention up to 8000 consecutive GCD cycles at 1 A/g. (Inset depicts the sequential GCD from 1st to 10th cycle at 1 A/g.) (d) Ragone plot of the SHASC. (Inset shows the variation of energy and power densities with current density.) (e) CV curves of the SHASC (at 2 mV/s) after 2, 5, 10, 15, 30, and 45 consecutive days. (f) CV (at 2 mV/s) and (g) GCD (at 1 A/g) of two serially connected SHASC. Magnetic hysteresis curves of (h) CoFe2O4 and (i) Fe-RGO at RT.

discharge) eventually illustrates excellent capacitive behavior of the device. Moreover, all of the GCD curves exhibit nearly equivalent voltage plateaus ensuring high reversibility of the ongoing redox process.66 A maximum Csp value of ∼155.8 F/g, specific capacity (Cs) of ∼69.25 mAh/g at 1 A/g current density, and an areal capacitance (CA) of ∼810 mF/cm2 at 5 mA/cm2 were obtained using the eqs E3, E3a, and E4, respectively, Supporting Information. As can be seen from Figure 3g, the calculated Csp still remains up to ∼122.2 F/g (Cs ≈ 54.3 mAh/g) even at a high current density of 20 A/g, revealing superior rate capability of the device. The GCD curves acquired after 1000, 5000, and 8000 GCD cycles at 1 A/g are shown in the inset of Figure 3g. The nature/shape of the curves remains analogous even after 8000 cycles, endorsing superior stability of the SHASC. Figure 3h shows excellent adherence of the as-deposited electroactive materials with the SS current collector. As can be seen, the device retains its outstanding electrochemical performance and rate capability under finger imparting, heel pressing, twisted state, and bending at several angles (15°, 30°, 45°, and 90°). At 20 A/ g, the SHASC retains ∼80.3, 82.3, 76.3, and 79.5% of its initial Csp value upon finger imparting, heel pressing, twisted state, and bending at 90°, respectively. Figure 3i illustrates the Csp retention (%) of the SHASC cell under repetitive bending at several angles. At 1 A/g, the device retains ∼99.7, 99.3, 98.9, 97.9, and 96.3% of its initial Csp even after 100 times repetitive bending at 15°, 30°, 45°, 90°, and 135°, respectively, demonstrating outstanding flexibility of the SHASC.5 The GCD curves obtained at 1 A/g during bending at several angles are displayed in the inset of Figure 3i. The nature of the curve remains analogous. This bendability of the SHASC is essential because during mechanical deformation of the SCSHASC power cell, the internal SHASC unit may also

120, 100, 80, 50, 10, 5, and 2 mV/s within 0−1.6 V are shown in Figure 3b. Typical asymmetric nature of the CV curves at all of the scan rates eventually illustrates admirable capacitive behavior of the SHASC. Besides, the nature of the CV curves corroborates a combined electrochemical characteristic (combination of electric double layer and Faradaic behaviors) owing to the existence of two dissimilar electrodes. The nature of the curves remains analogous even at high scan rates, signifying superior rate capability of the device. The comparative CV curves of the SHASC studied under several applied pressure (e.g., heel pressing and finger imparting mode at 2 mV/s) are represented in Figure 3c. The flexibility of the SHASC has also been assessed from the CV profiles under twisting and bending modes. As evident, the nature of the CV curves under several applied conditions remains virtually homologous. The area of the CV profiles remains nearly identical under bending and twisting modes while upon finger imparting and heel pressing, the area increases owing to enhanced electrolyte ion transport through the electrolyte membrane. Figure 3d represents the CV curves of the SHASC upon hand/elbow bending, shoe sole pressing, and after 8000 successive GCD cycles at 2 mV/s. The area under the CV curves increases apparently upon elbow bending and shoe sole pressing, whereas the area of the CV profile after 8000 GCD cycles remains virtually analogous (compared to the initial state) signifying superior stability of the power cell. Figure 3e describes the CV profiles of the SHASC during bending at different angles (0°, 30°, 45°, 90°, and 135°). Likewise, the area under the CV curves marginally expands with increasing bending angles, suggesting admirable flexibility of the SHASC. Figure 3f displays the GCD profiles of the SHASC under different current densities within 0−1.6 V. Typical asymmetric nature accompanied by a distinct plateau region (during F


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Figure 5. (a) Photograph of CoFe2O4@SS (positive) and Fe-RGO@SS (negative) electrodes, initially. (b) Electrodes after cutting/ripping. (c) Self-joining of the two respective electrodes through magnetic attraction (pull-up by own magnetic forces). Demonstration of the self-healing characteristics of the KOH-soaked Fe3+-PAA hydrogel membrane: (d) initially, (e) just after cutting, (f, g) after 1st and 6th healing. FE-SEM images of the surface of Fe3+-PAA: (h) initially, (i) just after cutting, (j) after partial healing, and (k) after complete healing. (l, m) CV curves (at 2 mV/s) and GCD profiles (at 1 A/g) of the SHASC: before healing, after 1st, 2nd, 4th, and 6th healing cycles. (n) Variation of Csp with healing cycles (inset displays the picture of SHASC after creation of wound and after frequent healing). (o) Nyquist plot of the SHASC: before healing, after 2nd, 4th, and 6th consecutive healing cycles. Stress vs strain curves: (p) KOH-soaked Fe3+-PAA membrane and (q) CPU encapsulate after different healing cycles. Photographs of the output voltage of the SHASC: (r) initial state, (t) after ripping, and (u) after healing. Photographs of a glowing red LED by the SHASC: (s) before cutting and (v) after healing.

∼10.3 Ω (after 1000 cycles) and 12.0 Ω (after 8000 cycles). Moreover, the initial impedance profile shows a more vertical line (higher slope) at low frequency in comparison to the profiles after 1000 and 8000 cycles, respectively. The lower Rct and ion diffusion resistance of the device suggest better reactivity, faster electrolyte ion transport, and decent exposure of the component electrodes toward electrolyte at initial state. After 1000 and 8000 cycles, the Rct and ion diffusion resistance slightly increases owing to increased inherent resistance of the component electrodes.5 Likewise, the device exhibits a solution resistance (intersection of the impedance profile with real impedance axis) of ∼0.90 Ω (initially), 0.96 Ω (after 1000 cycles), and 1.05 Ω (after 8000 cycles). The solution resistance marginally increases owing to sluggish diffusion of electrolytic ions after repetitive cycling. The equivalent circuit diagram fitted to the initial Nyquist plot is shown in the inset. The GCD cyclic stability measurement of the device was performed at 1 A/g for 8000 cycles within 0−1.6 V (Figure 4c). The interconnected microflowers of CoFe2O4 and stacked flakeslike Fe-RGO eventually obstruct the volume expansion during

suffer bending. All of these acquired outcomes suggest the capability of SHASC to uphold its own electrochemical performance against extreme bending applied during practical utility. The variation of CA with the alteration of current density for the SHASC (Figure 4a) also reveals excellent rate capability with ∼79% of its initial CA retention even at 100 mA/cm2 current density. The lightweight of the device has also been displayed in the inset. Figure 4b illustrates the Nyquist plots or complex plane impedance profiles: initially, after 1000 and after 8000 GCD cycles. The nature of the profiles is analogous to each other. Each profile comprises a typical semicircular arc at high frequency followed by a straight line at the low-frequency region. The diameter of the semicircular arc signifies the charge-transfer resistance (Rct), whereas the slope of the straight line at low frequency signifies resistance toward diffusion of electrolyte ions. A smaller semicircle/arc diameter indicates low Rct, whereas a straight line with higher slope suggests lower ion diffusion resistance. At initial state, the SHASC exhibits an Rct value of ∼9.5 Ω which is smaller than G


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damage.55 Figure 5d−g demonstrates the self-healing features of the KOH-soaked hydrogel membrane/separator. It has been perceived that the membrane still retains its inherent selfhealing characteristics even after sixth healing cycles (Figure 5g). The self-healing behavior of Fe 3+ -PAA hydrogel membrane can be attributed owing to the existence of strong interchain bonding amongst macromolecular PAA chains in the presence of Fe3+ ions. Numerous immobilized Fe3+ ions afford ionic cross-linking in between the PAA chains and improve the mechanical strength, whereas the free Fe3+ ions have a tendency to diffuse toward the ripped surface and strongly interact with the abundant carboxylate (COO−) moieties present in the PAA backbones of nearest surfaces, after sudden damage. Likewise, strong inter- and intramolecular H-bonding in between the PAA backbones improves the mechanical and healing properties of the membrane.3 Figure 5h−k demonstrates the typical surface morphological features of the Fe3+-PAA hydrogel separator at different stages of self-healing process. As demonstrated in Figure 5h, the surface of the Fe3+-PAA hydrogel membrane is moderately ruff. Figure 5i portrays the FESEM image of the membrane where a notch has been generated intentionally across the thickness. It has been recognized that the notch has been healed partially after 8 min under mild hand pressure at room temperature (Figure 5j). The time required for complete healing is ∼13−15 min, and evidently, the surface gains its initial morphology (as exposed in Figure 5k). To validate the synergistic effect of the self-healing components and magnetic electrodes on the electrochemical performance of the device, CV analysis was performed after different healing cycles at 2 mV/s fixed scan rate within a potential window of 0−1.6 V (Figure 5l). The shape of the CV curves remains virtually unaltered even after sixth healing cycles with a gentle diminution in the curve area, suggesting superior electrochemical performance retention of the SHASC after several healing cycles. The redox peak positions are also unaffected after several healing. Figure 5m illustrates the GCD profiles of the device acquired after different healing cycles at 1 A/g within 0−1.6 V. As can be realized, the nature of the GCDs remains almost unaltered even after sixth subsequent healing cycles. Fascinatingly, the device retains ∼97.1, 93.5, 91.8, and 91.1% of its original Csp after 1st, 3rd, 5th, and 6th consecutive healing cycles, respectively. Figure 5n demonstrates the variation of Csp with the number of self-healing cycles. The SHASC still exhibits a Csp value of ∼142 F/g (Cs ≈ 63.1 mAh/g) after sixth healing cycles. As a result of reconnection of the broken or ripped electrodes through magnetic force of attraction and self-healing features of the separator/encapsulate, the device retains its intrinsic electrochemical performance after several healing cycles (as revealed in the inset). Besides, the EIS analysis after several self-healing cycles (Figure 5o) was performed within 1 MHz to 100 mHz. All of the impedance spectra exhibit similar nature. The device displays a solution resistance of ∼14.1 Ω and Rct of ∼21.5 Ω after sixth healing cycles. Moderate increments in the Rct and diffusion resistance with the healing cycles are probably owing to the lack of proper reconnection between the ripped electrodes. Additionally, the mechanical properties of the KOH-soaked Fe3+-PAA hydrogel membrane before damage/wound and after 1st, 2nd, 4th, and 6th selfhealing cycles have been demonstrated systematically in Figure 5p. As can be realized, the mechanical properties of the membrane have been restored after sixth healing cycles. It exhibits an ultimate tensile strength (UTS) value of ∼268.2

vigorous electrochemical cycling (as supported in Figure S3). Thus, the device exhibits long-term stability against repetitive electrochemical cycling with ∼93% of initial Csp retention after 8000 GCD cycles. A moderate increment of Csp value (∼5.9%) up to ∼300 GCD cycles can be attributed to gentle release of the electrolyte from the hydrogel membrane and timedependent diffusion of the electrolytic ions through the inner pores of the electroactive materials. This phenomenon suggests complete utilization of the electrode surface by the electrolyte after ∼300 cycles. The incessant GCD profile up to 10 successive cycles (at 1 A/g) is shown in the inset. Figure 4d represents the Ragone plot of the SHASC (variation of energy density with power density). Assessment of the energy and power densities is essential for practical utility. Our SHASC provides a maximum gravimetric energy density of ∼55.4 Wh/ kg at a power density of ∼800 W/kg at 1 A/g (calculated by using eqs E5 and E6, Supporting Information). The energy density (estimated from the GCDs) still remains at ∼46.5 W h/kg even at an immense power density of 8000 W/kg (measured at 10 A/g) which manifests the feasibility of our power cell for realistic applications. A 3D Ragone plot demonstrating the simultaneous variation of energy and power densities with current density is depicted in the inset. A concise comparison of the achieved energy and power densities with analogous systems already published in the literature is tabulated in Table S1, Supporting Information. Additionally, the sustainability/lifespan of the SHASC has been demonstrated in Figure 4e. As can be seen, analogous nature of the CV plots (acquired at 2 mV/s) even after 45 days of device assembling with marginal decrease in the area signifying superior stability of the SHASC and the hydrogel electrolyte separator. Moreover, the voltage window can be extended by serially connecting two or more SHASCs. The obtained CV plots at 2 mV/s (Figure 4f) and the GCDs at 1 A/g (Figure 4g) of two serially connected SHASCs reveal an extended working voltage window of 0−3.2 V without any deterioration of their inherent nature. All of these results evidently illustrate the realistic utility of our SHASC in portable and flexible electronics. Besides, the magnetic response (hysteresis loop) of the component electroactive materials has been estimated to corroborate the self-joining phenomenon of the electrodes after cutting/ripping. Figure 4h shows the magnetization hysterics loop of CoFe2O4 at room temperature. The loop specifies the existence of ferromagnetic behavior.67 As evident, CoFe2O4 shows a saturation magnetization (Ms) value of ∼97 emu/g at 15 kOe magnetic field accompanied by a residual magnetization (Mr) of ∼24 emu/g and a coercivity (Hc) of ∼445 Oe. The room-temperature hysteresis loop of Fe-RGO (Figure 4i) also exhibits ferromagnetism with Ms, Mr, and Hc values of ∼56.3 emu/g (at 15 kOe), ∼2.18 emu/g, and ∼21.0 Oe, respectively.68 Figure 5a−c sequentially demonstrates the self-joining features of CoFe2O4@SS (+ve) and Fe-RGO@SS (−ve) electrodes after sudden cutting/ripping owing to the existence of permanent magnetic behavior. It is noteworthy that the magnetic response of bare SS fabric is negligible. The electroactive materials with permanent magnetic response (as explored previously) anchored on SS ensure adequate magnetic force that assists self-joining/reconnection of the broken electrodes. As displayed in Figure 5c, one part of the ripped electrode could easily lift-up the other one after damage and ensures reconnection. This classic phenomenon helps to restore the electrical conductivity of the electrodes after H


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Figure 6. (a) Output voltage (VO) generated by a single HPTENG upon frequent finger imparting. (Inset represents the magnified signal.) (b) Short-circuit current (ISC) generated by a single HPTENG upon finger imparting. (Inset shows the photograph of finger imparting on the top of HPTENG.) (c) VO and (d) ISC generated by single HPTENG upon frequent heel pressing. (Inset shows the photograph of heel pressing.) (e) Output voltage waveforms of a single HPTENG at varying frequencies. (f) Output performances of the HPTENG under various external load resistances. (Inset portrays the schematic circuit diagram.) (g) VO generated by the HPTENG upon sewing machine imparting. (Inset shows the photograph of sewing machine imparting.) (h) Durability and stability of the HPTENG for several weeks. (i) Charge−discharge profile of a 22 μF commercial capacitor charged by a single HPTENG upon continuous finger imparting. (Inset shows the schematic circuit diagram of capacitor charging.)

KPa, elongation at break (EB) of 448.5%, and Young modulus (YM) of ∼0.53 KPa, initially. Surprisingly, the UTS, EB, and YM are well maintained at ∼59.1, ∼86.7, and ∼60.4%, respectively, even after sixth healing cycles, suggesting the inherent self-healing features of the KOH-soaked hydrogel membrane. Additionally, the stress versus strain behavior of KOH-soaked Fe3+-PAA after 10 and 30 days (Figure S8a) reveals excellent mechanical durability. The swelling ratio (%) of the Fe3+-PAA membrane in aqueous 1 M KOH (calculated by employing eq E7, in the Supporting Information) at different immersing time intervals (Figure S8b) reveals that the membrane can reach its maximum swelling ratio after ∼18 min of immersion in the electrolyte medium. The mechanical properties of the CPU encapsulate before damage and after self-healing have also been demonstrated in Figure 5q. As can be seen, CPU restores its mechanical properties even after 6th healing cycles. It exhibits an UTS value of ∼0.35 MPa, EB of ∼753%, and toughness of ∼211.90 KPa, initially. Remarkably, the UTS, EB, and toughness are well sustained at ∼82.8, 95.1, and ∼77.5%, respectively, after 3rd healing. Likewise, ∼85.6, 93.8, and 69.5% retention of the initial UTS, EB, and toughness, accordingly, after 6th healing cycles ultimately validate the inherent self-healing features of the CPU encapsulate. This inherent self-healing behavior can be due to the formation of strong and reversible intermolecular Hbonds once the ripped interfaces are brought in contact.55 Figure 5r−v demonstrates the self-healing features of SHASC. Figure 5r displays that a single SHASC can attain ∼1.6 V before damage. The instantaneous glow of a single red lightemitting diode (LED) is shown in Figure 5s. Figure 5t depicts that the SHASC has been ripped with no output voltage. After proper healing, the device can further achieve ∼1.6 V (Figure

5u) and power-up a single red LED (Figure 5v). All of these outcomes eventually authorize the mechanical and electrical healing behavior of the supercapacitor unit. The capability of restoring its original electrical, electrochemical, and mechanical properties even after severe damage is essential for selfcharging device fabrication. The output performances of the as-fabricated arc-shaped HPTENG unit under different periodic compressive forces have been investigated methodically. As described earlier, our HPTENG comprises two triboelectric friction layers: polished SS fabric (+ve) and PVDF-HFP/SS-impregnated micropatterned PDMS film strip (−ve). An average output voltage (VO) of ∼274.3 V (Figure 6a) and a short-circuit current (ISC) of ∼29.0 μA (Figure 6b) were generated by a single HPTENG under periodic finger imparting/tapping state at F ≈ 17.6 N, contact pressure (σ) ≈ 7.34 KPa and f ≈ 3.80 Hz. (F and σ values are calculated from eqs E8−E10, accordingly, Supporting Information). When a compressive force/pressure is applied on the arc-shaped HPTENG, the two-component triboelectric friction layers come in contact followed by electron transfer from the less electronegative SS surface to the high electronegative PDMS composite film-strip surface (according to the triboelectric series). Upon removal of the force, a positive voltage and current response can be obtained, whereas on further pressing, it generates a negative voltage and current response. Thus, the HPTENG instantly generates alternating currents (AC) upon periodic compression. The inset of Figure 6b represents the real image of finger imparting on the arc-shaped HPTENG with an active area of 6 × 4 cm2. Furthermore, a single HPTENG can instantly produce an average VO of ∼42.5 V (Figure 6c) and ISC of ∼4.4 μA (Figure 6d) under repeated heel pressing at F ≈ 13.4 N, f ≈ 2.40 Hz. I


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ACS Applied Materials & Interfaces The inset of Figure 6d signifies heel pressing on HPTENG. The variation of VO with increasing imparting frequency for the HPTENG has been demonstrated in Figure 6e. Evidently, it exhibits admirable output performance within a wide range of applied frequency (∼2−10.2 Hz) under constant F ≈ 17.6 N. The applied frequency range coincides with the frequency of HPTENG excitation. An instant VO of ∼191.0, 274.3, 355.0, 368.0, and 378.0 V was produced by HPTENG at f ≈ 2.0, 3.8, 5.4, 8.1, and 10.2 Hz, accordingly. Under elevated magnitude of applied frequency, the VO increases (first abruptly and then moderately) owing to a prompt attainment of equilibrium state by the electrons flowing through the external circuit.20 The variation of VO and power densities of HPTENG with the load resistance are shown in Figure 6f. As can be seen, the VO value increases with increasing the electrical load resistance and ultimately reaches a saturation point (VO ≈ 274.3 V) when the load resistance is ∼10 MΩ (green curve). Consequently, the instant output power density value (calculated by using eq E11, Supporting Information) reaches a maximum of ∼313.5 μW/cm2 when the load resistance increases to ∼10 MΩ (red curve). Thus, the HPTENG is efficacious when the electrical load resistance is noticeably higher. The inset depicts the schematic circuit diagram of HPTENG connected with external load resistance. Besides, an instantaneous VO of ∼15.4 V was produced by the HPTENG upon repetitive sewing machine imparting at f ≈ 4.30 Hz (Figure 6g). To investigate the long-term utility of our HPTENG, durability experiment was executed under repetitive finger tapping (F ≈ 17.6 N, f ≈ 3.80 Hz) and the results are depicted in Figure 6h. It has been perceived that there is no significant loss of VO value (∼274.3 V) even after 6th week (∼20 min/week) signifying extreme durability, steady capacitor charging capability, and practical utility of the HPTENG. To compare the outcomes of the HPTENG with analogous PDMS-based arc-shaped TENG without PVDF-HFP (comprises a polished SS fabric as +ve and SS impregnated PDMS as −ve friction layer), the output performance of a single TENG was studied under similar finger tapping condition. An average VO of ∼152.3 V (Figure S9a), ISC of ∼8.9 μA (Figure S9b), and SCD (QD) value of ∼5 nC/cm2 (Figure S9c) were generated by a single TENG (6 × 4 cm2) under periodic finger imparting at F ≈ 17.6 N, f ≈ 3.80 Hz (QD value was calculated by using eq E12, Supporting Information). Similarly, the TENG can produce an average VO of ∼20.3 V (Figure S9d) and ISC of ∼3.5 μA (Figure S9e) upon repetitive heel pressing condition at F ≈ 13.4 N, f ≈ 2.40 Hz. The capability of our HPTENG to charge a 22 μF commercial capacitor after rectification through a full-wave bridge rectifier is revealed in Figure 6i. It is apparent that a single HPTENG can charge the capacitor gently up to ∼5 V upon continuous ∼3.5 min of finger tapping (F ≈ 17.6 N, f ≈ 3.80 Hz). The schematic circuit diagram for capacitor charging through a rectifier unit is displayed in the inset. The bridge rectifier converts the harvested AC to direct current (DC) before charging the capacitor. Additionally, a stacked nanogenerator was designed by parallelly connecting two HPTENG units to enhance the output performance. In comparison to a single HPTENG, the stacked device exhibits a higher VO of ∼419.6 V (Figure 7a) and ISC of ∼51.2 μA (Figure 7b) upon finger imparting at F ≈ 17.6 N, f ≈ 3.80 Hz. The device also exhibits nearly two times higher QD (32 nC/cm2) than that of a single HPTENG (QD ≈ 17.9 nC/cm2) during finger tapping (Figure S9f). Besides, the influence of PVDF-HFP to enhance the SCD can be estimated

Figure 7. (a) VO and (b) ISC generated by the stacked nanogenerator (with two parallelly connected HPTENGs) upon finger imparting. (c) Output voltage waveforms of the stacked nanogenerator at varying frequencies. (d) Charge−discharge profile of a 22 μF commercial capacitor charged by the stacked nanogenerator upon continuous finger imparting. (Inset shows the schematic circuit diagram of capacitor charging.)

by merely comparing Figure S9c,f. A threefold increase in the QD of HPTENG (in comparison to TENG) can be perceived. The VO of the stacked nanogenerator within a frequency range of 2−10.2 Hz under constant F ≈ 17.6 N is demonstrated in Figure 7c. Instantaneous VO of ∼371.0, 436.5, 476.0, 490.0, and 495.0 V were generated by the device at f ≈ 2.0, 3.8, 5.4, 8.1, and 10.2 Hz, respectively. As evident, the VO produced by the device reaches saturation at higher frequency. The switching polarity experiment of a single HPTENG was performed (Figure S10a) to authorize that the VO signals generated solely from HPTENG and not from the measurement unit. The VO signals generated after rectification through a full-wave bridge rectifier are illustrated in Figure S10b. The HPTENG produces an output DC voltage of 269 V upon rectification. The variation of VO with increasing applied force for a single HPTENG at a fixed frequency is presented in Figure S10c. Average VO values of ∼185.0, 224.0, 245.0, 267.0, and 274.3 V were produced at F ≈ 9.8, 12.5, 14.6, 16.4, and 17.6 N, respectively. This phenomenon can be attributed to enhanced contact between two opposite surfaces of the triboelectric friction layers ensuing higher VO upon increasing the magnitude of applied force. Introducing micropatterns on the active surface of the composite film-strip also expands the contact area causing more electrons to flow through the external circuit.69 The capability of our stacked nanogenerator device to charge a 22 μF commercial capacitor after rectification through a full-wave bridge rectifier is demonstrated in Figure 7d. It is apparent that the stacked device (with two HPTENGs) can efficiently charge the capacitor up to ∼10 V within ∼4 min of finger tapping. Because the quantity of transferred charges is considerably higher in the stacked device (than single HPTENG), it exhibits better charging efficiency. The schematic circuit diagram for capacitor charging through a rectifier unit is displayed in the inset. Thus, the stacked nanogenerator (with two HPTENG units) can efficiently convert ambient mechanical/bio-mechanical energy (e.g., human finger tapping, elbow bending, heel pressing etc.) J


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Figure 8. (a) Csp retention (%) of SHASC at different bending angles. (Inset shows the photographs of SHASC bent at several angles.) (b) Schematic circuit diagram of the SCSHASC as a power source for driving portable electronics. (c, d) Self-charge−discharge profiles of the SCSHASC power cell upon continuous imparting. (Inset photograph reveals power-up of a digital calculator during discharging.) Initial charging profiles of the SCSHASC: (e) at different tapping frequencies, (f) under various applied forces, and (g) under various compressive stresses produced by human body movements and sewing machine imparting (insets represent the photos of finger imparting, shoe sole pressing, heel pressing, elbow bending, and sewing machine imparting on the device). (h) Self-charging curves of the sandwich-type devices: SCSHASC and SCSHASC# (with two HPTENGs and two SHASC units). (Inset shows a schematic circuit diagram for SCSHASC#.)

that after ∼6 min of repetitive imparting/tapping, the potential of the device starts to rise gently and ultimately reaches to ∼1.6 V after ∼31 min of continuous tapping (Figure 8d). It is noteworthy that a bit longer time is required to complete the entire charging process possibly owing to the power limit of the internal HPTENGs and the inherent resistance of the SHASC unit. Upon removal of tapping force, the device discharges steadily and finally discharged back to almost “zero potential” within a discharge time (Δt) of ∼44 min under ∼10.5 μA constant discharge current. A tiny plateau region in the discharge profile illustrates the asymmetric nature of the SHASC. Inset photograph reveals power-up of a digital calculator by the device during discharging. The initial selfcharging profiles of SCSHASC under varying tapping frequency have been depicted in Figure 8e. As can be seen, the rate of charging increases with increasing of tapping frequency. The device can be charged to ∼1.35 V within only 16 min under a tapping frequency of ∼10.2 Hz, signifying outstanding charging efficiency. The effect of varying magnitude of applied force/stress on the charging efficiency of the device under a constant f ≈ 3.80 Hz is shown in Figure 8f. Upon elevating the magnitude of applied force, the charging time decreases, that is, the device takes lesser time to attain ∼1.6 V. Besides, the initial self-charging profiles under various compressive stresses generated by casual human body movements and sewing machine imparting are collectively demonstrated in Figure 8g. Profile-I (black) depicts the initial self-charging curve upon repetitive finger tapping at F ≈ 17.6 N and f ≈ 3.80 Hz. The device attains ∼0.7 V within ∼17 min under continuous finger tapping. Profile-II (red) and profile-III (green) show the self-charging curve upon repetitive shoe sole pressing and heel pressing state, individually at F ≈ 13.4 N, f ≈ 2.40 Hz, respectively. Upon shoe sole pressing, the device

directly to electricity which can further be utilized to charge the supercapacitors with proper circuit connection. Typical working mechanism of the sandwich-type SCSHASC power cell upon vertical contact separation is depicted in Figure S11. The GCD and impedance analyses of SHASC component electrodes after long and repetitive imparting cycles, as illustrated in section P, Supporting Information, reveal impressive mechanical stability or durability of the microstructures of CoFe2O4 and Fe-RGO. The retention of Csp (%) at several bending angles (Figure 8a) of the internal SHASC unit has been investigated. As can be realized, the SHASC retains ∼99.8, 99.6, 99.3, 99.0, 98.5, 98.0, and 97.4% of its initial Csp after 15°, 30°, 45°, 60°, 90°, 120°, and 135° bending, respectively. Thus, its ability to store charge in the bent state can be recognized. The bendability of the SHASC is sequentially displayed in the inset photographs. It is noteworthy that during mechanical deformation of the SCSHASC through external force/pressure, the internal SHASC unit suffers bending as well. To confirm the stability/bendability of SHASC, we have measured the bending behavior, that is, whether it is capable to maintain its own electrochemical performance against extreme bending/tapping. Figure 8b represents the schematic circuit diagram of SCSHASC power cell employed as sustainable power source. When switch “S1” is turned on and “S2” is off, the device harvests electrical energy on application of repetitive compressive force and consequently utilizes the energy to charge the inner SHASC unit through a rectifier. When switch “S1” turned off and “S2” turned on (after sufficient charge storage), the device acts as a sustainable power source to drive portable electronics. Figure 8c,d represents the self-charging/ discharging profiles of the SCSHASC under periodic finger tapping at F ≈ 17.6 N and f ≈ 3.80 Hz. It has been perceived K


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Figure 9. (a) Schematic demonstration for the feasible application areas. (b, c) VO of the SCSHASC and SCSHASC#. (d, e) Instant light-up of five red and green LEDs by the SCSHASC# after repeated imparting. Instant power-up of several portable electronic gadgets by the power cell: (f) small electronic display, (g) wrist watch, (h) mobile LED screen, and (i) commercial digital calculator.

attains ∼0.5 V, whereas upon heel pressing, it attains ∼0.42 V within 17 min. Profile-IV (blue) shows the initial self-charging curve upon continuous sewing machine imparting at f ≈ 4.30 Hz. The device attains ∼0.27 V within ∼17 min of imparting. Profile-V (brown) depicts the initial self-charging curve upon repeated hand/elbow bending condition. The device attains ∼0.09 V within ∼17 min of repeated elbow bending. Insets represent the photographs of finger imparting, shoe sole pressing, heel pressing, elbow bending, and sewing machine imparting. To utilize this sandwich-type design especially for high power applications, another power cell (SCSHASC#) has been assembled by integrating two serially connected SHASCs sandwiched in between two parallelly connected HPTENGs. Owing to the presence of two internal SHASC units, the SCSHASC# provides higher output voltage. As displayed in Figure 8h, the SCSHASC can be charged up to ∼1.6 V, whereas the SCSHASC# can be charged up to ∼2.56 V upon repetitive compression. All of these outcomes ultimately corroborate the triboelectric driven self-charging features of the power cell. Figure 9a demonstrates a schematic for the feasible application areas of our SCSHASC. Figure 9b,c represent the photographs of obtained V O by the SCSHASC and SCSHASC# with a rectifier unit (measured by a multimeter). The output power generated by these power cells upon repetitive compression-release has been exploited to drive various portable electronic appliances and light-up several commercial LEDs. The SCSHASC# can instantaneously lightup five commercial red LEDs (Figure 9d) and green LEDs (Figure 9e) after repetitive compression. The power cell is also proficient to instantly power-up small electronic display, wrist watch, mobile LED screen, and commercial digital calculator (as demonstrated in Figure 9f−i). On the basis of these outcomes, it can be inferred that our triboelectrificationenabled robust SCSHASC power cell can exert an immense contribution in self-powered multifunctional electronics. The overall features revealed by the SCSHASC# power cell under various applied deformations are demonstrated in three video clips in Supporting Information Videos.

4. CONCLUSIONS In summary, we propose facile and cost-effective fabrication of a triboelectrification empowered self-charging power cell that instantly scavenges ambient bio-mechanical energy and thereby stores it for sustainable power generation. The sandwich-type power cell consists of two units: one SHASC and two parallelly connected arc-shaped HPTENGs. Owing to the synergistic influence of CoFe2O4@SS and Fe-RGO@SS as positive and negative constituent electrodes with a Fe3+-PAA/ KOH self-healing hydrogel electrolyte separator, the SHASC exhibits a wide voltage range (0−1.6 V), Csp of ∼155.8 F/g (Cs ≈ 69.25 mAh/g) at 1 A/g, energy density of ∼55.4 Wh/kg, good rate capability, and cyclic stability. In addition, the CPUencapsulated internal SHASC unit can rejoin/self-heal even after severe damage/breakage during long and repetitive compressions. On the other hand, both the two parallelly connected HPTENGs consist of polished SS fabric as positive and a PVDF-HFP/SS-impregnated micropatterned PDMS film-strip as negative triboelectric friction layers. Owing to the presence of PVDF-HFP inside the composite film-strip and micropatterns on the friction surface, the QD increases enormously, and hence, these robust HPTENGs reveal high output performances upon repetitive compression. Finally, our SCSHASC power cell was charged up to ∼1.6 V within ∼31 min of continuous tapping. Intriguingly, the power cell can be charged in dual ways, that is, both electrically and mechanically. The SCSHASC# can instantly power-up several portable electronic appliances and LEDs. Thus, our ecofriendly, handy, and robust SCSHASC power cell with unique design exerts an immense contribution in modern multifunctional electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b19044. Materials details and characterizations, preparation of the ASC components and Fe3+-PAA/KOH, utilized equations, XPS and TGA analyses of Fe-RGO, elemental mappings and EDS patterns of CoFe2O4 and Fe-RGO, morphologies of CoFe2O4@SS and Fe-RGO@SS after L


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8000 GCD cycles and several finger imparting/tapping cycles, FTIR and Raman spectroscopic analysis, complete electrochemical analysis of CoFe2O4@SS and Fe-RGO@SS, estimation of the working potential range, comparison of Csp, energy/power density of our SHASC with other related outcomes reported in the literatures, mechanical durability and % swelling ratio of Fe3+-PAA/ KOH separator, output performances of the TENG upon finger tapping and heel pressing, VO of single HPTENG in reverse connection and rectified condition, working mechanism of the sandwich-type SCSHASC upon vertical contact separation, GCD and EIS of SHASC component electrodes after repetitive imparting (PDF) Characteristic self-charging and self-healing features revealed by the SCSHASC# power cell (AVI) Characteristic self-charging and self-healing features revealed by the SCSHASC# power cell (AVI) Characteristic self-charging and self-healing features revealed by the SCSHASC# power cell (AVI)

CA, areal capacity F, force f , frequency SCD (QD), surface charge density VO, output voltage ISC, short-circuit current NIPS, non-solvent induced phase separation GCD, galvanostatic charge−discharge Ag NPs, silver nanoparticles Ms, saturation magnetization Mr, residual magnetization Hc, coercivity AC, alternating current DC, direct current

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-3222283982. ORCID

Bhanu Bhusan Khatua: 0000-0002-1277-0091 Author Contributions †

S.P. and S.K.K. equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very much thankful to: (1) Indian Institute of Technology Kharagpur for financial support, (2) Krishnendu Sarkar for his support in current measurements, (3) Sanjeev Kumar Sharma for his support in magnetic measurements and (4) Sayan Ganguly for his support in mechanical property measurements as well.

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ABBREVIATIONS SCSHASC, self-charging and self-healing asymmetric supercapacitor CoFe2O4@SS, cobalt ferrite grown on a stainless steel fabric Fe-RGO@SS, iron oxides decorated reduced graphene oxide grown on a stainless steel fabric Fe3+-PAA/KOH, KOH soaked Fe3+ cross-linked polyacrylic acid SS, stainless steel fabric PVDF-HFP, poly(vinylidene fluoride-co-hexafluoropropylene) PDMS, polydimethylsiloxane SHASC, self-healing asymmetric supercapacitor CPU, carboxylated polyurethane DMF, dimethylformamide PP, polypropylene ASCs, asymmetric supercapacitors HPTENGs, high-performance triboelectric nanogenerators TENGs, triboelectric nanogenerators Csp, specific capacitance Cs, specific capacity M


Research Article

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Triboelectric Nanogenerator Driven Self-Charging and Self-Healing

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