Kai Dong and Zhong Lin Wang

1BeijingInstitute of Nanoenergy andNanosystems, Chinese Academy of Sciences, Beijing 100083, China

2College of Nanoscienceand Technology, University of ChineseAcademy of Sciences, Beijing100049, China

3Schoolof MaterialScienceandEngineering, GeorgiaInstitute of Technology,Atlanta, Georgia30332, USA

Abstract: Lightweight and flexible self-charging power systems with synchronous energy harvesting and energy storage abilities are highly desired in the era of the internet of things and artificial intelligences, which can provide stable, sustainable, and autonomous power sources for ubiquitous, distributed, and low-power wearable electronics. However, there is a lack of comprehensive review and challenging discussion on the state-of-the-art of the triboelectric nanogenetor (TENG)-based self-charging power textiles, which have a great possibility to become the future energy autonomy power sources. Herein, the recent progress of the self-charging power textiles hybridizing fiber/fabric based TENGs and fiber/fabric shaped batteries/supercapacitors is comprehensively summarized from the aspect of textile structural designs. Based on the current research status, the key bottlenecks and brighter prospects of self-charging power textiles are also discussed in the end. It is hoped that the summary and prospect of the latest research of self-charging power textiles can help relevant researchers accurately grasp the research progress,focus on the key scientific and technological issues, and promote further research and practical application process.

Key words: self-charging power textiles;triboelectricnanogenerators; energy harvesting; batteries/supercapacitors;energystorage; power management system

1.Introduction

The massive and rapid development of global wearable electronics is following the trend of miniaturization, portability, and functionalization, which promotes the rapid transformation of powersupply and demand mode. On theone hand,the excessive consumptionof fossil fuelswiththeconsequent ofresourcesshortage and environmentpollution have brought great challenges to global energy supply,which makes it an unprecedented urgency to explore a new way of carbon neutrality with the possibilities of developing renewable, sustainable, and green energy[1,2]. On the other hand, the conventional power generation from power plants with thefixed, concentrated, ordered and high-qualityfeatureshas beencentralized andthentransmitted overlong distances viaelectricity networks to local areas, whichis inconsistent with the portable,distributed,disordered andlow power energy supply requirements of the next-generation wearable electronics in the new era of internet of things (IoTs) and artificial intelligences (AIs)[3,4]. Therefore, a new energy supply strategy is urgently needed for future wearable electronics.

To tackle the above issues, researchers are committed to assemblingflexible, uninterruptible,and wearable self-charging power systems by integrating various energy-harvesting and energystoragedevices throughthecircuit management system, which can make environmental energy simultaneously scavenged and stored for self-sustainable power supply[5],asillustratedin Fig.1. In otherwords,wearableelectronicswith the ability of energy-autonomyisexpected to be self-powered by conformable energy storage devices. For energy harvesting, the idea is to utilize whatever is available in the environmentat which the device is deployed,at ornear whereit will be used[6]. Withconsidering the working status of each sensor, it is feasible to use the energy harvested from the environment, such as solar energy, body motion energy,vibration energy, thermalenergy, andchemicalenergy. Over the past decades, various energy harvesting strategies have emerged, such as the photovoltaic effect for harvesting solar energy[7−9], the thermoelectric and pyroelectric effects for harvestingthermalenergy[10,11],the piezoelectricandtriboelectric effects for harvesting mechanical energy[12,13], the biocatalytic effect for harvesting human swear energy[14,15], and the water molecules ionized effectforharvesting atmospheric moistureenergy[16,17],which have beenreceivinggreat attention in the field of research into renewable and sustainable self-powered systems[18]. In particular, triboelectric nanogenerator(TENG)isanew energyharvester that can convert small scale mechanical energies including human respiration[19], ultrasound energy[20], mechanical stress[21],heart beat energy[22], and so on, into electrical energy by a combination effectof triboelectrification and electrostaticinduction[23,24]. The periodic contact-separationand/or sliding movement between two tribo-materials with different abilities of gaining or losing electrical charges contributes to electricity generation[25]. With theadvantages of low cost,easy process, environmentally friendly, and diverse choice of materials, TENG has broad range of applications ranging from wearable power sources[26], physiological monitoring[27], human-machine interfaces[28], to artificial intelligences[2]. However, because of thenaturally uncontrollable and unstablefeatures of environmental mechanicalenergy, the converted electrical energy from energy harvesters is unstable and difficult to use as a direct power source for electronicdevices. Themost conventional technology is using electrochemical energy storage elements such as either lithium-ion batteries (LIBs) or supercapacitors (SCs) ormicrosupercapacitors(MSCs),whichis able to stabilize and control the power output for direct applications.On the contrary, rechargeable LIBsor SCs also requirecontinuous and stablepowersupplyto achieve sustainable operation. Therefore, sustainable self-charging power systems that cansynchronously harvestand store energy are ableto provide a maintenance-free secondary power supply for futuristic next-generation wearable applications.

Fig. 1. (Color online) Schematic illustration of self-charging power textiles, mainly including fiber/fabric-based energy harvesting units,fiber/fabric-based energy storage unit, and power management circuits.

At present, avarietyof combinationsof energyharvesting units and energy storage units have been reported to design self-charging powersystems, including solar cell-drivenphoto-rechargeable power cell(Fig.2(a))[9,29−31],thermoelectric generator coupled MSCs[32], triboelectric-driven selfchargingSC powersystem (Fig.2(b))[33],piezoelectric-driven electrochemical self-charging SC power cell (Fig. 2(c))[34], hybrid biofuel cells (BFCs) and SCs power system (Fig. 2(d))[35],and chemicallyself-charging aqueous ZIBs[36].Compared with other energy harvesting methods that depend on external conditions, such as sunlight, temperature, and auxiliary catalyst,TENGscanrun all day withoutextra burden or discomfort,which have more possibilities for application in wearable power supplyand multifunctional self-poweredsensing.However, conventional power systems with bulk and rigid configuration are not practical for use in wearable applications that require flexibilityand comfortability[37]. Textileswith the form of fibers and fabrics can withstand versatile complex mechanical deformations, which exhibit outstanding ability of structural retentionandfatigue resistanceduring wearing and washing. Textile TENGs with diverse structural designabilityand excellent performance represent an attractive interactive medium for human-oriented electronic integration[38]. Textile-based self-charging power systems with the form of fibers[39]and fabrics[27,40]are an important part of smart textiles in the future.Although therevieworperspectivearticleson self-chargingpowersystems havebeenwidely reported[41−45], there are few summaries on TENG-based self-charging power textile, which greatly limits its further research andapplicationdevelopment.

Here, the latestdevelopmentof self-charging powertextiles based on the combination of fiber/fabric energy harvesting TENGs with fiber/fabric energy storing units are comprehensively summarizedfrom theaspect of textile structure design, whichincludes fabricatingin single fibers,interweaving with TENG fabrics, interweaving with fiber TENGs and fiber SCs, developing from fabric substrates, preparing with fabric-related membranous structures, and designing with multi-module combination. The potentialchallenges and brighter prospects towards the further research and practical application of self-charging power textiles are briefly discussed at the end of this review. We firmly believe that selfcharging powertextiles will become an importantpart of the futurewearable energy supply.

2.Self-charging power textiles

2.1.All-in-one self-charging power fibers

1Dfiber-shaped electronicdevices withlight-weight, flexibility, andsmalldiameters fromtens to hundreds of micrometers have attracted broad interests in wearable electronic fields[46]. Recently, self-charging power systems have also been designedwith 1D fiber construction.Early studies oftenintegratethetwo independent systemsof TENGs and SCs by an external wire connection. For instance, a novel flexible self-charging power system prototype consisting of fiber SCs and fiber TENGsisreportedforthefirst time by integrating three SCswitha TENG via arectifierfor harvestingmechanical energy from human motion[47]. However, fiber TENGs and fiber SCs are relatively independent and fixed on membranous substrates, which greatly limit the structuralintegrity and wearableapplication ability. With thehelp ofnovel all-in-one fiber structural design, such as coaxial, twisting, and braiding,the self-charging power systems can be integrated into one fiber system, which endows them with excellent wearability andmechanical stability toadapt to avariety ofdeformations, such as bending, stretching, and distortion. Several works have already designed self-charging power fibers with a coaxial configuration. For example, a flexible coaxial fiber byfabricating a 1D TENGoutside and a SC inside,which can notonly harvest mechanical energy but alsostoreenergy[48].The all-in-one self-charging fiber utilizes carbon fiber bundles as the electrode material for the TENG and the active materialfor the SC, silicone rubberelastomer as the separator,triboelectric and encapsulationmaterial(Fig.3(a)). The proposed coaxial fiber with high flexibility and working stability can be further knitted as cloth for sustainably powering wearable electronics.Similarly, a multifunctional coaxialenergy-autonomy fiber composedof a fiber-shaped TENG outside and SC inside is developed towards synchronous energy harvesting, energy storage, and energy utilization[49]. As shown in Fig. 3(b),through a bridge rectifier asanAC-to-DC converter, thecoaxial self-charging power fibercan directlylight up LEDs and drive a temperature-humidity meter. The fiber TENGs and fiber SCs are usuallyseparated by a certain gap between them, whichcanmake themrelatively independent without interaction.However, theinserted gapusually increases thediameter of functional fiber devices, which also leads to an undesirable decrease in mechanical stability due to the weak interface. There are also studies devoted to the close integration of fiber TENGs and fiber SCs. A spontaneous hybrid smartfiberwith asymmetry coaxialstructurethat notonly scavenges surroundingmechanicalenergy, butalso synchronouslystores the harvestedenergyby charginga fiber SC without any external electric circuit is introduced[50]. As shown in Fig. 3(c), two carbon fiber bundle electrodes with the phosphoric acid (H3PO4)-polyvinyl alcohol (PVA) electrolyte are twisted together, which is further encapsulated by polymer polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE)solution as the triboelectric material using the brush printing method. The spontaneous self-charging mechanism of the hybrid fiber can bebrieflydescribed as the couplingeffect betweentriboelectric and electrochemical effects.

Fig. 2. (Color online)All-in-one integratedself-charging power systems based on different hybridizing modes, including(a) photorechargeable energy storage system.Reproducedwiththe permissionfrom Ref. [9]. Copyright2019, Elsevier. (b) Triboelectriccoupledwithmicrosupercapacitor self-chargingsystem. Reproducedwith thepermission fromRef.[33].Copyright 2020,Elsevier.(c) Piezoelectric-drivenelectrochemical self-charging SC power cell. Reproduced with the permission from Ref. [34]. Copyright 2020, Springer Nature Group. (d) Biofuel cell and SC hybrid self-charging system. Reproduced withthe permission fromRef. [35]. Copyright 2018, The RoyalSociety of Chemistry.

Fig.3. (Coloronline)All-in-oneself-chargingpowerfibers.(a) Aflexible coaxialself-chargingfiberwith a fiber-shaped TENG outsideand a fibershaped SC inside. Adapted withpermission from Ref. [48].Copyright2018, AmericanChemical Society.(b)Multifunctionalcoaxial energyautonomy fiber composed of an allfiber-shaped TENG, SC, and pressure sensor. Reproduced with permissionfrom Ref. [49]. Copyright 2021, American ChemicalSociety.(c) Ahybrid smartself-charging fiber with asymmetry coaxial structureby aspontaneousenergy generation andstorage.Reproducedwithpermission fromRef.[50]. Copyright 2020, Wiley.

2.2.Self-charging power fabrics interwoven with TENG fabrics

Considering the limited power output of single fiber TENGs,some research workshavebeencarried outtoimprove the power output densityof the energy harvesting units by preparing fabric-based TENGs through a variety of fabric interweaving techniques, such as weaving, knitting, braiding, and sewing. In such cases, self-charging power textiles with interwoven fabric TENGs and fiber LIBs or SCs are developed. For example,aself-chargingpower unitisrealized byintegration ofa textile TENG cloth and a flexibleLIB belt[51]. As shown in Fig. 4(a), a heartbeat meter strap worn at the chest is powered by a LIB belt, which is further charged or compensated by a TENG-cloth worn under the arm. The TENG cloth is woven by two kinds of fabric strips, one of whichisthe polyester fabriccoatedwithnickel, the other of whichis thenickel-coatedpolyesterfabric further covered by parylene layer.ConsideringthattheLIB belts aredifficultto be integrated into fabric systems, fiber-shaped SCs are designed to develop self-charging power textiles.Fig. 4(b)shows a proposed self-charging power textile that in one individual clothintegratesthree functional units, i.e.,fabric TENG for energyharvesting,fabric SC forenergy storage,and wearable electronics or sensorsfor application[52]. With the selfpowered charging fabric, the potential deficiencies of unstable output of TENGs and the short operation time of SCs can be simultaneously overcome. The fabric TENG is woven usingNi-coated polyester strapsandparylene-Ni-coatedpolyester straps as thewarpandweft yarns, respectively. Underthe contact and separation mode motion at 5 Hz, thefabric TENG can achieve 40 V open-circuit voltage and 5μA short-circuit current. The symmetric fiber SC is designed utilizing Nicoated polyester fibers as the electrodes, rGO as the active materials,and PVA/ H3PO4gel as the solidelectrolyte as wellas the separator.With the help of the bridgerectifiers, threeseries connected fiberSCscan be continuouslycharged by the fabric TENG. However, although the fabric TENGs designed in the form of cloth strips can increase the effective contact area and enhance the electrical output, they cannot meet with further applicationand industrialscale-up manufacture. Therefore,itis more desirableto designandprepare fabric TENGs throughtraditional andmaturetextileformingtechnologies, such as weaving, knitting, braiding, and etc. For example, a fully stretchable fabric TENG with knitted fabric structures and scale preparation ability is designed for harvesting human biomechanical energy in cloth and garments[53]. The large-scale fabricationtechnology of fabric TENGs is further extendedtoself-chargingpower textiles. By utilizingtraditional flying-shuttle woven craft, a one-piece self-charging power textile consisting of a fabric TENG and a woven SC for simultaneously harvesting and storing human motion energy is fabricated[54]. As shown in Fig. 4(c), the fabric TENG with the contact-separationmode iscomposedof aPTFEfabric and acarbon fabric, the latter of whichis connected throughthe external load. The fabric SC is fabricated with carbon fibers as the electrode, RuO2coated on carbon fibers as the active material, cotton fibers as the separator, and PVA/ H3PO4gel as the electrolyte. Under thefrequency of 1.5 Hz,theself-charging power fabric exhibits excellent charging performance with low charge leakage under a continuous charging process,demonstrating its potential ability to drivelow-power wearable electronics.

The contradiction between the alternating current (AC)electrical output form of TENGs and the direct current (DC)power demand severely limitstheirfurtherresearchdevelopment and potential application[55]. Fabric-based DC TENGs with high power output are gradually developed based on triboelectrification andair breakdowneffects[56,57]. A self-charging power textile with DC fabric TENG and symmetrical fiber SCs is designed[56]. As presented in Fig. 4(d), by taking advantageofthe electrostatic breakdown phenomenon, the DC TENG withPA nonconductive fibers as the warp yarns, and PA nonconductive fiber separated with PA conductive fibers as the weft yarns is fabricated with the plain woven structure.With thesize of 6.8 × 7 cm2, the fabric DC TENG canachieveVOCof 4500 V,ISCof 40μA, andQSCof 4.47μC. The solid-state symmetrical fiber SC is fabricated with carbon fibers as the electrode,PEDOT:PSS astheactivematerial,and PVA/H3PO4gelas theelectrolyte. With asimpleand efficient circuit design, the self-charging power textile can power the hydrothermograph and calculator.

2.3.Self-charging power fabrics interwoven with fiber TENGs and fiber SCs

It seems to be a more efficientway tointegrate fiber TENGsand fiberSCs directlyinto a fabricsystem, which greatly improves the integration of self-charging power systems. In previous studies, we have reported a highly stretchableand washable all-yarn-based self-charging knitting power textile that enables both biomechanical energy harvesting and simultaneously energy storing by hybridizing TENG andSCinto one fabric[58]. As shown inFig. 5(a),the highly stretchable and shape-adaptive self-chargingpowertextile is obtained by integrating energy harvesting fiber TENGs with a flexible energy storing fiber SCs via weft knitting technique.The fiber TENGis fabricated by coating siliconerubberon the surface of the conductive fiber. The all-solid-state symmetric fiber SC is fabricated by dip-coating carbon nanofiber and PEDOT:PSSsuccessivelyon acarbon fiberbundle.With the addition of a full-waverectifier,the self-charging powertextile can continuously drive a temperature-humidity meter or a calculator by hand tapping it. Similarly, a flexible and easy largescale production of single-electrodemode TENG and SC yarnbased self-charging power fabric for simultaneously converting and storing biomechanical energy is also presented[59], indicatingthattheproposeddesignisapromising sustainable power sourcefor wearable electronic devices.

Fig. 4. (Color online)Self-charging power textiles developed with interwoven TENG fabrics. (a)A novel integrated self-chargingpower unit consisting of aflexibleenergy harvesting TENGcloth anda flexible LIB belt. Reproduced with permissionfrom Ref.[51]. Copyright 2015, Wiley. (b) A textile self-charging powersystemdesignedby chargingafiberSCwith a TENG cloth.Reproducedwith permissionfromRef. [52].Copyright2016,Wiley. (c)Aone-piece self-chargingpower textileintegratinga fabric TENG and wovenSCforsimultaneously harvesting andstoringbodymotion energytosustainably drive wearableelectronics.Reproducedwithpermission from Ref.[54].Copyright2020,Elsevier. (d)Self-charging powerfabricintegrated withdirectcurrent TENGand fiberSCs.Reproduced withpermission fromRef.[56]. Copyright 2020, American Chemical Society.

Fig. 5. (Color online) Self-charging power textiles fabricated with fiber-based TENGs and fiber-based SCs. (a) A highly stretchable and washable all-yarn-based self-charging knitting power textile composed of fiber TENG and fiber SC. Reproduced with permission from Ref. [58]. Copyright 2017, American Chemical Society. (b) Self-charging power textile interwoven by all-yarn-based energy harvesting TENG and energy storing yarntype asymmetric SC. Reproduced with permission from Ref. [60]. Copyright 2019, Wiley. (c) All-in-one self-charging power textile developed by integrating fiber TENG with all-solid-state fiber-based asymmetric SC. Reproduced with permission from Ref. [63]. Copyright 2021, Elsevier.

In addition to the knitting technique, weaving is also an effectivemethod to integrate fiber TENGs and fiber SCs into the self-charging power fabric systems. For example, a selfcharging power textile enabled by yarn-based TENGs as the energy-harvesting devices and yarn asymmetric SCs as the energy storing unit is reported byPuet al.[60].As exhibited in Fig. 5(b), the energy-harvesting fibers are fabricated by encapsulatingPDMS on the surface oftheCu-coatedpolyester fiber, and the energy-storing fibers consist of a negative fiber electrode with hydrothermally self-assembled rGO/CNT coating and a positive electrode of electroplated Ni-Co bimetallic oxyhydroxide coating. The serially connected fiber SCs can be charged by the woven TENG to 2.4 V in 104 min at about 3 Hz, which can power an electronic watch. However, due to the large impedance mismatch between TENGs and SCs, the charging efficiency is estimated to be only 2.6%. With proper power management, the energy utilizationefficiency andcharging rate can befurther improved[61,62]. Similar research works canalso be foundinother literatures. Forinstance, an all yarn-type self-charging powersystem couplingwith TENGsand SCs is woven into cloth to continuouslyharvest and store bio-motion energy[63]. As demonstrated in Fig. 5(c), the fiber TENG is fabricated by wrapping PTFE fiber on the carbon fibers@PDMS/MnO2NW elastic fibers. The allsolid-state asymmetric fiber SC is designed with carbon fibers@ MnO2as the positive electrode, carbon fibers @ activated carbon as the negative electrode, and Na2SO4/PVA simultaneously playing as the solid electrolyte and separator. The fiber SC can be charged by the fiber TENG under different motions, which can power a red LED.

Fig. 6. (Color online)Self-charging power textiles developed from fabric substrates. (a)Wearable fabric-based integratedself-chargingpower supply system developed bystoring triboelectric energy harvestingenergyin anintegratedSC.Reproducedwith permissionfrom Ref. [65].Copyright2014,Wiley.(b) Stretchablecoplanarself-chargingpowertextile withresist-dyeing TENG andmicrosupercapacitors.Reproducedwith permissionfrom Ref. [66]. Copyright2020, American ChemicalSociety.(c) Integrating a TENGwitha zinc-ion battery with a 3D spacer fabricstructure. Reproduced with permissionfromRef. [67]. Copyright 2018, Wiley.

2.4.Self-charging power fabrics developed from fabric substrates

In order to achieve more efficient fabrication of self-chargingpower fabrics, commonfabricsareoften used as the substrates,on which a variety of conductiveandactivematerials are applied. The simplest way of developing self-charging power fabrics with fabric substrates is to directly stack the fabric TENG withsingle-electrodemode and fabric SCs together.Moreover, the structure design can befurther simplified by sharing electrodes. For example, Zhanget al. presented an all-fabric-based self-charging power cloth by integrating a wearable single-electrode TENG and a flexibleSC with a general carbonnanotube/cottonfabricelectrode[64].However, due to the stacking structure, the power generation of TENG induced from the mechanical loading will affect or even weaken the performance of the SC, which will reduce the stability of the self-charging device. Therefore, the design and preparation of self-charging power fabrics based on common fabric substrates require better structural design.

The earlier research work of self-charging power fabrics on fabric substrates is reported in 2014[65], which developed a wearable fabric-based integrated power-supply system that generates human motion energy using TENG and stores the generated energy in an integrated SC on a conductive carbon fabric, as shown in Fig. 6(a). The fabric TENG consists of two kinds of fabrics, one of which is fabricated by alternatively coating polyurethane and polyimide on a carbon fabric,theotherof which is created by patterningPDMS and Al on a carbonfabric. The symmetric fabric SC isdeveloped with sandwiched structurecomposed ofCF/CNT/RuO2electrode,PVA/H3PO4gel electrolyte, and CF/CNT/RuO2electrode. With the help of the rectifier circuit, the wearable energy supply system can be utilized either as self-powered activity monitors or as apowersupplyforexternal wearable sensors.As exhibitedin Fig. 6(a), by stitchingthe fabric-based TENGs andSCs into commercial clothing,the self-charging power fabriccan monitor and record the rectified output current and the charge accumulation from a subject simulating normal jogging procedures of stretching, walking, running, sprinting,andacool-downwalking. Basedonthesimilar method,Puet al.developed a stretchable coplanar self-chargingpowertextilewith TENGsand microsupercapacitors bothfabricated through a resist-dyeing-analogous method[66]. As shown in Fig. 6(b), the stretchable coplanar self-charging power textile is demonstrated for powering small electronics intermittently without extra recharging. We can also use a variety of textile structuresto achieve the design andfabrication of self-charging power fabrics.Forexample, Zhiet al. proposed an integrated flexible and wearable TENG and rechargeable ZIB system based on a flexible 3D spacer fabric as a promising wearable power unit for powering personal electronic devices[67]. As shown inFig. 6(c), the electrical output from the fabric TENGcanbe effectively stored inthefabric SC via the rectifyingcircuit,which canthenpower electronic watches. Based on the above analysis, it can be found that the versatile fabric structures provide an effective and scalable design carrier and implementation platform for self-charging power textiles.

2.5.Self-charging power textiles with fabric-related membranous configurations

Paper is often utilized as the substrate for building functionalized electronics, especially for self-charging power units[68,69], dueto its lightweight, low cost, environmental friendliness,and easeoffabrication[70]. Inspired by paperbased electronicdevices, textile related membrane structures are gradually used to design wearable and flexible electronics. Moreover, textile related membrane structures can also expand the design space and application scope of functionalwearable electronics,including self-charging power fabrics,whichhaveattractedgreatattentions.Nanofiber-based membranes are firstlyused to design self-charging power systems. For example, an ultralight and flexible self-charging power system via all electrospun paper based TENGs as energy harvesters and all electrospun paper based SCs as a storage device is proposed[71]. As illustrated in Fig. 7(a), the nanofiber-based TENG made intoarch shapeisderivedfrom one nonconductive polyacrylonitrile(PAN) nanofiber membrane as a triboelectric layer and conductive carbon nanofiber paper as electrodes. The nanofiber-based TENG is fabricated with conductive carbon nanofiber paper as capacitive materials and a nonconductive PAN nanofiber membrane as the separator.Similarly,a portable,flexible, and low-cost self-charging power systemconsistingof apaper-based TENGwith high output density as the energy harvester and a paperbased SC with a great areal capacitance as the energy storage device is developed to meet the energy demand for flexible and miniaturized green electronics[72]. As shown in Fig. 7(b), the paper-based TENG is fabricated with the cellulose paper/ polypyrrole (PPy) composite as both electrode and the positive friction layer, and a nitrocellulose membrane as the negative friction layer. The paper SC is made of cellulose paper/PPy composite active material and H3PO4/PVA gel electrolyte. The all paper-based self-charging power system can drive a segmented LED display and a temperature/humidityindicator.Inaddition tonanofiber membranes,self-chargingpower unitscanbe directly fabricated on common cloth.As shownin Fig. 7(c), awovencarbon fabric-based multifunctional TENGintegrate with P-doped Cu–Mn selenide nanowire-basedSCisdeveloped for meeting future energy demands of self-charging automobiles,electronics,andvarious outdoorapplications[73]. In addition to the stacking structure, TENGs and SCs can also be connected in the horizon direction. For instance, a highly compact selfcharging power unit by integrating TENG with MXene-based MSCs in a wearable and flexible harvester-storage module[74].Fig. 7(d) shows that the MSCs are prepared with MXene as the active and conductive material, and PVA/H3PO4gel as the electrolyte by employing a spray-coating method followed by a direct laser cutting process. A single-electrode mode TENG based on carbon fiber embedded silicone is designed for integration withthesilicone-encapsulatedMXene-based MSC,which can successfully power up a thermo-hygrometer.Itcanbe foundthat textile-relatedmembranous structure is also a broad carrier and combination form to design all-inone self-charging power textiles, which canrealize integratedpackage.

Fig. 7. (Color online)Fabric-based self-chargingpower systemswithmembranousconstructions. (a)Anultralight andflexible self-charging power system viaallelectrospunpaper basedon TENGs asenergy harvester and all electrospun paper based SCsasstorage device.Reproduced withpermission fromRef. [71].Copyright2017, Elsevier.(b)Paper-basedself-chargingpower system consistingof a paper-based TENGand a paper-based SC.ReproducedwithpermissionfromRef. [72].Copyright2019,American ChemicalSociety. (c) Anintegratedenergy harvestingand storage systemwith TENG-integratedSCstructure. Reproducedwithpermission fromRef. [73].Copyright2020, Elsevier.(d)A self-charging power unit by integrating MXene-basedMSCswith TENG. Reproduced withpermissionfrom Ref. [74]. Copyright 2018,Elsevier.

3.Multi-module combined self-charging power textiles

In manycases, the outputpowerfrom individual energy harvester cannotcompletelyfulfillthepowerrequirements of the wireless sensor networks, partly because the energy source may not always be stable or continuously available in reality[75]. However, a variety of energy sources coexist in most situations. Single-sourceenergyharvesters cannot harvest all of the energy sources and will waste part of the harvestable energy, thus hindering the maximization of their energy harvesting capability. Therefore, hybrid energy harvesterthat is able to harvest multipleenergy sources is developed. Thenumber of harvestable energy sources for thehybrid energy harvesters is increased and the output power is significantly improved when the harvesters suffer instability of a particularenergy source,comparedtothe caseforprevious single-source energyharvesters[76]. Hybridenergy harvesters based on TENGs provide a promising method to effectively use the environmental conditions for energy harvesting by combining two or more working mechanisms, generating high currentsby harvesting all types of available energyto meet the needs of distributedenergy units[77].

Based on thehybrid energy harvesting strategies,multimodulecombined self-charging power systemswith complementary and synergistic energy harvesters and commensurate energy storage modules for maximized efficiency and performance are also developed. Photovoltaic solar cells can be hybridized with TENGs, which can collect solar energy and mechanical energy at the same time. For example, a textilebased self-charging power system is realized by integrating fabric TENGs and fiber-shaped dye-sensitized solar cells(DSSCs) to scavenge the energy of human motion and solar energy, which are further stored in a lithium-ion battery for sustainable power supply application[78]. The self-charging systems coupling with photovoltaic and mechanical energy harvesting can also be designed in all fiber forms. An all-fiberbasedhybridized self-charging powertextileis proposed with the aimofsimultaneously collecting outdoor sunshine and random body motion energies and thenstoringthem in an energy storage unit[79].As illustratedin Fig. 8(a), a single fiber TENG is formed by connectingeach fiberDSSCs and fiberSCsunit toone another. According tothe charging curve, it can be found that the charging efficiencies of the self-charging power textile can be greatly improved. Similarly, a highly stretchable, wearable, and comfortable self-charging power system is fabricated to individually and simultaneously harvest solar energy and human motion energy,which are then stored in SCs to sustainably power electronics[80]. As exhibited inFig. 8(b), two generators including three fiber DSSCs in series and a TENG and two SCs in series can be effectively integrated into a flexible and stretchable bracelet, which can power several portable electronic devices such as LED lights, electronic watches, and temperature sensors.In addition tothecombination of solar cellsand TENGs,otherenergy harvesting combinationmethods have also been gradually explored and studied. Forinstance, aselfsustainablewearablemulti-modular e-textile bioenergy systemis developed by harvesting biochemicaland biomechanical energyusing sweat-basedBFCsand TENGs,andregulating the harvested energy via SCs for high-power output[81]. As shown in Fig. 8(c), an integrated microgrid system into one etextile platform that unites BFCs and TENGs with distinct and complementary energy conversion mechanisms based on human activities, along with SC modules for regulating the powering of wearable applications with both low and high power demand. Thecomplementary relationship between the two bioenergy harvesters thus compensates for the limitations of the BFCs due to delayed perspiration and of the TENGs due to the lack of motion. The SC modules regulate low-current,highvoltageinputsfromthe TENGmodules and high-current, low-voltage inputsfromtheBFC modules,with the optimal capacity to deliver sufficient power for designated applications while maintaining fast booting.

Fig. 8. (Color online) Self-charging power textiles with multi-modular energy harvestingmethods.(a) Self-powered textiles for wearable electronicsbyhybridizing fiber-shaped TENGs,solarcells,andSCs. Reproduced frompermissionfromRef. [79].Copyright2016,AAAS.(b) Highlyelastic self-chargingpower bracelet consisting of two energy harvestingdevices, i.e., TENG andFDSSC, andan energy storagedevice. Reproduced from permission from Ref. [80]. Copyright 2019, Elsevier. (c)Self-sustainable wearablemulti-modular E-textile by harvesting biochemicalandbiomechanical energy using sweat-basedBFCs and TENGsand regulatingtheharvested energy via SCs.Adaptedfrom permissionfromRef.[81]. Copyright 2021, Springer Nature Group.

4.Summary and discussions

In summary, the recent process of self-charging power textiles thatintegrate fiber/fabricenergy harvesting TENGswith fiber/fabric-shapedenergy storageLIBs/SCs are comprehensively summarized, which provides a promising energyautonomy strategy to the next-generation wearable electronics. According to the textile structure design, the TENG-based self-chargingpower textilescanbe dividedintofabrication in singlefibers,interweavingwith TENGfabrics,interweaving with fiber TENGs and fiber SCs, development from fabric substrates, preparation with fabric-related membranous structures, anddesignwith multi-module combination, each of which has been introducedand discussedwith representative examples. However, although significant improvements have been achieved in the research development of self-charging power textiles, some critical problems or huge challengesregarding theirfurtherstudy and potentialapplication are needed to be addressed, such as:

(1) Working performance. The relatively low output power of fiber/fabric TENGs is one of the key bottlenecks for self-charging power textiles. For energy storing devices, the self-discharge behaviors of SCs andthe high threshold voltage of LIBs also greatly reduce the overall energy conversion efficiency. In addition, the pulsed electrical output mode may affect ion diffusion andtransport across the isolating membrane. Tremendous efforts need to be made to improve the output performance of fiber/fabric TENGs as well as their combining ability with energy storage devices. Moreover, the performance stability of self-charging power textiles in the long-term working cycles is also very important. Reasonable packaging techniques are necessary to prevent the internal electrode and triboelectric charge from being interfered with by theexternalenvironment. Some polymer materialswith excellent hydrophobicity andstrong electrification ability areoften adoptedas packagingmaterials.

(2) Capacities/impedances mismatch. The energyharvesting TENGs with the electricaloutputcharacteristicsof high voltage and low current do not match with the stored form of electrical energy of energy storing units, which is low voltage and high current. Many efforts should be done in order to improve energy conversion efficiency.

(3) Power management systems/circuits. The power management circuits including hard modules are hard to integrate into cloth, which requires certain flexibility and stretchability. In addition, althoughvarious kinds of power management circuits, including full-wave rectification[82], half-wave rectification[83],inductance-capacitor (LC)buckconversionwith mechanical/electronicswitch[61,84],LC oscillation[85], switched capacitorconverter basedonfractal design[86,87], have been designed to match the impedance between them, the development of more effective management circuits is still significant and urgent.

(4) Self-charging mechanism. The working principles of energy harvesting TENGs and energy storing devices have been widely studied and reported, which has basically reached a consensus. However, the potentialworking mechanisms of TENG-based self-charging power textilesare still not fullyunderstood, which makes it difficult to explore new or higher performance self-charging devices.In most cases,self-charging mainly goes throughthree processes.Firstly,the mechanical energy is harvested and simultaneously converted into electric energy through the coupling effect of contact electrification and electrostatic induction. Secondly, the triboelectric signals with the characteristics of pulse, high voltage, low current, and alternating current (AC) require energy management circuits to transform into the electrical signals with stable, low voltage, steady current, and direct current (DC)mode.Finally, thestableDC output is storedin SCsor LIBsto power external electronic devices. More detailed explanations ofthe self-charging mechanismneed the help ofmore advancedequipment andtechnology.

(5) Performance evaluation criteria.At present,theperformance metrics of electrochemical energy storage (EES)devices have been widely reported, which mainly include energy density and power density[88]. However, challenges still remain daunting for establishing more sophisticated standard criteria for the evaluation EES systems. In addition, as a new technology, the properties of TENGs are evaluated based on different opinions without a unified standard, which makes it difficult to compare the performance of TENG-based self-charging power systems. Although some potential standards for evaluating the overall performanceof TENGs, such asQ–Vcurves[89], figure-of-merits(FOMs) includingstructural FOM and material FOM[90], maximized effective output energy density[91,92], and etc., their coverage and effectiveness still need a lot of verification as well as a further improvement. Therefore,the evaluation criterion of the overall performanceof self-chargingpower systems, includingcharging efficiencyshould be unified in the future.

(6) Integration into one cloth system. For terminal wearable use, the components of self-charging power textiles should be highly integrated into one clothing system, and should have certain shape adaptabilitytohumandaily motions. This is notonly related to thetextile-relateddesignof each unit, but also the connection and packaging of the circuit between the constituent units.

(7) Wearability and comfortability. The wearability and comfortability of self-charging power textiles are also particularly important since their serviceoccasion is always for wearable usage.Ingeneral,the additionoffunctionalattributes will lose part of the comfort, due to excessive chemical treatments and the occurrence of skin sensitive materials. In addition, external loads are required to stimulate the electricity generation of fiber/fabric TENGs, which will also cause discomfort tothe humanbody. Therefore, more attention shouldbe paid to wearability and comfortability onthe basis of satisfying good working performance.

(8) Cost-effectiveness analysis. Most of the reported selfcharging power textiles are still proof-of-concept prototypes,which are fabricated with expensive raw materials and complicated preparation processes. Itistheprimary concern andthe only way to realizethecommercialapplication of self-charging power textiles by reducing the cost of raw materials, simplifying the preparation process, and improving manufacturing efficiency.

Asanew research directionwithgreatapplication prospects,self-charging powertextiles provideauniquesolutionfor future energyautonomy energy supply and distributed self-powered sensing. Although it is full of difficulties and challenges in the road of practical application, we believe that the TENG-based self-charging power textiles will be extensivelyappliedinour dailylife inthe near future owing to the unremitting efforts by a largenumber ofresearchers around the world,especially for wearable electronicdevices and self-powered systems.

Acknowledgments

The authors are grateful for the support received from National Natural Science Foundation of China (Grant No.22109012), the Beijing Municipal Natural Science Foundation(Grant No. 2212052), andtheFundamentalResearch Funds for the CentralUniversities (Grant No. E1E46805).