Wearable supercapacitor self-charged by P(VDF-TrFE) piezoelectric separator

Yao Lu,Yuan Jiang,Zheng Lou,Ruilong Shi,Di Chen,Guozhen Shen,∗

a School of Mathematics and Physics,University of Science and Technology Beijing,Beijing,100083,China

b State Key Laboratory for Superlattices and Microstructures,Institute of Semiconductors,Chinese Academy of Sciences,Beijing,100083,China

c Robert Frederick Smith School of Chemical and Biomolecular Engineering,Cornell University,Ithaca,NY,14853,USA

ABSTRACT Flexible piezoelectric self-charging supercapacitors(PSCS)are capable of converting random mechanical energy directly into electric energy and providing sustainable power source for various wearable electronic devices.In this work,using PDMS-rGO/C hybrid membrane as the positive and negative electrodes,P(VDF-TrFE)film as the piezoseparator,a flexible and ultralight PSCS was developed to harvest the mechanical energy of human finger curvature and simultaneously store as chemical energy in supercapacitors.The PSCS can be efficiently charged from finger deformation in the presence of polarized P(VDF-TrFE)separator film.When the finger was bended to 90°,the fabricated PSCS achieved a potential window of 0.45 V with the discharging time of about 18.0 s,and a stable charging current of 6.4 μA,providing a promising power solution to the next-generation self-powered wearable electronics system.

Keywords:Wearable electronics Piezoelectric Separator Energy harvester Energy storage Self-charging supercapacitors

1.Introduction

An increasingly severe energy shortage and a lack of fossil fuel have accelerated the exploration and development of clean and environmental-friendly energy storage techniques including supercapacitors in the past decades[1-6].With the commercialization of lithium batteries and supercapacitors,many efforts have been made to develop an integrated system by hybridizing energy conversion and storage processes into one single process for collecting energy from the surrounding environment[7-11].Among various types of ambient energy available in the surrounding,mechanical energy with variable frequency and amplitude is the most valuable energy because of its great accessibility[12-15].In 2014,Wang et al.first reported a self-charging power cell by fabricating a particular lithium-ion battery using PVDF-PZT nanocomposite film as the piezoseparator.The cell could be efficiently charged up with the capacity of～0.01 μA in 240 s through the piezoelectrochemical mechanism [16].

Besides Li-ion batteries,supercapacitors,as a type of promising energy storage unit,have also been widely studied in the field of integrated self-powered system due to the outstanding cycling stability and long lifespan [17-21].For example,Sun et al.fabricated an ultralight and flexible self-charging power system via all electrospun paper based triboelectric nanogenerators as energy harvester and all electrospun paper based supercapacitors as storage device [22].Moreover,Wang et al.also reported a paper-based flexible solid supercapacitor biomimetic tactile sensor with a piezoelectrically active PVDF separator that was capable of self-charging solely by finger tapping,and presenting information of both the static and the dynamic pressure through electrical signal[23].However,it remains a challenge to fabricate portable and lightweight self-powered systems in order to meet the growing power demand of wearable micro-electronic devices.

Herein,we presented a piezoelectric self-charging supercapacitor device consisting of ultrathin PDMS-rGO/C film as symmetric electrodes,network-like P(VDF-TrFE) film as piezoelectric and separator and PVA/H3PO4as gel electrolyte.The fabricated PSCS stuck to the human finger can effectively collect the mechanical energy from deformation and store as the electrochemical energy in the supercapacitor.The piezoelectric potential across the polarized P(VDF-TrFE)separator raised as the finger bending angle increased.By bending the finger to 90°,the PSCS can charge itself to 0.45 V in about 17.0 s with a discharge time of about 18.0 s.Moreover,the fabricated integrated device also has a stable discharging current of about 6.4 μA,demonstrating the potential applications in future wearable and micro-electronic devices.

2.Experimental

2.1.Preparation of P(VDF-TrFE) film separator

The P(VDF-TrFE) nanofiber film was prepared by a typical doubleelectrode electrospinning process.First,appropriate P(VDF-TrFE) was dissolved in a mixture of N-Dimethylformamide (DMF) and acetone solvent to obtain the 20% (w/v) P(VDF-TrFE) transparent viscous solution (named as spinning fluid) under vigorous stirring for about 1 h.Subsequently,the spinning fluid was injected into a plastic syringe with a needle of 0.5 mm in diameter.Then the electrospinning process was conducted with a constant flow rate of 1 mL/h using a syringe pump.The DC voltage of 15 kV was applied between the syringe needle and two 200 mm apart adjacent aluminium foil electrodes,with a distance of 10 cm from the needle.Then the prepared P(VDF-TrFE) film was dried at 65 °C in an oven for 10 h to completely evaporate the residual solvent.

2.2.Preparation of functionalized PDMS-rGO/C membrane electrode

The GO/C ink was first prepared by mixing commercial carbon powder solution (0.2 mg mL−1) and graphene oxide solution(0.2 mg mL−1) with the volume ratio of 1:5 under ultrasonic agitation for about 30 min.The soft PDMS mold with a hollow square shape of 2 × 2 cm2was placed on the microfiltration membrane tiled over the filter.Then the prepared GO/C ink (200 μL) was pipetted into the square area to form a layer of GO/C film by the vacuum filtration process as shown in Fig.1a.After dried at 80°C for 1 h,a brown GO/C film was prepared (Fig.S1a).Subsequently,an ultrathin PDMS film with the thickness of about 50 μm (Fig.S1b) was obtained by spincoating 10 mL solution onto the clear silicon wafer at 2000 r/min for 30 s and then heat-treating at 100°C for 1 h.When peeled offfrom the Si wafer,the ultrathin PDMS film was cut into a substrate of 3×3 cm2.After pre-stretched 150% of its initial length along one direction,the freshly prepared GO/C film was pasted onto the PDMS substrate(Fig.1b).Fig.S1c displays a digital picture of the PDMS-GO/C membrane.Finally,the produced PDMS-GO/C membrane was further processed by hydroiodic acid vapour for about 10 min at 80°C,the GO was reduced into rGO and the waved PDMS-rGO/C membrane electrode in grey was prepared (Fig.1 c and Fig.S1d).

2.3.Assembly of the PSCS

The prepared P(VDF-TrFE) film piezoseparator coated with PVA/H3PO4gel electrolyte was sandwiched between two PDMS-rGO/C electrodes to form a symmetrical supercapacitor.Subsequently,the device was sealed and solidified in air at room temperature for about 2 h.Fig.1 d displays the schematic of the sandwich structured PSCS device in detail.In this system,the P(VDF-TrFE) film acted as both the separator and the energy harvester,and the PDMS-rGO/C acted as the symmetric electrodes for the supercapacitor.

3.Results and discussion

The scanning electron microscopy (SEM) was used to assist the characterization of the morphology of the PDMS-rGO/C electrode from the reduction procedure.Fig.2a shows the uniform waved structure of as-prepared electrode by pre-stretching of PDMS.The magnified image in Fig.2b shows that PDMS-rGO/C electrode was mainly composed of many rGO sheets with wrinkled surface.Fig.2c displays the typical cross-sectional SEM image of a single wave structure with the film thickness of 50 nm.The magnified SEM image in Fig.2d further confirms the rough surface structure of the film.Furthermore,TEM image in Fig.2e clearly shows that the carbon nanospheres with the diameter of about 50 nm and the rGO sheets were evenly distributed in the film products.To investigate the wettability of PDMS-rGO/C electrode,the contact angle of electrolyte on the film electrode was measured at room temperature.Fig.2f shows the contact angle was about 30.9° illustrating the excellent hydrophilicity of PDMS-rGO/C electrode due to its waved microstructure with rough surface.

In order to investigate the stability of the prepared PDMS-rGO/C electrodes,traditional supercapacitor was first fabricated based on the commercially available Celgard separator as the separator,PDMS-rGO/C as the electrodes and PVA/H3PO4gel as the electrolyte,respectively.As shown in Fig.S2,the CV curves of the fabricated SC tested in the double-electrode system with a scan speed of 3 V/s exhibits a regular rectangular shape.Under various bending states,the measured CV curves were almost overlapped and no obvious changes can be observed,illustrating the outstanding flexibility and stability of the PDMSrGO/C electrodes.

In addition,network-like β-phase P(VDF-TrFE)film was prepared by electrospinning process and the crystallinity and microstructure were characterized by XRD and SEM,respectively.From Fig.S3,only one peak at 19.2° appearing in XRD pattern could be indexed to (110) and(200) planes of the fully polarized β-phase P(VDF-TrFE) film [24,25].SEM image in Fig.S4a displays that the prepared film was composed of uniform nanofibers with the diameter of about 200 nm and the length of up to several tens of micrometers.The magnified SEM image in Fig.S4b displays the network-like structure of the P(VDF-TrFE) film suggesting its possible applications in supercapacitor as separator.

Symmetrical supercapacitor device was fabricated by using the P(VDF-TrFE) film as the separator,PVA/H3PO4as gel electrolyte and PDMS-rGO/C as the electrodes,respectively.The electrochemical performance of the device was measured and Fig.3a shows the CV curves at various scan rates from 0.5 to 3.0 V/s with a potential window of 0-1 V.Obviously,the CV loops were in similar rectangular shapes without any redox peaks,revealing that the electrochemical double layer capacitance (EDLC) behavior was actually originated from the fast,reversible successive surface electro-adsorption of ions,as well as proton incorporation.The galvanostatic charge-discharge test in a stable potential window between 0 to 1 V at various current densities from 25 to 250 μA cm−2is displayed in Fig.3b.All CD profiles show linear and relatively symmetric triangular shape,in accordance with the CV curves,indicating the EDLC behavior of the device with a high Coulombic efficiency.The areal specific capacitances evaluated from the CD curves were 44.6,36.6,29.8,26.5 and 23.1 μF cm−2,corresponding to the current density of 25,50,75,100 and 250 μA cm−2,respectively,as shown in Fig.3c.Fig.3d displays the calculated areal energy density and power density of the fabricated flexible supercapacitor with the largest areal energy density of about 0.078 μW h cm−2at the power density of 0.025 mW cm−2.Fig.3e shows the electrochemical impedance spectroscopy (EIS) of the device from 100 kHz to 0.01 Hz.The Nyquist plot displayed that the fabricated supercapacitor has a charge transport resistance of around 500 Ω.In addition,Fig.3f displays the cycling property of the device evaluated from the charge-discharge results at a current density of 250 μA cm−2.After 20000 cycles,the capacitance retention still retained about 98%of the initial value,suggesting the excellent cycling stability of the supercapacitor.Fig.3f inset displays the kinetics schematic of the symmetrical supercapacitor based on PDMS-rGO/C electrodes.First,due to the existence of C nanospheres,the rGO/C compounds with improved conductivity and expanded layer spacing provided efficient charge transfer paths.Moreover,the measured high BET surface area of 2630 m2g−1illustrated that the rGO/C film was advantageous for the charge absorption and storage.Thus,it is concluded that the outstanding electrochemical performance of the PDMS-rGO/C electrodes based supercapacitor is attributed to the synergistic effect between the rGO and C nanospheres [26,27].

To our best knowledge,β-phase P(VDF-TrFE) samples,generally prepared by poling under a high electric field with a typically high piezoelectricity,have been widely used in piezo-nanogenerators,piezotransistor and bio-sensors,etc.[28-33].Herein,the piezoelectric property of P(VDF-TrFE)separator by coating Au layer on both sides of the film was investigated.Under different levels of compressive force,Fig.4a displays the open-circuit voltage of β-phase P(VDF-TrFE) based energy harvesting device with an area of 1 × 2 cm2stuck on to the human finger.The open-circuit voltages increased from 0.5 to 0.9 V gradually as the bending angle increased from 30° to 90°.During each cycle the output voltage was almost unchanged,illustrating the excellent stability of the fabricated energy harvesting device.This phenomenon could be explained by piezoelectric effect of the P(VDF-TrFE)film caused by human finger deformation [34,35].Fig.4b shows the amplified curve of the dash area in Fig.4a.When introduced an external deformation to the device,the positive and negative piezoelectric voltages of 0.9 and −0.4 V were generated.The time lasted for about 1 s during the process and the positive voltage (0-0.9-0 V)maintained about 0.12 s.Fig.4c displays the varied output voltage of the device under different deformation frequencies with a finger bending angle of about 30°.The generated piezoelectric voltage raised from 0.3 to 0.9 V under the varied frequency from 1-5 Hz,which could be ascribed to the decrease in the impedance with the increased strain rate.The amplification curve in Fig.4d clearly exhibited the high generated piezoelectric potential of the device under high deformation frequency,and the biggest out voltage retained about 0.025 s at the highest frequency.The real-time short-circuit current of the energy harvesting device recorded under the finger bending angle of 90° was shown in Fig.4e.During the 10 cycles,the average current of 2.8 μA was obtained in 4.5 s,illustrating that the free electrons in electrode materials could flow into the external circuit due to the presence of piezoelectric field.From all of the above results,it may be concluded that the P(VDF-TrFE)separator can generate piezo-potential in the fabricated PSCS.

Moreover,the piezoelectric self-charging supercapacitor device was fabricated by using piezoelectric material P(VDF-TrFE)film as both the separator and the potential generator,PDMS-rGO/C as the positive and negative electrodes and PVA/H2SO4as the gel electrolyte,respectively.The self-charging performance of the PSCS device was further studied by sticking onto human finger,as shown in Fig.5,under different extends of finger deformation.When the human finger was flatten in Fig.5a,an initial voltage of about −0.15 V was obtained,which was possibly caused by the remnant polarization of the P(VDF-TrFE) piezoseparator during the poling process.When the finger bended to 30°,the voltage of PSCS began to increase from −0.15 V to the maximum of 0.05 V in 1.5 s.After relaxing the finger,the discharging time maintained about 4s as shown in Fig.5a.When the finger bended to 90°,the device showed an increase in voltage for 0.45 V and sustained the discharging time for about 18 s,confirming the self-charging capability of the as-fabricated PSCS device.Clearly,for this device,the mechanism of its function was concluded as the piezoelectric potentialdriven double electrical layer behavior (non-faraday process).Under the deformation state,the piezoelectric field in the P(VDF-TrFE) separator would drive the H+and SO42−ions toward to the positive and negative electrodes of the PSCS device[36,37].The increased potential window was generated by increasing the bending angle of the finger and the stress applied on the PSCS device.

In order to compensate for the self-charging voltage,the chargingdischarging current of PSCS device was also evaluated as shown in Fig.5b.Under the piezoelectric voltage,the charging current continued to rise to 4.2 μA from the 1.9 μA and remained stable for 2.4 s until the disappearance of the piezoelectric field.Finally,the discharging current still maintained for about 4.0 s without the presence of the piezoelectric field,and the calculated capacitance was about 240 μF cm−2from the above results.In addition,the increased charging current up to about 6.4 μA was obtained under higher piezoelectric potential,confirming that the self-charging process of PSCS device was caused by the piezoelectric effect.

4.Conclusions

In summary,a network-like P(VDF-TrFE) film from the simple and effective electrospinning process has been prepared.The piezoelectric P(VDF-TrFE)film,as an energy harvester,can also be used as a separator for the supercapacitor due to its porous structure,which is suitable for ion migration.Furthermore,a piezoelectric self-charging supercapacitor based on PDMS-rGO/C film as the symmetric electrodes,and PVA/H3PO4as the gel electrolyte,respectively,has been fabricated.The PSCS can charge itself from finger deformation stress with a high open circuit output voltage of～0.45 V and short circuit current of～6.4 μA.The result suggested that the self-charging system is promising for applications in future flexible and wearable electronic devices.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC,No.51672308,51972025 and 61888102).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2020.01.023.