Single-Layer Graphene-Based Transparent and Flexible

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Functional Nanostructured Materials (including low-D carbon)

Single-Layer Graphene-Based Transparent and Flexible Multifunctional Electronics for Self-Charging Power and Touch-Sensing System Sungwoo Chun, Wonkyeong Son, Gwangyeob Lee, Shi Hyeong Kim, Jong Woo Park, Seon Jeong Kim, Changhyun Pang, and Changsoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20143 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Materials & Interfaces

Single-Layer Graphene-Based Transparent and Flexible Multifunctional Electronics for Self-Charging Power and Touch-Sensing System Sungwoo Chun1†, Wonkyeong Son2†, Gwangyeob Lee3, Shi Hyeong Kim4, Jong Woo Park5, Seon Jeong Kim5, Changhyun Pang1,6, and Changsoon Choi2*

1

Department SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan

University, Suwon, Gyeonggi-do 16419, Republic of Korea 2

Division of Smart Textile Convergence Research, DGIST, Daegu 42988, Republic of Korea

3

Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic

of Korea 4

Research Institute of Industrial Technology, Ansan, 15588, Korea

5

Center for Self-powered Actuation, Department of Biomedical Engineering, Hanyang

University, Seoul 04763, Republic of Korea 6

School of Chemical Engineering, Sungkyunkwan University (SKKU), Gyeonggi-do 16419,

Republic of Korea

[†] S. C. and W. S. contributed equally to this work. *To whom correspondence should be addressed. E-mail: [email protected] (C.C.) 1

ACS Paragon Plus Environment

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Abstract Applications in the field of portable and wearable electronics are becoming multifunctional, and the achievement of transparent electronics extensively expands the applications into devices such as wearable flexible displays or skin-attachable mobile computers. Moreover, the self-charging power system (SCPS) is the core technique for realizing portable and wearable electronics. Here, we propose a transparent and flexible multifunctional electronic system in which both an all-in-one SCPS and a touch sensor are combined. A single-layer graphene (SLG) film was adapted as an electrode for the supercapacitor, touch sensor, and a triboelectric nanogenerator (TENG), thus making an electronic system that is ultrathin, lightweight, transparent, and flexible. Capacitive-type transparent and flexible electronic devices can be simultaneously used as an electrochemical double-layer capacitance (EDLC)-based supercapacitor and as a sensitive, fast-responding touch sensor in single device architecture by inserting a separator of polyvinyl alcohol (PVA)–LiCl-soaked polyacrylonitrile (PAN) electrospun mat on polyethylene naphthalate (PEN) between two symmetric SLG film electrodes. Furthermore, a transparent all-in-one SCPS was fabricated by laminating a TENG device with a supercapacitor, and high-performance electric power generation/storage capability is demonstrated.

Keywords: Self-charging power, Single-layer graphene, Supercapacitors, Energy harvesting, Energy storage 2


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1. Introduction Portable and wearable electronics have attracted tremendous interest because of their great potential for advanced smart electronic devices such as bendable touch screens,1 flexible circuits,2 wearable sensors and actuators,3–5 and flexible light emitting diodes (LEDs).6 For these devices, multifunctionality and wearability have become the most important and essential requirements. To achieve these requirements, electronic devices or systems should be highly flexible/stretchable, ultrathin, lightweight, and also be capable of performing various functions of integrated electronics including sensing, powering, storing, and self-cleaning at the same time. In particular, the achievement of transparent electronics is very challenging, but if accomplished, it would help enormously to expand the application fields for the portable and wearable electronics market: for example, wearable flexible displays or skin-attachable mobile computing devices. The core technique for realizing portable and wearable electronics is to develop a suitable and sustainable self-charging power system (SCPS) that combines energy-harvesting and energy-storage technologies.7–13 For energy-harvesting devices, considerable effort has been made that targets the harvesting of renewable energy sources in daily life by introducing piezoelectric,14 triboelectric,15 electromagnetic,16 and thermoelectric17 generators. Among the various energy-harvesting devices, the triboelectric nanogenerator (TENG) has been proven to harvest mechanical energy induced by human movements with high efficiency.18 Frictional operations between a wide variety of materials with different electron affinity can harvest the electric power based on a combination of contact electrification and electrostatic induction.18 With regard to energy-storage devices, supercapacitors and not conventional batteries have been spotlighted as a strong alternative for electric power storage in portable and wearable electronics because of their extraordinary attributes such as high power density and an outstanding cycle performance that originates from the surface charge storage mechanism.19 3



Recently, fiber, yarn, textile, and film have been adapted for various types of supercapacitors to achieve wearable energy-storage applications.20–22 Graphene is one of the strongest candidates for achieving transparent and portable/wearable SCPSs, owing to its extraordinary properties. It possesses the most important requirements for transparent portable/wearable SCPSs: it is ultrathin and lightweight, as well as showing the best optical transparency,23 mechanical strength,24 and electrical conductivity25 among known materials. As a result, single-layer graphene (SLG) has been reported for use as active or electrode materials for high-performance TENG26-27 or supercapacitor28 devices. In addition, SLG has been widely adopted for sensors, actuators, displays, circuits, and transistors.29–31 In this work, we report on transparent and flexible multifunctional electronics, which can 1) harvest mechanical energy and convert it into electrical energy, 2) effectively store the harvested energy, and 3) sense noncontact and contact touch by measuring capacitance change, by introducing the SLG electrode. The capacitive-type transparent and flexible electronic devices were first fabricated by inserting a separator of polyvinyl alcohol (PVA)–LiCl-soaked electrospun polyacrylonitrile (PAN) mat between two symmetric SLG electrodes on polyethylene naphthalate (PEN) substrates. The resulting devices simultaneously act as an electrochemical double-layer capacitance (EDLC)-based supercapacitor and as a sensitive, fastresponding touch sensor in a single device architecture, forming the top panel for the multifunctional electronics. Moreover, the TENG device for electric power harvesting as the bottom panel was easily laminated with the top panel and demonstrated a transparent all-in-one SCPS.

4


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2. Results and discussion The proposed multifunctional electronic devices are fabricated by assembling two independent transparent and flexible panels, schematically shown in Figure 1a. The top panel contains two symmetric SLG electrodes transferred onto flexible and transparent PEN substrates with an intermediate electrospun PAN mat soaked in PVA–LiCl as electrolyte and separator. The SLG film was obtained using a direct growing method with thermal chemical vapor deposition (CVD) (see Supporting Methods and Figure S1) and transferred on the PEN substrate using a wet transfer method (Figure 1b(ⅰ)) (see the Supporting Information for details). The electrical sheet resistances depend on the graphene film layers with the transfer repetitions and are characterized in Figure 1b(ⅱ). Although the resistances decrease as the graphene stack number increases, here we used a once-transferred graphene film (called SLG) to achieve high transparency as well as good electrical and mechanical performance. The PAN mat was synthesized by the electrospinning method.32 Herein, the electrospun PAN mat functions as a transparent separator. The scanning electron microscopy (SEM) image in Figure 1c(ⅰ) presents a randomly oriented web structure indicating that the mat, which consists of nanoscale-diameter fibers, can provide an effective porosity, which is channeled to the ion diffusion process. Accounted fiber diameter of the spun mat exhibited that more than 80% were in the range of 500–900 nm, indicating relatively uniform size distribution (see Figure 1c(ⅱ)). For electrochemical performance characterization, the PVA–LiCl gel and LiCl solutions were applied on the PAN mat to act as electrolytes. The SLG-based supercapacitor and sensor device was finally completed by sandwiching the quasi-solid-state PAN mat electrolyte and separator between the SLG-coated PEN plates. For details, see the Supporting Information. In this work, the top panel is used as a highly transparent and thin-film structured all-solid-state supercapacitor and capacitive touch sensor. The bottom panel is composed of a TENG device to harvest the electric energy. The PEN and polytetrafluoroethylene (PTFE) surfaces for friction 5



were coated with SLG electrodes and were assembled using an acryl bumper (of ~1 mm height) to isolate the surfaces without applying force. After laminating the top and bottom panels, the multifunctional electronics were completed by electrical wiring on both sides of the top and bottom panel, respectively. The optical transmittance values of various independent films were investigated, and the comparison is presented in Figure 1d. By comparing optical transmittances of pristine PEN (89.4% transparency) and SLG-mounted PEN (88.1% transparency) substrates, we found that the SLG results in decrease of 1.5% in transparency at 550 nm wavelength, which is comparable with a previous report.33 The insertion of the electrospun PAN mat soaked with PVA–LiCl on SLG/PEN does not cause any significant decrease in transparency after spin wetting by gel electrolyte, although the pristine PAN mat on SLG/PEN shows a decrease of ~ 3%. Then, it indicates that not only the SLG electrode but also the solid electrolyte-soaked separator displays a remarkably low loss in transparency of less than 1.7% in total. As a result, the top panel device comprising of two symmetric SLG electrodes and a PVA–LiCl-soaked electrospun mat shows an impressive total transmittance (77.4%). If a more transparent substrate was used, the entire top panel device would be extremely transparent. For supercapacitor devices, such a high transparency was hardly achievable for previously reported carbon nanotube (CNT)-based transparent supercapacitors.34–35 The transmittance for the bottom panel of the TENG device was also investigated, and the result indicates a transparency of 74.8 %. Finally, the multifunctional electronic device laminated with the top and bottom panels presented the transparency of 58.7 % (Figure 1d and the photograph in Figure 1e). With the exclusive use of flexible materials, the completed multifunctional device is fully flexible (Figure 1f). The electrochemical performance of the SLG-based transparent supercapacitor is shown in Figure 2. The main advantages of using an SLG electrode as a supercapacitor are as follows: 1) its two-dimensional (2D) structure enables a large surface area to serve as an extensive 6


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transport characteristic for electrolytes; 2) the ultrahigh electrical conductivity of the SLG electrode provides improved power and energy density because of low diffusion resistance; 3) its extraordinary mechanical properties make it flexible without insignificant degradation in performance under mechanical deformations, and 4) the supercapacitor device or system introducing the SLG electrode can be highly transparent, offering excellent optical transparency. The present advantages become clearer when compared to previously reported transparent supercapacitors, which are based on a brittle indium–tin–oxide (ITO) current collector and optically opaque pseudocapacitive active materials such as metal oxides36 or conducting polymers.37 Rectangular cyclic voltammetry (CV) curves at low scan rates from 10 to 100 mV/sec (Figure 2a) and 100 to 1,000 mV/sec (see Figure S2) without any observable redox peaks indicate the EDLC characteristic that is originated from the high-quality and pure SLG electrode layer.38 High power performance in the SLG-based supercapacitor can be also confirmed by highly retained rectangular CV curves without significant dent at higher scan rates from 1,000 to 10,000 mV/sec, as shown in Figure 2b. Specific areal capacitance calculated from CV curves was plotted versus scan rate (Figure 2c). The initial areal capacitance value for single electrodes from the CV curve at 10 mV/sec was 3.83 µF/cm2, which can be converted into 102 F/g of gravimetric capacitance. The areal and specific capacitances of present values are comparable with the literature39–40 and can be well retained up to very high charge/discharge rates, such that about 41.5 % of the capacitance was retained at an extremely high scan rate of 10,000 mV/sec. The capacitive characteristic was also demonstrated in galvanostatic charge/discharge curves (Figure 2d). The linear discharge characteristic of triangular shaped galvanostatic curves also supports the EDLC-based high power energy-storage mechanism of the present transparent supercapacitor.41 The capacitance retention performance versus charge/discharge cycles is characterized in Figure 2e. The transparent supercapacitor exhibited excellent cycle performance such that about 98.3% initial capacitance was retained after 10,000 7



charge/discharge cycles at 1,000 mV/sec scan rate. The initial and 10,000th discharged CV curves are compared in the inset of Figure 2e, showing the high cyclability of the present supercapacitor. The capacitance retention performance of the transparent thin-film supercapacitor during bending deformation is also investigated. Figure 2f displays photographic images of a SLG-based supercapacitor without or with bending. Figure 2g shows static CV curves measured before and after bending deformation. The transparent supercapacitor exhibited a stable electrochemical performance during the statically applied bending deformations; 95.6 % of the initial CV curve area was retained at application of 160 bending. The dynamically measured electrochemical performance is also important for wearable electronics target applications.42 Real time capacitance retention performance of the transparent supercapacitor was characterized under dynamically applied bending deformation (Figure 2h). Owing to high structural stability, the dynamic CV curve (at a scan rate of 100 mV/s) was recorded during measurement with even contour during two cycles of bending deformation and recovery. To characterize the electrochemical performance reliability of the transparent supercapacitor, the capacitance retention was characterized during repeated bending deformation cycles (Figure 2i). With application of 160 bending, about 99.8% of the initial capacitance was retained after the 1,000th bending deformation. During the CV curve measurements for the SLG based supercapacitor under bending deformations, static bending was applied first (figure 2g), and subsequently, dynamic bending was applied later (figure 2h, i). Therefore, initially applied, static bending led to first-increase in electrical resistance of the SLG electrodes, which could be a main reason for capacitance loss (~5%). However, postapplied dynamic bending did not generate additional resistance increase, resulting in high capacitance retention (99.8%). Additional EIS measurement was conducted for SLG electrode under bending deformations (figure S3) and the Nyquist curves were fitted using Randles cell model. The non8


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deformed SLG electrode exhibited about 370.7 ohm of Rs (equivalent series resistance, ESR), and this increased up to 457.6, and 516.8 ohm at 140, and 160 bending degrees, respectively. This might be originated from bending-induced resistance increase of SLG electrode, which is main reason for capacitance loss. On the other hands, Rct (charge transfer resistance) of the flat SLG electrode is calculated to be 182.3 ohm, and this value slightly decreased into 138.9, and 144 ohm at 140, and 160 bending degrees, respectively. It can be concluded that bending deformation effect on the electrode-electrolyte interface for SGL electrode is relatively ignorable. In addition to being an outstanding transparent supercapacitor, another advantage of the present SLG-based capacitive device (top panel) is its bifunctionality. The sandwich-structured capacitive device (SLG/electrospun mat/SLG) can be also used to sense mechanical touch because the applied touch induces a reversible change in the capacitance of dielectrics between two SLG electrodes. For this reason, the top panel device can be applied for a capacitive touch sensor as well as a supercapacitor simultaneously (Figure 3). Figure 3a presents the sensitive detection of noncontact and contact touches. There are no changes in capacitance when the conductive touching objective is more than 6 cm away from the touch sensor. When approaching 5 cm, the sensor causes a decrease in capacitance, indicating a threshold distance for sensor response. The capacitance values continuously decrease with decreasing distance between the sensor and the touching objective. With touching, a capacitance change (C = C−C0) of ~ 1.6 pF was measured from the initial capacitance (6.8 pF), indicating a capacitance change rate of –0.24. Unlike the conventional capacitive touch sensor, for which applied touch or pressure results in an increase in capacitance because of decreased distance between electrodes, our SLG-based touch sensor exhibits the opposite because it is based on measuring the change in capacitance of dielectrics between two SLG electrodes.43 The capacitance response represented by the rate of capacitance change with dynamically applied vertical 9



pressure (~ 10 kPa) with a human finger is shown in Figure 3b. The sensor shows consistent and reliable capacitance responses in pressing-to-releasing operations of time-dependent pulses with a pulse width and duration of 0.5 s. The response time of the sensor was also evaluated. The response time is a critical factor because it is related to the detection ability of dynamic pressure or vibration for touch or pressure sensors. With a touching pressure of 10 kPa, Figure 3c shows a millisecond-level ultrafast response time of below 2 ms for restoration with a measurement accuracy of 1 ms, which was similar to a previously reported response time of an SLG-based pressure sensor.44 This fast response property demonstrates that the sensor can detect a vibrating pressure of over 500 Hz without a significant signal loss; this is comparable to the frequency detection limit (

Single-Layer Graphene-Based Transparent and Flexible

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