Laser-Induced Graphene Triboelectric Nanogenerators | ACS Nano

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Laser-Induced Graphene Triboelectric Nanogenerators Michael G. Stanford, John T. Li, Yieu Chyan, Zhe Wang, Winston Wang, and James M. Tour ACS Nano, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Laser-Induced Graphene Triboelectric Nanogenerators Michael G. Stanford,1 John T. Li,1 Yieu Chyan,1 Zhe Wang,1 Winston Wang,1 James M. Tour1,2,3,4* 1Department

of Chemistry, 2Smalley-Curl Institute and the NanoCarbon Center, 3Department of

Materials Science and NanoEngineering, and 4Department of Computer Science, Rice University, 6100 Main Street, Houston, Texas 77005, United States Email: [email protected]

Abstract Triboelectric nanogenerators (TENGs) show exceptional promise for converting wasted mechanical energy into electrical energy. This study investigates the use of laser-induced graphene (LIG) composites as an exciting class of triboelectric materials in TENGs. Infrared laser irradiation is used to convert the surfaces of the two carbon sources, polyimide (PI) and cork, into LIG. This gives the bilayer composite films the high conductivity associated with LIG and the triboelectric properties of the carbon source. A LIG/PI composite is used to fabricate TENGs based on conductor-to-dielectric and metal-free dielectric-to-dielectric device geometries with open-circuit voltages > 3.5 kV and peak power > 8 mW. Additionally, a single sheet of PI is converted to a metal-free foldable TENG. The LIG is also embedded within a PDMS matrix to form a singleelectrode LIG/PDMS composite TENG. This single-electrode TENG is highly flexible, stretchable, and was used to generate power from mechanical contact with skin. The LIG composites present a class of triboelectric materials that can be made from naturally occurring and synthetic carbon sources.

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Keywords laser-induced graphene, triboelectric nanogenerator, TENG, cork, polyimide, wearable sensors

Wearable sensors and devices have received much attention in the recent decades for applications such as monitoring human health and surrounding environmental conditions.1–5 It is desirable to reduce weight and volume associated with energy storage media, such as batteries, that power wearable sensors and devices. One strategy to achieve this is by incorporating portable energy harvesting devices to make wearable devices self-powering. Triboelectric nanogenerators (TENGs) provide a method to convert mechanical movement to electrical energy by exploiting the triboelectric effect.6–9 Specifically, when two materials are brought into contact with one another, electrons can be exchanged from a tribo-positive to tribo-negative material due to factors including differences in material electronegativity, composition, environmental conditions, and contact process.10 Flexible11 and rigid12 TENGs can operate in a variety of configurations; however, relevant to this study, TENGs are often operated in contact-separation mode13 and single-electrode mode.14 In general, the TENG performance is characterized by both the voltage induced by mechanical contact and the output current. The power output characteristics13,15 can be described by the V-Q-x relationship, in which V is the voltage between electrodes, Q is the transferred charge, and x is the separation distance. TENGs should ideally pair materials on the opposite ends of the triboelectric series10 to generate a large static charge upon contact that results in a sufficient potential generation (V) when the tribo-materials begin to separate (x). The amount of charge (Q) transferred when TENG layers are brought in and out of contact is driven by the induced potential. High conductivity electrodes are desirable to maximize output current. To date, many electrodes have been applied to triboelectric materials by vacuum deposition techniques of precious metals.


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These deposition techniques can be costly, and to be commercially viable, they are generally reserved for high-end devices. Additionally, metal adhesion on PI (Kapton), a popular tribonegative material, is poor.16,17 This poses a difficulty since TENGs should be able to withstand thousands of mechanical flexing cycles without significant degradation in performance. Carbonbased materials such as graphene18 and carbon nanotubes19 have been used as electrodes in TENGs, but they require methods such as spin coating or vacuum filtration to form a continuous film. Graphene electrodes for TENG devices have also been made through chemical vapor deposition (CVD) growth on copper films, followed by spin coating the copper with the desired polymer, and then removing the copper through a wet etching process.20 However, CVD, spin coating, and wet processes can be costly, time consuming, and require specialized equipment. Composite films with carbon-based triboelectric materials and laser-induced graphene (LIG) provide a class of durable materials that can be promising for application in TENGs. LIG is formed by irradiating a carbon source with a laser, which photothermally converts the carbon to sp2-hybridized carbon.21,22 Carbon sources for the formation of LIG have been demonstrated using synthetic materials such as PI and poly(etherimide)21 as well as naturally occurring materials such as wood, cotton, cork, and other materials of high lignin content.22 Temperatures of greater than 2500 °C under laser exposure results in the rapid outgassing of non-carbon elements that gives the LIG a porous micro-structure and high surface area. Conveniently, LIG can be fabricated by irradiating carbon sources with a CO2 (10.6 m) laser, which is commonly found in machine shops as a laser cutter. The process is often done in open air. This enables the direct-writing of LIG structures in 2D and 3D geometries.23 Due to its physical robustness, thermal and chemical stability, ease of synthesis, and conductivity, LIG can be directly written into a variety of electronic device patterns. The ability to readily make a low-cost nonmetallic conductor on an insulating film


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has led to the application of LIG in a variety of recent technologies, such as microsupercapacitors,24,25 water splitting electrodes,26 water filtration,27 wearable sensors,28 strain sensors,29 soft actuators,30 photodetectors,31 and gas sensors,32 to name a few. The ease of generating LIG on substrates such as PI, a popular tribo-negative material, makes it attractive for generating flexible TENGs. Additionally, LIG can be easily embedded in other materials, such as PDMS and PTFE, to form a wide variety of LIG-based composites.28,33,34 In this study, multiple architectures for TENGs using LIG composites are demonstrated. Specifically, PI and cork are lased to form bilayer composite films of LIG with carbon precursors. The composites exhibit the triboelectric properties of the carbon source and the high conductivity of LIG to form a high performance TENG electrode. The LIG/PI composite film is demonstrated as a high-performance electrode in a standard dielectric-to-conductor TENG, and metal-free dielectric-to-dielectric TENGs, where the type of TENG refers to the interface at which contact electrification occurs. These devices demonstrated power outputs of 2.0 and 0.76 Wm-2, respectively. Additionally, LIG/PDMS composites are fabricated to form stretchable and flexible single-electrode TENGs that can generate power by contact with skin. The LIG/PDMS composite in a shoe insert geometry is made to demonstrate that LIG-based composites can be used as effective TENG materials for harvesting mechanical energy.

Results and Discussion Figure 1a shows a schematic for the formation of LIG from a carbon precursor. Specifically, the carbon source is irradiated with a 10.6 µm CO2 laser that photothermally converts the near-surface to graphene. Characteristics of these films have been extensively studied.21,22 The porous microstructure of the LIG is shown in Figure 1b. Surface-bound percolating flakes of LIG provide


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a continuous network that exhibits a relatively low sheet resistance and can be used as a metal-free electrode material, where the sheet resistance is highly dependent upon laser conditions but generally are in the range of 5-115 /sq.35,36 Figure 1c and 1d show photographs of LIG that has been synthesized using a piece of PI and cork as carbon sources, respectively. The LIG film is adhered to the underlying PI and cork and is robust to mechanical bending and contact. The LIG thickness is ~ 35 µm. Cross-section images can be found in the Supporting Information Figure S2. Sheet resistances for the PI- and cork-derived LIG are ~ 40 E) > and ~115 E) >& respectively. Figure 1e and 1f shows the Raman spectra for the LIG films synthesized from PI and cork. Notably, a sharp 2D peak is exhibited at ~ 2690 cm-1, indicating that the film is comprised of LIG. The 2D graphene peak is upshifted in comparison to graphite, thus confirming formation of graphene as opposed to simply graphite. Additionally, LIG can be generated by lasing a carbon source and directly transferring to another material to form a LIG-based composite material.34 A schematic and additional details for the composite fabrication can be found in the Supporting Information and Methods section. The LIG is robust to mechanical flexing, and the electrical properties as a function of bending and compression cycles are reported in the Supporting Information Figure S3. Additionally, encapsulated LIG adhesion is shown to be more resistant to delamination in comparison to a metallic thin film as shown in Supporting Information Figure S4.


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Due to the high performance of the LIG/PI composite, other TENG architectures using this material were further explored. PI and PDMS are common materials used in the fabrication of TENGs because they lie toward the negative end of the triboelectric series, which indicates that they are prone to gain electrons, and hence negative charge, when brought into contact with materials that are relatively tribo-positive. Therefore, LIG/PI and LIG/PDMS bilayer composites can be used to form a variety of TENGs that are flexible, durable, and require minimal fabrication steps. Figure 3 shows a schematic for a standard dielectric-to-conductor TENG that uses a LIG/PI film as one electrode (LIG) and dielectric layer (PI) and then Al as a triboelectrically positive conductor material. Again, the fabrication of the LIG layer requires no vacuum processing, which is otherwise common for metal deposition, and it also avoids the need of solution suspension or vacuum filtration techniques typically needed to form electrodes from other 2D materials. When the PI layer (dielectric) is brought into contact with the Al (conductor), contact electrification (V) occurs as charges are exchanged at the interface. As the layers are separated (x), electrical potential will cause current to flow (Q) from the triboelectrically positive Al, through an external load to the LIG electrode via electrostatic induction. As the films are brought back into contact, current will flow back through the external load to the Al layer to balance the charge at the Al/PI interface. Therefore, repeated contact and separation between the two electrodes of the TENG generates power with an AC behavior. A detailed theoretical description of the V-Q-x relationship for power generation from TENGs has been provided by previous works.14,15


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Initial state LIG PI Aluminum

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Figure 3. Mechanism for power generation by dielectric-to-conductor TENG using a LIG/PI bilayer composite film. See text for details of mechanism.

Figure 4 shows the output characteristics of the LIG/PI/Al based TENGs. Open-circuit voltage (VOC) of ~ 3.5 kV is generated during operation for a device with a contact area of ~ 36 cm2, as shown in Figure 4b. Short-circuit current (ISC) of ~ 60 µA is also generated (Figure 4c). The average voltages and currents generated as a function of load resistance are reported in Figure 4d and exhibit typical behavior in which VOC increases and ISC decreases with increasing load resistance. The generated power was determined (P = VI) for the TENG and reported in Figure 4e. The peak power output was approximately 8.5 mW (~ 2.4 Wm-2) at a resistance of 70 1E Peak power has been experimentally shown to occur due to resistance matching between the TENG and the load, thus indicating internal resistance is on the order of 70 1E for this TENG.37 Comparison to recent literature shows that the power output generated by the LIG/PI/Al based TENG is similar in comparison to other contact separation TENGs38 and other classes of conductive tribo-negative 9 ACS Paragon Plus Environment

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materials.39 This power was generated with a flat surface TENG, and additional surface texturing could easily increase peak power output by effectively increasing the surface area.8 Additionally, multilayered TENGs have also been fabricated to increase the power output of TENGs, and a similar approach could be applied to the LIG composite materials.9,40 Further optimization of the lasing conditions to increase the electrical conductivity of the LIG offers another route to increase power output, as well as to decrease the thickness of composite materials which serve as dielectric layers.41 The TENG readily simultaneously powered 60 blue and red LEDs in series, which was the maximum number of LEDs tested. Additional details about the LED circuit and Web Enhanced Object S1 can be found in the Supporting Information and Figure S5. The LIG/PI composites demonstrate impressive performance in TENGs because it uses the tribo-negative properties of the PI and simultaneously acts as a carbon precursor for the laser-induced formation of LIG electrodes.


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Dielectric-to-dielectric TENG In order to fabricate truly metal-free TENGs, other LIG-based architectures are demonstrated. Figure 5a shows the structure for a LIG-based TENG that used a single-folded sheet of LIG/PI bilayer composite. To fabricate the device, LIG was lased on both sides of a PI film and then spraycoated with polyurethane (see Methods for additional details). The polyurethane coats the LIG, prevents damage to the film upon contact, and acts as a tribo-positive material in comparison to PI. Therefore, this device operates as a dielectric-to-dielectric TENG since contact electrification occurs at the PI/polyurethane interface. A schematic showing the power generation mechanism is shown in the Supporting Information Figure S6. Specifically, when the tribo-negative PI is brought into contact with the relatively tribo-positive polyurethane, electrons are transferred to the PI. When the electrodes are separated, electrostatic induction causes current to flow through an external load to balances the charges. The TENG is operated by folding and forcing contact between the two sides of the TENG (Figure 5b). Figure 5c shows that a VOC of ~ 1 kV is generated for the 9 cm2 device. The device was bent/contacted 5,000 times to demonstrate reliability and durability, and the VOC experienced no statistically relevant variation. This device powered ~ 40 LEDs in series (the maximum number of LEDs tested). Web Enhanced Object S2 of the device in operation can be found in the Supporting Information. Additionally, full characterization as a function of load resistance for the rigid device with the same architecture is provided in the Supporting Information Figure S7. ISC and VOC of 40 µA and 1.75 kV were achieved with a maximum power output of ~ 2.75 mW at 40 1E load resistance.


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therefore the LIG/PDMS composite was used to form a STENG which behaves as a conductive tribo-negative material. The LIG and PDMS sides of the bilayer composite both exhibit highly tribo-negative behavior, as shown in Supporting Information Figure S8. The robust LIG/PDMS platform is far easier to fabricate than the typical vacuum-deposited metallic electrodes on PDMS that easily crack and experience poor adhesion.43 Figure 6a shows the operating mechanism for the device. The LIG layer is connected to ground through an external load. When the PDMS layer is contacted by a more tribo-positive material, such as skin, negative static charge accumulates at the surface. When the external material is removed, current is generated via electrostatic induction. Equivalently, when the external material approaches the STENG, electrostatic induction drives current in the opposite direction. To collect output data, a simple 36 cm2 square geometry of LIG was embedded within the PDMS as shown in Figure 6b. With the direct-write capabilities of the commercial laser cutter, more complex geometries can be tailored for other applications. The sheet resistance of the embedded LIG layer is approximately 120 E) >& and it shows no variation after 5,000 bending cycles or 5000 contacts at a force of 0.5 N (Figure S3). Figure 6c shows the VOC for the STENG when contacted with a variety of materials that are tribo-positive in comparison to PDMS. Skin and nitrile demonstrate the highest VOC, which is promising for making STENGs that generate power from human contact. VOC and ISC as a function of load resistance when contacted with nitrile are reported in Figure 6d and the power output is reported in Figure 6e. A peak power output of 1.2 mW (0.33 Wm-2) is exhibited at a load resistance of 70 1E Web Enhanced Object S1 of the STENG powering a series LEDs can be found in the Supporting Information. Additionally, LIG can be embedded in tribo-positive materials, such as melamine (Supporting information Figure S9). This demonstrates that LIG can be embedded in many materials for application in TENGs.


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Many silicone-based elastomers and rubbers are highly tribo-negative, and are commonly incorporated as support materials in clothing, such as shoe soles. To demonstrate that the LIG/PDMS composite STENG can be used to generate power from human movement, a STENG shoe sole insert was made (Figure 7a) and incorporated into a flip-flop for electrical power generation (Figure 7b). The direct-write capabilities of the commercial laser cutter enables simple fabrication of a geometry that can be used as a shoe insert. A circuit diagram for the power generating flip-flop is shown in Figure 7c. Specifically, the STENG was connected to a bridge rectifier which provides unipolar charging to a capacitor. The bridge rectifier was also connected to a copper pad adhered to the bottom of the sandal which acted as an Earth ground. The connections to the STENG and copper pad are shown by the white and yellow alligator clips, respectively, in Figure 7d. To demonstrate the effects of running or walking with a bare heel on the sole STENG, the flip-flop embedded STENG was contacted at a frequency of 2 Hz with bare skin. Figure 7e shows the charging performance of 0.1 µF, 1 µF and 100 µF capacitors that were charged with the STENG sole insert. The STENG stored electrical energy on a capacitor at a rate of ~ 2.4 × 10-2 mJ/s, hence demonstrating potential as a portable power generator. The STENGembedded flip-flop was worn on a ~1 km walk (Figure 7f) to further demonstrate the applicability of the LIG/PDMS composite as a portable power generation system. The device generated a peak VOC of ~760 V by contact with skin as shown in Figure 7e. By walking ~ 1 km, 0.22 mJ of electrical energy was stored on a capacitor. This demonstrates that the wearable LIG-based STENG generates sufficient electrical power for low power consumption devices and sensors. Walking ~ 2 km produced a dirty STENG that did not store energy as well (Figure S10), hence it is important for the surfaces to be cleaned or encapsulated for maximum efficiency.


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~ 0.5 N. (f) Map showing route walked at Rice University to charge capacitor with the STENG. (g) VOC generated from TENG embedded within flip-flop.

Conclusions In summary, LIG-based composites have been demonstrated as an exciting class of robust materials for the easy fabrication of TENGs. The LIG is simply incorporated into composites with tribo-negative materials by direct laser conversion on PI, or by embedding LIG within PDMS. LIG can also be formed from natural carbon-based materials, such as cork, to enable additional TENG architectures. Using the LIG/PI bilayer composite, contact-separation TENGs were fabricated using minimal processing steps. A metal-free flexible TENG was fabricated from a single sheet of PI by lasing opposite sides and spray coating the LIG with polyurethane. This enabled the formation of a dielectric-to-dielectric TENG that demonstrated reliable performance over thousands of bending cycles. LIG was also embedded within PDMS to form a flexible and stretchable single electrode STENGs. This STENG generated impressive power output when contacted with skin and was demonstrated as a portable power system. LIG-based composites incorporated in TENGs show promise for output, flexibility, and the durability required for portable energy harvesters.

Methods LIG synthesis and characterization LIG on PI was produced by irradiation of McMaster-Carr Kapton PI 127 µm film with a 10.6 µm 75-Watt CO2 pulsed laser mounted in a Universal Laser Systems XLS10MWH laser cutter at a scan rate of 15 cm/s and 1% duty cycle. To generate the LIG on cork, the cork was lased at a scan


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rate of 30 cm/s and 3-4% duty cycle. For all laser exposures, a 1000 PPI image density was employed. After synthesis of the composite film, durability testing ensued. The LIG/PI composite was subjected to 5000 bending cycles at a curvature radius of 4 cm, and the sheet resistance was measured at every 100 cycles. The device then underwent 5000 0.50 N compression cycles, and sheet resistance was again measured every 100 cycles. The same testing was conducted for LIG/PDMS composites. LIG/PI composite TENG fabrication The 60 × 60 mm TENG originated from a single piece of PI, of which a single side was laser irradiated to form a conductive LIG electrode. The TENG was mounted on a glass plate with double-sided tape for testing. For testing the LIG/PI/Al dielectric-to-conductor device, aluminum foil was used as a tribo-positive electrode. Contacts were made to the TENGs by clamping with a standard electrical alligator clip. Flexible metal-free LIG/PI composite TENG The 60 × 60 mm TENG was fabricated by lasing two pieces of PI, of which a single side was irradiated on each to form two conductive LIG electrodes. The top TENG electrode was mounted on a glass plate with double-sided tape for testing. The bottom LIG/PI composite was coated with Minwax (0 27426 33050) Clear Gloss Fast-Drying Polyurethane Aerosol spray to enhance resistance to abrasion. Hence the device acts as a dielectric-to-dielectric TENG since charges are exchanged at the polyurethane/PI interface when brought into contact. A flexible TENG of similar architecture was fabricated on a single continuous piece of PI. The flexible TENG was fabricated by lasing 30 × 30 mm LIG squares as electrodes on each side


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of PI, coating the LIG with Minwax Fast-Drying Polyurethane Aerosol spray to enhance resistance to abrasion, and then folding the piece of PI to allow for PI-LIG contact. Single electrode LIG/PDMS composite TENG A single electrode TENG was made from a LIG/PDMS composite. This composite was fabricated by the method shown in Figure S1. Specifically, PI was lased at a scan rate of 15 cm/s and 5% duty cycle at 1000 PPI image density to form a layer of LIG. A two-part PDMS (Dow SYLGARDTM 184 Silicone Elastomer) was then cast in a mold on top of the LIG/PI film.42 The PDMS was allowed to cure for 2 h at a temperature of 90 °C. After cooling, the PDMS was peeled from the PI, thus transferring the LIG to the PDMS film. The PDMS film thickness was approximately 0.5 mm. The LIG/PDMS composite was treated as a single-electrode TENG TENG characterization The current and voltage generated by ~ 2 N of force was recorded for 1, 10, 40, 70, 100, 200, and 300 1E load resistance that was added in series with ceramic resistors. The resulting data was plotted as power vs load resistance curves. Voltage data was collected using a Hantek DSO4102C digital storage oscilloscope, and current data was recorded with an Agilent B1500A semiconductor analyzer. As a demonstration, the TENG was connected to a breadboard with a bridge rectifier and used to power a series of 1.9 - 3.0 V LEDs.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional Methods, Schemes and Figures for TENGs and STENGs (PDF). Web Enhanced Object S1: LEDs powered from LIG/PI/Al TENG: LEDs_LIG-PI-AI.avi (AVI) Web Enhanced Object S2: Flexible TENG: Flexible TENG Video.avi (AVI)


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Web Enhanced Object S3: Single electrode TENG: Single-electrode TENG.avi (AVI)

Acknowledgements We thank the Air Force Office of Scientific Research for support (FA9550-14-1-011). The use of the STENG in the flip-flop in this work did not meet the definition of human subject research at Rice University, therefore no IRB protocol approval was needed. We gratefully acknowledge the support of Universal Laser Systems for their generously providing the XLS10MWH laser system with Multiwave Hybrid™ technology that was used for this research. The Universal Laser Systems staff kindly provided helpful advice.

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