Letter www.acsami.org
Triboelectricity Generation from Vertically Aligned Carbon Nanotube Arrays Moses Oguntoye,† Michael Johnson,‡ Lawrence Pratt,† and Noshir S. Pesika*,† †
Department of Chemical and Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States ‡ Department of Chemistry, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States S Supporting Information *
ABSTRACT: We explore the use of vertically aligned carbon nanotube (VACNT) arrays as an electrode in a triboelectric nanogenerator (TENG) that harvests mechanical energy and converts it to electrical energy. When VACNT arrays 1 cm2 in area are mechanically contacted with PET and PTFE counter electrodes in vertical contact−separation mode, currents up to 0.16 and 0.21 μA and voltages up to 1.42 and 3.20 V are obtained, respectively. The VACNT TENG output remains stable even after more than 20 000 continuous contact cycles. A 0.47 μF capacitor is successfully charged to 4.5 V in 60 s using a VACNT-PTFE triboelectric nanogenerator (TENG) prototype. KEYWORDS: energy harvesting, triboelectricity, self-powering, nanogenerator, vertically aligned carbon nanotubes
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generated current and associated open-circuit voltage is dependent on a number of factors including the relative positions of the contacting materials on the triboelectric series, contact force, frequency of contact, contact area (or roughness of the contacting surfaces).7 The triboelectrification principle together with the many different perpetual contact motions involved in everyday activities make the TENG a suitable and sustainable potential source of energy for systems and applications that require low power. Some examples of energy harvesting using the TENG include the design of the shoe insole TENG for generating electricity from the human walking motion developed by Hou and co-workers8 as well as the printed circuit board (PCB) integrated TENG for harvesting the energy from keyboard typing.9 Among many other challenges to the development of the TENG, one major challenge is the choice of materials. The triboelectric series serves as a guide but is largely qualitative in its rankings of materials. It has been suggested that using engineered nanomaterials and nanocomposites can lead to TENGs with high instantaneous power.10 Vertically aligned carbon nanotube (VACNT) arrays are particularly attractive as potential electrode materials for application in triboelectric nanogenerators because of their nanoscale roughness and their excellent mechanical properties.11 Both features are desirable for materials undergoing repeated compression cycles to generate triboelectricity. In addition, VACNT arrays are stable
urrent and anticipated energy supply shortages have been the driving force for extensive research into the design and integration of new types of energy systems that draw energy from their normal operation or from ambient sources. These systems, also known as energy harvesters, have been well-studied and shown to be mainly dependent on the scientific principles of piezoelectricity, electrostatic induction and electromagnetic induction.1−3 Photoelectric and thermoelectric energy harvesters have also attracted some interest.4,5 Recently, triboelectrification has shown some promise with respect to its ability to produce sufficient amounts of energy to power small devices. Systems made to harvest triboelectric energy were first developed by the Wang group in 2012 and this led to the creation of the triboelectric nanogenerator (TENG).6 The TENG works based on the principle of the triboelectric effect, which is the generation of electrical surface charges when two different materials are brought into frictional contact. In the TENG, two different materials are brought in full contact and this creates a shared electron cloud due to adhesion. When the materials are fully or partially separated, the electron cloud is broken in an imbalanced manner thereby producing triboelectric charges on the surfaces of these materials.7 If an external circuit connects these two materials, a current will flow to re-establish electron balance. Repeating the contact and separation motion of the materials can generate an alternating current that can be used to power electronic devices.7 Materials are ranked on the triboelectric series according to their propensity to accept or donate electrons to the shared cloud in the triboelectrification process.7 The magnitude of the © XXXX American Chemical Society
Received: September 26, 2016 Accepted: October 4, 2016
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DOI: 10.1021/acsami.6b11457 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 2a, b show the current and voltage output of the PETVACNT TENG. A 0.16 μA maximum short-circuit current and a 1.42 V maximum open-circuit voltage are obtained. This corresponds to a maximum instantaneous power of 0.23 μW. For the PTFE-VACNT TENG system, shown in Figure 2c, d, a maximum short-circuit current of 0.21 μA and maximum opencircuit voltage of 3.20 V are measured respectively corresponding to a maximum instantaneous power of 0.67 μW. These values were obtained when the electrodes are contacted using a 4 N load at a 0.14 Hz frequency with a 40 mm/s separation speed. VACNT arrays are capable of withstanding compressive forces much greater than the load exerted in this study, at higher frequencies and for multiple cycles without failure.11 However, because of the limitations of the tribometer used in this study to apply predetermined loads, the contact force was limited to 4 N. Despite the relatively small force and frequency used in our experiments, we obtain maximum values of both short circuit current and open circuit voltage that are comparable to results using some other electrodes in previous studies. Table S1 provides a comparison of the output currents and voltages of the VACNT TENG and other previously studied unstacked TENG assemblies. Moreover, as also shown in the Video S1, the PTFE-VACNT TENG is capable of reaching about 15 V output voltage by establishing contact manually at higher frequency (roughly 4 N and a higher frequency of about 1 Hz). Under a wide range of resistive electrical loads, we see that the VACNT TENG is robust as shown by an almost constant current. The voltage obtainable rises with resistance as expected. The load curves showing the behavior of the TENG across a range of mechanical and electrical loads are shown in Figures S1 and S2, respectively. Because VACNT arrays are good electrical conductors, the working principle of the VACNT TENG is expected to follow the working principle of the polymer−metal TENG described elsewhere.14 The voltage measured represents the electrical potential difference between the two electrodes once the positive and negative triboelectric charges are separated.15 A schematic of the proposed mechanism of current tribogeneration in the VACNT TENG is shown in Figure 3. At the onset, the two electrodes are in full contact and the current is effectively zero between points 1 and 2. This contact results in adhesion between the two electrodes such that when they are partially or fully separated, there will be a net positive charge on one electrode’s surface (VACNT array) and a net negative charge on the other electrode’s surface (PET and PTFE are well-known triboelectrically negative materials7) due to an imbalanced sharing of the electrons in the adhesive electron cloud. At the onset of separation (point 2 to 3), an instantaneous current peak is observed representative of the flow of electrons (through an external circuit) from the current collector of the negative material to the positive material to offset the potential difference. This flow of electrons, however, produces a deficit of electrons on the outer surface of the negative material leaving it with a net positive charge (points 3 to 4). Thus, the two surfaces of the negative electrode (which is an insulator) have net charges of opposite signs (points 3 to 4). When contact is re-established (points 4 to 1), another instantaneous peak is seen because electrons flow again through the VACNT array (acting just as a conductor now), and also through the external circuit in the opposite direction from one side of the negative material to the other to re-establish charge equilibrium. This completes one cycle. The two current peaks
and inert in many environments and are therefore not susceptible to degradation.12,13 In this work, we demonstrate the potential application of VACNT arrays as TENG electrodes. We show that VACNT arrays are triboelectrically active given their significant current and voltage outputs with both polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) counter electrodes, which are well-known triboelectrically active materials. Also, to demonstrate its potential application, a 0.47 μF capacitor is charged using an assembled VACNT TENG device. To the best of our knowledge, this is the first investigation into the potential application of VACNT arrays in the generation of triboelectricity. VACNT electrodes are fabricated directly on stainless steel foils acting as current collectors. Counter electrodes for the VACNT TENG were fabricated by coating a thin gold current collector layer on PTFE film for the PTFE electrode and ITO current collector on PET film for the PET electrode. Current and voltage measurements were made while using a CETR universal material tester (tribometer) to achieve repeated contact between the two electrodes. More details on the preparation of electrodes and characterization methods are included in the Supporting Information. The VACNT TENG is assembled as shown in the schematic in Figure 1a. The foam tape spacers, which are double-sided
Figure 1. (A) Schematic illustration of a VACNT TENG device in operation; (B) photograph of assembled TENG on a glass slide; (C) SEM side-view image of a VACNT array grown for 90 min; (D) TEM image of a single CNT.
adhesive tapes, keep the two electrodes from sticking together after contact while also ensuring equal spacing between the two electrodes during separation. A continuous oscillating vertical mechanical motion depicted by the arrows represents the source of the mechanical energy (i.e., the tribometer) that is converted to electricity by the TENG. The entire assembly is placed on a glass slide as shown in Figure 1b and the two electrodes have a contact area of about 1 cm2. SEM and TEM images of the as-grown VACNT arrays are shown in Figure 1c, d, respectively. The SEM image shows that the VACNT are self-aligned and are grown to a length of about 1 mm. The TEM image shows that the individual nanotubes in the VACNT are multiwalled in nature with a diameter of about 20 nm. B
DOI: 10.1021/acsami.6b11457 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 2. (A) Current and (B) voltage measurements for PET-VACNT TENG. (C) Current and (D) voltage measurements for the PTFE-VACNT TENG. The contact area between the electrodes was 1 cm2 and the frequency was 0.14 Hz. Applied load was 4 N and contact and retraction speeds were 5 and 40 mm/s, respectively.
Figure 3. Schematic illustration of the operation of a TENG and the mechanism of tribocurrent generation.
have opposite signs because the electrons flow in opposite directions. The separation peak current is larger than the contact peak current. This is expected because the electrode separation is happening at a faster rate (40 mm/s on the tribometer) than the contact (5 mm/s on the tribometer) and thus the flow rate of electrons during separation is higher than during contact resulting in higher currents.15 However, the amount of charge transferred during separation and during contact are effectively equal as obtained through the integration of one cycle of current peaks shown in the Figure S3. Moreover, we studied the ability of the VACNT TENG to maintain its current and voltage output levels for multiple compression cycles. Durability is also an important factor to consider when designing TENGs. Our VACNT TENG was
tested for more than 20 000 contact cycles and there was no significant reduction in performance observed. We however observed a bending of the tips of the individual nanotubes but this likely had a positive effect on the performance since it ensures higher surface area of contact. The results of these tests are shown in Figures S4 and S5. To demonstrate the practical application of the assembled PTFE-VACNT TENG, we charged a 0.47 μF capacitor by manual contact. The TENG was connected to the capacitor according to the electric circuit shown in Figure 4a. The charging curve shown in Figure 4b reveals that the capacitor is charged up to about 4.5 V in 1 min. In summary, we have tested and shown that VACNT arrays are triboelectrically active and are viable candidates for use in C
DOI: 10.1021/acsami.6b11457 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
(2) Torres, E. O.; Rincón-Mora, G. A. Electrostatic EnergyHarvesting and Battery-Charging CMOS System Prototype. IEEE Trans. Circuits Syst. 2009, 56 (9), 1938−1948. (3) Yang, B.; Lee, C.; Xiang, W.; Xie, J.; Han He, J.; Kotlanka, R. K.; Low, S. P.; Feng, H. Electromagnetic Energy Harvesting from Vibrations of Multiple Frequencies. J. Micromech. Microeng. 2009, 19 (3), 35001. (4) Zhang, Y.; Wang, H.; Liu, Z.; Zou, B.; Duan, C.; Yang, T.; Zhang, X.; Zheng, C.; Zhang, X. Optical Absorption and Photoelectrochemical Performance Enhancement in Si Tube Array for Solar Energy Harvesting Application. Appl. Phys. Lett. 2013, 102 (16), 163906. (5) Hudak, N. S.; Amatucci, G. G. Small-Scale Energy Harvesting through Thermoelectric, Vibration, and Radiofrequency Power Conversion. J. Appl. Phys. 2008, 103, 101301. (6) Fan, F. R.; Tian, Z. Q.; Lin Wang, Z. Flexible Triboelectric Generator. Nano Energy 2012, 1 (2), 328−334. (7) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors. ACS Nano 2013, 7 (11), 9533−9557. (8) Hou, T. C.; Yang, Y.; Zhang, H.; Chen, J.; Chen, L. J.; Lin Wang, Z. Triboelectric Nanogenerator Built inside Shoe Insole for Harvesting Walking Energy. Nano Energy 2013, 2 (5), 856−862. (9) Han, C.; Zhang, C.; Tang, W.; Li, X.; Wang, Z. L. High Power Triboelectric Nanogenerator Based on Printed Circuit Board (PCB) Technology. Nano Res. 2015, 8 (3), 722−730. (10) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology and Self-Powered Sensors − Principles, Problems and Perspectives. Faraday Discuss. 2014, 176, 447−458. (11) Bradford, P. D.; Wang, X.; Zhao, H.; Zhu, Y. T. Tuning the Compressive Mechanical Properties of Carbon Nanotube Foam. Carbon 2011, 49 (8), 2834−2841. (12) De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339 (6119), 535−539. (13) Wang, J. Carbon-Nanotube Based Electrochemical Biosensors: A Review. Electroanalysis 2005, 17 (1), 7−14. (14) Wang, S.; Lin, L.; Wang, Z. L. Nanoscale-Triboelectric-Effect Enabled Energy Conversion for Sustainable Powering of Portable Electronics. Nano Lett. 2012, 12 (12), 6339−6346. (15) Zhu, G.; Pan, C.; Guo, W.; Chen, C. Y.; Zhou, Y.; Yu, R.; Wang, Z. L. Triboelectric-Generator-Driven Pulse Electrodeposition for Micropatterning. Nano Lett. 2012, 12 (9), 4960−4965.
Figure 4. (A) Electrical circuit for charging capacitor using TENG; (B) charging curve of 0.47 μF capacitor. TENG contact area was 1 cm2 and frequency of contact was approximately 1 Hz.
fabricating TENGs. We demonstrated the potential of applying VACNT arrays as electrodes in TENG systems with two different counter electrodes (PET and PTFE) culminating in charging a 0.47 μF capacitor to 4.5 V in approximately 1 min. Future work involves exploring sliding contact and actual mechanical system integration to fully harness the potential of the VACNT TENG.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11457. Detailed description of experimental methods, comparison of our VACNT TENG to other nonstacked TENG assemblies, mechanical and electrical load capabilities, as well as a visual and performance characterization of the TENG over 20 000 cycles (PDF) Video S1 showing TENG working (AVI)
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AUTHOR INFORMATION
Corresponding Author
*Email:
[email protected]. Tel: 614 316-2744. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSF EPSCOR Cooperative Agreement EPS-1003897, with additional support from the Pfund program of the Louisiana Board of Regents.
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REFERENCES
(1) Erturk, A.; Hoffmann, J.; Inman, D. J. A Piezomagnetoelastic Structure for Broadband Vibration Energy Harvesting. Appl. Phys. Lett. 2009, 94 (25), 254102. D
DOI: 10.1021/acsami.6b11457 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
