Natural Leaf Made Triboelectric Nanogenerator for ... · A bionic Leaf-TENG was fabricated with filter paper and sodium chloride solution to illustrate the mechanism. The filter paper

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Natural Leaf Made Triboelectric Nanogenerator for Harvesting Environmental Mechanical Energy

Yang Jie, Xueting Jia, Jingdian Zou, Yandong Chen, Ning Wang,* Zhong Lin Wang,* and Xia Cao*

DOI: 10.1002/aenm.201703133

mechanical energy exists extensively in our daily life, such as vibration, wind, and waves.[3] However, most of this energy is wasted and long-neglected. For one thing, environmental mechanical energy is random in amplitude, low in frequently, widely distributed, unstable, and random. Therefore, the design of effective and envi-ronmental-friendly energy harvesters still has lots of technical and economic chal-lenges.[4] Besides, IoT has made a great progress with the development of sensor, wireless communication, and micro/nano-technology.[5] IoT enables collecting and processing data among the whole network, which consists of huge numbers of widely distributed sensors or smart devices. But how to supply adaptive and sustainable

energy for nodes of IoT is a big task even though each node only needs a tiny power. According to the characteristic of wide distribution, harvesting environmental mechanical energy is an attractive approach to powering the nodes of IoT.[6]

Up to now, several technologies have been utilized to harvest various mechanical energy mainly based on electromagnetic, electrostatic, piezoelectric, and triboelectric mechanism.[5,7] In 2012, the triboelectric nanogenerator (TENG) as a new approach was first invented to harvest mechanical energy by coupling contact electrification and electrostatic induction.[8] With the fast development in recent years, TENG has shown tremendous application in the fields of micro/nanoenergy, self-powered sys-tems, and blue energy.[9] On the basis of different mechanical energy, there are four basic working modes of TENG: vertical contact-separation mode, lateral sliding mode, single electrode mode, and freestanding triboelectric-layer mode.[9b,10] Different modes of TENG can improve the adaptability, but more impor-tant advantage of TENG is abundant choices of materials. Each material has its electron affinity, and the contact electrification relies on the difference of two materials.[11] In order to optimize the adaptability of TENG, lots of studies have been carried on to seek new materials that are flexible, transparent, degradable, or recyclable etc.[12]

In the ambient environment, the vibration is a prevalent mechanical energy source and the leaves are common dis-tributed materials. Here, we presented a novel leaf assem-bled TENG (Leaf-TENG) with environmental friendliness by using natural leaves as the triboelectric material and electrode. The Leaf-TENG has natural and simple structure with widely green raw materials, which improves the adaptability as envi-ronmental mechanical energy harvester. When the natural

Distributed environmental mechanical energy is rarely collected due to its fluctuating amplitudes and low frequency, which is usually attributed as “random” energy. Considering the rapid development of the Internet of things (IoT), there is a great need for a large number of distributed and sustainable power sources. Here, a natural leaf assembled triboelectric nanogenerator (Leaf-TENG) is designed by utilizing the green leaf as an electrification layer and electrode to effectively harvest environmental mechanical energy. The Leaf-TENG with good adaptability has the maximum output power of ≈45 mW m−2, which is capable of driving advertising LEDs and commercial electronic temperature sensors. Besides, a tree-shaped energy harvester is integrated with natural Leaf-TENG to harvest large-area environmental mechanical energy. This work provides a new prospect for distributed and environmental-friendly power sources and has potential applications in the IoT and self-powered systems.

Dr. Y. Jie, X. T. Jia, Prof. X. CaoResearch Center for Bioengineering and Sensing TechnologyBeijing Key Laboratory for Bioengineering and Sensing TechnologySchool of Chemistry and Biological EngineeringBeijing Municipal Key Laboratory of New Energy Materials and TechnologiesUniversity of Science and Technology BeijingBeijing 100083, ChinaE-mail: [email protected]. D. Zou, Y. D. Chen, Prof. Z. L. WangBeijing Institute of Nanoenergy and NanosystemsChinese Academy of SciencesNational Center for Nanoscience and Technology (NCNST)Beijing 100083, ChinaE-mail: [email protected]. N. WangCenter for Green InnovationSchool of Mathematics and PhysicsUniversity of Science and Technology BeijingBeijing 100083, ChinaE-mail: [email protected]. Z. L. WangSchool of Material Science and EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332-0245, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201703133.

Triboelectric Nanogenerators

Besides the well-known energy need at large scale, the develop-ment of sensor networks and Internet of things (IoT) propose the need for widely distributed and mobile power sources. Although the power for each can be very small, the number can be huge in the order of millions to billions.[1,2] Environmental

Adv. Energy Mater. 2018, 8, 1703133

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Leaf-TENG worked in single electrode mode, the peaks of open-circuit voltage and short-circuit current can respectively reach about 230 V and 9.5 µA under the optimization condi-tion. And the maximum output power can approximately reach ≈45 mW m−2 with the external resistance of around 1 × 107 Ω. In addition, the Leaf-TENG can also work in the freestanding mode, which was integrated into the tree-shaped energy harvester for large-area environmental mechanical energy harvesting. Furthermore, the electrical energy collected by Leaf-TENG can be stored or directly power the distributed sensors or smart devices. This design of natural Leaf-TENG takes full advantage of the characteristics of natural leaves and environ-mental mechanical energy, which have potential in the energy harvesting and self-powered systems.

The schematic diagram of natural Leaf-TENG is given in Figure 1a. The natural leaf plays dual roles as triboelectric layer and electrode. Natural plants are abundant and widely distrib-uted in our environment, and the natural leaves with large area were collected in the experiments. A metal electrode is con-nected with the leaf for electrical connection. And the polymer sheet is chosen as the contact layer, which has a significant dif-ference in electron affinity compared with the surface of leaf. As shown in Figure 1b, the natural leaf from Hosta plantaginea (Hosta) that is large and bright glossy was collected to assemble the leaf-TENG. In order to investigate the performance of Leaf-TENG, the natural leaf was attached to the poly(methyl meth-acrylate) (PMMA), which has excellent impact strength and light weight. And the crocodile clip as electrode was connected to the leaf.

Figure 1c shows a cross section of the leaf from Hosta, which is porous with cells. The external surface of leaf is covered with waxy cuticle and epidermal cells. The upper surface is flat and dense to reduce the water evaporation, whereas there are more stomata for gas exchange in the back surface. In the interior of leaf, the main component is mesophyll that can be generally divided into palisade tissue and spongy tissue. As the natural leaf is full of water and electrolyte inside, it can be used as con-ductive liquid in the Leaf-TENG. Figure 1d shows the irregular surface of Hosta leaf, which can increase the effective contact area during the triboelectrification. And the inset gives further insight into the microstructure of the external surface, where a typical morphology of stomata can be found.

In order to simplify the structure and improve the adapt-ability, the Leaf-TENG was mainly investigated by connecting to the ground to work in the single electrode mode. The mecha-nism of the Leaf-TENG working in the single electrode mode is schematically illustrated in Figure 2a. When the leaf is contacted with a polymer material (PMMA) under an external mechan-ical force, contact electrification occurs at the interface due to their different electron affinities (Figure 2a-i). Since the surface electron affinity of leaf (Hosta) is lower than PMMA, negative triboelectric charges will be transferred from leaf to PMMA, which can be retained for a long time.[13] As shown in Figure 1c, the leaf has the outside surface and the inside electrolyte, which can be used as a triboelectric layer and conductive liquid, respectively. By releasing the force, the PMMA is separated from the surface of leaf and the changed electric potential dif-ference will induce the movement of the ions in the interior


Figure 1. The structure of natural leaf assembled TENG. a) Schematic diagram of Leaf-TENG. b) Photographs of Hosta leaf and Leaf-TENG directly assembled with Hosta leaf and PMMA. c) Photomicrograph of cross section of Hosta leaf, the inset shows structure of cross section of common leaves. d) SEM images of morphology of Hosta leaf.



of leaf (Figure 2a-ii). Meanwhile, there will be a polarized electrical double layer at the metal/electrolyte interface, which can drive the electron flow in the external circuit. When two surfaces reach the separated state, the electric potential reaches maximum value (Figure 2a-iii). The electrical double layer has a high capacitance and the TENG has high inherent impedance, so there is no electrochemical reaction.[12g,14] Then, the PMMA is forced to approach to leaf, the change of electric potential dif-ference will be reversed so that the electron moves in the oppo-site direction (Figure 2a-iv). When two surfaces contact again, another working cycle starts and there will be an alternating current with the periodical contact separation.[15]

A bionic Leaf-TENG was fabricated with filter paper and sodium chloride solution to illustrate the mechanism. The filter paper is composed of cellulose, which is the major component of plant cell walls. And the sodium chloride solution and the polyethylene film were chosen to act as conductive electrolyte and bionic leaf surface. According to the schematic diagram in Figure 1b, the bionic Leaf-TENG was assembled and tested in the single electrode mode. (The detailed procedure is pre-sented in the experimental section.) A homemade motor system was used to supply the controlled mechanical force with the movement frequency of 2 Hz and peak velocity of 0.333 m s−1. (The displacement-time curve is shown in the Figure S1, Sup-porting Information) Figure 2b,c, respectively, show the open-circuit voltage and short-circuit current of bionic Leaf-TENG (concentration of sodium chloride solution: 0.001 mol L−1), which has the similar output characteristic of natural Leaf-TENG.

The conductivity of electrolyte was measured, which shows the sodium chloride solution (0.002 mol L−1) has similar conductivity with the sample (0.11 g Hosta leaf + 6 mL H2O) (Table S1, Supporting Information). And the outputs of bionic Leaf-TENG with different concentration of sodium chloride solution were measured (Figure S2, Supporting Information), which indicates that the concentration of electrolyte has little effect on the output due to the high inherent impedance of TENG.[14]

To investigate the output performance of Leaf-TENG, several factors like contact area, mechanical frequency, and external resistance were compared. In the experiment, an optimization parameter was chosen with a contact area of 8 × 8 cm2, the movement frequency of 2 Hz and peak velocity of 0.333 m s−1. As shown in Figure 3a,b, the peaks of open-circuit voltage and short-circuit current of Leaf-TENG are, respectively, about 230 V and 9.5 µA. The enlarged views show the output characteristic, which is accordance with the above mechanism (Figure 2a). Besides, the as-designed natural Leaf-TENG has better perfor-mance than the bionic Leaf-TENG because the surface of nat-ural leaf has bigger effective contact area and bigger difference in electron affinity. Figure 3c shows the electrical outputs of Leaf-TENG increased on increasing the area of contact surface, which is in consistent with the former studies.[12b,16] Although the outputs increase with the larger area in a certain range, direct scale-up processing is not suitable for the Leaf-TENG working in the single electrode mode due to the effect of electrostatic shield.[15]


Figure 2. The working mechanisms of Leaf-TENG. a) Schematic illustration of working principles of Leaf-TENG in the single electrode mode. i: contact state; ii: separating state; iii: separated state; iv: approaching state. b) Open-circuit voltage and c) short-circuit current of bionic Leaf-TENG.



Figure 3d shows the relation between the electrical output of Leaf-TENG and the working frequency. As the same as previous studies of TENG,[17] the short-circuit current increased with higher mechanical frequency, while the open-circuit voltage remained relatively stable. Most of environmental mechanical energy is low-frequency and random, so this Leaf-TENG can harvest it effectively. As shown in Figure 3e, the performance of Leaf-TENG was tested for a week to investigate its durability. As the collected leaves are not mechanically strong, they would be dried and fragile with the continuous water evaporation and mechanical contacts. But the Leaf-TENG had certain dura-bility for several days in the laboratory environment. Besides, the stability of Leaf-TENG was also tested for ≈10 000 cycles, which shows a good stability (Figure S3, Supporting Informa-tion). The electrical output of Leaf-TENG (Hosta leaf) was more fluctuant than the Leaf-TENG (Populus leaf) because the Hosta leaf has poorer mechanical strength. For further evaluating the performance of Leaf-TENG, different external resistance was connected to examine the resistance dependence. The voltage increased with the increment of external resistance from 0 to 4 × 108 Ω, while the current had a reverse trend (Figure S4,

Supporting Information). The maximum output power was approximately reached ≈45 mW m−2 when the loading resist-ance was around 1 × 107 Ω, as shown in Figure 3f.

As the effect of contact electrification is closely related to the two dissimilar contact materials, different leaves were collected and different contact materials were used to investigate the per-formance of Leaf-TENG. Figure 4a shows the electrical output of Leaf-TENG assembled with various leaves when contacted with PMMA. The voltage and current depend on the species of leaves, which have different surface electron affinity. In order to survive and propagate in nature, different leaves have unique and superior surface structures, which have the vena-tion embedded in the mesophyll (insets of Figure 4a). Besides, most of leaves have hierarchical micro/nanostructures that are beneficial to enhance the performance of Leaf-TENG (Figure S5, Supporting Information). It could be concluded that a majority of natural leaves can be used to assemble Leaf-TENG, which reflects the general applicability in energy harvesting.

The electrical output of Leaf-TENG is also affected by the difference of contact materials in electron affinities, so a series of contact materials were tested. As shown in Figure 4b, the


Figure 3. The output performance of as-designed Leaf-TENG. a) Open-circuit voltage and b) short-circuit current of Leaf-TENG, insets show the enlarged views. c) The relation between the area of Leaf-TENG and the electrical output. d) The relation between the working frequency of Leaf-TENG and the electrical output. e) The performance of Leaf-TENG as a function of time. f) The power density with the external loading resistance.



Leaf-TENG assembled with Hosta leaf had better electrical output when it was contacted with PMMA. The higher elec-trical output of Leaf-TENG is generated with a larger difference in the electron affinities and effective contact area between two contact materials.[18] According to the designed Leaf-TENG, dif-ferent contact materials can be tested to optimize the electrical output. Furthermore, the natural leaves might be destroyed during the successive contact-separation process in the experi-ments. As shown in Figure 4c, multiple Leaf-TENG assembled with Hosta leaf were tested under the optimized condition and the electrical output of Leaf-TENG was fluctuant. On the one hand, the leaves collected in different time have individual diversity; on the other hand, contact electrification between the leaves and PMMA was largely affected by humidity.[19] The effect of relative humidity on the outputs of Leaf-TENG was investigated, which shows that the Leaf-TENG has low out-puts under the high humidity (Figure S6, Supporting Informa-tion). Considering the random characteristic of environmental mechanical energy, to integrate this natural Leaf-TENG with energy management system is a better choice to be the environ-mental mechanical energy harvester.

The Leaf-TENG can effectively harvest environmental mechanical energy, so it has wide applications in the

self-powered systems and sensors especially for powering the nodes of IoT. Figure 5a shows the experimental setup with a linear motor, which was used to supply mechanical energy for the Leaf-TENG. And the electricity generated by Leaf-TENG can power the LEDs directly (Video S1, Supporting Information). Considering the random and fluctuation of environmental mechanical energy, the electrical output of Leaf-TENG can be regulated and stored in capacitors or batteries.[20] As shown in the Figure 5b, commercial capacitors with various capacities were charged by Leaf-TENG, and the inset is the equivalent cir-cuit of charging system. The voltage of 1 µF capacitor can reach up to 10 V around 60 s, while the voltage of 10 µF capacitor can be charged to 1.5 V. The capacitors with larger capacities had better storage capacity, but they took longer time to reach the required voltage. So it is important to choose the appropriate match capacitor according to different Leaf-TENG. Further-more, the electrical energy collected by Leaf-TENG can be used to power widely distributed sensors or smart devices. As shown in Figure 5c, an electronic temperature sensor was powered by a 100 µF commercial capacitor, which was charged by the Leaf-TENG. After charged by Leaf-TENG for about 8 min, the capac-itor drove the electronic temperature sensor to sense external environment temperature (Video S2, Supporting Information).


Figure 4. The effect of materials and stability of Leaf-TENG. a) The electrical output of Leaf-TENG (PMMA) assembled with various leaves, insets show the photographs of corresponding leaf. b) The electrical output of Leaf-TENG (Hosta leaf) in response to different contact materials. c) The electrical output of multiple Leaf-TENG (Hosta leaf, PMMA) assembled in different time.


© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1703133 (6 of 7)Adv. Energy Mater. 2018, 8, 1703133

With tremendous achievements of nanogenerators, a tree-design blue print of self-powered systems and energy sciences has been presented.[9b] Figure 5d shows a natural Leaf-TENG driven by hand tapping to light advertising LEDs (Video S3, Sup-porting Information). Distributed display board for safety tips or advertisements is necessary in many places, so this natural Leaf-TENG provides a way to supply sustainable power by harvesting environmental mechanical energy. Besides working in the single electrode mode, the Leaf-TENG can also work in the freestanding mode. Figure 5e shows two types of natural Leaf-TENG (Hosta leaf and M.denudata leaf) working in the freestanding mode to light LEDs (Videos S4 and S5, Supporting Information). This work mode of Leaf-TENG can conveniently harvest mechan-ical energy without grounding, which is suited for tree-shaped

design. Considering that most of leaves have relatively small area,[21] connecting Leaf-TENG in parallel can also enhance the electrical output. Figure 5f shows a tree-shaped energy harvester assembled with natural Leaf-TENG (M.denudata leaf) that can drive multiple LEDs (Video S6, Supporting Information). The electrical output of Leaf-TENG was rectified before being con-nected in parallel, which can effectively help to power the elec-tronic devices or be stored in energy storage devices.

In summary, we have developed a simple, low-cost, and environmental friendly Leaf-TENG for effective environmental mechanical energy harvesting. The maximum output power of natural Leaf-TENG can approximately reach ≈45 mW m−2 with an external resistance of around 1 × 107 Ω, when it worked in single electrode mode under the optimization condition. And

Figure 5. The demonstrations of Leaf-TENG for mechanical energy harvesting and for charging electronics. a) Photograph showing the experimental setup with linear motor and LEDs lighted by Leaf-TENG. b) Voltage profile of capacitors (1; 2.2; 10 µF) charged by Leaf-TENG, the inset shows equiva-lent circuit. c) Photograph showing that electricity generated from Leaf-TENG was stored by a capacitor (100 µF) to drive the electronic temperature sensor. d) Demonstration of a natural Leaf-TENG driven by hands to light advertising LEDs. e) Demonstration of two types of natural Leaf-TENG (Hosta leaf and M.denudata leaf) working in the freestanding mode to light LEDs. (d,e) Tree/Self powered system schematic, Reproduced with permission.[2b] Copyright 2015, Elsevier. f) Demonstration of tree-shaped energy harvester assembled with natural Leaf-TENG (M.denudata leaf) that can drive multiple LEDs.


© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1703133 (7 of 7)Adv. Energy Mater. 2018, 8, 1703133

a bionic Leaf-TENG was fabricated to investigate the working mechanism of Leaf-TENG based on contact electrification. Besides, the natural Leaf-TENG was demonstrated to power LEDs and charge the capacitors, which shows its good ability to harvest mechanical energy. And a tree-shaped energy harvester was assembled with natural Leaf-TENG in the freestanding mode to harvest large-area environmental mechanical energy. This work not only presents a novel approach to effectively harvesting environmental mechanical energy, but also has great application prospects for designing self-powered system in the IoT.

Experimental SectionFabrication of Natural Leaf-TENG: Natural leaves (Hosta, M.denudate,

Lotus etc.) were collected on campus, washed with deionized water for several times and fully dried at room temperature. Then the leaf was cut into 8 × 8 cm2 as optimized contact area in the test experiment, and attached to the PMMA with the same area. A metal electrode was connected with the leaf for electrical connection. And polymer sheets (PMMA, Kapton, PTFE etc.) with corresponding area were chosen as the contact layer.

Fabrication of Bionic Leaf-TENG: The qualitative filter paper was cut into 8 × 8 cm2, washed with deionized water for several times and fully dried at room temperature. Then different concentrations of sodium chloride solution were prepared. The filter paper was immersed in the solution for 10 min and sealed in a packaging bag that is made of polyethylene. After being attached to the PMMA, a metal electrode was connected with the paper for electrical connection.

Characterization: The surface morphology of natural leaves was characterized by Nikon Eclipse Ti Inverted Microscope, Quanta FEG 450 SEM and Nova NanoSEM 450. For the measurement of electrical outputs of Leaf-TENG, a homemade motor system was used to supply the external mechanical force. The electrical outputs of Leaf-TENG were collected by a Keithley 6514 system electrometer with computer measurement software written in LabVIEW.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsY.J. and X.T.J. contributed equally to this work. The authors thank to the support of national key R&D project from Minister of Science and Technology, China (2016YFA0202702), the National Natural Science Foundation of China (NSFC Nos. 21275102, 51272011, 21575009, and 21605004), the “Thousands Talents” Program for Pioneer Researcher and His Innovation Team, China, and the National Natural Science Foundation of China (Grant Nos. 51432005).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsenergy harvesting, environmental friendly, leaves, triboelectric nanogenerators

Received: November 9, 2017Revised: December 7, 2017

Published online: January 26, 2018

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Natural Leaf Made Triboelectric Nanogenerator for ... · A bionic Leaf-TENG was fabricated with filter paper and sodium chloride solution to illustrate the mechanism. The filter paper