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DOI: 10.1039/D5ME00055F (Paper) Mol. Syst. Des. Eng., 2025, Advance Article

Simulation and experimental evaluation of a freestanding triboelectric layer nanogenerator for self-powered electronics†

Kuldeep Singh , Akshpreet Kaur , Preetika Sharma and Gaurav Sapra *
Department of Electrical and Electronics Engineering, UIET, Panjab University, Chandigarh, India. E-mail: gaurav.sapra@pu.ac.in

Received 8th April 2025 , Accepted 23rd June 2025

First published on 18th July 2025

Abstract

With the world swiftly evolving towards technology, the relentless quest for sustainable energy sources has garnered significant attention. Amidst the scientific advancements, triboelectric nanogenerators (TENGs) have emerged as a beacon of hope, providing a promising solution through energy generation from the ambient environment. This paper focuses on the development of a freestanding triboelectric layer (FTL) TENG for increasing output power energy by incorporating a PTFE layer. A comprehensive investigation is performed that involves mathematical modeling and COMSOL simulation of the FTL TENG. Experimentally, the novel FTL TENG is designed and fabricated using fur and PTFE/Cu electrodes. The results show a two-fold enhancement in the performance of the FTL TENG upon incorporating a PTFE layer over copper electrodes. The simulation results accurately predict voltage build up under open-circuit conditions and substantial current flow under short-circuit conditions, closely mirroring experimental results. The study also demonstrates the practicality of the FTL TENG in powering electronic devices through successful capacitor charging. This research underscores the FTL TENG's reliability as a clean and sustainable energy harvesting solution with potential applications across various fields, thereby illuminating the path towards self-powered electronic devices.


Design, System, Application

This work presents a novel freestanding triboelectric layer (FTL) triboelectric nanogenerator (TENG), optimized through strategic material selection and structural configuration to enhance mechanical-to-electrical energy conversion. The device utilizes fur and a PTFE/Cu electrode combination, leveraging the triboelectric series to maximize charge transfer efficiency. The addition of a PTFE layer over copper electrodes results in a significant two-fold increase in output performance, demonstrating a practical and generalizable material optimization strategy. The results demonstrate an increase in the open circuit voltage of the TENG from 6.8 V to 16.9 V when the frequency is escalated from 1 Hz to 3 Hz. Through integrated mathematical modelling and COMSOL simulations, the system demonstrates consistent open-circuit voltage generation and reliable current flow under short-circuit conditions, closely aligned with experimental outcomes. Designed for compatibility with low-power electronic circuits, the FTL TENG charges capacitors effectively and powers devices such as digital calculators. With the potential for integration into portable, wearable, and remote sensing applications, this work lays the foundation for next-generation self-powered systems. Future directions include exploring advanced triboelectric materials, scalable fabrication, and real-world deployment, positioning this technology as a promising candidate in the growing field of sustainable energy harvesting.

1. Introduction

The age of globalisation has prompted significant advancements in technology in the fields of artificial intelligence, internet of things (IoT) and portable electronics, calling for clean and sustainable sources of energy. The excessive use of fossil fuels and other finite energy sources has left a deep impression on economic, social and environmental well-being.1 Responding to this crucial situation, researchers have been focusing on revolutionising the existing scenario by developing innovative energy harnessing and generating methods from the ambient environment. The evolution of energy scavenging technologies has produced a vast array of portable and tailored power solutions over the past two centuries. The technological advancement in energy harvesting technologies has produced a variety of customised power solutions.2,3 However, despite this incessant growth in the field, there are still challenges that need to be addressed specifically in terms of portability, operational lifetime, recyclability, reusability and cost-effectiveness of energy storage technologies like fuel cells, lithium-ion batteries, etc.4 Subsequently, traditional energy storage systems require periodic replacement, constant maintenance and recharging, posing considerable economic, logistical and waste disposal burdens.5–7 In the quest for uninterrupted and cleaner power solutions with extended lifespans, energy conversion technologies such as electromagnetic generators,8 piezoelectric nanogenerators,9 triboelectric nanogenerators,10 thermoelectric generators,11 solar panels,12 and biofuel cells have emerged as promising solutions to harvest energy from the ambient environment – water,13 wind,14 sunlight,15 mechanical energy, etc.16 Mechanical energy is an excellent candidate due to its abundance and accessibility. Triboelectric nanogenerators (TENGs) have emerged as a pioneering technology among the mechanical energy scavenging technologies owing to their high electric output, easy fabrication and high energy conversion efficiency.17,18 In recent years, TENGs have been employed in a wide variety of applications such as artificial synaptic devices,19 cardiac pacemakers,20 unconventional sequential logic,21 artificial intelligence,22 wireless devices,23 etc. by exploiting innovative materials that include PET, PMMA, and OHP sheets,24 TiO2,25 2D materials,26 etc. The operating principle of TENGs involves the triboelectric effect. ‘Tribo’ is basically a Greek word meaning friction.27 TENGs convert mechanical energy into electrical energy due to friction between two materials of opposite tribopolarity.28 The two materials are selected from the triboelectric series in such a way that the larger the difference between the two materials, the greater is the output voltage.29–31 As shown in Fig. 1, there are mainly four operational modes of these innovative devices: the lateral sliding mode, contact separation mode, single electrode mode and freestanding layer mode (FTL). Each mode of the TENG is designed uniquely to be well-suited for capturing different forms of mechanical energy from the environment. Notably, out of all the operating modes of the TENG, the FTL mode exhibits high power output.32–34 This mode utilises an innovative structural design for scavenging kinetic energy from the surroundings. Yet, enhancing electrical output with design characteristics of robustness, miniaturisation and stability remain challenging. The FTL mode had been chosen in this study, based on the natural decoupling ability of charge generation from the electrode contact point to reduce potential charge leakage and wear. This system also provides stable and repeatable sliding motion thereby increasing triboelectric charge density and has potential scalability, positioning it as a compelling framework for evaluating prolonged operational stability.
image file: d5me00055f-f1.tif
Fig. 1 Operating modes of the TENG.

This paper focuses on the development of a novel TENG PTFE-fur tribo-pair to improve electric output of the FTL TENG. For a deeper understanding of the underlying principles, a mathematical model has been studied followed by the COMSOL simulation of the FTL TENG under both open and short-circuit conditions, thereby providing a comprehensive analysis of its performance characteristics. Furthermore, the paper discusses the fabrication process of the FTL TENG and presents experimental results, which includes open circuit voltage output of the TENG corresponding to different frequencies of sliding motions. Notably, the study showcases the potential of FTL TENGs for powering electronic devices such as a digital calculator by charging capacitors. Therefore, this work contributes significantly into the field of mechanical energy harvesting, shedding light on sustainable energy solutions to meet the growing energy demands of our modern electronic-centric society.

2. Freestanding triboelectric layer (FTL) mode of the TENG

The freestanding triboelectric-layer mode of the TENG possesses the capability of capturing energy from physical or mechanical movement without requiring a connected electrode. To achieve this, two symmetrical electrodes are positioned beneath a dielectric layer, considering the electrode's dimensions being comparable in proportion to the size of the moving entity. There is a slight distance between the electrode and object. When the object gets closer to or moves away from the electrodes, an uneven charge distribution is generated within the medium, assuming the object was previously charged through a triboelectric process. This induces electrons to move across the two electrodes to equalise the local potential distribution. The to-and-fro motion of the object leads to the oscillation of electrons among the paired electrodes, leading to an AC current output. In contrast to the single-electrode mode, this mode does not suffer from a screening effect and the electrostatically-triggered electron transfer can match the quantity of triboelectric charges on the freestanding layer. One significant advantage of this mode is that there is an absence of direct physical touch between the two triboelectric layers, unlike the lateral sliding mode. As a result, there is minimal material wear or heat generation during extended continuous operation, making the freestanding mode exceptionally robust. The FTL mode of the TENG can be employed for energy harvesting from various sources, including vibration, rotational motions, computer mouse usage, airflow, human walking, or vehicle movement. Furthermore, it has been transformed into autonomous vibration sensors and active micro-actuators.

3. Mathematical modelling and simulations

3.1 Mathematical modelling of the FTL TENG

The theoretical model of the freestanding triboelectric layer (FTL) mode is as shown. Metals 1 and 2 are arranged in the same plane as the two electrodes, separated by a gap ‘g’ (Fig. 2(a)). In this configuration, there is a freestanding dielectric layer positioned on top of a metal electrode, with a vertical distance ‘h’ between them. The structure's breadth is represented by the symbol ‘w’. In this mode, the electrical potential on the lower dielectric surface is not uniform, making it challenging to view it as a single node. The combined charges on the two electrodes sum up to σwdk, and it's anticipated that only a limited area of dk on the lower dielectric surface contains the tribo-charges. Using quasi-static approximations, the capacitance between a localised dielectric segment of width dk and metal i is modelled as:
 
image file: d5me00055f-t1.tif(1)
where hi(k) is the effective vertical distance from the dielectric region to metal i, depending on position k. These capacitances represent simplified 1D approximations based on Gauss's law and parallel-plate field assumptions. Under short-circuit conditions, dQ1 and dQ2 denote the differential induced charges on metal 1 and metal 2, respectively.
 
image file: d5me00055f-t2.tif(2a)
 
image file: d5me00055f-t3.tif(2b)
Employing the superposition principle in electrostatics, for metal 1 and metal 2, the sums of the induced charges owing to the whole tribo-charged dielectric surface can be expressed as an integral over infinitesimal charged segments along the dielectric. This gives us the sum of all contributions stemming from the interaction of the dielectric with the electrodes as follows:
 
image file: d5me00055f-t4.tif(3a)
 
image file: d5me00055f-t5.tif(3b)
where, σ is the tribo-charge density. Hence, QSC,final is represented as:
 
image file: d5me00055f-t6.tif(4)
The core-working concept of the freestanding mode TENG is the changing of x in this working mode.
image file: d5me00055f-f2.tif
Fig. 2 (a) Model of the FTL TENG and (b) design of the FTL TENG.

3.2 COMSOL simulations of the FTL TENG

The simulation of the freestanding triboelectric layer (FTL) mode of the triboelectric nanogenerator (TENG) using COMSOL Multiphysics offers a comprehensive and versatile approach to understanding its electrical performance. The simulation process begins with the creation of a 2D model of the FTL TENG within COMSOL. The geometry includes two electrodes separated by a gap of 1 mm with a PTFE layer on top of it and fur as a sliding layer. The dimensions are assigned in accordance with the physical dimensions of the FTL TENG as shown in Table S1.† The thickness of the copper electrode, PTFE and fur is assigned to be 0.1 mm, 0.1 mm and 1 mm respectively. The material properties of the copper electrode, negative dielectric layer of PTFE, and positive dielectric layer of fur are assigned using the built-in module. An infinite domain was established and the entire structure was assumed to be exposed to air. The surface charge density of PTFE-fur tribo-charged surface is assumed to be ±50 μC m−2. Mechanical input is modeled through a parametric sweep simulating linear displacement of the triboelectric layer to replicate motion-induced charge transfer. Boundary conditions include grounded outer domains and floating potential on the active electrode to simulate charge transfer. A physics-controlled mesh with extra coarse element size is employed and simulations are validated for convergence through stepwise mesh refinement and parametric stability checks. The simulation is conducted under both open-circuit and short-circuit conditions to comprehensively assess the FTL TENG's behaviour.

4. Design and fabrication

The schematic of the MP TENG design is shown in Fig. 2(b). The design and fabrication of a freestanding triboelectric nanogenerator (TENG) represent a crucial step in harnessing mechanical energy sustainably. This process begins with the selection of appropriate materials and tools to create a TENG capable of generating electricity through the triboelectric effect. Key materials include triboelectric materials like PTFE, a dielectric material like fur, conductive electrodes such as copper sheets with conductive adhesive, a substrate material like acrylic or epoxy glass fiber, and basic tools like scissors, cutting tools, double-sided adhesive tape, conductive wires, and a multimeter for testing. The TENG design focuses on the freestanding layer mode, which consists of two essential components: a positive charge affinity triboelectric layer with a metal electrode film and a negative charge affinity triboelectric layer with an acrylic sheet substrate. In this innovative approach, the TENG is constructed with flexibility in mind. Two copper electrodes, each measuring 2 cm by 1.5 cm, are securely affixed to an epoxy glass fiber sheet measuring 3 cm by 2.5 cm as shown in Fig. 3(a). A PTFE layer, measuring 3 cm by 2.5 cm, is adhered to the copper electrodes using clear tape on edges as depicted in Fig. 3(b). To complete the sliding layer, a fur sheet is attached to an acrylic sheet measuring 3 cm by 3 cm (Fig. 3(c)). The design incorporates two identical TENG portions arranged in a manner that facilitates sliding between the PTFE and fur layers, enabling efficient charge transfer between surfaces and electron flow through the copper electrodes. This design and fabrication process lays the foundation for the creation of a functional and reliable freestanding TENG, offering the potential for sustainable energy harvesting across various applications.
image file: d5me00055f-f3.tif
Fig. 3 Fabrication of the FTL TENG: (a) bottom layer, (b) bottom layer of the FTL TENG with the PTFE layer, and (c) top layer of the FTL TENG with fur as the dielectric.

5. Results and discussion

5.1 Simulation outcomes

The COMSOL simulations are evaluated under open circuit and short circuit conditions and discussed in section 5.1.1 and 5.1.2 respectively.
5.1.1 Open circuit condition. In the triboelectric nanogenerator (TENG), the “open circuit condition” refers to the scenario when there is no external electrical load connected to the TENG. In this state, the TENG generates an open-circuit voltage but does not provide any electrical current flow to an external circuit. In the freestanding mode of the TENG, the device operates without being anchored to any external structure. The freestanding triboelectric-layer mode requires simply a freely moving element. The uneven potential distribution arises from the movement of the freely moving portion, causing electrons to move between the two electrodes, which ultimately leads to the generation of electrical output as shown in Fig. 4(a). Under the open circuit conditions, the generated electrical potential (voltage) builds up between the triboelectric layers, but since there is no closed loop to complete the circuit, there is no current flow. The charge remains separated, leading to a voltage across the terminals of the TENG without any current passing through the external circuit. The variation of voltage on the surface of electrodes is plotted by using FEM in COMSOL software and the peak value of voltage is found to be 90 volts as shown in Fig. 4(b). The output voltage between copper electrodes is of alternating nature due to the linear motor of top fur layer.
image file: d5me00055f-f4.tif
Fig. 4 (a) Simulated model of the FTL TENG under open circuit conditions and (b) open circuit voltage of the FTL TENG.
5.1.2 Short circuit condition. In the triboelectric nanogenerator (TENG), the “short circuit condition” refers to the scenario when the output terminals of the TENG are directly connected together without any electrical load in the circuit. In this state, the TENG generates a short-circuit current, but the voltage across the terminals is typically very low or zero. Consequently, under short-circuit conditions, the triboelectric nanogenerator (TENG) generates a substantial current flow through the short-circuit path. The magnitude of this short-circuit current is influenced by several factors, such as the mechanical motion applied, the materials utilised, and the spacing between the triboelectric layers. By connecting an appropriate load to the TENG, the device operates under closed-circuit conditions, and the electrical energy generated by the TENG can be efficiently utilised to power external devices or charge energy storage systems. Understanding the short circuit behaviour of the freestanding mode TENG is crucial for designing the device, optimising its performance, and ensuring efficient power transfer under different operating conditions. The potential variation over the surfaces of electrodes is drawn using FEM in COMSOL software and the graph of charges is drawn on both electrodes and the charges which flow through the external circuit is also plotted using COMSOL. The graphs of charge on each electrode and between two electrodes are shown in Fig. 5.
image file: d5me00055f-f5.tif
Fig. 5 (a) Short circuit model of the FTL TENG, (b) charge on electrode 2, (c) charge on electrode 1, and (d) charge between both electrodes.

5.2 Experimental results

The output performance of the fabricated FTL TENG was assessed by connecting it to a digital storage oscilloscope (DSO) to capture voltage signals. The contact and separation of the TENG were achieved by sliding the top fur layer on the PTFE layer. The open circuit voltage corresponding to different sliding frequencies (1 Hz, 2 Hz and 3 Hz) was recorded and is presented in Fig. 6(a)–(c) respectively. During slow sliding at 1 Hz, the maximum peak-to-peak voltage recorded on the TENG was 6.8 V. For normal sliding at 2 Hz, it reached 10.04 V, and during fast sliding at 3 Hz, it reached 16.9 V. It is evident that the output voltage of the TENG is significantly influenced by the applied external sliding motion. The output voltage of the FTL TENG is found to be highly dependent on the sliding frequency. In Fig. 6(d), the open circuit voltage is compared corresponding to the FTL TENG with and without PTFE layer and it is therefore observed that the incorporation of the PTFE layer enhances the output voltage by approximately two-fold. Fig. 6(e) shows the durability test of the FTL TENG for 500 cycles performed by employing lead screw mechanism and a stepper motor. To demonstrate the practicality of the FTL TENG, an electronic circuit is designed consisting of a bridge rectifier for converting AC voltage from the TENG to DC voltage that can be further fed to capacitors for energy storage (Fig. 7(a)). For data acquisition, the FTL TENG is interfaced with an Arduino Uno and MATLAB software for plotting the charging curves corresponding to different capacitors (2.2 μF, 3.3 μF, 5 μF and 10 μF) as shown in Fig. 7(b). It is observed that under a sliding frequency of 3 Hz, 10 μF is charged to 3.5 V in approximately 18 seconds, which is then discharged to power a digital calculator as shown in Fig. 7(c). The average power output and power density of the FTL TENG are observed to be 3.4 μW (P = CV2/2t) and 11.3 mW m−2 respectively.
image file: d5me00055f-f6.tif
Fig. 6 (a) Open circuit voltage at 1 Hz, (b) open circuit voltage at 2 Hz, (c) open circuit voltage at 3 Hz, (d) graph of open circuit voltage with different frequencies and (e) durability test of the FTL TENG for 500 cycles.

image file: d5me00055f-f7.tif
Fig. 7 (a) Electronic circuit diagram of power management unit to power a digital calculator, (b) charging curves of different capacitors, and (c) the FTL TENG powering a digital calculator.

5.3 Validation of experimental results with simulation outcomes

The experimental outcomes of the freestanding triboelectric layer (FTL) mode align closely with the simulation results, validating the efficacy and accuracy of this energy harvesting technology. The simulations mimic the open circuit voltage build-up, short circuit current generation, and the influence of sliding frequency on open circuit voltage output. However, the difference in simulation and experimental output is attributed to parasitic losses and variety of other factors such as contact inefficiency, surface roughness, material property deviations, air gaps, and environmental influences. Furthermore, the successful integration of the FTL TENG with the Arduino Uno and MATLAB software for powering a digital calculator through capacitor charging highlights its capability to power portable electronic devices. This convergence between simulation and experimentation underscores the FTL TENG's potential as a reliable and sustainable energy source, poised to revolutionise mechanical energy conversion for a wide range of applications. Various applications of the FTL TENG and their corresponding output power is tabulated in Table S1 (see ESI† document).

6. Conclusions

The freestanding triboelectric layer (FTL) mode has emerged as an exceptionally promising avenue for harvesting mechanical energy, as evident from the close alignment between experimental results and simulation outcomes. The capability of generating substantial output voltage by varying sliding frequencies and efficiently powering an electronic device underscores the practicality and potential of this innovative technology. The FTL TENG's compatibility with portable electronic devices and its capacity to convert mechanical energy into electrical power position it as a sustainable and versatile energy source for diverse applications. The inconsistencies and losses during rectification has limited its power output. Future work will focus on tapping the full potential of this cutting-edge technology by advancing it to DC-TENGs with innovative energy storage systems and exploring real-world deployment scenarios for addressing the increasing demand of energy sources in our progressively electronic driven world.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Kuldeep Singh: experimental investigation, formal analysis, and writing – original draft; Akshpreet Kaur: experimental investigation, writing – review & editing, and revision; Preetika Sharma: supervision; Gaurav Sapra: conceptualisation, resources, supervision, and project administration.

Conflicts of 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

Authors would like to express their sincere gratitude to Scheme for Promotion of Academic and Research Collaboration (SPARC) and UK-India Education and Research Initiative (UKIERI) for their generous grant that supported this research (SPARC-UKIERI/2024-2025/P3118). Akshpreet Kaur is thankful for the financial support in the form of the DST INSPIRE Fellowship (DST/INSPIRE/Fellowship/2018/IF190152) by the Department of Science and Technology (DST), Ministry of Science and Technology, Government of India. The authors are thankful to MHRD, Government of India for giving the grant to the Department of Electrical and Electronics Engineering, UIET, Panjab University Chandigarh for Design Innovation Centre (DIC) vide letter no. 17-11/2015-PN.1 for providing data acquisition facility. The authors acknowledge the support of an undergraduate student Manthan Sharma for his assistance in the development of the electronic circuit.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5me00055f
This journal is © The Royal Society of Chemistry 2025
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