The Unified Theory of Triboelectric Nanogenerators—A Pathway to Practical Applications

<b>Background</b><br/>Triboelectric nanogenerators (TENGs) convert mechanical movements into electrical pulses using triboelectric charging and electrostatic induction. TENGs are categorized as either contact-mode or sliding-mode architectures, and a wide range of device designs have been introduced for each of these categories. Due to high instantaneous power (&gt;500 W/m²) and energy conversion efficiency (&gt;50%) [1], TENGs are considered a leading power source and a sensing technology for future wearable electronics, especially in health monitoring applications.<br/>Designing efficient and reliable TENGs for practical applications has been challenging due to lack of detailed knowledge on their theory and optimization. The classical theoretical models for TENG use idealised assumptions such as parallel-plate capacitor approximations which deviate from real-world devices, making it difficult to accurately simulate or optimise their output generation. We introduced the distance-dependent electric field (DDEF) model, causing a paradigm shift from classical models to Maxwell’s equations based electric field modelling for TENG [2,3,4]. This model, for the first time, demonstrated accurate modelling of non-planar TENGs and the optimisation of all primary parameters which govern their output behaviour. However, this initial model was restricted to contact-separation mode TENGs.<br/>Herein, for the first time, we present the unified DDEF theory which can accurately represent both contact and sliding mode 3D TENGs. We compare the output trends of different TENG working modes using this theory to assess their relative performances. A series of optimisation techniques is derived to improve electrical outputs and to reduce internal impedances, which are critical for future TENG applications.<br/><b>Methods</b><br/>Gauss’s law is used to model the electric field behaviour of finite charged surfaces via analytical methods to derive the unified DDEF equation, which is applied to triboelectric contact surfaces and electrode interfaces to evaluate the electrode potentials [4]. These potentials are modelled against the relative movement between the triboelectric surfaces, which is used to assess the charge, current, voltage and power outputs from the TENG. This unified DDEF model is combined with the Norton’s theorem and TENG power transfer theory to maximize electrical outputs and minimize internal impedance [2]. The theoretical results are validated experimentally to assess the effectiveness of these predictions.<br/><b>Results and discussion</b><br/>The experimental and theoretical results indicate a close match, proving the accuracy of the unified DDEF model. This new theory, for the first time, encompasses all possible TENG configurations via a single electric field equation, allowing a common platform for TENG performance comparison. The results indicate that, at constant speed movements, contact-mode TENGs perform relatively better than the sliding-mode TENGs. The optimisation techniques derived here improves the power generation and reduce impedance of the TENG by over 60 times [3], creating a step increase in efficiency and bringing them closer to real-life applications.<br/><b>References</b><br/>[1] Wu et al., Adv. Energy Mater. 2018, 1802906<br/>[2] Dharmasena et al., Adv. Energy Mater. 2018, 8(31), 1802190<br/>[3] Dharmasena et al., Nano Energy, 2021, 90, 106511<br/>[4] Dharmasena et al., Energy Environ. Sci. 2017, 10(8), 1801-1811