Reporter Li Huiyu

As a brand-new energy technology, the triboelectric nanogenerator (TENG) based on the coupling of triboelectricity and electrostatic induction effects can directly convert the tiny mechanical energy in the environment into electrical energy. For example, it can collect air or water flow, engine rotation, and even It is the energy generated by triboelectric in people walking, talking, heartbeat, muscle contraction, and other sports.

With the development of electronic information technology, the volume and power consumption of electronic devices and sensors are getting smaller. The emergence of disruptive technology like TENG will also make many devices in the future no longer need batteries and obtain energy from their working environment，they can generate electricity for themselves.

However, due to the high voltage, low current, and AC pulse output characteristics of TENG, there is currently no TENG that can independently, continuously, and stably supply power to electronic devices. Power management circuits and energy storage devices are required to achieve stable output.

For this reason, the research group of Prof. Cheng from the Key Laboratory of Special Functional Materials of the Ministry of Education/School of Materials of Henan University designed a general passive power management circuit based on Pulsed-TENG, with the maximum output impedance. By using Pulsed-TENG, the output impedance is reduced to the greatest extent, and the output energy is always maximized, which solves the problem of high output impedance in traditional TENG. The research results have recently been published in the international journal Nano Energy.

Advantage: zero output impedance

Compared with electromagnetic generators, TENG has higher energy conversion efficiency at lower operating frequencies; compared with piezoelectric nanogenerators, its output voltage and energy are increased by 3 to 4 orders of magnitude. The materials used are Energy sources that are also more extensive. The effective combination of TENG and electronic devices in a specific working environment to develop self-driven electronic devices or systems is the development direction of wearable electronic devices and sensors in the Internet of Things.

In fact, as far as the working principle is concerned, Pulsed-TENG is the same as the conventional TENG, and both are the coupling of the triboelectric effect and the electrostatic induction effect. However, Pulsed-TENG has a significant advantage-the output impedance is zero.

"Conventional TENG has the characteristics of high output voltage, low current, high output impedance (in the order of megaohms), and AC pulse output." The first author of the paper, the Key Laboratory of Special Functional Materials of the Ministry of Education/School of Materials, Henan University Dr. Guangqin Gu told China Science Daily that the equivalent circuit of a conventional TENG is a series connection of a constant voltage source and a variable capacitor. The rate of change of the capacitor is determined by the frequency of motion of the TENG. However, TENG usually works at low frequencies (several hertz to tens of hertz), which makes its electrical output controlled only by the frequency of motion when the load is small and has nothing to do with the output load, which also causes the high output impedance of TENG root cause.

In response to the above problems, Cheng's group introduced a synchronously triggered mechanical switch at TENG’ output, and developed Pulsed-TENG. According to reports, the equivalent circuit of Pulsed-TENG is a series connection of a constant voltage source, a constant capacitor, and a switch. Its electrical output has nothing to do with the TENG's motion frequency and is only controlled by the output load, so its output impedance is zero. This also allows Pulsed-TENG to maintain the maximum output voltage and energy without being affected by external impedance.

"This is different from the ubiquitous high internal resistance of TENG, which brings new enlightenment for future research in this field." Zhang Chi, a researcher at the Beijing Institute of Nano Energy and Systems, Chinese Academy of Sciences, commented on China Science News. TENG has a large power density but restricted by its high output impedance characteristics, its direct charging efficiency for energy storage devices is extremely low, and it cannot directly supply power to electrical devices. However, Pulsed-TENG's effective power management technology progress and breakthroughs It is expected to solve the technical bottleneck of TENG's practical application.

From "active" to "passive"

Before this, many groups have developed various active power management circuits, which greatly improved the energy storage efficiency of TENG's charging of capacitors or batteries. However, due to large output impedance, there is a serious impedance mismatch with the power management circuit, so it is necessary to introduce active devices in the power management circuit, which also increases the complexity and energy consumption of the circuit.

In this regard, Cheng’s group prepared Pulsed-TENG with unidirectional output and designed a passive power management circuit using its impedance matching and unidirectional output characteristics. The circuit structure is simplified, but a total energy storage efficiency of 48.0% is obtained in the actual charging process.

However, Guangqin Gu told reporters that due to the complex structure of the one-way switch, its use environment is greatly restricted, and it is difficult to match the widely used TENG such as the turntable type, so it cannot be used to collect various forms of mechanical energy in the environment. Therefore, they also need to develop a universal passive power management circuit that matches Pulsed-TENG to efficient manage and store of TENG output energy.

In this work, Cheng's group has designed a universal passive power management circuit based on Pulsed-TENG. In the simulation, they found that when the matching impedance is 0.001Ω, the output voltage and energy can still reach the maximum value. This shows that the output energy of Pulsed-TENG can be maximized without being affected by the load resistance, which solves the problem of impedance mismatch between TENG and power management resistance.

Guangqin Gu said that they studied energy storage efficiency of the Pulsed-TENG passive power management circuit based on electrostatic vibration switches through simulation and actual testing. The simulation results show that its total energy storage efficiency can reach 83.6%. In the actual charging test, the energy storage efficiency is 57.8%. This circuit’s electrical energy can drive electronic devices such as commercial calculators and temperature and humidity sensors.

In this regard, Chi Zhang believes that this data also represents that the power management circuit designed by him is already a strategy that can be adopted for actual industrial applications. Besides, the simple circuit design to achieve effective power management is simple and easy to implement, providing a theoretical basis for future arraying and large-scale applications.

Industrialization still faces challenges.

As it solves the high output impedance of TENG, Pulsed-TENG will have important application prospects in self-power energy packs based on TENG, self-power sensors, and their systems and networks.

"But for now, there are still many problems to be solved if Pulsed-TENG wants to realize the industrial application. First, it faces the design and preparation of multi-mode, high-performance Pulsed-TENG." Guangqin Gu said frankly.

In this regard, Cheng's group has begun research. Guangqin Gu said that they would will start from the design of Pulsed-TENG and integrate all current technical means and methods in the TENG field to increase the charge density of the TENG dielectric triboelectric layer, improve its output characteristics, reduce triboelectrical resistance and wear, and improve its durability, stability. Besides, a variety of mechanical switches will be developed to make them suitable for more modes and more types of Pulsed-TENG, thereby expanding the application range of Pulsed-TENG.

Not only that, Guangqin Gu said that the current power management circuit only efficiently stores the electric energy output by TENG in a capacitor, but the capacitor is only a temporary storage device for electric energy. It cannot output a stable voltage to the outside. Therefore, it is also necessary to develop a more efficient and multi-functional power management circuit that can not only output a stable voltage but also manage and distribute the output power to meet the needs of different sensors.

"There are always many difficulties in the industrial application of every new invention. This is a process that new technologies must go through to consumers." In Zhang Chi's view, TENG-based air purifiers, automobile exhaust gas treatment devices and night running lights Shoes have been successfully incubated. At present, Pulsed-TENG has achieved a lot of results in the experimental stage. The key to marketization is to find the places where such inventions are really needed, that is, "pain points."

Thus, Chi Zhang suggests starting from Pulsed-TENG's collection of low-frequency and low-amplitude characteristics and looking for places where such applications are needed. Besides, to implement the project as soon as possible, the initial prototype design is also essential. He suggested that the prototype should be developed and promoted while the laboratory results are reported.

Related paper information: https://doi.org/10.1016/j.nanoen.2019.104372

China Science News (2020-01-15 3rd Edition Energy and Chemical Industry)