What are nanogenerators? Explaining their types, applications, and potential

The burgeoning field of harvesting untapped energy within our living environment is gaining momentum as the global quest to supplant fossil fuels with clean, renewable alternatives intensifies. Innumerable mechanical, thermal, or electromagnetic energy sources pervade our surroundings, with anything in motion holding potential for energy harvesting. This spectrum spans from colossal mechanical forces, such as oceanic wave power, to minute phenomena like raindrops or the biomechanical energy derived from heartbeats, respiration, and blood circulation. Nanogenerators could also harvest energy form sources as diverse as temperature fluctuations and sound waves.
The challenge lies in devising methods to harness these seemingly boundless energy reserves and use them to power self-sufficient and sustainable electronic devices, such as sensors, wearables, and implantable medical devices.
This research field has expanded rapidly in recent years due to the potential of nanogenerator breakthroughs as sustainable and renewable sources of energy. In this article, we explain the different types of nanogenerators for energy harvesting, how they work, their purpose, and examples of applications where they are used.

Types of Nanogenerators

There are several types of nanogenerators, each with their own unique advantages and disadvantages. The most common types are piezoelectric and triboelectric, but there are also electromagnetic, pyroelectric, and tribo-thermoelectric nanogenerators.

Piezoelectric Nanogenerators (PENGs)

PENGs use piezoelectric materials, such as zinc oxide nanowires, lead zirconate titanate, or polyvinylidene fluoride (PVDF) films. When these materials experience mechanical stress or pressure, their crystal structure changes, leading to a charge separation and, thus, an electric charge generation. This effect is highly sensitive, enabling PENGs to convert even slight mechanical deformations into electrical energy. This characteristic makes them suitable for applications in energy harvesting from various sources, such as vibrations, human motion, and fluid flow.
Examples include piezoelectric nanogenerators for self-powered flexible sensors and nanogenerators incorporated into wound dressings that generate electricity in response to movement and thereby accelerate wound healing and tissue regeneration.

Triboelectric Nanogenerators (TENGs)

Triboelectric nanogenerators use a combination of the triboelectric effect and electrostatic induction to generate small amount of electrical power from mechanical motion such as rotation, sliding or vibration. The triboelectric effect takes advantage of the fact that certain materials become electrically charged after they come into moving contact with a surface made from a different material. The triboelectric effect causes a charge transfer between the materials, and when they are separated, an electric field is generated.
The electricity generated by triboelectric devices could replace or supplement batteries for a broad range of potential applications. Triboelectric nanogenerators have a broad range of applications, from wearable electronics to energy harvesting from environmental sources like wind or rain.
This video shows how a zinc oxide nanowire nanogenerator is used to generate electricity from the contraction and expansion of the beating heart of a living rat. (Video: Dr. Zhong Lin Wang, Georgia Institute of Technology)
Piezoelectric and triboelectric nanogenerators are the main types of nanogenerators that are actively researched. However, there are also other types that deserve mentioning:

Electromagnetic Nanogenerators (EMGs)

EMGs operate on the principle of electromagnetic induction, where a change in the magnetic field generates an electric current in a nearby conductor. In EMGs, this is typically achieved by using a microcoil and a micro-magnet. The relative motion between the coil and the magnet, caused by external mechanical vibrations or movements, induces an electric current in the coil. This technology has been miniaturized to nanoscale dimensions, allowing EMGs to be integrated into a variety of devices, such as sensors, actuators, and wireless communication systems.

Pyroelectric Nanogenerators (PyENGs)

PyENGs harness the pyroelectric effect, in which certain materials generate an electric charge in response to a change in temperature. This effect was first described by the Greek philosopher Theophrastus in 314 B.C., when he noticed the gemstone tourmaline produced static electricity and attracted bits of straw when heated. Heating and cooling rearrange the molecular structure of certain materials, including tourmaline, and create an imbalance of electrons that generates an electric current.
Materials like lithium tantalate, barium titanate, and PVDF exhibit strong pyroelectric properties. When the temperature of these materials fluctuates, the change in the crystal structure's charge distribution results in an electric charge. PyENGs can, therefore, capture waste heat or environmental temperature variations as an energy source for powering low-energy devices.
A working prototype of a pyroelectric nanogenerator was already demonstrated more than ten years ago.
pyroelectric nanogenerator
The coupling of the pyroelectric and semiconducting properties in ZnO creates a polarization electric field and charge separation along the ZnO nanowire as a result of the time-dependent change in temperature. (© ACS)

Tribo-Thermoelectric Nanogenerators (TTENGs)

TTENGs combine the principles of triboelectric and thermoelectric effects, allowing them to generate electricity from both mechanical and thermal energy. A typical TTENG consists of two layers: one triboelectric material and one thermoelectric material. The triboelectric layer generates an electric charge through contact and separation, while the thermoelectric layer produces an electric current due to the temperature difference between the two layers. This dual-mode energy harvesting capability enables TTENGs to be used in applications where both mechanical motion and temperature gradients are present, such as body heat harvesting or industrial waste heat recovery.

Hybrid Nanogenerators

Hybrid nanogenerators merge two or more types of energy-harvesting mechanisms, such as piezoelectric, triboelectric, electromagnetic, or thermoelectric, to improve efficiency and power output. By exploiting multiple energy conversion mechanisms, hybrid nanogenerators can capture a broader range of energy sources and achieve higher overall performance. These devices have been used in various applications, including wearable technology, environmental monitoring systems, and medical devices like pacemakers or drug delivery systems.
For instance, researchers have demonstrated a
hybrid piezo-triboelectric nanogenerator to sustainably power portable electronics with mechanical motion.

Applications of Nanogenerators

Nanogenerators have the potential to revolutionize the way we generate and use energy by providing a sustainable and renewable source of electricity. They have as self-powered devices such as sensors, wearable electronics, and medical implants. Here are some examples of application areas for nanogenerators:
Wearable Electronics: Nanogenerators can be integrated into fabrics, clothing, or accessories, providing a continuous power source for wearable devices such as smartwatches, fitness trackers, and health monitoring systems. For example, TENGs embedded in smart textiles can harvest energy from body movement, while PENGs in shoe insoles can generate power from the pressure applied during walking or running.
Triboelectric textile interfaces. RF TENG audio control interface embroidered on an omniphobic polo shirt. The audio player, embedded in the textile, is powered using the biomechanical energy harvested from the wearer using the RF TENGs on the sides of the polo shirt. (Video: FlexiLab, Purdue University)
Environmental Monitoring: Nanogenerators can power self-sufficient sensor networks for monitoring air quality, water quality, or structural health. For instance, TENGs or EMGs can harvest energy from wind or water flow to power remote sensors in hard-to-reach locations, reducing the need for battery replacement or external power sources.
Medical Devices: Nanogenerators can be integrated into implantable medical devices, such as pacemakers, neurostimulators, or drug delivery systems, to provide a continuous power source without the need for battery replacement surgery. PENGs or hybrid nanogenerators can harvest energy from bodily functions, such as heartbeats or muscle movements, to maintain the device's functionality.
Internet of Things (IoT): Nanogenerators can supply power to rigid as well as flexible IoT devices, such as wireless sensors, actuators, and communication modules, allowing for long-lasting and self-sufficient networks. By converting ambient energy sources, like vibrations or temperature fluctuations, into electricity, nanogenerators can help minimize the maintenance and energy costs associated with large-scale Internet-of-Things deployments.
Energy Harvesting: Nanogenerators can be employed to capture and convert waste energy from various sources, such as industrial machinery, transportation systems, or urban infrastructure. By converting wasted mechanical or thermal energy into usable electricity, nanogenerators can contribute to more sustainable and efficient energy management practices.
Consumer Electronics: Nanogenerators can be integrated into consumer electronics, such as smartphones, tablets, or remote controls, to extend battery life or even eliminate the need for batteries altogether. For example, PENGs or TENGs can be embedded into a device's casing to harvest energy from user interactions, like pressing buttons or swiping screens.

Advantages and Disadvantages of Nanogenerators

Nanogenerators offer several advantages over traditional energy harvesting devices, such as their small size, flexibility, versatility and ability to exploit even the tiniest energy sources. They can be integrated into various materials and structures, allowing for seamless and unobtrusive energy harvesting. They also offer the potential for long-term and self-sufficient power generation, reducing the need for battery replacement or recharging.
However, they also face several challenges, such as low energy conversion efficiency, limited power output, and high fabrication cost. Moreover, their commercialization and scalability remain major challenges, as the production of nanogenerators at a large scale is still a significant hurdle.