Background
Nowadays, almost all of the various electronic products around us, such as smartphones, laptops, wearables, etc., cannot be powered by batteries. However, batteries suffer from problems such as limited lifespan, limited endurance, the need for repeated charging, and safety hazards. As a result, batteries have also become one of the key factors affecting the performance and user experience of modern electronic products.
For this reason, scientists have been actively researching and developing new power solutions that allow electronics to get rid of batteries. Previously, I have also introduced many cases in this regard. Next, let's look at a few classic cases:
(a) The University of Washington invented the world's first battery-free cell phone, which can get a few microwatts of energy from radio signals or light in the surrounding environment to ensure normal cell phone calls.
(ii) A team of researchers at Harvard University's Wise Institute for Biologically Inspired Engineering and the John Paulson School of Engineering and Applied Science have created a battery-free origami robot that can be wirelessly energized and controlled by a magnetic field to develop complex, repeatable movements.
(c) A team of researchers from the Chinese Academy of Sciences, Chongqing University, Georgia Institute of Technology, and the National Taiwan University of Science and Technology, inspired by the traditional Chinese art of paper-cutting, developed a lightweight, paper-cutting-style friction electric nanogenerator (TENG) that captures the energy of the human body's movements to power electronic products.
(d) Researchers at Michigan State University have developed a flexible device consisting of a ferroelectric electret nanogenerator (FENG) that allows electronic devices to harvest energy directly from human movement.
Innovation
Today, I would like to introduce you to a new scientific research advancement that allows electronics to get rid of batteries.
Recently, the Massachusetts Institute of Technology (MIT), in conjunction with other scientific institutions (Universidad Politécnica de Madrid, U.S. Army Research Laboratory, Universidad Carlos III de Madrid, Boston University, University of Southern California), developed the first fully flexible device that can convert the energy of WiFi signals into electricity, which can power electronic products.
The device, which converts alternating electromagnetic waves into direct current, is called a "rectifier antenna". In a paper published in the journal Nature, researchers have demonstrated a new type of rectifier antenna.
Technology
The rectifier antenna employs a flexible radio-frequency (RF) antenna that captures electromagnetic waves (including those carrying WiFi signals) in an alternating waveform. This antenna is then connected to a new device made of a "two-dimensional semiconductor" just a few atoms thick. This AC signal is transmitted to the semiconductor, which converts it into a DC voltage that can be used to power electronic circuits or charge batteries.
In this way, the battery-free device passively captures the ubiquitous WiFi signal and converts it into a useful DC power source. Further, the device is flexible and can be prepared in a "roll-to-roll" process, allowing it to cover very large areas.
All rectifier antennas rely on a component called a "rectifier" that converts an AC input signal into a DC power source. Traditional rectifier antennas use silicon or gallium arsenide for the rectifier. These materials can cover the WiFi band, but unfortunately they are rigid. Although it is relatively inexpensive to use these materials to make small devices, it is too expensive to use them to cover large areas, such as the surfaces of buildings and walls. Researchers have long been trying to solve these problems. But few of the flexible antennas reported so far operate at low frequencies and are unable to capture and translate signals at gigahertz frequencies, yet most relevant cell phone and WiFi signals are at this frequency.
To construct their rectifier, the researchers used a new two-dimensional material called molybdenum disulfide (MoS2). At just three atoms thick, it is one of the world's thinnest semiconductors, and can be used to build flexible semiconductor components, such as processors.
In doing so, the team took advantage of a "peculiar" behavior of molybdenum disulfide: when exposed to a specific chemical, the material's atoms rearrange themselves, behaving like switches and creating a phase transition from a semiconductor to a metallic material. This structure, also known as a "Schottky diode", utilizes the principle of the "semiconductor-metal junction" formed when a metal is in contact with a semiconductor.
"By designing MoS2 as a two-dimensional semiconductor-metal junction, we constructed an atomically thin, ultrafast Schottky diode that reduces series resistance and parasitic capacitance in tandem," said first author Xu Zhang, a postdoctoral fellow in electrical engineering and computers, who will soon become an assistant professor at Carnegie Mellon University. "
Parasitic capacitance is an unavoidable situation in electronic devices. This is a situation where a specific material stores a small amount of charge that will slow down the circuit. Therefore, the lower the parasitic capacitance, the faster the rectifier will be and the higher the operating frequency. The parasitic capacitance in the Schottky diode designed by the researchers is an order of magnitude smaller than the parasitic capacitance in state-of-the-art flexible rectifiers. As a result, the diode can convert signals faster, and can capture and convert wireless signals up to 10 GHz.
Zhang said, "This design will lead to a fully flexible device that is fast enough to cover most of the RF bands of the electronics we use every day, such as WiFi, Bluetooth, cellular LTE, etc."
The work reported by the researchers provides a blueprint for other flexible devices that convert WiFi into electricity with sufficiently large output and efficiency. Based on the input power of the WiFi input signal, the maximum output efficiency of current devices is about 40 percent. At typical WiFi power levels, MoS2 rectifiers have an energy efficiency of about 30%. In comparison, the best current silicon and gallium arsenide rectifier antennas (made from the more expensive and rigid materials silicon and gallium arsenide) achieve efficiencies of almost 50% to 60%.
Value
Tomás Palacios, one of the paper's co-authors and director of the MIT/MTL Center for the Study of Graphene Devices and 2D Systems at the Microsystems Technology Laboratory at the Massachusetts Institute of Technology, said, "What if we developed an electronic system that could wrap around a bridge, or What if we developed electronic systems that could wrap around a bridge, or cover an entire highway, or cover the walls of an office, and bring electronic intelligence to every object around us? How would you power these electronics? We propose a new way to power these electronic systems of the future, by harvesting the energy from WiFi in a way that can be simply integrated over a large area and bring intelligence to every object around us."
Early applications for this rectifier antenna proposed by the scientists include powering flexible and wearable devices, medical devices, and "Internet of Things" sensors. For example, flexible smartphones will be a hot new market for major technology companies. In experiments, when the researchers placed the device in an environment with typical WiFi signal power levels (around 150 microwatts), it produced 40 microwatts of power. That's enough power to light up a simple mobile display or power a silicon chip.
Another possible scenario is to power data communications for implantable medical devices, said Jesús Grajal, one of the paper's co-authors and a researcher at the Universidad Politécnica de Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and send health data back to a computer for diagnosis.
"Ideally, you wouldn't want to power these systems with batteries, because if the battery leaks lithium, then the patient could die," Grajal said. Harvesting energy from the environment to power these small labs inside the body and data communication with external computers has clear advantages."
Currently, the team is planning to build more complex systems and increase efficiency.
References
1http://news.mit.edu/2019/converting-wi-fi-signals-electricity-0128
2Xu Zhang, Jesús Grajal. Jose Luis Vazquez-Roy, Ujwal Radhakrishna, Xiaoxue Wang, Winston Chern, Lin Zhou, Yuxuan Lin, Pin-Chun Shen, Xiang Ji, Xi Ling, Ahmad Zubair, Yuhao Zhang, Han Wang, Madan Dubey, Jing Kong, Mildred Dresselhaus and Tomás Palacios. Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi- band wireless energy harvesting. band wireless energy harvesting . Nature, 2019 DOI: 10.1038/s41586-019-0892-1