Now all kinds of electronic products around us, such as smart phones, laptops and wearable devices, are almost inseparable from battery power supply. However, the battery has some problems such as limited service life, limited endurance, repeated charging and potential safety hazards. Therefore, the battery has also become one of the key factors affecting the performance and user experience of modern electronic products.

To this end, scientists have been actively developing new power supply schemes to get rid of the batteries of electronic products. The author has also introduced many cases in this field before. Next, let's look at several classic cases:

(1) The world's first battery-free mobile phone invented by the University of Washington, USA, can obtain several microwatts of energy from the radio signals or light in the surrounding environment, and ensure the normal call of the mobile phone.

(2) A team of researchers from Weiss Institute of Bioinspired Engineering of Harvard University and John Paulson College of Engineering and Applied Science created an origami robot without batteries, which can provide energy and control wirelessly through a magnetic field and launch repetitive and complex actions.

(3) A group of researchers from China Academy of Sciences, Chongqing University, Georgia Institute of Technology and Taiwan Province University of Science and Technology, inspired by the traditional paper-cutting art in China, developed a light paper-cutting triboelectric nano-generator (TENG), which can collect the energy of human movement and supply power for electronic products.

(4) Researchers at Michigan State University in the United States have developed a flexible device consisting of ferroelectric electret nano-generators (FENG), which allows electronic devices to collect energy directly from human movements.

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Today I'm going to introduce a new scientific research progress to get rid of batteries for electronic products.

Recently, MIT and other scientific research institutions (Madrid Institute of Technology, US Army Research Laboratory, Carlos III University in Madrid, Boston University, University of Southern California) have developed the first completely flexible device, which can convert the energy of WiFi signals into electric energy and power electronic products.

A device that can convert alternating electromagnetic waves into direct current is called a "rectifying antenna". In a paper published in Nature, researchers showed a new type of rectifying antenna.

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The rectifier antenna uses a flexible radio frequency (RF) antenna to capture electromagnetic waves with alternating waveforms (including electromagnetic waves carrying WiFi signals). Then, the antenna is connected to a new device made of a "two-dimensional semiconductor" only a few atoms thick. This AC signal is transmitted to the semiconductor and converted into DC voltage by the semiconductor, which can be used to power electronic circuits or charge batteries.

In this way, battery-free devices passively capture ubiquitous WiFi signals and convert them into useful DC power. In addition, the device is flexible and can be manufactured by a "roll-to-roll" process, so that a very large area can be covered.

All rectifier antennas rely on an element called "rectifier", which converts AC input signals into DC power. The traditional rectifier antenna uses silicon or gallium arsenide as rectifier. These materials can cover the WiFi band, but unfortunately they are rigid. Although it is relatively cheap to make small equipment with these materials, it is too expensive to cover large areas, such as building surfaces and walls. Researchers have long tried to solve these problems. However, the reported flexible antennas rarely work at low frequencies and cannot capture and convert signals with gigahertz frequency. However, most related mobile phones and WiFi signals are at this frequency.

To build their rectifier, the researchers used a new two-dimensional material called "MoS2". It is only three atoms thick and is one of the thinnest semiconductors in the world. MoS2 can be used to build flexible semiconductor components such as processors.

In doing so, the research team took advantage of a "strange" behavior of molybdenum disulfide: when exposed to a specific chemical substance, the atoms of the material will rearrange and behave like a switch, resulting in a phase transition from semiconductor to metal. This structure, also known as Schottky diode, is made by using the principle that metal and semiconductor contact to form a "semiconductor-metal junction".

Xu Zhang, the first author of the paper and a postdoctoral fellow in electronic engineering and computer science (who will soon become an assistant professor at Carnegie Mellon University), said: "By designing MoS2 as a two-dimensional semiconductor-metal junction, we have constructed an ultra-high-speed Schottky diode with atomic thickness, which can simultaneously reduce the series resistance and parasitic capacitance."

In electronic devices, parasitic capacitance is inevitable. In this case, a specific material stores a small amount of charge, which will slow down the speed of the circuit. Therefore, the lower the parasitic capacitance, the faster the rectifier speed and the higher the working frequency. The parasitic capacitance in Schottky diode designed by researchers is one order of magnitude smaller than that in the most advanced flexible rectifier at present. Therefore, the signal conversion speed of this diode is faster, and it can collect converted 10GHz wireless signals.

Zhang said: "This design will bring a completely flexible device, which is fast enough to cover most radio frequency bands of electronic devices we use every day, such as WiFi, Bluetooth, cellular LTE and so on."

The work reported by the researchers provides a blueprint for other flexible devices that convert WiFi into electricity. These flexible devices have sufficient output and efficiency. According to the input power of the WiFi input signal, the maximum output efficiency of the current device is about 40%. At the typical WiFi power level, the energy efficiency of MoS2 rectifier is about 30%. In contrast, the efficiency of the best silicon and gallium arsenide rectifying antennas (made of more expensive rigid materials, silicon and gallium arsenide) is almost 50% to 60%.

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Tomás Palacios, one of the co-authors of the paper and director of the MIT /MTL Graphene Devices and Two-dimensional Systems Research Center, said: "What if we develop an electronic system that can surround bridges, cover the whole road, or cover office walls, and bring electronic intelligence to everything around us? How do you power these electronic products? We propose a new way to power these future electronic systems, collect the energy of WiFi in a simple and large-area integrated way, and bring intelligence to every object around us. "

The early applications of this rectifier antenna proposed by scientists include power supply for 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 the experiment, when researchers put the device in a typical environment with the power level of WiFi signal (about 150 microwatts), it can generate 40 microwatts of power. This power is enough to light up a simple mobile display screen or provide power for silicon chips.

Jesús Grajal, one of the co-authors of the paper and a researcher at Madrid Polytechnic University, said that another possible solution is to supply power for data communication of implantable medical devices. For example, researchers began to develop pills that can be swallowed by patients and send health data back to the computer for diagnosis.

Grajal said: "Ideally, you don't want to use batteries to power these systems, because if the batteries leak lithium, patients may die. It has obvious advantages to collect energy from the environment, provide power for these small laboratories in the body, and communicate with external computers. "

At present, the team is planning to build more complex systems and improve efficiency.

reference data

1http://news.mit.edu/2019/conversion -wi-fi- signal-electricity -0 128.

2Xu Zhang, Jesús Grajal, Jose Luis-Roy, Ujwal Radhakrishna, Wang Xiaoxue, Winston Chern,,, Shen Pinchun,, Ling, Ahmed zubair,, Wang Han, Madan Dube, Kong Jing, Mildred Dresselhaus and Thomas palacios. Flexible silicon rectifying antenna supporting two-dimensional MoS2 is used for wireless energy acquisition in Wi-Fi band. Naturally, 2019 doi:10.1038/s 41586-019-0892-1