You may have heard of Nikola Tesla's idea for wireless energy transmission, in which a large amount of power could be transmitted over long distances. Although the idea is technically feasible, the main issue was the huge power losses made while transmitting such a high amount of electricity over such long distances, consequently making wireless transmission inefficient.
For short distances and low power transmission, however, wireless energy transmission can actually be efficient and affordable. Such technology could provide power for electronic devices, smartphones, and laptops, even without batteries.
To demonstrate the possibilities, a team of researchers from MIT recently developed the first flexible device capable of converting electromagnetic alternating current (AC) from Wi-Fi signals into direct current (DC) energy for powering electronic devices. This apparatus, called a “rectenna”, uses a flexible radio frequency (RF) antenna to absorb electromagnetic signals.The antenna is connected to a novel gadget made using a tiny (just a few atoms thick) two-dimensional semiconductor that converts the absorbed AC signal to DC voltage suitable for powering electronic devices.
The mechanical and electrical properties of 2D flexible materials make them very attractive for flexible electronics. These malleable materials can be produced using roll-to-roll fabrication, enabling them to cover large areas. They also allow the development of intelligent electronic systems that can be wrapped around a bridge or walls or buildings, allowing for the intelligent control and monitoring of everything around us. Such an electronic system could be supplied by new Wi-Fi energy transmission devices that can be easily scaled.
The energy conversion is performed using rectifiers,commonly made of silicon (Si)and gallium arsenide (GaAs). Flexible rectennas operate at low frequencies, howeversmartphones and Wi-Fi signals work in the GHz frequency range (2.4 and 5.9 GHz) andthe silicon microparticles in flexible diodes have a cut-off frequency of 1.6 GHz. The low on/off current ratio results in an unreliable turn-on voltage, which is unacceptable for large-scale production. These materials are also quite rigid and can be very expensive if they are meant to cover a large area,such as entire walls.
To get around this issue, the researchers developed flexible, battery-free rectennas using a 2D semiconducting material called molybdenum disulphide (MoS2), which hasa cut-off frequency of 10 GHz. MoS2 is very mechanically robust, while large-scale production is low in cost.
“This device is, to our knowledge, the first flexible rectifier with a cutoff frequency in the X-band, and it fully covers the global-satellite-positioning band (1.58 GHz and 1.22 GHz), the cellular-communications fourth-generation (4G) long-term-evolution band (1.7 GHz and 1.9 GHz), Bluetooth (2.4 GHz), the Wi-Fi channels (2.4 GHz and 5.9 GHz)8 and even the next-generation (5G) radio system,” the research team explained.
The flexible MoS2 rectifiers have an efficiency of 10percent to 40 percent percent for input power of15 dBm and 5dBm (decibel-milliwatts). The efficiency is quite competitive to the state-of-the-art rigid Si and GaAs Schottky diodes, which have a power efficiency of 5percent–70percent at the same frequency and range of input power. The maximum efficiency of 40.1percent is obtained at 2.4 GHz and -0.7 dBm of RF input power. The input power of the receiver antenna is 156 µW (-8 dBm). The power efficiency at 2,4 GHz is about 31percent (at -8 dBm), which provides output power of 48.4 µW. This is enough to light up an LED or drive silicon chips.
The output power was achieved in an academic laboratory and has not yet been optimized. Through material engineering and circuit optimization, the power efficiency of the rectennas could be improved even further.
The most promising applications for the technology can be could be supplying power to wearable electronics, medical devices, IoT sensors, and more. It could even provide electricity forimplantable medical devices. To learn more, check out the journal article, “Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting,” published in Nature Research.