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Posted on September 12, 2017

Material turning sunlight, heat, and movement into electricity

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Dr Yang Bai and co-workers from the Microelectronics Research Unit of the University of Oulu, Finland, have presented in their recent paper in Advanced Materials a perovskite based ceramic material that is able to collect three ambient energy sources and convert them to electricity simultaneously - all with one material.
People are surrounded by many forms of energy: sunlight, the heat in the atmosphere, and even your own movements. Such energies that are normally wasted can be collected and converted to electricity, and thus potentially help to power portable and wearable devices, from biometric sensors to smart watches. This technology is called Energy Harvesting (EH).
For a long time in EH Technology's history, people could only use one particular class of materials to harvest its corresponding energy, e.g. semiconductors (such as solar cells) for sunlight, combinations of magnets and coils (such as dynamos and wind turbines) for movement, and certain types of alloys (such as thermoelectric cells) for heat, etc.
Sometimes, however, just one type of energy is not enough in practice. A given form of energy is not always handy to harvest - maybe it is cloudy or you are staying still and do not want to move around. Other researchers have developed devices that can harness multiple forms of energy, but they require multiple materials, adding bulk to what's supposed to be a small and portable device.
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Materials with perovskite structure have extremely versatile properties which enables harvesting of one or two types of energy at a time - but not simultaneously. One type of perovskite may be good at harvesting sunlight via photovoltaic effect to efficiently convert solar energy into electricity. However, the same perovskite is not good for harvesting motion and/or heat. Vice versa, another type may be adept at harnessing energy from heat fluctuations and pressure/deformation arising from motion, but having wide band gaps making solar energy harvesting practically impossible.
Thanks to the new discovery, a single perovskite material called KNBNNO is able to harness three forms of energy simultaneously. For the first time people can practically use piezoelectric effect - a mechanical-electrical energy conversion mechanism, pyroelectric effect - a temperature fluctuation-electricity conversion mechanism and photovoltaic effect - a light-electricity conversion mechanism to harvest sunlight, heat and movement, simultaneously, solely through a single piece of material.
KNBNNO is synthesized by doping a proper amount of BNNO (a perovskite with oxygen vacancies present in its microstructure) into KNN (a widely used lead-free material exhibiting good piezoelectric and pyroelectric properties). The experiments showed KNBNNO has a narrow band gap which can induce good photovoltaic properties to harvest sunlight. Meanwhile, KNBNNO's piezoelectric and pyroelectric properties are found to be very close to those of KNN, meaning they are the same good at harvesting heat and movement. However, KNN's band gap is very wide, making it impossible to be used for harvesting sunlight. Similarly, KNBNNO's superior 3-in-1 property has beaten that of all other counterpart materials, including silicon based solar cells and conventional piezoelectric/pyroelectric materials - PZT. This is a big step in both fields of narrow band gap semiconductor and strong ferroelectrics (usually strong piezoelectric and pyroelectric materials). A material was considered difficult to yield both good properties of them, but now it is proved practical, triggering a lot of physical phenomena and corresponding theories to be deeply (re-)investigated.
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One of the most promising futures of this kind of material is it would likely supplement the batteries on electronic devices, improving energy efficiency and reducing how often they need to be recharged. One day, multi-energy harvesting may mean you won't have to plug in your gadgets anymore. Batteries for small devices may even become obsolete. This will push the development of the Internet of Things and smart cities, where power-consuming sensors and devices can be energy sustainable.
This research is funded by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant (No. 705437).
Source and top image: University of Oulu
Learn more at the next leading event on the topic: Energy Independent Electric Vehicles 2017 External Link on 27 - 28 Sep 2017 in TU Delft, Delft, Netherlands hosted by IDTechEx.