Materials for energy harvesting: At the forefront of a new wave
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troduction Society in the future will require materials and methodologies for energy harvesting to enable smart systems and embedded automation. As the Internet of Things (IoT) becomes a reality, there will be a need for trillions of sensors1 to enable artificial intelligence (AI) applications, for example, to automate caretaking for aging populations. There is also a need to effectively power increasingly ubiquitous mobile personal devices that are already an integral part of everyday life. Changing batteries for a trillion sensors is not feasible; even recharging batteries of everyday mobile devices is quite a challenge currently. It is thus vital to develop energy-harvesting materials and technologies that can dynamically harvest energy from the surroundings to generate electrical power for sensors and devices. Energy harvesting can target a variety of environmental resources (e.g., the utilization of mechanical vibrations and motion, magnetic field, and heat). Figure 1 shows examples of energy-harvesting devices demonstrated in the literature.2 The performance of energy-harvesting devices capturing such broadband low-amplitude energy from the environment is critically dependent upon the figure of merit of the material. Further increases in the efficiency of these devices will be realized by improving material performance, which in many cases remains magnitudes behind the desired values. In this issue of the MRS Bulletin, we feature six articles describing the progress and challenges at the forefront of developing
cutting-edge energy-harvesting devices, as illustrated in Figure 2.3 Given the increasing relevance of wearables in our lives, some of the articles also focus on the flexibility of energy harvesters compatible with textiles, wearables, and the human body.
Energy harvesting from heat It is well known that out of the primary energy that we consume, only one-third is effectively used, and the majority of energy is lost as waste heat (Figure 3).4 In the 20th century, humankind learned to achieve unprecedented control over charge carriers (electrons), spins, and photons, however, advanced control over phonons and thermal energy is still not satisfactory and remains one of the important scientific challenges for the 21st century. The majority of waste heat is estimated to have temperatures below 150°C,5 and thermoelectrics, which can compactly convert heat to electricity through solid-state devices using the Seebeck effect without scaling, are quite promising. The Seebeck effect is the phenomenon where electrical voltage is generated by the diffusion of charge carriers when a temperature difference is applied to a material (Figure 3). With wearable thermoelectrics, generating electricity by utilizing human body heat is another attractive application. One difficulty in achieving high conversion efficiency in thermoelectric materials is that the figure of merit ZT = (S2σT)/κ comprises of competing requirements in physical
Takao Mori, National Institute for Materials Science, Japan; [email protected] Shash
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