sensors Article Pyroelectric Energy Conversion and Its Applications—Flexible Energy Harvesters and Sensors Atul Thakre 1, Ajeet Kumar 1, Hyun-Cheol Song 2 , Dae-Yong Jeong 3,* and Jungho Ryu 1,4,* 1 School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Korea; [email protected] (A.T.); [email protected] (A.K.) 2 Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea; [email protected] 3 Department of Materials Science & Engineering, Inha University, Incheon 22212, Korea 4 Institute of Materials Technology, Yeungnam University, Gyeongsan 38541, Korea * Correspondence: [email protected] (D.-Y.J.); [email protected] (J.R.); Tel.: +82-53-810-2474 (J.R.) Received: 6 April 2019; Accepted: 7 May 2019; Published: 10 May 2019 Abstract: Among the various forms of natural energies, heat is the most prevalent and least harvested energy. Scavenging and detecting stray thermal energy for conversion into electrical energy can provide a cost-effective and reliable energy source for modern electrical appliances and sensor applications. Along with this, flexible devices have attracted considerable attention in scientific and industrial communities as wearable and implantable harvesters in addition to traditional thermal sensor applications. This review mainly discusses thermal energy conversion through pyroelectric phenomena in various lead-free as well as lead-based ceramics and polymers for flexible pyroelectric energy harvesting and sensor applications. The corresponding thermodynamic heat cycles and figures of merit of the pyroelectric materials for energy harvesting and heat sensing applications are also briefly discussed. Moreover, this study provides guidance on designing pyroelectric materials for flexible pyroelectric and hybrid energy harvesting. Keywords: pyroelectric materials; thermal energy harvesters; flexible 1. Introduction Energy has been a prime concern of the scientific community and industrial areas worldwide. As the demand for self-powered autonomous electronics with low power consumption has grown drastically in the electronics industry, energy harvesters have received immense focus and are being widely studied. Solid-state batteries, which are commonly used as an energy source in devices, require frequent and periodic maintenance such as recharging or replacement. Thus, various scavenging methods (harvesting the energy from stray energy sources) have been proposed to provide sustainable energy supplies for small electronic devices [1–10]. For example, piezoelectric energy harvesters from mechanical energy [2], thermal energy harvesters utilizing temperature differences through thermoelectricity or pyroelectricity [11], magnetic energy harvesters utilizing the stray magnetic noise from magneto-electric properties [12], and solar cells using the photovoltaic effect from solar light are areas of focus [13]. Among the above forms of stray energy, heat is ubiquitous and serves as a low-grade waste [14]. To convert thermal energy into usable electricity, thermoelectricity and pyroelectricity can be utilized. Thermoelectric materials have been employed to convert the spatial thermal gradient into electrical energy, i.e., the Seebeck effect [11]. Meanwhile, pyroelectricity is a phenomenon in which temperature fluctuations in the environment are converted into electrical energy [15]. Pyroelectric materials need Sensors 2019, 19, 2170; doi:10.3390/s19092170 www.mdpi.com/journal/sensors Sensors 2018, 18, x FOR PEER REVIEW 2 of 27 46 merit) at room temperature. Thus, pyroelectric energy harvesting (PyEH) is preferable for 47 harvesting low-grade thermal energy and at low temperatures. 48 The typical application arena of pyroelectric energy conversion is a thermal sensor, which can 49 detect thermal signals at the moment. However, it can also be used as a thermal energy harvester if 50 the conversion efficiency and total converted energy are sufficiently high to charge electrical energy Sensors 2019, 19, 2170 2 of 25 51 storage devices, such as a supercapacitor or battery. Although the PyEH concept was introduced in 52 the 1960s, it still remains a comparatively less studied area [17–23]. The reports estimate that in 53 2009,a temporal over 50% temperature of the total gradient consumed just energy as thermoelectric was wasted materials as heat, needwhich a is spatial mostly gradient from electrical [11,16]. 54 Variationpower generation in the temperature and automobile of the pyroelectricsystems [24]. material In addition, causes a large a net dipoleamount moment, of heat whichenergy furtheris lost 55 throughresults in electrical the accumulation appliances, of chargessuch as at refrigerators, the electrode, air separating conditioning application systems, targets and suchheat aspumps. small 56 Althoughscale microgenerators the study of withPyEH small accounts dimensions for a just (small small enough fraction for of spatial the total temperature amount of fluctuations). studies on 57 Thermoelectricpyroelectric materials, materials it can have be a lowernoted ZTfrom (“Z” Figure is Io ff1e’s that figure over of the merit) past at few room years, temperature. the number Thus, of 58 pyroelectricresearch articles energy focused harvesting on it (PyEH)has increased. is preferable Further for harvestingstudy of PyEHs low-grade would thermal clearly energy benefit and the at 59 utilizationlow temperatures. of this wasted heat. 60 InThe the typical 21st century, application wearable arena and of pyroelectric implantable energy electronics conversion have isgained a thermal considerable sensor, which attention can 61 [25–33].detect thermal To power signals these at electronics, the moment. which However, are necessarily it can also besmall, used flexible, as a thermal and endurable, energy harvester onboard if 62 powerthe conversion sources earefficiency required. and totalAs with converted other energyautonomous are suffi devices,ciently highPyEHs to chargecould electricalbe the optimal energy 63 poweringstorage devices, solution such for as asuch supercapacitor devices. Several or battery. reports Although have demonstrated the PyEH concept significant was introduced generated in 64 outputthe 1960s, power it still densities remains a(in comparatively the range lessof ~ studied µW∙cm area-3 to [17 mW–23].∙cm The-3) reportsusing estimatepyroelectric that inenergy 2009, 65 conversion,over 50% of which the total can consumed be used to energy drive devices, was wasted such as as heat, liquid which color is displays mostly from(LED), electrical light-emitting power 66 diodesgeneration (LED), and and automobile wireless systemsdevices [34,35]. [24]. In Am addition,ong various a large energy amount harvester of heat energy candidates, is lost although through 67 PyEHselectrical have appliances, great potential such as refrigerators,for such applications, air conditioning they are systems, the least and explored heat pumps. area. Although In addition, the 68 energystudy of harvesters PyEH accounts are clearly for a just necessary small fraction for the of theflexibility total amount of wearable of studies and on pyroelectricimplantable materials,devices. 69 Manyit can bescientific noted from groups Figure have1 that reported over the various past few demonstrations years, the number of offlexible research pyroelectric articles focused or hybrid on it 70 pyroelectric–piezoelectrichas increased. Further study energy of PyEHs harvesters would in clearly recent benefityears [36–46]. the utilization of this wasted heat. 500 Others Energy Harvesting Applications 400 92.97% 300 7.03% 200 Number of Papers 100 data till Jan, 2019 0 2001 2004 2007 2010 2013 2016 2019 Year 71 Figure 1. The histogram distribution of the number of papers on the pyroelectric materials published 72 Figurein the last1. The two histogram decades. distribution The inset image of the shows number a pieof papers chart foron the numberpyroelectr ofic papers materials reporting published the 73 inapplication the last two of thermal decades. energy The inset harvesters image (red) shows through a pie pyroelectricity.chart for the number The data of havepapers been reporting taken from the 74 “Webapplication of Science” of thermal database energy (https: harvesters//www.webofknowledge.com (red) through pyroelectricity.), and the actual The datadata may have vary been from taken the data shown here. Sensors 2019, 19, 2170 3 of 25 In the 21st century, wearable and implantable electronics have gained considerable attention [25–33]. To power these electronics, which are necessarily small, flexible, and endurable, onboard power sources are required. As with other autonomous devices, PyEHs could be the optimal powering solution for such devices. Several reports have demonstrated significant generated output power densities (in the range of ~ µW cm 3 to mW cm 3) using pyroelectric energy conversion, which can be used to drive · − · − devices, such as liquid color displays (LED), light-emitting diodes (LED), and wireless devices [34,35]. Among various energy harvester candidates, although PyEHs have great potential for such applications, they are the least explored area. In addition, energy harvesters are clearly necessary for the flexibility of wearable and implantable devices. Many scientific groups have reported various demonstrations of flexible pyroelectric or hybrid pyroelectric–piezoelectric energy harvesters in recent years [36–46]. Over the past two decades, many pyroelectric materials have been extensively studied, as shown