The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Textiles have been concomitant of human civilization for thousands of years. With the advances in chemistry and materials, integrating textiles with energy harvesters will provide a sustainable, environmentally friendly, pervasive, and wearable energy solution for distributed on-body electronics in the era of Internet of Things. This article comprehensively and thoughtfully reviews research activities regarding the utilization of smart textiles for harvesting energy from renewable energy sources on the human body and its surroundings. Specifically, we start with a brief introduction to contextualize the significance of smart textiles in light of the emerging energy crisis, environmental pollution, and public health. Next, we systematically review smart textiles according to their abilities to harvest biomechanical energy, body heat energy, biochemical energy, solar energy as well as hybrid forms of energy. Finally, we provide a critical analysis of smart textiles and insights into remaining challenges and future directions. With worldwide efforts, innovations in chemistry and materials elaborated in this review will push forward the frontiers of smart textiles, which will soon revolutionize our lives in the era of Internet of Things.

Smart Textiles for Electricity Generation.

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations.

The programmable nature of smart textiles makes them an indispensable part of an emerging new technology field. Smart textile-integrated microelectronic systems (STIMES), which combine microelectronics and technology such as artificial intelligence and augmented or virtual reality, have been intensively explored. A vast range of research activities have been reported. Many promising applications in healthcare, the internet of things (IoT), smart city management, robotics, etc., have been demonstrated around the world. A timely overview and comprehensive review of progress of this field in the last five years are provided. Several main aspects are covered: functional materials, major fabrication processes of smart textile components, functional devices, system architectures and heterogeneous integration, wearable applications in human and nonhuman-related areas, and the safety and security of STIMES. The major types of textile-integrated nonconventional functional devices are discussed in detail: sensors, actuators, displays, antennas, energy harvesters and their hybrids, batteries and supercapacitors, circuit boards, and memory devices.

Smart Textile-Integrated Microelectronic Systems for Wearable Applications

Review of wearable thermoelectric energy harvesting: From body temperature to electronic systems

Fiber-based flexible thermoelectric energy generators are 3D deformable, lightweight, and desirable for applications in large-area waste heat recovery, and as energy suppliers for wearable or mobile electronic systems in which large mechanical deformations, high energy conversion efficiency, and electrical stability are greatly demanded These devices can be manufactured at low or room temperature under ambient conditions by established industrial processes, offering cost-effective and reliable products in mass quantity This article presents a critical overview and review of state-of-the-art fiber-based thermoelectric generators, covering their operational principle, materials, device structures, fabrication methods, characterization, and potential applications Scientific and practical challenges along with critical issues and opportunities are also discussed

Fiber‐Based Thermoelectric Generators: Materials, Device Structures, Fabrication, Characterization, and Applications

The progress of spontaneous energy generation from ubiquitous moisture is hindered the low output current and intermittent operating voltage of the moisture‐electric generators. Herein a novel and efficient ionic hydrogel moisture‐electric generator (IHMEG) is developed by rational combination of poly(vinyl alcohol), phytic acid, and glycerol‐water binary solvent. Thanks to the synergistic effect of notable moisture‐absorption capability and fast ion transport capability in the ionic hydrogel network, a single IHMEG unit of 0.25 cm2 can continuously generate direct‐current electricity with a constant open‐circuit voltage of ≈0.8 V for over 1000 h, a high short‐current density of 0.24 mA cm−2, and power density of up to 35 µW cm−2. Of great importance is that large‐scale integration of IHMEG units can be readily accomplished to offer a device with voltage up to 210 V, capable of directly driving numerous commercial electronics, including electronic ink screen, metal electrodeposition setup, and light‐emitting‐diode arrays. Such prominent performance is mainly attributed to the enhanced moisture‐liberated proton diffusion proved by experimental observation and theoretical analysis. The ionic hydrogel with high cost‐efficiency, easy‐to‐scaleup fabrication, and high power‐output opens a brand‐new perspective to develop a green, versatile, and efficient power source for Internet‐of‐Things and wearable electronics.

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Ionic Hydrogel for Efficient and Scalable Moisture‐Electric Generation

Predicting performance of fiber thermoelectric generator arrays in wearable electronic applications

Flexible stimuli-responsive materials are deformable, stretchable, light-weight, and desirable for smart personal protective equipment (PPE), and as the primarily functional components in the wearable system which spontaneously respond to surrounding variations. These materials enable the traditional PPE who provide passive protection to be smart with the abilities of sensing, actuating, surface changing and self-healing, which enhances the protection and reduces the unintentional occupation injuries. This article presents a critical review of the structure, properties, fundamental mechanisms and current development of flexible stimuli-responsive materials and their potential/present applications to smart PPE, covering strain, pressure, temperature, and gas sensors, biopotential electrodes, exosystems, switchable wetting surfaces and biosafety masks. Scientific and practical challenges along with critical issues and opportunities are also discussed.

Lisha Zhang

Papers

Smart Textile-Integrated Microelectronic Systems for Wearable Applications

Fiber‐Based Thermoelectric Generators: Materials, Device Structures, Fabrication, Characterization, and Applications

Ionic Hydrogel for Efficient and Scalable Moisture‐Electric Generation

Predicting performance of fiber thermoelectric generator arrays in wearable electronic applications

Flexible stimuli-responsive materials for smart personal protective equipment