Thermoelectric Silver-Based Chalcogenides

Abstract

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Keywords: multinary alloys; near-room-temperature thermoelectric materials; silver-based chalcogenides; thermal energy harvesting; waste heat recovery.

Figure 1

Number of publications in a quadrennial period, as reported by the Web of Science when using the topic “silver thermoelectric” in the search engine.

Figure 2

a) Schematic illustration of engineering strategies for optimizing silver‐based chalcogenide thermoelectric materials. b) Schematic comparison of various binary, ternary, quaternary, hybrid, and composite silver‐based chalcogenide thermoelectric materials for the applications of waste heat harvesting and their temperature range of operation.

Figure 3

Preparative methodologies of silver‐based chalcogenides based on solid‐, liquid‐, and vapor‐state reaction. Solid‐state reaction methods include mechanical milling, melt alloying, zone melting, Bridgman method, additive manufacturing, high‐pressure preparation, and spark plasma sintering. Liquid‐state reaction preparations usually involve wet‐chemical routes in aqueous solutions/organic solvents through colloidal synthesis, hydrothermal/solvothermal precipitation, microwave‐/ultrasound‐assisted preparation or template‐assisted ion exchange reactions. Vapor‐state reactions encompass a variety of physical and chemical vapor deposition techniques.

Figure 4

Crystal structures for a) monoclinic Ag₂S and b) orthorhombic Ag₂Se. Reproduced under Creative Common CC BY 4.0 license.^{[ 154 ]} Copyright 2021, The Authors. Published by AAAS. c) Flexibility‐zT phase diagram for Ag₂S‐Ag₂Se‐Ag₂Te system. d) Twisted Ag₂S_0.5Se_0.5 and Ag₂S_0.8Te_0.2 samples in various shapes. e) Temperature dependence of zT for Ag₂S_{1− x} Se _x (x = 0/white, 0.1/green, 0.3/purple, and 0.5/cyan), Ag₂S_0.8Te_0.2 (red), and Ag₂S_0.5Se_0.45Te_0.05 (yellow). f) Optical image of a six‐couple flexible Ag₂S_0.5Se_0.5/Pt‐Rh thermoelectric device. Reproduced with permission.^{[ 20 ]} Copyright 2019, Royal Society of Chemistry.

Figure 5

a) Dark‐field transmission electron microscopy (TEM) image of Ag₂Se showing the grain and pore distribution. b) Schematic view for the nanopore distribution at grain interfaces. c) High‐resolution TEM image taken along the [$\overline{1}$01] zone axis and its fast Fourier transform (FFT) pattern. d) Enlarged image from the orange square area in (c) in which the “O” and “M” represent the orthorhombic phase and metastable phase of Ag₂Se, respectively. High‐resolution TEM image showing e) high‐density dislocations and f) nanosized grains with semi‐coherent interfaces. g) Schematic illustration of Ag ₂ Se with hierarchical pore architectures consisting of high‐density pores, a metastable phase, nanosized grains, semi‐coherent grain boundaries, high‐density dislocations, and localized strains, resulting in high thermoelectric performance at room temperature. Reproduced with permission.^{[ 75 ]} Copyright 2020, American Chemical Society.

Figure 6

a) Schematic aqueous synthesis of Ag₂Se at room temperature. b) Thermoelectric zT as a function of temperature for Ag₂Se and x%Cu⁺:Ag₂Se (x = 1.0, 1.5 and 2.0) samples. Temperature dependent c) electrical conductivity (σ) and d) Seebeck coefficient (S) for Ag₂Se and Ag⁰:Ag₂Se pellets. Reproduced with permission.^{[ 43 ]} Copyright 2022, American Chemical Society.

Figure 7

a,b) Crystal structure of AgBiS₂ in a cubic unit cell: a) undistorted and b) small distortions of central cations away from octahedral center. The gray, violet, and orange colors represent Ag, Bi, and S atoms, respectively. c) Atom‐projected phonon density of states (PhDOS) for AgBiS₂. d) Temperature‐dependent lattice thermal conductivity (κ _lat) of cubic AgBiS₂. The dashed line is the theoretical minimum of lattice thermal conductivity (κ _min ≈ 0.34 W m⁻¹ K⁻¹) of AgBiS₂. Reproduced with permission.^{[ 91 ]} Copyright 2019, American Chemical Society.

Figure 8

a) Crystal structures of Ag₂Se and KAg₃Se₂:2D KAg₃Se₂ is viewed as a dimensionally reduced derivative of 3D Ag₂Se. Reproduced with permission.^{[ 21 ]} Copyright 2018, American Chemical Society. b) Crystal structures and c) thermoelectric performance of SnSe, AgSnSbSe₃, and AgSnSbSe_1.5Te_1.5. Reproduced with permission.^{[ 116 ]} Copyright 2020, American Chemical Society.

Figure 9

Temperature dependent a) Seebeck coefficient (α), b) electrical conductivity (σ), c) lattice thermal conductivity (κ _L), d) thermoelectric zT of Ag_{1+2 x} InSe_{2+ x} (x = 0–0.40). An inset in (c) is the total thermal conductivity. Reproduced with permission.^{[ 127 ]} Copyright 2020, American Chemical Society. e) Temperature‐dependent zT and thermal conductivity of (GeSe)_0.03(AgBiSe₂)_0.97 and pristine AgBiSe₂ alloys. f) Backscattered electron image of AgBiSe₂ and inverse fast Fourier transform (FFT) image, showing the formation of Bi₂Se₃ nanoprecipitate embedded in the Ge‐doped AgBiSe₂ matrix. Reproduced with permission.^{[ 40 ]} Copyright 2017, Elsevier. g) Rock‐salt cubic structure of (SnSe)_0.5(AgSbSe₂)_0.5. h) Magnitude of Se distortion along [111] direction with increasing temperature. Reproduced with permission.^{[ 126 ]} Copyright 2021, American Chemical Society.

Figure 10

Schematic colloidal synthesis of alloyed and hybrid nanoparticles based on the Ag‐Au‐Se ternary system. Temperature dependent a) electrical conductivity (σ), b) Seebeck coefficient (S), c) thermal conductivity (κ), and d) thermoelectric zT of binary Ag₂Se nanomaterial, Ag–Au–Se ternary nanocomposite, and Ag₂Se ingot reference (bulk Ag₂Se). Reproduced with permission.^{[ 46 ]} Copyright 2016, American Chemical Society.

Figure 11

a) High resolution TEM image of Ag₂Te_0.6S_0.4 showing a typical crystallite in the amorphous matrix. b) SEM image of the fracture surface showing crossing shear bands. c) Room temperature Hall mobility in comparison with other amorphous inorganic materials. Reproduced with permission.^{[ 79 ]} Copyright 2020, AAAS.

Figure 12

a) Thermoelectric performance of the flexible Ag/Ag₂Se/nylon composite film and the assembled thermoelectric generator (TEG). Reproduced with permission.^{[ 130 ]} Copyright 2021, American Chemical Society. b) Schematic illustration of phonon scattering mechanisms in the PVP/Ag₂Se composite thermoelectric film. Reproduced with permission.^{[ 133 ]} Copyright 2021, Elsevier.

Figure 13

a) Schematic illustration of six‐leg TEG and thermoelectric performance of TEG fabricated with the flexible Ag₂Se/Ag/CuAgSe/nylon composite films. Reproduced with permission.^{[ 132 ]} Copyright 2020, Royal Society of Chemistry. b) Schematic illustration and thermoelectric performance of the TEG fabricated with the flexible Ag₂Se/nylon film. Digital photo of 4.3 mV voltage produced from four‐leg Ag₂Se/nylon‐based TEG from a temperature difference of 6.7 K between the wrist and the environment. Reproduced with permission.^{[ 129 ]} Copyright 2020, American Chemical Society. c) Schematic illustration of the additive manufacturing‐based fabrication process: printing−painting−sintering and device structures of three different 3D‐TEG prototypes of cuboid shape, cylindrical gear shape and sawtooth shape. Reproduced with permission.^{[ 143 ]} Copyright 2022, American Chemical Society.