Thermoelectric Energy Conversion Research Articles

Novel Hybrid Modeling and Analysis Method for Steam Reforming Solid Oxide Fuel Cell System Multifault Degradation Fusion Assessment.

Steam reforming solid oxide fuel cell (SOFC) systems are important devices to promote carbon neutralization and clean energy conversion. It is difficult to monitor system working conditions in real time due to the possible fusion fault degradation under high temperatures and the seal environment, so it is necessary to design an effective system multifault degradation assessment strategy for solid oxide fuel cell systems. Therefore, in this paper, a novel hybrid model is developed. The hybrid model is built to look for the system fault reason based on first principles, machine learning (radial basis function neural network), and a multimodal classification algorithm. Then, stack, key balance of plant components (afterburner, heat exchanger, and reformer), thermoelectric performance, and system efficiency are studied during the progress of the system experiment. The results show that the novel hybrid model can track well the system operation trend, and solid oxide fuel cell system working dynamic performance can be obtained. Furthermore, four fault types of solid oxide fuel cell systems are analyzed with thermoelectric parameters and energy conversion efficiency based on transition and fault stages, and two cases are also successful by using the built model to decouple the multifault degradation fusion. In addition, the solid oxide fuel cell multifault degradation fusion assessment method proposed in this paper can also be used in other fuel cell systems.

Atom Probe Tomography Advances Chalcogenide Phase‐Change and Thermoelectric Materials

Main‐group chalcogenides show outstanding performance for phase‐change data storage and thermoelectric energy conversion applications. A common denominator for these different property requirements is ascribed to the metavalent bonding (MVB) mechanism. Atom probe tomography (APT) provides a unique way to distinguish MVB from other bonding mechanisms by determining the bond‐breaking behavior. Specifically, an unusually high probability to dislodge several fragments upon one successful laser pulse (probability of multiple events [PME]) is found in metavalently bonded crystalline phase‐change and thermoelectric materials. In contrast, amorphous phase‐change materials and poor thermoelectrics usually show lower PME values. This indicates that the large optical and electrical contrast between the crystalline and amorphous chalcogenides is attributed to a transition of chemical bonding. A strong correlation between high thermoelectric performance and large PME is also established. Besides, APT can investigate structural defects on the subnanometer scale. These characteristics reveal the interdiffusion of elements in interfacial phase‐change materials and revisit its switching mechanism. The complex role of structural defects such as grain boundaries in tuning the thermoelectric properties can also be unraveled by investigating the local composition and bonding mechanism at defects. This review demonstrates that APT is a powerful technique for designing phase‐change and thermoelectric materials.

Synthetic Strategies toward High Entropy Materials: Atoms-to-Lattices for Maximum Disorder.

High-entropy materials are a nascent class of materials that exploit a high configurational entropy to stabilize multiple elements in a single crystal lattice and to yield unique physical properties for applications in energy storage, catalysis, and thermoelectric energy conversion. Initially, the synthesis of these materials was conducted by approaches requiring high temperatures and long synthetic time scales. However, successful homogeneous mixing of elements at the atomic level within the lattice remains challenging, especially for the synthesis of nanomaterials. The use of atom-up synthetic approaches to build crystal lattices atom by atom, rather than the top-down alteration of extant crystalline lattices, could lead to faster, lower-temperature, and more sustainable approaches to obtaining high entropy materials. In this Perspective, we discuss some of these state-of-the-art atom-up synthetic approaches to high entropy materials and contrast them with more traditional approaches.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

AbstractComprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p‐type cubic AgSnSbTe3 which shows an innately ultra‐low lattice thermal conductivity (κlat) of 0.47–0.27 Wm−1 K−1 and a high electrical conductivity (1238 – 800 S cm−1) in the temperature range 294–723 K. We investigated the origin of the low κlat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe3. Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence‐band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe3.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3.

Comprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p-type cubic AgSnSbTe3 which shows an innately ultra-low lattice thermal conductivity (κlat ) of 0.47-0.27 Wm-1 K-1 and a high electrical conductivity (1238 - 800 S cm-1 ) in the temperature range 294-723 K. We investigated the origin of the low κlat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe3 . Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence-band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe3 .

Size and kink effects on thermal conductivity in nickel nanowires

The potential applications of nanowires in thermal management and thermoelectric energy conversion have sparked extensive research on thermal transport in various nanowires. Nickel nanowires, with their unique properties and promising applications, have been extensively studied. However, the influence of size, particularly the impact of kink structures, on the thermal transport behavior in nickel nanowires remains unclear. In this paper, we employed electron-beam lithography and liftoff techniques to fabricate suspended nickel nanowires with varying sizes and kinks to experimentally investigate the size and kink effect on the thermal conductivity. The experimental results revealed that the thermal transport behavior of nickel nanowires is significantly influenced by both size and kink effects. Notably, as the nanowire size decreases, the thermal conductivity also decreases. Furthermore, we discovered that the thermal conductivity can be adjusted by altering the number and angle of kinks. Increasing the number of kinks from 18 to 36 resulted in a significant decrease in thermal conductivity. In contrast, as the kink angle decreased from 157° to 90°, the thermal conductivity also decreased. However, intriguingly, when the kink angle was further decreased from 90° to 43°, the thermal conductivity increases. This non-monotonic change in thermal conductivity with the kink angle provides an interesting insight into the intricate behavior of heat carriers in kinked nickel nanowires. Additionally, we found that varying the alloy elements can profoundly alter the thermal conductivity of nanowires with kinks. These results offer valuable insights into the behaviors of heat carriers, including electrons and phonons, during heat transfer in nickel nanowires.

Decorated dislocations lead to dynamically optimized thermoelectric performance in N-type PbTe

A high thermoelectric energy conversion efficiency requires a large power factor and a low thermal conductivity over a broad temperature range. Optimizing the temperature-dependent carrier concentration and introducing structural defects are effective methods to realize these goals. In this work, we demonstrate that Ag-decorated dislocations can facilitate the dissolution of dopants into the PbTe matrix at elevated temperatures to optimize the carrier concentration in conjunction with the static doping effect of Sb. These spatially aligned and chemically decorated dislocations strongly scatter medium-to low-frequency phonons even at a rather low number density on the order of 1010 cm−2. Moreover, these dislocations show little scattering effect on electrons, maintaining a high carrier mobility. As a consequence, a maximum thermoelectric figure-of-merit, zT, of 1.5 at 750 K and an excellent average zT value of 1.1 between 300 and 800 K are obtained in Sb and dilute Ag2Te co-doped n-type PbTe. This work demonstrates that decorated dislocation networks could optimize the electrical and thermal transport properties of thermoelectrics in a wide temperature window.

3D Printing of Bi2Te3-Based Thermoelectric Materials with High Performance and Shape Controllability.

Thermoelectric (TE) energy conversion technology provides a promising way to improve the efficiency of fossil energy by generating electricity from low-grade waste heat. With regard to these applications, thermoelectric generators (TEGs) should be designed from system integration perspectives to simultaneously improve heat transfer efficiency and system simplification as well as the robust mechanical properties. However, typical TEGs fabricated by conventional methods barely accomplish such requirements. Herein, high-quality TEGs were assembled by combining the well-flowable spherical bismuth telluride (BT) powdered precursors and selective laser melting (SLM) technology. By optimizing the electronic and phonon transport properties through defect engineering driven by 3D printing, a high figure of merit was accomplished for 1.27 (p-type) and 1.13 (n-type) in BT. This achievement is primarily attributed to the nonequilibrium solidification mechanism, which leads to the formation of multiscale defects during the 3D printing process. The introduction of these multiscale defects enables the effective scattering of wide frequency phonons, leading to a substantial reduction in lattice thermal conductivity. Meanwhile, robust mechanical properties were obtained in the printed p-type/n-type BT TE materials parallel to the building direction (BD) with a compressive strength reaching 257/250 MPa by employing the fine grain structure and the high density of nanotwins introduced during the SLM process. A well shape-controllable and high-performance TEG was designed using 3D-printed BT half-rings, and an output power of 134 mW was achieved at a temperature gradient of 38.9 °C. Our study opens a new route for the great potential of TE materials based on standard commercial SLM 3D printing technology for low-grade waste heat emitted from structures with heterogeneous shapes.

Modulation doping of p-type Cu12Sb4S13 toward improving thermoelectric performance

The commercial viability of thermoelectric (TE) devices relies heavily on two factors: cost reduction and efficiency enhancement. In this study, we first produce p-type Cu12Sb4S16–x (x = 0, 3, 4) using a low-temperature bottom-up approach and demonstrate Cu12Sb4S13 to show the best TE performance among the three tested compositions. Subsequently, the TE energy conversion efficiency of Cu12Sb4S13 is further enhanced by optimizing its electronic band structure through the incorporation of small amounts of tellurium. At an optimal Te content of 5 mol%, more than a twofold increase in the TE figure of merit (zT) is obtained. To gain insight into the mechanism of improvement on the transport properties of the material, we compare the interphase transport mechanism by incorporating nanodomains of different metals (Ag and Cu) into the Cu12Sb4S13 matrix. The improved electrical conductivity obtained with Cu12Sb4S13-Te nanocomposites is attributed to a charge flooding of the Cu12Sb4S13 surface. In contrast, excessive downward band-bending at the interphases of Ag/Cu metal-semiconductor drastically reduces the electrical conductivity. Besides, a weighted mobility (μw) analysis shows a dominant thermal activation of carriers in Cu12Sb4S13-Te nanocomposites. In this material, a strong decrease in lattice thermal conductivity is also found, which is associated with a phonon-carrier scattering mechanism. Our work shows the importance of proper band-engineering in TE nanocomposites to decouple electrical and thermal transport to enhance TE performance, and the efficacy of μw for electrical and thermal transport analysis.

Proton-Coupled Electron Transfer Aided Thermoelectric Energy Conversion and Storage.

Low-grade heat is ubiquitous in the environment and its thermoelectric conversion by the ionic conductors remains a challenge because of the low efficiency and poor sustainability. Here we demonstrate that the thermoelectric performances can be boosted by combining the Soret effect of protons and proton-coupled electron transfer (PCET) reaction of benzoquinone and hydroquinone in hydrogels. An overall enhancement of thermopower (25.9 mV K-1), power factor (5 mW m-1 K-2), figure of merit (>2.4) and continuity of power output is achieved. Moreover, an energy-storage function can be achieved by the redox couple, and a retained power output of 27.7%, or 14 mW m-2 for more than 3 hours is obtained by the re-balance of PCET reactants in the hydrogel after the removal of the temperature gradient.

Oxide Strontium-Barium Perovskites Ceramics: Examinations of Structural Phase Transitions and Potential Application as Oxygen Carriers

The structural properties of selected (Ba1−xSrx)PbO3 ceramics were examined at 14–1148 K using X-ray powder diffraction (XRD). These materials are attractive due to their variety of applications, such as, for example, high-temperature thermoelectric energy conversion. Attention was paid to this paper as a continuation of the previous examinations of higher Sr2+ concentrations. The type of perovskite distortion and temperatures of the structural phase transitions (SPTs) were determined from the splitting of certain pseudocubic lines. At this point, for example (Ba0.3Sr0.7)PbO3 showed three temperature-induced SPTs. When the amount of Sr increased in the samples, no phase transition was observed, which is contrary to the data previously demonstrated in the literature. The quality of the ceramics was examined by scanning electron microscopy-energy dispersion X-ray spectroscopy (SEM-EDS), demonstrating their homogeneity and uniform elements dispersion. As a result of profound crystal investigations, confirmed by thermogravimetric analysis and quadrupole mass spectroscopy (TGA-QMS), a phase diagram was prepared for the (Ba1−xSrx)PbO3 system based on our former and recent study. Also, the investigation of a new application for the (Ba1−xSrx)PbO3 family is presented in this paper for the first time. The TGA analysis was conducted on Illinois#6 hard coal to evaluate the capability of perovskites to be used in the chemical looping combustion (CLC) process in a range of temperatures 1073–1173 K. Due to its thermal stability and reactivity, Ba0.9Sr0.1PbO3 is the material with the greatest potential to be applied as an oxygen carrier. The combination of strontium and barium offers encouraging results compared to the pure barium and strontium lead oxide perovskites.

Experimental Investigation of Coupled Transport Mechanisms in a PEM Based Thermoelectric Energy Converter

Thermoelectric energy converters based on galvanic cells (TGC) offer the possibility of direct conversion of low-temperature waste heat into electrical energy and could therefore be a promising approach for an increase in the overall efficiency of energy conversion. Due to an externally applied heat source, a temperature gradient across the electrolyte is induced, leading to a gradient in the chemical potential of the species and an electrical potential difference between the electrodes. The aim of approaching an internal equilibrium state leads to various coupled molecular transport mechanisms taking place in the electrolyte, impacting the open circuit voltage (OCV) and the performance of the TGC. By applying the theory of non-equilibrium thermodynamics (NET) to describe these coupled processes, the interactions that occur can be characterized in more detail. In this work, a polymer electrolyte membrane (PEM)-based TGC with two H2/H2O electrodes of different temperatures and gas compositions is experimentally investigated. By controlling the gradients in temperature and concentration, different impacts on the resulting OCV can be identified. In addition, we present the measured coupling coefficient, representing the singular relation between the transport of the hydrogen ions inside the membrane and the electrical potential difference between the electrodes for a wide variety of working conditions.

Unraveling Thermal Transport Properties of MoTe2 Thin Films Using the Optothermal Raman Technique.

Understanding phonon transport and thermal conductivity of layered materials is not only critical for thermal management and thermoelectric energy conversion but also essential for developing future optoelectronic devices. Optothermal Raman characterization has been a key method to identify the properties of layered materials, especially transition-metal dichalcogenides. This work investigates the thermal properties of suspended and supported MoTe2 thin films using the optothermal Raman technique. We also report the investigation of the interfacial thermal conductance between the MoTe2 crystal and the silicon substrate. To extract the thermal conductivity of the samples, temperature- and power-dependent measurements of the in-plane E2g1 and out-of-plane A1g optical phonon modes were performed. The results show remarkably low in-plane thermal conductivities at room temperature, at around 5.16 ± 0.24 W/m·K and 3.72 ± 0.26 W/m·K for the E2g1 and the A1g modes, respectively, for the 17 nm thick sample. These results provide valuable input for the design of electronic and thermal MoTe2-based devices where thermal management is vital.

The amplitudons of dynamical as well as stable binary cubic crystals for hot-electron cooling and enhanced thermoelectric effect

Dynamical properties of materials is determined using force constant expansions in the Hiphive package and density functional theory as implemented in the Gpaw code. Thermal conductivity of alkaline earth tellurides was determined, and the narrower width between amplitudons and electrical modes exhibited more supersonic cooling and heat transportation, and our findings are in perfect agreement with experimental research. When compared to the other binary crystals, BaTe appears to offer a dual advantage of shorter phonon lifetime at normal temperature as well as a speedier thermoelectric energy conversion due to the tiny gap between amplitudons and electric modes.

Theoretical Study of Thermoelectric Properties of a Single Molecule of Diphenyl-Ether

Taking into consideration the research that has been conducted on the optical and electrical properties of molecular systems, especially the good thermoelectric energy conversion at a nanometric scale that such systems have presented, here we present a new alternative by using a particular diphenyl-ether molecule as a functional device. Such a molecular system is modeled as a planar segment coupled to two electrodes in the first-neighbor approximation within a tight-binding Hamiltonian. We study the electrical and thermal properties of diphenyl-ether molecules such as the electric current, electrical and thermal conductance, Seebeck coefficient, and figure of merit, in the strong and weak coupling regimes, considering different structural configurations and variations with temperature. Our results could be valuable for laboratory applications and/or verification since we characterize the diphenyl-ether molecule as a semiconductor device for different structural models.

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials.

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu2CoSnS4-xSex, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu2CoSnS4-xSex makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu2CoSnS4-xSex only intensified this effect resulting in a lower lattice component of the thermal conductivity (κL) for Se-rich samples. As a result of the enhanced power factor S2ρ-1 and the low κL, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.

Preliminary conceptual design of an electronuclear system for space applications

The purpose of this paper is to present a preliminary conceptual design of a nuclear power system for space applications. This design was produced within the framework of a collaboration between CNES and CEA. The targeted electrical power of the system is 10 kWe for a period of at least 10 years. The scalability towards larger (30 kWe) and lower (about 1 kWe) power has been investigated. The system must be technologically “affordable” and robust over a 10-year horizon, which de facto eliminates the choice of too innovative and unqualified technologies in conditions representative of the missions; the system must be able to be launched in Ariane 6 while respecting the launcher mass and size capacities. Given the intermediate power level targeted and the potential use of the system on the Moon or Mars, the CEA favored design options associated with a compact and monolithic radiator (allowing it to be landed) rather than minimizing the system mass. Taking into account the desired characteristics for the nuclear power system, the CEA proposes main conceptual design features which includes: a thermal spectrum core whose fuel is UO2 enriched to 19.75% in U235, zirconium fuel cladding, a zirconium hydride (ZrH2) moderator, and a beryllium reflector. The proposed shield is made of tungsten and lithium hydride (LiH). A core cooling system made up of 150 steel heat pipes operating with a sodium–potassium alloy (NaK) has been pre-designed and a thermoelectric energy converter made up of SiGe thermo-elements (hot temperature of 950 K and cold temperature of 700 K) generates electrical power. A “hot” carbon composite radiator lined with titanium NaK heat pipes rejects the heat of the system. This system has intrinsic redundancies conferred by the heat pipes and the large number of thermo-elements of the converter as well as high passivity, in particular in its ability to adapt its power to electricity demand. In its version limited to 10 kWe, the system whose height is about 5 m, enables to keep a height of 13 m for a payload to be launched in the Ariane 6 fairing and its mass is estimated at about 2 tons. To access a version of the system capable of producing 30 kWe, an additional mass estimated at around 1 ton is to be expected as well as a lower useful height in the fairing under the system, which would then be 10 m.

Effects of hydrothermal synthesis conditions on material properties of Cu-Se compounds for thermoelectric and photothermal energy conversion

Copper selenide (CuSe) is a promising material for these applications due to its unique properties such as meaningful electrical conductivity, Seebeck coefficient, and photothermal conversion efficiency. Here, a systematic investigation into the hydrothermal synthesis and characterization of copper selenides (CuSe) was performed for thermoelectric and photothermal applications. Depending on the feeding ratio of the Cu and Se precursors and reducing agent, CuSe2, CuSe, and Cu2Se were synthesized at a relatively low temperature and within a limited time frame. The crystal growth and morphology of the compounds were composition-dependent, which led to variations in their electrical conductivity, Seebeck coefficient, and thermoelectric properties. Synthesized Cu-Se compounds also showed promising photothermal conversion characteristics, which were utilized to increase the energy harvesting of the thermoelectric module under light exposure. This study provide important insights into the relationship between the synthesis conditions and the material properties, as well as the potential of CuSe-based materials for thermoelectric and photothermal applications.

Flexible Active Peltier Coolers Based on Interconnected Magnetic Nanowire Networks.

Thermoelectric energy conversion based on flexible materials has great potential for applications in the fields of low-power heat harvesting and solid-state cooling. Here, we show that three-dimensional networks of interconnected ferromagnetic metal nanowires embedded in a polymer film are effective flexible materials as active Peltier coolers. Thermocouples based on Co-Fe nanowires exhibit much higher power factors and thermal conductivities near room temperature than other existing flexible thermoelectric systems, with a power factor for Co-Fe nanowire-based thermocouples of about 4.7 mW/K2m at room temperature. The effective thermal conductance of our device can be strongly and rapidly increased by active Peltier-induced heat flow, especially for small temperature differences. Our investigation represents a significant advance in the fabrication of lightweight flexible thermoelectric devices, and it offers great potential for the dynamic thermal management of hot spots on complex surfaces.

Frontispiece: Metal Complex Molecular Junctions as Thermoelectric Devices

Thermoelectric energy conversion is of interest as an energy harvesting technology. Metal complex molecular junctions are promising architectures for thermoelectric devices because they allow fine tuning of the transmission spectra, for example by varying the metal centers, metal–metal interactions and mechanical manipulations. For more details, see the Concept article by Y. Tanaka. (DOI: 10.1002/chem.202300472).