Thermoelectrocatalysis: an emerging strategy for converting waste heat into chemical energy.

This perspective defines and explores an innovative waste heat harvesting strategy, thermoelectrocatalysis (TECatal), emphasizing materials design and potential applications in clean energy, environmental, and biomedical technologies.

High-performance cryo-temperature ionic thermoelectric liquid cell developed through a eutectic solvent strategy.

Ionic thermoelectric (i-TE) liquid cells offer an environmentally friendly, cost effective, and easy-operation route to low-grade heat recovery. However, the lowest temperature is limited by the freezing temperature of the aqueous electrolyte. Applying a eutectic solvent strategy, we fabricate a high-performance cryo-temperature i-TE liquid cell. Formamide is used as a chaotic organic solvent that destroys the hydrogen bond network between water molecules, forming a deep eutectic solvent that enables the cell to operate near cryo temperatures (down to -35 °C). After synergistic optimization of the electrode and cell structure, the as-fabricated liquid i-TE cell with cold (-35 °C) and hot (70 °C) ends achieve a high power density (17.5 W m-2) and a large two-hour energy density (27 kJ m-2). In a prototype 25-cell module, the open-circuit voltage and short-circuit current are 6.9 V and 68 mA, respectively, and the maximum power is 131 mW. The anti-freezing ability and high output performance of the as-fabricated i-TE liquid cell system are requisites for applications in frigid regions.

Polaron interfacial entropy as a route to high thermoelectric performance in DAE-doped PEDOT:PSS films.

Enhancing the thermoelectric transport properties of conductive polymer materials has been a long-term challenge, in spite of the success seen with molecular doping strategies. However, the strong coupling between the thermopower and the electrical conductivity limits thermoelectric performance. Here, we use polaron interfacial occupied entropy engineering to break through this intercoupling for a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) thin film by using photochromic diarylethene (DAE) dopants coupled with UV-light modulation. With a 10-fold enhancement of the thermopower from 13.5 µV K-1 to 135.4 µV K-1 and almost unchanged electrical conductivity, the DAE-doped PEDOT:PSS thin film achieved an extremely high power factor of 521.28 µW m-1 K-2 from an original value of 6.78 µW m-1 K-2. The thermopower was positively correlated with the UV-light intensity but decreased with increasing temperature, indicating resonant coupling between the planar closed DAE molecule and PEDOT. Both the experiments and theoretical calculations consistently confirmed the formation of an interface state due to this resonant coupling. Interfacial entropy engineering of polarons could play a critical role in enhancing the thermoelectric performance of the organic film.

Performance boost for bismuth telluride thermoelectric generator via barrier layer based on low Young's modulus and particle sliding.

The lack of desirable diffusion barrier layers currently prohibits the long-term stable service of bismuth telluride thermoelectric devices in low-grade waste heat recovery. Here we propose a new design principle of barrier layers beyond the thermal expansion matching criterion. A titanium barrier layer with loose structure is optimized, in which the low Young's modulus and particle sliding synergistically alleviates interfacial stress, while the TiTe2 reactant enables metallurgical bonding and ohmic contact between the barrier layer and the thermoelectric material, leading to a desirable interface characterized by high-thermostability, high-strength, and low-resistivity. Highly competitive conversion efficiency of 6.2% and power density of 0.51 W cm-2 are achieved for a module with leg length of 2 mm at the hot-side temperature of 523 K, and no degradation is observed following operation for 360 h, a record for stable service at this temperature, paving the way for its application in low-grade waste heat recovery.

Wide-temperature-range thermoelectric n-type Mg3(Sb,Bi)2 with high average and peak zT values.

Mg3(Sb,Bi)2 is a promising thermoelectric material suited for electronic cooling, but there is still room to optimize its low-temperature performance. This work realizes >200% enhancement in room-temperature zT by incorporating metallic inclusions (Nb or Ta) into the Mg3(Sb,Bi)2-based matrix. The electrical conductivity is boosted in the range of 300-450 K, whereas the corresponding Seebeck coefficients remain unchanged, leading to an exceptionally high room-temperature power factor >30 µW cm-1 K-2; such an unusual effect originates mainly from the modified interfacial barriers. The reduced interfacial barriers are conducive to carrier transport at low and high temperatures. Furthermore, benefiting from the reduced lattice thermal conductivity, a record-high average zT > 1.5 and a maximum zT of 2.04 at 798 K are achieved, resulting in a high thermoelectric conversion efficiency of 15%. This work demonstrates an efficient nanocomposite strategy to enhance the wide-temperature-range thermoelectric performance of n-type Mg3(Sb,Bi)2, broadening their potential for practical applications.

General Figures of Merit ZQ for Thermoelectric Generators Under Constant Heat-In Flux Boundary.

The thermoelectric figure of merit ZT bridges the efficiency and material parameters for a thermoelectric device operating under constant temperature of the hot- and cold-source thermal boundary (Type-I TB). However, many application scenarios fall under the constant heat-in flux (qh ) and constant cold-source temperature (Tc ) thermal boundary (Type-II TB), for which a figure of merit is absent for more than half a century. This study aims to fill this gap and propose a figure of merit ZQD for the thermoelectric devices under the Type-II TB condition, defined as Z Q D = ( Z T c Z T c + 1 ) ( h κ ) ( q h T c ) $Z{Q}_{\mathrm{D}} = ( {\frac{{Z{T}_{\mathrm{c}}}}{{Z{T}_{\mathrm{c}} + 1}}} )( {\frac{h}{\kappa }} )( {\frac{{{q}_{\mathrm{h}}}}{{{T}_{\mathrm{c}}}}} )$ , where Z, h, and κ are the traditional figure of merit, leg height, and thermal conductivity, respectively. The effectiveness of ZQD is verified through both numerical calculations and experiments, which are more accurate and practical than ZT. Furthermore, a system-level figure of merit ZQS is suggested after considering the external thermal resistance. Finally, optimization strategies for thermoelectric systems based on ZQS are proposed, showing a 30% enhancement in the efficiency. ZQD and ZQS are expected to be widely used in the thermoelectric field.

Ultrasensitive mechanical/thermal response of a P(VDF-TrFE) sensor with a tailored network interconnection interface.

Ferroelectric polymers have great potential applications in mechanical/thermal sensing, but their sensitivity and detection limit are still not outstanding. We propose interface engineering to improve the charge collection in a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) copolymer (P(VDF-TrFE)) thin film via cross-linking with poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS) layer. The as-fabricated P(VDF-TrFE)/PEDOT:PSS composite film exhibits an ultrasensitive and linear mechanical/thermal response, showing sensitivities of 2.2 V kPa-1 in the pressure range of 0.025-100 kPa and 6.4 V K-1 in the temperature change range of 0.05-10 K. A corresponding piezoelectric coefficient of -86 pC N-1 and a pyroelectric coefficient of 95 µC m-2 K-1 are achieved because more charge is collected by the network interconnection interface between PEDOT:PSS and P(VDF-TrFE), related to the increase in the dielectric properties. Our work shines a light on a device-level technique route for boosting the sensitivity of ferroelectric polymer sensors through electrode interface engineering.

Bi-Deficiency Leading to High-Performance in Mg3 (Sb,Bi)2 -Based Thermoelectric Materials.

Mg3 (Sb,Bi)2 is a potential nearly-room temperature thermoelectric compound composed of earth-abundant elements. However, complex defect tuning and exceptional microstructural control are required. Prior studies have confirmed the detrimental effect of Mg vacancies (VMg ) in Mg3 (Sb,Bi)2 . This study proposes an approach to mitigating the negative scattering effect of VMg by Bi deficiency, synergistically modulating the electrical and thermal transport properties to enhance the thermoelectric performance. Positron annihilation spectrometry and Cs -corrected scanning transmission electron microscopy analyses indicated that the VMg tends to coalesce due to the introduced Bi vacancies (VBi ). The defects created by Bi deficiency effectively weaken the scattering of electrons from the intrinsic VMg and enhance phonon scattering. A peak zT of 1.82 at 773 K and high conversion efficiency of 11.3% at ∆T = 473 K are achieved in the optimized composition of Mg3 (Sb,Bi)2 by tuning the defect combination. This work demonstrates a feasible and effective approach to improving the performance of Mg3 (Sb,Bi)2 as an emerging thermoelectric material.

Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg3 Sb2 -Based Thermoelectric Materials.

Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg3 Sb1.5 Bi0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy.

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 µW m K-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, εmax, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher εmax values, which significantly improved the processability of the solution-derived films.

Modulation of electrical and thermal transports through lattice distortion in BaTi1-xNbxO3solid solutions.

The electron and heat transports in solids are through the movement of carrier electrons and quantized lattice vibrations (phonons), which are sensitive to the lattice distortion and ionized impurities, and are essential aspects for the development of novel thermoelectric materials. In this study, we systematically investigated the modulations of electrical and thermal conductivities of BaTi1-xNbxO3solid solution (BTNO, 0 ≤ x ≤ 1) epitaxial films. At room temperature, BaTiO3belongs to tetragonal perovskite and exhibits electron conduction through doubly degenerated Ti 3d-t2gorbitals upon doping, while BaNbO3belongs to cubic perovskite and exhibits metallic electron conduction through partially filled triply degenerate Nb 4d-t2gorbitals. By controlling the Ti/Nb ratio, we found a dual modulation effect on both the lattice structures and conduction band, which affects the electrical and thermal conductivities. Similar to the SrTi1-xNbxO3solid solution (STNO, 0 ≤ x ≤ 1) system, a phase transition was detected atx âˆ¼ 0.5, at which both the electron and heat transports exhibit abrupt changes. Unlike the transition in STNO, which was attributed to a polaronic phase transition, the transition in BTNO was due to contributions from both the lattice distortion and polaron effect. By controlling the lattice distortion, conduction band, and polaronic phase transitions, the electrical and thermal conductivity of BTNO epitaxial films are modulated within a much greater range than those of the STNO epitaxial films. Due to the double contribution of electron carriers and phonon to thermal conductivity (κ), the maximumκmodulation ratio of BTNO epitaxial films was ∼6.9. Our research provides an effective route to design electrical/thermal management materials.

Large Transverse and Longitudinal Magneto-Thermoelectric Effect in Polycrystalline Nodal-Line Semimetal Mg3 Bi2.

Topological semimetals provide new opportunities for exploring novel thermoelectric phenomena, owing to their exotic and nontrivial electronic structure topology around the Fermi surface. Herein, the discovery of large transverse and longitudinal magneto-thermoelectric (MTE) effects in Mg3 Bi2 is reported and predicted to be a type-II nodal-line semimetal in the absence of spin-orbit coupling (SOC). The maximum transverse power factor is 2182 µW m-1 K-2 at 13.5 K and 6 Tesla. The longitudinal power factor reaches up to 3043 µW m-1 K-2 , which is 20 times higher than that in a zero-strength magnetic field and is also comparable to state-of-the-art MTE materials. By compensating the Mg loss in Mg-rich conditions for tuning the carrier concentration close to intrinsic state, the sample fabricated in this study exhibits a large linear non-saturating magnetoresistance of 940% under a field of 14 Tesla. Using density functional calculations, the authors attribute the underlying mechanism to the parent linear-dispersed nodal-line electronic structure without SOC and the anisotropic Fermi surface shape with SOC, highlighting the essential role of high carrier mobility and open electron orbits in the moment space. This work offers a new avenue toward highly efficient MTE materials through defect engineering in polycrystalline topological semimetals.

Few-layer bismuth selenide cathode for low-temperature quasi-solid-state aqueous zinc metal batteries.

The performances of rechargeable batteries are strongly affected by the operating environmental temperature. In particular, low temperatures (e.g., ≤0 °C) are detrimental to efficient cell cycling. To circumvent this issue, we propose a few-layer Bi2Se3 (a topological insulator) as cathode material for Zn metal batteries. When the few-layer Bi2Se3 is used in combination with an anti-freeze hydrogel electrolyte, the capacity delivered by the cell at -20 °C and 1 A g-1 is 1.3 larger than the capacity at 25 °C for the same specific current. Also, at 0 °C the Zn | |few-layer Bi2Se3 cell shows capacity retention of 94.6% after 2000 cycles at 1 A g-1. This behaviour is related to the fact that the Zn-ion uptake in the few-layer Bi2Se3 is higher at low temperatures, e.g., almost four Zn2+ at 25 °C and six Zn2+ at -20 °C. We demonstrate that the unusual performance improvements at low temperatures are only achievable with the few-layer Bi2Se3 rather than bulk Bi2Se3. We also show that the favourable low-temperature conductivity and ion diffusion capability of few-layer Bi2Se3 are linked with the presence of topological surface states and weaker lattice vibrations, respectively.

Vacancy Modulating Co3 Sn2 S2 Topological Semimetal for Aqueous Zinc-Ion Batteries.

Weyl semimetals (WSMs) with high electrical conductivity and suitable carrier density near the Fermi level are enticing candidates for aqueous Zn-ion batteries (AZIBs), meriting from topological surface states (TSSs). We propose a WSM Co3 Sn2 S2 cathode for AZIBs showing a discharge plateau around 1.5 V. By introducing Sn vacancies, extra redox peaks from the Sn4+ /Sn2+ transition appear, which leads to more Zn2+ transfer channels and active sites promoting charge-storage kinetics and Zn2+ storage capability. Co3 Sn1.8 S2 achieves a specific energy of 305 Wh kg-1 (0.2 Ag-1 ) and a specific power of 4900 Wkg-1 (5 Ag-1 ). Co3 Sn1.8 S2 and Znx Co3 Sn1.8 S2 benefit from better conductivity at lower temperatures; the quasi-solid Co3 Sn1.8 S2 //Zn battery delivers 126 mAh g-1 (0.6 Ag-1 ) at -30 °C and a cycling stability over 3000 cycles (2 Ag-1 ) with 85 % capacity retention at -10 °C.

Homo-composition and hetero-structure nanocomposite Pnma Bi2SeS2 - Pnnm Bi2SeS2 with high thermoelectric performance.

Nanocomposite engineering decouples the transport of phonons and electrons. This usually involves the in-situ formation or ex-situ addition of nanoparticles to a material matrix with hetero-composition and hetero-structure (heC-heS) interfaces or hetero-composition and homo-structure (heC-hoS) interfaces. Herein, a quasi homo-composition and hetero-structure (hoC-heS) nanocomposite consisting of Pnma Bi2SeS2 - Pnnm Bi2SeS2 is obtained through a Br dopant-induced phase transition, providing a coherent interface between the Pnma matrix and Pnnm second phase due to the slight structural difference between the two phases. This hoC-heS nanocomposite demonstrates a significant reduction in lattice thermal conductivity (~0.40 W m-1 K-1) and an enhanced power factor (7.39 µW cm-1 K-2). Consequently, a record high figure-of-merit ZTmax = 1.12 (at 773 K) and a high average figure-of-merit ZTave = 0.72 (in the range of 323-773 K) are achieved. This work provides a general strategy for synergistically tuning electrical and thermal transport properties by designing hoC-heS nanocomposites through a dopant-induced phase transition.

Enhanced Thermoelectric Performance by Strong Phonon Scattering at the Heterogeneous Interfaces of the Mg2Sn/Mg3Sb2 High-Content Nanocomposite.

Nano approaches are practical strategies to boost the thermoelectric figure of merit due to the strong phonon scattering from the grain boundaries and nanoinclusions. Here, we have reported a strong phonon scattering at the heterogeneous interfaces of Mg2Sn/Mg3Sb2 high-content nanocomposites (HCnCs). As a result, a significantly reduced lattice thermal conductivity of 1.09 W m-1 K-1 was observed in the equimolar Mg2Sn/Mg3Sb2 HCnC, 80% lower than pure Mg2Sn and 25% lower than pure Mg3Sb2. As a result, a high ZT ∼ 1.13 at 773 K was achieved in the Mg2Sn/Mg3Sb2 HCnC. Furthermore, various defects, including solid solutions, nanoinclusions, and misfit dislocations, were observed in both the Mg3Sb2 phase and the Mg2Sn phase through the microstructure characterization.

Wearable Thermoelectric Materials and Devices for Self-Powered Electronic Systems.

The emergence of artificial intelligence and the Internet of Things has led to a growing demand for wearable and maintenance-free power sources. The continual push toward lower operating voltages and power consumption in modern integrated circuits has made the development of devices powered by body heat finally feasible. In this context, thermoelectric (TE) materials have emerged as promising candidates for the effective conversion of body heat into electricity to power wearable devices without being limited by environmental conditions. Driven by rapid advances in processing technology and the performance of TE materials over the past two decades, wearable thermoelectric generators (WTEGs) have gradually become more flexible and stretchable so that they can be used on complex and dynamic surfaces. In this review, the functional materials, processing techniques, and strategies for the device design of different types of WTEGs are comprehensively covered. Wearable self-powered systems based on WTEGs are summarized, including multi-function TE modules, hybrid energy harvesting, and all-in-one energy devices. Challenges in organic TE materials, interfacial engineering, and assessments of device performance are discussed, and suggestions for future developments in the area are provided. This review will promote the rapid implementation of wearable TE materials and devices in self-powered electronic systems.

Fiber-Based Thermoelectric Materials and Devices for Wearable Electronics.

Fiber-based thermoelectric materials and devices have the characteristics of light-weight, stability, and flexibility, which can be used in wearable electronics, attracting the wide attention of researchers. In this work, we present a review of state-of-the-art fiber-based thermoelectric material fabrication, device assembling, and its potential applications in temperature sensing, thermoelectric generation, and temperature management. In this mini review, we also shine some light on the potential application in the next generation of wearable electronics, and discuss the challenges and opportunities.

Leaf-Inspired Flexible Thermoelectric Generators with High Temperature Difference Utilization Ratio and Output Power in Ambient Air.

The inherently small temperature difference in air environment restricts the applications of thermoelectric generation in the field of Internet of Things and wearable electronics. Here, a leaf-inspired flexible thermoelectric generator (leaf-TEG) that makes maximum use of temperature difference by vertically aligning poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and constantan thin films is demonstrated. Analytical formulae of the performance scales, i.e., temperature difference utilization ratio (φth) and maximum output power (Pmax), are derived to optimize the leaf-TEG dimensions. In an air duct (substrate: 36 °C, air: 6 °C, air flowing: 1 m s-1), the 10-leaf-TEG shows a φth of 73% and Pmax of 0.38 µW per leaf. A proof-of-concept wearable 100-leaf-TEG (60 cm2) generates 11 µW on an arm at room temperature. Furthermore, the leaf-TEG is flexible and durable that is confirmed by bending and brushing over 1000 times. The proposed leaf-TEG is very appropriate for air convection scenarios with limited temperature differences.

Enhanced Piezocatalytic Activity of Sr0.5Ba0.5Nb2O6 Nanostructures by Engineering Surface Oxygen Vacancies and Self-Generated Heterojunctions.

Piezocatalysis provides a promising strategy for directly converting weak mechanical energy into chemical energy. In this work, we report a simple one-step hydrogen reduction route for the simultaneous generation of surface defects and heterojunctions in Sr0.5Ba0.5Nb2O6 nanorods fabricated by a molten salt synthesis method. The as-fabricated Sr0.5Ba0.5Nb2O6/Sr2Nb2O7 nanocomposites with controllable oxygen vacancies exhibited excellent piezocatalytic activity under ultrasonic vibration, with an about 7 times enhancement of the rate constant (k = 0.0395 min-1) for rhodamine B degradation and an about 10 times enhancement of the water-splitting efficiency for hydrogen generation (109.4 µmol g-1 h-1) for the optimized sample (H2 annealed at 500 °C) compared to pristine Sr0.5Ba0.5Nb2O6 nanorods. This work demonstrates the essential role of a well-modulated oxygen vacancy concentration in the piezocatalytic activity and provides an inspiring guide for designing self-generated heterojunction piezocatalysts.