Controlling Electrostatic Interaction in PEDOT:PSS to Overcome Thermoelectric Tradeoff Relation

A high power factor must be achieved to improve the thermoelectric (TE) output of organic TE materials though the tradeoff between electrical conductivity and the Seebeck coefficient is a serious obstacle to the further development of these materials. Here, systematic control of the electrostatic interaction between a conducting polymer and a dopant induces a positive deviation from this TE tradeoff relation so that the electrical conductivity and the Seebeck coefficient simultaneously increase. Upon reduction of the electrostatic interaction, substantial changes in the film morphology, chain conformation, and crystalline ordering are observed, all of which critically affect the TE charge transport. As a result, the electrostatic interaction control is found to be an effective strategy to enhance the power factor, overcoming the tradeoff between TE parameters. Adapting this strategy to poly(3,4‐ethylenedioxythiophene):polystyrene‐sulfonate results in a remarkable power factor (=700.2 µW m^?1 K^?2 ) and figure of merit ZT (=0.25).     

Transport Properties of Polymer Semiconductor Controlled by Ionic Liquid as a Gate Dielectric and a Pressure Medium

An effective way of using ionic liquid as a gate dielectric as well as a pressure medium to tune the transport of an exemplary polymer semiconductor, poly(2,5‐bis(3‐tetradecyl‐thiophene‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT‐C14) is presented. Working as gate dielectrics, the ionic liquids exhibit the well‐known ability to induce dense carriers (>10²⁰ cm^?3) in the polymer film contributing to the high conductivity (≈10² S cm^?1). In addition, it is found that the ionic liquid works as a pressure medium at the highly charged state, leading to significant enhancement of conductivity. By combining both gating and pressuring, a crossover of transport properties is observed from one‐dimensional to three‐dimensional hopping, as the clear indication that the polymer film has accessed the regime adjacent to the transition region between insulator and metal. These results show an effective way of utilizing pressure effect of ionic liquid as a new degree of freedom in controlling transport of polymers, a method having strong potential to be generalized for even broader range of materials.     

Simultaneous Increases in Electrical Conductivity and Seebeck Coefficient of PEDOT:PSS Films by Adding Ionic Liquids into a Polymer Solution

The electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films were simultaneously improved by adding an ionic liquid (IL) into a polymer solution of the polymers. The maximum electrical conductivity of such a PEDOT:PSS/IL film reached 174 S cm⁻¹, more than an order of magnitude higher than that of pure PEDOT:PSS film, and the maximum Seebeck coefficient was up to 30 μV K⁻¹, more than twice the value for pure PEDOT:PSS film. This behavior is different from conventional thermoelectric (TE) materials, whose TE properties are strongly correlated, such as increasing electrical conductivity with increasing carrier concentration, usually resulting in a logarithmic decrease in Seebeck coefficient. Atomic force microscopy images of the PEDOT:PSS/IL films indicated that the ILs induced formation of a particular three-dimensional structure of highly conducting PEDOT grains, resulting in improvement of the TE performance of PEDOT:PSS films.     

Development of Flexible Micro-Thermo-electrochemical Generators Based on Ionic Liquids

The unfavourable relationship between electrical and thermal conductivity limits the choice of solid-state materials for thermoelectric generators (TEG). Among ionic liquids (IOL), it appears that a large variety of thermoelectric (TE) materials with promising high Seebeck coefficients have potential for development. Furthermore, the novel solid-on-liquid deposition technology (SOLID) allows the encapsulation of liquid TE materials to create new, highly integrated TEG devices. Following this vision, this paper studies a large number of IOLs looking at TE-relevant parameters such as thermal and electrical conductivity, Seebeck coefficient and temperature-dependent viscosity. We show that positive and negative Seebeck coefficients can be obtained, depending on the molecular structure and the viscosity of the IOL. The properties of single-junction TEGs are presented in terms of I–V characteristics correlated with the IOL properties. We prove that the limiting effect of conversion efficiency is the current density that can be extracted from a device rather than the Seebeck coefficient.     

Shape Persistent,Highly Conductive Ionogels from Ionic Liquids Reinforced with Cellulose Nanocrystal Network

Shape-persistent, conductive ionogels where both mechanical strength and ionic conductivity are enhanced are developed using multiphase materials composed of cellulose nanocrystals and hyperbranched polymeric ionic liquids (PILs) as a mechanically strong supporting network matrix for ionic liquids with an interrupted ion-conducting pathway. The integration of needlelike nanocrystals and PIL promotes the formation of multiple hydrogen bonding and electrostatic ionic interaction capacitance, resulting in the formation of interconnected networks capable of confining a high amount of ionic liquid (≈95 wt%) without losing its self-sustained shape. The resulting nanoporous and robust ionogels possess outstanding mechanical strength with a high compressive elastic modulus (≈5.6 MPa), comparable to that of tough, rubbery materials. Surprisingly, these rigid materials preserve the high ionic conductivity of original ionic liquids (≈7.8 mS cm⁻¹), which are distributed within and supported by the nanocrystal network-like rigid frame. On the one hand, such stable materials possess superior ionic conductivities in comparison to traditional solid electrolytes; on the other hand, the high compression resistance and shape-persistence allow for easy handling in comparison to traditional fluidic electrolytes. The synergistic enhancement in ion transport and solid-like mechanical properties afforded by these ionogel materials make them intriguing candidates for sustainable electrodeless energy storage and harvesting matrices.     

Electrostatic Control of the Thermoelectric Figure of Merit in Ion-Gated Nanotransistors

Semiconductor nanostructures have raised much hope for the implementation of high-performance thermoelectric generators. Indeed, they are expected to make available reduced thermal conductivity without a heavy trade-off on electrical conductivity, a key requirement to optimize the thermoelectric figure of merit. Here, a novel nanodevice architecture is presented in which ionic liquids are employed as thermally-insulating gate dielectrics. These devices allow the field-effect control of electrical transport in suspended semiconducting nanowires in which thermal conductivity can be simultaneously measured using an all-electrical setup. The resulting experimental data on electrical and thermal transport properties taken on individual nanodevices can be combined to extract ZT, guide device optimization and dynamical tuning of the thermoelectric properties.     

Improved Thermoelectric Performance of Free-Standing PEDOT:PSS/Bi2Te3 Films with Low Thermal Conductivity

Free-standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS)/Bi₂Te₃ thermoelectric (TE) composite films have been successfully prepared by a simple physical mixing method with different contents of Bi₂Te₃. x-Ray diffraction (XRD) and scanning electron microscopy were used to analyze the phase composition and microstructure of the composite films. Their TE performance from 100 K to 300 K was systematically investigated. The maximum electrical conductivity of the composite polymer film reached up to 421 S/cm when the film contained 10 wt.% Bi₂Te₃, corresponding to the highest power factor of 9.9 μW/m/K², while their Seebeck coefficient fluctuated smoothly in a tiny range (14.2 μV/K to 18.6 μV/K). In addition, a relatively low thermal conductivity of 0.07 ± 0.02 W/m/K has been obtained. The maximum figure of merit of the composite reached up to 0.04 at room temperature, which is a relatively high value in the organic TE field.     

[Pmim][SCN]离子液体电解质的合成和性质研究

采用两步法合成了1-戊基-3-甲基咪唑硫氰酸盐([Pmim][SCN])新型离子液体电解质,测定了该电解质的物理化学性质。并用这种新型离子液体电解质与活性炭电极组装成模拟超级电容器,研究了所制超级电容器的电化学性能。结果表明:所制离子液体电导率较高,密度和表面张力都随温度升高而减小,模拟超级电容器的工作电压可达4.0 V,比电容可达421.05 F/cm3,充放电效率为96.3%,且该离子液体具有很好的与常见有机溶剂互溶的能力,具有成为超级电容器用电解质的应用潜力。     

Simultaneous Enhancement in Electrical Conductivity and Thermopower of n‐Type NiETT/PVDF Composite Films by Annealing

Nickel ethenetetrathiolate (NiETT) polymers are promising n‐type thermoelectric (TE) materials, but their insolubility requires the use of an inert polymer matrix to form films, which is detrimental to the TE performance. In this work, the use of thermal annealing as a post‐treatment process simultaneously enhances the electrical conductivity from 6 ± 2 to 23 ± 3 S cm^?1 and thermopower from ?28 ± 3 to ?74 ± 4 µV K^?1 for NiETT/PVDF composite films. Spectroscopic characterization reveals that the underlying mechanism involves removal of residual solvent and volatile impurities (carbonyl sulfide and water) in the NiETT polymer backbone. Additionally, microscopic characterization reveals morphological changes caused by a densification of the film that improves chain packing. These effects result in a 25 × improvement in power factor from 0.5 to 12.5 µW m^?1 K^?2 for NiETT/PVDF films and provide insight into the composition of these coordination polymers that maintain their stability under ambient conditions.     

Low-Temperature,Solution-Based,Scalable Synthesis of Sb2Te3 Nanoparticles with an Enhanced Power Factor

Nanostructured thermoelectric (TE) materials, for example Sb₂Te₃, PbTe, and SiGe-based semiconductors, have excellent thermoelectric transport properties and are promising candidates for next-generation TE commercial application. However, it is a challenge to synthesize the corresponding pure nanocrystals with controlled size by low-temperature wet-chemical reaction. Herein, we report an alternative versatile solution-based method for synthesis of plate-like Sb₂Te₃ nanoparticles in a flask using SbCl₃ and Te powders as raw materials, EDTA-Na₂ as complexing agent, and NaBH₄ as reducing agent in the solvent (distilled water). To investigate their thermoelectric transport properties, the obtained powders were cold compacted into cuboid prisms then annealed under a protective N₂ atmosphere. The results showed that both the electrical conductivity (σ) and the power factor (S ² σ) can be enhanced by improving the purity of the products and by increasing the annealing temperature. The highest power factor was 2.04 μW cm^?1 K^?2 at 140°C and electrical conductivity remained in the range 5–10 × 10³ S m^?1. This work provides a simple and economic approach to preparation of large quantities of nanostructured Sb₂Te₃ with excellent TE performance, making it a fascinating candidate for commercialization of cooling devices.     

Degenerately Doped Semi-Crystalline Polymers for High Performance Thermoelectrics

Thermoelectric (TE) energy conversion in conjugated polymers is considered a promising approach for low-energy harvesting and self-powered temperature sensing. To enhance the TE performance, it is necessary to understand the relationship between the Seebeck coefficient (α) and electrical conductivity (σ). Typical doped polymers exhibit α–σ relationship that is distinct from that of inorganic materials due to their large structural and energetic disorder, which prevents them from achieving the maximum TE power factor (PF = α²σ). Here, an ideal α–σ relationship in the Kang–Snyder model following a transport parameter s  = 1 is demonstrated with two degenerately doped semi-crystalline polymers, poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) using a sequential doping method. The results allow the realization of the PFs reaching theoretic maxima (i.e., 112.01  µ W m⁻¹ K⁻² for PPDT2FBT and 49.80  µ W m⁻¹ K⁻² for PCPDTSBT) and close to metallic behavior in heavily doped films. Additionally, it is shown that the PF maxima appear when the doping state switches from non-degenerate to degenerate. Strategies towards an optimal α–σ relationship enable optimization of the PF and provide an understanding of the charge transport of doped polymers.     

Double Doping of Semiconducting Polymers Using Ion-Exchange with a Dianion

The interactions between counterions and electronic carriers in electrically doped semiconducting polymers are important for delocalization of charge carriers, electronic conductivity, and thermal stability. The introduction of a dianions in semiconducting polymers leads to double doping where there is one counterion for two charge carriers. Double doping minimizes structural distortions, but changes the electrostatic interactions between the carriers and counterions. Polymeric ionic liquids (PIL) with croconate dianions are helpful to investigate the role of the counterion in p-type semiconducting polymers. PILs prevent diffusion of the cation into the semiconducting polymers during ion exchange. The redox-active croconate dianions undergo ion exchange with doped semiconducting polymers depending on their ionization energy. Croconate dianions are found to reduce doped films of poly(3-hexyl thiophene), but undergo ion exchange with a polythiophene with tetraethylene glycol side chains, P(g₄2T-T), that has a lower ionization energy. The croconate dianion maintains crystalline order in P(g₄2T-T) and leads to a lower activation energy for the electrical conductivity than PF₆⁻ counterions. The control of the doping level with croconate allows optimization of the thermoelectric performance of the semiconducting polymer. The thermal stability of the doped films of P(g₄2T-T) is found to depend strongly on the nature of the counterion.     

Effect of Cooling Conditions on the Microstructure and Thermoelectric Properties of Zn/Si-Codoped InSb

InSb is a good candidate thermoelectric (TE) material owing to its high carrier mobility and narrow band gap around 0.18 eV. However, a high figure of merit (ZT) value has not been achieved with InSb because of its high lattice thermal conductivity (κ _lat). To reduce the κ _lat of InSb, we prepared a ZnIn₁₈SiSb₂₀ alloy by Zn/Si codoping into the In lattice sites of InSb. Polycrystalline samples of ZnIn₁₈SiSb₂₀ were prepared by a solid-state reaction method combined with hot pressing. To investigate the microstructures and TE properties resulting from different cooling conditions, samples were prepared by water quenching or slow cooling after an annealing process. The different cooling conditions led to different ZnIn₁₈SiSb₂₀ microstructures and TE properties. The electrical transport properties showed that both samples exhibited metal-like behavior and p-type conduction. The thermal conductivity values of the quenched and slow-cooled samples at room temperature were 8.7 W m^?1 K^?1 and 11.7 W m^?1 K^?1, respectively. A maximum ZT value of 0.23 was obtained at 723 K for the quenched ZnIn₁₈SiSb₂₀ sample.     

Ionic Thermoelectric Properties of Reconstructed Lamellar Vanadium Pentoxide Membranes

In recent years, the application of ionic thermoelectric (TE) materials to convert low-grade waste heat into electricity has become a subject of intense scientific research. However, most of the efforts are focused on organic polyelectrolytes or ionic-liquids embedded in polymeric gels. Here, for the first time, it is demonstrated that nanofluidic membranes of reconstructed layered materials like vanadium pentoxide (V₂O₅) exhibit excellent ionic-TE characteristics. The high Seebeck coefficient (S = 14.5 ± 0.5 mV K^-1) of the V₂O₅ membrane (VO-M) is attributed to temperature gradient-induced unidirectional transport of protons through the percolated network of 2D nanofluidic channels. The TE characteristics of VO-M show nearly 80% improvement (S = 26.3 ± 0.7 mV K^-1) upon functionalizing its percolated network with ionic polymers like poly(4-styrenesulfonic acid) (PSS). Further, unlike organic polymer-based TE systems, VO-M not only sustains exposure to high temperatures (≈200 °C, 5 min) but also protects the PSS molecules intercalated into its interlayer space. Moreover, V₂O₅-based TE materials can self-repair any damage to their physical structure with the help of a tiny water droplet. Thus, nanofluidic membranes of reconstructed layered materials like VO-Ms demonstrate vast robustness and great ionic-TE performance, which can provide a novel platform for scientific studies and futuristic applications.     

Flexible Thermoelectric Polymer Composites Based on a Carbon Nanotubes Forest

Polymer‐based composites are of high interest in the field of thermoelectric (TE) materials because of their properties: abundance, low thermal conductivity, and nontoxicity. In applications, like TE for wearable energy harvesting, where low operating temperatures are required, polymer composites demonstrate compatible with the targeted specifications. The main challenge is reaching high TE efficiency. Fillers and chemical treatments can be used to enhance TE performance of the polymer matrix. The combined application of vertically aligned carbon nanotubes forest (VA‐CNTF) is demonstrated as fillers and chemical post‐treatment to obtain high‐efficiency TE composites, by dispersing VA‐CNTF into a poly (3,4‐ethylenedioxythiophene) polystyrene sulfonate matrix. The VA‐CNTF keeps the functional properties even in flexible substrates. The morphology, structure, composition, and functional features of the composites are thoroughly investigated. A dramatic increase of power factor is observed at the lowest operating temperature difference ever reported. The highest Seebeck coefficient and electrical conductivity are 58.7 µV K^?1 and 1131 S cm^?1, respectively. The highest power factor after treatment is twice as high in untreated samples. The results demonstrate the potential for the combined application of VA‐CNTF and chemical post‐treatment, in boosting the TE properties of composite polymers toward the development of high efficiency, low‐temperature, flexible TEs.     

Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi2Te3 Thin Films

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10^?4 W/K² m and 35 × 10^?4 W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.     

Solid Polymer Electrolytes Based on Ionic Graft Polymers: Effect of Graft Chain Length on Nano‐Structured,Ionic Networks

The length of graft chains in graft polymers is controlled in order to dictate the formation of a nanochannel network of ions in a non‐ionic matrix. Graft polymers were prepared by copolymerization of styrene with poly(sodium styrene sulfonate) (PSSNa) macromonomers. The latter were prepared with controlled molecular weight and narrow polydispersity by stable free radical polymerization. Phase separation of ionic aggregates occurs to a greater extent in films prepared from amphiphilic polymers possessing longer graft chains. Films prepared from polymers containing low ion content comprise of isolated ionic domains and exhibit low ionic conductivity. Increasing the ion content with the membrane, by increasing the number density of ionic graft chains in the polymer, results in ionic domains that coalesce into a network of nanochannels, and a dramatic increase in ion conductivity is observed. The ionic network is developed to a greater extent for films based on longer ionic graft chain polymers; an observation explained on the basis of phase separation.     

锂无机固态电解质研究进展

对商业化的传统液态电池存在的问题及无机固态电解质相较于传统液态电解质在安全性和能量密度等方面的优势进行了简单阐述.介绍了目前研究较多的硫化物型、钠超离子导体(NASICON)型、钙钛矿型、石榴石型和锂超离子导体(LISICON)型五种无机固态电解质的化学组成和晶体结构及锂离子导电机制,并对其离子电导率、电化学窗口、电子...     

Thermoelectric Generators Based on Ionic Liquids

Looking at energy harvesting using body or waste heat for portable electronic or on-board devices, Ionic liquids are interesting candidates as thermoactive materials in thermoelectric generators (TEGs) because of their outstanding properties. Two different kinds of ionic liquid, with alkylammonium and choline as cations, were studied, whereby different anions and redox couples were combined. This study focussed on the intention to find non-hazardous and environmentally friendly ionic liquids for TEGs to be selected among the thousands that can potentially be used. Seebeck coefficients (SEs) as high as ? 15 mV/K were measured, in a particular case for an electrode temperature difference of 20 K. The bottleneck of our TEG device is still the abundance of negative SE liquids matching the internal resistance with the existing positive SE-liquids at series connections. In this paper, we show further progress in finding increased negative SE liquids. For current extraction from the TEG, the ionic liquid must be blended with a redox couple, allowing carrier exchange in a cyclic process under a voltage which is incuced by the asymmetry of the generator in terms of hot and cold electrodes. In our study, two types of redox pairs were tested. It was observed that a high SE of an ionic liquid/redox blend is not a sufficient condition for high power output. It appears that more complex effects between the ionic liquid and the electrode determine the magnitude of the final current/power output. The physico-chemical understanding of such a TEG cell is not yet available.     

Evaluation of the Structure and Transport Properties of Nanostructured Antimony Telluride (Sb2Te3)

Antimony telluride, (Sb₂Te₃), and its doped derivatives are considered to be among the best p-type thermoelectric (TE) materials for room temperature (300–400 K) applications. However, it is still desirable to develop rapid and economical routes for large-scale synthesis of Sb₂Te₃ nanostructures. We report herein a high yield, simple and easily scalable synthetic method for polycrystalline Sb₂Te₃ nanostructures. Prepared samples were compacted into dense pellets by use of spark plasma sintering. The products were characterized by x-ray diffraction and scanning electron microscopy. To investigate the anisotropic behavior of Sb₂Te₃ TE transport property measurements were performed along and perpendicular to the direction of compaction. Thermal conductivity, electrical conductivity, and Seebeck coefficient measurement over the temperature range 350–525 K showed that the anisotropy of the material had a large effect on TE performance.