High-efficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical energy.They can also be used in solid-state refrigeration devices.The conversion of waste heat into electrical energy may play an important role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels and reduce greenhouse gas emissions.

Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2-2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m K at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.© 2021. The Author(s).

Synthetic polymers are ubiquitous across both the industrial and consumer segments of the world economy. Catalysts enable rapid, efficient, selective, and even stereoselective, formation of desired polymers from any of a host of candidate monomers. While numerous molecular catalysts have been shown to be effective for these reactions, separation of the catalysts from reaction products is typically difficult - a potentially problematic complication that suggests instead the use of heterogeneous catalysts. Many of the most effective heterogeneous catalysts, however, comprise supported collections of reaction centres that are decidedly nonuniform in their composition, siting, and activity. Nonuniformity complicates atomic-scale evaluation of the basis for catalytic activity and thus impedes scientific hypothesis-driven understanding and development of superior catalysts. In view of the fundamental desirability of structural and chemical uniformity at the meso, nano, and even atomic scale, crystallographically well-defined, high-porosity metal-organic frameworks (MOFs) have attracted attention as model catalysts and/or catalyst-supports for a wide variety of chemical transformations. In the realm of synthetic polymers, catalyst-functionalized MOFs have been studied for reactions ranging from coordination-mediated polymerization of ethylene to visible-light initiated radical polymerizations. Nevertheless, many polymerization reactions remain to be explored - and, no doubt, will be explored, given the remarkable structural and compositional diversity of attainable MOFs. Noteworthy emerging studies include work directed toward more sophisticated catalytic schemes such as polymer templating using MOF pore architectures and tandem copolymerizations using MOF-supported reaction centres. Finally, it is appropriate to recognize that MOFs themselves are synthetic polymers - albeit, uncoventional ones.

Carbon capture from flue gas and natural gas offers a green path to construct a net-zero emissions economic system. Selective adsorption-based gas separation by employing metal-organic frameworks (MOFs) is regarded as a promising technology due to the advantages of simple processing, easy regeneration and high efficiency. We synthesized two Zirconium MOFs (UiO-66 and UiO-66-NH2) nanocrystals for selective capture and further removal of CO2 from flue gas and natural gas. In particular, UiO-66-NH2 nanocrystals have a smaller grain size, a large amount of defects, and pending –NH2 groups inside their pores which display effective CO2 selective adsorption abilities over CH4 and N2 with the theoretical separation factors of 20 and 7. This breakthrough experiment further verified the selective adsorption-based separation process of natural gas and flue gas. In one further step, we used the Monte Carlo simulation to investigate the optimized adsorption sites and energy of CO2, N2 and CH4 molecules in the gas mixture. The significantly large adsorption energy of CO2 (0.32 eV) over N2 (0.19 eV) and N2 (0.2 eV) may help us to reveal the selective adsorption mechanism.

The recent achievement of the high thermoelectric figure of merit in nanograined materials is attributed to the successful optimization of the consolidation process. Despite a thermal conductivity reduction, it has been experimentally observed that the porous nanograined materials have lower thermoelectric figure of merit than their bulk counterpart due to significant reduction in the electrical conductivity. In this paper, nanoscale porosity effects on electron and phonon transport are modeled to predict and explain thermoelectric properties in porous nanograined materials. Electron scattering at the pores is treated quantum mechanically while phonon transport is treated using a classical picture. The modeling results show that the charge carriers are scattered more severely in nanograined materials than the macroscale porous materials, due to a higher number density of scattering sites. Porous nanograined materials have enhanced Seebeck coefficient due to energy filtering effect and low thermal conductivity, which are favorable for thermoelectric applications. However, the benefit is not large enough to overcome the deficit in the electrical conductivity, so that a high sample density is necessary for nanograined SiGe.

A series of conductive porous composites were obtained by the polymerization of 3,4-ethylenedioxythiophene (EDOT) in the cavities of MIL-101(Cr). By controlling the amount of EDOT loaded into the host framework, it was possible to modulate the conductivity as well as the porosity of the composite. This approach yields materials with a reasonable electronic conductivity (1.1 × 10(-3) S·cm(-1)) while maintaining high porosity (SBET = 803 m(2)/g). This serves as a promising strategy for obtaining highly nanotextured conductive polymers with very high accessibility for small gas molecules, which are beneficial to the fabrication of a chemiresistive sensor for the detection of NO2.

Metal-organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the "through-bond" approach, continuous chains of coordination bonds between the metal centers and ligands' functional groups create charge transport pathways. In the "extended conjugation" approach, the metals and entire ligands form large delocalized systems. The "through-space" approach harnesses the π-π stacking interactions between organic moieties. The "guest-promoted" approach utilizes the inherent porosity of MOFs and host-guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.

For three-dimensional (3D) metal-organic frameworks (MOFs), the presence and nature of structural defects has been recognized as a key factor shaping the material's physical and chemical behavior. In this work, the formation of the "missing linker" defects has been addressed in the model biphenyl-4,4'-dicarboxylate (bpdc)-based Zr MOF, UiO-67. The defect showed strong dependence on the nature of the modulator acid used in the MOF synthesis; the defects, in turn, were found to correlate with the MOF physical and chemical properties. The dynamic nature of the Zr6 (node)-monocarboxylate bond showed promise in defect functionalization and "healing", including the formation of X-ray-quality "defect-free" UiO-67 single crystals. Chemical transformations at defect sites have also been explored. The study was also extended to the isoreticular UiO-66 and UiO-68' systems.