As a mid-temperature thermoelectric material, CuGaTe2 features a distinctive crystal structure, environmentally friendly and high tunability, all of which contribute to its promising thermoelectric performance. However, due to its intrinsically low electrical conductivity and high lattice thermal conductivity, the thermoelectric performance of pristine CuGaTe2 is unsatisfactory. In this work, Cu-deficient Cu0.96GaTe2 was alloyed with different contents of AgSbTe2 via a combined approach involving melting, ball milling, and spark plasma sintering. The Cu-poor condition introduces Cu vacancies, thereby providing more hole carriers. AgSbTe2 alloying leads to a further increase in the carrier concentration. Despite of the decrease in carrier mobility, the electrical conductivity was evidently enhanced. On the other hand, Sb doping increases the density of states at the Fermi level, leading to enhancement of the Seebeck coefficient. Moreover, Cu vacancies and AgSbTe2 alloying both cause a significant decrease in sound velocity, thereby decreasing the lattice thermal conductivity to an ultralow value of 0.29 W m-1 K-1 at 743 K. Finally, the optimized (Cu0.96GaTe2)0.9(AgSbTe2)0.1 sample demonstrates the highest thermoelectric performance, achieving an average zT of 0.41, which represents a 215.4% improvement over pristine CuGaTe2. Its peak zT reaches 1.39 at 743 K, corresponding to a 124.2% improvement relative to pristine CuGaTe2.

To address the key obstacles of high carrier concentration and high lattice thermal conductivity in p-type GeTe, we incorporated Sb2Se3 into the GeTe matrix for synergistic optimization. X-ray diffraction confirms that Sb incorporation promotes the rhombohedral-to-cubic phase transition, increasing band degeneracy and enhancing the Seebeck coefficient. Sb and Se co-doping tunes the hole carrier concentration to an optimal level while maintaining high carrier mobility, and jointly modulates the phonon band structure to strengthen phonon scattering and reduce sound velocity. Consequently, an ultralow lattice thermal conductivity of 0.36 W m-1 K-1 at 723 K and a peak zT of 2.22 are achieved. This work demonstrates the effectiveness of Sb2Se3 incorporation in regulating electronic and phononic transport, providing a viable strategy for designing high-performance lead-free thermoelectric materials.

SnTe is widely recognized as a promising alternative to environmentally harmful PbTe for thermoelectric applications at moderate and high temperatures. Nevertheless, due to the ultrahigh hole concentration as well as lattice thermal conductivity, the thermoelectric performance of pure SnTe is greatly constrained. In our work, a novel strategy was put forward to optimize the relatively poor thermoelectric performance of SnTe by simultaneously incorporating In2Se3 and Cd. The Seebeck coefficient was apparently improved through the whole temperature range originating from the introduction of resonant energy levels achieved by In doping, while lattice thermal conductivity was decreased to a relatively low level of similar to 0.49 W m-1 K-1 at 873 K, owing to a decrease in sound velocity and strong scattering on phonon caused by nanopores and dislocations after doping. Profiting from the synergistic optimization thermoelectric performance strategies, a peak zT of 1.32 at 873 K was achieved in Sn0.96Cd0.04Te+1%In2Se3, which represents a remarkable 89% increase over pristine SnTe. And an average zT of 0.66 from 303 to 873 K was also obtained. Our work provides a new strategy for multifunctional synergistic modulation of the thermoelectric properties of SnTe-based materials.

Modulation of the band structure is essential for achieving superior thermoelectric (TE) and optical properties in semiconductor materials. Based on first-principles calculations and Boltzmann transport theory, this work comprehensively explores the modulation of the TE and optical properties of Bi2Te3 monolayers through atomic substitution with S, Se, Y, and La. The results demonstrate that while substituting the middle-layer Te atoms with S or Se largely enhances the electrical transport of the Bi2Te3 monolayer by boosting carrier mobility, this approach also deteriorates its lattice thermal conductivity by facilitating phonon propagation. In contrast, substituting the Bi atomic layer with Y or La significantly suppresses phonon transport, with the BiLaTe3 monolayer achieving an ultralow lattice thermal conductivity of 0.21 W m-1 K-1 at 500 K owing to the short phonon relaxation time and low group velocity. Consequently, the p-type Bi2Te2Se and n-type BiLaTe3 monolayers exhibit maximum ZT values of 3.66 and 1.00 at 500 K, respectively, which are 2.1 times and 1.5 times those of the corresponding Bi2Te3 monolayer. Besides, optical analysis indicates that with the exception of the BiLaTe3 monolayer, all other monolayers present high optical absorption peaks within the visible spectrum. These findings highlight that Bi2Te3 monolayers modified by atomic substitution show promising prospects in optical absorption and thermoelectric devices.

Conventional doping approaches for enhancing thermoelectric performance typically rely on multi-element co-doping, which complicates both the fabrication process and the interpretation of underlying mechanisms. In this study, we demonstrate that a single dopant, Sb, can effectively modulate the thermoelectric properties of PbSe through a concentration-dependent regulatory mechanism. Sb dopant at low concentrations (x <= 1.5 %) suppresses Pb vacancies in the structure which not only enhances doping efficiency but also maintains high carrier mobility at elevated carrier concentrations. All these significantly boost the electrical transport properties. In contrast, excess Sb induces additional Pb vacancies at higher concentration (x > 1.5 %), which reduce both carrier concentration and mobility. By suppressing the formation of Pb vacancy with low dose of Sb content, a significantly improved ZT(max) value of similar to 1.43 is attained at 785 K for Pb0.997Sb0.003Se sample, representing an approximately 2-fold improvement over the pristine PbSe (ZT(max) similar to 0.55).

Abstract As a phonon-liquid electron-crystal (PLEC), AgCuTe exhibits unique advantages due to its complex crystal structure and inherently low lattice thermal conductivity. However, the high concentration of Ag/Cu cation vacancies in AgCuTe leads to an excessively high carrier concentration, which restricts its thermoelectric performance. In this study, both the phase composition and carrier concentration were tailored by tuning the Ag/Cu content in AgCuTe. As the Ag/Cu content increases, the phase composition evolves from Ag2Te and Cu2Te secondary phases to a predominant AgCuTe phase. Seebeck coefficient measurements reveal that AgCuTe exhibits a conduction type transition upon tailoring the Ag/Cu content. The Ag/Cu-deficient samples exhibit n-type semiconducting behavior at room temperature. Increasing the Ag/Cu content gradually reduces the Ag2Te secondary phase, which in turn decreases electron carriers. Meanwhile, positron annihilation measurements reveal that more cation vacancies are introduced with increasing Ag/Cu content, which afford a high concentration of hole carriers and thus switch samples to p-type conduction. At temperatures higher than 527 K, all the samples exhibit a pure cubic AgCuTe phase and show p-type conduction behavior. The increased Ag/Cu content reduces Hall carrier concentration, leading to enhanced power factor and decreased electronic thermal conductivity. In addition, the increased Ag/Cu content further reduces the lattice thermal conductivity by decreasing the sound velocity and enhancing lattice anharmonicity. Collectively, these factors realize synergistic optimization of thermoelectric performance. The (AgCu)1.06Te sample achieves a remarkable zTmax value of 1.39 at 737 K, with an average zT (zTave) of 1.29 between 523 and 737 K, which show enhancement of 74 and 94% compared with the pristine AgCuTe, respectively. The Ag/Cu excess samples also demonstrate excellent thermal stability up to 873 K. This work provides a strategy for further optimizing the thermoelectric performance of AgCuTe-based materials.

Polyimide precursors undergo thermal rearrangement at a high temperature of 450 degrees C to form thermally rearranged polybenzoxazoles (TR-PBOs), which are promising materials for gas separation membranes due to their excellent permselective properties. In this work, the free volume characteristics were systematically modulated by incorporating non-thermally rearrangeable diamine monomers featuring diverse bridging group configurations and molar ratios. Positron annihilation lifetime spectroscopy (PAS) revealed that increasing the proportion of thermally rearrangeable diamine or enhancing the steric hindrance of the bridging groups in non-thermally rearrangeable diamines resulted in a modest expansion of free volume cavity size and a pronounced increase in fractional free volume (FFV). Moreover, the introduction of non-thermally rearrangeable diamines effectively improved the mechanical robustness of the resulting membranes. Pure-gas permeation measurements demonstrated an ideal linear correlation between the logarithm of gas permeability and the reciprocal of FFV (1/FFV), confirming the decisive role of free volume architecture in gas transport behavior. Notably, the separation performance of most membrane samples exceeded the 2008 Robeson upper bound for the H2/CH4 gas pair.

Metal organic frameworks (MOF) are widely incorporated into mixed matrix membranes (MMMs) for CO2 separation. However, the relationship between MOF-induced free volume characteristics and transport behavior remains insufficiently understood. In this work, Pebax MH 1657-based MMMs were prepared using a dual ligand aluminum-based MOF (CAU-PBH), derived from CAU-10-H through partial substitution of isophthalic acid by pyridine-3,5-dicarboxylic acid, as a filler to investigate its effect on the free volume and CO2 transport properties of the Pebax membranes. It exhibits a microporous structure and a polar chemical environment containing carboxylate and N-heterocyclic groups, which are favorable for CO2 adsorption and polymer-filler compatibility. The optimized MMM containing 5 wt% CAU-PBH showed a CO2 permeability of 95.10 Barrer and a CO2/N2 selectivity of 66.64 at 0.2 MPa and 35 °C, approaching the 2008 Robeson upper bound. Quantitative positron annihilation lifetime spectroscopy (PALS) analysis revealed that CAU-PBH incorporation modified the free volume characteristics of Pebax through changes in polymer chain packing and crystallization behavior. The MOF-induced changes in free volume characteristics were closely associated with changes in gas diffusivity, while CO2 solubility increased markedly. The regulated free volume characteristics, together with the intrinsic micropores and CO2 affinity sites of CAU-PBH, enhanced CO2 sorption and contributed to selective transport pathways, thereby improving CO2 separation performance. These results indicate that the separation enhancement cannot be attributed to a simple change in free volume alone, establishing a PALS-based structure-transport correlation between MOF-induced free volume characteristics and CO2 transport behavior in Pebax MMMs.

A series of carbon molecular sieve (CMS) membranes were prepared by crosslinkable rearrangable 6FDA-based polyimide precursors to study the effects of crosslinked and rearranged structures on the pore structure and gas separation performance of CMS membranes after pyrolysis. Fourier transform infrared (FT-IR) and thermogravimetric-mass spectrum (TG-MS) show that the precursor releases small molecules during pyrolysis to form an oxygen and nitrogen-containing aromatic heterocyclic structure. X-ray diffraction (XRD) showed that the stacking efficiency of the heterocyclic planar structure was improved with the decrease of the rearrangement part and the increase of the crosslinking part, resulting in the decrease of the chain spacing. Positron annihilation lifetime measurements reveal that the CMS membranes have similar structure with that of graphite containing high density of ultramicropores with size in the range of 5.7-6.1 & Aring;. The size of ultramicropore decreases gradually with the increase of the crosslinking ratio, which is in agreement with the results of nitrogen adsorption and XRD measurements. Compared with the polyimide precursors, the gas permeability of the CMS membranes is greatly enhanced. The CMS-DABA-0 membrane has an ultrahigh CO2 permeability up to 18,265 barrer. A positive correlation between the gas diffusivity and the ultramicropore volume of the CMS membranes was found. The gas permeability deteriorates with the increase in the crosslinking ratio, while the gas selectivity increases significantly. Notably the permeability selectivity of CO2/N2 and CO2/CH4 pairs in CMS-DABA-0.5 exceeds the 2019 upper bound due to the modulation of ultramicroporous structure by structure of PI precursors, suggesting the potential application of CMS membranes for future CO2 separation.

Developing novel polymer membranes with high gas permeability and excellent resistance to aging and plasticization remains a significant challenge in membrane technology. A novel rigid spirobifluorene (SBF) monomer was rationally designed and synthesized for incorporation into the backbone of polymers of intrinsic microporosity (PIM-1). The introduction of SBF units simultaneously enhanced gas permeability and plasticization resistance of the resulting membranes. As a result, the PIM-1 copolymer containing 5 mol% TFSBF (5% SBF@PIM-1) membrane exhibited a substantially increased CO2 permeability of 4077 Barrer, corresponding to a 84% increase relative to pristine PIM-1, and improved resistance to physical aging and CO2-induced plasticization. The physical aging behaviors has been moderately reduced as evidenced by a 27% reduction in CO2 permeability after seven days, in contrast to a 45% loss observed for pristine PIM-1. Moreover, the SBF@PIM-1 membranes demonstrated enhanced resistance to CO2-induced plasticization, with the plasticization pressure shifted from 13 bar for PIM-1 to 20 bar. Structure-property relationship analysis reveals that the rigid SBF units effectively stabilize the polymer matrix, mitigating free volume collapse and thereby improving long-term membrane durability. These results provide valuable insights into the rational molecular design of next-generation gas separation membranes with simultaneously enhanced permeability, stability, and resistance to plasticization.

As a lead-free p-type thermoelectric material, GeTe exhibits considerable potential for sustainable energy conversion in the medium-temperature range. However, pristine GeTe possesses excessively high carrier concentration and high lattice thermal conductivity, which severely deteriorate its thermoelectric performance. Doping is a versatile strategy for precisely tuning carriers, which also introduces defects to suppress lattice thermal conductivity. However, these same defects often impede carrier mobility, creating an inherent tradeoff that can counteract the overall enhancement of thermoelectric performance. In this work, Ge1-x-ySbxYbyTe samples were synthesized by co-doping Sb and Yb via a high-temperature melting method. Co-doping can tune the carrier concentration to an optimal range, while the carrier mobility shows only slight decrease. Both Sb and Yb promote the rhombohedral-to-cubic phase transition of GeTe, thereby changing the band structure and enhancing the Seebeck coefficient. In addition, Sb and Yb doping not only impede the phonon transport by lowering down the sound velocity, but also enhance the lattice anharmonicity, thus remarkably reducing the lattice thermal conductivity. For the optimal sample Ge0.92Sb0.06Yb0.02Te, the lattice thermal conductivity reaches as low as 0.348 W m-1 K-1 at 723 K, with a peak zT value of 2.42 and an average zT of 1.33 within 323-773 K. This study confirms that Sb/Yb co-doping is an effective strategy to enhance the thermoelectric performance of GeTe, providing a reference for the development of high-performance lead-free thermoelectric materials.

GeTe-based binary chalcogenides have been proved to be potential thermoelectric materials. Their performance can be greatly enhanced by appropriate doping, which not only optimizes carrier concentration, but also suppresses lattice thermal conduction and in some cases enhances band degeneracy. However, controversy exists in explaining the experimental findings. The major discrepancy lies in the mechanism of carrier optimization by doping. The effect of doping on band structure is also inconsistent. In addition, the origin of the suppressed lattice thermal conductivity by doping is also unclear. Herein, a systematic investigation of the versatile roles of elemental doping in R-GeTe was performed using first-principles calculations. The results indicate that Ge vacancies are the source of intrinsically high carrier concentration in R-GeTe. 8 typical dopants were selected to investigate their effect on the defect formation, band structures and phonon transport properties. Among them, Bi and Sb are effective in suppressing Ge vacancies, which can easily optimize carrier concentration, while Cd, Mg, Mn and Yb can enhance the band degeneracy. Moreover, Bi and Sb are very effective in suppressing the lattice thermal conductivity by lowering down the sound velocity and increasing the lattice anharmonicity. Therefore, the internal mechanism of these dopants to optimize the thermoelectric properties of R-GeTe is clarified by theoretical calculations. Our results provide a robust theoretical guidance on the selection of suitable dopants in R-GeTe, which is conducive to the realization of its excellent thermoelectric performance and accelerating its commercial application.

Thermoelectric materials enable direct thermal-to-electrical energy conversion for waste heat recovery, yet their figure of merit ZT is constrained by intrinsic transport trade-offs. Herein, we target CuCrTi2Se6, a new narrow-bandgap semiconductor and quaternary chalcogenide derived from two-dimensional transition-metal dichalcogenides (TMDs), and tune its coordination via Ag doping, driving its peak ZT to 1.0 at 773 K and single-leg efficiency to 6.3% at ΔT = 500 K. The larger atomic mass and size of Ag generate local stress fields that drive Cu migration from octahedral to tetrahedral interlayer sites, shortening Cu-Se bonds, enhancing bond covalency, and increasing carrier mobility from 28 to 35 cm2·V-1·s-1. Concurrently, Ag doping reduces Cu-vacancy formation energy, increasing hole concentration while elevating valence band degeneracy to enhance the Seebeck coefficient. On the phononic side, weak Ag-Se bonds induce lattice softening, and strong point-defect scattering from Ag-Cu mass/strain fluctuations synergistically reduces lattice thermal conductivity from 0.46 to 0.31 W·m-1·K-1. Benefiting from the simultaneous optimization of electronic and phononic transport, Cu0.95Ag0.05CrTi2Se6 achieves a peak ZT nearly 70% higher than pristine CuCrTi2Se6. This work establishes coordination environment regulation as an effective strategy for tuning chemical bonding and achieving coupled optimization of thermoelectric transport in layered materials.

In GeTe, the cation doping can effectively regulate the band structure and carrier concentration, suppress phonon transport, thereby achieving a significant improvement in thermoelectric performance. However, the research on the phonon transport properties of GeTe by anion doping is relatively scarce. Here, we systematically studied the effects of five types of anion doping (S, Se, Cl, Br and I) on the phonon transport properties of R-GeTe through first-principles calculations. The results indicate that anion doping in R-GeTe can effectively reduce the lattice thermal conductivity, especially for Cl doping. At 300 K, the average 𝜅𝐿 decreases from the intrinsic value of 3.42 to 1.08, 2.27, 0.16, 1.42, and 1.31 W m-1 K-1 for S, Se, Cl, Br, and I doping, respectively. The decrease of lattice thermal conductivity caused by doping is strongly correlated with the shortening of phonon relaxation time, indicating that phonon relaxation time plays a dominant role in inhibiting heat transport of R-GeTe. Among them, the sharp decrease in lattice thermal conductivity caused by Cl doping is mainly due to the greatest differences in electronegativity, ionic radius and atomic mass between Cl and Te. Moreover, after doping, the system has a larger Grüneisen parameter and a shorter phonon relaxation time, thereby exhibiting the lowest lattice thermal conductivity. This work provides direct evidence for the reduction of lattice thermal conductivity of chalcogenides by anion doping, and provides a clear direction for further optimizing the thermal transport properties of materials.

Both anharmonicity and phonon scattering have significant effects on the lattice thermal conductivity of materials. Through the calculation on the double-anion chalcohalides MnXS2Cl (X=Sb,Bi), it is found that both MnSbS2Cl and MnBiS2Cl exhibit low intrinsic lattice thermal conductivity, which are only 0.40 and 0.46 W m-1 K-1 at 300 K, respectively. In order to clarify the origin of the low lattice thermal conductivity in this kind of material, the phonon correlation quantities are analyzed in detail by the phonon Boltzmann transport theory. It is discovered that the presence of double-anion causes distortion in the crystal structure and the asymmetry in chemical bonding. The bonding asymmetry leads to strong coupling between acoustic-optical branches and therefore more phonon scattering processes, decreasing the phonon relaxation time. The weak X-Cl bonding also leads to low phonon group velocity. These two factors greatly reduce the lattice thermal conductivity of the MnXS2Cl compounds. In addition, due to the more suitable carrier effective mass and the achievement of band convergence in the conduction band, MnXS2Cl exhibits superior electronic transport properties for n-type materials. Combined with the low intrinsic lattice thermal conductivity, the optimal ZT value of MnSbS2Cl can reach 2.7 at 800 K, indicating that it has excellent thermoelectric performance. This study provides a theoretical basis for elucidating the thermal conductivity characteristics of mixed-anion chalcohalides, and demonstrates the potential applications of such materials in the thermoelectric field.

One potential solution for the transport of hydrogen (H2) is the injection of hydrogen into natural gas pipelines. Therefore, it is imperative to develop an efficient purification technology. Membrane separation has great potential to meet this challenge due to its effective energy consumption and cost. Here, a series of mixed matrix membranes (MMMs) containing ZIF-71 fillers of different sizes are reported for faster H2 permeation. The uniform distribution of nanosized ZIF-71 (0.1 mu m) in 6FDA-DAM provides an attractive diffusion channel, allowing the membrane to show rapid H2 permeation of 1050 Barrer and good H2/CH4 separation factor of 43. This performance is markedly superior to that of the larger-sized ZIF-71 (1.0 mu m and 3.5 mu m) in 6FDA-DAM and the samesized ZIF-71 (0.1 mu m) in 6FDA-Durene and PEI, and also exceeds the upper bound. Moreover, the long-term stable H2/CH4 separation suggests a high potential for practical applications. The findings here demonstrate the importance of the filler size, which has a strong influence on the formation of mass transfer channels, and also provide straightforward method for the development of high-performance MMMs.

In this work, a series of Ni/Co-MOFs with high specific capacitances were synthesized as anode materials using a one-step hydrothermal reaction method. NaOH in different amounts (3, 4, 5, and 6 mmol) was added during the synthesis to tune the pore structure of Ni/Co-MOFs. It was found that the Ni/Co-MOF-3 with a NaOH amount of 5 mmol exhibits the largest specific surface area and pore volume, which provides more active sites for the electrochemical reaction and facilitates ion diffusion at the interface of the electrolyte solution/active material, thus increasing the capacitance of the electrode material. The electrochemical test results show that the specific capacitance of Ni/Co-MOF-3 reaches 1361 F g-1 at 1 A g-1. Impressively, the specific capacitance is still as high as 1214 F g-1 when the current density increases from 1 to 20 A g-1, with a high capacitance retention rate of about 89.2%. In addition, Ni/Co-MOF-3 and activated carbon were used as positive and negative materials, respectively, to assemble an asymmetric capacitor, which has a specific capacitance of 134.4 F g-1 at 1 A g-1 and an energy density of 47.69 Wh kg-1 at a power density of 800 W kg-1. The specific capacitance is 60% of the initial specific capacitance at 5 A g-1 after 10,000 cycles. At the same time, two such asymmetric capacitors can light up the red LED indicator for more than 30 min after being connected in series with a full charge, indicating that they have outstanding application potential.

As a typical argyrodite compound, Ag8SnSe6 is a very promising candidate for thermoelectric materials due to its inherently ultralow lattice thermal conductivity. However, the main obstacle to further improving its thermoelectric properties is its low intrinsic carrier concentration. Appropriate doping may increase the carrier concentration, but the electronic thermal conductivity will also increase simultaneously, which partially compensates for the optimized power factor. This is particularly prominent for materials with ultralow lattice thermal conductivity. In this study, SnI2 was introduced into Ag8SnSe6, which provides more carriers and thus successfully improves the electrical properties. The maximum power factor at 748 K is enhanced from 4.63 μW cm-1 K-2 to 5.83 μW cm-1 K-2 after adding 3 at. % of SnI2. Meanwhile, the substitution of I for Se weakens the chemical bonding strength, resulting in a lowered sound velocity. In addition, doping of I at the Se site also enhances the bond asymmetry, which causes a stronger lattice anharmonicity. As a result, an extremely low lattice thermal conductivity of 0.058 W m-1 K-1 is achieved at 748 K. Eventually, a high zT value of 1.36 was obtained through the above synergistic optimization, which is a 39% improvement compared to the pristine sample (0.98). Our study demonstrates that chemical bonding engineering is an effective way to further suppress the lattice thermal conductivity of Ag8SnSe6 and also provides guidance for improving the thermoelectric properties of other argyrodite compounds.

Phase transition of CuAl 2 O 4 to CuAlO 2 during the sintering process of Al 2 O 3 /CuO nanoceramics at high temperatures (973 - 1373 K) were investigated by positron annihilation lifetime spectroscopy (PALS), X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD results showed that the CuAl 2 O 4 phase was first synthesized at 973 K and then decomposed into CuAlO 2 at 1373 K during the sintering process of Al 2 O 3 /CuO. SEM revealed that a lot of agglomerated small particles were formed at 1273 K, exhibiting octahedral structure features of CuAl 2 O 4 crystals, but transformed into a distinct layered structure of CuAlO 2 crystals after phase transition occurred at 1373 K. PALS found that positron lifetime parameters exhibited a decreasing trend throughout the entire process with increasing sintering temperature, indicating a decrease of both small vacancy-sized and vacancy cluster-sized defects, and a significant decrease occurred during the transition from CuAl 2 O 4 at 1273 K to CuAlO 2 at 1373 K. These findings align with the results obtained from XRD and SEM analyses.