Extremely Low Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Performance in α-Cu2Se through the Incorporation of Porous Carbon.

In this work, a series of Cu2Se/x wt % porous carbon (PC) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) composite materials were synthesized by ball milling and spark plasma sintering (SPS). The highly ordered porous carbon was synthesized by a hydrothermal method using mesoporous silica (SBA-15) as the template. X-ray diffraction results show that the incorporation of porous carbon induces a phase transition of Cu2Se from the ß phase to the α phase. Meanwhile, the addition of porous carbon reduces the carrier concentration from 2.7 × 1021 to 2.45 × 1020 cm-3 by 1 order of magnitude. The decrease of the carrier concentration leads to the reduction of electrical conductivity and the increase of the Seebeck coefficient, which results in the enhancement of the power factor. On the other hand, the incorporation of porous carbon into Cu2Se increases the porosity of the composites and also introduces more interfaces between the two materials, which is evidenced by positron annihilation lifetime measurements. Both pores and interfaces greatly enhance phonon scattering, leading to extremely low lattice thermal conductivity. In addition, the decrease of electrical conductivity also causes a sufficient reduction in electronic thermal conductivity. Due to the above synergistic effects, the thermoelectric performance of the Cu2Se/PC composite is significantly enhanced with a maximum ZT value of 0.92 at 403 K in the Cu2Se/1 wt % PC composite, which is close to that of the Bi2Te3-based materials. Our work shows that α-Cu2Se has great potential for near-room-temperature thermoelectric materials.

Dandelion-Like CuCo2O4@ NiMn LDH Core/Shell Nanoflowers for Excellent Battery-Type Supercapacitor.

Dandelion-like CuCo2O4 nanoflowers (CCO NFs) with ultrathin NiMn layered double hydroxide (LDH) shells were fabricated via a two-step hydrothermal method. The prepared CuCo2O4@NiMn LDH core/shell nanoflowers (CCO@NM LDH NFs) possessed a high specific surface area (~181 m2·g-1) with an average pore size of ~256 nm. Herein, the CCO@NM LDH NFs exhibited the typical battery-type electrode material with a specific capacity of 2156.53 F·g-1 at a current density of 1 A·g-1. With the increase in current density, the rate capability retention was 68.3% at a current density of 10 A·g-1. In particular, the 94.6% capacity of CCO@NM LDH NFs remains after 2500 cycles at 5 A·g-1. An asymmetric supercapacitor (ASC) with CCO@NM LDH NFs//activated carbon (AC) demonstrates a remarkable capacitance of 303.11 F·g-1 at 1 A·g-1 with excellent cycling stability. The coupling and synergistic effects of multi-valence transition metals provide a convenient channel for the electrochemical process, which is beneficial to spread widely within the realm of electrochemical energy storage.

Enhancing Thermoelectric Performance of AgSbTe2-Based Compounds via Microstructure Modulation Combining with Entropy Engineering.

Modulation of the microstructure and configurational entropy tuning are the core stratagem for improving thermoelectric performance. However, the correlation of evolution among the preparation methods, chemical composition, structural defects, configurational entropy, and thermoelectric properties is still unclear. Herein, two series of AgSbTe2-based compounds were synthesized by an equilibrium melting-slow-cooling method and a nonequilibrium melting-quenching-spark plasma sintering (SPS) method, respectively. The equilibrium method results in coarse grains with a size of >300 µm in the samples and a lower defect concentration, leading to higher carrier mobility of 10.66 cm2 V-1 s-1 for (Ag2Te)0.41(Sb2Te3)0.59 compared to the sample synthesized by nonequilibrium preparation of 1.83 cm2 V-1 s-1. Moreover, tuning the chemical composition of nonstoichiometric AgSbTe2 effectively improves the configurational entropy and creates a large number of cation vacancies, which evolve into dense dislocations in the samples. Owing to all of these in conjunction with the strong inharmonic vibration of lattice, an ultralow thermal conductivity of 0.51 W m-1 K-1 at room temperature is achieved for the (Ag2Te)0.42(Sb2Te3)0.58 sample synthesized by the equilibrium preparation method. Due to the enhanced carrier mobility, optimized carrier concentration, and low thermal conductivity, the (Ag2Te)0.42(Sb2Te3)0.58 sample synthesized by the equilibrium preparation method possesses the highest ZT of 1.04 at 500 K, more than 60% higher than 0.64 at 500 K of the same composition synthesized by nonequilibrium preparation.

Synergetic Optimization of Electrical and Thermal Transport Properties by Cu Vacancies and Nanopores in Cu2Se.

In this study, a series of Cu2+x-yInySe (-0.3 ≤ x ≤ 0.2 and 0 ≤ y ≤ 0.05) samples were prepared by melting and the spark plasma sintering method. X-ray diffraction measurements indicate that the Cu-deficient samples (x = -0.3 y = 0 and x = -0.2 y = 0) prefer to form the cubic phase (ß-Cu2Se). Adding excessive Cu or introducing In atoms into the Cu2Se matrix triggers a phase transition from the ß to α phase. Positron lifetime measurements confirm the reduction in Cu vacancy concentration by adding excessive Cu or introducing In atoms into Cu2Se, which causes a dramatic decrease in carrier concentration from 1.59 × 1021 to 5.0 × 1019 cm-3 at room temperature. The samples with In contents of 0.01 and 0.03 show a high power factor of about 1 mW m-1 K-2 at room temperature due to the optimization of the carrier concentration. Meanwhile, the excess Cu content and doping of In atoms also favor the formation of nanopores. These pores have strong interaction with phonons, leading to remarkable reduction in lattice thermal conductivity. Finally, a high ZT value of about 1.44 is achieved at 873 K in the Cu1.99In0.01Se (x = 0 and y = 0.01) sample, which is about twice that of the Cu-deficient sample (Cu1.7Se). Our work provides a viable insight into tuning vacancy defects to improve efficiently the electrical and thermal transport performance for copper-based thermoelectric materials.

Exfoliated Ultrathin ZnIn2 S4 Nanosheets with Abundant Zinc Vacancies for Enhanced CO2 Electroreduction to Formate.

Electrocatalytic conversion of carbon dioxide (CO2 ) is promising for balancing carbon cycles while producing value-added feedstocks. Herein, ultrathin ZnIn2 S4 nanosheets with abundant Zn vacancies are demonstrated for electrochemically reducing CO2 to formate. Specifically, a partial current density of 245 mA cm-2 with a near-unity faradaic efficiency of 94 % for formate generation was achieved over the ultrathin ZnIn2 S4 nanosheets in a flow cell configuration. Experimental and theoretical results revealed that abundant Zn vacancies in the ultrathin ZnIn2 S4 nanosheets with a high electrochemically active surface area synergistically optimized the intermediate binding energy and contributed to the boosted selectivity and activity. This work may provide useful understandings in designing efficient catalysts for selective CO2 electroreduction.