Anke Weidenkaff

Enhanced thermoelectric performance of β-Zn4Sb3 based nanocomposites through combined effects of density of states resonance and carrier energy filtering

It is a major challenge to elevate the thermoelectric figure of merit ZT of materials through enhancing their power factor (PF) and reducing the thermal conductivity at the same time. Experience has shown that engineering of the electronic density of states (eDOS) and the energy filtering mechanism (EFM) are two different effective approaches to improve the PF. However, the successful combination of these two methods is elusive. Here we show that the PF of β-Zn4Sb3 can greatly benefit from both effects. Simultaneous resonant distortion in eDOS via Pb-doping and energy filtering via introduction of interface potentials result in a ~40% increase of PF and an approximately twofold reduction of the lattice thermal conductivity due to interface scattering. Accordingly, the ZT of β-Pb0.02Zn3.98Sb3 with 3u2009vol.% of Cu3SbSe4 nanoinclusions reaches a value of 1.4 at 648 K. The combination of eDOS engineering and EFM would potentially facilitate the development of high-performance thermoelectric materials.

Theoretical and experimental investigations of the thermoelectric properties of Bi2S3

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500u2009μW K−2 m−1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500u2009μW K−2 m−1 at room temperature for a chlorine equivalence of 0.25 mol.u2009% BiCl3. The obtained results...

High thermoelectric performance of tellurium doped paracostibite

Paracostibite (CoSbS) has recently been identified as a promising thermoelectric (TE) material, yet its full potential remains to be attained. We present herein an integrated method based on high throughput DFT computations validated with experiments that has allowed us to identify tellurium on antimony sites as a much more effective dopant than the formerly used nickel on cobalt sites. By carrying out a systematic adjustment optimization of the experimental parameters, we achieve a power factor as high as 2.7 mW m−1 K−2 at 543 K, which is maintained up to 730 K. This is, to the best of our knowledge, the largest value reported for polycrystalline metal sulfides.

Design Guidelines for High-Performance Particle-Based Photoanodes for Water Splitting: Lanthanum Titanium Oxynitride as a Model.

Semiconductor powders are perfectly suited for the scalable fabrication of particle-based photoelectrodes, which can be used to split water using the sun as a renewable energy source. This systematic study is focused on variation of the electrode design using LaTiO2 N as a model system. We present the influence of particle morphology on charge separation and transport properties combined with post-treatment procedures, such as necking and size-dependent co-catalyst loading. Five rules are proposed to guide the design of high-performance particle-based photoanodes by adding or varying several process steps. We also specify how much efficiency improvement can be achieved using each of the steps. For example, implementation of a connectivity network and surface area enhancement leads to thirty times improvement in efficiency and co-catalyst loading achieves an improvement in efficiency by a factor of seven. Some of these guidelines can be adapted to non-particle-based photoelectrodes.

CuO promoted Mn2O3-based materials for solid fuel combustion with inherent CO2 capture

We experimentally demonstrate the promising redox and oxygen release characteristics of a novel bimetallic Cu–Mn oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) based CO2 capture. The new material was prepared via a co-precipitation technique and showed a higher oxygen partial pressure than pure CuO and a higher oxygen carrying capacity than Mn2O3, thus, synergistically combining the advantages of the individual metal oxides. The promising CLOU characteristics of the new material were demonstrated further by combusting charcoal fully in a fluidized bed and producing a pure stream of CO2.

Designing strontium titanate-based thermoelectrics: insight into defect chemistry mechanisms

Driven by a need to develop low-cost and thermally stable materials for thermoelectric applications, donor-substituted strontium titanate is considered as a promising alternative to traditional thermoelectrics. The complex defect chemistry of SrTiO3-based materials imposes various limitations on identifying the relevant effects exerted on the electronic band structure and heat transfer, being a subject of debate and intensive research. Based on combined XRD, SEM/EDS, HRTEM, XPS, and TGA studies and measurements of thermoelectric properties, this work uncovers the particular role of various structural defects in electrical and thermal transport in Sr1±yTi0.9Nb0.1O3±δ, selected as a model system. Introduction of A-site cation vacancies provides a synergistic effect of combining fast charge transport in the perovskite lattice and suppressing the thermal conductivity mostly due to simultaneous generation of oxygen vacancies. The presence of oxygen vacancies promotes more efficient phonon scattering compared to Ruddlesden–Popper-type layers. These findings provide a link between structural and thermoelectric properties, offering further prospects for seeking highly performing SrTiO3-based thermoelectrics by tailoring the defect chemistry mechanisms.

Compatibility approach for the improvement of oxide thermoelectric converters for industrial heat recovery applications

New ceramic Ca3Co3.9O9+δ u2009/CaMn0.97W0.03O3−δ thermoelectric generators with different cross section areas Ap and An of the p- and the n-type leg are fabricated, characterized, and tested at high temperatures in long-term tests. The variation of the measured power output and the efficiency with changing Ap/An ratio is discussed and compared with calculations based on the measured material properties. The highest conversion efficiencies are reached for ratios close to the one predicted by the compatibility approach, whereas an improper choice of Ap/An leads to a strong reduction of the efficiency. A volume power density of 1.4u2009W/cm3 and an efficiency of 1.08% are found for the most promising generator (temperature difference ΔT= 734u2009K and Ap/An= 1.12). The results reveal the major importance of the Ap/An ratio for the conversion efficiency and subsequently cost and weight reduction issues, both crucial for a large scale application of thermoelectric converters. Additionally, the oxide generators proved to b...

Thermal conductivity of half-Heusler superlattices

Thin films and superlattices (SLs) of TiNiSn and ZrHfNiSn layers have been grown by dc magnetron sputtering on MgO (100) substrates to reduce the thermal conductivity, aiming for improvement of the thermoelectric figure of merit ZT. The thermal conductivity of 1 Wm−1K−1 was measured by the differential 3ω method for an SL with a periodicity of 8.8 nm. In addition to x-ray diffraction analysis of the SL crystal structure, smooth interfaces were confirmed by scanning/transmission electron microscopy.

Methanol steam reforming catalysts derived by reduction of perovskite-type oxides LaCo1−x−yPdxZnyO3±δ

Methanol steam reforming (MSR) catalysts are derived from perovskite-type oxides LaCo1−x−yPdxZnyO3±δ by reductive pretreatment. The unsubstituted LaCoO3±δ (LCO) and LaCo1−x−yPdxZnyO3±δ (Co substituted with Pd and/or Zn) are synthesized by a citrate method and characterized by different techniques. The perovskite-type oxides exhibit a rhombohedral crystal structure and a comparable surface area (≈8.5 (±2) m2 g−1). The temperature-programmed reduction (TPR) shows low (100 °C 450 °C) temperature reduction events that correspond to partial and complete reduction of the non-rare-earth metal ions, respectively. At high temperatures, Pd–Zn alloy nanoparticles are formed exclusively on Pd- and Zn-containing LaCo1−x−yPdxZnyO3±δ, as evident from high angular annular dark-field scanning transmission electron microscopy (HAADF-STEM). The CO2-selective MSR performance of the catalysts strongly depends on the reductive pretreatment temperature, catalyst composition (i.e., the Pdu2006:u2006Zn molar ratio and the degree of Co substitution) and reaction temperature. Only LaCo1−x−yPdxZnyO3±δ catalysts show a low-temperature CO2 selectivity maximum between 225 and 250 °C, while all catalysts present similar high-temperature selectivity maxima at T > 400 °C. The former is missing on LCO, LaCo1−xPdxO3±δ or LaCo1−yZnyO3±δ. Pd–Zn nanoparticles facilitate Zn(OH)2 and Co(OH)2 formation exclusively on LaCo1−x−yPdxZnyO3±δ, as evident from in situ XRD under steam atmosphere. This indicates the important role of Pd–Zn nanoparticles in the low-temperature CO2 selectivity, which is improved from 0 to 76% at 225 °C on LCO and LaCo0.75Pd0.125Zn0.125O3±δ, respectively. The high-temperature CO2 selectivity is governed by the bulk catalyst composition and the occurrence of reverse water gas shift reaction.

Recent Developments inβ-Zn4Sb3Based Thermoelectric Compounds

Thermoelectricity has been recognized as an environmentally friendly energy conversion technology due to its ability to directly achieve conversion between heat and electricity for a long time. β- has attracted considerable interest as promising thermoelectric material in the moderate temperature range (500u2009K–900u2009K), which is the temperature range of most industrial waste heat sources. In this paper, first we present the structure of β- and the traditional doping strategy used to enhance its performance. Next, we review the details of some new methods utilized for improving the thermoelectric properties of β- and its thermal stability as well as reliability. Finally, the review finishes with highlighting some promising strategies for future research directions in the material.