This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and ...synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.
This review discusses recent developments in high-performance bulk thermoelectrics, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies.
One of the intellectual challenges for next generation thermoelectric materials revolves around the synthesis and fabrication of hierarchically organized microstructures that do not appreciably ...compromise the innate high power factor of the chosen thermoelectric system, but significantly reduce lattice thermal conductivity to enhance the overall figure of merit, ZT. An effective emerging strategy is to introduce nanostructures into bulk thermoelectric materials, which allow for diverse phonon scattering mechanisms to reduce thermal conductivity. In this review, we present key examples to show the intricate but tractable relationship across all relevant length-scales between various microstructural attributes (point, line, interfacial and mesoscale defects; as well as associated elastic and plastic strain) and lattice thermal conductivity in systems based on PbTe matrices. We emphasize the need for an overarching panoscopic approach that enables specific design strategies for the next generation of thermoelectric materials.
Currently, there is considerable interest in developing advanced rechargeable batteries that boast efficient distribution of electricity and economic feasibility for use in large-scale energy storage ...systems. Rechargeable aqueous zinc batteries are promising alternatives to lithium-ion batteries in terms of rate performance, cost, and safety. In this investigation, we employ Cu
(HHTP)
, a two-dimensional (2D) conductive metal-organic framework (MOF) with large one-dimensional channels, as a zinc battery cathode. Owing to its unique structure, hydrated Zn
ions which are inserted directly into the host structure, Cu
(HHTP)
, allow high diffusion rate and low interfacial resistance which enable the Cu
(HHTP)
cathode to follow the intercalation pseudocapacitance mechanism. Cu
(HHTP)
exhibits a high reversible capacity of 228 mAh g
at 50 mA g
. At a high current density of 4000 mA g
(~18 C), 75.0% of the initial capacity is maintained after 500 cycles. These results provide key insights into high-performance, 2D conductive MOF designs for battery electrodes.
Electrode materials based on conversion reactions with lithium ions have shown much higher energy density than those based on intercalation reactions. Here, nanocubes of a typical metal oxide (Co3O4) ...were grown on few-layer graphene, and their electrochemical lithiation and delithiation were investigated at atomic resolution by in situ transmission electron microscopy to reveal the mechanism of the reversible conversion reaction. During lithiation, a lithium-inserted Co3O4 phase and a phase consisting of nanosized Co–Li–O clusters are identified as the intermediate products prior to the subsequent formation of Li2O crystals. In delithiation, the reduced metal nanoparticles form a network and breakdown into even smaller clusters that act as catalysts to prompt reduction of Li2O, and CoO nanoparticles are identified as the product of the deconversion reaction. Such direct real-space, real-time atomic-scale observations shed light on the phenomena and mechanisms in reaction-based electrochemical energy conversion and provide impetus for further development in electrochemical charge storage devices.
Due to their unique optoelectronic properties and potential for next generation devices, monolayer transition metal dichalcogenides (TMDs) have attracted a great deal of interest since the first ...observation of monolayer MoS2 a few years ago. While initially isolated in monolayer form by mechanical exfoliation, the field has evolved to more sophisticated methods capable of direct growth of large-area monolayer TMDs. Chemical vapor deposition (CVD) is the technique used most prominently throughout the literature and is based on the sulfurization of transition metal oxide precursors. CVD-grown monolayers exhibit excellent quality, and this process is widely used in studies ranging from the fundamental to the applied. However, little is known about the specifics of the nucleation and growth mechanisms occurring during the CVD process. In this study, we have investigated the nucleation centers or “seeds” from which monolayer TMDs typically grow. This was accomplished using aberration-corrected scanning transmission electron microscopy to analyze the structure and composition of the nuclei present in CVD-grown MoS2–MoSe2 alloys. We find that monolayer growth proceeds from nominally oxi-chalcogenide nanoparticles which act as heterogeneous nucleation sites for monolayer growth. The oxi-chalcogenide nanoparticles are typically encased in a fullerene-like shell made of the TMD. Using this information, we propose a step-by-step nucleation and growth mechanism for monolayer TMDs. Understanding this mechanism may pave the way for precise control over the synthesis of 2D materials, heterostructures, and related complexes.
The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of ...thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally friendly means of energy savings and power generation. The thermoelectric efficiency is ...determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZTdev ~1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.
SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising ...from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit ZT. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficient, and (b) enlarging the energy band gap. Thus, alloying with Cd atoms enables a form of valence band engineering that improves the high-temperature thermoelectric performance, where p-type samples of SnCd0.03Te exhibit ZT values of ∼0.96 at 823 K, a 60% improvement over the Cd-free sample. Finally, we introduce endotaxial CdS or ZnS nanoscale precipitates that reduce the lattice thermal conductivity of SnCd0.03Te with no effect on the power factor. We report that SnCd0.03Te that are endotaxially nanostructured with CdS and ZnS have a maximum ZTs of ∼1.3 and ∼1.1 at 873 K, respectively. Therefore, SnTe-based materials could be ideal alternatives for p-type lead chalcogenides for high temperature thermoelectric power generation.
The role of conventional graphene-oxide in biosensing has been limited to that of a quenching substrate or signal transducer due to size inconsistencies and poor supramolecular response. We overcame ...these issues by using nanoscale GOs (nGO) as artificial receptors. Unlike conventional GO, nGOs are sheets with near uniform lateral dimension of 20 nm. Due to its nanoscale architecture, its supramolecular response was enhanced, with demonstrated improvements in biomacromolecular affinities. This rendered their surface capable of detecting unknown proteins with cognizance not seen with conventional GOs. Different proteins at 100 and 10 nM concentrations revealed consistent patterns that are quantitatively differentiable by linear discriminant analysis. Identification of 48 unknowns in both concentrations demonstrated a >95% success rate. The 10 nM detection represents a 10-fold improvement over analogous arrays. This demonstrates for the first time that the supramolecular chemistry of GO is highly size dependent and opens the possibility of improvement upon existing GO hybrid materials.
We report a novel hierarchical microstructure in the PbSe-CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with
ZT
ave
∼ 0.83 across the 400-923 K ...temperature range, the highest reported for p-type, Te-free PbSe systems. We have investigated the local atomic structure as well as the microstructure of a series of PbSe-
x
CdSe materials, up to
x
= 10%. We find that the behavior of the Cd atoms in the octahedral rock salt sites is discordant and results in off-center displacement and distortion. Such off-centered Cd in the PbSe matrix creates (1) L-Σ electronic energy band convergence, (2) a flattened L band, both contributing to higher Seebeck coefficients, and (3) enhanced phonon scattering, which leads to lower thermal conductivity. These conclusions are supported by photoemission yield spectroscopy in air (PYSA), solid state
111
Cd,
77
Se NMR spectroscopy and DFT calculations. Above the solubility limit (>6%CdSe), we also observe endotaxial CdSe nano-precipitates with core-shell architecture formed in PbSe, whose size, distribution and structure gradually change with the Cd content. The nano-precipitates exhibit a zinc blende crystal structure and a tetrahedral shape with significant local strain, but are covered with a thin wurtzite layer along the precipitate/matrix interface, creating a core-shell structure embedded in PbSe. This newly discovered architecture causes a further reduction in lattice thermal conductivity. Moreover, potassium is found to be an effective p-type dopant in the PbSe-CdSe system, leading to an enhanced power factor, a maximum
ZT
of ∼1.4 at 923 K for Pb
0.98
K
0.02
Se-6%CdSe.
Off-centering and core-shell nanoscale CdSe precipitates lead to high thermoelectric performance for p-type, Te-free PbSe systems.
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