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N-type Mg3Sb2-based Zintl compounds have attracted considerable interest in recent years for their high thermoelectric performance. Mg3Sb2-based compounds inherently have p-type transport properties because of the presence of intrinsic Mg vacancies. Therefore, eliminating Mg vacancies and increasing the electron concentration are crucial for achieving high-performance n-type Mg3Sb2-based materials. The addition of excess Mg in the initial composition and the doping of chalcogens (Te, Se, and S) at the Sb site have been the primary methods used to date. Here, we demonstrate that n-type conduction was successfully achieved by a simple Y doping at the Mg site without adding extra Mg in the initial composition. Neutron diffraction analysis shows that Y preferentially goes to the Mg (Ⅱ) site and that almost all of the Mg vacancies are eliminated, giving rise to a strong donor effect to create n-type conduction. By eliminating the vacancies, the carrier concentration is easily optimized through the combination of Y and Mg, leading to a peak ZT of ∼1.8 at 773 K in Mg3.02Y0.02Sb1.5Bi0.5.

Airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via air-conditioning systems poses a significant threat for the continued escalation of the current coronavirus disease (COVID-19) pandemic. Considering that SARS-CoV-2 cannot tolerate temperatures above 70 °C, here we designed and fabricated efficient filters based on heated nickel (Ni) foam to catch and kill SARS-CoV-2. Virus test results revealed that 99.8% of the aerosolized SARS-CoV-2 was caught and killed by a single pass through a novel Ni-foam-based filter when heated up to 200 °C. Additionally, the same filter was also used to catch and kill 99.9% of Bacillus anthracis, an airborne spore. This study paves the way for preventing transmission of SARS-CoV-2 and other highly infectious airborne agents in closed environments., Graphical abstract Image 1, Highlights • A heated air disinfection system was fabricated based on commercial Ni foams. • The Ni-foam-based filters showed ∼100% ability for catching and killing of SARS-CoV-2. • The air disinfection system is significant to control spread of COVID-19.

Superconductivity intrinsic to the intercalated black phosphorus (BP) with a transition temperature Tc of 3.8 K, independent of the intercalant, whether an alkali or an alkaline earth element, has been reported recently by R. Zhang et al. (2017). However, the reported Tc and the field effect on the superconducting (SC) transition both bear great similarities to those for the pure Sn, which is commonly used for BP synthesis under the vapor transport method. We have therefore decided to determine whether a minute amount of Sn is present in the starting high purity BP crystals and whether it is the culprit for the small SC signal detected. Energy-dispersive X-ray spectroscopy results confirmed the existence of Sn in the starting high purity BP crystals purchased from the same company as in R. Zhang et al. (2017). We have reproduced the SC transition at 3.8 K in Li- and Na-intercalated BP crystals that contain minute amounts of Sn when prepared by the vapor transport method but not in BP crystals that are free of Sn when prepared by the high-pressure method. We have therefore concluded that the SC transition reported by R. Zhang et al. (2017) is associated with the Sn but not intrinsic to the intercalated BP crystals., Comment: 8 pages, 6 figures

Hydrogen evolution electrocatalyst made from earth-abundant elements for electrocatalytic water splitting is essential for sustainable and clean hydrogen economy. At present, how to make efficient catalysts with superior catalytic activity from cheap raw materials in large scale remains a great challenge. Here, we report a new nickel phosphide-based hybrid nanosheet arrays electrocatalyst, synthesized by a one-step phosphorization of commercial nickel (Ni) foam, for hydrogen evolution in water splitting. In acidic medium, the nickel phosphide nanosheet arrays exhibit very stable and fast hydrogen evolution kinetics with a relatively low overpotentials of 61 mV and 121 mV to achieve current densities of −10 and −100 mA cm−2, respectively, with a Tafel slope of 51 mV dec−1, showing comparable performance to the most efficient non-noble metal based hydrogen evolution electrocatalysts reported so far. This robust electrocatalyst, made from commercially available materials, holds a potential for the industrialization of clean hydrogen energy.

Defects, which can scatter the carriers as well as alter the carrier concentration, could greatly impact the material's electrical conductivity. Here we report the anomalous temperature dependences of electrical conductivity observed in n-type Te-doped Mg3Sb2-based materials. These anomalies are induced by the variations of Hall carrier concentration and mobility, both of which are influenced by the defects that closely relate to the Te dopant. Our results demonstrate that Te concentration plays a critical role in manipulating the defect chemistry of n-type Mg3Sb2-based materials. The intricate relationship among the defects, composition, Fermi level, and temperature significantly complicates our understanding on the thermoelectric properties of n-type Mg3Sb2-based materials. Our work indicates that further enhancement in thermoelectric performance of this material is possible if the defects can be effectively controlled.

Mg3Sb2-based Zintl compounds have attracted extensive attention as potential thermoelectric materials due to their earth-abundant elements. However, pure and intrinsic p-type Mg3Sb2 manifests a poor thermoelectric performance because of its high electrical resistivity. It is reported that Ag doping in Mg sites can increase the mobility and carrier concentration of p-type Mg3Sb2; however, high thermal conductivity still limits the improvement of the ZT value. In the present work, Zn and Ag co-doping in Mg sites was carried out to optimize the thermoelectric performance of p-type Mg3Sb2. Experimental results revealed that the carrier concentration and mobility of Mg3Ag0.01Sb2 significantly increased after Zn doping, leading to an improvement of the power factor. Simultaneously, lattice thermal conductivity was significantly reduced due to the large mass difference between Zn and Mg. The Mg2.39Zn0.6Ag0.01Sb2 sample with an optimal doping concentration of 3.95 × 1019 cm−3 achieved a maximum ZT value of 0.84 at 773 K. Hence, Zn and Ag co-doping is an effective method to improve the thermoelectric performance of p-type Mg3Sb2.

Excellent thermoelectric performance has been realized in trace-elements-doped n-type Mg3Bi2-xSbx materials, but the structural origin for such improved performance hasn't been fully studied, inhibiting this effective approach to better performance. Here we present the characterizations and analyses of the atomic and electronic structures associated with La dopants in polycrystalline Mg3·2Bi1·5Sb0.5 using synchrotron X-ray diffraction and X-ray absorption spectra, as well as the material's thermoelectric performance. Interstitial oxygen was observed inside the cavities of the Mg6Bi6 hexagonal cylinders in un-doped Mg3·2Bi1·5Sb0.5, which may account for its semiconducting behavior, while the La can effectively depresses the interstitial oxygen, resulting in the more negative valence state of Bi from −2.20 to −3.35. Furthermore, the thermoelectric performance of La-doped Mg3·2Bi1·5Sb0.5 has been significantly improved due to the optimized carrier concentration and the increased Hall mobility, exhibiting a zT value of ∼0.75 at 525 K.

Achieving superconducting joinings with large current capacities is a challenge for magnetic resonance imaging applications of YBa2Cu3O7-δ (YBCO) coated conductors (CCs). Here we report a novel method to fabricate a superconducting joining between two YBCO CCs using a joining strap consisting of YBCO, Ag, a buffer, and a substrate. The YBCO layer exfoliated from the buffer stack of a YBCO CC can retain more than 90% of its original critical current (Ic). This is particularly due to the Ag layer serving as a fast oxygen diffusion path to promote the recovery of superconductivity of the YBCO layer during oxygenation annealing. The atomic arrangement at the joined YBCO interface is well aligned along the (001) plane, enabling superior superconducting performance. The method reported here provides a practicable solution to achieve persistent current mode operation in second-generation high-temperature superconducting magnet applications.

Since 2015, when we first reported that the half-Heusler alloy NbCoSb with a nominal 19-valence-electrons composition exhibits the moderate thermoelectric properties of a heavily doped n-type semiconductor, we have been studying the preparation of pure-phase materials. Here we report, for the first time, the effects of Nb and Sb deficiencies on the phase composition and thermoelectric properties of NbCoSb. The experimental results show that phase-pure material can be achieved if the composition is slightly Nb and Sb deficient and that intrinsic vacancy defects have a remarkable effect on the thermoelectric performance. The ZT value is doubled and a maximum ZT of 0.8 is achieved in Nb0·97CoSb0.99 at 973 K, benefiting from the elimination of impurity phases and the optimization of carrier concentration. This work proves that introducing vacancy defects is an effective strategy to improve the thermoelectric properties of some half-Heusler alloys, especially of nominal 19-electron half-Heusler compounds.

Low-grade heat harvesting by thermoelectric technology can contribute to efficient energy utilization. However, the large-scale application of thermoelectric devices is partially hindered by their unsatisfactory performance and relatively high cost. Here we prepared a cost-effective n-type Mg3Bi2-based compound with a peak zT of 1.24 at 573 K. A unicouple comprised of n-type Mg3.2Bi1.29Sb0.7Te0.01 and p-type Bi0.2Sb1.8Te3 was constructed and its energy conversion efficiency was evaluated. The unicouple exhibited a record-high efficiency of (9.0 ± 0.5)% or (8.3 ± 0.4)% [considering the radiation heat loss] under a temperature difference of 265 K at the hot-side temperature of 573 K. Our results demonstrate the great potential of Mg3Bi2-based materials for low-grade heat recovery.

Rechargeable aqueous Zn-based batteries are highly desirable for future applications in large-scale energy storage since they are inexpensive and safe in comparison with lithium-ion batteries (LIBs). Additionally, the high energy density of Zn batteries, nearly comparable to that of LIBs, stands out in all types of aqueous batteries. Fast charge, extremely important in practical application, is another typical characteristic in aqueous batteries compared to LIBs with organic electrolyte, but little attention has been paid to it thus far. Herein, ultrafast charge of the cathodes in Zn batteries are realized through the rapid conversion of low-valence transition-metal ions to their high-valence solid oxides using a simple high-potential deposition strategy. In particular, the Mn-based cathode exhibits a charge time that is only around 1/40 of that by traditional constant-current charge method, while high capacity is acquired simultaneously due to the multivalent conversion.

We develop a model based on first principles calculations and experimental data for the thermoelectric properties of p -type CaMg 2 Bi 2 . The thermoelectric performance was assessed and optimized based on the model. The model predicts bipolar reduction of ZT at high temperatures (e.g. > 800 K). This indicates the thermoelectric performance of the material can be further optimized by tuning the carrier concentration beyond the bipolar regime.

Thermoelectric figure of merit ZT has been greatly improved in the past decade via band engineering to enhance power factor or nanostructuring to reduce thermal conductivity, but less attention has been paid to other significant factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here we show that Te doping on the Sb site, as an n-type strong donor, could significantly suppress the high-temperature bipolar effect in the nanostructured Zintl Zr3Ni3Sb4, which can be ascribed to the combination of high majority-carrier concentration and enlarged band gap. A relatively good ZT of ∼0.6 at 773 K for Te doping can be achieved and that is almost double of the previous reported ZT by Cu doping. In addition, the role of carrier scattering mechanism on the low-temperature electrical transport properties is also pointed out, where both carrier mobility and power factor of Te doping, due to the detrimental effect of ionized impurity scattering, are lower than that of Cu doping in which the mixed acoustic phonon and ionized impurity scattering dominates.

Obtaining outstanding results is the most important step for research, but getting these results published in a journal that can generate the greatest impact to the scientific community and society is just as important as obtaining the results. I did a simple investigation about the papers related to thermoelectrics that were published in different journals between the years of 2000 and 2017. I noticed that there is not a journal with more than 5% of the papers on thermoelectrics. To better serve the community and better inform the society of the importance of thermoelectrics, we now start a new journal, Materials Today Physics, to hopefully publish papers on thermoelectrics with a much higher percentage but open to other novel discoveries on materials and physics.

Thermoelectric materials have attracted extensive interest in the last two decades due to their potential applications in waste-heat recovery from industrial processes, automobiles, and renewable energy sources. Among the various candidate materials, Zintls have recently gained significant interest because of their high thermoelectric figure of merit (ZT) for potential use in thermoelectric power generation. In this review, we first briefly summarize some of the most intensely studied Zintl families with unique and diverse anionic frameworks, ranging from isolated moieties to one-dimensional (1D) chains of tetrahedra or frames of ribbons to 2D layered structures, along with discussion of their challenges and possibilities for further improvements. Second, the 2D layered CaAl 2 Si 2 -type Zintl phases are discussed in more detail, from fundamental crystal structure and electronic band structure to the approaches that have been successfully used to enhance the thermoelectric performance. Finally, we provide an overview of the recent progress in thermoelectric Zintl materials, particularly the most recent exciting development in achieving high ZTs in n-type Zintls, and what can be realistically expected for advancing this class of materials into practical applications.

The MCoSb-based (M = Hf, Zr) half-Heusler compounds were recognized as a promising p-type thermoelectric (TE) material for more than 2 decades although the base compound is intrinsically n-type. Here we investigate the TE properties of Ni-substituted n-type MCoSb. The anomalous changes of carrier concentration and lattice thermal conductivity with higher amount of Ni indicate the presence of atomic disorder. Peak power factor of ∼33 μW cm−1 K−2 and peak ZT of 0.6 are obtained in ZrCo0.9Ni0.1Sb. Further substitute Zr by Hf suppresses the lattice thermal conductivity and yields a peak ZT exceeding 1 in the composition Zr0.5Hf0.5Co0.9Ni0.1Sb at 1073 K. Thus the MCoSb compounds possess promising TE properties by both n- and p-type doping, which is unique among the half-Heusler based TE materials.

As reserves of conventional light oil become depleted, recovery of viscous oil is urgently needed to meet increasing energy demands worldwide before clean energy sources are fully developed. Current oil extraction technologies suffer from low efficiency, high cost, and environmental concerns. Attempts to use nanotechnology in this field have thus far been recognized to have only auxiliary effects, rather than playing a major role. Here we report a reactive sodium nanoparticle fluid able to in situ recover highly viscous crude oil very effectively even at room temperature. Its high recovery performance is based on a chemical reaction that allows the nanofluid to exhibit multiple benefits in displacing subsurface oil: heat, hydrogen gas, and sodium hydroxide. In addition, multi-stage nanofluid injection is found to be superior to a single injection mode. Thus, it opens a new way to extract and process crude oil.

The conventional doping strategy for thermoelectric materials generally focuses on a shallow donor/acceptor model with the energy level close to the band edge as for electronic devices. However, thermoelectric devices operate over a large temperature difference, and the optimal carrier concentration increases with increasing temperature. A shallow level cannot meet the requirement over a large temperature range. Here, an innovative strategy of introducing an intermediate level is proposed. Such an intermediate level introduces more carriers with increasing temperature, consistent with the trend of increasing optimal doping concentration with temperature, enabling larger power factor over a broader temperature range. Furthermore, the intermediate level typically requires more impurities, leading to increased phonon scattering. This strategy allows simultaneous optimization of carrier concentration over a wide temperature range and suppression of thermal conductivity via stronger point-defect phonon scattering. Experimental results from heavily-doped ZrCoSb employing shallow, intermediate, and deep levels successfully corroborate this strategy, where simultaneously improved power factor and figure of merit are obtained by introducing an intermediate level. Our work indicates that the performance of known thermoelectric materials should be reevaluated by introducing an intermediate level to unleash their full potential.

Cubic boron arsenide (BAs) is attracting greater attention due to the recent experimental demonstration of ultrahigh thermal conductivity \k{appa} above 1000 W/mK. However, its bandgap has not been settled and a simple yet effective method to probe its crystal quality is missing. Furthermore, traditional \k{appa} measurement methods are destructive and time consuming, thus they cannot meet the urgent demand for fast screening of high \k{appa} materials. After we experimentally established 1.82 eV as the indirect bandgap of BAs and observed room-temperature band-edge photoluminescence, we developed two new optical techniques that can provide rapid and non-destructive characterization of \k{appa} with little sample preparation: photoluminescence mapping (PL-mapping) and time-domain thermo-photoluminescence (TDTP). PL-mapping provides nearly real-time image of crystal quality and \k{appa} over mm-sized crystal surfaces; while TDTP allows us to pick up any spot on the sample surface and measure its \k{appa} using nanosecond laser pulses. These new techniques reveal that the apparent single crystals are not only non-uniform in \k{appa}, but also are made of domains of very distinct \k{appa}. Because PL-mapping and TDTP are based on the band-edge PL and its dependence on temperature, they can be applied to other semiconductors, thus paving the way for rapid identification and development of high-\k{appa} semiconducting materials.

Stainless steel (SS) has been widely incorporated into electrodes or used as a conductive scaffold in industrial water electrolysis. Operations using such SS-bearing materials proceed under substantial overpotentials above the thermodynamic requirement due to the sluggish kinetics of the anodic oxygen evolution reaction (OER). Traditional surface engineering of SS involves a corrosion process to optimize its composition and modify its surface morphology, but its catalytic properties remain unsatisfactory even after employing this technique. Here, we introduce an effective route, based on surface decoration, to construct a ternary phase composed of nickel iron selenide (NiFeSe) uniformly distributed on a stainless-steel mat (SSM). NiFeSe/SSM was found to require an overpotential of only 260 mV to drive a current density of 1,000 mA cm−2 at 75°C in alkaline solution, one of the best results among SS-based and even transition metal-based catalysts for OER. Additionally, this NiFeSe/SSM electrode shows negligible potential degradation after operating at 1,000 mA cm−2 for 55 h. It is suggested that Se in the nickel iron selenide directly binds to the SSM through strong covalent bonding during selenization, which guarantees low charge-transfer resistance and excellent durability against the gas bubbles produced during operation at large current densities. This work opens a new route for performing surface engineering on SS or other conductive substrates to achieve superior OER performance.

The strong adsorption of intermediates such as OH* and H* species on the surface of ruthenium (Ru) significantly impedes its electrocatalytic activity for hydrogen evolution reaction (HER) in alkaline media. Here, we propose the introduction of cobalt phosphide (CoP) to efficiently regulate the electronic configuration of Ru. The resultant CoP nanowire array decorated with ultrafine Ru nanoparticles on a conductive carbon cloth substrate (denoted as Ru–CoP/CC) displays comparable catalytic performance to the state-of-the-art Pt/C catalyst for HER in 1 M KOH solution, requiring an extremely low overpotential of only ~21 mV to achieve a current density of 10 mA cm−2, exhibiting a Tafel slope as low as ~49 mV/dec, and maintaining excellent stability over 24 h of electrolysis. Density functional theory calculations on the simulated charge distribution reveal that the regulation of the surface electronic structure of Ru by CoP decreases the adsorption energy of OH* and H* intermediates on the surface of Ru and ultimately contributes to the outstanding electrocatalytic activity of the Ru–CoP/CC electrode for HER.

A strategy inspired by plant growth was applied as a novel and facile way to fabricate nanoscale composites for high-performance electrodes employed in supercapacitors. A binder-free cactus-like nanostructure CoNiO2@Co3O4@Co2N grown on N-doped reduced graphene oxide (rGO)-wrapped nickel foam, in which the N-doped rGO sheet acts as the ground, providing active sites for the composite to grow, and the nickel foam acts as the root system, providing both Ni ions to form the bimetal oxide CoNiO2 and storage for the electrolyte ions, was designed. As the active material, the nanostructured CoNiO2@Co3O4@Co2N not only provides more ion-accessible surfaces but also contributes its battery-mimicking faradaic reaction to the high electrochemical performance of the as-prepared electrode. The synergistic effects of the nanostructure, materials, and energy storage mechanism result in the high capacitance [2615.32 F g−1 (6.76 F cm−2)] and excellent rate performance of the as-prepared electrode. This in situ growth system also provides a novel design concept to obtain effective, environmental friendly, and inexpensive composite materials for high-performance energy storage devices.

Boron arsenide (BAs) has been the least investigated cubic III-V compound, but it has recently attracted significant attention since the confirmation of its unusually high thermal conductivity above 1000 W/m-K. However, determining how to achieve growth of a BAs single crystal on the centimeter scale remains unsolved, which strongly limits further research into, and potential applications of, this interesting material. Here we report our technique to grow a 7-mm-long BAs single crystal via the chemical vapor transport method by applying an additional nucleation site. The different thermal conductivity values obtained from BAs single crystals grown on nucleation sites of different compositions show the importance of choosing the proper nucleation-site material. We believe these findings will inspire further research into the growth of this unique semiconductor., Comment: 8 pages, 3 figures

Because graphene oxide is so important for many applications and also the precursor for large quantity of graphene, a low cost and environmentally friendly method using much less acid is essential to the future of graphene oxide and graphene. It is a great challenge to reduce the acid usage to avoid the safety risk, but tremendous benefits could be achieved if this were made possible for the industrial-scale synthesis of graphene oxide and graphene. In this work, we report a fast and simple method that uses much less acid than the state of the art to obtain nanoscale graphene oxide by using the heat released by the reaction. With the introduction of dry ice, the reaction temperature can be well controlled, thus stabilizing the highly oxidative and explosive Mn2O7 intermediate in order to avoid safety risk.