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The formula for maximum efficiency ( η ₘₐₓ) of heat conversion into electricity by a thermoelectric device in terms of the dimensionless figure of merit ( ZT ) has been widely used to assess the desirability of thermoelectric materials for devices. Unfortunately, the η ₘₐₓ values vary greatly depending on how the average ZT values are used, raising questions about the applicability of ZT in the case of a large temperature difference between the hot and cold sides due to the neglect of the temperature dependences of the material properties that affect ZT . To avoid the complex numerical simulation that gives accurate efficiency, we have defined an engineering dimensionless figure of merit ( ZT ) ₑₙg and an engineering power factor ( PF ) ₑₙg as functions of the temperature difference between the cold and hot sides to predict reliably and accurately the practical conversion efficiency and output power, respectively, overcoming the reporting of unrealistic efficiency using average ZT values.

Commercial hydrogen production by electrocatalytic water splitting will benefit from the realization of more efficient and less expensive catalysts compared with noble metal catalysts, especially for the oxygen evolution reaction, which requires a current density of 500 mA/cm² at an overpotential below 300 mV with long-term stability. Here we report a robust oxygen-evolving electrocatalyst consisting of ferrous metaphosphate on self-supported conductive nickel foam that is commercially available in large scale. We find that this catalyst, which may be associated with the in situ generated nickel–iron oxide/hydroxide and iron oxyhydroxide catalysts at the surface, yields current densities of 10 mA/cm² at an overpotential of 177 mV, 500 mA/cm² at only 265 mV, and 1,705 mA/cm² at 300 mV, with high durability in alkaline electrolyte of 1 M KOH even after 10,000 cycles, representing activity enhancement by a factor of 49 in boosting water oxidation at 300 mV relative to the state-of-the-art IrO₂ catalyst.

With the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material ‘hematene’ obtained from natural iron ore hematite (α-Fe2O3), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.

Thermal management becomes increasingly important as we decrease device size and increase computing power. Engineering materials with high thermal conductivity, such as boron arsenide (BAs), is hard because it is essential to avoid defects and impurities during synthesis, which would stop heat flow. Three different research groups have synthesized BAs with a thermal conductivity around 1000 watts per meter-kelvin: Kang et al. , Li et al. , and Tian et al. succeeded in synthesizing high-purity BAs with conductivities half that of diamond but more than double that of conventional metals (see the Perspective by Dames). The advance validates the search for high-thermal-conductivity materials and provides a new material that may be more easily integrated into semiconducting devices. Science , this issue p. 575 , p. 579 , p. 582 ; see also p. 549 Boron arsenide has an ultrahigh thermal conductivity, making it competitive with diamond for thermal management applications. Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that departs from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 watts per meter-kelvin and an average bulk value reaching 900 watts per meter-kelvin in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon-band engineering and can only be explained by higher-order phonon processes. These findings yield insight into the physics of heat conduction in solids and show BAs to be the only known semiconductor with ultrahigh thermal conductivity.

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Achieving higher carrier mobility plays a pivotal role for obtaining potentially high thermoelectric performance. In principle, the carrier mobility is governed by the band structure as well as by the carrier scattering mechanism. Here, we demonstrate that by manipulating the carrier scattering mechanism in n-type Mg₃Sb₂-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved. In this work, Fe, Co, Hf, and Ta are doped on the Mg site of Mg3.2Sb1.5Bi0.49Te0.01, where the ionized impurity scattering crosses over to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼16 to ∼81 cm²·V−1·s−1 is obtained, thus leading to a notably enhanced power factor of ∼13 μW·cm−1·K−2 from ∼5 μW·cm−1·K−2. A simultaneous reduction in thermal conductivity is also achieved. Collectively, a figure of merit (ZT) of ∼1.7 is obtained at 773 K in Mg3.1Co0.1Sb1.5Bi0.49Te0.01. The concept of manipulating the carrier scattering mechanism to improve the mobility should also be applicable to other material systems.

Improvements in thermoelectric material performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor has been less successful in comparison. In this work, a peak power factor of ∼106 μW·cm−1·K−2 is achieved by increasing the hot pressing temperature up to 1,373 K in the p-type half-Heusler Nb0.95Ti0.05FeSb. The high power factor subsequently yields a record output power density of ∼22 W·cm−2 based on a single-leg device operating at between 293 K and 868 K. Such a high-output power density can be beneficial for large-scale power generation applications.

The efficiency of sunlight-driven reduction of carbon dioxide (CO 2 ), a process mimicking the photosynthesis in nature that integrates the light harvester and electrolysis cell to convert CO 2 into valuable chemicals, is greatly limited by the sluggish kinetics of oxygen evolution in pH-neutral conditions. Current non-noble metal oxide catalysts developed to drive oxygen evolution in alkaline solution have poor performance in neutral solutions. Here we report a highly active and stable oxygen evolution catalyst in neutral pH, Brownmillerite Sr 2 GaCoO 5 , with the specific activity about one order of magnitude higher than that of widely used iridium oxide catalyst. Using Sr 2 GaCoO 5 to catalyze oxygen evolution, the integrated CO 2 reduction achieves the average solar-to-CO efficiency of 13.9% with no appreciable performance degradation in 19 h of operation. Our results not only set a record for the efficiency in sunlight-driven CO 2 reduction, but open new opportunities towards the realization of practical CO 2 reduction systems. Solar-to-fuel conversion under mild conditions offers a renewable means to store energy. Here authors report a Brownmillerite oxide for neutral-pH oxygen evolution that, when integrated with an anodized silver cathode, enables a 13.9% energy-conversion efficiency for light-driven CO 2 reduction

To raise the superconducting-transition temperature (Tc) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with Tcs up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high Tcs. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced Tc without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its Tc up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT , has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor ( PF ) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg ₂Sn-based material, Mg ₂Sn ₀.₇₅Ge ₀.₂₅, is a good example to meet the dual requirements in efficiency and output power. It was found that Mg ₂Sn ₀.₇₅Ge ₀.₂₅ has an average ZT of 0.9 and PF of 52 μW⋅cm ⁻¹⋅K ⁻² over the temperature range of 25–450 °C, a peak ZT of 1.4 at 450 °C, and peak PF of 55 μW⋅cm ⁻¹⋅K ⁻² at 350 °C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with T ₕ = 400 °C and T c = 50 °C to be of 10.5% and 6.6 W⋅cm ⁻² under a temperature gradient of 150 °C⋅mm ⁻¹, respectively. Significance Thermoelectric materials have been extensively studied for applications in conversion of waste heat into electricity. The efficiency is related to the figure-of-merit, ZT = ( S ²σ / κ ) T , where S , σ , and κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Pursuing higher ZT for higher efficiency has been the focus by mainly reducing the thermal conductivity. In this paper, we point out, for a given ZT , higher power factor ( S ²σ ) should be pursued for achieving more power because power is determined by ( T ₕ − T c) ²( S ²σ )/ L , where T ₕ, T c, and L are the hot and cold side temperatures, and leg length, respectively. We found a new material, Mg ₂Sn ₀.₇₅Ge ₀.₂₅, having both high ZT and high power factor.