Showing papers by "Zhifeng Ren published in 2021"

Thermoelectric cooling materials.

Abstract: Solid-state thermoelectric devices can directly convert electricity into cooling or enable heat pumping through the Peltier effect. The commercialization of thermoelectric cooling technology has been built on the Bi2Te3 alloys, which have had no rival for the past six decades around room temperature. With the discovery and development of more promising materials, it is possible to reshape thermoelectric cooling technology. Here we review the current status of, and future outlook for, thermoelectric cooling materials. Thermoelectric materials can generate electricity from waste heat but can also use electricity for cooling. This Perspective discusses coefficients of performance for these systems and the state-of-the-art for materials, and suggests strategies for the discovery of improved thermoelectric materials.

Rational design of core-shell-structured CoPx@FeOOH for efficient seawater electrolysis

Abstract: Hydrogen generation by seawater electrolysis is a sustainable approach to renewable-energy conversion which requires efficient catalyst to address challenges such as competing chlorine evolution reaction, chloride corrosion, and catalyst poisoning. Here, core-shell-structured CoPx@FeOOH is designed for selective OER in seawater. This catalyst has high conductivity, large surface area, improved turnover frequency, and optimal absorption energy to OER intermediates, which together lead to excellent catalytic activity. The enhanced chemical stability and corrosion resistance ensure its catalytic performance in seawater. Specifically, it requires overpotentials of 283 and 337 mV to attain current densities of 100 and 500 mA cm−2, respectively, in 1 M KOH seawater, with durability over 80 h of continuous testing without producing any hypochlorite. The CoPx||CoPx@FeOOH pair requires voltages of 1.710 and 1.867 V to attain current densities of 100 and 500 mA cm−2 with a high Faradaic efficiency, showing its great promise for fuel-gas production from seawater.

Towards tellurium-free thermoelectric modules for power generation from low-grade heat

Abstract: Thermoelectric technology converts heat into electricity directly and is a promising source of clean electricity. Commercial thermoelectric modules have relied on Bi2Te3-based compounds because of their unparalleled thermoelectric properties at temperatures associated with low-grade heat (<550 K). However, the scarcity of elemental Te greatly limits the applicability of such modules. Here we report the performance of thermoelectric modules assembled from Bi2Te3-substitute compounds, including p-type MgAgSb and n-type Mg3(Sb,Bi)2, by using a simple, versatile, and thus scalable processing routine. For a temperature difference of ~250 K, whereas a single-stage module displayed a conversion efficiency of ~6.5%, a module using segmented n-type legs displayed a record efficiency of ~7.0% that is comparable to the state-of-the-art Bi2Te3-based thermoelectric modules. Our work demonstrates the feasibility and scalability of high-performance thermoelectric modules based on sustainable elements for recovering low-grade heat. Though earth abundant magnesium-based materials are attractive for thermoelectrics (TEs) due to their device-level performance, realizing efficient modules remains a challenge. Here, the authors report a scalable route to realizing Mg-based compounds for high performance TE modules.

Rational design of oxygen evolution reaction catalysts for seawater electrolysis

Abstract: Water electrolysis provides a promising route to produce high energy density hydrogen. Compared with the limited amount of fresh water, seawater is an abundant resource that has attracted increasing attention for electrolysis. However, seawater electrolysis has thus far suffered from degraded activity and stability, and from low oxygen evolution reaction (OER) selectivity, due to the existence of chloride ions and insoluble solids in seawater. This short review summarizes trends in the rational design of OER catalysts, providing some effective strategies, including constructing 3D hierarchical porous structures, employing protective layers, and engineering surface wettability, to synthesize efficient and stable OER catalysts for seawater electrolysis. Finally, a perspective regarding designing high-performance catalysts for seawater electrolysis is also provided.

CALPHAD as a powerful technique for design and fabrication of thermoelectric materials

Abstract: Phase diagrams have always been used as a roadmap for materials research in terms of melting, casting, crystal growth, joining, solid-state reaction, heat treatment, phase transformation, and so on. CALPHAD (CALculation of PHAse Diagram) offers a theoretical instruction through a plausible simulation based on Gibbs energies, which could bypass some difficult experiments, extrapolate to a multicomponent-system or metastable region to get complete thermodynamic and kinetic properties, and improve the consistency and accuracy of the measured phase diagram. The present work takes thermoelectric materials as a paradigm, illustrating how to interpret and utilize phase diagrams as well as other thermodynamic information to design target materials, with the aid of multi-component CALPHAD-base databases and software. Two main aspects including phase design and microstructure modulation are delivered by the fundamentals of CALPHAD modeling and concrete examples of thermoelectric alloys. Opinions about the challenges and potentials of its applications in thermoelectric systems are also pointed out.

Recent advances in flexible thermoelectrics

Abstract: Recently, flexible thermoelectric (TE) materials and devices have attracted extensive attention due to their capability to convert heat into electricity directly and their conformal contact with arbitrarily shaped heat sources, demonstrating great promise for application in self-powered portable/wearable low power consuming electronics. Here, we review the state of the art in the development of flexible TEs, including TE modules and materials themselves. The remaining challenges that limit the practical application of flexible TE devices are discussed, and possible solutions and suggestions to guide future development are also provided in this perspective.

The challenge of tuning the ratio of lattice/total thermal conductivity toward conversion efficiency vs power density

Abstract: Minimizing the lattice thermal conductivity of thermoelectric materials is essential for preserving the temperature difference during the operation of thermoelectric devices incorporating these materials. During the past two decades, there has been substantial improvement in the thermoelectric figure of merit (zT) due to reduced lattice thermal conductivity. Employing alloying effects in solid-solution compounds is the most common and practical approach for inhibiting lattice thermal conductivity. This Perspective takes the n-type Mg3Sb2−xBix thermoelectric alloys as examples, addressing their lattice thermal conductivity and corresponding zT as functions of their Bi concentration. Additionally, we seek to understand the effect of the lattice contribution to total thermal conductivity for most thermoelectric materials currently being researched. The lattice/total thermal conductivity ratio at the temperature corresponding to the peak zT shows weak material dependence, widely ranging from 0.5 to 0.75, which implies that the lattice thermal conductivity of most thermoelectric materials can be decreased further to improve thermoelectric performance. On the other hand, thermoelectric materials with relatively low ratios exhibit high power factors in their operating temperature ranges, which is ascribed to their excellent electrical performance. These observations provide guidelines to tune transport properties for future applications in thermoelectric power generation.

Hybrid Transition-Metal Oxide and Nitride@N-Doped Reduced Graphene Oxide Electrodes for High-Performance, Flexible, and All-Solid-State Supercapacitors.

Abstract: Nanoscale composites for high-performance electrodes employed in flexible, all-solid-state supercapacitors are being developed. A series of binder-free composites, each consisting of a transition bimetal oxide, a metal oxide, and a metal nitride grown on N-doped reduced graphene oxide (rGO)-wrapped nickel foam are obtained by using a universal strategy. Three different transition metals, Co, Mo, and Fe, are separately compounded with nickel ions, which originate from the nickel foam, to form three composites, NiCoO2 @Co3 O4 @Co2 N, NiMoO4 @MoO3 @Mo2 N, and NiFe2 O4 @Fe3 O4 @Fe2 N, respectively. These as-prepared active materials have similar regular variation patterns in their properties, including better conductivity and battery-mimicking pseudocapacitance, which result in their high whole-electrode capacitance performance [2598.3 F g-1 (39.85 F cm-2 ), 3472.6 F g-1 (41.43 F cm-2 ) and 1907.5 F g-1 (3.41 F cm-2 ) for the composites incorporating Co, Mo, and Fe, respectively]. The as-assembled flexible, all-solid-state NiCoO2 @Co3 O4 @Co2 N//KOH/PVA//NiCoO2 @Co3 O4 @Co2 N device can be easily bent and exhibits high energy density and power density of 92.8 Wh kg-1 and 1670.4 W kg-1 , respectively. The universality of this design strategy could allow it to be employed in producing hybrid materials for high-performance energy-storage devices.

Interfacial Superconductivity Achieved in Parent AEFe2As2 (AE = Ca, Sr, Ba) by a Simple and Realistic Annealing Route.

Abstract: Materials with interfaces often exhibit extraordinary phenomena exemplified by rich physics, such as high-temperature superconductivity and enhanced electronic correlations. However, demonstrations...

Lattice melting and superconductivity in a group IV-VI compound

Abstract: Inspired by the rich physical properties of IV-VI compounds, we choose polycrystalline ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ to investigate its structural, vibrational, and electrical transport properties under pressure up to 50 GPa. The structural transitions from the $B1$ to $Pnma$ phase and then to the $B2$ phase in this sample are verified by the x-ray diffraction and Raman scattering measurements. The formation of the intermediate phase is suggested to be mediated by Peierls distortion, and the broad hump in the temperature-dependent resistivity in the intermediate phase gives further evidence of this phenomenon. When the material evolves into the $B2$ phase, superconductivity is observed to emerge, accompanied by suppressing the broad hump of resistivity at intermediate temperatures. Meanwhile, Hall coefficient measurements indicate that the carrier type changes during the structural transitions. These results suggest that the superconductivity in the $B2$ phase for this material is originated by ``melting'' the Peierls lattice distortion. By extending the present findings to other similar IV-VI semiconductors, we propose that all group IV-VI compounds could exhibit superconductivity in their $B2$ phase due to the lattice melting at high pressures.

Development of a high-temperature (295–900 K) Seebeck coefficient Standard Reference Material

Abstract: We report the development of a high-temperature Seebeck coefficient Standard Reference Material (SRM) for use in instrument validation and interlaboratory data comparison in the temperature range of 295–900 K to support the research, development, and production of materials and devices related to thermoelectric-based energy conversion applications. We describe the synthesis, anneal–quench procedure, and physical characterization of a p-type boron-doped polycrystalline silicon–germanium alloy with a nominal composition of Si80Ge20. For the certification measurements, we describe the measurement protocols, statistical analysis, the certified Seebeck coefficient values, comprehensive uncertainty budgets, and metrological traceability. Our extensive efforts to identify, reduce, and quantify measurement uncertainties will be emphasized. This new SRM complements SRM 3451 Low-Temperature Seebeck Coefficient Standard (10–390 K) to provide certified reference materials traceable to the International System of Units (SI) for Seebeck coefficient measurements within the temperature range 10–900 K.

Influence of cation size on the thermoelectric behavior of salt-doped organic nanocomposite thin films

Abstract: Developing and understanding novel doping strategies for thermoelectric materials is imperative to efficiently convert waste into a useful voltage. One such method for improving the power factor of polymer nanocomposites is through salt doping. The cation size of a monovalent salt dopant was varied in a layer-by-layer (LbL)-assembled film composed of poly(diallyldimethylammonium chloride) (PDDA) and double-walled carbon nanotubes (DWNTs) stabilized by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) [PEDOT:PSS]. Doping a 20 bilayer PDDA/DWNT-PEDOT:PSS film doped with 3 mmol CsCl yields the maximum power factor of 485 ± 29 μW m−1 K−2. This value was obtained through a five times increase in the electrical conductivity with a minimal decrease in the Seebeck coefficient relative to the undoped film. Cs+ is believed to more heavily dope the carbon nanotube network due to its relatively larger hydrophobicity, while also separating PEDOT from PSS due to charge screening. This study demonstrates the significance of the salt dopant identity, and the insight herein can likely be applied more broadly to improve a variety of organic thermoelectric materials.

Thermal Conductivity of BAs under Pressure

Abstract: Boron arsenide (BAs) is an ultrahigh thermal conductivity material with special phonon-phonon scattering properties. At ambient pressure, the bunching of acoustic phonon branches in BAs leads to a small phase-space for three-phonon scattering. Density functional theory predicts that this acoustic phonon bunching effect is sensitive to pressure. To explore this physics, we measure the thermal conductivity of BAs from 0 to 25 GPa using time-domain thermoreflectance in a diamond anvil cell. We characterized two BAs samples with ambient thermal conductivities of 320 and 480 W m-1 K-1. Our experiments show that the thermal conductivity of both samples depends weakly on pressure from 0 to 25 GPa. We attribute the weak dependence of the thermal conductivity of BAs on pressure to the weak pressure dependence of phonon-phonon scattering rates. Our experimental results are consistent with DFT predictions that three-phonon scattering rates increase from 0 to 25 GPa, while four-phonon scattering rates decrease.