Showing papers by "Zhifeng Ren published in 2020"

Ultrafast room-temperature synthesis of porous S-doped Ni/Fe (oxy)hydroxide electrodes for oxygen evolution catalysis in seawater splitting

Abstract: Developing energy- and time-saving methods to synthesize active and stable oxygen evolving catalysts is of great significance to hydrogen production from water electrolysis, which however remains a grand challenge. Here we report a one-step approach to grow highly porous S-doped Ni/Fe (oxy)hydroxide catalysts on Ni foam in several minutes under room temperature. This ultrafast method effectively engineers the surface of Ni foam into a rough S-doped Ni/Fe (oxy)hydroxide layer, which has multiple levels of porosity and good hydrophilic features and exhibits extraordinary oxygen evolution reaction (OER) performance in both alkaline salty water and seawater electrolytes. Specifically, the S-doped Ni/Fe (oxy)hydroxide catalyst requires low overpotentials of 300 and 398 mV to deliver current densities of 100 and 500 mA cm−2, respectively, when directly used as an OER catalyst in alkaline natural seawater electrolyte. Using this OER catalyst together with an efficient hydrogen evolution reaction catalyst, we have achieved the commercially demanded current densities of 500 and 1000 mA cm−2 at low voltages of 1.837 and 1.951 V, respectively, for overall alkaline seawater electrolysis at room temperature with very good durability. This work affords a cost-efficient surface engineering method to steer commercial Ni foam into robust OER catalysts for seawater electrolysis, which has important implications for both the hydrogen economy and environmental remediation.

Atypical Oxygen-Bearing Copper Boosts Ethylene Selectivity toward Electrocatalytic CO2 Reduction.

Abstract: Oxygen-bearing copper (OBC) has been widely studied for enabling the C-C coupling of the electrocatalytic CO2 reduction reaction (CO2RR) since this is a distinctive hallmark of strongly correlated OBC systems and may benefit many other Cu-based catalytic processes. Unresolved problems, however, include the instability of and limited knowledge regarding OBC under realistic operating conditions, raising doubts about its role in CO2RR. Here, an atypical and stable OBC catalyst with a hierarchical pore and nanograin-boundary structure was constructed and was found to exhibit efficient CO2RR for the production of ethylene with a Faradaic efficiency of 45% at a partial current density of 44.7 mA cm-2 in neutral media, and the ethylene partial current density is nearly 26 and 116 times that of oxygen-free copper (OFC) and commercial Cu foam, respectively. More importantly, the structure-activity relationship in CO2RR was explored through a comprehensive analysis of experimental data and computational techniques, thus increasing the fundamental understanding of CO2RR. A systematic characterization analysis suggests that atypical OBC (Cu4O) was formed and that it is stable even at -1.00 V [(vs the reversible hydrogen electrode (RHE)]. Density functional theory calculations show that the atypical OBC enables control over CO adsorption and dimerization, making it possible to implement a preference for the electrosynthesis of ethylene (C2) products. These results provide insight into the synthesis and structural characteristics of OBC as well as its interplay with ethylene selectivity.

Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride

Abstract: Materials with high thermal conductivity (κ) are of technological importance and fundamental interest. We grew cubic boron nitride (cBN) crystals with controlled abundance of boron isotopes and measured κ greater than 1600 watts per meter-kelvin at room temperature in samples with enriched 10B or 11B. In comparison, we found that the isotope enhancement of κ is considerably lower for boron phosphide and boron arsenide as the identical isotopic mass disorder becomes increasingly invisible to phonons. The ultrahigh κ in conjunction with its wide bandgap (6.2 electron volts) makes cBN a promising material for microelectronics thermal management, high-power electronics, and optoelectronics applications.

Hydrogen Generation from Seawater Electrolysis over a Sandwich-like NiCoN|NixP|NiCoN Microsheet Array Catalyst

Abstract: Seawater electrolysis presents a transformative technology for sustainable hydrogen production and environmental remediation. However, the lack of active and robust hydrogen evolution reaction (HER...

Robust Hydrogen-Evolving Electrocatalyst from Heterogeneous Molybdenum Disulfide-Based Catalyst

Abstract: Molybdenum disulfide-based layered materials are promising electrocatalysts for hydrogen production from water electrolysis if their catalytic performance can be further improved by increasing the ...

VS4 with a chain crystal structure used as an intercalation cathode for aqueous Zn-ion batteries

Abstract: Non-aqueous lithium-ion batteries are currently widely used throughout society, but aqueous batteries could be more feasible for grid-scale applications or even electric cars when factors like cost and safety are taken into consideration. Rechargeable aqueous zinc-ion batteries are promising energy storage devices due to their high energy density, safety, environmental friendliness, and low cost. However, their development for commercial applications remains in the beginning stages because of the limited options among positive electrodes exhibiting adequate capacity and cycle life. Furthermore, their energy-storage mechanisms are not yet well established. Here, vanadium tetrasulfide (VS4) with a beneficial one-dimensional atomic-chain structure is reported to be able to serve as a favorable intercalation cathode material for high-performance Zn-ion batteries. The energy-storage mechanism was investigated both theoretically and experimentally. The maximum capacity of this material reaches 310 mA h g−1 and 85% of this capacity remains even after 500 cycles, which is promising for future practical applications.

Facile synthesis of nanoparticle-stacked tungsten-doped nickel iron layered double hydroxide nanosheets for boosting oxygen evolution reaction

Abstract: Developing highly efficient and cost-effective water oxidation catalysts to enhance the oxygen evolution reaction (OER) remains a significant challenge for electrochemical water splitting. Here, ultrathin tungsten-doped nickel iron layered double hydroxide (Ni–Fe–W LDH) nanosheets, which are composed of nanoparticles with size of 2 to 6 nanometers, have been directly grown on nickel foam (NF) using a facile and scalable water bath reaction. When applied as a catalyst for OER catalysis, this self-supported Ni–Fe–W LDH/NF shows superb OER activity, requiring low overpotentials of only 247 and 320 mV to attain current densities of 100 and 1000 mA cm−2, respectively, with a small Tafel slope of 55 mV dec−1 in 1.0 M KOH electrolyte. The high catalytic activity is attributed to the unique open porous structure, high density of crystalline–amorphous phase boundaries, abundant active sites, and enhanced conductivity resulting from the combination of W doping and Ni–Fe LDH. This work presents a general approach toward the large-scale fabrication of inexpensive trimetallic LDH-based OER electrocatalysts.

Recent Advances in Self-Supported Layered Double Hydroxides for Oxygen Evolution Reaction.

Abstract: Electrochemical water splitting driven by clean and sustainable energy sources to produce hydrogen is an efficient and environmentally friendly energy conversion technology. Water splitting involves hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), in which OER is the limiting factor and has attracted extensive research interest in the past few years. Conventional noble-metal-based OER electrocatalysts like IrO2 and RuO2 suffer from the limitations of high cost and scarce availability. Developing innovative alternative nonnoble metal electrocatalysts with high catalytic activity and long-term durability to boost the OER process remains a significant challenge. Among all of the candidates for OER catalysis, self-supported layered double hydroxides (LDHs) have emerged as one of the most promising types of electrocatalysts due to their unique layered structures and high electrocatalytic activity. In this review, we summarize the recent progress on self-supported LDHs and highlight their electrochemical catalytic performance. Specifically, synthesis methods, structural and compositional parameters, and influential factors for optimizing OER performance are discussed in detail. Finally, the remaining challenges facing the development of self-supported LDHs are discussed and perspectives on their potential for use in industrial hydrogen production through water splitting are provided to suggest future research directions.

High-Performance Ag-Modified Bi0.5Sb1.5Te3 Films for the Flexible Thermoelectric Generator.

Abstract: Bi–Sb–Te-based semiconductors possess the best room-temperature thermoelectric performance, but are restricted for application in the wearable field because of their inherent brittleness, rigidity, and nonscalable manufacturing techniques. Therefore, how to obtain thermoelectric materials with excellent thermoelectric properties and flexibility through the batch production process is a serious challenge. Here, we report the fabrication of flexible p-type thermoelectric Ag-modified Bi0.5Sb1.5Te3 films on flexible substrates using a facile approach. Their optimized power factors are ∼12.4 and ∼14.0 μW cm–1 K–2 at 300 and 420 K, respectively. These high-power factors mainly originate from the optimized carrier transport of the composite system, through which a high level of electrical conductivity is achieved, whereas a remarkably improved Seebeck coefficient is simultaneously obtained. Bending tests demonstrate the excellent flexibility and mechanical durability of the composite films, and their power facto...

Achieving high room-temperature thermoelectric performance in cubic AgCuTe

Abstract: Although there has been significant progress in developing high-temperature thermoelectric materials, seeking promising near-room-temperature candidates has been extremely difficult, and the discovery of such materials, which would be beneficial for low-grade waste-heat power generation and cooling near room temperature, has been rarely reported. Here we report the enhanced near-room-temperature performance (ZTmax = ∼1.1 at 350 K, ZTavg = ∼1.0 between 300 K and 673 K) of copper chalcogenide (AgCu)0.995Te0.9Se0.1 by successfully stabilizing the face-centered cubic (FCC) phase at room temperature. Surprisingly low lattice thermal conductivity (∼0.4 W m−1 K−1) and a good power factor (∼13.8 μW cm−1 K−2) are simultaneously achieved near room temperature due to the unique properties of the FCC phase. A competitive conversion efficiency of 11% is obtained in a (AgCu)0.995Te0.9Se0.1-based single leg at a low temperature difference of 400 K. The high thermal stability and low operating temperature, combined with the economically competitive efficiency, will greatly promote the application of (AgCu)0.995Te0.9Se0.1-based devices in power generation from low- and medium-grade waste heat. The results also indicate a new strategy to improve the near-room-temperature performance and stability of copper chalcogenide thermoelectric materials and a new direction for further research.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline {\alpha}-MgAgSb

Abstract: Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in {\alpha}-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in {\alpha}-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

Bi0.5Sb1.5Te3-based films for flexible thermoelectric devices

Abstract: Bismuth antimony telluride (Bi0.5Sb1.5Te3)-based films have excellent thermoelectric (TE) properties at around room temperature. However, tremendous challenges remain for applying these materials in flexible TE devices due to their inherent brittleness and rigidity. Here we report the fabrication of flexible p-type Bi0.5Sb1.5Te3-based heterostructure films exhibiting optimized power factor and thermal conductivity values of ∼23.2 μW cm−1 K−2 and ∼0.8 W m−1 K−1, respectively, at 300 K. It is shown that the intrinsic TE parameters can be partially decoupled for this heterostructure system, in which new phases (Te and Sb2Te3 nanoinclusions) and numerous interfaces among Bi0.5Sb1.5Te3, Te, and Sb2Te3 are introduced. High mechanical durability and flexibility with less than 10% degradation in performance after 700 bending cycles (a bending radius of 5 mm) were achieved. Finally, a flexible TE generator assembled using these Bi0.5Sb1.5Te3-based heterostructure films and traditional n-type Bi2Te3 films exhibits a high power density of 897.8 μW cm−2 at a relatively small ΔT of 40 K. This result can provide insight into the scalable fabrication of high-performance flexible TE generators for energy harvesting.

Thermoelectric Properties of Zintl Phase YbMg2Sb2

Abstract: The goal of waste heat recovery using high-performance thermoelectric materials has promoted the development of promising materials with enhanced performance. Here, YbMg2Sb2 is studied as one such ...

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α -MgAgSb

Abstract: Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials. In order to optimize thermoelectric (TE) materials which are used to convert thermal energy and electrical energy, the underlying physics needs to be understood. Here, the authors show that by exploiting static local structure distortion, transverse acoustic phonons can be suppressed resulting in high performing TE materials.

N-Type Mg3Sb2-xBix Alloys as Promising Thermoelectric Materials

Abstract: N-type Mg3Sb2-x Bi x alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects resulting from alloying Mg3Bi2 with Mg3Sb2, including narrowed bandgap, decreased effective mass, and increased carrier mobility, are summarized. Subsequently, defect-controlled electrical properties in n-type Mg3Sb2-x Bi x are revealed. On one hand, manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed for Mg3Sb2-x Bi x thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in power generation and cooling applications.

Optical properties of cubic boron arsenide

Abstract: The ultrahigh thermal conductivity of cubic boron arsenide (BAs) makes it a promising material for next-generation electronics and optoelectronics. Here, we report measured optical properties of BAs crystals, including the complex dielectric function, refractive index, and absorption coefficient in the ultraviolet, visible, and near-infrared wavelength range. The data were collected at room temperature using spectroscopic ellipsometry and transmission and reflection spectroscopy. We further calculated the optical response using density functional theory and many-body perturbation theory, considering quasiparticle and excitonic corrections. The computed values for the direct and indirect bandgaps (4.25 eV and 2.07 eV) agree well with the measured results (4.12 eV and 2.02 eV). Our findings pave the way for using BAs in future electronic and optoelectronic applications that take advantage of its demonstrated ultrahigh thermal conductivity and predicted high ambipolar carrier mobility.

Achieving high-performance p-type SmMg2Bi2 thermoelectric materials through band engineering and alloying effects

Abstract: Thermoelectric Zintl phases have attracted increasing attention in the past few decades, with good thermoelectric performance observed in many different families. Due to their intrinsic low lattice thermal conductivity, p-type CaAl2Si2 (1-2-2)-type Zintl phases, which also exhibit relatively higher electrical transport performance, have been demonstrated to be promising thermoelectric materials for mid- to high-temperature applications. Here we investigate the thermoelectric performance of p-type SmMg2Bi2, a new member of this 1-2-2 Zintl family. Band structure calculations reveal that the calculated band gap of SmMg2Bi2 is smaller in comparison to that of other Bi-based Zintl phases, which inevitably contributes to the bipolar effect clearly observed at higher temperature. Further successful substitution of Eu and Yb is effective in suppressing the bipolar effect and ensures achievement of superior electronic performance, resulting in a peak figure of merit (ZT) of ∼0.9 at 773 K. The current work has successfully expanded the family of Bi-based p-type 1-2-2 Zintls, and could play an essential role in stimulating further investigation of other Zintl compounds.

Pressure-Dependent Behavior of Defect-Modulated Band Structure in Boron Arsenide.

Abstract: The recent observation of unusually high thermal conductivity exceeding 1000 W m-1 K-1 in single-crystal boron arsenide (BAs) has led to interest in the potential application of this semiconductor for thermal management. Although both the electron/hole high mobilities have been calculated for BAs, there is a lack of experimental investigation of its electronic properties. Here, a photoluminescence (PL) measurement of single-crystal BAs at different temperatures and pressures is reported. The measurements reveal an indirect bandgap and two donor-acceptor pair (DAP) recombination transitions. Based on first-principles calculations and time-of-flight secondary-ion mass spectrometry results, the two DAP transitions are confirmed to originate from Si and C impurities occupying shallow energy levels in the bandgap. High-pressure PL spectra show that the donor level with respect to the conduction band minimum shrinks with increasing pressure, which affects the release of free carriers from defect states. These findings suggest the possibility of strain engineering of the transport properties of BAs for application in electronic devices.