Showing papers by "Zhifeng Ren published in 2018"

High Performance Bifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall Water Splitting

Abstract: Water electrolysis is an advanced energy conversion technology to produce hydrogen as a clean and sustainable chemical fuel, which potentially stores the abundant but intermittent renewable energy sources scalably. Since the overall water splitting is an uphill reaction in low efficiency, innovative breakthroughs are desirable to greatly improve the efficiency by rationally designing non-precious metal-based robust bifunctional catalysts for promoting both the cathodic hydrogen evolution and anodic oxygen evolution reactions. We report a hybrid catalyst constructed by iron and dinickel phosphides on nickel foams that drives both the hydrogen and oxygen evolution reactions well in base, and thus substantially expedites overall water splitting at 10 mA cm−2 with 1.42 V, which outperforms the integrated iridium (IV) oxide and platinum couple (1.57 V), and are among the best activities currently. Especially, it delivers 500 mA cm−2 at 1.72 V without decay even after the durability test for 40 h, providing great potential for large-scale applications.

Water splitting by electrolysis at high current densities under 1.6 volts

Abstract: Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent waste heat, wind, or solar energies is one of the easiest and cleanest methods for high-purity hydrogen production and an effective way to store the excess electrical power. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions (HER and OER, respectively) is the high overpotential required, especially for the OER. We report an exceptionally active and durable OER catalyst yielding current densities of 500 and 1000 mA cm−2 at overpotentials of only 259 mV and 289 mV in alkaline electrolyte, respectively, fulfilling the commercial criteria of the OER process. Together with a good HER catalyst, we have achieved the commercially required current densities of 500 and 1000 mA cm−2 at 1.586 and 1.657 V, respectively, with very good stability, dramatically lower than any previously reported voltage. This discovery sets the stage for large-scale hydrogen production by water splitting using excess electrical power whenever and wherever available.

Advances in thermoelectrics

Abstract: Thermoelectric generators, capable of directly converting heat into electricity, hold great promise for tackling the ever-increasing energy sustainability issue. The thermoelectric energy conversio...

Unusual high thermal conductivity in boron arsenide bulk crystals

Abstract: 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.

Hierarchical CoP/Ni5P4/CoP microsheet arrays as a robust pH-universal electrocatalyst for efficient hydrogen generation

Abstract: Highly active catalysts composed of earth-abundant materials, performing as efficiently as Pt catalysts, are crucial for sustainable hydrogen production through water splitting. However, most efficient catalysts consist of nanostructures made via complex synthetic methods, making scale-up quite challenging. Here we report an effective strategy for developing a very active and durable pH-universal electrocatalyst for the hydrogen evolution reaction (HER). This catalyst is constructed using a sandwich-like structure, where hierarchical cobalt phosphide (CoP) nanoparticles serve as thin skins covering both sides of nickel phosphide (Ni5P4) nanosheet arrays, forming self-supported sandwich-like CoP/Ni5P4/CoP microsheet arrays with lots of mesopores and macropores. The as-prepared electrocatalyst requires an overpotential of only 33 mV to achieve a benchmark of 10 mA cm−2, with a very large exchange current density and high turnover frequencies (TOFs) in acid media, superior to most electrocatalysts made of metal phosphides, well-known MoS2 and WS2 catalysts, and it performs comparably to state-of-the-art Pt catalysts. In particular, this electrocatalyst shows impressive operational stability at an extremely large current density of 1 A cm−2, indicating its possible application toward large-scale water electrolysis. Additionally, this electrocatalyst is very active in alkaline electrolyte (71 mV at 10 mA cm−2), which demonstrates its pH universality as a HER catalyst with outstanding catalytic activity. This simple strategy does not involve any solvothermal and hydrothermal processes, paving a new avenue toward the design of robust non-noble electrocatalysts for hydrogen production, aimed at commercial water electrolysis.

Discovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency.

Abstract: Thermoelectric materials are capable of converting waste heat into electricity. The dimensionless figure-of-merit (ZT), as the critical measure for the material’s thermoelectric performance, plays a decisive role in the energy conversion efficiency. Half-Heusler materials, as one of the most promising candidates for thermoelectric power generation, have relatively low ZTs compared to other material systems. Here we report the discovery of p-type ZrCoBi-based half-Heuslers with a record-high ZT of ∼1.42 at 973 K and a high thermoelectric conversion efficiency of ∼9% at the temperature difference of ∼500 K. Such an outstanding thermoelectric performance originates from its unique band structure offering a high band degeneracy (Nv) of 10 in conjunction with a low thermal conductivity benefiting from the low mean sound velocity (vm ∼2800 m s−1). Our work demonstrates that ZrCoBi-based half-Heuslers are promising candidates for high-temperature thermoelectric power generation. Identifying new compounds with intrinsically high conversion efficiency is the key to demonstrating next-generation thermoelectric modules. Here, Zhu et al. report the discovery of p-type ZrCoBi-based half Heuslers with thermoelectric conversion efficiency of 9% and large high-temperature stability.

Trimetallic NiFeMo for Overall Electrochemical Water Splitting with a Low Cell Voltage

Abstract: We report the development of an efficient and earth-abundant catalyst for electrochemical overall water splitting. Trimetallic NiFeMo alloy is synthesized by hydrothermal deposition from inorganic precursors and subsequent low-temperature thermal annealing. A complete cell made of NiFeMo electrodes on nickel foam exhibits a low voltage of 1.45 V at 10 mA/cm2 as a result of low overpotentials for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). High-resolution transmission electron microscopy reveals that nanometer-sized single-crystal domains of Ni, Fe, and Mo are intimately integrated at the atomic level, which enables a synergistic effect of metallic Ni, Fe, and Mo for efficient HER, while self-formed Ni–Fe–Mo (oxy)hydroxides on the surface of the NiFeMo anode become active sites for OER. Such a multimetallic alloy and its (oxy)hydroxides represent a typical HER/OER catalyst couple, and our method provides a new route to develop efficient low-cost metallic alloys for overall w...

Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping

Abstract: 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.

Deep defect level engineering: a strategy of optimizing the carrier concentration for high thermoelectric performance

Abstract: Thermoelectric properties are heavily dependent on the carrier concentration, and therefore the optimization of carrier concentration plays a central role in achieving high thermoelectric performance. The optimized carrier concentration is highly temperature-dependent and could even possibly vary within one order of magnitude in the temperature range of several hundreds of Kelvin. Practically, however, the traditional doping strategy will only lead to a constant carrier concentration, and thus the thermoelectric performance is only optimized within a limited temperature range. Here, we demonstrate that a temperature-dependent carrier concentration can be realized by simultaneously introducing shallow and deep defect levels. In this work, iodine (I) and indium (In) are co-doped in PbTe, where iodine acts as the shallow donor level that supplies sufficient electrons and indium builds up the localized half-filled deep defect state in the band gap. The indium deep defect state traps electrons at a lower temperature and the trapped electrons will be thermally activated back to the conduction band when the temperature rises. In this way, the carrier concentration can be engineered as temperature-dependent, which matches the theoretically predicted optimized carrier concentration over the whole temperature range. As a result, a room temperature ZT of ∼0.4 and a peak ZT of ∼1.4 at 773 K were obtained in the n-type In/I co-doped PbTe, leading to a record-high average ZT of ∼1.04 in the temperature range of 300 to 773 K. Importantly, since deep defect levels also exist in other materials, the strategy of deep defect level engineering should be widely applicable to a variety of materials for enhancing the thermoelectric performance across a broad temperature range.

Amorphous NiFe layered double hydroxide nanosheets decorated on 3D nickel phosphide nanoarrays: a hierarchical core–shell electrocatalyst for efficient oxygen evolution

Abstract: The rational design of efficient and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) plays a paramount role in hydrogen production by water electrolysis. Here we report a 3D hierarchical core–shell nanostructured OER electrocatalyst, in which amorphous NiFe layered double hydroxide (LDH) nanosheets are decorated on 3D conductive nickel phosphide nanoarrays. The integrated 3D core–shell electrode simultaneously offers excellent electrical conductivity for fast electron transfer, a large surface area with numerous active edge sites, and a hierarchical nanostructure for rapid release of gas bubbles, thus contributing to outstanding catalytic performance: low overpotentials (197, 243, and 283 mV for current densities of 10, 100, and 300 mA cm−2, respectively), a small Tafel slope (46.6 mV dec−1), and superior stability, which are better than those of almost all reported LDH-based OER catalysts. When this hybrid catalyst is combined with nickel phosphide for overall water splitting, the two-electrode cell achieves current densities of 10 mA cm−2 at 1.52 V and 100 mA cm−2 at 1.68 V in alkaline media, which are even superior to those of benchmark IrO2 and Pt. This work paves an effective approach to design 3D hierarchical hybrid electrocatalysts for energy conversion and storage.

High thermoelectric performance of α-MgAgSb for power generation

Abstract: Solid-state thermoelectric devices enable the conversion of waste heat and solar energy into electricity, providing an alternative route for energy harvesting to tackle the challenges of energy sustainability. α-MgAgSb-based materials have recently gained popular attention as a new promising p-type candidate for low-temperature (between room temperature and 550 K) applications. A high figure of merit (ZT) of 1.2–1.4 at 550 K was achieved, along with experimental demonstration of a record high conversion efficiency of ∼8.5% under a cold- and hot-side temperature difference of 225 K, which holds the realistic prospect for power generation. In this review, we summarize the recent progress in both material-level understanding and device-level measurement of the α-MgAgSb system. First, the phase transitions and common fabrication methods are briefly introduced. Then, the origin of the “phonon glass electron crystal” behavior of α-MgAgSb is thoroughly elucidated with regard to some critical factors, such as the crystal structure, lattice dynamic properties, defect chemistry, microstructure, and band structure. Afterwards, the effective strategies to further enhance the ZT are illustrated, including optimizing the carrier concentration and intrinsic defect engineering. In addition, other feasible methods in theory are also presented. Finally, the conversion efficiency on a single-leg device is presented. In the outlook section, the currently unsolved questions as well as future directions and challenges for this material system are discussed.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers.

Abstract: Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material-the combination of electrical conductivity and Seebeck coefficient-measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron-acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.

Discovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency

Abstract: United States. Department of Energy. Office of Science. Basic Energy Sciences (Award Number: DE-SC0001299)

Significant Role of Mg Stoichiometry in Designing High Thermoelectric Performance for Mg3(Sb,Bi)2-Based n-Type Zintls

Abstract: Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds Mg3+xSb1.5Bi0.5, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical-aberration-corrected (CS-corrected) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on Mg3.025Sb1.5Bi0.5 has enabled good n-type thermoelectric properties, which is...

Nano-microstructural control of phonon engineering for thermoelectric energy harvesting

Abstract: Manipulating the thermal conductivity of solids is important for practical applications. Due to the fact that phonons in thermoelectric materials have longer mean free paths (MFPs) than electrons, strengthening phonon scattering to reduce lattice thermal conductivity (κlat) becomes the most straightforward and effective approach to enhance the thermoelectric figure of merit, ZT, which determines the maximum device efficiency. Phonons have a wide range of MFPs in semiconductors, and different dimensions of lattice defects can be targeted to scatter particular phonons with distinct relaxation times. Designing hierarchical nano-microstructures, spanning from point defects to volume defects, would be beneficial to achieve low κlat via a full spectrum of phonon scattering. Herein, we review the formation and underlying mechanisms for lattice defects and highlight the role of all-scale hierarchical nano-microstructure on phonon engineering. Existing challenges in simulations are also discussed.

Recent progress towards high performance of tin chalcogenide thermoelectric materials

Abstract: Thermoelectric materials have been extensively studied for decades to help resolve the global energy shortage and environmental problems. Many efforts have been focused on the improvement of the figure of merit (ZT) for highly efficient power generation. Lead telluride is one of the materials with high ZT, but lead toxicity is always a concern, which has inspired research on lead-free tin chalcogenides. ZT values as high as ∼2.6 at 923 K for SnSe single crystals and ∼1.6 at 923 K for Sn0.86Mn0.14Te(Cu2Te)0.05-5 atm% Sn were recently reported, attracting extensive attention for potential applications. In this review, we present the progress in SnTe, SnSe, and SnS, mainly discussing the effective tuning of the electron and phonon transport based on the intrinsic properties, along with the challenges for further optimization and applications. For SnTe, successful strategies, including resonant doping, band convergence, defect engineering, etc., are discussed. For SnSe, we focus on the analysis of the intrinsic low thermal conductivity due to strong anharmonicity and a high Seebeck coefficient because of the multi-valley bands. For SnS, high performance is expected considering its similar band structure and crystal structure to SnSe.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers

Abstract: United States. Department of Energy. Office of Science. Basic Energy Sciences (Award # SC0001299/DE-FG02-09ER46577 (for fundamental research on electron–phonon interaction in thermoelectric materials))

Robust and selective electrochemical reduction of CO2: the case of integrated 3D TiO2@MoS2 architectures and Ti–S bonding effects

Abstract: Developing efficient and affordable catalysts toward electrochemical reduction of CO2 to valuable chemicals is of great significance for energy and environmental sustainability, which can be efficaciously achieved by catalyst structure steering. In this study, we propose an ingenious strategy to modulate MoS2 favorable for CO2 reduction by fabricating an integrated three-dimensional (3D) TiO2@MoS2 architecture containing Ti–S bonds, the formation of which, revealed by density functional theory calculations, has changed the electric properties of the MoS2 layer and the adsorption characters of Mo exposed edges. The modulated MoS2 is vigorous for CO2 reduction due to the decrease of both the binding energy of CO2 and the energy barriers of CO2 reduction reaction pathways. Experimentally, the integrated 3D TiO2@MoS2 architectures can act as efficient and stable catalysts for selective reduction of CO2 to CO. The optimized composite showed a negligible onset overpotential of 100 mV for CO formation in KHCO3 solution, and a maximum faradaic efficiency of ∼82% for CO at −0.7 V vs. RHE with a large partial current density for CO of 68 mA cm−2. Additionally, the integrated 3D electrodes exhibited superior stability during CO2 reduction. This study will shed light on the modification of electrocatalysts for efficient CO2 reduction through structure steering.

Ultrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti).

Abstract: Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm-1 K-2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb-based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm-2 is achieved based on a single-leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.

Seeded growth of boron arsenide single crystals with high thermal conductivity

Abstract: Materials with high thermal conductivities are crucial to effectively cooling high-power-density electronic and optoelectronic devices. Recently, zinc-blende boron arsenide (BAs) has been predicted to have a very high thermal conductivity of over 2000 W m−1 K−1 at room temperature by first-principles calculations, rendering it a close competitor for diamond which holds the highest thermal conductivity among bulk materials. Experimental demonstration, however, has proved extremely challenging, especially in the preparation of large high quality single crystals. Although BAs crystals have been previously grown by chemical vapor transport (CVT), the growth process relies on spontaneous nucleation and results in small crystals with multiple grains and various defects. Here, we report a controllable CVT synthesis of large single BAs crystals (400–600 μm) by using carefully selected tiny BAs single crystals as seeds. We have obtained BAs single crystals with a thermal conductivity of 351 ± 21 W m−1 K−1 at room temperature, which is almost twice as conductive as previously reported BAs crystals. Further improvement along this direction is very likely.

Impurity-derived p-type conductivity in cubic boron arsenide

Abstract: Cubic boron arsenide (c-BAs) exhibits an ultrahigh thermal conductivity (κ) approaching 1300 Wm−1 K−1 at room temperature. However, c-BAs is believed to incorporate high concentrations of crystal imperfections that can both quench κ and act as sources of unintentional p-type conductivity. Although this behavior has been attributed to native defects, we show here, using optical and magnetic resonance spectroscopies together with first-principles calculations, that unintentional acceptor impurities such as silicon and/or carbon are more likely candidates for causing the observed conductivity. These results also clarify that the true low-temperature bandgap of c-BAs is 0.3 eV higher than the widely reported value of ∼1.5 eV. Low-temperature photoluminescence measurements of c-BAs crystals reveal impurity-related recombination processes (including donor-acceptor pair recombination), and electron paramagnetic resonance experiments show evidence for effective mass-like shallow acceptors. Our hybrid density functional calculations indicate that native defects are incapable of giving rise to such signals. Instead, we find that group-IV impurities readily incorporate on the As site and act as shallow acceptors. Such impurities can dominate the electrical properties of c-BAs, and their influence on phonon scattering must be considered when optimizing thermal conductivity.

Study on anisotropy of n-type Mg3Sb2-based thermoelectric materials

Abstract: The recent discovery of a high thermoelectric figure of merit (ZT) in an n-type Mg3Sb2-based Zintl phase triggered an intense research effort to pursue even higher ZT. Based on our previous report on Mg3.1Nb0.1Sb1.5Bi0.49Te0.01, we report here that partial texturing in the (001) plane is achieved by double hot pressing, which is further confirmed by the rocking curves of the (002) plane. The textured samples of Mg3.1Nb0.1Sb1.5Bi0.49Te0.01 show a much better average performance in the (00l) plane. Hall mobility is significantly improved to ∼105 cm2 V−1 s−1 at room temperature in the (00l) plane due to texturing, resulting in higher electrical conductivity, a higher power factor of ∼18 μW cm−1 K−2 at room temperature, and also higher average ZT. This work shows that texturing is good for higher thermoelectric performance, suggesting that single crystals of n-type Mg3Sb2-based Zintl compounds are worth pursuing.

Bio-derived three-dimensional hierarchical carbon-graphene-TiO 2 as electrode for supercapacitors.

Abstract: This paper reports a novel loofah-derived hierarchical scaffold to obtain three-dimensional biocarbon-graphene-TiO2 (BC-G-TiO2) composite materials as electrodes for supercapacitors. The loofah scaffold was first loaded with G and TiO2 by immersing, squeezing, and loosening into the mixed solution of graphene oxide and titania, and then carbonized at 900 °C to form the BC-G-TiO2 composite. The synergistic effects of the naturally hierarchical biocarbon structure, graphene, and TiO2 nanoparticles on the electrochemical properties are analyzed. The biocarbon provides a high interconnection and an easy accessibility surface for the electrolyte. Graphene bridged the BC and TiO2 nanoparticles, improved the conductivity of the BC-G-TiO2 composite, and increased the electron transfer efficiency. TiO2 nanoparticles also contributed to the pesudocapacitance and electrochemical stability.

Poly(sodium 4-styrenesulfonate) Stabilized Janus Nanosheets in Brine with Retained Amphiphilicity.

Abstract: Maintaining colloidal stability in unfriendly environments while retaining surface chemical properties is challenging for fundamental science and crucial for many applications. Here, we report for the first time that by using a low concentration of poly(sodium 4-styrenesulfonate) (PSS), graphene-based amphiphilic Janus nanosheets (AJNs) can be stabilized in high salt brine (3 wt % NaCl and 0.5 wt % CaCl2), whereas the interfacial behavior of the nanosheets is not affected. The adsorption of PSS on the hydrophilic and hydrophobic surfaces of AJNs in brine was investigated experimentally and by molecular dynamics simulations. Simulations further showed that the spatial configuration of absorbed PSS molecules with sulfonate functional groups facing outward favored the generation of electrosteric repulsive interactions. Calculations of the interaction energy between PSS molecules and the nanosheet revealed surface charge as a key parameter to stabilize AJNs in the salt environment, as demonstrated by the case...