Showing papers by "Zhifeng Ren published in 2019"

Non-noble metal-nitride based electrocatalysts for high-performance alkaline seawater electrolysis

Abstract: Seawater is one of the most abundant natural resources on our planet. Electrolysis of seawater is not only a promising approach to produce clean hydrogen energy, but also of great significance to seawater desalination. The implementation of seawater electrolysis requires robust and efficient electrocatalysts that can sustain seawater splitting without chloride corrosion, especially for the anode. Here we report a three-dimensional core-shell metal-nitride catalyst consisting of NiFeN nanoparticles uniformly decorated on NiMoN nanorods supported on Ni foam, which serves as an eminently active and durable oxygen evolution reaction catalyst for alkaline seawater electrolysis. Combined with an efficient hydrogen evolution reaction catalyst of NiMoN nanorods, we have achieved the industrially required current densities of 500 and 1000 mA cm−2 at record low voltages of 1.608 and 1.709 V, respectively, for overall alkaline seawater splitting at 60 °C. This discovery significantly advances the development of seawater electrolysis for large-scale hydrogen production. Seawater electrolysis is a promising approach to produce hydrogen fuel and is also of great significance to seawater desalination. Here, the authors prepare 3D core-shell metal-nitride catalysts from earth-abundant elements for high-performance alkaline seawater electrolysis.

Flexible Electronics: Stretchable Electrodes and Their Future

Abstract: Flexible electronics, as an emerging and exciting research field, have brought great interest to the issue of how to make flexible electronic materials that offer both durability and high performance at strained states. With the advent of on-body wearable and implantable electronics, as well as increasing demands for human-friendly intelligent soft robots, enormous effort is being expended on highly flexible functional materials, especially stretchable electrodes, by both the academic and industrial communities. Among different deformation modes, stretchability is the most demanding and challenging. This review focuses on the latest advances in stretchable transparent electrodes based on a new design strategy known as kirigami (the art of paper cutting) and investigates the recent progress on novel applications, including skin-like electronics, implantable biodegradable devices, and bioinspired soft robotics. By comparing the optoelectrical and mechanical properties of different electrode materials, some of the most important outcomes with comments on their merits and demerits are raised. Key design considerations in terms of geometries, substrates, and adhesion are also discussed, offering insights into the universal strategies for engineering stretchable electrodes regardless of the material. It is suggested that highly stretchable and biocompatible electrodes will greatly boost the development of next-generation intelligent life-like electronics.

High thermoelectric cooling performance of n-type Mg 3 Bi 2 -based materials.

Abstract: Thermoelectric materials have a large Peltier effect, making them attractive for solid-state cooling applications. Bismuth telluride (Bi2Te3)–based alloys have remained the state-of-the-art room-temperature materials for many decades. However, cost partially limited wider use of thermoelectric cooling devices because of the large amounts of expensive tellurium required. We report n-type magnesium bismuthide (Mg3Bi2)–based materials with a peak figure of merit (ZT) of ~0.9 at 350 kelvin, which is comparable to the commercial bismuth telluride selenide (Bi2Te3–xSex) but much cheaper. A cooling device made of our material and p-type bismuth antimony telluride (Bi0.5Sb1.5Te3) has produced a large temperature difference of ~91 kelvin at the hot-side temperature of 350 kelvin. n-type Mg3Bi2-based materials are promising for thermoelectric cooling applications.

Discovery of TaFeSb-based half-Heuslers with high thermoelectric performance

Abstract: Discovery of thermoelectric materials has long been realized by the Edisonian trial and error approach. However, recent progress in theoretical calculations, including the ability to predict structures of unknown phases along with their thermodynamic stability and functional properties, has enabled the so-called inverse design approach. Compared to the traditional materials discovery, the inverse design approach has the potential to substantially reduce the experimental efforts needed to identify promising compounds with target functionalities. By adopting this approach, here we have discovered several unreported half-Heusler compounds. Among them, the p-type TaFeSb-based half-Heusler demonstrates a record high ZT of ~1.52 at 973 K. Additionally, an ultrahigh average ZT of ~0.93 between 300 and 973 K is achieved. Such an extraordinary thermoelectric performance is further verified by the heat-to-electricity conversion efficiency measurement and a high efficiency of ~11.4% is obtained. Our work demonstrates that the TaFeSb-based half-Heuslers are highly promising for thermoelectric power generation. The discovery of thermodynamically stable thermoelectric materials for power generation has relied on empirical methods that were not effective. Here, the authors apply the inverse design approach to identify and experimentally realize TaFeSb-based half Heuslers with high thermoelectric performance.

Realization of higher thermoelectric performance by dynamic doping of copper in n-type PbTe

Abstract: It is a great challenge to optimize a material's thermoelectric performance due to the strong correlation between its thermoelectric-transport properties, especially the electrical-transport properties. Optimizing the peak zT using a constant carrier concentration is commonly adopted because of the difficulty in realizing the optimum temperature-dependent carrier concentration, but this is not meaningful for real applications, in which the average zT value over the working temperature range is much more important. Here we propose an effective strategy involving the dynamic doping effect of interstitial Cu atoms to fully optimize the electrical-transport properties of n-type PbTe over a wide temperature range. By using Cu intercalation, the temperature-dependent carrier concentration of PbTe is found to well match the theoretically optimal profile. Furthermore, high carrier mobility is largely maintained because the dynamic behavior of the interstitial Cu does not alter the band structure and therefore change the effective mass. Consequently, a peak zT of ∼1.3 and a calculated leg efficiency of 12% were achieved for the sample with 0.2 at% Cu. Based on our findings, we further proposed a concept of ‘interstitial engineering’ to reinforce the dynamic doping effect, which is of fundamental importance for optimizing the thermoelectric properties.

A universal synthesis strategy to make metal nitride electrocatalysts for hydrogen evolution reaction

Abstract: Transition-metal nitrides have increasingly attracted interest for use as electrocatalysts in water splitting due to their superior catalytic activity and stability. However, the development of a general and simple strategy to synthesize metal nitrides remains challenging. Here we report a facile strategy for the synthesis of various porous monometallic and bimetallic nitrides on different substrates for the hydrogen evolution reaction (HER) in alkaline media. The best monometallic nitride of CoN supported on the Ni foam delivered current densities of 10 and 100 mA cm−2 at overpotentials of 95 and 212 mV, respectively in 1 M KOH. This performance was further improved through Ni-doping to form bimetallic nitrides of NiCoN, the best of which exhibited excellent HER performance with low overpotentials of 48 and 149 mV at current densities of 10 and 100 mA cm−2, respectively, along with superior stability in 1 M KOH. The enhanced performance is mainly attributed to the synergistic effect of Co and Ni, a larger surface area with more active sites, and improved electrical conductivity for more efficient charge transfer. This work demonstrates a particularly facile and general approach to synthesize porous transition metal nitrides with advanced HER performance.

Highly Efficient Hydrogen Evolution from a Mesoporous Hybrid of Nickel Phosphide Nanoparticles Anchored on Cobalt Phosphosulfide/Phosphide Nanosheet Arrays.

Abstract: Facile design of low-cost and high-efficiency catalysts with earth-abundant and cheap materials is desirable to replace platinum (Pt) for the hydrogen evolution reaction (HER) in water splitting, but the development of such HER catalysts with Pt-like activity using simple strategies remains challenging. A mesoporous hybrid catalyst of nickel phosphides nanoparticles and cobalt phosphosulfide/phosphide (CoS|Ni|P) nanosheet arrays for HER is reported here, which is developed by a facile three-step approach consisting of electrodeposition, thermal sulfurization, and phosphorization. This hybrid catalyst is highly robust and stable in acid for HER, and is distinguished by very low overpotentials of 41, 88, and 150 mV to achieve 10, 100, and 1000 mA cm-2 , respectively, as well as a small Tafel slope (45.2 mV dec-1 ), and a large exchange current density (964 µA cm-2 ). It is among the most efficient earth-abundant catalysts reported thus far for HER. More importantly, this electrocatalyst has electrochemical durability over 20 h under a wide range of current densities (up to 1 A cm-2 ) in acidic conditions, as well as very high turnover frequencies of 0.40 and 1.26 H2 s-1 at overpotentials of 75 and 100 mV, respectively, showing that it has great potential for practical applications in large-scale water electrolysis.

Zintl-phase Eu2ZnSb2: A promising thermoelectric material with ultralow thermal conductivity.

Abstract: Zintl compounds are considered to be potential thermoelectric materials due to their “phonon glass electron crystal” (PGEC) structure. A promising Zintl-phase thermoelectric material, 2-1-2–type Eu2ZnSb2 (P63/mmc), was prepared and investigated. The extremely low lattice thermal conductivity is attributed to the external Eu atomic layers inserted in the [Zn2Sb2]2- network in the structure of 1-2-2–type EuZn2Sb2 ( P 3 ¯ m 1 ) , as well as the abundant inversion domain boundary. By regulating the Zn deficiency, the electrical properties are significantly enhanced, and the maximum ZT value reaches ∼1.0 at 823 K for Eu2Zn0.98Sb2. Our discovery provides a class of Zintl thermoelectric materials applicable in the medium-temperature range.

Enhancement of thermoelectric performance across the topological phase transition in dense lead selenide.

Abstract: Alternative technologies are required in order to meet a worldwide demand for clean non-polluting energy sources. Thermoelectric generators, which generate electricity from heat in a compact and reliable manner, are potential devices for waste heat recovery. However, thermoelectric performance, as encapsulated by the figure of merit ZT, has remained at around 1.0 at room temperature, which has limited practical applications. Here, we study the effects of pressure on ZT in Cr-doped PbSe, which has a maximum ZT of less than 1.0 at a temperature of about 700 K. By applying external pressure using a diamond anvil cell, we obtained a room-temperature ZT value of about 1.7. From thermoelectric, magnetoresistance and Raman measurements, as well as density functional theory calculations, a pressure-driven topological phase transition is found to enable this enhancement. Experiments also support the appearance of a topological crystalline insulator after the transition. These findings point to the possibility of using compression to increase not just ZT in existing thermoelectric materials, but also the possibility of realizing topological crystalline insulators. By applying a pressure of 2.8 GPa using a diamond anvil cell, a topological phase transition is found to occur in Cr-doped PbSe. This enables a thermoelectric figure of merit ZT of 1.7 at room temperature.

High Thermal Conductivity in Boron Arsenide: From Prediction to Reality

Abstract: Modern first-principles calculations predict that the thermal conductivity of boron arsenide is second only to that of diamond, the best thermal conductor, which may be of benefit for waste heat management in electronic devices. With the optimization of single-crystal growth methods, large-size and high-quality boron arsenide single crystals have been grown and thermal conductivity measurements have verified the related predictions. Benefiting from the increased size and improved qualities, additional properties have been characterized. Important factors related to boron arsenide, remaining challenges, and the future outlook are addressed in this minireview.

Understanding the asymmetrical thermoelectric performance for discovering promising thermoelectric materials.

Abstract: Thermoelectric modules, consisting of multiple pairs of n- and p-type legs, enable converting heat into electricity and vice versa. However, the thermoelectric performance is often asymmetrical, in that one type outperforms the other. In this paper, we identified the relationship between the asymmetrical thermoelectric performance and the weighted mobility ratio, a correlation that can help predict the thermoelectric performance of unreported materials. Here, a reasonably high ZT for the n-type ZrCoBi-based half-Heuslers is first predicted and then experimentally verified. A high peak ZT of ~1 at 973 K can be realized by ZrCo0.9Ni0.1Bi0.85Sb0.15. The measured heat-to-electricity conversion efficiency for the unicouple of ZrCoBi-based materials can be as high as ~10% at the cold-side temperature of ~303 K and at the hot-side temperature of ~983 K. Our work demonstrates that the ZrCoBi-based half-Heuslers are highly promising for the application of mid- and high-temperature thermoelectric power generation.

n-Type TaCoSn-Based Half-Heuslers as Promising Thermoelectric Materials.

Abstract: Half-Heusler compounds are recognized as promising thermoelectric materials for high-temperature power generation, but their relatively high lattice thermal conductivity impedes further improvement...

Giant Poisson's Effect for Wrinkle-Free Stretchable Transparent Electrodes.

Abstract: The next generation of flexible electronics will require highly stretchable and transparent electrodes, many of which consist of a relatively stiff metal network (or carbon materials) and an underlying soft substrate. Typically, such a stiff-soft bilayer suffers from wrinkling or folding when subjected to strains, causing high surface roughness and seriously deteriorated optical transparency. In this work, a network with a giant effective Poisson's ratio on a soft substrate is found to be under biaxial tension upon deformation, and thus does not wrinkle or fold, but maintains smooth surfaces and high transparency. Soft tactile sensors employing such network electrodes exhibit high transparency and low fatigue over many stretching cycles. Such a giant Poisson's ratio has the same effect in other systems. This work offers a new understanding of surface instabilities and a general strategy to prevent them not only in flexible electronics, but also in other materials and mechanical structures that require flat surfaces.

The effect of carbon quantum dots on the electrocatalytic hydrogen evolution reaction of manganese–nickel phosphide nanosheets

Abstract: Transition-metal phosphides (TMPs) are good electrocatalysts for the hydrogen evolution reaction (HER) due to their high catalytic efficiency and low cost. Carbon quantum dots (CQDs) deposited on top of TMPs could make them even better for the HER by increasing the surface area and the number of active sites. Here we report a method to synthesize CQD-modified manganese–nickel phosphide (CQDs/MnxNi5−xP4) for efficient and stable HER activity using inexpensive raw materials. In 0.5 M H2SO4, CQDs/MnxNi5−xP4 requires a low overpotential of only 31 mV to achieve a current density of 10 mA cm−2, as well as having a low Tafel slope of 41.0 mV dec−1, a large exchange current density of 1.753 mA cm−2, and good stability, making it better than most reported transition-metal-based catalysts. Moreover, CQDs/MnxNi5−xP4 also displays high activity and stability in alkaline solution, revealing that the ancillary role played by CQDs could be beneficial under both acidic and alkaline conditions. Based on our results, we believe that CQDs have great potential to be applied to other materials with various morphologies and structures for designing high-performance HER catalysts.

Mechanical properties of boron arsenide single crystal

Abstract: As the only semiconductor material exhibiting ultrahigh thermal conductivity under ambient conditions, cubic boron arsenide (BAs) is currently attracting great interest. Thanks to the development of high-quality BAs single crystal growth techniques, investigation of its basic physical properties has now become possible. Here, the mechanical properties of BAs single crystals are studied by experimental measurements combined with first-principles calculations. A Vickers hardness of 22 GPa suggests that BAs is a hard material, although not among the hardest. The bulk and Young's moduli are measured to be 142 and 388 GPa, respectively. These important mechanical performance parameters, in conjunction with the unusual high thermal conductivity, show great potential for BAs to serve in next-generation semiconductor applications.

Manipulation of Ni Interstitials for Realizing Large Power Factor in TiNiSn‐Based Materials

Abstract: Half-Heusler compounds have been discovered with various beneficial characteristics[1–3] (e.g., high power factor, novel orbital interactions, unique atomic arrangement, robust mechanical properties, and low toxicity) for use as efficient thermoelectric materials working at high temperatures (800–1000 K).[4–9] The most fascinating property is their high power factor originating from the weak electronacoustic phonon couplings induced by the symmetry-protected orbital interactions, which favors a delicate relationship between high conductivity and a large Seebeck coefficient.[3] Correspondingly, a record-high power factor (with the peak value ≈100 μW cm−1 K−2 obtained in Ti-doped NbFeSb-based materials) highlights the superior electrical performance of the half-Heusler materials.[10] Among this class of materials, TiNiSnbased compounds have generated much interest,[11–14] although their relatively low carrier mobility (below 20 cm2 V−1 s−1) and inferior power factor (≈30 μW cm−1 K−2) have impeded their development for highperformance.[15–20] Since their counterpart MNiSn (M = Zr, Hf) compounds commonly possess large power factor values (≈50 μW cm−1 K−2) resulting from higher carrier mobility (over 30 cm2 V−1 s−1),[21–23] it is reasonable that the power factor of TiNiSn-based compounds can be improved via carrier mobility enhancement. TiNiSn has a MgAgAs-type structure[24] comprised of four interpenetrating face centered cubic (fcc) sublattices, as shown in Figure 1a, where Ti, Ni, and Sn are located at the Wyckoff positions: Ti at 4a (0, 0, 0), Ni at 4c (1/4, 1/4, 1/4), and Sn at 4b (1/2, 1/2, 1/2), leaving the fourth fcc sublattice at 4d (3/4, 3/4, 3/4) unoccupied. When this vacant position is filled with extra Ni, the full-Heusler structure TiNi2Sn is formed, as shown in Figure 1b. In this half-Heusler material, Ni plays a unique role in either phonon or electron transport. Based on first-principle calculations, interstitial Ni atoms are energetically more favorable than other types of defects.[19,25] Neutron diffraction results have also confirmed partial occupancy of the 4d position by a small amount of Ni in the 1:1:1 stoichiometric TiNiSn.[15,20] Defect engineering has been identified as an effective strategy for improving thermoelectric performance by tailoring electron and phonon transport. TiNiSn is unique due to its naturally formed Ni interstitials, where the interstitial atoms enable strong phonon scattering that results in reduced lattice thermal conductivity, although an adverse effect on mobility is inevitable. Rather than pursuing the conventional strategy of strengthening the interstitial scattering to improve the performance of TiNiSn-based materials, an attempt is made to minimize the atomic disorder in order to enhance the mobility, which in turn favors a higher power factor. The altered bandgap, and electrical and thermal properties demonstrate that the interstitials can be effectively controlled by intentionally reducing the amount of Ni. Benefiting from the manipulation of the interstitials, significantly enhanced mobility is achieved in the Ni-deficient composition, resulting in peak power factor of ≈50 μW cm−1 K−2, which is comparable to the best n-type halfHeusler compounds. Additionally, the well-designed composition employing Ni interstitial manipulation and heavy-element doping exhibits peak ZT of ≈0.73, higher than that of all other reported TiNiSn-based materials. The unique role of interstitials in either electron or phonon transport is emphasized, and further encouragement for engineering thermoelectric properties by manipulating intrinsic disorder, especially in materials with complex structures, is provided.

Electrochemical Performance of Free-Standing and Flexible Graphene and TiO2 Composites with Different Conductive Polymers as Electrodes for Supercapacitors.

Abstract: The advantage of using composite electrode materials for energy storage is, to a large extent, the synergistic role of their components. Our work focuses on the investigation of the interactions of each phase, exploring the patterns found with the change of materials to provide theoretical or experimental foundations for future study. Here, conductive polymers (CPs), including polyaniline (PANi), polypyrrole (PPy), and polythiophene (PTh), as well as reduced graphene oxide (rGO), and TiO2 with the different crystalline phases of anatase and rutile were applied to form a series of free-standing and flexible binary or ternary composite electrodes. The electrochemical behaviors of these composite electrodes are presented. The results indicate that the synergistic improvement in electrochemical performance is due to the incorporation of the different components. CPs significantly increase the current density of these composite films and contribute their pseudocapacitance to improve the specific capacitance, but lead to a decline in cycle stability. After introducing TiO2 , both the specific capacitance and the cycle-stability of rGO-TiO2 -CP were synergistically improved. A CP can magnify the pseudocapacitance behavior of any of the TiO2 crystalline phases, and interactions vary with the specific CP and the specific TiO2 crystalline phase employed. The synergistic effects of the as-prepared composites were theoretically predicted and explored.

Large reduction of thermal conductivity leading to enhanced thermoelectric performance in p-type Mg3Bi2–YbMg2Bi2 solid solutions

Abstract: Intensifying the phonon scattering via point defect engineering has been demonstrated to be particularly effective in minimizing the lattice thermal conductivity for enhancing thermoelectric performance. In this work, significant phonon scattering has been realized by alloying YbMg2Bi2 with Mg3Bi2. The substantial mass difference between the host atom Yb and the alloying atom Mg leads to an intense phonon scattering effect that significantly reduces the lattice thermal conductivity. The room-temperature lattice thermal conductivity decreases from ∼2.7 W m−1 K−1 for YbMg2Bi1.96 to ∼0.8 W m−1 K−1 for Yb0.7Mg0.3Mg2Bi1.96, a reduction of ∼70%. Benefiting from the greatly reduced thermal conductivity, the average ZT has been effectively improved from ∼0.46 for YbMg2Bi1.96 to ∼0.61 for Yb0.8Mg0.2Mg2Bi1.96, an enhancement of ∼33%. In addition, the predicted maximum heat-to-electricity conversion efficiency can be increased from ∼7% for YbMg2Bi1.96 to ∼10% for Yb0.8Mg0.2Mg2Bi1.96.

Optical properties of cubic boron arsenide

Abstract: The ultrahigh thermal conductivity of boron arsenide makes it a promising material for next-generation electronics and optoelectronics. In this work, we report measured optical properties of cubic boron arsenide 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 as well as transmission and reflection spectroscopy. We further calculate the optical response using density functional and many-body perturbation theory, considering quasiparticle and excitonic corrections. The computed values for the direct and indirect band gaps (4.25 eV and 2.07 eV) agree well with the measured results (4.12 eV and 2.02 eV). Our findings contribute to the effort of using boron arsenide in novel electronic and optoelectronic applications that take advantage of its demonstrated ultrahigh thermal conductivity and predicted high ambipolar carrier mobility.

High-pressure phases of boron arsenide with potential high thermal conductivity

Abstract: Zinc-blende (zb) boron arsenide (BAs) has been confirmed to have impressively high thermal conductivity. However, studies on its phase transitions under pressure have been few. Here, through a recently developed structure search method, we predicted that many polytypes with the structural features of cubic and hexagonal diamond, which are known to be unstable even up to very high pressures for carbon and boron nitride, can become stable at low pressures and might be retained to ambient conditions. Moreover, some of these BAs polytypes are calculated to have impressively high thermal conductivities at ambient conditions and the thermal conductivities for zb- and $2H$-BAs will decrease with increasing pressure, which are mainly attributed to the stronger third anharmonic interaction. The current study will open up a route to the search for high thermal conductors.

New Way to Synthesize Robust and Porous Ni1-xFex Layered Double Hydroxide for Efficient Electrocatalytic Oxygen Evolution.

Abstract: Traditional catalysts are usually synthesized by sputtering, electrochemical deposition, or hydrothermal methods, and they also need to be combined with substrates to obtain the working electrodes....