Grain size control is very important for enhancing the electric properties of CaCu3Ti4O12 (CCTO) ceramics. The effect of grain sizes on the dielectric and non-ohmic properties of the CCTO ceramics ...was investigated. Firstly, CCTO precursor powders were synthesized by the sol-gel process. Then the CCTO ceramics with different grain sizes were obtained under the same condition by using the powders calcined at different temperature. The XRD patterns of the sintered CCTO ceramics show the cubic perovskite with pure CCTO phases. The SEM results indicate that the mean grain sizes range of the CCTO ceramics are about 2.03(±0.66) ∼ 9.02(±3.57) μm. The electrical properties of the CCTO ceramics demonstrate that the change of the grain size obviously affects the non-ohmic characteristics and dielectric properties. And the larger grain size is beneficial to enhance the dielectric constant (ε ≈ 3 × 104, at a frequency of 1k Hz), while the smaller grain size is beneficial to improve its breakdown field (Eb ≈ 6010 V/cm) and nonlinear coefficient (α ≈ 24), which can be explained by the barrier behavior of Schottky-type model and the grain boundary properties of the internal barrier layer capacitor (IBLC) model. This work may provide an effective way to design high dielectric constant and nonlinear coefficient CCTO materials from the viewpoint of controlling grain size through the preparation parameter of the powders.
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•CCTO precursor powders were synthesized by the sol-gel process.•The CCTO ceramic grain sizes increase with the calcination temperature increasing.•Grain size control is very important for enhancing the electric properties.•The larger grain size is beneficial to enhance the dielectric constant.•The smaller grain size is helpful to improve the non-ohmic characteristics.
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The elaborative design and construction of first-rank bifunctional oxygen electrocatalysts featuring low price, high activity and strong stability is critical for the large-scale ...applications of rechargeable Zn-air batteries. Here, a resultful strategy is proposed for fabricating nitrogen-doped 1D beaded-like structure carbon nanofibers uniformly decorated with nitrogen-doped CuCo2O4 nanoparticles (N-CuCo2O4@CNFs) toward boosting oxygen evolution reaction/oxygen reduction reaction (OER/ORR) catalysis. Taking advantage of the synergistic effect between interconnected 1D hierarchical porous carbon nanofiber structure and high catalytic activity of N-doped CuCo2O4 nanoparticles derived from bimetallic MOFs, the N-CuCo2O4@CNFs catalysts possess enhanced reaction kinetics and preferable charge transfer ability. Impressively, the obtained catalysts exhibit prominent electrocatalytic ability and superior stability for OER/ORR, even surpass the commercial RuO2 and Pt/C. More significantly, the Zn-air batteries employing the N-CuCo2O4@CNFs-800 as cathode display a higher power density of 175.6 mW cm−2, a lower charge-discharge voltage gap of 0.82 V at 10 mA cm−2, as well as a better cycling stability with respect to those of Pt/C + RuO2 mixture, demonstrating the great potential of N-CuCo2O4@CNF as a high-efficiency catalyst for clean energy devices.
In this work, we developed a novel system of isovalent Zr4+ and donor Nb5+ co-doped CaCu3Ti4O12 (CCTO) ceramics to enhance dielectric response. The influences of Zr4+ and Nb5+ co-substituting on the ...colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr4+ and Nb5+ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu3Ti3.8-xZrxNb0.2O12 (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr4+ and Nb5+ ions than that of single-doped with Zr4+ or Nb5+ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~105) at a frequency range of 102–105 Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr4+ and Nb5+ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus.
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•A novel type of CaCu3Ti3.8-xZrxNb0.2O12 ceramics were successfully fabricated by a solid-state reaction process.•CaCu3Ti3.8-xZrxNb0.2O12 ceramics sintered at 1060 °C for 10 h show a very small grain size of 0.86 ~ 2.05 μm.•Extremely enhanced giant dielectric response of CaCu3Ti4O12 is achieved by co-doping Zr4+ and Nb5+ ions.•CaCu3Ti3.8-xZrxNb0.2O12 ceramics exhibit obvious dielectric relaxation behavior.
Nitrogen-doped MnO/graphene nanosheets (N-MnO/GNS) hybrid material was synthesized by a simple hydrothermal method followed by ammonia annealing. The samples were systematically investigated by X-ray ...diffraction analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and atomic force microscopy. N-doped MnO (N-MnO) nanoparticles were homogenously anchored on the thin layers of N-doped GNS (N-GNS) to form an efficient electronic/ionic mixed conducting network. This nanostructured hybrid exhibited a reversible electrochemical lithium storage capacity as high as 772 mAh g(-1) at 100 mA g(-1) after 90 cycles, and an excellent rate capability of 202 mA h g(-1) at a high current density of 5 A g(-1). It is expected that N-MnO/GNS hybrid could be a promising candidate material as a high capacity anode for lithium ion batteries.
(1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) ceramics have been reported to exhibit large dielectric response in the vicinity of the multi-phase-coexisting point (i.e. triple point). However, ...the reason for large dielectric response in such a material system is still unclear and thus awaits explanation. In this paper, we investigate the reason for large dielectric response by studying the phase transition behavior around the triple point of BZT-xBCT ceramics. Our results show that the transition enthalpy nearly vanishes and the associated specific heat shows discontinuity on the triple point, which suggest tricritical behavior (i.e. crossover point from first to second order phase transition) for such a triple point. Further Rayleigh analysis indicates that strong dielectric response is due to large reversible contribution which may be caused by phase transition. Moreover, TEM study shows a mottled domain structure with numerous nanodomains close to tricritical triple point, which reveals a polarization isotropic state. In addition, a six-order Landau free energy modeling demonstrates that the energy barrier between paraelectric and ferroelectric phases nearly vanishes on the tricritical triple point, which facilitates large polarizability in the presence of external electric field and is thus responsible for large dielectric permittivity in BZT-xBCT.
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Exploring transition metal-based, highly efficient and bifunctional catalysts for electrochemical water splitting remains a challenging task. Herein, ZnCo2S4 nanosheet array supported on nickel foam ...(ZnCo2S4/NF) is fabricated and demonstrated to be efficacious electrocatalyst for both oxygen evolution and hydrogen evolution reactions. The resulting catalyst affords overpotentials of only 278 mV to achieve 10 mA cm−2 towards oxygen evolution and 185 mV to reach 10 mA cm−2 towards hydrogen evolution with superior stability in alkaline solution. The assembled two-electrode water electrolyzer using ZnCo2S4/NF can reach a current density of 10 mA cm−2 by 1.66 V cell voltage.
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•ZnCo2S4 nanoarray supported on nickel foam (ZnCo2S4/NF) is fabricated and acts as catalyst electrode for water splitting.•The as-prepared ZnCo2S4/NF exhibits remarkable OER and HER activities.•The two-electrode water electrolyzer based on ZnCo2S4/NF needs only 1.66 V to achieve 10 mA cm-2.•ZnCo2S4/NF shows superior electrochemical durability and long-term stability.
Developing highly efficient and cost-effective catalysts for electrochemically oxidizing biomass-derived 5-hydroxymethylfurfural (HMF) into value-added 2,5-furandicarboxylic acid (FDCA) is of great ...importance. Herein, we report a controllable nitrogen doping strategy to significantly improve the catalytic activity of Co3O4 nanowires for highly selective electro-oxidation of HMF into FDCA. The nitrogen doping leads to the generation of defects including nitrogen dopants and oxygen vacancies in Co3O4 nanowires, which is conducive to the formation of catalytically active sites. As a result, the electro-oxidation potential for HMF is only 1.38 V (vs. RHE) when the current density reaches 50 mA/cm2. More importantly, the conversion rate of HMF is as high as 99.5%, and the yield of FDCA is up to 96.4%.
In this work, we report a controllable N-doping strategy to significantly improve the electrocatalytic activity of Co3O4 nanowires for highly selective oxidation of HMF into FDCA. Display omitted
A novel sodium-ion capacitor (NIC) was assembled using graphitic mesocarbon microbead anode and activated carbon cathode in diglyme-based electrolyte. Charge/discharge tests indicate that sodium ions ...can reversibly co-intercalated with diglyme solvent into graphite anode and show good rate performance. The energy densities of the NICs are as high as 93.5 and 86.5 Wh kg−1 at 573 and 2832 W kg−1 (equal to 4 C and 50 C) in the voltage window at 1–4 V, respectively. By optimizing the voltage ranges, the capacity retention of the NIC at 20 C is 98.3% even after 3000 cycles. Such superior electrochemical performance should be attributed to the reversible intercalated/deintercalated reaction of sodium ions and the formation of ternary graphite intercalation compounds in diglyme-based electrolyte. The present work pioneers new realms of hybrid energy storage system with high energy density, high power density and long cycle life.
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•A NIC is constructed with graphitic MCMB anode and activated carbon cathode.•The sodium ions can reversibly intercalate into MCMB in diglyme-based electrolyte.•The optimized NIC delivers a maximum energy density of 93.5 Wh kg−1 at 573 W kg−1.•The NIC still reaches 86.5 Wh kg−1 at 2832 W kg−1.•The NIC retains 98.3% after 3000 cycles at a rate of 20 C.
Large scale solar‐driven hydrogen production is a crucial step toward decarbonizing society. However, the solar‐to‐hydrogen (STH) conversion efficiency, long‐term stability, and cost‐effectiveness in ...hydrogen evolution reaction (HER) still need to be improved. Herein, an efficient approach is demonstrated to produce low‐dimensional Pt/graphene‐carbon nanofibers (CNFs)‐based heterostructures for bias‐free, highly efficient, and durable HER. Carbon dots are used as efficient building blocks for the in situ formation of graphene along the CNFs surface. The presence of graphene enhances the electronic conductivity of CNFs to ≈3013.5 S m−1 and simultaneously supports the uniform Pt clusters growth and efficient electron transport during HER. The electrode with a low Pt loading amount (3.4 µg cm−2) exhibits a remarkable mass activity of HER in both acidic and alkaline media, which is significantly better than that of commercial Pt/C (31 µg cm−2 of Pt loading). In addition, using a luminescent solar concentrator‐coupled solar cell to provide voltage, the bias‐free water splitting system exhibits an STH efficiency of 0.22% upon one‐sun illumination. These results are promising toward using low‐dimensional heterostructured catalysts for future energy storage and conversion applications.
The Pt/graphene‐carbon nanofibers (CNFs)‐based heterostructures are prepared for the cost‐effective, highly efficient, and durable hydrogen evolution reaction. Carbon dots are used for the in situ formation of graphene along the CNFs surface by self‐cross‐linking. The presence of graphene enhances the electronic conductivity of the CNFs and simultaneously supports the uniform growth of Pt clusters on the G‐CNFs.
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