Facile Synthesis of NiCo2O4 Nanowire Arrays/Few-Layered Ti3C2-MXene Composite as Binder-Free Electrode for High-Performance Supercapacitors

Herein, a 3D hierarchical structure is constructed by growing NiCo₂O₄ nanowires on few-layer Ti₃C₂ nanosheets using Ni foam (NF) as substrate via simple vacuum filtration and solvothermal treatment. Ti₃C₂ nanosheets are directly anchored on NF surface without binders or surfactants, and NiCo₂O₄ nanowires composed of about 15 nm nanoparticles uniformly grow on Ti₃C₂/NF skeleton, which can provide abundant active sites and ion diffusion pathways for enhancing electrochemical performance. Benefiting from the unique structure feature and the synergistic effects of active materials, NiCo₂O₄/Ti₃C₂ exhibits a high specific capacitance of 2468 F g⁻¹ at a current density of 0.5 A g⁻¹ and a good rate performance. Based on this, an asymmetric supercapacitor (ASC) based on NiCo₂O₄/Ti₃C₂ as positive electrode and activated carbon (AC)/NF as negative electrode is assembled. The ASC achieves a high specific capacitance of 253 F g⁻¹ at 1 A g⁻¹ along with 91.5% retention over 10,000 cycles at 15 A g⁻¹. Furthermore, the ACS presents an outstanding energy density of 90 Wh kg⁻¹ at the power density of 2880 W kg⁻¹. This work provides promising guidance for the fabrication of binder-free, free-standing and hierarchical composites for energy storage application.     

Electrochemical synthesis of Fe3O4-PB nanoparticles with core-shell structure and its electrocatalytic reduction toward H2O2

The Fe₃O₄-Prussian blue (PB) nanoparticles with core-shell structure have been in situ prepared directly on a nano-Fe₃O₄-modified glassy carbon electrode by cyclic voltammetry (CV). First, the magnetic nano-Fe₃O₄ particles were synthesized and characterized by X-ray diffraction. Then, the properties of the Fe₃O₄-PB nanoparticles were characterized by CV, electrochemical impedance spectroscopy, and superconducting quantum interference device. The resulting core-shell Fe₃O₄-PB-modified electrode displays a dramatic electrocatalytic ability toward H₂O₂ reduction, and the catalytic current was a linear function with the concentration of H₂O₂ in the range of 1 × 10⁻⁷~5 × 10⁻⁴ mol/l. A detection limit of 2 × 10⁻⁸ (s/n = 3) was determined. Moreover, it showed good reproducibility, enhanced long-term stability, and potential applications in fields of magnetite biosensors.     

The NiFe2O4 - MgFe2O4 series as electrode materials for electrochemical reduction of NO x

Solid solutions of spinel-type oxides with the composition (x = 0.0, 0.3, 0.5, 0.6, 1.0) were prepared with the glycine-nitrate combustion synthesis (x = 0.0, 0.3, 0.5, 0.6) and the citric-acid combustion synthesis (x = 1.0). The oxides were used as electrode materials in a pseudo-three-electrode setup in the temperature range of 400–600 °C. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the electrochemical behavior in 1% NO and 10% O₂. Measurements show that NiFe₂O₄ has relatively high cathodic activity in both NO and O₂, whereas MgFe₂O₄ shows much higher activity in NO compared to O₂. MgFe₂O₄ was also measured with cyclic voltammetry in 1% NO₂ and different gas mixtures of NO and O₂ at 300 and 400 °C. Results show that the cathodic activities (−0.6 V) are relatively high with current ratios, , ranging from 10.1–167.7 and with a maximum at 400 °C. Dilatometry measurements were performed on the materials in air up to 1,000 °C, and they showed that the Curie temperature could be detected for all samples. Four-point DC resistivity measurements at elevated temperatures show that Ni_0.4Mg_0.6Fe₂O₄ has the highest conductivity, whereas Ni_0.7Mg_0.3Fe₂O₄ and NiFe₂O₄ have the highest conductivity at lower temperatures.     

Effect of ionic interactions on the oxidation of Fe(II) with H2O2 in aqueous solutions

The oxidation of Fe(II) with H₂O₂ has been measured in NaCl and NaClO₄ solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L^–1). The rate constants, k (M^–1-sec^–1), d[Fe(II)]/DT=-k[Fe(II)][₂O₂]at pH=6.5 have been fitted to equations of the formlog k = log k₀+ AI ^1/2+BI+CI ^1/2/T Where log k₀=15.53-3425/T in water; A=–2.3, –1.35; B=0.334, 0.180; and C=391, 235, respectively, for NaCl (=0.09) and NaClO₄ ( =0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH) ₄ ^– >HCO ₃ ^– >ClO ₄ ^– >Cl^–>NO ₃ ^– >SO ₄ ^2– and are attributed to the relative strength of the interactions of Fe²⁺ or FeOH⁺ with these anions. The FeB(OH) ₄ ⁺ species is more reactive while the FeCO ₃ ⁰ , FeCl⁺, FeNO ₃ ⁺ and FeSO ₄ ⁰ species are less reactive than the FeOH⁺ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O₂ except for B(OH) ₄ ^– . The effect of pH on the logk was found to be a quadratic function of the concentration of H⁺ or OH^– from pH=4 to 8. These results have been attributed to the different rate constants for Fe²⁺ (k₀) and FeOH⁺ (k₁) which are related to the measured k by, k=k_0Fe + k_1FeOH, where _i is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH⁺ compared to Fe²⁺. k₀ is independent of composition and ionic strength but k₁ is a function of ionic strength and composition due to the interactions of FeOH⁺ with various anions.     

Ni/NiCo2O4电极的制备及其析氧反应性能

采用溶胶-凝胶法制备NiCo₂O₄尖晶石粉体, 然后以多孔Ni 为基体, 通过复合溶胶涂覆结合烧结制备Ni/NiCo₂O₄ 涂层电极. 运用扫描电子显微镜(SEM)、能量色散谱(EDS)和X 射线衍射(XRD)表征粉体以及Ni/NiCo₂O₄涂层电极的组成和结构. 采用循环伏安(CV), 稳态极化(LSV), 电化学阻抗谱(EIS), 恒电位阶跃以及恒电位长时间电解研究涂层电极在5 mol·L^-1 KOH溶液中的电催化析氧反应(OER). 结果表明: Ni/NiCo₂O₄涂层电极与多孔Ni 电极对比, 具有低的析氧过电位、高的比表面积和高的稳定性能; 其中比表面积增大了28.69倍,表观活化能在不同过电位分别降低了166.78和162.15 kJ·mol^-1.     

Responsive Ag@NiCo2O4 Nanowires Anchored on N-Doped Carbon Cloth as Array Electrodes for Nonenzymatic Glucose Sensing

The development of responsive materials in a predictable manner is high on the list of the material industry’s trends. In this work, responsive Ag@NiCo₂O₄ nanowires were, firstly, anchored on N-doped carbon cloth (NC) and, then, employed as array electrodes for a nonenzymatic glucose-sensing application. The results showed that the highly conductive NiCo₂O₄ nanowires supported Ag nanoparticles and exhibited high conductivity and electrocatalytic properties. The fully exposed crystalline planes of Ag nanoparticles provided more active surface sites. As a result, the assembled Ag@NiCo₂O₄-NC electrodes for the glucose-sensing evaluation delivered a selectivity of 2803 μA mM⁻¹ cm⁻² and a detection limit of 1.065 μM, which outperformed the literature-reported Ag- and NiCo₂O₄-based glucose-sensing catalysts.     

Charge-discharge behavior of K2FeO4 electrodes in concentrated KOH solutions

The charge-discharge behavior of the K₂FeO₄ electrodes with different current collectors is investigated by various electrochemical techniques. Due to higher electrochemical stability of platinum, the K₂FeO₄ electrode using platinum lattice as current collector displays better primary discharge performance than those with silver lattice and nickel foam as current collectors. The galvanostatic charge-discharge experiments indicate that the K₂FeO₄ electrodes have low electrochemical reversibility, which may result from their relatively lower oxygen evolution potentials or the effects of the oxidation/reduction of current collectors. The experimental results of intermittent discharge, cycle voltammetry, and XRD show that the discharge reaction of the K₂FeO₄ electrode may consist of two steps. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 4, pp. 355–360. The text was submitted by the authors in English.     

NOx储存催化剂Pt/BaAl2O4-Al2O3的XAFS研究

采用共沉淀-浸渍法在不同载体焙烧温度下,制备了不同Al/Ba原子比的Pt/BaAl2O4-Al2O3系列样品.用XRD, XANES, EXAFS,以及NSC (NOx storage capacity）测定等手段对样品的微观结构和NOx储存性能进行了详细的表征.样品中Ba物种是以BaAl2O4和BaCO3两种混合物相的形式存在,且伴随着载体焙烧温度和Ba含量的降低, BaAl2O4物相的分散度变高, NOx储存活性也随之提高,这表明BaAl2O4相的分散度与样品的NOx储存性能密切相关,小颗粒的BaAl2O4相是NOx的主要储存活性中心.在样品中, Pt物种以金属原子簇形式存在,分散度很高,其Pt－Pt壳层配位数较标样Pt粉有显著下降, Pt－Pt键长变短,出现了纳米收缩现象.高分散的小颗粒金属Pt原子簇为捕获和氧化NOx的主要活性中心.     

Response Surface Methodology Optimization in High-Performance Solid-State Supercapattery Cells Using NiCo2S4–Graphene Hybrids

In this work, NiCo₂S₄–graphene hybrids (NCS@G) with high electrochemical performance were prepared using a hydrothermal method. The response surface methodology (RSM), along with a central composite design (CCD), was used to investigate the effect of independent variables (G/NCS, hydrothermal time, and S/Ni) on the specific capacitances of the NCS@G/Ni composite electrodes. RSM analysis revealed that the developed quadratic model with regression coefficient values of more than 0.95 could be well adapted to represent experimental results. Optimized preparation conditions for NCS@G were G/NCS = 6.0%, hydrothermal time = 10.0, and S/Ni = 6.0 of NCS@G (111) sample. The maximum specific capacitance of NCS@G (111)/Ni fabricated at the optimal condition is about 216% higher than the best result obtained using the conventional experimental method. The enhanced capacitive performance of the NCS@G (111) sample can be attributed to the synergistic effect between NCS nanoparticles and graphene, which has the meso/macropores conductive network and low diffusion resistance. Notably, the NCS@G (111) could not only provide numerous reaction sites but also prevent the restacking of graphene layers. Furthermore, a supercapattery cell was fabricated with an (G + AC)/Ni anode, a NCS@G (111)/Ni cathode, and a carboxymethyl cellulose–potassium hydroxide (CMC-KOH) gel electrolyte. The NCS@G (111)//(G + AC) demonstrates an outstanding energy density of 80 Wh kg⁻¹ at a power density of 4 kW kg⁻¹, and a good cycling performance of 75% after 5000 cycles at 2 A g⁻¹. Applying the synthesis strategy of RSM endows remarkable capacitive performance of the hybrid materials, providing an economical pathway to design promising composite electrode material and fabricate high-performance energy storage devices.     

The thermal behaviour of the hydroxide mixtures used for the synthesis of MgFe2O4 spinel

The thermal behaviour of hydroxide mixtures, precursors to the synthesis of MgFe₂O₄ spinel powders, was investigated.The mixtures of hydroxides were prepared by coprecipitation reaction from nitrate solutions with an Mg/Fe atomic ratio of 12. The results were related to the thermal behaviour of separately precipitated components of the hydroxide mixtures. Samples prepared by mechanically mixing the separately precipitated hydroxides were also studied.The spinel formation temperature was identified by performing XRD analysis on powder samples heated to different temperatures. The presence of spinel from the thermal decomposition of coprecipitated mixtures was detected at a temperature as low as 380°C. The separate formation of MgO and Fe₂O₃ from the thermal decomposition of mechanical mixtures was observed; nevertheless the formation of MgFe₂O₄ through a solid state reaction between the oxides was noted at a temperature as low as 500°C.The Authors wish to thank Dr. L. Petrilli and Mr. F. Dianetti for carrying out the elemental chemical analyses at the Microanalysis Service of the C. N. R. laboratories of the Area della Ricerca di Roma.     

脊椎状NiCo2O4纳米棒的制备及其析氧和氧还原性能研究

以水热法并进一步焙烧合成脊椎状NiCo₂O₄纳米棒,通过透射电子显微镜（TEM）、X射线衍射仪（XRD）、X射线光电子能谱仪（XPS）和热重分析仪（TG）等来表征其结构形态及热稳定性.采用线性扫描法（LSV）、循环伏安（CV）研究所制备催化剂的在玻碳和旋转圆盘电极上的电催化活性：在0.1 mol·L^-1 KOH溶液中的电催化析氧反应（OER）和电催化氧还原反应（ORR）.研究结果表明,所制备的脊椎状NiCo₂O₄纳米棒有大量的不饱和态,200℃焙烧制备的脊椎状NiCo₂O₄纳米棒析氧过电位最小可达309 mV,Tafel斜率145.6 mV/dec,其氧还原极限电流密度在1600 rmp可达到5.095 mA·cm^-2,电子转移数在3.2~3.8之间,接近四电子转移机理,其优良电化学性能可能是由于暴露了更多的边缘缺陷的缘故.     

Synthesis and electrochemical properties of chemically substituted LiMn2O4 prepared by a solution-based gel method

Lithium manganese oxide, LiMn(2)O(4), and its substituted samples LiM(0.05)Mn(1.95)O(4) (M=Al, Co, and Zn) were first prepared by a cost-saving and effective new solution-based gel method using a mixture of acetate and ethanol as the chelating agent. The physical properties of the synthesized samples were investigated by thermogravimetry/differential thermal analysis, X-ray diffraction, and scanning electronic microscopy. The as-prepared powders were used as positive materials for a lithium-ion battery, whose charge/discharge properties and cycle performance were examined. The results revealed that all the substituted samples had better cycle performance than pure LiMn(2)O(4). Among these synthesized materials, the LiCo(0.05)Mn(1.95)O(4) sample had the best cycle performance. After 30 cycles, its capacity loss was only 3%. Therefore, cyclic voltammetry and electrochemical impedance spectroscopy were employed to characterize the reactions of Li ion insertion into and extraction from LiCo(0.05)Mn(1.95)O(4) electrodes.     

松针状NiCo2O4@碳布复合材料在锂硫电池中的应用

以碳布（CC）作为柔性基底,采用水热法在其表面原位生长松针状网络结构NiCo₂O₄,制得NiCo₂O₄@CC复合材料,并应用于锂硫电池。NiCo₂O₄在碳纤维表面竖直生长形成三维纳米针簇网络,为硫的存储提供更多的空间,有效缓解硫电极的体积膨胀。通过吸附实验,证明了NiCo₂O₄@CC能有效吸附多硫化物,从而抑制多硫化物的穿梭效应。与CC/S相比（933 mAh·g^-1）,NiCo₂O₄@CC/S复合材料用于锂硫电池具有更优异的电池性能,在0.1C下初始放电比容量高达1 467 mAh·g^-1,在0.2C下初始放电比容量为1 098 mAh·g^-1,经200次循环后,放电比容量仍然保持在879 mAh·g^-1,平均每圈衰减率为0.09%,表现出良好的循环性能。     

Ionic liquid-assisted electrochemical determination of pyrimethanil using reduced graphene oxide conjugated to flower-like NiCo2O4

The novel hierarchical flower-like superstructure NiCo₂O₄/reduced graphene oxide (rGO) hybrids have been successfully synthesized with a facile one-step hydrothermal process for the determination of fungicide pyrimethanil (PMT). For comparison, various structures of NiCo₂O₄/rGO including hexagonal nanoplates and nanorods were also synthesized. Among them, three-dimensional (3D) flower-like NiCo₂O₄/rGO exhibited the highest electrocatalytic activity for the oxidation of PMT. With the synergistic effect of [OMIM]PF₆ ionic liquid (IL), the electrochemical sensor film (NiCo₂O₄/rGO/IL) further facilitated interfacial electron transfer and enhanced electrocatalytic activity for the oxidation of PMT. Under the optimum conditions, the electrochemical sensor exhibited two linear ranges of 0.1–10.0 μmol/L and 20.0–140 μmol/L for PMT with a low detection concentration of 11.0 nmol/L. Besides, the interference, repeatability, reproducibility and stability measurements were also evaluated. The proposed method was successfully applied to the detection of PMT in water, seawater, fruits and vegetables with good recovery ranging from 93% to 105%, and possessed potential applications in the analysis of real samples.     

Synthesis and electrical properties of dense Bi2Al4O9

Bi₂Al₄O₉ ceramics are difficult to sinter to greater than 80% theoretical density due to peritectic decomposition at 1,070 °C. A novel processing method is discussed where a high-bismuth oxide-based liquid is used as a sintering aid. After sintering, the high bismuth oxide phase is removed by leaching with 40% acetic acid. The resulting samples are phase pure and ∼91% dense. The grain size varies in a wide range with the average grain size of ∼1 μm. The electrical properties of these ceramics were measured as functions of temperature (550–850 °C) and oxygen partial pressure (6×10⁻⁶–1 atm). The total conductivity was separated into electronic and ionic contributions. The low ionic conductivity indicates that the material is not an ‘intrinsically defective fast ion conductor’. The ionic conductivity is due almost exclusively to compensating oxygen vacancies related to impurities. With increasing temperature and decreasing oxygen partial pressure, the electronic conduction dominates over the ionic conduction.     

Effect of ionic interactions on the rates of reduction of Cu(II) with H2O2 in aqueous solutions

The rates of reduction of Cu(II) with H₂O₂ have been measured in NaCl and NaBr solutions and mixtures with NaClO₄ as a function of pH (6 to 9), temperature (5 to 45°C) and ionic composition (0.1 to 6M). The effect of pH on the rates was found to be independent of temperature and ionic composition. The rates increased as a function of [H⁺] raised to the power of 1.3 to 1.6. Speciation calculations indicate that this pH dependence can be attributed to Cu(OH)₂ being the reactive species. The rate constants in NaCl and NaBr and mixtures with NaClO₄ were independent of ionic strength, but proportional to the halide concentration raised to the power of 2.0 (0.2 to 2.6M). These results can be attributed to Cu(OH)₂Cl ₂ ²⁻ being the reactive species to reduction with H₂O₂. The Cu(I) halide complexes formed from the reduction are not easily oxidized with O₂ or H₂O₂. The faster rates in Br⁻ solutions, which form stronger complexes with Cu⁺, support this contention. Measurements made in NaCl with added NaHCO₃, NaB(OH)₄ EDTA, NTA and glycine were also made. These measurements indicate that the CuL complexes (L=B(OH) ₄ ⁻ , CO ₃ ²⁻ , EDTA, NTA, and glycine) are not very reactive to reduction with H₂O₂. The addition of Mg²⁺ or Ca²⁺ caused the rates to increase due to the formation of MgL or CaL complexes and the resultant release of reactive Cu²⁺.     

Effect of sodium oxide doping on solid-solid interactions between V2O5 AND Al2O3

A V₂O₅/Al₂O₃ mixed solids sample was prepared with a molar ratio of 0.41 Na₂O (4 and 10 mol%) was added in the form of sodium nitrate prior to calcination in air in the temperature range 500–1000C. Solid-solid interactions between V₂O₅ and Al₂O₃ were studied using DTA and TG curves and their derivatives together with XRD techniques.The results obtained showed that Na₂O interacted with V₂O₅ at temperatures starting from 500C to yield a sodium/vanadium compound, Na_0.3V₂O₅ which remained stable and decomposed in part by heating at 1000C. V₂O₅ exists in orthorhombic and monoclinic forms in the case of pure mixed solids and those containing 4 mol% of Na₂O and preheated at 500C, and in monoclinic form in the case of the mixed solid doped with 10 mol% of Na₂O.Heating of pure and doped mixed oxide solids at 650C resulted in the conversion of most of the V₂O₅ into AlVO₄. Doping with sodium oxide enhanced the solid-solid interaction between V₂O₅ and Al₂O₃ at 650C to produce AlVO₄. The produced AlVO₄ decomposed completely on heating at 700C to form -Al₂O₃ and V₂O₅, (orthorhombic and monoclinic forms).The presence of Na₂O was found to decrease the relative intensity of the diffraction lines of -Al₂O₃ (corundum) produced at 750C which indicated some kind of hindrance of the crystallization process.Heating of pure and doped mixed solids at 1000C resulted in a further crystallization of acorundum together with V₂O₅ and sodium vanadate, Na_0.3V₂O₅. However, the intensities of diffraction lines relative to those of the sodium vanadium compound were found to decrease markedly by heating at 1000C, indicating partial thermal decomposition into vanadium and aluminium oxides.     

Synthesis and electrochemical properties of spinel LiMn2O4 prepared by the rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders were successfully synthesized by a simple rheological phase method. The thermal behavior and structure properties of the powders prepared by the rheological phase method compared with the solid-state reaction were investigated by thermogravimetry, powder X-ray diffraction , scanning electron microscopy and transmission electron microscopy. According to the results of the electrochemical tests, it is obvious that the sample resulting from the rheological phase method shows higher discharge capacity and better cycling stability than one formed in the solid-state reaction. The cyclic voltammogram and columbic efficiency curves also confirm that the product by the rheological phase method has a good cycling performance due to its fine cubic spinel structure and morphology.     

Synthesis and characterization of phosphate-modified LiMn2O4 cathode materials for Li-ion battery

<正>LiMn_2O_4 spinel cathode materials were modified with 2 wt.%Li-M-PO_4(M=Co,Ni,Mn) by polyol synthesis method.The phosphate surface-modified LiMn_2O_4 cathode materials were physically characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS).The charge-discharge test showed that the cycling and rate capacities of LiMn_2O_4 cathode materials were significantly enhanced by stabilizing the electrode surface with phosphate.