LiFePO4在不同pH值水溶液中电化学性能表现及其衰减机理

LiFePO₄在含Li⁺水溶液中的电化学性能稳定性与水溶液的pH值密切相关,当溶液的pH值达到11后LiFePO₄在充放电循环过程中的容量衰减十分明显. 通过循环伏安测试、交流阻抗测试、电极充放电性能测试、非原位X射线衍射测试以及化学分析的方式对其容量衰减机理进行了研究. 结果表明LiFePO₄在pH=7的LiNO₃水溶液中具有相对最高的电化学稳定性,但是LiFePO₄材料在水溶液中较之其在有机电解液中依然会有较差的电化学性能表现. 认为LiFePO₄在水介质中的容量衰减现象归因于其在持续充放电过程中的Li、Fe、P溶解,同时电极表面也会附着一层沉淀物. 这些最终导致了材料晶体结构的破坏、电极极化的增大以及电极容量的衰减.     

ϑ-Sensors: A new concept for advanced solid-state ionic gas sensors

A new principle of solid state electrochemical sensors based on the kinetics of controlled chemical reactions of the gas with the electroactive species of a solid electrolyte is presented and demonstrated for the measurement of CO₂ partial pressures. The reaction may be for many gases modified by the formation of intermediate product phases.A periodic voltage or current signal is applied to the electrolyte. The current corresponds to alternating chemical reactions at the electrodes in contact with the gas. The new concept avoids the often complicated application of reference electrodes and shows characteristic signals for different types of gases and allows in this way to overcome problems of cross-sensitivity. The experimental results show high accuracy with an approximately linear relationship between the current and the CO₂ partial pressure and also show extraordinarily fast response.     

Study of adsorption of oxygen on β-Al2O3 + Au and β-Al2O3 + Pt: Work function measurements—proposition of a model

To improve the understanding of the electrochemical effects observed on an original potentiometric gas sensor, interactions of oxygen with the device were investigated. This gas sensor is made of a solid electrolyte (treated Na-β-alumina) associated with two metallic electrodes (gold and platinum) located in the same gas mixture. Adsorption of charged oxygen species, considered responsible for the electrical response developed by the sensor, was investigated by work function measurements. Results showed that charged oxygen species only form on partially gold or platinum covered solid electrolyte. Comparison of these results with those obtained in a previous calorimetric study of interactions between oxygen and the same materials suggests the existence of at least two different oxygen species adsorbed on the surface of the sensitive element. The first one, located on the solid electrolyte surface, is neutral and characterized by an endothermal reaction of formation. The second one is charged and probably produced at the gas/solid electrolyte/metallic electrode interface. A mechanism based on the concept of “three phase boundary” and similar to the “reverse spillover” phenomenon is proposed to account for the adsorption of these oxygen species.     

High-voltage asymmetric supercapacitors operating in aqueous electrolyte

The performance of supercapacitors based on different materials with pseudocapacitive properties such as several conducting polymers (ECPs), amorphous manganese dioxide (a-MnO₂), and activated carbon is reported. Composite electrodes of high resiliency and good electronic conductivity were obtained by mixing the active materials with carbon nanotubes. The various limitations of all the above-mentioned materials, when used as negative and positive electrodes in traditional symmetric systems, are shown. It is demonstrated that a successful application of ECPs and a-MnO₂ in supercapacitor technologies is possible only in an asymmetric configuration, i.e. with electrodes of different nature for positive and negative polarizations. Several types of asymmetric capacitors were developed by combining ECPs, a-MnO₂, and activated carbon and characterized in aqueous electrolyte by galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. The best device considering the specific energy and power is the asymmetric supercapacitor using a-MnO₂ and poly(3,4-ethylenedioxythiophene) (PEDOT) for the positive and negative electrodes, respectively. It has an operating voltage of 1.8 V, which is attributed to different operating potentials of both electrodes, and good electrochemical stability in neutral aqueous electrolyte. According to the voltage value, the energy density of the asymmetric capacitor at a current density of 250 mA/g is found to be 13.5 W h/kg, which is about ten times more than for a symmetric capacitor based on PEDOT in an aqueous medium. The asymmetric capacitor provides two times higher power than a symmetric capacitor based on activated carbon in organic electrolyte, and is thus extremely promising for the development of environmentally friendly systems. PACS 82.45.Wx; 82.45.Yz; 82.47.Uv; 82.35.Cd     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

Properties of TiO2 as photoelectrode for hydrogen generation using solar energy

The present paper describes electrochemical characteristics of photo-electrochemical cell during light-on and light-off regimes. The cell involves TiO₂ as photo-anode and Pt as cathode both immersed in a aqueous electrolyte. The photo-voltage was determined for two different cell structures both equipped with TiO₂ as photo-anode. The first cell involved a homogeneous electrolyte system, formed of Na₂SO₄, and photo-anode made of pressed polycrystalline TiO₂. The second cell involved an electrolyte of different pH at both electrodes (ΔpH=14.6) and TiO₂ thin film (deposited in Ti) as photo-anode. Maximum photo-voltage of these cells was 0.7 V and 1.3 V, respectively.     

Pseudo-capacitive properties of LiCoO<Subscript>2</Subscript>/AC electrochemical capacitor in various aqueous electrolytes

LiCoO₂ particles were synthesized by a sol-gel process. X-ray diffraction analysis reveals that the prepared sample is a single phase with layered structure. A hybrid electrochemical capacitor was fabricated with LiCoO₂ as a positive electrode and activated carbon (AC) as a negative electrode in various aqueous electrolytes. Pseudo-capacitive properties of the LiCoO₂/AC electrochemical capacitor were determined by cyclic voltammetry, charge–discharge test, and electrochemical impedance measurement. The charge storage mechanism of the LiCoO₂-positive electrode in aqueous electrolyte was discussed, too. The results showed that the potential range, scan rate, species of aqueous electrolyte, and current density had great effect on capacitive properties of the hybrid capacitor. In the potential range of 0–1.4 V, it delivered a discharge specific capacitance of 45.9 Fg^–1 (based on the active mass of the two electrodes) at a current density of 100 mAg^–1 in 1 molL^–1 Li₂SO₄ aqueous electrolyte. The specific capacitance remained 41.7 Fg^–1 after 600 cycles.     

Formation of Submicron Zinc Particles by Electrodeposition

The influence of the cationic composition on the electrochemical deposition of zinc on plane cathodes was investigated. Electrochemical deposition of zinc from aqueous feed needs high purity of the feed electrolyte owing to sensitivity of the hydrogen overvoltage. In contrast to sulfate‐based electrolytes, the hydrogen overvoltage in acetic acid can be kept high in electrodeposition of zinc and several heavy metals such as nickel. As a consequence, electrochemical deposition of zinc is possible even at nickel concentrations of 50 mg/l and above. Depending on the electrolyte composition in acetate‐based systems the deposition of granular zinc of different particle size, from the micron to the submicron range, is possible on plane electrodes. The chemical composition of granular deposits from several acetate‐based electrolytes was analysed by inductively coupled plasma (ICP) analysis and their structure and morphology were detected by scanning electron microscopy (SEM) and energy‐dispersive X‐ray spectrometry (EDXS).     

Electrocatalytic properties of glassy carbon electrodes modified with hydroxy derivatives of 9,10-anthraquinone for oxygen reduction reaction

The electrochemical and electrocatalytic behavior of glassy carbon electrodes modified by one mono and four dihydroxy derivatives of anthra-9,10-quinone compounds have been investigated by cyclic voltammetric technique. The stability of the modified electrodes was ascertained in acidic and neutral media. The surface morphology of modified electrode was characterized by scanning electron microscope. The influence of pH on the electrochemical and electrocatalytic behavior was studied and pH?6.0 or 7.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic ability for oxygen reduction with overpotential ranging from 388 to 547?mV lower than that at a plain glassy carbon electrode. Hydrodynamic volatammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used, and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions.     

Electrode reactions at oxygen,noble metal / stabilized zirconia interfaces

The polarization behaviour of electrodes of the type “oxygen, noble metal / stabilized zirconia”, comprising different zirconia-based materials as electrolyte, platinum or gold as metal component and an oxygen containing gas atmosphere, was investigated at elevated temperatures under equilibrium and non-equilibrium conditions by means of impedance spectroscopy. Massive metal contacts were used as part of the working electrodes. Under non-polarized conditions, the experimental results for platinum indicate a basically uniform reaction mechanism in a vast range of temperature and oxygen partial pressure, involving the surface diffusion of dissociatively adsorbed oxygen on platinum towards the electrochemical reaction sites on the electrolyte surface as rate-determining step. The experimental findings for gold are consistent with the occurrence of two competing reaction mechanisms, namely a charge transfer controlled process and a surface diffusion controlled process, each of them prevailing in different regimes of temperature and oxygen partial pressure. Under polarized conditions, a significant decrease of the polarization resistance takes place, followed by the onset of low frequency loops in the impedance spectra. In the case of cathodic polarization, the onset voltage can be correlated with the partial electron conductivity of the electrolyte, thus confirming the hypothesis of direct participation of electronic species of the electrolyte in the electrode reaction under biased conditions. At moderate temperatures, the polarization induced changes in the electrode properties exhibit a slow relaxation behaviour. This can be attributed to the successive annihilation of additional metastable electrochemical reaction sites having been created during the preceding polarization treatment. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy     

Experimental limitations in impedance spectroscopy: Part VI. Four-point measurements of solid materials systems

Four-point impedance spectroscopy of solid materials systems is severely hampered by unavoidable voltage-divider effects associated with the reference electrodes. As demonstrated by test circuit studies and experiments with Pt/YSZ/Pt cells (with embedded silver reference electrodes), high impedance reference electrodes can produce distorted and erroneous impedance data. The relationships between these data and the sample properties (conductivity, dielectric constant) and instrument limitations (input impedance/capacitance) are derived. Successful four-point impedance measurements on conductive systems require large effective dielectric constants, which may be unattainable in bulk solids, but are often associated with internal interfaces.     

Effect of pH on the ultrasonic degradation of ionic aromatic compounds in aqueous solution

The sonolysis of 4-nitrophenol (4-NP) and aniline in O2-saturated aqueous solutions was performed at 610 kHz with ultrasonic power of 25 W and aqueous temperature of 15 +/- 1 degrees C. The initial rate of degradation of both 4-NP and aniline in sonolysis of aqueous media follows pseudo-first-order reaction kinetics. Investigation of the H2O2 generation rate in phosphate buffer media (0.01 M) over the range of pH 2-9 revealed a maximum yield at pH approximately 3.2. The pH, which results in modification of the physical properties (including charge) of molecules with ionisable functional groups, plays an important role in the sonochemical degradation of chemical contaminants. For hydrophilic substrates, the neutral species more easily diffuse to and accumulate at the hydrophobic interface of liquid-gas bubbles in comparison with their corresponding ionic forms. As a consequence, the degradation rate of 4-NP under ultrasonic irradiation decreases with increasing pH. In contrast, the disappearance rate of aniline exhibits a maximum under alkaline conditions due to the high solubility of the ionic anilinium ion and the (potentially) preferential movement of the uncharged form to the interface. Additionally, the rate of reaction of the uncharged aniline molecule (which dominates at pH > 4.6) with hydroxyl radicals is reported to be about three times as fast as the rate of reaction of the cationic anilinium species.     

Spatiotemporal self-organization in the oscillatory HCOOH oxidation on a Pt ribbon electrode – Theory and experiments

Since the current density near the edges of ribbon and disk electrodes is enhanced, the resulting stationary and non-stationary double layer potential is generally inhomogeneous in all electrochemical reactions. We investigate the impact of this edge effect induced spatial inhomogeneity on the pattern formation of the oscillatory formic acid oxidation on thin Pt ribbon electrodes. In order to be able to theoretically describe the spatiotemporal behavior of the double layer potential distribution, we derive and discuss the properties of the electrochemical ribbon coupling function for various distances of the reference electrode. The resulting reaction–migration equation is analyzed in connection with a chemical model accounting for the specific reaction mechanism of the formic acid oxidation. The interaction of structural inhomogeneity, chemically induced temporal instability and nonlocal spatial coupling due to ion migration gives rise to novel types of spatiotemporal behavior. The results compare favorably with experiments conducted so far, which are presented as well and can be explained within the framework of reaction–migration equations.     

Formation of a passivating film at the lithium-PEO-LiCF3SO3 interface

The formation of a passivating film on lithium electrodes is demonstrated using ac impedance analysis. The film is formed by an electrochemical reaction between the lithium electrode and the electrolyte, which consists of poly (ethylene oxide) and LiCF₃SO₃. Effects of the salt concentration in the electrolyte and temperature on the nature and conductivity of such films are described. Data obtained from the literature for equivalent systems was interpreted according to the proposed film formation mechanism. The rate-determining step in the dissolution or deposition process of the lithium may, in some cases, be defined by the interphase film.     

Fluorescent properties of thiochrome in solvents of different polarity

Spectral and fluorescent properties of thiochrome in solvents of different polarity were studied. It was found that the pK_a value of the transition between the cationic and neutral forms of thiochrome in aqueous solutions increased from ∼5.5 to 9.7 upon photoexcitation. It is supposed that protonation takes place in the excited state of the molecule resulting in fluorescence quenching of the thiochrome neutral form in aqueous solutions at neutral pH values. The fluorescence quantum yield of thiochrome increased by ∼2.2 times upon the transition from aqueous solutions to alcohols or polar aprotic solvents. It was found that an increase of the solvent polarity led to an increase in the Stokes shift from 3200 to 4200 cm⁻¹ for the thiochrome neutral form emission. The change in the dipole moment upon excitation into the S1-state was estimated to be less than 3D.     

真空热处理碳纳米管的储氢性能研究

研究了真空热处理对多壁碳纳米管(MWNTs)电化学储氢性能的影响.采用化学气相沉积法(CVD)制备碳纳米管，碳纳米管与LaNi₅储氢合金按质量比1∶10混合，制作成CNTs-LaNi₅电极.电解池采用三电极体系，6mol/L KOH为电解液，Ni(OH)₂为正极，Hg/HgO为参比电极.实验结果表明，在相同的充放电条件下，850℃时CNTs-LaNi₅电极的储氢性能最好，克容量最大为503.6mAh/g，相应的平台电压高达1.18V.从500—850℃随着温度升高，放电量有较大幅度的增加，但到950℃时放电量反而下降.由此可见，碳纳米管的热处理温度对碳纳米管的电化学储氢性能有着较大的影响. 关键词： 碳纳米管(CNTs) 储氢性能 5合金')" href="#">LaNi₅合金 化学气相沉积法(CVD法)     

Structure elucidation of fluorescent H1 antihistamines by ultraviolet resonance Raman spectroscopy: solvent structures of tripelennamine and mepyramine

Ultraviolet Resonance Raman (UVRR) spectroscopy—a Raman technique that combines high sensitivity with high selectivity and does not suffer from background fluorescence—is applied to the fluorescent H₁ antihistamines tripelennamine (TRP) and mepyramine (MEP) in aqueous solution to elucidate their molecular structure as a function of pH. In a previous investigation of these compounds (C. Tardioli, G. Gooijer G. van der Zwan, J. Phys. Chem. B, 113 , (2009), 6949), the presence of gauche conformers caused by intramolecular interaction of the protonated alkylamine tail with the pyridine nitrogen was assumed to explain the pH dependence of the fluorescence properties. In order to validate this assumption, use is made of the resonant excitation of the aminopyridine chromophore in TRP and MEP. In that way, structural information associated with the vibrations of that moiety can be obtained, and the changes it undergoes upon protonation can be monitored. Assignment of the vibrations was achieved with the help of a number of other compounds, and quantum chemical calculations. N,N‐Dimethylaminopyridine (2DMP) and its mono‐protonated form (2DMPH⁺) were investigated, since this molecule was shown to have optical properties closely resembling those of the aminopyridine moiety in TRP and MEP. Assignment of the vibrations of 2DMP was accomplished by comparison with the resonance Raman spectra of two other reference structures, 2‐aminopyridine and dimethylaniline—for which ordinary Raman data are available—and by Gaussian calculations. UVRR spectra of TRP and MEP could be readily interpreted on the basis of vibrational assignments of the parent chromophores, i.e. 2DMP and 2DMPH⁺. Vibrations of the aminopyridine chromophore in TRP and MEP at neutral pH, where the aminoalkyl chain is protonated, are modified when compared to the vibrational pattern recorded for a fully neutral molecule in alkaline solution. This implies an electronic redistribution in the ring originating from internal hydrogen bonding between the aminoalkyl tail and the aminopyridine chromophore. Copyright © 2010 John Wiley & Sons, Ltd.     

ATR-SEIRAS--an approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes

Quasi-single crystalline gold films of 20 nm thickness and preferential (1 1 1) orientation on Si hemispheres were modified by controlled potentiostatic deposition of Pd (sub-ML, ML, multi-L) from sulphate and/or chloride-containing electrolyte. The electrochemical properties of these model electrodes were characterised for hydrogen and (hydrogen-) sulphate adsorption as well as for surface oxide formation by cyclic voltammetry. Conditions were developed to fabricate defined and stable Pd monolayers. In situ ATR-SEIRAS (Attenuated Total Reflection Surface Enhanced Infrared Reflection Absorption Spectroscopy) experiments were carried out to describe the electrochemical double layer of Pd modified gold film electrodes in contact with aqueous 0.1 M H₂SO₄ with focus on interfacial water and anion adsorption. Based on an analysis of the non-resonant IR background signal the potential of zero charge is estimated to 0.10-0.20 V (vs. RHE). CO was found to be weakly physisorbed in atop sites on Au(1 1 1-20 nm)/0.1 M H₂SO₄ only in CO saturated electrolyte. CO, deposited on a quasi-single crystal gold film modified with 1 ML Pd, is chemisorbed with preferential occupation of bridge sites and atop positions at step edges. Saturated CO adlayers, as obtained by deposition at 0.10 V, contain isolated water species and are covered by a second layer of hydrogen bonded water. Potentiodynamic SEIRAS experiments of CO electro-oxidation on Pd-modified gold film electrodes demonstrate clearly the existence of a “pre-oxidation” region. They also provide spectroscopic evidence that isolated water and weakly hydrogen bonded water are consumed during the reaction and that atop CO on defect sites is a preferential reactant. The simultaneous in situ monitoring of the potential- and time-dependent evolution of characteristic vibrational modes in the OH- and CO-stretching regions are in agreement with the Gilman (“reactant pair”) mechanism of CO oxidation.     

高硼掺杂金刚石膜电极的电化学应用研究

概述了高硼掺杂金刚石膜电极的电化学研究的最新进展，介绍了高硼掺杂金刚石膜电极的制备，金刚石膜电极在水介质中的电化学行为，金刚石膜电极在废水处理，微量有机化合物成分探测和蜂窝状金刚石电极双电层电容器方面的应用。     

A study of the capacity fade of porous NiO/Ni foam as negative electrode for lithium-ion batteries

High energy density materials such as NiO that undergoes conversion reaction hold promise for lithium (Li)-ion batteries (LIBs). However, porous NiO experiences substantial volume change due to the diffusion-induced stress during electrochemical operation, which causes mechanical fractures and morphological changes of porous NiO electrodes, leading to capacity fade through internal short circuit (ISCr). In this study, both non-destructive and destructive operations were used to visualize and quantify the origins and evolutions of the capacity fading of porous NiO/Ni foam electrodes. Results indicated that charge transfer resistance was dominant among all the internal resistances before ISCr, whereas solid electrolyte interface (SEI) resistance was dominant after ISCr of LIBs. The generation of the large amount of heat and pressure during ISCr caused the volume expansion and the formation of the micro-cracks in the struts of the porous NiO/Ni foam electrodes. Together with the electrolyte decomposition, this led to capacity fade. The results of this study provide insights for developing of NiO/Ni electrode for LIBs.