Electrochemical Noise Measurement of Polymer Membrane Fuel Cell under Load

Electrochemical noise of a polymer membrane hydrogen–air fuel cell under different currents is measured. Frequency and amplitude dependences of the current-noise power spectral density are calculated. In the frequency interval from 10 to 200 Hz a linear segment of the frequency characteristic has a slope of ?2. The current-noise power spectral density is found to be proportional to the 4th power of the fuel cell loading DC current. Thus found frequency dependence of the fuel cell electrochemical noise was shown to differ markedly from that of the electrochemical impedance real component.     

The effect of partial charge transfer on the electrochemical turbulent noise

Theoretical analysis of the effect of electrode potential on the spectral density of random alternating current emerged in electrochemical cell under the action of turbulent pulsations of the electrolyte solution velocity is carried out. An impedance model of metal electrode dissolution reaction, including two adsorption stages, is suggested, with allowance for the oxidized ion diffusion in electrolyte solution. It is known that in terms of the Ershler-Randles model, at low frequencies the experimentally measured slope of bilogarithmic frequency dependence of spectral density equals 3, which is characteristic of the diffusion control; at high frequencies the slope equals 4, which is characteristic of the kinetic control. It is shown that for the model of impedance of the two-stage adsorption oxidation process, in the middle segment of the spectrum the local slope must decrease down to 2, provided the first oxidation stage, which proceeds within the inner electrical double layer, is slow; the local slope must increase up to 6 (or 5, for diffusion control), provided the second oxidation stage (the partially oxidized ion desorption to solution) is slow. The “height” and “width” of the slope local changes appeared explicitly depending on the parameters of the partial charge transfer. This makes the turbulent noise method somewhat superior to the impedance method in the studying of the above-specified reaction type.     

Software and Instrumentational Methods of Enhancing the Resolution in Electrochemical Noise Measurements

Several methods of enhancing the signal-to-noise ratio for instrumentation designed to measure electrochemical noise are practically tested. The experiments are carried out using model RC-circuits and lielectrolyte electrochemical cells. Strong limitations in the tested objects’ impedance values are found due to the input current noise of the instrumentation, especially during the parallel connection of several channels. The advantages of a two-channel scheme for automatically compensating the instrument’s self noise are demonstrated. Different methods of lowering the dispersion of the frequency dependences of the spectral power density of electrochemical noise are compared. It is shown that averaging over segments with an overlap is the most effective method but averaging over frequencies can lead to large distortions when investigating electrochemical systems.

     

Frequency dependence of hysteresis associated with the electromechanical performance of PVDF film

The thickness strains, the electric displacements, and the hysteresis exhibited by poly(vinylidene fluoride) films under sinusoidal applied electric fields were measured over a range of frequencies from 0.05 to 100 Hz. The loops of strain‐ and electric displacement versus electric field exhibit a shape and hysteresis that undergoes a continuous and reversible change with frequency. At lower frequencies (<5 Hz), the shape and large hysteresis are characteristic of a ferroelectric with degrees of remanent polarization. At higher frequencies (>5 Hz), the shape and slight hysteresis of the loops are representative of a nonremanently polarized ferroelectric. This dependence of shape and hysteresis on frequency is attributed to the difference in rates between the rapid rate by which the polarization of domains of aligned dipoles is reversed in a periodic field and the slower rate by which the polarization is made remanent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3207–3214, 2007     

Noise-power spectra of optical and acoustic emission signals from an inductively coupled plasma

Noise power spectra of emission signals from an ICP discharge have been measured. Below 5 Hz the noise power spectra show a marked dependence on the type of nebulizer used. While a strict 1/f character was not observed the spectra were clearly dominated by low frequency components. Above 5 Hz the noise power spectra were, broadly speaking, independent of the nebulizer. Distinct peaks were observed in the noise power spectra in the 200–400 Hz region. The exact position and intensity of these peaks were dependent on R.F. power, coolant gas flow rate and torch design. In addition the intensity of these peaks is dependent on the exact spatial region viewed by the spectrometer. The position of these peaks correspond exactly to similar peaks found in the acoustic noise power spectrum of the ICP. It is believed these peaks are the result of plasma rotation.     

A highly sensitive nonenzymatic glucose sensor based on nickel oxide–carbon nanotube hybrid nanobelts

In this study, we demonstrated a highly sensitive electrochemical sensor for the determination of glucose in alkaline aqueous solution by using nickel oxide single-walled carbon nanotube hybrid nanobelts (NiO–SWCNTs) modified glassy carbon electrode (GCE). The hybrid nanobelts were prepared by the deposition of SWCNTs onto the Ni(SO₄)_0.3(OH)_1.4 nanobelt surface, followed by heat treatment at different temperatures ranging from 400 °C to 600 °C. The NiO–SWCNTs hybrid nanobelts modified electrode prepared at 500 °C displays enhanced electrocatalytic activity towards glucose oxidation, revealing a synergistic effect between the NiO and the deposited SWCNTs. The as-fabricated nonenzymatic glucose sensor exhibits excellent glucose sensitivity (2,980 μA cm^?2 mM^?1), lower detection limit (0.056 μM, signal/noise [S/N] ratio?=?3), and wider linear range (0.5–1,300 μM). Moreover, the sensor has been successfully used for the assay of glucose in serum samples with good recovery, ranging from 96.4 % to 102.4 %.     

腐蚀金属电极的电化学频域测量研究进展──金属腐蚀与防护研究所电化学研究介绍之一

腐蚀金属电极的电化学频域测量研究进展──金属腐蚀与防护研究所电化学研究介绍之一曹楚南（金属腐蚀与防护国家重点实验室,金属腐蚀与防护研究所,沈阳１１００１５）腐蚀电化学是研究与腐蚀有关的电化学问题,具体的研究对象为腐蚀金属电极。虽然电化学中的基本原理和...     

Theoretical consideration of the anomalous temperature dependence of the surface tension of pure liquid gallium

The surface tension of pure liquid gallium in the temperature range 303–503 K (303 K is the melting point) was previously measured using the noninvasive method of capillary wave spectroscopy (CWS). The result of this experiment showed that the value of surface tension increases from 303 to 345 K indicating a negative surface excess entropy (S ^σ), and decreases linearly from ~345 to 503 K confirming a negative slope, and thus a positive S ^σ. This unusual behavior of Ga is not known for other liquid metals such as Bi, Pb, Hg, Sn and Al. The reported experimental behavior is modeled here. A theoretical equation for calculating the surface tension of liquid Ga, based upon formulating a proper partition function that includes the rotational part, is derived and described. The theory predicted no maximum in the temperature-dependence of the surface tension, as seen in the experiment, where the analysis was done over a large temperature range (325–503 K). The value obtained from this mathematical expression indicates that the temperature variation of surface tension has no positive slope within the temperature range 303–345 K. At T > 345 K, the surface tension shows the usual linear temperature-dependence with a negative slope. Therefore, the equation is only applicable for the latter temperature range. A comparison between the theoretical and experimental values of surface tension of liquid Ga is discussed.     

Convective Current in a Four-Electrode Electrochemical Cell at Various Boundary Conditions at Anodes

The frequency dependence of the transfer function of an electrochemical cell is studied experimentally and theoretically in the frequency region 0.005 to 1 Hz under conditions of controlled convective diffusion, at various boundary conditions on the anodes. It is shown that the results are independent of the conditions on the anodes at frequencies exceeding a diffusion frequency. On the other hand, the effect of the boundary conditions al low frequencies is substantial. In particular, it is shown that it is feasible to design a cell with a conversion efficiency indefinitely increasing in the direction of lower frequencies.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 987–992.Original Russian Text Copyright © 2005 by Agafonov, Nesterov.     

An electrochemical process intensified by bipolar iron particles for nitrate removal from synthetic groundwater

An intensified electrochemical process in an undivided cell using Cu–Zn alloy as cathode and Ti/IrO₂–Pt as anode combined with bipolar iron particles (electro-iron system) has been developed. The performance of nitrate reduction was evaluated using synthetic groundwater. Results showed that the nitrate-N dropped rapidly from 50 to less than 10 mg/L within 100 min in the developed system at current densities in the range of 5–30 mA/cm². Sodium chloride addition was found to have a positive effect on the system performance. No nitrite-N was detected during the electrolysis in the presence of sodium chloride. The concentration of total iron ion in the solutions was found to be less than 0.25 mg/L after 100 min electrolysis. Furthermore, the electrical energy consumption for nitrate reduction in the electro-iron system was saved by approximately 29.4–34.8 % at 5–30 mA/cm². The developed system has been proved to promote electrochemical nitrate reduction and greatly improve the electrical energy efficiency.     

Comparative study of turbulent mass transfer in the viscous sublayer using electrochemical method and direct numerical simulations

In this study we present, analyze and compare the power spectral density of the wall shear stress in a turbulent plane channel flow obtained with different techniques. Experimentally the instantaneous wall shear stress was measured with the electrochemical technique using different probes, which give approximately the same results after applying the transfer function for correction of the probe??s inertia. Numerically, the time evolution of the wall shear stress has been determined using direct numerical simulations (DNS) and large eddy simulations (LES). The results of DNS are in a good agreement with the electrochemical flow measurements. However the power spectra of the wall shear stress obtained with LES shows deviations with respect to DNS at high frequencies because of the spatial filtering inherent to the LES technique.     

Theoretical analysis of the telegraph-like fluctuations in bridged electrochemical contacts

Steady-state current-voltage characteristic of an electrochemical bridged contact and the telegraph noise power are calculated in terms of the model of two redox states. It is shown that the overvoltage dependence of the tunnel current at a constant bias voltage is S-shaped, which was observed in works on the electron tunneling in similar systems. The overvoltage dependence of the contact conductance has a maximum near the equilibrium potential of the bridge-molecule redox group. The overvoltage dependence of the noise power at zero frequency has a maximum whose position is determined by the bias voltage.     

Detecting Mechanochemical Degradation of Nitrocellulose by Combining Dynamic Mechanical Analysis with Mass Spectrometry

Summary: Dynamic mechanical analysis is combined with mass spectrometry to study nitrocellulose under oscillating strain. At a constant temperature (150–160 °C) and frequency (400–600 Hz) nitrocellulose fractures demonstrating a modulus drop and release of products with m/z: 30 and 44. At linear heating (2 °C · min⁻¹) and a frequency of 10–50 Hz similar products are released in two steps, the second of which demonstrates a modulus drop and a temperature increase indicating ignition.

Data for an isothermal dynamic mechanical analysis–mass spectrometry experiment performed at 160 °C.     

Rubber networks containing unattached macromolecules. II. Viscoelastic properties in small strains of the system ethylene–propylene terpolymer with ethylene–propylene copolymer

Mixtures of crosslinkable ethylene–propylene terpolymer with saturated ethylene–propylene copolymer (molecular weights 3.6, 16.7, and 45 × 10⁴) containing up to 50% by weight of copolymer were crosslinked by sulfur, leaving the saturated copolymer unattached and free to reptate in the terpolymer network. Stress relaxation in small simple elongations (stretch ratio about 1.15) and dynamic Young's modulus at frequencies from 3.5 to 110 Hz were measured at temperatures from 10 to 50°C. Comparison with the properties of the terpolymer crosslinked without added copolymer showed contributions to stress relaxation and mechanical loss attributable to the unattached species. The time required in stress relaxation for the portion of the modulus attributable to the unattached species to decay to half its plateau value, t_1/2, is approximately proportional to the 3.5 power of the molecular weight; t_1/2 appears to be slightly smaller for networks containing 50% than for those containing 25% unattached component.     

Hydrogen diffusivity in the massive LaNi5 electrode using voltammetry technique

The Randles–Sev?ik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi₅ intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 °C), and then voltammetry measurements were carried out at various, very slow potential scan rates (υ?=?0.01–0.1 mV?·?s^?1). At potentials more noble than the equilibrium potential of the H₂O/H₂ system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor α?=?½. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I _a ^(peak) ?=?f(υ ^1/2) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi₅ material after cathodic exposure to be C _0,H?=?0.071 mol?·?cm^?3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D _H?=?2.0?·?10^?9 cm²s^?1. Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H₂O/H₂ system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi₅ alloy are close to 1 mA?·?cm^?2, irrespective of the electrode potential scan rate.     

Effect of Cu content on structure,hydrogen storage properties and electrode performance of LaNi4.1-x Co0.6Mn0.3Cu x alloys

The effect of Cu content on structure, hydrogen storage, and electrochemical properties of LaNi_4.1-x Co_0.6Mn_0.3Cu _x alloys has been investigated. For sample, A, B, C, and D are used to represent alloys (x?=?0, 0.15, 0.3, and 0.45), respectively. The results indicate that the four alloys are all single-phase alloy with LaNi₅ phase of CaCu₅ hexagonal structure, the hydrogen storage capacities of the alloy are about 1.49 wt% (A), 1.48 wt% (B), 1.43 wt% (C), and 1.25 wt% (D) at 303 K. With the increase of Cu content (x) from A to D, hydrogen desorption plateau pressure and pressure hysteresis decrease. Alloy electrode A shows better activation property and higher capacity (334.44 mAh/g). The addition of Cu improves the cyclic stability of the alloy electrodes when x?=?0?~?0.45. However, their self-discharge properties and high-rate dischargeability (HRD) decrease with the increase of x. Further, electrochemical kinetics and electrochemical impedance spectroscopy (EIS) analysis show that the reaction of alloy electrode is controlled by charge transfer step, and the adding of Cu benefits the electrode properties in alkaline solution.     

Electrochemical Determination of Pentachlorophenol Using a Glassy Carbon Electrode Modified with a Film of CuS Nanocomposite-Chitosan

A new electrochemical sensor was fabricated by modifying the glass carbon electrode surface with CuS nanocomposites and chitosan for the determination of pentachlorophenol. CuS nanocomposites obtained by a solvothermal method were composed primarily of CuS with hexagonal phase and Cu₂Cl(OH)₃ with a tetragonal phase. The results indicated that CuS nanocomposites possessed good electrochemical activity. After optimizing the experimental conditions, the linear dependence of current vs. pentachlorophenol concentration was reached in a range from 1.88 × 10^?6–7.50 × 10^?5 mol/L pentachlorophenol, and the detection limit was 6.25 × 10^?7 mol/L. The electrode displayed a high degree of stability and reproducibility. A new, simple, rapid, and highly sensitive electrochemical detection method of pentachlorophenol was established.     

New PVC-membrane electrode based on charge-transfer inclusion complex between I2 and 1,4,8,11-tetraazacyclotetracdecane for selective determination of I3 − ions

A new triiodide ion-selective electrode based on charge-transfer complex of iodine with 1,4,8,11-tetraazacyclotetracdecane as membrane carrier was prepared. The best performance of electrode observed with the membrane having the ligand–PVC–NPOE–NaTPB composition 3.0:33:66:0.5 mg which worked well over a wide concentration range 2.8 × 10^?6–1.4 × 10^?1 M with a Nernstian slope of 59.6 ± 0.3 mV per decade and a detection limit of 1.2 × 10^?6 M. This electrode showed a response time of 25 s and was used over a period of 2 months. The electrode exhibits an anti-Hofmeister selectivity sequence with a performance for triiodide at pH 1.5–10.2. The response mechanism of the electrode was investigated by UV–Vis spectroscopic technique. The electrode was successfully applied as an indicator electrode in the potentiometric titration of ascorbic acid and hydroquinone in drugs.     

A facile nitrogen-doped carbon encapsulation of CoFe2O4 nanocrystalline for enhanced performance of lithium ion battery anodes

Nitrogen-doped (N-doped) carbon encapsulation of CoFe₂O₄ nanocrystalline is achieved by a simple pressure-assisted pyrrole pyrolysis method. The CoFe₂O₄/N-doped carbon nanocomposite (CFO/NC) delivers a capacity of 646.2 mAh g^–1 after 80 cycles at 0.1 C, exhibits stable cycling performance at various rates from 0.2 to 1.6 C and retains a capacity of 662.8 mAh g^–1 as the rate returns back to 0.1 C, showing significantly improved lithium storage reversibility compared to the bare CFO. A different lithiation mechanism of CFO/NC above and below the plateau relative to CFO in the first discharge is analyzed in detail based on the potential profiles and cyclic voltammogram curves. Morphology characterization of the cycled electrodes confirms much better integrity of CFO/NC electrode due to the buffer effect of N-doped carbon coating. Electronic conductivity and electrochemical impedance spectroscopy measurements indicate enhanced electrode reaction kinetics of CFO/NC. All the results contribute to its improved electrochemical performance.