Ac Perturbation Research Articles

Kinetics of Lithium - Ion Transfer at Carbon-Electrolyte Interface in Presence of Conducting Nano-Fillers

Besides highly successful in portable electronic devices, Li-ion batteries are regarded as key technology for power sources for next generation HEVs, PHEVs and EVs. However, batteries in transportation vehicles are required to deliver sufficiently high power with longer lifetime and better safety. Graphitic carbon is the most successful and commonly used candidate for negative electrodes in Li-ion batteries. However, this material is known to have moderate rate capability due to slow kinetics [1, 2]. It has also been proposed that it is not only graphite which limits high rate performance, rather other parameters such as binder amount, electrolyte solvents and conducting salts are also likely responsible [3-5]. The surface charge-transfer kinetics in graphite/electrolyte interface studied by Xu [3] has shown that the activation energy for interfacial Li-ion transfer with LiPF6 in EC:DMC (1:1) as electrolyte was 64 kJ mol-1 which is high compared to solid phase transport [6] and liquid electrolyte transport of Li-ion [7]. This large activation energy is attributed to desolvation of ions on SEI layer [6]. In order to achieve good performance in terms of realizing higher power density, the interface charge transfer kinetic must improve. In this work, we investigate the kinetics of Li-ion transfer for mesocarbon mocrobeads (MCMB) carbon active material with nano-conducting fillers such as carbon black (CB) and multiwalled carbon nanotubes (MWCNTs) in a CR2016 type coin. It has earlier been shown that a synergistic improvement in electrical and electrochemical performance occurs with partial substitution of CB by CNT [8]. In this work, we examine the effect of conductive fillers on charge-transfer resistance, Rct, for various temperatures. A thermal chamber with temperature controller as shown in Figure 1 was designed and fabricated to conduct experiments with reproducible EIS results in a wide temperature range. The working electrodes were fabricated with 88% MCMB, 4% conducting additives (CB, CNT or CB+CNT) and 8% PVDF coated on copper foil. The steps followed for fabricating the electrodes and cells have been described in detail elsewhere [8]. Before each impedance test the cells were cycled at 0.2C-rate for five cycles between 2.0 V to 0.005 V to ensure interfacial film formation, subsequent to which the cells were stabilized at 0.2 V by keeping them at rest for three hours after discharging them with 0.05C-rate to 0.2V. The EIS spectra were measured at OCP of 0.2 V with an AC perturbation of amplitude 5mV in the range 30mHz to 200kHz. The conductivity of interfacial Li-ion transfer (1/Rct) obeys the Arrhenius equation, 1/Rct=A0exp(Ea/RT). The symbols A0, Ea, R and T stand for frequency factor, activation energy, gas constant and absolute temperature. In this study, the activation energy is used to quantify the kinetics of interfacial Li-ion transfer as its value indicates essential kinetics for solid/electrolyte interface process without the effects of temperature, surface area and ion activity. Figure 2 shows the Arrhenius plot of interfacial conductivity (1/Rct) obtained from impedance plot for different conducting fillers at various temperatures. The activation energies evaluated were 56.2 kJmol-1, 53.6 kJmol-1 and 49.6 kJmol-1 for electrodes containing conducting nano-fillers namely CB, CNT and hybrid CB+CNT (1:3), respectively. The lower activation energy or improved kinetic performance of electrode containing CNT nano-fillers is attributed to the ease of ion transfer to active material surface from bulk electrolyte through SEI layer. Further improvement of kinetics for hybrid conducting fillers (CB+CNT) electrodes is attributed to the formation of dense, thin and ionically conductive surface film that can pave the way for rapid migration of Li-ion and faster electron transfer for interfacial reaction.

Versatile Duplex Electrochemical Sensor for the Detection of CO2 and Relative Humidity Using Room Temperature Ionic Liquid

In this work, we present a versatile and a robust electrochemical duplex sensor for detection of CO2 and humidity using a novel sensing material – Room temperature ionic liquid (RTIL). Rising environmental concerns have led to the development of sensors to monitor environmental conditions such as CO2, VOC's, particulate matter, temperature, and relative humidity. Requirement of sensor performance metrics such as low power, high stability, increased sensitivity has led to the investigation of RTIL as a suitable candidate for the CO2 and humidity sensing for usability in a broad range of applications from air quality monitoring in HVAC systems, weather prediction, and monitoring the environment in green houses. RTIL's are essentially solvent free electrolytes consisting of an organic cations and inorganic anions with tunable chemical and physical properties for sensing. The innovation of this study is to interface RTIL's with a metal electrode sensing platform which results in the formation of multi stack electrochemical double layer (EDL). CO2 or water molecules settle within the interstitial spaces formed between the RTIL moieties on the application of a bias potential. AC perturbation of the system causes charge redistribution in the EDL as a result of CO2 or water adsorption which is studied through an AC based technique- Electrochemical impedance spectroscopy (EIS). The frequency response of the EDL provides an insight of the impedance changes of the EDL as the environment changes around them. We investigated the effects of ambient and elevated CO2 concentrations across a temperature range of 25- 65°C at 45% humidity on two fluorinated RTIL’s- EMIM[TF2N] and EMIM[FAP]. Performance comparison was based on the impedance response and dynamic behavior of the system based on the changes in the anionic symmetry. Both, EMIM[TF2N] and EMIM[FAP] behave similarly but EMIM[TF2N] shows a higher sensitivity to CO2 across temperatures. We also report the effect of varying humidity from 25% to 65% across temperature range of 25- 65°C on the performance of a non-fluorinated RTIL- MMIM[MeSO4]. MMIM[MeSO4] shows a larger change in impedance response with increasing humidity percentages at elevated temperatures. RTIL’s have the potential to be integrated with semiconductor technology for the sensitive detection of CO2 concentrations and humidity ranges across varying temperatures. Our work demonstrates the potential of a new strategy for achieving low power environmental sensors.

Effects of halide anions on adsorption and 2D condensation of 5-fluorocytosine at hanging mercury drop electrode

Self-assembled layers, and 2D condensed molecular films as their specific cases, are known to be strongly sensitive to external conditions such as temperature, concentration of self-assembling substance(s) in solution, as well as composition of the supporting electrolyte in terms of the kind and concentration of present ions. In this work we have studied effects of a series of four sodium halides on the 2D condensation of 5-fluorocytosine (5-FC) at the hanging mercury drop electrode. This fluorinated nucleobase was selected for the study with respect to its unique 2D condensation properties (previously described in NaCl), showing remarkable capacitance pits in two potential regions (one around the potential of zero charge and the other within highly negative potential region). NaF electrolyte was not able to support 2D condensation of 5-FC, while in NaI and NaBr the 2D condensation was facilitated in comparison with the NaCl electrolyte. Furthermore, we have studied the formation of the 2D condensed film at different temperatures and ac perturbation frequencies to obtaining a complex view into the 5-FC 2D condensation processes. Our results show great importance of the composition of supporting electrolyte, and/or other experimental conditions on the properties of 2D condensed layers.

Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells

The partial replacement of methylammonium by formamidinium and cesium in organolead trihalide materials is of great importance to improve the performance and stability of photovoltaic solar cells. However, the effect of multiple cations on the cell functioning and their electrical characteristics remains to be clarified. By using the impedance spectroscopy technique, we have investigated the electrical response to a small ac perturbation applied to solar cells implementing hybrid perovskites with various compositions, polarized over a large potential range. The solar cell preparation protocols have been optimized to reach power conversion efficiencies higher than 17%. The impedance response has been investigated both under light and in the dark to discriminate the light sensitive parameters. The spectra have been carefully analyzed using an ad hoc equivalent circuit, and the data have been discussed in the light of the existing literature. The spectra showed no intermediate frequency inductive loop due to...

Assessment of copper surface coverage with corrosion inhibitor using AFM-based local electrical measurements

ABSTRACTThe paper presents a new method of assessment of metal surface coverage with corrosion inhibitor and thus of inhibitor protective performance. It is based on the atomic force microscopy measurement performed in a contact mode. Apart from topography images the proposed approach allows acquisition of local DC maps and local electrical impedance spectra via application of DC bias voltage or AC perturbation signal between the conductive AFM tip and the substrate. Potentialities of this technique in inhibitor performance monitoring were illustrated on the example of copper/benzotriazole system exposed to elevated humidity environment.

The Effect of Cations on Carbon Steel Corrosion in Chloride Media

The corrosion of carbon steel in chloride media has been investigated extensively in literature [1, 2]. However, the role of cations in carbon steel corrosion behavior is not widely reported. In the present work, the effect of Na+, K+ and NH4 +cations on the corrosion of carbon steel in chloride media is investigated via. potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and field emission scanning electron microscopy (FESEM) measurements. All the electrochemical measurements were carried out using a three electrode system, and after open circuit potential reaches a stable value. The carbon steel was used as working electrode. The reference and counter electrode were Ag/AgCl (3M KCl) and platinum wire respectively. The concentration of chloride is 10 wt.% for all the solutions. The polarization curves were obtained by scanning the electrode potential from -250 mV to 500 mV w.r.t OCP at a scan rate of 1 mV/s. The impedance spectra were obtained by applying an AC perturbation signal of 10 mV in the frequency range 100 kHz to 10 mHz. The polarization measurements (Fig. 1) reveals that the corrosion rate is higher in case of ammonium chloride (NH4Cl) solution following the order NH4 +> K+ > Na+. Further, the polarization measurements, in case of chloride solutions containing Na+ and K+ cation shows the formation of passive layer on the carbon steel surface (the regime of insignificant current increase with potential) followed by breakdown of the same (the regime of increase in current) indicating susceptibility to pitting corrosion. While the anodic polarization curve in the presence of NH4 +cation does not show any sign of passivation. This implies that the carbon steel is more prone to uniform corrosion. These results suggest the formation of passive layer on the carbon steel surface and the mechanistic reaction pathway of carbon steel dissolution in chloride media is strongly influenced by the cations present in the solution. The EIS measurements performed at open circuit potential, shows two time constant for all the cations (Fig.2). However, for NaCl and KCl system, the two time constants are merged and appears to be a single capacitive loop. The impedance data is analysed by an appropriate electrical equivalent circuit (EEC) and the EEC parameters are retrieved. Impedance results also confirms that the corrosion rate is higher in NH4Cl solution while lower in NaCl solution. So the presence of cations affect both the corrosion rate and the corrosion mechanism. Keywords: Carbon steel, cations, chloride media, polarization, EIS

The impact of high-frequency-high-current perturbations on film formation at the negative electrode-electrolyte interface

Long term ageing experimental results show that degradation resulting from coupled DC and AC current waveforms lead to additional degradation of lithium-ion batteries above that experienced through pure DC cycling. More profoundly, such experiments show a dependency of battery degradation on the frequency of AC perturbation. This paper addresses the underlying causality of this frequency dependent degradation. Cell autopsy techniques, namely X-ray photoelectron spectroscopy (XPS) of the negative electrode surface film, show growth of surface film components with the superimposition of an AC waveform. XPS results show that high frequency AC perturbations lead to the increased formation of a passivating film. In order to determine the cause of this increased film formation, a heterogeneous electrochemical model for the LiNiCoAlO2/C6 lithium ion battery coupled with governing equations for the electrical double-layer and solid electrolyte interface film growth is developed. Simulation results suggest that the increased growth of surface film is attributed to frequency dependent heat generation. This is due to ion kinetics in the double layer which are governed by the Poisson-Boltzmann equation. Additional thermal and reference cell relaxation experiments are undertaken that further corroborates the conclusion that heat generation within the battery is a function of the AC excitation frequency through resistive dissipation and the entropy of the cell reaction.

Observation of l-cysteine enhanced zinc dissolution during cathodic polarization and its consequences for corrosion rate measurements

The corrosion inhibiting and accelerating effect of l-cysteine on Zn were correlated with the formation of Zn oxides by means of a novel coupling of impedance spectroscopy (EIS) with atomic emission spectroelectrochemistry (AESEC). A plateau of anodic Zn dissolution was discovered in the cathodic branch of the polarization curve, hidden by the large cathodic current. The effect of l-cysteine at all pH values and all concentrations was to increase the plateau current. The effect was most pronounced at high pH suggesting a synergy between hydroxide and l-cysteine. Due to the presence of Zn plateau current, Tafel extrapolation resulted in a wrong estimation of corrosion currents. The contribution of the anodic Zn oxidation in the plateau region to the total current was inversely proportional to the frequency of the AC perturbation.

Kinetics of the charge ordering in magnetite below the Verwey temperature

In this work the kinetics of the charge ordering in magnetite (Fe3O4) below the Verwey transition temperature TV is investigated in time and energy domain. After excitation by a one-second voltage pulse to destruct the charge-ordered state below TV, an alternating current (AC) is used to perturb its recovery process. Upon warming up to above a temperature Tr(< TV) the charge order recovers despite the ac perturbation, because the temperature-dependent relaxation time becomes shorter than the polarity change of the ac. From the frequency dependence of Tr(f), an activation energy of ΔE = 126 meV is extracted. Below Tr the real part of the ac conductance Greal follows the relation Greal ∼ fα with α = 0.98 ± 0.18, suggesting that the charge reordering is driven by correlated hopping transport. The relaxation time τ = 1/f(Tr) of the charge-ordered state is determined for temperatures between 70 and 98 K and is extrapolated to τ(TV) = 1.6 ms, continuously slowing down its dynamics as the temperature is decreased.

Resonant extended states in driven quasiperiodic lattices: Aubry-Andre localization by design

We consider a quasiperiodic Aubry-Andre (AA) model and add a weak time-space-periodic perturbation. The undriven AA model is chosen to be well in the localized regime. The driving term controls the effective number of propagation channels. For a spatial resonance which reduces the reciprocal space dynamics to an effective one-dimensional two-leg ladder, the ac perturbation resonantly couples certain groups of localized eigenstates of the undriven AA model and turns them into extended ones. Slight detuning of the spatial and temporal frequencies off resonance returns these states into localized ones. We analyze the details of the resonant extended eigenstates using Floquet representations. In particular, we find that their size grows linearly with the system size. Initial wave packets overlap with resonant extended eigenstates and lead to ballistic spreading.

Electrodeposited Manganese Oxide for Electrochemical Capacitor: Effect of Substrates on the Capacitive Behavior of Electrodes

Electrochemical capacitor (EC), also named as supercapacitor or ultracapacitor have been known as one of the potential energy storage systems. In recent years, many research focus on two modes of electrochemical capacitor: the pseudocapacitor and the electrochemical double layer capacitor [1]. The types of electrode materials are carbon, transition metal oxides and conducting polymers. Manganese oxides have attracted increasing interest in the field of active electrode material for ECs due to their low cost, abundance, environmental friendly nature and potentially replacing ruthenium oxide in the future [2]. However, these materials exhibit the inherent disadvantage of low conductivity and low specific capacitance. The improvement pursed in manganese oxides mainly concerns electronic conductivity, the specific capacitance and rate capability. Several methods have been developed for this improvement, consisting of nanostrutured material, deposited thin film, Ni or Co doped materials and composite material (with carbon, other metal oxides such FeOx, NiO…) [2-4]. In order to improve its electrochemical properties (cycling stability, charge/discharge capacity), all the factors related to the method synthesis such as stoichiometry, crystal structure and morphology of the active materials need to be wholely investigated. In this work, thin films of manganese oxide were synthesized by electrochemical deposition on three substrates such as: graphite (EMD), modified graphite by two layers of hydrophobic layer and hydrophilic layer (EMD2L) and stainless steel (EMDSS). The initial solution was MnSO4 aqueous solution and the electrodeposition were carried by using Cyclic Voltammetry (CV) and Chronoamperometry (CA).The results shows that the specific capacitances (Csp) of the prepared electrodes were determined by cyclic voltammetry measured in 1 M Na2SO4 at different potential scan rates: 1, 5, 10, 25 and 50 mV/s. The deposited film on graphite substrate exhibited Csp of 250 – 99 F/g. The hydrophilic layer improves the specific capacitance of the modified electrode that was about 405 – 108 F/g. By contrast, thin film on staineless steel shows the Csp value between 258 -102 F/g. The highest value of EMD2L may be attributed to the porous surface of the hydrophilic layer modified electrode providing a larger surface area, enhancing the interface between the aqueous electrolyte and the electrode materials. Electrochemical impedance spectroscopy (EIS) measurements were also carried 100 kHz-0.1 Hz at open circuit potential (OCP) with an AC perturbation of 10 mV. With graphite and modified graphite substrate, EIS shows the charge transfer resistance (Rct) about 52 mΩ and 144 mΩ. With stainless steel, manganese oxide films were deposited by CV in changing time deposition. The EIS spectra of these films shows Rct from 20 to 40 Ω corresponding to the variation of time deposition (5, 7, 9, 11, 13 min, respectively). The Csp of EMD 2L obtained values of 51 F.g-1, 74 F.g-1, 118 F.g-1, 218 F.g-1 with good stability upon cycling. This result can be explained by the hydrophilic property of electrode which improved the ionic transport in aqueous electrolyte. And high porosity of electrodeposited surface increased the conduction pathway of deposited MnO2. Keywords:electrochemical capacitor, EMD, electrocatalyst Acknowledgement: The authors gratefully acknowledge Office of Naval Research Global (ONRG) and VNU-HCM for financial support throught grants N62909-13-1-N235 and HS2013-76-01. References : [1] J. R. Miller, P. Simon, Science, 321 (2008).[2] W. Wei, X. Cui, W. Chen, D. G. Ivey, Chem. Soc. Rev. 40 (2011).[3] P. Simon, Y. Gogotsi, Nature Materials, 7 (2008).[4] A. T. Ha, M. L. P. Le, T. P. T. Nguyen, V. M. Tran, ECS Transactions, 45 (20) (2013).

Nickel Oxide Carbon Nanofiber Composite for Electrochemical Oxidation of Methanol

The electrospinning of polyvinyl polyvinylpyrrolidone (PVP) with a polyaniline (PANI) and graphene sol mixture produced uniform, smooth fibers. These electrospun fibers were stabilized for 2 h at 200 °C and then carbonized at 800 °C for 5 h. Composites were prepared by depositing Ni(OH)2 on the carbon nanofibers (CNFs) and calcining them at different temperatures. The composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of the calcination temperatures on the electrochemical catalytic properties towards methanol oxidation was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The optimum calcination temperature for the CNF/NiO composite that yielded highest oxidation peak currents for the MeOH oxidation reaction was found to be 500 oC. The analysis of the chronoamperometry measurements confirmed a significant increase in the real surface area for the CNF/NiO composite as the calcination temperature increases. The real surface area is increased at calcination temperatures of 400 and 500 oC more than 20 times higher than that calcined at 300 oC. This explains the higher electrocatalytic properties for the CNF/NiO composite as the calcined temperatures increases. The oxidation of MeOH was not found to be a purely diffusion controlled process where the square root of the scan rate followed a linear relationship with the currents of the anodic peaks. The CNF/NiO composite has shown, not only, a good stability as the potential was cycled between a large potential window, but also, the electrocatalytic properties was enhanced. This occurred as a result of the surface activation during the cyclic process. The impedance parameters were calculated from the EIS measurements proved that this system has two time constants; one for adsorption and the other for the charge transfer. Figure . Nyquist plots of the experimental impedance data (scattering dots) measured potentiostatically at 0.5 V (SCE) in a frequency range between 100K and 0.1 Hz with an AC perturbation of 5 mV and the fitting results (solid lines) for CNF-supported Ni oxide composite calcined at 500 oC in (A) 6 M KOH and (B) 6M KOH + 1 M MeOH aqueous solutions at 25 oC. The inset image is the electrical equivalent circuit used for fitting the impedance spectra.

The Influence of AC Perturbations on EIS Measurement of Copper/LDPE Composite with High Resistance

Copper/LDPE composites are novel materials for intrauterine devices and they are the dispersion-type composites with high resistances. When the electrochemical impedance spectroscopy was used to evaluate the properties of materials with high resistances, in order to reduce the fluctuations in the measured data, the AC perturbation should be optimized. In this paper, the effects of the AC perturbations on the electrochemical impedance spectroscopy of copper/LDPE were discussed and the optimal AC perturbation was confirmed. The results have shown that the optimal perturbation of the electrochemical impedance spectroscopy of copper/LDPE composite, in the earlier period was 50mV, after achieving the saturation point the perturbation would change to 5mV.

Enhancing vibration measurements by Mössbauer effect

The measurement of the Mossbauer effect in a system excited with a periodic perturbation can provide information about it. For that purpose, the Mossbauer absorption of a source-absorber set which hyperfine parameters are well known, is measured at a constant relative velocity (i.e. at a defined spectral energy). The resulting Mossbauer absorption periodic signal provides information of the sample ac perturbation response. This approach has been used time ago to measure small tympanic vibrations (mechanical perturbations). In this work we present an extension of the vibration experiments, by measuring them at various absorber-source relative velocities within a constant-velocity strategy. As a demonstration test, the frequency response of a piezoelectric diaphragm in the 100 Hz–5 kHz range is obtained with a custom electronic counter. The experiments are performed using a 57Co(Rh) source and a 25-μm-thick stainless-steel absorber fixed to a piezoelectric diaphragm. Phase shifts and amplitude vibrations with velocities in the range from 1.5 μm/s to 20 mm/s are well characterized, extending the linearity limit well beyond the earlier suggested one of 1 mm/s.

Effects of the Junction Temperature on the Dynamic Resistance of White LEDs

This paper deals with the thermal characteristics of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curve of GaN-based white light-emitting diodes (LEDs), focused on the variations of the dynamic resistance. The final goal of this study is to improve the static and dynamic operations of the LED driver within a wide range of temperatures. Four LEDs from different manufacturers were chosen for this study. The first part of this paper shows the thermal characterization of the forward voltage at a given injected current. After that, the experimental data are fitted in order to calculate the junction temperature accurately. Then, a small-signal analysis where the LEDs are supplied with dc current and an ac perturbation superimposed at the operation point under variable junction temperature is covered. This analysis allows the dynamic resistance to be experimentally determined for a wide junction temperature range. Furthermore, the experimental data have been fitted in order to establish the relationship between junction temperature and dynamic-resistance variation, so the dynamic resistance can be determined for a given operation point. Finally, an illustrative example is presented as a case study in order to analyze the implications of the dynamic resistance on the output-current ripple and on the closed-loop operation of a LED driver. The experimental results confirm that the junction temperature shift induces a variation in the dynamic resistance, which might have a significant effect on the output-current ripple and closed-loop performance in certain LED fixtures.

Application of the Kramers Kronig relations to locally resolved impedance data of polymer electrolyte fuel cells

The Kramers Kronig (KK) relations are applied to locally resolved impedance data as obtained from a simple, analytic ‘down the channel’ impedance model of a polymer electrolyte fuel cell (PEFC) air cathode. It is shown that the transport and superposition of oxygen concentration oscillations in gas flow direction can lead to non-causal local cell impedance spectra, while the calculated local cell admittance always fulfills the KK relations. This is explained by the homogeneous distribution of the ac perturbation voltage over the electrode area due to the high electrical conductivity of the bipolar plates. Hence, the KK relations must be applied to the locally resolved admittance data of PEFCs to check if the measured spectra can be a physical response of the system.

Theoretical Analysis of the Two-Electron Transfer Reaction and Experimental Studies with Surface-Confined CytochromecPeroxidase Using Large-Amplitude Fourier Transformed AC Voltammetry

A detailed analysis of the cooperative two-electron transfer of surface-confined cytochrome c peroxidase (CcP) in contact with pH 6.0 phosphate buffer solution has been undertaken. This investigation is prompted by the prospect of achieving a richer understanding of this biologically important system via the employment of kinetically sensitive, but background devoid, higher harmonic components available in the large-amplitude Fourier transform ac voltammetric method. Data obtained from the conventional dc cyclic voltammetric method are also provided for comparison. Theoretical considerations based on both ac and dc approaches are presented for cases where reversible or quasi-reversible cooperative two-electron transfer involves variation in the separation of their reversible potentials, including potential inversion (as described previously for solution phase studies), and reversibility of the electrode processes. Comparison is also made with respect to the case of a simultaneous two-electron transfer process that is unlikely to occur in the physiological situation. Theoretical analysis confirms that the ac higher harmonic components provide greater sensitivity to the various mechanistic nuances that can arise in two-electron surface-confined processes. Experimentally, the ac perturbation with amplitude and frequency of 200 mV and 3.88 Hz, respectively, was employed to detect the electron transfer when CcP is confined to the surface of a graphite electrode. Simulations based on cooperative two-electron transfer with the employment of reversible potentials of 0.745 ± 0.010 V, heterogeneous electron transfer rate constants of between 3 and 10 s(-1) and charge transfer coefficients of 0.5 for both processes fitted experimental data for the fifth to eighth ac harmonics. Imperfections in theory-experiment comparison are consistent with kinetic and thermodynamic dispersion and other nonidealities not included in the theory used to model the voltammetry of surface-confined CcP.

(Invited) Investigation of Early Degradation Processes at Coated Metals by AC-Scanning Electrochemical Microscopy

Alternating-current scanning electrochemical microscopy (AC-SECM) is employed to characterize the early stages of the degradation reactions occurring in metal-coating systems upon exposure to an aqueous environment. The spatial resolution of the technique results from the measurement of changes in the resistance of the thin electrolyte layer comprised between the tip and the surface of the coating due to variations in the tip-substrate distance. Resistance measurements are conducted at various frequencies of the AC perturbation signal effectively allowing topographic changes to be monitored as a function of time. Furthermore, AC-SECM can be used to determine the tip-substrate distance without the addition of redox mediators to the electrolyte, which might affect the chemical properties of the system. In this way, the effect of chloride ions from the aqueous phase to induce either the heterogeneous absorption of water by the coating, or its accumulation at the metal-substrate interface, has been imaged.

The Effect of Direct Axis Current on the Performance of the Synthetic Loading Technique

Synthetic loading is a technique that forces the machine under test to operate at rated speed, torque, and flux without the need for the test machine to be connected to an external load. The technique has been successfully applied to permanent magnet machines but it requires an inverter voltampere (VA) rating that is 1.51 pu of the rated value due to the need for instantaneous phase currents and voltages that are higher than the rated value. In order to reduce the VA rating, it is possible to inject direct axis current. This has important consequences. It reduces the ac perturbation current thereby reducing the peak instantaneous phase current. Also, by reducing the ac perturbation current, the rotor speed variation is reduced, thereby reducing the instantaneous phase voltage requirement. Both these effects reduce the VA rating of the inverter. However, this paper demonstrates through simulation and experiment that such a scheme increases the iron losses in the machine. This is important as it alters the distribution of losses which is not a desired consequence. The effect of direct axis current on the iron loss and the machine efficiency during synthetic loading is assessed and evaluated as is the effect on the minimum dc link voltage and the inverter VA requirements used to conduct the synthetic loading technique. Simulation and experimental results are used to demonstrate the effects and validate the conclusions that while direct axis current injection reduces the inverter VA rating, it reduces the accuracy of the synthetic loading technique.

A variable series resistance mechanism to explain the negative capacitance observed in impedance spectroscopy measurements of nanostructured solar cells

A simple model for conductivity modulation in a recombination diode is discussed to explain the observation of negative capacitance at forward bias observed in many optoelectronic devices. We formulate the basic recombination-modulation model in terms of an equivalent circuit for small ac perturbation and discuss the application of the results in measurements of impedance spectroscopy of dye-sensitized solar cells, organic solar cells and related systems.