Studies of the pulse charge of lead-acid batteries for PV applications: Part I. Factors influencing the mechanism of the pulse charge of the positive plate

The mechanism of the positive plate charge in pulse regime was studied in model lead-acid cells with one positive and two negative plates (8 Ah each) and Ag/Ag₂SO₄ reference electrodes. The results showed that the evolution of the electrode potential is much slower on the positive plate than on the negative plate. Regardless of this fact, the calculated capacitive current of charge and self-discharge of the electrochemical double layer (EDL) during the “ON” and “OFF” half-periods of the pulse current square waves is comparable with the charge current amplitude. The result is due to the high values of the EDL on the surface of the lead dioxide active material. The influence of different factors like state of charge, state of health, pulse frequency, current amplitude and open circuit stay before the polarization was discussed. The previously determined optimal frequency of 1 Hz was associated with a maximum in the average double layer current on frequency dependence. The average double layer current is also maximal at SOC between 75 and 100%. The exchange of the constant current polarization with pulse polarization does not change substantially the mechanism and the overvoltage of the oxygen evolution reaction on the positive plate. The mechanism of the self-discharge of the EDL was also estimated analyzing long-time PPP transients (up to 2 h). It was found that when the PPP is lower than 1.2 V the preferred mechanism of EDL self-discharge is by coupling with the lead sulphate oxidation reaction. At higher values of PPP the EDL self-discharge happens via oxygen evolution. The high faradic efficiency of the pulse charge is due to the chemical oxidation of the Pb(II) ions by the O atoms and OH radicals formed at the oxygen evolution both during the “ON” and “OFF” periods.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part II. Impedance of the positive plate revisited

In the second part of this publication series, dedicated to the pulse charge of the lead-acid battery, a special attention is paid to the impedance spectrum of the positive plate as a source for estimation of the electrostatic capacitance of the double layer (C_dl) on the surface of the positive active mass. The impedance spectra were measured at open circuit for different states of charge (SoC) in H₂SO₄ with specific gravity 1.24 and 1.28 g ml⁻¹. A substantial difference was observed in the impedance spectra of partially charged and partially discharged positive plates keeping the same value of the SOC. The impedance data were subjected to inductance error correction, followed by differential impedance analysis (DIA). Considering the results from DIA, the recently published equivalent circuits of the positive plate in charged and in discharged state and the gel-crystal model of the lead dioxide, we proposed a model of the positive plate in partial state of charge (PSoC). The analysis of the obtained experimental results using this model and DIA show that the double layer capacitance is not frequency distributed. The influence of the state of charge and state of health on the model parameters is discussed. One of the most interesting results is the dependence of C_dl on SOC—it features a hysteresis at which the values of C_dl during the charge are 5–6 times higher than the corresponding ones during the discharge. This result was discussed in terms of changes in the double layer structure considering the gel-crystal model of the lead dioxide. During the discharge in H₂SO₄ with specific gravity 1.28 g ml⁻¹ a passivation process was detected as a high frequency pseudo-inductive loop in the Nyquist plots in PSoC. The passivation time constant is higher at 50–60% SOC and decreases to zero in the end of the discharge. During the charge in both electrolytes, pseudo-inductive time constant was observed too. It was attributed to the phenomena of the dehydration of Pb(OH)₄, an intermediate in the reaction scheme of the PbSO₄ oxidation. The state of health influences mostly the ohmic resistance R_Ω, the charge transfer resistance R_ct and the parameters of the constant phase element accounting the diffusion in the pores (CPE_diff), when the plate is well charged.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part III. Electrolyte concentration effects on the electrochemical performance of the positive plate

In the third part of this work the effects of the sulphuric acid concentration on the positive plate discharge capacity, impedance and oxygen overvoltage are discussed. It has been found that the full discharge capacity of the positive plate is available down to electrolyte concentrations of 3 mol l⁻¹ (s.g. 1.18 g ml⁻¹). At further acid dilution, capacity of the positive plate declines, keeping the utilization of the sulphuric acid about 50%. Decreasing the acid concentration, the oxygen overvoltage decreases with a factor of 12–18 mV M⁻¹, excluding the effect of the equilibrium potential of the oxygen electrode as a function of pH. The capacitance of the electrical double layer decrease linearly with the dilution of the sulphuric acid suggesting strong adsorption effects. This suggestion has been confirmed from the measurements of potential of the zero charge of the positive plate, which increases from 1.11 to 1.34 V vs. Ag/Ag₂SO₄ in the region 1.11–4.60 M H₂SO₄. From the measurement of the time constant of the electronic transfer through the gel part of the lead dioxide (T_gel) as a function of the acid concentration and the applied potential, a change in the mechanism of the lead dioxide hydration has been estimated—below 1 M H₂SO₄T_gel increases sharply, showing sharp increases of the extent of the hydration. The dilution of the electrolyte increases substantially the value of average double layer current in the beginning of the charge. During the pulse overcharge at the employed frequency of 1 Hz, the average double layer current is equal to the pulse amplitude, suggesting that the maximal efficiency of the pulse charge is reached.     

Influence of carbon microstructure on the Li–O2 battery first‐discharge kinetics

Defects in the carbon microstructure have been reported to enhance the discharge performance of Li–O₂ battery. However, systematic studies correlating the presence of defects with the discharge kinetics have not addressed the variation of carbon electrode surface areas. In this work, carbon blacks and carbon nanofibers with different defect densities were investigated for their discharge properties. The electrolyte‐accessible areas of the carbon electrodes were obtained from Cyclic voltammetry measurements. The microstructure and surface areas of the carbons were characterized by Raman spectroscopy, electron microscopy, and N₂ isotherm. Linear sweep voltammetry and galvanostatic discharge experiments consistently demonstrated that graphitic carbons have more negative onset potentials and more negative discharge potentials at the same current density than defective carbons. The linear sweep voltammetry data were normalized to the carbon masses, Brunauer–Emmet–Teller surface areas, and double layer capacitance‐derived areas for comparison. Plot of inverse charge transfer resistance and double layer capacitance from electrochemical impedance spectroscopy measurements were used to extract current density values without knowledge of electrode areas. The current densities from impedance measurements exhibited good agreement with the data from linear sweep experiments. The electrochemical experiments conclusively showed that defects on the graphitic microstructure increase the discharge kinetics of the Li–O₂ battery. Copyright © 2016 John Wiley & Sons, Ltd.     

Development of a method to estimate the lifespan of proton exchange membrane fuel cell using electrochemical impedance spectroscopy

This paper proposes a new method for estimating the state and lifespan of fuel cells in operation by fuel cell equivalent impedance modeling by electrochemical impedance spectroscopy (EIS) and observing degradation. The performance change of fuel cells takes place in the form of changes in each parameter value comprising an equivalent AC impedance circuit; monitoring such changes allows for the prediction of the state and lifespan of a fuel cell. In the experiments, the AC impedance of high-temperature proton exchange membrane (PEM) fuel cells was measured at constant time intervals during their continuous operation for over 2200 h. The expression for the lifespan of a fuel cell was deduced by curve fitting the changes in each parameter to a polynomial. Electric double layer capacitance and charge transfer resistance, which show the reduction reaction of the cathode, were used as major parameters for judging the degradation; a method of using time constants is proposed to more accurately estimate the degree of degradation. In addition, an algorithm that can evaluate the soundness and lifespan of a fuel cell is proposed; it compares the measured time constant of the fuel cell being tested with that of average lifespan fuel cell.     

柴油机交流变频起动振荡充电的过渡过程分析

用交流同步主发电机作为柴油机的起动电动机实现柴油机起动是一种切实可行的交流起动方案。分析变频起动的振荡充电过程,找出电压及电流的变化规律,指出充电电路中电阻、电容、电感对充电过程及柴油机起动过程的影响。     

Investigation of porous silicon thin films for electrochemical hydrogen storage

In this study, nanoporous silicon (PS) layers have been elaborated and used for hydrogen storage. The effect of the thickness, porosity and specific surface area of porous silicon on the amount of hydrogen chemically bound to the nanoporous silicon structures is studied by Infrared spectroscopy (FTIR), cyclic voltammetry (CV), contact angle and capacitance –voltage measurements. The electrochemical characterization and hydrogen storage were carried out in a three-electrode cell, using sulfuric acid 3 M H₂SO₄ as electrolyte by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. The results indicate the presence of two oxidation peaks at 0.2 V and 0.4 V on the anodic side corresponding to hydrogen desorption and a reduction peak at −0.2 V on the cathodic side corresponding to the sorption of hydrogen. Moreover, the EIS studies performed on PS electrode in 3 M H₂SO₄ show that the hodograph contains a semicircle at high frequency region and a line in the lower frequency zone. An equivalent circuit has been proposed; the values of the equivalent circuit elements corresponding to the experimental impedance spectra have been determined and discussed. Finally, the highest hydrogen storage in PS was 86 mAh/g. This storage capacity decreases by only 7% of the initial capacity value, after 40 cycles.     

Hydrogen evolution activity of Ni-Mo coating electrodeposited under magnetic field control

Enhancement of catalytic activity of Ni-Mo alloy for hydrogen evolution reaction (HER) has been observed when the alloy is electrodeposited under magnetic field, B control. Using cyclic voltammetry at low scan rate and electrochemical impedance spectroscopy (EIS) measurements, the significant modification of the activity of the alloy has been characterized in acidic and in basic water at 25 °C and in 33% NaOH solution at 85 °C. For the deposit obtained with B, the polarization curves show high current densities values (about 50% higher then without B), low ohmic drop and overpotential shift to noble way more then 100 mV. EIS measurements show a low polarization resistance and a high double layer capacitance for the alloy deposited under magnetic field control. In basic media, with or without B, the EIS measurements exhibit a supplementary inductive type relaxation.     

Catalyst degradation diagnostics of proton exchange membrane fuel cells using electrochemical impedance spectroscopy

A previously validated equivalent circuit model, in which two resonant circuits were inserted to represent the processes in the catalyst layers, is applied to fit the electrochemical impedance spectroscopy results of a single proton exchange membrane fuel cell exposed to accelerated stress test targeting catalyst degradation. The simulation results of the applied equivalent circuit model show very good agreement with the experimental data. The applied model is able to extract contributions of each of the model elements to the cell degradation. The obtained results indicate that the cathode catalyst layer resonant loop parameters, together with the cathode charge transfer resistance and cathode double-layer capacitance, change the most during the accelerated stress test. If each of the elements of the cathode resonant loop can be associated with physical processes inside the catalyst layer, the model may be used to give more insight into the degradation effects on functioning of the catalyst layer. From the conducted electrochemical impedance spectroscopy analysis, it seems that the low-frequency intercept in Nyquist plot shows the most significant change with degradation, so it may be used directly as a sufficient indicator of fuel cell performance degradation due to catalyst layer degradation.     

Study of molybdenum electrodes for hydrogen evolution reaction

The molybdenum electrode, Mo, has been investigated for hydrogen production via water electrolysis in 10 vol.% aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) using electrochemical impedance spectroscopy (EIS). The EIS measurements show that the Mo system has much higher interfacial capacitance, and correspondently the electrical double layer formed on this electrode is thicker than those formed on nickel or platinum. The positive displacement of potential of zero charge (PZC) values indicates the specific adsorption of the imidazolium cation on the Mo surface. This study provides an elegant explanation for the better performance of Mo electrodes in the hydrogen evolution reaction (HER): the BMI cation acts as an intermediate for the proton transfer from water to the electrode surface, thereby decreasing the overpotential of HER. This model explains the synergism between Mo and the BMI cation in the HER process.     

Investigation of ceramic Fe2O3〈Ta〉 photoelectrodes for solar energy photoelectrochemical converters

Photocurrent and electrochemical impedance spectroscopy of a polycrystalline semiconductor photoelectrode, Fe₂O₃〈Ta〉, was carried out. The analysis of the frequency dispersion of the real and imaginary parts of the complex impedance allowed us to obtain an equivalent circuit for the electrochemical cell. The capacitance of the space-charge layer in the semiconductor electrode was isolated, and the limiting step of the electrode process was determined. Measurements of the temperature dependencies of the electroconductivity were used to determine the activation energy for the mobility of the charge carriers. A pair of simultaneously illuminated n-Fe₂O₃〈Ta〉 and p-Cu₂O photoelectrodes were shown to split water spontaneously.     

A nickel—cadmium cell residual charge analyser

This paper describes a portable unit for measuring the charge remaining in nickel—cadmium secondary cells. Exhaustive frequency response tests have confirmed that cell impedance varies very little with charge state, with the possible exception of that at very low frequencies (< 50 mHz). In the interim before further work in this area is carried out, a microprocessor-based test unit has been built which uses a current pulse discharge method to arrive at a residual charge reading. When the cell is discharged according to a particular regime, the unit produces results accurate to within 10 – 15% over the entire range of charge. Further development involving the inclusion of cell history parameters promises to make the unit useful for military and other applications.     

Measurement of the maximum charge and discharge powers of a nickel/metal hydride battery for hybrid electric vehicles

The determination of the maximum acceptable charge power and power output is of special significance in the development of hybrid electric vehicles. Theoretically, the maximum acceptable charge power and the power output can be defined as those relating to the maximum current levels before the occurrence of any side reaction. A new method has been developed to measure these maximum currents for nickel/metal hydride batteries used in hybrid electric vehicles. The method involves three step: (i) measurement of the transient voltage vs. current relation during charge or discharge by a sequence of pulse currents; (ii) calculation of the overall battery internal impedance at different times and current magnitudes; (iii) determination of the maximum current from the minimum point of the internal impedance. This method is based on the principle that, with increasing current level, mass transport becomes the rate-limiting step. Any extra increase in current can only cause the occurrence of a side reaction which will result in an increase in the battery internal impedance. The maximum current can thus be determined by the minimum internal impedance from a plot of this parameter against current. Experimental results show that the maximum current strongly depends on battery state-of-charge and also, battery structure. Increase in the surface area of the battery plates is an efficient way to increase the charge-acceptance and power output of the battery, and also to reduce the internal impedance.     

An EIS alternative for impedance measurement of a high temperature PEM fuel cell stack based on current pulse injection

In this paper a method for estimating the fuel cell impedance is presented, namely the current pulse injection (CPI) method, which is well suited for online implementation. This method estimates the fuel cell impedance and unlike electrochemical impedance spectroscopy (EIS), it is simple to implement at a low cost. This makes it appealing as a characterization method for on-line diagnostic algorithms. In this work a parameter estimation method for estimation of equivalent electrical circuit (EEC) parameters, which is suited for on-line use is proposed. Tests on a 10 cell high temperature PEM fuel cell show that the method yields consistent results in estimating EEC parameters for different current pulse at different current loads, with a low variance. A comparison with EIS shows that despite its simplicity the response of CPI can reproduce well the impedance response of the high and intermediate frequencies.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

Impedance characterization of high temperature proton exchange membrane fuel cell stack under the influence of carbon monoxide and methanol vapor

This work presents a comprehensive mapping of electrochemical impedance measurements under the influence of CO and methanol vapor contamination of the anode gas in a high temperature proton exchange membrane fuel cell, at varying load current. Electrical equivalent circuit model parameters based on experimental evaluation of electrochemical impedance spectroscopy measurements were used to quantify the changes caused by different contamination levels. The changes are generally in good agreement with what is found in the literature. It is shown that an increased level of CO contamination resulted in an increase in the high frequency and intermediate frequency impedances. When adding CO and methanol to the anode gas, the low frequency part of the impedance spectrum is especially affected at high load currents, which is clearly seen as a result of the high load current resolution used in this work. The negative effects of methanol vapor are found to be more pronounced on the series resistance. When CO and methanol vapor are both present in anode gas, the entire frequency spectrum and thereby all the equivalent circuit model parameters are affected. It is also shown that the trends of contamination effects are similar for all the test cases, namely, CO alone, methanol alone and a mix of the two, suggesting that effects of methanol may include oxidation into CO on the catalyst layer.     

Equivalent circuit components of nickel-metal hydride battery at different states of charge

Equations that describe the voltage variations with time of rechargeable batteries during charging and discharging were used to determine the component values of the equivalent circuit of nickel-metal hydride batteries under different states of charge (SOC). The equivalent circuit of the battery was described as an ideal voltage source in series with a resistor and the parallel combination of a resistor and a capacitor. The battery model used different values of resistance and capacitance, in the parallel combination, during the different phases of the discharge-rest-charge-rest sequence. The results show that the resistances in the equivalent circuit are approximately constant with variations in the SOC. For the discharge and charge phases the capacitor value increased and decreased, respectively, as the SOC decreased. The value of the capacitor in the parallel RC circuit is an indicator of the battery SOC.     

Equivalent circuit components of nickel-cadmium battery at different states of charge

Equations that describe the voltage variations with time of a rechargeable battery during charging and discharging were used to determine the component values of the equivalent circuit of nickel-cadmium batteries under different states of charge (SOC). The equivalent circuit of the battery was described as an ideal voltage source in series with a resistor and the parallel combination of a resistor and a capacitor. The battery model used different values of resistance and capacitance, in the parallel combination, during the different phases of the discharge-rest-charge-rest sequence. The results show that the series resistance is approximately constant with variations in the SOC while the resistor in the parallel RC circuit increases as the SOC decreases. For the discharge and charge phases the capacitor value increased and decreased, respectively, as the SOC decreased. The value of the resistor or capacitor in the parallel RC circuit is an indicator of the battery SOC.