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 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.     

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.     

Estimation of the kinetic parameters of processes at the negative plate of lead-acid batteries by impedance studies

Impedance characteristics of the negative electrode of lead-acid battery were derived on the basis of fundamental interfacial processes occurring at the electrode. The solution of the governing equations was presented in terms of a simple equivalent circuit consisting of resistive and capacitive loops in which charge transfer and sulfate layer formation and also mass transfer (Warburg) elements are considered. The kinetic parameters were deduced by fitting the theoretically derived impedance to the experimental data. Impedance at various States of Charge (SoC) was also examined.     

Carbon honeycomb grids for advanced lead-acid batteries. Part I: Proof of concept

The carbon honeycomb grid is proposed as innovative solution for high energy density lead acid battery. The proof of concept is demonstrated, developing grids suitable for the small capacity, scale of valve-regulated lead acid batteries with 2.5-3 Ah plates. The manufacturing of the grids, includes fast, known and simple processes which can be rescaled for mass production with a minimum, investment costs. The most critical process of green composite carbonisation by heating in inert, atmosphere from 200 to 1000 °C takes about 5 h, guaranteeing the low cost of the grids. An AGM-VRLA, cell with prototype positive plate based on the lead-2% tin electroplated carbon honeycomb grid and, conventional negative plates is cycled demonstrating 191 deep cycles. The impedance spectroscopy, measurements indicate the grid performance remains acceptable despite the evolution of the corrosion, processes during the cycling.     

Influence of carbons on the structure of the negative active material of lead-acid batteries and on battery performance

It has been established that addition of carbon additives to the lead negative active material (NAM) of lead-acid batteries increase battery charge acceptance in hybrid electric vehicle mode of operation. The present work studies three types of activated carbons and two types of carbon blacks with the aim to evaluate their efficiency in improving the charge acceptance of lead-acid batteries. It has been established that the size of carbon particles and their affinity to lead are essential. If carbon particles are of nanosizes, they are incorporated into the bulk of the skeleton branches of NAM and may thus increase the latter's ohmic resistance. Their content in NAM should not exceed 0.2-0.5 wt.%. At this loading level, carbon grains are adsorbed only on the surface of NAM contributing to the increase of its specific surface area and thus improving its charge acceptance. When carbon particles are of micron sizes and have high affinity to lead, they are integrated into the skeleton structure of NAM as a structural component and act as super-capacitors, i.e. electric charges are concentrated in them and then the current is distributed along the adjacent branches of the lead skeleton with the lowest ohmic resistance. This eventually improves the charge acceptance of the negative battery plates.     

Influence of expander components on the processes at the negative plates of lead-acid cells on high-rate partial-state-of-charge cycling. Part II. Effect of carbon additives on the processes of charge and discharge of negative plates

Lead-acid batteries operated in the high-rate partial-state-of-charge (HRPSoC) duty rapidly lose capacity on cycling, because of sulfation of the negative plates. As the battery operates from a partially discharged state, the small PbSO₄ crystals dissolve and precipitate onto the bigger crystals. The latter have low solubility and hence PbSO₄ accumulates progressively in the negative plates causing capacity loss. In order to suppress this process, the rate of the charge process should be increased.In a previous publication of ours we have established that reduction of Pb²⁺ ions to Pb may proceed on the surface of both Pb and carbon black particles. Hence, the reversibility of the charge-discharge processes improves, which leads to improved cycle life performance of the batteries in the HRPSoC mode. However, not all carbon forms accelerate the charge processes. The present paper discusses the electrochemical properties of two groups of carbon blacks: Printex and active carbons. The influence of Vaniseprse A and BaSO₄ (the other two components of the expander added to the negative plates) on the reversibility of the charge-discharge processes on the negative plates is also considered. It has been established that lignosulfonates are adsorbed onto the lead surface and retard charging of the battery. BaSO₄ has the opposite effect, which improves the reversibility of the processes on cycling and hence prolongs battery life in the HRPSoC duty. It has been established that the cycle life of lead-acid cells depends on the type of carbon black or active carbon added to the negative plates. When the carbon particles are of nano-sizes (<180 nm), the HRPSoC cycle life is between 10,000 and 20,000 cycles. Lignosulfonates suppress this beneficial effect of carbon black and activated carbon additives to about 10,000 cycles. Cells with active carbons have the longest cycle life when they contain also BaSO₄ but no lignosulfonate. A summary of the effects of the three expander components on the elementary processes during charge of negative lead-acid battery plates is presented at the end of the paper.     

Ageing behaviour of electrochemical double layer capacitors: Part II. Lifetime simulation model for dynamic applications

Based on the results of the experimental study in Part I [1], a holistic simulation model that combines electrical and thermal simulation of electrochemical double-layer capacitor (EDLC) modules with an ageing model is presented. This simulation model allows analysing self-accelerating degradation effects caused by elevated voltages and temperatures. Furthermore, the divergence of cell performance in a stack of cells can be investigated which makes the model a valuable tool for cell and stack design as well as for testing operating strategies and cooling systems.     

Ageing behaviour of electrochemical double layer capacitors: Part I. Experimental study and ageing model

Different types of commercially available electrochemical double layer capacitors (EDLCs) were analysed in accelerated ageing tests by impedance spectroscopy. From these measurements the parameters of an impedance model were determined. The characteristic change of the impedance parameters is discussed and an ageing model for EDLCs is developed.     

Influence of expander components on the processes at the negative plates of lead-acid cells on high-rate partial-state-of-charge cycling. Part I: Effect of lignosulfonates and BaSO4 on the processes of charge and discharge of negative plates

This study investigates the influence of the organic expander component (Vanisperse A) and of BaSO₄ on the performance of negative lead-acid battery plates on high-rate partial-state-of-charge (HRPSoC) cycling. Batteries operating in the HRPSoC mode should be classified as a separate type of lead-acid batteries. Hence, the additives to the negative plates should differ from the conventional expander composition. It has been established that lignosulfonates are adsorbed onto the lead surface and thus impede the charge processes, which results in impaired reversibility of the charge-discharge processes and hence shorter cycle life on HRPSoC operation, limited by sulfation of the negative plates. BaSO₄ exerts the opposite effect: it improves the reversibility of the processes in the HRPSoC mode and hence prolongs the cycle life of the cells. The most pronounced effect of BaSO₄ has been registered when it is added in concentration of 1.0 wt.% versus the leady oxide (LO) used for paste preparation. It has also been established that BaSO₄ lowers the overpotential of PbSO₄ nucleation. The results of the present investigation indicate that BaSO₄ affects also the crystallization process of Pb during cell charging. Thus, BaSO₄ eventually improves the performance characteristics of lead-acid cells on HRPSoC cycling.     

A novel flow battery—A lead-acid battery based on an electrolyte with soluble lead(II): Part VI. Studies of the lead dioxide positive electrode

The structure of thick lead dioxide deposits (approximately 1 mm) formed in conditions likely to be met at the positive electrode during the charge/discharge cycling of a soluble lead-acid flow battery is examined. Compact and well adherent layers are possible with current densities >100 mA cm⁻² in electrolytes containing 0.1–1.5 M lead(II) and methanesulfonic acid concentrations in the range 0–2.4 M; the solutions also contained 5 mM hexadecyltrimethylammonium cation, C₁₆H₃₃(CH₃)₃N⁺. From the viewpoint of the layer properties, the limitation is stress within the deposit leading to cracking and lifting away from the substrate; the stress appears highest at high acid concentration and high current density. There are, however, other factors limiting the maximum current density for lead dioxide deposition, namely oxygen evolution and the overpotential associated with the deposition of lead dioxide. A strategy for operating the soluble lead-acid flow battery is proposed.     

A novel flow battery—A lead-acid battery based on an electrolyte with soluble lead(II): V. Studies of the lead negative electrode

The structure of lead deposits (approximately 1 mm thick) formed in conditions likely to be met at the negative electrode during the charge/discharge cycling of a soluble lead-acid flow battery is examined. The quality of the lead deposit could be improved by appropriate additives and the preferred additive was shown to be the hexadecyltrimethylammonium cation, C₁₆H₃₃(CH₃)₃N⁺, at a concentration of 5 mM. In the presence of this additive, thick layers with acceptable uniformity could be formed over a range of current densities (20–80 mA cm⁻²) and solution compositions. While electrolyte compositions with lead(II) concentrations in the range 0.1–1.5 M and methanesulfonic acid concentrations in the range 0–2.4 M have been investigated, the best quality deposits are formed at lower concentrations of both species. Surprisingly, the acid concentration was more important than the lead(II) concentration; hence a possible initial electrolyte composition is 1.2 M Pb(II) + 5 mM C₁₆H₃₃(CH₃)₃N⁺ without added acid.     

Advances in Fe(VI) charge storage: Part II. Reversible alkaline super-iron batteries and nonaqueous super-iron batteries

Reversible thin film Fe(VI/III) cathodic charge/discharge storage in alkaline batteries is presented. Whereas ultra-thin (e.g., 3 nm) Fe(VI/III) films exhibit a high degree of reversibility, thicker films are increasingly passive toward the Fe(VI) charge transfer. An extended conductive matrix facilitates a 100-fold enhancement in charge storage for reversible Fe(VI/III) super-iron thin films. The thicker (100s of nanometers) films deposited on extended conductive matrixes composed of high-surface-area Pt, Ti, and Au can sustain high reversibility, which provides the possibility of using Fe(VI) salts as the cathode materials for rechargeable Fe(VI)/metal hydride batteries. Super-iron cathodes can also be discharged in conjunction with a Li anode in nonaqueous media. Optimization of the nonaqueous primary super-iron/Li batteries is summarized. Fe(VI) cathodes are also reversible in nonaqueous electrolyte systems. The charge/discharge process of super-iron cathodes in nonaqueous media involves both the lithiation/delithiation of the active mass and the reduction/oxidation of the Fe(VI/III), while only the thin film Fe(VI/III) electrodes can sustain high reversibility involving the full theoretical capacity in the nonaqueous batteries.     

Lithium insertion in graphite from ternary ionic liquid-lithium salt electrolytes: I. Electrochemical characterization of the electrolytes

In this paper we report the results of chemical-physical investigation performed on ternary room temperature ionic liquid-lithium salt mixtures as electrolytes for lithium-ion battery systems. The ternary electrolytes were made by mixing N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl) imide (PYR₁₃FSI) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR₁₄TFSI) ionic liquids with lithium hexafluorophosphate (LiPF₆) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The mixtures were developed based on preliminary results on the cyclability of graphite electrodes in the IL-LiX binary electrolytes. The results clearly show the beneficial synergic effect of the two ionic liquids on the electrochemical properties of the mixtures.     

Fluid dynamics performance of different bipolar plates: Part II. Flow through the diffusion layer

After studying the velocity and pressure fields inside bipolar plates with different geometries, the analysis has been extended to include the gas passage across the backing or gas diffusion layer (GDL). The gas flow emerging from the porous layer has been monitored using acetone vapor laser-induced fluorescence. The different configurations tested are a parallel commercial case, a set of parallel diagonal channels, a branching cascade-type, and a serpentine distribution of parallel channel blocks. The experimental results have been compared with the predictions obtained from a computational numerical simulation. This study has served to determine the most suitable topology among the tested ones, and has also revealed that knowing the velocity map inside the bipolar plate may not be sufficient to decide if the gas distribution over the catalyst is going to be homogeneous and if a fuel cell is going to operate in an efficient way.     

Review of the micro-tubular solid oxide fuel cell: Part I. Stack design issues and research activities

Fuel cells are devices that convert chemical energy in hydrogen enriched fuels into electricity electrochemically. Micro-tubular solid oxide fuel cells (MT-SOFCs), the type pioneered by K. Kendall in the early 1990s, are a variety of SOFCs that are on the scale of millimetres compared to their much larger SOFC relatives that are typically on the scale of tens of centimetres. The main advantage of the MT-SOFC, over its larger predecessor, is that it is smaller in size and is more suitable for rapid start up. This may allow the SOFC to be used in devices such as auxiliary power units, automotive power supplies, mobile electricity generators and battery re-chargers.The following paper is Part I of a two part series. Part I will introduce the reader to the MT-SOFC stack and its applications, indicating who is researching what in this field and also specifically investigate the design issues related to multi-cell reactor systems called stacks. Part II will review in detail the combinations of materials and methods used to produce the electrodes and electrolytes of MT-SOFC's. Also the role of modelling and validation techniques used in the design and improvement of the electrodes and electrolytes will be investigated. A broad range of scientific and engineering disciplines are involved in a stack design. Scientific and engineering content has been discussed in the areas of thermal-self-sustainability and efficiency, sealing technologies, manifold design, electrical connections and cell performance optimisation.     

Short stack modeling of degradation in solid oxide fuel cells: Part II. Sensitivity and interaction analysis

In the first part of this two-paper series, we presented a numerical model of the impedance behaviour of a solid oxide fuel cell (SOFC) aimed at simulating the change in the impedance spectrum induced by contact degradation at the interconnect-electrode, and at the electrode–electrolyte interfaces. The purpose of that investigation was to develop a non-invasive diagnostic technique to identify degradation modes in situ. In the present paper, we appraise the predictive capabilities of the proposed method in terms of its robustness to uncertainties in the input parameters, many of which are very difficult to measure independently. We applied this technique to the degradation modes simulated in Part I, in addition to anode sulfur poisoning. Electrode delamination showed the highest robustness to input parameter variations, followed by interconnect oxidation and interconnect detachment. The most sensitive degradation mode was sulfur poisoning, due to strong parameter interactions. In addition, we simulate several simultaneous two-degradation-mode scenarios, assessing the method's capabilities and limitations for the prediction of electrochemical behaviour of SOFC's undergoing multiple simultaneous degradation modes.     

Simulation of methane conversion to syngas in a membrane reactor. Part II: Model predictions

A one-dimensional non-isothermal model was employed in the simulation of partial oxidation of methane to syngas in a dense oxygen permeation membrane reactor. The model predicts that if methane is consumed completely in the reactor, a temperature runaway occurs. The reactor inlet temperature is chosen as a major factor to demonstrate the correlativity of the reactor performance and this phenomenon. A borderline inlet temperature (BIT) is defined. Simulation results showed that when the reactor inlet temperature approaches this value, an optimized reactor performance is achieved. This temperature increases with the increase of the air flow rate and carbon space velocity. The surface exchange kinetics at the oxygen-rich side has a small effect on this temperature, while that at the oxygen-lean side has a significant effect.     

Microstructure changes in the catalyst layers of PEM fuel cells induced by load cycling: Part I. Mechanical model

To achieve a better understanding of the degradation phenomena of polymer electrolyte membrane fuel cells (PEMFCs), it is imperative to understand the mechanism of microstructure changes in the catalyst layer. To this end, a rate-dependent isotropic plasticity model with temperature- and humidity-dependent material properties is proposed to describe the viscoplasticity of the catalyst layer components. To understand the mechanism of such changes caused by the cycling of start-up and shutdown during the operation of a PEMFC, the material model, combined with the cohesive zone model and the contact model, is solved using the finite element method. The cohesive zone model and the frictional contact model are used to describe the evolution of interfaces between the protonic and the electronic conducting phases. Numerical simulation, based on the representation of the microstructure in the catalyst layers, shows that there is competition between crack initiations in the bulk material of the protonic phase and delamination between different phases. This competition plays an important role in microstructure changes in the catalyst layers. The reduction of connectivity between the protonic and the electronic conducting phases, which may explain the decrease of performance after certain duty cycles, could be contributed to by cracking or delamination.