The impedance of alkaline manganese cells and their relationship to cell performance. II. Correlation with short-circuit current and initial pulse discharge behaviour

The theory relating the short-circuit or flash current behaviour of alkaline manganese and Leclanché cells to their internal resistance values has been reviewed. It is shown that the relationship SCC=VOC/R _i pertains for both cell types where SCC is the short-circuit current, VOC the voltage at open circuit andR _i the internal cell resistance which is a composite of several components. In the case of alkaline manganese cells these can be independently resolved usingin situ impedance measurements into three major components: the electrolyte resistance within the anode-separator-cathode porous matrices; the resistance of the cathode (MnO₂ + graphite mixture); and the resistance of the nickel oxide layer on the surface of the nickel-plated steel positive current collector (cell container). In the case of Leclanché cells three components also control the internal resistance, but these cannot be so easily resolved. They are: the electrolyte resistance within the cathode separator matrices; the resistance of the cathode (MnO₂ + carbon); and contact resistance between the cathode and positive current collector (carbon rod). Equivalent circuits for both alkaline manganese and Leclanché cells are proposed.Galvanostatic 2-A pulse discharge measurements have been made on LR20 alkaline manganese cells and directly correlated with the impedance measurements, thus providing confirmatory evidence for the equivalent circuit proposed. It is shown that the resistance calculated from the potential drop at 10 ms correlates closely with the internal resistance and hence short-circuit current value. It is also shown that ohmic polarization at long times (10 s) constitutes 67% of the total potential loss within the cell. Hence for a typical repetitive 2 A/10 s pulse discharge regime, the discharge life is critically dependent on the cell internal resistance value.     

In situ electrochemical scan to study the behavior of the asymmetric (single-side) pasted positive plate as used in automotive lead-acid batteries

The performance of a asymmetric (single-side) pasted positive plate was studied by measuring the current density and potential distributions on both sides of the plate. It was found that the current densities on both sides are almost the same during the early discharge at the 3 C discharge rate. As the discharge proceeds, the current decreases gradually on the non-pasted side and increases on the pasted side. Since more active mass exists on the pasted side, the matrix resistance is higher, causing the polarization to increase by 55–90 mV at the 3 C discharge rate. On the non-pasted side, the active mass undergoes a deeper depth of discharge (DOD). This leads to the shedding of active mass, which shortens the battery life. Since the positive grid on the non-pasted side is exposed, its corrosion rate is faster.     

Determination of transport and electrochemical kinetic parameters of M-H electrodes

Electrochemical and transport properties of La_0.65Ce_0.35Ni_3.55Co_0.75Mn_0.4Al_0.3 electrode were investigated in alkaline solution. The exchange current density, polarization resistance and the symmetry factor were determined from polarization curves obtained at low overpotentials. The symmetry factor was estimated to be 0.55 ± 0.01 and is independent of the state of charge. The equilibrium potential of the electrode was found to depend upon the hydrogen content in the alloy. The constant current discharge technique was used to determine the hydrogen diffusion coefficient in the alloy. The estimated value of /a ² at 0.1 C discharge rate was 1.39 × 10^–4s^–1.     

The impedance of alkaline manganese cells and their relationship to cell performance. III. Correlation with high-rate pulse discharge capacity

The extent to which the initial impedance characteristics of a batch of LR6 alkaline manganese cells determine their life and therefore capacity during a typical 2 A/10 s pulse discharge regime has been investigated, and the importance of thermodynamic factors have also been considered. It is shown that the potential drop (E-V _pulse) for the initial discharge cycle can be calculated approximately from a knowledge of the initial internal resistance value, and the recovery voltage,V _rec, can be calculated using a simple thermodynamic theory for the homogeneous phase discharge of -MnO₂. During subsequent cycles the polarization of the cathode-can assembly remains approximately constant at 300 mV while that of the anode-separator system increases progressively from 100 mV to >300 mV. The constancy of the former parameter can be attributed to constancy in the cathode contribution to the internal resistance, whereas the changes in the latter can be ascribed to increases in anode resistance polarization and anode concentration polarization. Minimization of cell internal resistance and anode polarization are therefore of primary concern if cell performance is to be maximized.Nomenclature E initial open-circuit voltage - V _pulse cell voltage att=10 s - V _pulse cell voltage att=10 s for the first pulse - V _rec open-circuit voltage at the end of a 50-s recovery period - V total polarization of the cell - V _A anode polarization (anode-separator system) - V _C cathode polarization (cathode-can assembly) - ohmic polarization - _NT charge-transfer polarization - _C concentration polarization - R _i cell internal resistance - R _e electrolyte resistance - R _part ^cath contact resistance between cathode particles or within the particles themselves - R ^cath effective resistance of cathode-can assembly - R _i ^cath contact resistance at the interface between the nickel oxide phase and the cathode (MnO₂ + graphite mixture) - R _phase ^cath resistance of the nickel oxide phase on the surface of the nickel-plated steel positive current collector (cell can) - R ₂ ^cath contact resistance at the interface between the nickel oxide layer on the can surface and the can itself - R high frequency intercept on complex plane impedance diagram - R diameter of the complex plane impedance semicircle - f ^* characteristic frequency at the top of the complex plane semicircle - C effective parallel capacitance in the equivalent circuit for a cell attributed to the cathode-can assembly - c _MnO2 concentration of MnO₂ at any point in the discharge - cMnO ₂ ⁰ maximum MnO₂ concentration at 100% efficiency - c _MnOOH concentration of MnOOH at any point in the discharge - c _MnOOH ⁰ maximum MnOOH concentration at 100% efficiency - proton-electron spatial correlation coefficient - I total current - i _R current through resistanceR - i _c current through capacitor - V _p voltage drop across parallel R-C circuit - A anode - C cathode - obs observed - calc calculated     

Multidimensional Modelling of Polymer Electrolyte Fuel Cells under a Current Density Boundary Condition

A three‐dimensional numerical model of the polymer electrolyte fuel cell (PEFC) is applied to study current distribution and cell performance under a current density boundary condition. Since the electronic resistance in the along‐channel direction in the current collector plate is much larger than in the other two directions, i.e., 50 mΩ cm² vs. 0.5 mΩ cm², it significantly affects current flow, and current and cell voltage distributions in a PEFC. An identical polarization curve results with two different boundary conditions, constant cell voltage and constant current density, however, the current density profiles in the along‐channel direction differ significantly; it is much flatter for the constant current boundary condition. Increasing the electronic conductivity of the bipolar plate diminishes the difference in the current density distribution under the two boundary conditions. The results also point out that an experimental validation of a PEFC model based on the polarization curve alone is insufficient, and that detailed current density distribution data in the along‐channel direction is essential.     

Effects of phase transition on discharge properties of PLZST antiferroelectric ceramics

The effects of electric field‐induced phase transition on discharge properties of Pb_0.94La_0.04[(Zr_0.52Sn_0.48)_0.84Ti_0.16]O₃ antiferroelectric (AFE) ceramics were investigated. Due to the forward phase transition, high polarization and energy density are achieved. The backward phase transition results in nonlinear increase of current in underdamped circuit. The stored charge (14.2 μC under 40 kV/cm at 22°C) can be released completely in very short duration due to the low remanent polarization. With increasing temperature, the polarization and releasable energy decline. However, the current amplitude reaches maximum at 40°C, which is attributed to the backward phase transition. The maximum current and power density are as high as 143.8 A/cm² and 2.4 MW/cm³, which indicates the potential of the ceramics for pulsed capacitors.     

Hypochlorite electro-generation. I. A parametric study of a parallel plate electrode cell

A parametric study is described of a parallel plate Ti/PbO₂/x mol dm^–3 NaCl/Ti hypochlorite cell, for which the cell voltage, current efficiency, and energy yield (mol ClO^– kWh^–1) were examined as functions of current density, chloride concentration, and electrolyte flow rate, inlet temperature and pH.The cell was found to behave ohmically, with current efficiencies of 85–99% for 0.5 mol dm^–3 NaCl electrolyte, a typical chloride concentration for sea water. However, the hypochlorite energy decreased substantially with increased current density, reflecting the large contribution of the electrolyte ohmic potential drop to the cell voltage.The behaviour of the Ti/PbO₂ anode was found to be irreproducible, and low temperature (say 278K)/high current density operation was irreversibly detrimental both in terms of the anode potential/cell voltage and current efficiency.Nomenclature b polarization resistance (ohm m²) - d _min interelectrode spacing to minimize the cell voltage (m) - f(x) volume fraction of gas at levelx f - _av average volume fraction of gas - F Faraday constant (96487 C mol^–1) - h electrode length/height (m) - i(x) current density at positionx (A m^–2) - i _av average current density (A m^–2) - I cell current (A) - P pressure of gas evolved at electrodes (N m^–2) - R universal gas constant (8.314 J mol^–1K^–1 ) - R _eff total ohmic resistance of electrolyte and gas in cell (ohm) - s bubble rise rate (m s^–1) - chloride ion transport number - T electrolyte temperature (K) - w electrode width (m) - x distance from bottom of electrodes (m) - z number of Faradays per mole of gas evolved - (x) overpotential at positionx (V) - resistivity of gas free electrolyte (ohm m) - (x) resistivity at levelx of electrolyte containing bubbles (ohm m)     

Negative plate discharge in lead acid batteries. Part I: General analysis, utilization and energetic coefficients

The process of negative plate discharge in lead acid batteries from two manufacturers has been investigated at low current densities. The discharge curves and specific capacities, at several H₂SO₄ concentrations (2.3–7.0 M) and current densities (0.5–3.2 mA cm^–2), showed that the negative plate discharge includes solid state reactions. When these results were put beside those previously obtained for positive plates, they showed that the processes of positive and negative plate discharges are different and an explanatory theory has been proposed. By analysing the utilization coefficient together with the specific capacities, the different roles of the nucleation processes at the positive and negative plates were revealed. Finally, the determination of the energetic coefficient proved to be useful in evaluating the quality of manufacture of the active material.     

Corrosion behaviour of Ti-6Al-4V in phosphoric acid

Ti-6A1-4V shows distinct active-passive behaviour in phosphoric acid over a wide concentration range. The cathodic polarization curves are similar over a wide range of acid concentration and temperatures. The alloy undergoes active dissolution and turns passive in the negative potential region followed by a wide range of passivity at all acid concentrations at different temperatures. Increasing acid concentration up to 11 M results in an increase in critical current density (i _cr). The passive current density (i _p) increases up to an acid concentration of 9 M while at 13 M i _cr and i _p decrease appreciably. A significant increase in both i _cr and i _p occurs with increase in solution temperature. The passive specimen remained stable for a long time when exposed to phosphoric acid under open circuit conditions.     

Corrosion protection of stainless steel by a new and low-cost organic coating obtained from cashew nutshell liquid

In this work, the corrosion protection of 316L steel was promoted by an electro-synthesized polymer obtained from the technical cashew nutshell liquid (t-CNSL). Spectroscopic techniques confirmed the polymer formation. The polymer was dispersed in the ethyl acetate solvent and used to form coatings on 316L steel substrates. The coated samples were subjected to electrochemical tests in a saline environment. The coated electrode with poly(t-CNSL) polymer was exposed to the corrosive medium for 24 days, and superior corrosion protection was observed compared with the uncoated sample. The open circuit potential measurements showed that the coated sample possessed a more positive corrosion potential when compared with the uncoated substrate. The electrochemical impedance spectroscopy results indicated that the coated electrode's polarization resistance (R_p) recorded ~1.0 MΩ cm² after 24 days of exposure. A decrease in polarization resistance was observed with the exposure time due to the presence of micropores in the t-CNSL coating. The polarization curves exhibited that the coated electrode with poly(t-CNSL) has lower corrosion current density and less negative corrosion potential than the uncoated steel electrode. Therefore, t-CNSL favors the manufacture of thin poly(t-CNSL) coatings for corrosion protection purposes besides being a low-cost material.     

Study on surface modification of AB5-type alloy electrode with polyaniline by electroless deposition

A new polyaniline (PANI)-coated technique was adopted for a AB₅-type alloy (La_0.64Ce_0.25Pr_0.03Nd_0.08Ni_4.19Mn_0.31Co_0.42Al_0.23) in order to improve its electrochemical and kinetic properties. FE-SEM observation and FT-IR analysis results revealed that the PANI electroless deposited to the surface of alloy particles. Through the PANI-coating the initial discharge capacity increased from 299 to 331 mAh/g and the high rate discharge ability (HRD) increased from 8.5 to 45.0% at discharge current density of 1440 mA/g. For kinetic properties, linear polarization, EIS, anodic polarization and cyclic voltammetry measurements suggested that charge-transfer resistance decreased and the hydrogen absorption rate of the alloys increased after PANI-coating.     

Significance of logarithmic throwing index: a theoretical approach

An expression for the metal distribution ratio in electroplating systems as a function of the primary current density ratioL in the formM=L [W(1–r)/(1+K)] is derived.W,r andK are three dimensionless parameters related to the current efficiency ratio, the concentration polarization and activation polarization during the metal discharge. The function [W(1–r)/1+K] is compared with 1/A, the logarithmic throwing index empirically determined by Chin. The metal distribution ratio calculated by the use of the above formula is compared with the experimentally observed values. The close agreement between the two within an accuracy of 10% proves the validity of the equation derived. The logarithmic throwing power of electroplating systems is thus confirmed on theoretical grounds.Nomenclature A Logarithmic Throwing Index —inverse of the slope of the plot of logM versus logL - b Tafel slope. Slope of the equation =a + b logi - d_n Current efficiency in percent for metal deposition at near cathode - d _f Current efficiency in percent for metal deposition at far cathode - E The overall cell potential - E _n The potential drop in the electrolyte between the anode and near cathode - E _f The potential drop in the electrolyte between the anode and far cathode - e _a Dynamic anode potential - e _n Dynamic potential at the near cathode at a current densityi _n - e _f Dynamic potential at the far cathode at a current densityi _n - f a fraction = - i The average current density (A dm^–2) - i _n The primary current density at the near cathode when there is no polarization(A dm^–2) - i _f The primary current density at the far cathode when there is no polarization(A dm^–2) - i _n The secondary current density at the near cathode (A dm^–2) - i _f The secondary current density at the far cathode (A dm^–2) - i _{H n} The partial cathode current density at the near cathode for parallel cathodic reactions other than metal discharge (A dm^–2) - i _{H f} The partial cathode current density at the far cathode for parallel cathodic reactions other than metal discharge (A dm^–2) - i _{M n} The partial cathode current density for metal discharge at the near cathode - i _{M f} The partial cathode current density for metal discharge at the far cathode - K A dimensionless parameter =b/2.3E _f - l Linear Ratio =l _f/l _n ori _n i _f - l _n Linear distance of the near cathode (cm) - l _f Linear distance of the far cathode (cm) - M Metal distribution ratio - m _n Weight of metal deposited on the near cathode - m _f Weight of metal deposited on the far cathode - R Secondary current distribution ratio=i _n/i_f - r A dimensionless parameter related toK andf and given byf=(1/L) ^r/K - W A dimensionless parameter related current efficiency ratioR ^W–1 =d _n/d_f - Specific resistivity of the electrolyte( cm^–2) - _n The overpotential at the near cathode (V) - _f The overpotential at the far cathode - i ₀ The exchange current density     

Spontaneous passivation observations during scale formation on mild steel in CO2 brines

Previous study revealed localized corrosion in CO₂ environments was driven by a galvanic cell established between pit surfaces and scaled surrounding area. In order to underpin the understanding of the galvanic mechanism of localized corrosion, the root cause of potential differences between these two surfaces, passivation of mild steel, in CO₂ environments was investigated using transmission electron microscopy technique and electrochemical techniques including potentiodynamic polarization, cyclic polarization and open circuit potential techniques. Potentiodynamic polarization experiments showed that the passivation of the carbon steel surface favorably occurred at pH > 7 and facilitated with the presence of FeCO₃ scale. Cyclic polarization tests showed that polarization rate had an important influence on passivation behavior. At a slower polarization rate, lower passivation potential and current density were observed. Spontaneous passivation was evidenced by a significant increase of corrosion resistance and an open circuit potential without any externally applied current or potential during electrode immersion. This process is affected by pH, temperature, presence of CO₂ and iron carbonate. Nevertheless, iron carbonate film is not the only one responsible for passivation, as demonstrated from depassivation tests where passivity was lost without losing the existing iron carbonate film. Transmission electron microscopy technique was used to determine the structure of the passive layer. An extra phase, most likely magnetite, was observed to be beneath the iron carbonate scale and at the crystal grain boundaries which passivated the mild steel.     

Study of electrochemical hydrogen charge/discharge properties of FePO4 for application as negative electrodes in hydrogen batteries

We report the electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ as active material in KOH electrolyte. Crystalline and amorphous FePO₄ were synthesized by an alcohol-assisted precipitation method, and the powders obtained were characterized by X-ray diffraction. X-ray photoelectron spectroscopy is used to investigate the mechanism of hydrogen charge/discharge behavior of FePO₄. The electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ were investigated for potential application as negative electrodes in rechargeable hydrogen batteries. In galvanostatic discharge/charge mode at 25 °C, the crystalline FePO₄ showed a maximum discharge capacity of 109 mA h g⁻¹, while the amorphous FePO₄ showed a maximum discharge capacity of 81.4 mA h g⁻¹. The electrochemical kinetic properties, exchange current density, and proton diffusivity were calculated using linear polarization measurement and the potential-step method.     

Structure and electrical properties of sputtered TiO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript> bilayer composite dielectrics upon annealing in nitrogen

The high-k dielectric TiO₂/ZrO₂ bilayer composite film was prepared on a Si substrate by radio frequency magnetron sputtering and post annealing in N₂ at various temperatures in the range of 573 K to 973 K. Transmission electron microscopy observation revealed that the bilayer film fully mixed together and had good interfacial property at 773 K. Metal-oxide-semiconductor capacitors with high-k gate dielectric TiO₂/ZrO₂/p-Si were fabricated using Pt as the top gate electrode and as the bottom side electrode. The largest property permittivity of 46.1 and a very low leakage current density of 3.35 × 10^-5 A/cm² were achieved for the sample of TiO₂/ZrO₂/Si after annealing at 773 K.     

Performance degradation studies on PBI/H3PO4 high temperature PEMFC and one-dimensional numerical analysis

Five hundred hours continuous aging test at constant discharge current (640 mA cm⁻²) was performed on PBI/H₃PO₄ high temperature PEMFC unit cell, electrochemical techniques-linear sweep voltammetry (LSV) and AC impedance measurement were used to investigate the changes of electrochemical surface area (ESA) and high frequency resistance (internal resistance) with time. Initial experimental results showed that during 500 h continuous aging test the main reason for cell performance degradation is the decrease of ESA caused by sintering. In addition, a one-dimensional mathematical model was constructed, the concentration distributions of cathode reactant gases (O₂ and gaseous H₂O) were calculated and polarization curves recorded during aging test were simulated based on the model, the simulated polarization curves compare well with the experimental results.     

Cathodic modification as a means of improving the corrosion resistance of alloys

Cathodic modification is defined and its origin is discussed. The criteria necessary for cathodic modification are listed for the base alloy as well as the cathodic alloying component, and the principle of cathodic modification is treated in detail. The mechanism of cathodic modification is also described extensively. Various mechanisms that account for the redistribution of alloying elements on the corroded surface are compared and discussed, and the most likely one is pointed out. It is concluded that cathodic modification is a very powerful electrochemical means of improving the corrosion resistance of alloys, particularly of stainless steels and titanium-based alloys in non-oxidizing acid media.Nomenclature i _p Passive current density - i _cr Critical current density - i _co Corrosion current density - i _cath Current density of cathodic process - i _tr Transpassive current density - E _co Corrosion potential - E _p Passivation potential - E _O ^A Anodic potential of base metal or alloy - E _O ^C Potential of cathodic component - E _tr Transpassive potential     

Anodic oxidation of tellurium in NaOH solutions

The potential of tellurium anodes has been measured as a function of current density in 0.01-3 N NaOH at 30°C. The results indicate the oxidation of tellurium into tellurous acid. The initial discharge of hydroxyl ions governs the overall reaction rate within the concentration range 0.01-1 N. In concentrations above 1 N, the dual chemical-electrochemical formation of TeO is suggested to be rate-determining in the low current density range. At higher currents, hydroxyl-ion discharge again controls the reaction rate.

Accumulation of tellurous acid on the electrode surface at high anodic polarization leads to the interference of resistance overpotential, whose magnitude in dilute solutions is so large that the potential rapidly approaches 40 V. At such a potential, tellurium is probably oxidized to the trioxide.     

Optimization of pulsed electrodeposition of aluminum from AlCl3-1-ethyl-3-methylimidazolium chloride ionic liquid

In this study, Al was electrodeposited on a platinum substrate at room temperature from an ionic liquid bath of EMIC containing AlCl₃ using potentiostatic polarization (PP), galvanostatic polarization (GP), monopolar current pulse polarization (MCP) and bipolar current pulse polarization (BCP). Transition of current or potential during galvanostatic or pulse polarization revealed that the initial stage of the deposition process was controlled by a nucleation process depending on the polarization condition. For example, the average size of Al deposits decreased with increasing current density in the case of GP. FE-SEM observation showed that dense and compact Al deposits with a smooth surface were obtained by the current pulse method. Roughness factor evaluated from electrochemical impedance measurement confirmed the smooth surface of these deposits. Adhesion strength of Al deposits was greatly improved using BCP in which an anodic pulse was combined with a cathodic pulse for electrodeposition. In this study, the optimal parameters for BCP were found to be I_C = −16.0 mA cm⁻², I_A = 1.0 mA cm⁻², r_C (duty ratio) = 0.5, and f = 2 Hz. The mechanisms of electrodeposition by these three methods are discussed.     

Double-layer capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na₃AlF₆–Al₂O_3(sat) melts at 1010° C. The double-layer capacitance (C _dl) was used as an indicator of the wetted surface area. For unpolarized anodes,C _dl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheC _dl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenC _dl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm^–2, 0.20–0.44 V per decade and 0.1–0.5 A cm^–2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theC _dl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.