Electrode Kinetic Parameters Research Articles

Effect of gelling on the electrode kinetics of the Pb/PbSO 4 and hydrogen-electrode reactions in maintenance-free lead/acid batteries

The electrode kinetics of the Pb/PbSO 4 and the hydrogen-evolution reactions on porous lead electrodes in sulfuric acid has been studied using a quantitative analysis of galvanostatic transients in the linear polarization domain. Significant changes are observed in the kinetics with the addition of thixotropic agents (e.g., sodium silicate) that are often employed in the manufacture of maintenance-free lead/acid batteries. The effects of these thixotropic agents on the charge-transfer and ohmic components of the Pb/PbSO 4 reaction and charge-transfer components of hydrogen-electrode reaction are analysed with the help of a ‘pore-plugging’ model. This study allows a non-destructive determination of the electrode kinetic parameters and the internal resistance contribution from pore-plugging. This is useful when designing maintenance-free lead/acid batteries.

Digital simulation of galvanostatic current-potential data for gas-diffusion electrodes and estimation of electrode-kinetic parameters

A computerized non-linear-least-squares regression procedure to analyse the galvanostatic current-potential data for kinetically hindered reactions on porous gas-diffusion electrodes is reported. The simulated data fit well with the corresponding measured values. The analytical estimates of electrode-kinetic parameters and uncompensated resistance are found to be in good agreement with their respective values obtained from Tafel plots and the current-interrupter method. The procedure circumvents the need to collect the data in the limiting-current region where the polarization values are usually prone to errors. The polarization data for two typical cases, namely, methanol oxidation on a carbon-supported platinum-tin electrode and oxygen reduction on a Nafion-coated platinized carbon electrode are successfully analysed.

Investigation of the influence of residual uncompensated resistance and incomplete charging current correction on the calculation of electrode kinetics when global and convolution analysis methods are used

The global analysis and convolution potential sweep voltammetry (CPSV) procedures for the analysis of electrode kinetic parameters are compared under the influence of residual uncompensated resistance and incomplete charging current correction. An initial examination of simulated data containing these known artifacts indicates that both procedures are similarly susceptible to the effects of uncompensated resistance. CPSV may be unaffected by the effects of incomplete charging current correction, whereas the global analysis procedure does experience some inaccuracies and a combination of both procedures is recommended in this circumstance. The effect of unequal diffusion coefficients is also examined.

Study of hydrogen evolution on selected PTFE-bonded porous electrodes

The hydrogen evolution reaction was studied in 1M NaOH solution at 70°C on various PTFE-bonded electrode materials. Using steady-state galvanostatic measurements and a.c. impedance spectroscopy, the electrode properties and kinetic parameters were determined. It was found that the a.c. behavior could be described by a constant phase element model and that the hydrogen evolution reaction proceeds via the Volmer-Heyrovsky mechanism. No catalytic effect of PTFE was found. The observed changes in performance could be explained by changes in the roughness of the electrode materials.

Effect of sputtered film of platinum on low platinum loading electrodes on electrode kinetics of oxygen reduction in proton exchange membrane fuel cells

Localization of Pt electrocatalyst by sputter deposition of a thin film (500 Å) on the front surface of a fuel cell electrode containing a supported electrocatalyst (20% Pt/C, 0.4 mg cm −2 loading) has been known to exhibit higher fuel cell performance as compared to that on the electrode without the sputtered film. This study compares the electrode kinetic parameters, electrochemically active surface areas, activation energies and reaction orders for the oxygen reduction reaction (ORR) in the sputtered and unsputtered electrodes in proton exchange membrane fuel cells as functions of temperature and pressure. Comparison of the cell performance at 5 atm and 95°C indicates an almost 4 fold improvement in ORR activity at 0.9 V vs. rhe and a similar 3.6 fold improvement in the exchange current densities. The increment in the electrochemically active surface area was about two fold, thereby indicating that factors beyond a surface area increment were responsible for the observed activity enhancement in the ORR. Evaluation of ORR electrode kinetics as a function of temperature indicated a lower activation energy for ORR on the sputtered electrode, as compared to that on the unsputtered electrode. The reaction orders for ORR were, however, the same for both electrodes and were similar to previously obtained values at the Pt microelectrode/Nafion interface, thereby indicating no change in the rate determining step for ORR. This paper also presents the morphological characterization of the electrode/membrane interface using the SEM/EDAX technique, which clearly signifies the two types of Pt—unsupported and supported.

Electrode kinetics of oxygen reduction at carbon-supported and unsupported platinum microcrystallite/Nafion® interfaces

The electrode kinetics of oxygen reduction at the platinum microcrystallite/Nafion® interface was investigated as a function of temperature and pressure. These studies were conducted using porous gas-diffusion electrodes, containing unsupported platinum (10 Mg cm −2) or carbon-supported platinum (0.4 mg cm −2), in proton exchange membrane fuel cells, using H 2 and O 2 as reactants. The effects of platinum loading and of Nafion impregnation on the electrode kinetic parameters were elucidated. Over the range of current densities from 1 to 1000 mA cm −2 (geometric), the Tafel slope was ca. −60 mV per decade and was practically independent of temperature, O 2 pressure, and type of electrode. The platinum electrode was still in the potential regime where oxygenated species are present on the surface (Temkin conditions). The reaction order with respect to O 2 is unity and the activation energy for O 2-reduction was ca. 80 kJ mol −1. These electrode kinetic parameters are in good agreement with those obtained at the platinum microelectrode/Nafion interface, as previously determined in our laboratories.

Indirect Electrochemical Processes at a Rotating Disk Electrode: Catalytic Alkaline Cyanide Oxidation

Rotating‐disk‐electrode polarization curves for the oxidation of cyanide on a copper oxide film at moderately alkaline pH have been fit by a reaction‐plane model of the fast homogeneous reaction between cyanogen and hydroxide. The results indicate that when the ratio of bulk hydroxide concentration to that of bulk cyanide is low, the main electrode reaction product is cyanogen. This cyanogen is transported away from the electrode until it encounters hydroxide and reacts very quickly to form cyanate and cyanide. Electrode kinetic parameters for the heterogeneous cyanogen formation reaction on the copper oxide film have been estimated.

Redox measurements in aqueous solutions — A theoretical approach to data interpretation, based on electrode kinetics

Redox electrodes are commonly used to measure redox potentials in complex natural systems. In this paper, an approach based on electrode kinetics to interpreting measured redox potentials is presented. Redox couples dissolved in aqueous solutions are classified as sensor effective redox couples (SERC's), which exchange electrons with the electrode surface at a significant rate, and sensor ineffective redox couples (SIRC's), which do not control the potential of the electrode. Going through the corresponding mathematical formalism, we arrive at a Butler-Volmer-type equation which states that a quantitative interpretation of redox potentials is not possible without understanding the nature of SERC's and their electrode kinetic parameters. A sensitivity analysis of effects of these unknown parameters on the redox potential is presented. Implications for interpretations of potential readings in natural samples are discussed. Finally, outlooks for necessary future experimental studies are given.

Parameter Sensitivity and Optimization Predictions of a Hydrogen/Oxygen Alkaline Fuel Cell Model

A mathematical model is used to predict parameter sensitivities and optimal design parameters for a hydrogen/oxygen alkaline fuel cell. A sensitivity analysis of the various transport and electrode kinetic parameters indicates which parameters have the most influence on the predicted current density and over which range of potentials these parameters affect the fuel‐cell performance the most. This information can be used to decide which parameters should be optimized or determined more accurately through further modeling or experimental studies. The effect of various design parameters on the limiting current density are investigated to determine if optimal values exist for the parameters. The optimal electrode thicknesses for the anode and cathode reaction layers and the gas‐ and liquid‐phase porosity in the cathode reaction layer are determined by maximizing the power density. These parameter sensitivities and optimal design parameters can help in the development of better three‐phase electrodes and separators for the alkaline fuel cell.

Electrochemical behaviour of vanadium(V)/vanadium(IV) redox couple at graphite electrodes

The electrochemical behaviour of the V(V)/V(IV) couple has been studied at a graphite disc electrode in sulfuric acid using both cyclic and rotating-disc voltammetry. The results from the latter technique have revealed that the cathodic and anodic characteristics of this redox couple are quite different. The diffusion coefficient for V(IV), 2.14×10−6 cm2 s−1, is independent of the vanadium concentration. For V(IV) oxidation, the electrode kinetic parameters i0 and α have values of 2.47×10−4 A cm−2 and 0.71, respectively. The exchange current density, i0, for the V(V)/V(IV) reaction has been obtained at both graphite felt and reticulated vitreous carbon electrodes.

A novel approach for estimating the electrode kinetic parameters of gas-diffusion electrodes using the inflection point in steady-state current/potential data

A method is proposed for deriving electrode kinetic parameters (i.e., Tafel slopes, exchange-current densities and limiting currents) using the inflection point in steady-state current/potential data, without reaching the limiting-current region. The procedure is applied to the oxygen reduction reaction on a Nafion-bound platinized carbon electrode, a CoTMPP (cobalt tetramethoxyphenylporphyrine) catalyzed carbon electrode, and a pyrochlore oxide based electrode.

High energy efficiency and high power density proton exchange membrane fuel cells — electrode kinetics and mass transport

The development of proton exchange membrane (PEM) fuel cell power plants with high energy efficiencies and high power densities is gaining momentum because of the vital need of such high levels of performance for extraterrestrial (space, underwater) and terrestrial (power source for electric vehicles) applications. Since 1987, considerable progress has been made in achieving energy efficiencies of about 60% at a current density of 200 mA/cm 2 and high power densities (⪢ 1 W/cm 2) in PEM fuel cells with high (4 mg/cm 2) or low (0.4 mg/cm 2) platinum loadings in electrodes. This article focuses on: (i) methods to obtain these high levels of performance with low Pt loading electrodes — by proton conductor impregnation into electrodes, localization of Pt near front surface; (ii) a novel microelectrode technique which yields electrode kinetic parameters for oxygen reduction and mass transport parameters; (iii) demonstration of lack of water transport from anode to cathode; (iv) modeling analysis of PEM fuel cell for comparison with experimental results and predicting further improvements in performance; (v) recommendations of needed R&D for achieving the above goals.

Investigations of the O 2 Reduction Reaction at the Platinum/Nafion® Interface Using a Solid‐State Electrochemical Cell

Research in solid polymer electrolyte fuel cells is gaining momentum because of the prospects of attaining high energy efficiencies and power densities essential for transportation and space applications. The most advanced solid polymer electrolytes for these fuel cells are the perfluorosulfonate ionomers (PFSIs), such as Du Pont's Nafion and the Dow PFSIs. The high oxygen solubility, chemical stability, proton conductivity and permselectivity exhibited by Nafion and the Dow PFSIs make them ideal candidates as electrolytes for fuel cells. Furthermore, the minimal anion adsorption on electrodes from fluorinated acids enhances oxygen reduction kinetics. The objectives of this work were to determine the concentration and diffusion coefficient of oxygen in Nafion, and the electrode kinetic parameters for the reduction of oxygen at the Pt/Nafion interface under totally solid‐state conditions (i.e., no contacting liquid electrolyte phase). Cyclic voltammetric and potentiostatic transient measurements were made at the Pt/Nafion interface. From cyclic voltammetric measurements, the purity of Nafion was ascertained and the roughness factor of the electrode was calculated. The slow sweep experiments yielded the Tafel parameters for oxygen reduction. From the two‐section Tafel plot, the calculated exchange current densities were found to be higher than those obtained at any other Pt/acid interface. From an analysis of the potentiostatic transients, the calculated values of oxygen solubility and diffusion coefficient in Nafion were higher than previously reported. These differences in mass‐transfer data were attributed to differences in water content of the Nafion membrane.

Electrochemical Studies on Protective Thin Co3 O 4 and NiCo2 O 4 Films Prepared on Titanium by Spray Pyrolysis for Oxygen Evolution

Electrochemical studies were done on thin and films on Ti by cyclic voltammetric and potentiostatic polarization techniques. The films were quite conductive and prevented the underlying surface from playing a harmful role during measurements. The results indicated the formation of a surface quasi‐reversible redox couple, CoIV/CoIII, in solution. The reaction order with respect to OH− ion concentration was found to be nearly 2. The Tafel slopes decreased with increasing concentrations. was observed to evolve oxygen gas at a lower overpotential as compared to that of . Electrode kinetic parameters suggest similar mechanisms for the oxygen evolution for both the oxides.

Electrochemical behaviour of some organotin(IV) compounds by rotating disc voltammetry in non‐aqueous solvents

AbstractThe electrochemical behaviour of three series of organotin(IV) compounds having the general formula RySnX4−y (R = Me, nBu or Ph, y = 2 or 3, X = Cl; R = Me or Ph, y = 3, X = NO3, N3, NCS, NCO, OAc or OH) have been examined in the non‐aqueous solvents dimethylformamide and acetonitrile, at rotating disc electrodes of glassy carbon and gold electrodes. The reduction of tin(IV) in the above series gave a single wave and obeyed the Levich plots at both electrodes in the two solvents. The electrode kinetic parameters and halfwave potentials have been determined, and the effect of solvents and substituents on these parameters have been explored.

Study of the kinetics of oxygen reduction at a sintered carbon electrode with a silver catalyst in alkaline medium

The present work deals with a model of hydrogen-oxygen electrochemical cell with a sintered carbon cathode containing a silver catalyst in an alkaline medium. The discharge curves of the cell connected to a variable discharge resistance were measured; their analysis based on a theory of the model cell yielded information about the electrode kinetic parameters.

Estimation of Electrode Kinetic Parameters of the Lithium/Thionyl Chloride Cell Using a Mathematical Model

A one‐dimensional mathematical model for the lithium/thionyl chloride primary cell is used in conjunction with a parameter estimation technique, in order to estimate the electrode kinetic parameters of this electrochemical system. The electrode kinetic parameters include the anodic transfer coefficient and exchange current density of the lithium oxidation, and ; the cathodic transfer coefficient and the effective exchange current density of the thionyl chloride reduction, and , and a morphology parameter, ξ. The parameter estimation is performed on simulated data first in order to gain confidence in the method. Data reported in the literature for a high‐rate discharge of an experimental lithium/thionyl chloride cell is used for an analysis.

Electroanalytical studies of ferrocene and substituted ferrocene in non-aqueous solvents by rotating disc voltammetry

Ferrocene, ferrocene carboxyaldehyde, acetylferrocene and benzoylferrocene were investigated by anodic rotating disc voltammetry at platinum, gold and glassy carbon electrodes in dimethyl sulphoxide and acetonitrile. All displayed a single wave and obeyed the Levich plots. The limiting current is linearly proportional to concentration, affording an excellent and rapid determination of these compounds in the presence of tetraethylammonium perchlorate (0.1 M) as supporting electrolyte. The electrode kinetic parameters were determined and mechanisms are suggested.

Equilibrium Fluctuation Analysis of Potassium Ferro/Ferricyanide Redox System on Platinum Disk Microelectrodes Using Field Effect Transistors

Spontaneous voltage fluctuations of the equilibrium potential of the ferrocyanide/ferricyanide redox couple at a Pt disk microelectrode have been measured with field effect transistors in the range of . Heterogeneous rate constants of the charge transfer reaction as well as the Warburg coefficients have been calculated from these measurements using a crossa‐correlation analysis. Stochastic and deterministic techniques for determination of electrode kinetic parameters have been compared. The deterministic procedure was a modified impedance technique in which the electrochemical system was perturbed by a lowa‐amplitude random signal applied to the electrode and the real and the imaginary parts of the equivalent electrical circuit were evaluated from the transfer function. Despite the very different nature of the origin of the signal the results obtained by these two techniques are in excellent agreement. An unusual concentration dependence of the Warburg impedance has been observed which is probably related to the small size of the electrode.

ChemInform Abstract: ELECTRODE KINETICS OF FUEL OXIDATION AT COPPER IN MOLTEN CARBONATE

AbstractAls Alternative zu den üblichen Nickel‐Elektroden wird Kupfer als vielversprechendes kostengünstigeres Anodenmaterial in Erwägung gezogen.