Electrode Kinetics Of Oxygen Reduction Research Articles

A “proton pump” concept for the investigation of proton transport and anode kinetics in proton exchange membrane fuel cells

A novel concept for the measurement of proton transport properties and electrode kinetics in proton exchange membrane fuel cells (PEMFC) is presented. The “proton pump” is essentially a fuel cell operated with pure nitrogen or very low hydrogen partial pressure instead of oxygen-containing gas on the cathode side, avoiding the complicated electrode kinetics of oxygen reduction. In this first study using this concept, we investigated the proton transport in high temperature PEMFC based on polybenzimidazole (PBI)/phosphoric acid membranes. The impedance spectra of the proton pump allow the clear distinction between anode and cathode kinetics and proton transport in the membrane. Identifying and analyzing the contribution of the anodic processes in the impedance spectra enabled the quantitative investigation of anode kinetics based on the Butler–Volmer equation. The proton transport was investigated in more detail in the current saturation region, where proton transport turned out to be the limiting process in case of sufficient H 2 supply at the anode. The maximum proton transport capacity of the PBI/phosphoric acid membrane was found to be comparable to those of Nafion ® membranes.

A Study of Bismuth Ruthenate as an Electrocatalyst for Bifunctional Air Electrodes

The electrode kinetics of oxygen reduction and evolution on Ru, and electrodes were studied in 0.1M. The transport and current shielding properties of the rotating ring‐disk electrode technique were used to obtain the kinetic variables as well as to determine the stability of these electrodes. Relatively high catalytic activity for reduction was observed on an Ru electrode. The catalytic activity of Ru was considerably reduced on the surface modified by “irreversibly adsorbed” Bi. The inhibition of reduction was accompanied by a change in the reaction mechanism from a predominately direct “four‐electron pathway,” characteristic for reduction on pure Ru, to a significant branching of the reaction to the “peroxide pathway” on the electrode. exhibited low catalytic activity for reduction and relatively high activity for peroxide decomposition and/or reduction. Although less active for oxygen evolution than either Ru or electrodes, the electrode is more stable during evolution than either of these electrodes, with no dissolution products detected at the ring electrode.

Mass Transport Phenomena in Proton Exchange Membrane Fuel Cells Using O 2 / He , O 2 / Ar , and O 2 / N 2 Mixtures: I . Experimental Analysis

This investigation on proton exchange membrane fuel cells (PEMFCs) consists of two parts: (i) an experimental analysis of the oxygen reduction reaction, using an , , or gas mixture as the cathodic reactant, to determine the effects of oxygen partial pressure, temperature, and pressure on the contributions to activation and mass‐transport overpotentials; and (ii) a theoretical analysis of oxygen reduction to: first, interpret the experimental results with ,, and gas mixtures as the cathodic reactant, and, second, elucidate the effects of the structure of the electrode and of the physicochemical parameters of the reactant gas, as well as of temperature and pressure, on the oxygen electrode potential vs. current density behavior. For the experimental analysis, which is presented in this Part of the paper, the performance evaluation of PEMFCs (in single cells with the geometric area of the electrodes being 50 cm2) was carried out using , , and gas mixtures as the cathodic reactants. Experiments were also carried out using air and oxygen. Cell potential vs. current density measurements were made over the temperature range of 50 to 90°C and pressure range of 1 to 5 atm. These studies showed that mass‐transfer limitations are less in PEMFCs with , than with or gas mixtures. Furthermore, at an oxygen composition greater than 40% of in the gas mixture, the pseudo‐linear region of the potential vs. current density plot is extended at least up to 1 A/cm2. With air as the cathodic reactant, mass‐transport effects are minimal at pressures of 3 atm and higher. Temperature effects are less significant than pressure effects, because in the former case there is a compensating effect of higher temperature and lower partial pressure of oxygen on the electrode kinetics of oxygen reduction.

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.

Temperature Dependence of the Electrode Kinetics of Oxygen Reduction at the Platinum/Nafion® Interface—A Microelectrode Investigation

A knowledge of the temperature dependence of the electrode‐kinetic parameters for oxygen reduction at the platinum/ proton exchange membrane (PEM) interface and of mass‐transport parameters of oxygen in the PEM is of vital importance in analyzing the performance of proton exchange‐membrane fuel cells. The microelectrode technique which was previously developed to determine these parameters at the platinum/Nafion® interface at 25°C was used in the present investigation at the same electrode/electrolyte interface. This study was carried out in the temperature range of 30–80°C and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the activation energy for oxygen reduction at the platinum/Nafion® interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid (TFMSA) interface but less than values obtained at the and interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These parameters also depend on the water content of the membrane. The conductivity of the membrane increases with temperature until it reaches a plateau at a temperature of 80°C; the dependence of the conductivity on temperature was correlated with the variation of water content of Nafion with temperature.

Electrode kinetics of oxygen reduction on gold in molten carbonate

The kinetics of oxygen reduction on gold in pure Li 2CO 3 and Li + K carbonate eutectic melts were studied using various techniques. The steady-state current in the potentiostatic mode was very agitation-dependent, indicating that the mass transfer effect was significant even at low polarization. The stability limits of the carbonate melt and the reaction sequence of oxygen reduction were examined by the potential scan method. The kinetic parameters were determined using the potential step method. The exchange current density was found to be two orders of magnitude higher than the value suggested by potential scan measurements. The standard exchange current density in the eutectic is 11.0 mA/cm 2 at 650°C; that in pure Li 2CO 3 at 750°C is 26.3 mA/cm 2. The exchange reaction orders of O 2 and CO 2, which are similar in the two melts, are quite different from the theoretical values according to the peroxide or superoxide mechanisms. The slow neutralization reaction of oxide by CO 2, which has a strong effect on the electrode potential, is the main reason for this discrepancy.

The Platinum Microelectrode/Nafion Interface: An Electrochemical Impedance Spectroscopic Analysis of Oxygen Reduction Kinetics and Nafion Characteristics

The kinetics of oxygen reduction at the platinum/proton‐exchange‐membrane interface is of direct importance in solid‐polymer‐electrolyte fuel‐cell research and development. Previous studies at a platinum microelectrode/Nafion® interface yielded electrode‐kinetic parameters for this reaction. The objectives of this study were to use electrochemical impedance spectroscopy (EIS) to study the oxygen‐reduction reaction under lower humidification conditions than previously studied. The EIS technique permits the discrimination of electrode kinetics of oxygen reduction, mass transport of in the membrane, and the electrical characteristics of the membrane. Electrode‐kinetic parameters for the oxygen‐reduction reaction, corrosion current densities for Pt, and double‐layer capacitances were calculated. The production of water due to electrochemical reduction of oxygen greatly influenced the EIS response and the electrode kinetics at the Pt/Nafion interface. From the finite‐length Warburg behavior, a measure of the diffusion coefficient of oxygen in Nafion and diffusion‐layer thickness was obtained. An analysis of the EIS data in the high‐frequency domain yielded membrane and interfacial characteristics such as ionic conductivity of the membrane, membrane grain‐boundary capacitance and resistance, and the uncompensated resistance.

Electrode Kinetics of Oxygen Reduction in Lithium Carbonate Melt: Use of Impedance Analysis and Cyclic Voltammetric Techniques to Determine the Effects of Partial Pressure of Oxygen

The effects of the partial pressure of oxygen and temperature on the oxygen reduction on a submerged gold electrode in a lithium carbonate melt were investigated using cyclic voltammetry and impedance analysis. The values for the mass‐transfer parameters, , obtained from cyclic voltammetry and impedance analysis were in good agreement. The reaction orders for oxygen at 800°C were calculated to be about 0.3 for the exchange current density and 0.5 for the product; these values are consistent with the mechanism proposed in the literature for oxygen reduction in melt.

Electrode kinetics of oxygen reduction on platinum in trifluoromethanesulphonic acid

The kinetics of oxygen reduction at Pt in trifluoromethanesulphonic acid (TFMSA) (0.05–6.0 M) and in 1.0 M TFMSA with addition of small concentrations of phosphoric acid (0.003–0.1 M) was investigated using the rrde technique. In TFMSA, the oxygen reduction current on the oxide-covered Pt was found to be smaller than that on the oxide-free Pt surface. This result is consistent with the greater amonut of hydrogen peroxide produced on the oxide-covered Pt. A reaction order of one-half with respect to the oxygen concentration for the oxygen reduction reaction was obtained from the ring-disc data. Addition of increasing amounts of phosphoric acid to TFMSA resulted in a progressive decrease in the oxygen reduction current and in an increase of the reaction order with respect to oxygen. On the basis of these experimental results the reaction mechanism proposed for the oxygen reduction of Pt in TFMSA is the fast dissociative adsorption of oxygen, followed by the slow electron transfer step.

Electrode kinetics of oxygen reduction A theoretical and experimental analysis of the rotating ring-disc electrode method

Electrode kinetics of oxygen reduction A theoretical and experimental analysis of the rotating ring-disc electrode method

Electrode kinetics of oxygen reduction: A theoretical and experimental analysis of the rotating ring-disc electrode method

The previous theoretical treatments of the rotating ring-disc electrode method to distinguish between the mechanisms of electroreduction of O 2 to H 2O with and without the formation of H 2O 2 as an intermediate, were examined. A new expression was derived for I d1/( I d1− I d) as a function of ω −1/2 (where I d1 is the disc limiting current, I d is the disc current and ω is the rotational speed of electrode) for five possible reaction models. This, along with the corresponding expressions for I d/ I r vs. ω −1/2 ( I r is the ring current), enables the calculations of the individual rate constants for the intermediate steps of O 2 reduction. The experimental data of I d and I r were obtained for O 2 reduction on platinum in 0.55 M H 2SO 4 at 25°C. By use of these experimental results in the present theoretical treatment, it is shown that: (1) the most applicable model over the entire potential region was the one suggested by Damjanovic, Genshaw and Bockris; (2) the models involving the adsorption/desorption of H 2O 2 were applicable only over a narrow region of potential; and (3) the models involving the chemical decomposition of H 2O 2 were inconsistent with the dependence of I d1/( I d1− I d) vs. ω −1/2.

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/ rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H 2SO 4 ∼ CF 3SO 3H > H 3PO 4 > HClO 4. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH − ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.

Electrode kinetics of oxygen reduction on oxide-free platinum electrodes

The cathodic V log i curves for O2 reduction on oxide-free Pt electrodes were determined in oxygen-saturated purified acid (HClO4, pH 1–3) and alkaline (NaOH, pH 13) solutions. It was found that ∂V/∂ log i = −2·3RT/F, ∂V/∂ log pO2 = −2·3RT/F and ∂V/∂ pH = −100 mV. Coverage of electrodes by oxygen-containing species was determined by superimposing a constant cathodic current over the working electrode. Intermediate values of coverages were found to be nearly linearly dependent on the potential in the Tafel region.The kinetics mechanism of O2 reduction on oxide-free Pt electrode are different from that reported for the oxide-covered electrode. Reaction mechanism are discussed in terms of Langmuir and Temkin adsorption isotherms. It is suggested that under Temkin conditions the rate-determining step in the over-all four-electron process is the first electron transfer.