Daniel Simonsson

Proton Conductivity of Nafion 117 as Measured by a Four‐Electrode AC Impedance Method

Nafion 117{reg_sign} is a proton conducting ion-exchange membrane which is now receiving much attention due to its use as an electrolyte in the polymer-electrolyte-membrane fuel cell (PEMFC), which is of great interest for electric vehicle propulsion. The proton conductivity of Nafion 117 was measured under various conditions of humidity and temperature using a four-electrode ac impedance method. The conductivity of this membrane without heat-treatment was ca. 7.8 {times} 10{sup {minus}2} S/cm at ambient temperature and 100% relative humidity; it varied strongly with the humidity and heat-treatment of the membrane. After heat-treatment, the membrane showed a slight dependence of conductivity on temperature. From 21 to 45 C, its conductivity at a given relative humidity decreased with increasing temperature, while from 45 to 80 C it increased with temperature.

Ruthenium dioxide as cathode material for hydrogen evolution in hydroxide and chlorate solutions

Ruthenium Dioxide as Cathode Material for Hydrogen Evolution in Hydroxide and Chlorate Solutions

A mathematical model for the porous lead dioxide electrode

A theoretical model for the porous lead dioxide electrode is proposed on the basis of the macrohomogeneous model for porous electrodes.The structural changes during discharge, due to precipitation of lead sulphate, are considered.The two main structural effects, plugging of the pores and gradual insulation of the active electrode surface by the reaction product, lead sulphate, are both considered by relating them to the local degree of discharge.The numerical results show that, at high current densities, the discharge capacity is limited by both structural and transport restrictions.At the end of discharge a layer of lead sulphate crystals blocks the electrode surface in the outer layers of the electrode. The current can then neither be transferred across this insulated surface nor reach remaining active material in the inner parts of the electrode because of acid depletion, which is furthermore accelerated by the decreasing porosity.

Comparison of structural models for gas-solid reactions in porous solids undergoing structural changes

Abstract In many gas-solid reactions the porous solid undergoes structural changes due to differences in molar volume between solid reactants and solid products. In this paper some new models are proposed which take into account the effects of structural changes on the reaction rate. The models differ in their geometric descriptions of the initial structure of the porous solid but they all take into consideration the decreasing diffusion rate in the growing ash layer and the decreasing gas-solid interfacial area at swelling of the solid structure during reaction. The different models are compared with each other and with previously suggested models. A model that regards the initial solid structure as an aggregate of truncated spheres in contact with each other as in the initial stage of sintering seems to be the most realistic and flexible model of those investigated. This model has been used to predict conversion-time curves for the sulfation of limestone. The predicted curves are in good agreement with experimental results reported in literature. It is also shown that a semi-empirical approach can give essentially the same results with less numerical effort.

Inhibition of cathode reactions in sodium hydroxide solution containing chromate

The formation of a film on the cathode surface due to electroreduction of chromate and its effect on other cathode reactions have been studied by cyclic voltammetry using rotating disc electrodes o ...

Comparison of electrochemical and surface characterisation methods for investigation of corrosion of bipolar plate materials in molten carbonate fuel cell: Part I. Electrochemical study

Abstract The corrosion of bipolar plate and current collector materials in molten carbonate fuel cells (MCFC) was investigated. Tafel extrapolation, linear polarisation resistance, chronopotentiometry and electrochemical impedance spectroscopy (EIS) were used to determine corrosion rates of iron and iron-based alloys in molten carbonate melts for both cathode and anode MCFC environments. During the initial stages, agreement among the methods was good but could have differed after a corrosion layer formed on the surface. The corrosion rate for a fully immersed sample at the open circuit potential seemed to be controlled by external mass transfer of water. Iron–chromium alloys exhibit reasonable corrosion resistance on the cathode side, provided that the chromium concentration exceeds 25%.

Theoretical and experimental studies of free convection and stratification of electrolyte in a lead-acid cell during recharge

Abstract Free convection and stratification of the electrolyte in a lead-acid cell with porous electrodes and during recharge were studied theoretically and experimentally. The concentration field was measured by means of Holographic Laser Interferometry (HLI) and the velocity field by means of Laser Doppler Velocimetry (LDV). A two-dimensional mathematical model was also developed for mass transfer and electrolyte motion during the process. It was assumed for simplicity that the electric current density and porosities of the electrodes were constant and uniformly distributed in the electrodes. The results from the experiments were compared with numerical results obtained from the mathematical model. The agreement was found to be good. A simplified mathematical model, as an alternative to the full numerical problem, was also developed. The results of the simplified model proposed here proved to be in good agreement with the results from the full numerical solution, albeit for sufficiently large times.

The Effect of Chromate Addition on Cathodic Reduction of Hypochlorite in Hydroxide and Chlorate Solutions

The cathodic reduction of chromate and its effect on the reduction of hypochlorite have been studied by cyclic voltammetry, using a rotating disk electrode of platinum in IM NaOH solution as well as in an electrolyte with a composition and temperature which were typical for industrial chlorate synthesis. It was found that in both electrolytes a thin film of presumably Cr(OH)3, with a thickness of only one or two molecular layers, is formed. After formation of this film in a cathodic sweep of the potential, the hypochlorite reduction reaction is almost completely inhibited in the reversed sweep, up to the potential region for dissolution of the formed film. From the experimental results and a theoretical analysis of the mass transfer rate of hypochlorite ions to the cathode surface, it is concluded that the addition of chromate to the electrolyte in the industrial chlorate process leads to the formation of a thin film on the cathode surface, which hinders electron transfer at the reduction of hypochlorite.

A flow-by packed-bed electrode for removal of metal ions from waste waters

The design and performance of a full-scale, particulate flow-by electrode is described. The mass transfer rate in the electrode is high and can be estimated for different operating conditions by means of the correlation Sh=1.46Re0.72Sc1/3 The bed is effective for waste waters with a specific conductivity above 10−3 mho cm−1. Noble metals can be electrodeposited easily, even if bound in strong complexes, while deposition of zinc from acid solutions is highly pH-dependent.The scale-up of a packed-bed electrochemical reactor for industrial applications is achieved by using a multi-bed cell based on the filter press principle with the appropriate number of bed electrodes.

Influence of gas phase mass transfer limitations on molten carbonate fuel cell cathodes

The purpose of this paper is to elucidate to what extent mass transfer limitations in the gas phase affect the performance of porous molten carbonate fuel cell cathodes. Experimental data from porous nickel oxide cathodes and calculated data are presented. One and two-dimensional models for the current collector and electrode region have been used. Shielding effects of the current collector are taken into account. The mass balance in the gas phase is taken into account by using the Stefan–Maxwell equation. For standard gas composition and normal operating current density, the effect of gas-phase diffusion is small. The diffusion in the gaseous phase must be considered at operation at higher current densities. For low oxygen partial pressures, the influence of mass transfer limitations is large, even at low current densities. To eliminate the influence of conversion on polarization curves recorded on laboratory cell units, measurements should always be performed with a five to tenfold stoichiometric excess of oxygen. Two-dimensional calculations show rather large concentration gradients in directions parallel to the current collector. However, the influence on electrode performance is still small, which is explained by the fact that most of the current is produced close to the electrolyte matrix.