Electrochemical mass-transfer studies, model

Model Reactions Used in Electrochemical Mass-Transfer Studies... 

The diffusion model [5, 6] has been developed for mass transfer study within the system bipolar plate - gas diffusion layer (with micro-porous sublayer) - electrocatalytic layer . It was shown that the current density distribution is a complex function and depends mainly on the electrochemical parameters of the MEA (electrocatalytic layer activity) and... 

When we begin to investigate an electrochemical system, we normally know little about the processes or mechanisms within the system. Electrochemical impedance spectroscopy (EIS) can be a powerful approach to help us establish a hypothesis using equivalent circuit models. A data-fitted equivalent circuit model will suggest valuable chemical processes or mechanisms for the electrochemical system being studied. From Chapter 1, we know that a fuel cell is actually an electrochemical system involving electrode/electrolyte interfaces, electrode reactions, as well as mass transfer processes. Therefore, EIS can also be a powerful tool to diagnose fuel cell properties and performance. 

Sections II-X are not exhaustive. For example, they contain no reference to metallocarboranes or to macrocyclic species containing metal—carbon bonds, such as the vitamin B12 models (32). However, we have attempted to include material which relates electrochemistry to other fields. For example, we note the formation of electron-transfer products by 60Co y irradiation, negative ion mass spectrometry, and photoelectron spectroscopy. We also describe pairs of complexes apparently related by one-electron transfer but which have not been studied electrochemically. Clearly, such studies would provide confirmatory evidence for the proposed formulations. 

Computer modeling of convection has had mixed success. Many convection problems, particularly those involving laminar flow, can readily be solved by special computer programs. However, in situations where turbulence and complex geometries are involved, computer analysis and modeling are still under development. Mass transfer analogies can play a key role in the study of convective heat transfer processes. Two mass transfer systems, the sublimation technique and the electrochemical technique, are of particular interest because of their convenience and advantages relative to direct heat transfer measurements. 

Understanding of gas-liquid flow in electrochemical systems is very important for system optimization, enhance mass transport and thus gas release efficiency. There are relatively little theoretical studies available in the literature which considers process as a two-phase flow problem. Zeigler and Evans[2] applied the drift - flux model of Ishii[3] to electrochemical cell and obtained velocity field, bubble distribution, mass transfer rate. Instead of treating the bubbles as a second phase, they obtained bubble distribution from concentration equation. Dahikild [4] developed an extensive mathematical model for gas evolving electrochemical cells and performed a boundary layer analysis near a vertical electrode. 

Electrochemical systems where the mass transport of chemical species is due to diffusion and electromigration were studied in previous chapters. In the present chapter, we are going to consider the occurrence of the third mechanism of mass transfer in solution convection. Although the modelling of natural convection has experienced some progress in recent years [1], this is usually avoided in electrochemical measurements. On the other hand, convection applied by an external source forced convection) is employed in valuable and popular electrochemical methods in order to enhance the mass transport of species towards the electrode surface. Some of these hydrodynamic methods are based on electrodes that move with respect to the electroljAic solution, as with rotating electrodes [2], whereas in other hydrodynamic systems the electrolytic solution flows over a static electrode, as in waU-jet [3] and channel electrodes [4]. 

Porous media finds extensive application in chemical engineering. In certain cases they are simply used to increase the mass transfer rate between two distinct phases, while in certain other cases they are used to disperse the catalyst effectively. Catalytic packed beds are an integral part of any chemical production industry. Solid Oxide Fuel Cells are class of electrochemical devices where porous media finds important application. Over the years many models have been developed to study the transport processes in porous media, starting from simple Fickian approach to complex Dusty Gas Model GDGM). However, very little is done to validate the accuracy of these models under reaction conditions, especially with multi-component species mixtures. 

Modeling carbon monoxide poisoning is normally categorized under mass transport studies. Wang and Savinell [17] created a model of a carbon monoxide-poisoned anode that used variable membrane conductivity data of Springer et al. [15,16]. Only the mass transfer, proton migration, and electrochemical reaction of the anode catalyst layer were modeled. The local current production was found using ... 

Mercury electrodeposition is a model system for experimental studies of electrochemical phase formation. On the one hand, the product obtained is a liquid drop, corresponding very well with the liquid drop model of classical nucleation theory. Besides, electron transfer is fast [61] and therefore the growth of nuclei is controlled by mass transport to the electrode surface [44]. On the other hand, the properties of the mercuryjaqueous solution interface have been the object of study for over a century and hence are fairly well understood. The high overpotential for proton reduction onto both mercury and vitreous carbon favor the study of the process over a wide range of overpotentials. In spite of the complications introduced by the equilibrium between the Hg +, Hg2 " ", and Hg species, this system offers an excellent opportunity to verily the fundamental postulates of the electrochemical nucleation theory. In fact, the dependence of the nucleation rate on the oxidation state of the electrodepositing species is fiiUy consistent with theory critical nuclei appear with similar sizes and onto similar number densities of active sites... 

The model accoxmts for detailed species mass transport, heat transfer in the solids as well as in the reactants, potential losses in the gas diffusion layers and membrane, electrochemical kinetics, and the transport of water through the membrane. In addition, the model accounts for the physics of phase change in that the rate of evaporation is a function of the amount of liquid water present and the level of undersaturation. Finally, the model is not limited to relatively low humidity reactants, as was the case in prior studies, and can be used to simulate eonditions representative of actual fuel cell operation. 

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