Current densities reactant diffusion process

Concentration Polarization As a reactant is consumed at the electrode by electrochemical reaction, there is a loss of potential due to the inability of the surrounding material to maintain the initial concentration of the bulk fluid. That is, a concentration gradient is formed. Several processes may contribute to concentration polarization slow diffusion in the gas phase in the electrode pores, solution/dissolution of reactants/products into/out of the electrolyte, or diffusion of reactants/products through the electrolyte to/from the electrochemical reaction site. At practical current densities, slow transport of reactants/products to/from the electrochemical reaction site is a major contributor to concentration polarization ... 

The performance of PEMFC is often presented by the polarization curve that shows the voltage output as a function of current density. Fig. 8 shows a typical polarization curve of PEMFC. As the PEMFC processes charge-transfer reactions and the diffusion of the reactants to and products from the electrochemical interface, the transport and kinetics within the cell determine the polarization characteristics of PEMFC. In the practical PEMFC, the terminal cell potential V... 

Mass transport in MREF is a combination of steady state and non-steady state diffusion processes. The mass transfer limited current density (i,) is related to the reactant concentration gradient (Cb-Cs) and to the diffusion layer thickness (8) by Nemst using the following equation ... 

The current density measured from the rotating electrode is contributed by both the current densities of electrode electron-transfer reaction and the reactant diffusion. In order to obtain the kinetic parameters of these two processes and their associated reaction mechanisms based on the experiment data, both the theories of electrode electron-transfer reaction and reactant diffusion should be studied and understood. In this chapter, the general theories for electrode kinetics of electron-transfer reaction and reactant diffusion will be given in a detailed level, and we hope these theories will form a solid knowledge for a continuing study in the following chapters of this book. 

Actually, the reactant transport by these three processes (diffusion, migration, and convection) occurs at the same time along the x direction. The total oxidant transport current density... 

Models that include all parts of a fuel cell are typically two- or three-dimensional and reflect many of the physical processes occurring within the fuel cell. In a real PEM fuel cell geometry, the gas diffusion layers are used to enhance the reaction area accessible by the reactants. The effect of using these diffusion layers is to allow a spatial distribution in the current density on the membrane in both the direction of bulk flow and the direction orthogonal to the flow but parallel to the membrane. This two-dimensional distribution caimot be modeled with the well-used two-dimensional models, (like models that developed by the researchers in Ref. [48-52]), where the mass-transport limitation is absent in the third direction. 

The model presented here is a comprehensive full three-dimensional, non-isothermal, singlephase, steady-state model that resolves coupled transport processes in the membrane, eatalyst layer, gas diffusion eleetrodes and reactant flow channels of a PEM fuel cell. This model accounts for a distributed over potential at the catalyst layer as well as in the membrane and gas diffusion electrodes. The model features an algorithm that allows for a more realistie representation of the loeal activation overpotentials which leads to improved prediction of the local current density distribution. This model also takes into aeeount convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases, electrochemical reactions and the transport of water through the membrane. 

Few pore network models of GDL materials have been applied to calculate the material s relative diffnsivity after an invasion process. This could be due to the fact that most models apply inlet conditions snch as nniform pressure or uniform flnx, where it is assnmed that liquid water is present at each throat at the catalyst layer GDL interface. However, with a nniform pressure boundary condition, Gostick et al modeled diffusion at various stages of saturation and calculated the limiting current density due to reactant transport across the GDL (Fig. 10.13). They were able to do this for two reasons. First, they did not consider the effect of the inlet reservoir on reactant transport. Therefore, reactants could diffuse through inlet throats as long as they had not yet been invaded by water. Secondly, they investigated a scenario where there was a thin film of air in liquid-saturated pores and throats through which reactants could diffuse. 

Effective operation and precise imderstanding of PEM fuel cells can be hindered by the wide scale range of the physical processes, for instance from flow phenomena in millimeter-size stractures to reactant diffusion followed by adsorption on the catalyst clusters. The interrelated character of the various physicochemical processes is another difficulty for the technological/scientific issues for instance, water management depends on temperature and humidity, then on heat and water production rates, thus on current density. .. which can be locally affected by formation of liquid water, i.e. insufficient water management (Fig. 12.1). 

Concentration polarization represents the energy losses associated with mass transport effects. For instance, the performance of an electrode reaction may be inhibited by the inability for reactants to diffuse to or products to diffuse away from the reaction site. In fact, at some current, the limiting current density a situation will be reached wherein the current will be completely limited by the diffusion processes (see Fig. 42.2). Concentration polarization can be represented by... 

Assuming electrochemical equilibrium for the charge transfer step, the potential drop at the electrode/electrolyte interface will be determined by the concentration of the reactants in its direct vicinity according to the Nernst equation. For the consumption of a redox constituent due to the electrochemical reaction, its concentration in front of an electrode will change in a simple case linearly with the distance. This decrease of the concentration occurs within the Nernst diffusion layer of thickness from in the bulk of the electrolyte to c adjacent to the electrode surface (Figure 1.22). Applying Ficks diffusion law together with Faraday s law leads to Equation 1.118 for the diffusion-limited current density i of a cathodic process at the electrode like Ox + e —> Red. 

Esquivel et al. (2010) present an all-polymer micro-DMFC fabricated with a SU-8 photoresistor. This development exploits the capability of SU-8 components to bond to each other by a hot-pressing process and obtain a compact device. The device is formed by a MEA sandwiched between two current collectors. The MEA consists of a porous SU-8 membrane filled with a proton-exchange polymer and covered by a thin layer of carbon-based electrodes with a low catalyst loading (1.0 mg/cm ). The current collectors consist of two metalhzed SU-8 plates provided with a grid of through-holes that make it possible to deliver the reactants to the MEA by diffusion. The components were then bonded to obtain a compact micro-DMFC. With this assembly, using a 4 M methanol concentration at a temperature of 40°C, a maximum power density of 4.15 mW/cm was obtained. 

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