Author: admin
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The Gas–Liquid Interface in Porous Electrodes
There are many instances when both gas and liquid fill the void volume of a porous electrode. Here, we introduce important concepts to describe these two fluids contained in the pores. A key aspect of porous media relevant for our studies is capillarity or capillary action. The capillarity can be understood from the ability of…
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Current Distributions in Porous Electrodes
As noted in Chapter 4, a uniform current distribution is often desired. Generally, the calculation of the current distribution for porous electrodes is not amenable to analytical solutions. There are a few exceptions—most notably the one-dimensional treatment of a porous electrode in the absence of concentration gradients, which is described by the secondary current distribution. In…
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Impact of Porous Electrode on Transport
Clearly, the porous structure has an effect on transport through the media. The way to treat this influence is to define effective transport properties that can be related to the bulk transport properties discussed in Chapter 4. The need for the effective transport properties can be seen with the cartoon illustrated in Figure 5.4. There are two…
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Characterization of Porous Electrodes
An idealized porous electrode with straight cylindrical pores was considered above. Next our aim is to describe and characterize porous media in a way that is broadly applicable. The empty space of the electrode available to the electrolyte (or gas phase) is the void volume fraction or porosity, ε. This parameter is the most important feature…
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Mathematical Description of Porous Electrodes
As we examine porous electrodes, we first consider a simple geometry consisting of straight cylindrical pores in an electrically conductive matrix as shown in Figure 5.1. The idea behind using a three-dimensional porous structure is to increase the amount of surface area in a given electrode volume. Let’s assume that we have an electrode that is…
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Membrane Transport
Electrolyte and Electrode Separation The electrodes in electrochemical cells are physically separated, and electrolyte occupies the space between the two electrodes. The manner in which this separation is implemented can vary and has significant impact on transport. Let’s inspect the space between electrodes for three electrochemical systems to highlight the differences. The first system we…
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Current Distribution
tential as shown in Section 4.3. In that section, we solved for the potential drop in a one-dimensional (symmetric) geometry for fast kinetics. Previously, we examined the impact of the surface reaction on the potential losses at each of the two electrodes, in addition to losses associated with the ohmic drop in solution. Rapid kinetics…
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Concentration Overpotential
In Section 4.3, we presented the following expression for the cell potential during discharge in the absence of concentration gradients: (4.53) We also examined how to solve problems with concentration gradients at various levels of approximation. The cell voltage comes naturally from the solution of the coupled equations for the potential field and concentrations. However,…
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Convective Mass Transfer
In cases where migration can be neglected (e.g., excess supporting electrolyte), traditional methods and correlations for mass transfer can be used to determine reaction rates and the current density, which of course are related through Faraday’s law. In this section, we consider convective mass transfer for geometries that are of interest for electrochemical systems. It…
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Transference Numbers, Mobilities, and Migration
In Equation 4.3, both the ion mobility and a diffusion coefficient appear. Both the concentration and the potential impact the electrochemical potential (μi). While the details are beyond the scope of this text, it is the gradient of the electrochemical potential that is the true driving force for transport. Therefore, we might expect the diffusivity and mobility…