Category: 04. 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…

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…

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,…

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…

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…

In order to solve most transport problems, expressions for the flux, such as the Nernst–Planck equation, are incorporated into material balances or conservation equations. Here we derive a balance for a single species over a control volume of size ΔzΔxΔy as shown in Figure 4.3. The balance takes the form (4.9) The rate of accumulation in the…

The most widely used expression for the flux in electrochemical systems is the Nernst–-Planck equation, (4.3) The flux of species, i, is the combination of three terms: migration, diffusion, and convection. The Nernst–Planck equation is similar to Equation 4.2 but adds a contribution that arises from the gradient in electrical potential called migration. For charged species, the force…

The transport of material by diffusion is due to the random thermal movement of molecules and is described by Fick’s law: (4.1) where Ji is the molar flux [mol m−2 s−1] of species i. The flux represents the rate at which material passes through a plane of unit area. It is a vector quantity with both direction and magnitude. In…