Origin of Faradaic Current in Voltammetry

Let’s assume we have a solution that is 1.0 mM in Fe(CN)63– is 1.0 mM. The standard state potential for the reduction of Fe(CN)63– to Fe(CN)64– is +0.356 V. If we apply a potential of +0.550 V the concentration of Fe(CN)63– at the electrode’s surface decreases to zero and we establish a concentration gradient between the solution at the electrode’s surface and the bulk solution. As shown here, this concentration gradient creates a driving force that transports Fe(CN)64– away from the electrode and that transports Fe(CN)63– to the electrode. A faradaic current continues to flow until there is no difference between the concentrations of Fe(CN)63– at the electrode and in bulk solution.

Figure11.39

Although the potential at the working electrode determines if a faradaic current flows, the magnitude of the current is determined by the rate of the resulting oxidation or reduction reaction. Two factors contribute to the rate of an electrochemical reaction: the rate at which the reactants and products are transported to and from the electrode—what we call mass transport—and the rate at which electrons pass between the electrode and the reactants and products in solution.

As shown here, diffusion across a concentration gradient is one form of mass transport. Before applying the potential (t = 0) the concentration of Fe(CN)63– is the same at all distances from the electrode’s surface. After applying the potential, its concentration at the electrode’s surface decreases to zero and Fe(CN)63– diffuses to the electrode from bulk solution. The longer we apply the potential, the greater the distance over which diffusion occurs. The dashed red line shows the extent of the diffusion layer at time t3. These profiles assume that neither convection (due to stirring) or migration (resulting from the electrostatic attraction or repulsion between charged species and the electrode surface) contribute significantly to the mass transport of Fe(CN)63–.

Figure11.40

As shown here, even in the presence of convection from stirring, diffusion is the only significant form of mass transport close to the electrode’s surface. At distances greater than δ, convection is the only significant form of mass transport, maintaining a homogeneous solution in which the concentration of Fe(CN)63– at δ is the same as its concentration in bulk solution.

Figure11.41

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