Electrochemistry of Stimulation Electrodes: Part I: Page 6

Now consider a redox couple where the density of states, describing the acceptor state and that describing the donor state, are widely separated. The large separation between the densities of states is a characteristic of an irreversible reaction. Assuming that the concentration of the two forms of the species are equal, an electrode immersed in an aqueous solution containing this redox couple would assume a potential E0, half way between the two density of state representations. As the potential of the immersed electrode is made more negative, Fermi level raised, no electron transfer occurs until the Fermi level of the electrode begins to overlap the acceptor density of states. A similar situation occurs when the electrode is made more positive. Over the potential region where electron transfer does not occur, charge is moved onto the electrode/electrolyte interface, double layer charging. When the electrode is made more negative, negative charge is added to the metal and positive charge is moved closer to the electrode in the electrolyte medium. The opposite occurs when the electrode potential is made positive, when an anodic current is applied. Potential excursions in the region were electron transfer does not occur are treated as charging and discharging a capacitor and are is modeled as capacitor. A typical value for double layer capacitance is ~20µF/cm2. The current flowing in the circuit is a combination of double layer capacitance current, if the potential at the interface is changing as a function of time, and electron transfer across the interface. Plotting the current flowing in the electrode circuit, as a function of interface potential, is shown in the figure. The currents are similar to the previous case except that the onset of current flow occurs at greater potential values. Since capacitive current is very small in this representation, the sweep rate, dV/dt, must be small.

Summary of Concepts Involved in Electron Transfer
Current injected into a tissue medium is derived from capacitive currents that result from double layer charging and from electron transfer between the stimulating electrode and molecular species in the electrolyte medium. For electron transfer to take place, molecules in the medium must either accept electrons from the electrode or donate electrons to the electrode. Radiationless electron transfer requires that the energy level of the electron be the same in the metal and molecular structure. Energy levels for the reactant molecules are fixed, where as the electron energy levels for the metal electrode can be raised or lowered by charging and discharging the electrode with an external power source. When the electrode potential is changing as a function of time and no electron transfer is taking place the current flowing in the tissue medium is due to ion migration in response to charging and discharging the double layer.