The role of self-exchange reactions in ionic diffusion in thermocells

A thermo-electrochemical cell (or thermocell) uses redox couples as electrolyte and symmetric metal electrodes to generate power from a heat source. Self-exchange reactions take place between redox couples in the bulk electrolyte of thermocells. A self-exchange reaction is a chemical process where an electron moves from one chemical specie to the other, changing the oxidation states of both species. These reactions are usually very fast. The species must diffuse into close proximity for the self-exchange reactions to occur. When a redox electrolyte is used in a thermocell, diffusion is the process that makes possible the flux of ions in the cell in order to maintain the electric current. This diffusion is enhanced by self-exchange currents, which effectively reduce the distance over which individual ions must diffuse. Using the redox couple ferri/ferrocyanide we strive to understand the balance of electronic (self-exchange) and ionic (diffusion) conduction in a thermocell, and discover ways to enhance current and improve the efficiency of thermocells. Fundamental electrochemical studies were conducted an interpreted with models available in the literature to help elucidate the contribution of self-exchange reactions to the overall flux of ions in the thermocell.  The studies revealed that the contribution to the generated current attributed to self-exchange or electronic conduction is significant compared to contribution attributed to ionic diffusion.  The results also suggest that catalytic dispersants such as carbon nanotubes (CNTs) could be used to mediate self-exchange reactions and enhance the effective ionic diffusion in thermocells.