Heat and internal energy changes at electrodes and junctions in thermocells

A new thermodynamic theory of thermocells has been applied to cells with Ag/AgCl electrodes, and a monovalent chloride compound in aqueous solution as electrolyte. All local heat and energy changes in the cells are calculated for the first time. The results show large heat effects at the electrodes, several orders of magnitude larger than the electric work obtained from the cell. Internal energy changes and Thomson effects are much smaller, however, comparable in magnitude to the electric work. This emphasizes the importance even of small effects. Heat effects are characterized by mass changes (thermodynamic heat changes) or by material changes (Peltier or Thomson effects). The Peltier effects show a systematic variation from about {minus}21 kJ F{sup {minus}1} to 6 kJ F{sup {minus}1} as the cation changes from Li{sup +} to Cs{sup +}. The transported entropy of KCl seems constant for concentrations below 0.1 mol kg{sup {minus}1}. The transported entropy is largely dependent on the cation present. The variations can be correlated to inverse ionic radii. The theoretical reason why constant entropies appear to explain thermocell potentials for temperature differences up to 70{degrees}C is given. The theory suggests that a new experimental procedure should be used for determination of thermocell potentials. It is concluded that it is not feasible to measure directly the heat evolution in the electrolyte in these systems. Single-ion entropies are furthermore shown inadequate for evaluation of thermal properties relevant to electrochemical engineering.