Electrochemical Calorimetry of the Zinc and Bromine Electrodes in Zinc‐Bromine and Zinc‐Air Batteries

Abstract

Direct calorimetric measurements are reported on three half-cell reactions of interest in zinc battery thermodynamics: the Zn/Zn 2+ reaction in aqueous Br-solution; the BrJBr-reaction at Pt and the Zn/ZnO-ZnO~-reaction in alkali. An experimental procedure is described for determining the single reaction heats, observable as TAS quantities, in both directions of the above reactions, thus providing a method for eliminating, by difference, the Joule heating effects which are unavoidable in the course of measurements at finite currents. Corrections for entropies of transport are made and discussed. Under certain conditions, where Zn corrosion can occur with evolution of H2, the TAS heat change for that process is also determinable. Some heat changes for overall cell reactions are also determined~ While enthalpy changes for reversible electrochemical reactions can be obtained from EMF measurements and their temperature coefficients by means of the Nernst equation through the Gibbs-Helmholtz relation, such values are necessarily for the overall, two-electrode reaction. Direct calorimetric measurements, subject to some corrections for ionic entropies of transport, can be applied to evaluation of the heat changes that are associated with the electrochemical reactions at the electrodes of a cell. Such directly measurable heats are, however, associated with the "TAS" quantity for the cell reaction, rather than the AH.It is of interest to recall that the actual enthalpy change, AH, for an electrode reaction is not measurable calorimetrically like it would be for a corresponding overall chemical reaction. This is because the AH normally constitutes the main component of the AG value determining the EMF of the cell. From measurements of the latter at various temperatures, ~ can, of course, be determined using a form of the Gibbs-Helmholtz equation.Direct calorimetric measurements made at a suitably designed electrode configuration with an appropriately arranged thermal detection probe, and separated anode and cathode compartments, can also give information on the TZkS quantity for single electrode processes which are of interest in a general way in electrochemical thermodynamics and ionic solution electrochemistry. In some papers (1-3), these TAS quantities have been referred to as electrolytic Peltier heats, and values for several redox reactions at electrodes have been reported (4-5). The first reliable work of this kind was done by Lange and Hesse (6) and Lange and Monheim (7) some years ago, but, for many years since that time up until several years ago, few other papers have appeared on this topic.Renewed interest in electrochemical calorimetry has arisen on account of the importance of measurements of overall and component heat changes in battery processes. Such measurements are essential for understanding the thermochemistry of battery reactions and in assessing practical energy efficiencies as well as evaluating heat changes associated with various contributions to irreversibility in battery charge and discharge cycle...

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