ROBERT C. PLUMB
chemical exemplified
Racing Car Batteries
pure solid or liquid phases, as in the silver-zinc couple
Illustrating the Nernst equation and Faraday's law Informationpmuided by R. E. Combs, Goodyear Tire and Rubber Co. and J. M. Connelly, GouldInc., Burgess Diuision In 1972 many racing crews in the Indianapolis 500 switched from lead-acid batteries or magnetos, as sources of current for engine ignition, to silver-zinc batteries. The superiority of the silver-zinc battery rests upon two fundamental chemical principles. It's adoption stems from space-age developments in hattery technology which make the hattery very reliable. What characteristics would one seek in an ideal engine ignition hattery for a racing car?-a constant voltage as the charge is drained off and the greatest possible energy output per unit weight of hattery. The Nernst equation and Faraday's "second law" point the directions which one should pursue in seeking such a hattery. An electrochemical cell voltage E will differ from the standard cell voltage GO if the activity quotient Q is different from unity.
The cell voltage will change during discharge if the activities of the reactants or products change. For example, in a lead-acid storage battery, the cell reaction is
The cell voltage is
and it decreases by -0.12 V for a ten-fold change in sulfuric acid concentration. We see that one can make an electrochemical cell in which the voltage will not decrease during discharge by choosing reactants and products which have constant activity, independent of the quantities of the species which are present. If all species are
this condition is realized. The racing car battery uses precisely this reaction. Silver-silver oxide electrodes and zinc-zinc hydroxide electrodes are separated by plastic spacers which are permeable to the hydroxide ions of a 40% KOH electrolyte. At the cathodes the reaction is
The hydroxide ion migrates to the anode compartment where the anodic reaction
takes olace. The chemical activities of all soecies remain essentially constant as the reactions proceed; hence the voltaee does not decrease as the cell discharees. H O does ~ the silver-zinc alkaline battery compare with the lead-acid hattery on a weight basis? One mole of the silver-zinc hattery reactants weighs 315.2 g and, if allowed to react reversibly a t standard conditions, will drive 2 moles of electrons through an opposing potential difference of 1.59 V doing 73 kcal of work. One mole of the lead-acid hattery reactants, weighing 642.5 g, can do 87 kcal of work. Thus silver-zinc is 70% better than lead-acid on a weight basis when reversibly discharged. In actual use of course the cells are not discharged reversihly; one must take into account the internal resistance of the cells. As a result of differences in specific resistances of the metals, of differences in mobility of the ions, and of differences in concentration polarization the silver-zinc hattery is even more superior to the lead-acid battery under operating conditions: A further advantage of the silver-zinc battery, not only for racing car application hut also for use in rockets, missiles, satellites, and torpedoes, for which they were developed, is the greater mechanical strength. Lead is much softer than the other metals and mechanical stresses of acceleration and vibration can cause deformation and result in hattery failure.
Volume 50, Number 12, December 1973 / 857
