Oscillatory Phenomena on Solid Electrodes

Oscillatory behavior observed as periodic potential transients at constant current or periodic current transients at constant potential is found frequently when more than two parallel electrode reactions are coupled. Usually, an upper and a lower current-potential curve limit the oscillation region. These two curves represent stable states [139] according to the theory of stability of electrode states [140]. Oscillatory phenomena occurring during the oxidation of certain fuels on solid electrodes are discussed in this section. The discussion is not extended to porous electrodes because the theory of the diffusion electrode has not been developed to the point to allow an adequate description of the complex coupling of parallel electrode reactions and mass transport processes in the liquid and gaseous phase. 

The following interpretation of the oscillatory behavior observed during the galvanostatic oxidation of fuels like CH3OH, CH2O and HCOOH is proposed by the author for large fuel concentrations when the influence of mass transport processes is negligible. The net reaction 

CgHpOq is compensated by the larger rate constant of the process of formation at less positive potentials. The oxygen layer is reduced when the appropriate potential for the given solution is passed during the second part of the cycle. The inhibition of the fuel oxidation to CO2 is removed. The initial state of the surface for the beginning of another cycle is approached. 

An analysis of oscillatory kinetics in the electrochemical oxidation of formate and ethylene was given recently by Wojtowicz, Marincic, and Conway [142]. In the formate case the oscillations are associated with electrochemical reactions. An autocatalytic step for the removal of the oxygen layer was considered consistent with the conditions for oscillations under certain circumstances. For ethylene oxidation it was shown that only imaginary solutions are possible under certain conditions of adsorption and mass transport control. Necessarily the derivations depend strongly upon the form of the kinetic expressions and upon the reaction mechanisms. The reader is referred for details to reference 142. 

Stonehart, P. Proc. 5th International Symposium on Power Sources 1966, p. 509. 

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