Fadadaic Rectification and Electrode Processes

Abstract

The potentiostatic methods (voltage-step method), galvanostatic methods, coulostatic methods (charge-pulse method), faradaic impedance methods, and faradaic rectification methods have been developed in these fifteen years for the study of fast electrode kinetics. Indirect methods such as photo-electric current by a flash light or a voltammetry using vigorous mechanical stirring possibly be used for this purpose.These relaxation methods are based on measurement of the reaction rate under high rate of mass transfer, which is achieved by diffusion with steep concentration gradient or by vigorous stirring. So far as diffusion is the sole mode of mass transfer, the average flux of reducing species diffusing toward a plane electrode is given by 2Co(D/πt)I/2, where C0 is the bulk concentration of the reducing species, D its diffusion coefficient, and t the time elasped since the biginning of electrolysis. In a process with mixed control by mass and charge transfer, this conclusion is qualitatively valid, and diffusion is no longer rate-determining in a measurement at a sufficiently short time interval.An overall electrode reaction is schematically given by a diagram as shown in Fig. 1. In relaxation methods, it is expected that chemical reactions, specific adsorption of the reactant or/and product, or the charge transfer process will be rate-determining, provided the the time of measurement is sufficiently short. It is noteworthy that the datum obtained at a time interval has to be analysed in relation to a corresponding rate-determining step. In pluse techniques mentioned above, measurement at a short time interval has restriction from charging of the double layer capacity, transient of the pulse from the net-work composing the cell circuit, and the rise-time of a pulse and detector used. The doublepulse galvanostatic method enables measurement at a very short time interval up to about few microsec. by using an ingeneous method of compensating double layer charging. A cathode ray oscilloscope usually used in these method is accompanied by meager precision and accuracy. The faradaic impedance method seems to be the most accurate one, while the upper limit of the frequency is about 20 kc which corresponds to the time interval of 5x10-5 sec. The faradaic rectification method is promising at very high frequencies up to 50 or possibly to 100 me (10-8 sec.), being unaffected by transients and double layer charging by using the double-pulse method or null method. The apparent cxchange current density evaluated by this method was 10 amp./cm2 in the electrode process of Hg(I) in 1.1 M HClO4, which corresponds to an apparent standard rate constant of kao =100 cm./sec. for an electrode process involving both soluble oxidant and reluctant of equal concentration of Kao = 5x10-4 M (n=2, Do = DR =10-5 cm2/sec.) or to kaK =1000 cm./sec. for CK =510-5 M. The theory of the faradaic rectification has been developed by Barker1 and Matsuda- Delahay, and more extensively studied by Delahay-Senda-Weis. This review will cover rather an experimental point of view including the method of analysis and experimental techniques.