Principles of transient techniques

Generally the current density j that is measured is determined both by the rate of the electrochemical reaction and by the transport of the reacting species to the interface. Prom an electrochemist s point of view there is little interest in transport processes as such, and we would like to eliminate their effect on the data. For this purpose it is convenient to define a few quantities. 

If transport were infinitely fast, the concentrations c x and c ed of nonadsorbing reacting species would be the same at the interface as in the bulk. The measured current density would solely be determined by the reaction, and would equal the kinetic current density. 

The other limiting case is that of an infinitely fast reaction, when the current is determined by transport only. It is customary to call such a reaction reversible, and denote the corresponding current density, which is determined by transport alone, as the reversible current density jrev. It is determined by the transport, usually by diffusion, 

Since transport and electrochemical reactions are in series, the slower process determines the overall current. Hence we can obtain the rate constants of the reaction only, if the reversible current jrev is not much slower than the kinetic current. This limits the magnitude of the reaction rates that can be measured with any given method. 

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In order to get closer from the catalytic conditions (for example in DeNO reaction exhaust gases are to be treated) investigators developed reactor cells allowing the infrared study of catalysts underflow. The principle of transient technique is then to introduce... 

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