Double layer charging effects

The tip current depends on the rate of the interfacial IT reaction, which can be extracted from the tip current vs. distance curves. One should notice that the interface between the top and the bottom layers is nonpolarizable, and the potential drop is determined by the ratio of concentrations of the common ion (i.e., M ) in two phases. Probing kinetics of IT at a nonpolarized ITIES under steady-state conditions should minimize resistive potential drop and double-layer charging effects, which greatly complicate vol-tammetric studies of IT kinetics. 

A fundamental disadvantage of controlled-current techniques is that double-layer charging effects are frequently larger and occur throughout the experiment in such a way that correction for them is not straightforward. Treating data from multicomponent systems and stepwise reactions is also more complicated in controlled-current methods, and the waves observed in E-t transients are usually less well-defined than those of potential sweep i-E curves. 

Fig. 51. Scheme of the double-layer charging effect at three different sweep rates on LSV. Constant bulk concentration of the electroactive species c% as well as the surface area S and the double-layer capacitance Qi are assumed ratio of the sweep rates Vj V2 V3 = 1 4 9 charging currents 1, to I<-3 and peak currents Ip, to Ip3 in the same series. 

The major problem associated with chronopotentiometric measurements is the determination of r, owing to the double layer charging effects. In the absence of these effects the transition from one potential determining process to another would be essentially instantaneous, and r could be easily determined. This is not, however, true of real experiments the charging of the double layer requires a finite time as given by the solution of Equation (2.66) ... 

EXPERIMENTAL PROBLEMS ASSOCIATED WITH LINEAR SWEEP AND CYCLIC VOLTAMMETRY 6.7.1 Double layer charging effects... 

T 1, then semiinfinite diffusion conditions prevail, and the conventional electrochemical results are valid and can be used in data analysis. However if t > 1, then the full finite diffusion problem must be solved. The condition t 1 corresponds to the short-time regime. If data are captured in this reigme, then the mathematics becomes much simpler, but other complexities, such as double-layer charging effects, must be considered. Finite diffusion effects almost certainly come into play on longer time scales. 

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