Potentiostatic Charging

The concentration profile and a typical anodic transient for potentiostatic control are shown schematically in Fig. 3. Whether obtained potentiostatically or galvanostatically, the transients are typically characterized by four parameters—breakthrough time inflection point time (/,), half-rise time (/1/2), and time lag 

The diffusion of hydrogen in the absence of trapping can be represented by the classical form of Pick s second law  

Solving the differential equation subject to conditions (12)-(14) yields an expression for the concentration C(x, 0  

This approximation is valid up to / = 0.965/qo, so it is feasible to calculate the diffusivity from Eq. (18) by determining the time required to achieve a given fraction of the steady-state permeation rate. 

In some cases, a long time may be required to reach the steady state. However, the ratio of currents for two short times and 2 can be obtained from Eq. (18) as 

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Figure 9. Discharge curves at 4 mA cm of the three types of supercapacitors a) polypyrrole/LiC104 -propylene carbonate (PC)/polypyrrole b) polypyrrole/ LiC104-PC/poly(3-methylthiophene) c) poly(dithieno[3,4-6 3, 4 -rf ]thiophene)/ (C2Hs)4NBF4-PC/poly(dithieno[3,4-b . i, A -d]thiophene), potentiostatically charged at 1.1 V, 1.15 V, and 3.0 V, respectively.

A potentiostatic charging method was used by Gerischer and Mehl (70a). In this method, the potential of the electrode is changed fast (10 sec) from one constant value to another and the current-time... 

Calvanostatic Charging Galva-nostatic charging tends to be used more often than potentiostatic charging in... 

The diffusivity can be calculated from the breakthrough time by the same equation as that for potentiostatic charging with pure diffusion control. 

Palladium The high solubility of hydrogen coupled with rapid kinetics for hydrogen entry have made palladium the focus of numerous studies. It has widely been assumed in these studies that the subsurface concentration in palladium and palladium coatings instantaneously increases to a constant value at the start of charging. However, the more appropriate input condition for potentiostatic charging appears to be a step function in flux, as shown in Fig. 11 [53]. 

This review focuses on the use of electrochemical techniques for studying hydrogen ingress under conditions of galvanostatic and potentiostatic charging at foil and bulk electrodes. Previous... 

As with potentiostatic charging, multiple values of D can be determined from successive transients obtained for different charging conditions. In a generalized series of transients, the. th transient is initiated after the (.s—l)th transient has reached a steady state.At t = ts, the sth initial hydrogen concentration attained under galvanostatic charging is... 

This time again provides a convenient parameter for computing the diffusivity. The relationship between and thickness for galvanostatic charging is similar to that for potentiostatic charging except that the constant is different. ... 

As with potentiostatic charging, diffusion control of the permeation rate can be verified using these equations, since the time characteristics should be proportional to the square of the thickness in this case too. 

The input boundary condition for potentiostatic charging, as discussed earlier in Section III, has been considered in terms of constant concentration and constant flux, while galvanostatic charging has been modeled assuming a constant flux condition. Both of these boundary conditions ignore the outward flux (/ex) the input surface. If hydrogen ingress is not controlled solely by diffusion and if the outward flux exerts an influence on the kinetics of the overall process, the input boundary condition can be written as ... 

The electrochemical stability of the input surface during galvanostatic or potentiostatic charging in permeation experiments... 

An additional test for diffusion control is provided by the dependence of the steady-state anodic current on membrane thickness. In potentiostatic charging, the hydrogen concentration is fixed in the case of pure diffusion control, and therefore should be proportional to 1/L (see Section III). This test is not necessarily applicable for a constant flux boundary condition because C /L,... 

Figure 17. Comparison of experimental (solid curve) and theoretical permeation transients for Armco iron. (After Ref. 48. Reprinted with permission from Scripta MetalL, Copyright 1980, Pergamon Press pic.) Experimental data (a) Potentiostatic charging in 0.5 M H2SO4. (After Ref. 96. Reprinted by permission of the publisher, The Electrochemical Society, Inc.) (b) Galvanostatic charging at 2mAcm in 0.1 M NaOH. (After Ref. 97. Reprinted with permission from Metall Trans., The Minerals, Metals, and Materials Society.)...

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