Steady-state potentiostatic method

The validity of the SECV method was first tested for a stage II PEVD sample at 550°C. Thus, the results from SECV and the steady-state potentiostatic method could be compared. 

Because of the geometric change at the working electrode due to growth of the PEVD product, the results from a steady-state potentiostatic study are not applicable to stage I of PEVD. In this study, a solid electrolyte cyclic voltammetry (SECV) method was applied for two reasons ... 

Steady-state measurements can be made under both galvanostatic and potentiostatic conditions. It is irrelevant for the results of the measurements whether the current or the potential was set first. But in certain cases in which the polarization (/ vs. E) curve is nonmonotonic and includes a falling section (BC in Fig. 12.4), the potentiostatic method has important advantages, since it allows the potential to be set to any point along the curve and the corresponding current measured. But when the galvanostatic method is used, an increase in current beyond point B causes a jump in potential to point D (i.e., the potential changes discontinuously from the value Eg to the value Eg,) and the entire intermediate part of the curve is inaccessible. 

The surface of the electrode must remain constant as the potential is changed in a series of potentiostat, steady-state measurements. Apart from difficulties connected with impurity adsorption, which are reduced if the experiments are carried out sufficiently quickly (Cliapter 8), it may be that thermodynamically the most stable state of the surface changes with potential most commonly by means of oxide formation. If this is suspected, it is helpful to keep observing the electrode surface during the potential measurements. The methods used must be in situ spectroscopic ones (see Section 7.5.15) FTIR or ellipsometiy are the most readily applied (see Sections 7.5.15.2 and 7.5.16, respectively). 

The steady-state method has advantages in its freedom from double-layer charging and in the simplicity of light and current measurements. The transient method is mechanically simpler and does not require a dual potentiostat (Chap. 6). Moreover, the rate of electrolysis is smaller, and hence solutions may suffer more slowly from the buildup of contaminants arising from side reactions. 

The mean standard rate constant was k° = (2.1 + 0.2) x 10 3 cm s 1 and showed that SECM is a powerful method to determine the rate constant. The curve fitting and calculation of the offset are crucial for reproducible result. The special advantage of the method is its relative immunity to inaccuracies introduced by uncompensated resistance or limited rise time of potentiostats since the analysis occurs under steady-state conditions and very low total currents. 

The first step in the experimental procedure consists of preparative electrolysis of the aromatic compound A to A . The preparative potentiostat is then disconnected and a UME is inserted into the cathodic compartment. The steady-state oxidation current of A is recorded as a function of time for a certain time period to ascertain that the stability of A is high. If this is indeed the case, the alkyl halide RX is added to the solution while it is stirred for a few seconds to assure that homogeneous conditions apply for the reaction of Eq. 90. The recorded current is observed to decay exponentially towards zero. A plot of In / versus t is shown in Figure 16 for four different combinations of aromatic compounds and sterically hindered alkyl halides. From the slopes of the straight lines, -2A etCrx, A et values can readily be obtained. The method is useful for the study of relatively slow reactions with kET < 10 M- s-. ... 

Let us consider, for example, the simple nernstian reduction reaction in Eq. (221) and a solution containing initially only the reactant R. Before any electrochemical perturbation the electrode rest potential Ej is made largely positive to E . At time zero the potential is stepped to a value E2, sufficiently negative to E , so that the concentration of R is close to zero at the electrode surface. After a time 6, the electrode potential is stepped back to El, so that the concentration of P at the electrode surface becomes zero. When this potentiostatic perturbation, represented in Fig. 21a, is applied in a steady-state method, the R and P concentration profiles are linear and depend only on the electrode potential but not on time, as shown in Fig. 20a (for k 0). Yet when the same perturbation is applied in transient methods, the concentration profiles are curved and time dependent, as evidenced in Fig. 21b. Thus it is seen from this figure that a step duration at Ei, much longer than the step duration 0 at E2, is needed for the initial concentration profiles to be restored. This hysterisis corresponds to the propagation of the diffusion perturbation within the solution, which then keeps a memory of the past perturbation. This information is stored via the structuring of the concentrations in the space near the electrode as a function of the elapsed time. 

O ) polarography uses a dropping mercury cathode and the current is measured 00 the rotating-disc method uses a spinning platinum disc as the cathode and the current in the steady state situation at a series of rotation speeds is measured Hi) the potentiostatic method uses fixed electrodes and the fall of current with time is measured. 

This latter method can be called potentiostatic, and its results in general correspond more to the steady-state condition of practical use than do the potential sweep treatments that involve potential changes all the time measurements are being made. 

In contrast to the EIS method, the Tafel-extrapolation, Tafel-curve-modeling and polarization-resistance methods are conducted under essentially dc conditions. In these cases, in generating the appropriate Eexp versus log iex or iex curve, the potentiodynamic potential scan rate is very slow, or the time between potentiostatic potential steps is very long. The common practice is a potential scan rate of 600 mV/h or an equivalent step rate of 50 mV every 5 min. Underthese conditions, it is assumed that a steady-state, extemal-current-density results at every discrete potential. Consequently, every element in the electrical circuit is purely resistive in nature, and therefore, the applied potential and resultant extemal-current-density are exactly in phase. Since the impedance (normalized with respect to specimen area) is dEexp/diex, under these conditions, the impedance, Z, at Ecorr is given by (see Eq 6.29) ... 

For a correct use of this method, which can be applied using either the three-wire or the four-wire technique, the electric characteristics of the equipment must be examined very carefully in order to define the experimental procedures. In principle, no problems are encountered if use is made of potentiostats like EG G s mods. 173 and 273 or of the Solartron mod. 1286 electrochemical interface. The electrochemical system, however, must be polarized by means of a current square wave of such duration as to permit the polarization potential to reach a steady-state value. 

In general, potentiostatic or potential sweep methods are preferred over a galvano-static approach which has the tendency to oscillate and destabilize the steady state conditions within the pore. A method devised by Schmucki et al. [33] involves the... 

The techniques for characterizing the kinetics of electrode reactions can be classified into steady-state and transient methods. The steady-state methods involve the measurement of the current-potential relationships at constant current (galvanoslatic control) or constant potential (potentiostatic control) conditions and measuring the response, which is either the potential or the current after a steady state is achieved. The non-steady-state methods involve the perturbation of the system from an equilibrium or a steady-state condition, and follow the response of the system as a function of time using current, potential, charge, impedance, or any other accessible property of the interface. Relaxation methods are a subclass of perturbation methods. 

Current-overvoltage data can be obtained galvanostatically by applying a constant current and measuring the potential under steady-state conditions, or potentiostatically, by imposing a constant potential and recording the current. Current is applied stepwise, both in the upward and downward directions, to establish reproducibility. The galvano-static method is simpler than the potentiostatic method, but modem instmments can employ either method. 

The polarization curve can also be obtained by the potential sweep method, employing a potentiostat and a function generator at low sweep rates of about 20mVmin . However, preliminary experiments at various sweep rates are recommended to obtain a quasi-steady-state polarization curve, which is close to the true steady-state curve. 

Bruce etui, established a potentiostatic polarization method for solid polymer electrolytes [450], which is also used for diluted solutions because of its simphcity. For infinitely dilute electrolytes it was shown that this method is suitable for hquids as well [451]. Applying a small constant potential to a solution between nonblocking electrodes leads to decrease of the initial current value until a steady-state value is reached. The steady-state current is caused by the cations [450], so the cation transference number can be easily determined by dividing the cationic current by the initial current. Because electrode surfaces or rather passivating layers vary with time, this inaccurate description can be corrected by impedance measurements shortly before and after the potentiostatic polarization [452]. For small polarization potentials (< 10 mV), the steady-state current hs and initial current Iq are described as [450]... 

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