Potentiostats scanning rate

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

As the scan rate v increases, an additional kinetic bottleneck is introduced. Because the electrons move slowly through the electrode, there is a perceptible time lag between the potential at the voltage source (generally a potentiostat) and that at the electrode solution interface. Since all potentials experience this lag , the peaks are shifted to more extreme potentials, thus giving the CV a stretched appearance. 

The homogeneous catalysis method is suitable to measure rate constants over a very wide range, up to the diffusion limit. The lower limit is determined by interferences, such as convection, which occur at very slow scan rates. It is our experience that, unless special precautions are taken, scan rates below lOOmV/s result in significant deviations from a purely diffusion-controlled voltammetric wave. For small values of rate constants (down to 10 s ), other potentiostatic techniques are best suited, such as chronoamperometry at a rotating disk electrode UV dip probe and stopped-flow UV-vis techniques. ... 

After assembly, the sandwich-type cell is heated to 50-70°C and maintained at a fixed selected value during acquisition of electrochemical data. The potential is controlled using an RDE 3 Pine potentiostat equipped with a built-in signal generator. Cyclic voltammograms were recorded at scan rates of 5 to 100 mV/s. All potentials are reported with respect to Li[C/R],... 

While long-term potentiostatic tests can be useful, they are extremely time-consuming and expensive. Potentiostaircase tests can be performed in lieu of potenti-odynamic tests, though at equivalent potential scan rates, the results should be identical. In addition, the time frames involved are usually still very short compared to the projected life. Thus a number of approaches have been developed for accelerating the process of initiation in order to determine the potential below which initiated sites will repassivate (i.e., Ew). 

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

Although electrochemical characterizations have recently been performed on single intercalation particles, in most cases composite powdery electrodes containing a mixture of intercalation particles, electrically conductive additives (e.g., carbon black) and PVDF binder have also been used. In order to obtain consistent results and to reach comprehensible intercalation mechanisms in these electrodes, basic electroanalytical characterizations such as slow-scan rate -> cyclic voltammetry (SSCV), -> potentiostatic intermittent titration (PITT) (or -> galvanostatic intermittent titration, GITT), and -> electrochemical impedance spectroscopy (EIS) should be applied in parallel or in a single study. 

Some deviation of AEp is typically observed experimentally due to solution resistance, especially at higher scan rates. Many potentiostats can compensate for this phenomenon with internal resistance compensation. When ipip 1 and/or when AEp > 59/n mV, the electrochemical process is quasireversible. If no return wave is observed in the cychc... 

Although the potentiostat has an adjustable IR compensation circuit, this was not used. When it was used, the scan rate varied. Compensation did not seem to have much effect. Some researchers have developed their own IR compensation circuit for the high resistance solution ( ), some added a supporting electrolyte (12 11) And some simply ignored it (H). Since the behavior of electrolytes under supercritical conditions is not well known, no supporting electrolyte was added to eliminate the IR drop in this experiment. Furthermore, the IR circuit was not used for this study either, since the desired electrochemical data, such as exchange current density, open circuit potential, and transfer coefficients, can be obtained from the polarization curves without IR compensation. 

Type 430 stainless steel (Fe, 16 to 18 wt% Cr, 0.12 wt% C maximum) is used as an ASTM standard material to certify the performance of potentiostats in accurately and reproducibly determining polarization curves (Ref 27). The environment is 1 N H2SO4 at 30 °C, and the scan rate is specified as 600 mV/h. To meet the standard, a measured polarization curve determined using the reference standard must fall within the band shown in Fig. 5.30. An advantage in using this alloy is the large... 

Standard ASTM potentiodynamic anodic polarization plot for certification of potentiostat performance. Type 430 stainless steel in 1 N H2S04at 30 °C. Scan rate of 600 mV/h. Test curve is to lie within the shaded region. Redrawn from Ref 27... 

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) ... 

With a potentiostat the potential at the working electrode is linearly increased from 1.0 to 1.6 V and then decreased back to 0 V. In the first interval 1 is oxidized to the radical cation l+ with a peak potential of p.a = 1-38 V. 1 is stable in this solvent and is reduced in the reverse scan back to 1 at p,c = 1-32 V. The ratio of the current for reduction and oxidation ip c-ip.a = 1 indicates the stability of the radical cation. All of 1, that is formed by oxidation of 1 is reduced back to 1. This behavior is termed chemically reversible. Upon addition of 2,6-lutidine, the radical cation 1 reacts with the nucleophile to afford 2 , which is further oxidized to a dication, which yields the dication with 2,6-lutidine. This can be seen in the decrease of /p,c fp,a and an increase of due to the transition from an le to a 2e oxidation. From the variation of the ratio ip.c-ip,n with the scan rate, the reaction rate of the radical cation with the nucleophile can be determined [9]. This can also be aehieved by digital simulation of the cyclovoltammogram, whereby the current-potential dependence is calculated from the diffusion coefficients, the rate constants for electron transfer and chemical reactions of substrate and intermediates at the electrode/electrolyte interface [10]. With fast cyclovoltammetry [11] scan rates of up to 10 Vs- can be achieved and the kinetics of very short-lived intermediates thus resolved. 

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