Galvanostatic response

Figure 11. Transient galvanostatic response of the rates of ethylene oxidation r, (O) and deep oxidation rt (0) and of the cell overvoltage AV for reactor RC 9. Electrolyte breakdown occurred at 40 min. Conditions RC 9, 420°C, i — 100 pA, P0l = 0.095 bar, PET = 0.012 bar.

Figure 3. Galvanostatic response of a Zn electrode during electropolymerization of PPy at j = 20 A/dm in an aqueous solution of 2 M NaSac + 0.5 M pyrrole at pH 5. Rotation speed of the electrode 4000 rpm...

M. Keddam and C. Pallotta, Galvanostatic response of the passive film on iron in acidic media, Electrochim. Acta 30 469, (1985). 

Fig. 7.45 Impedance spectrum and approximate equivalent circuit of a cell with blocking electrodes. (The galvanostatic response was given in Fig. 7.25.) The simplifications apply to the particular frequency ranges. The discussion is analogous to that for Fig. 7.25 Short/long times correspond to high/low frequencies. The approximations axe not valid in the region of the broken line and do not correspond to the accurate calculation (cf. also Fig. 7.44) [431].

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

It was quickly observed that the catalytic rate response during galvanostatic transients can be reasonably well approximated by the response of a first order system, i.e. by ... 

The electrochemically induced creation of the Pt(lll)-(12xl2)-Na adlayer, manifest by STM at low Na coverages, is strongly corroborated by the corresponding catalyst potential Uwr and work function O response to galvanostatic transients in electrochemical promotion experiments utilizing polycrystalline Pt films exposed to air and deposited on (T -AbCb. 3637 Early exploratory STM studies had shown that the surface of these films is largely composed of low Miller index Pt(lll) planes.5... 

Figure 5. Evolution of cathodic response to 10 pA cm" galvanostatic pulse as Ecorr increases during biofouling of 316L stainless steel in fresh river water. Data shown by solid lines and circles indicate points generated from curve lit to Eq. (6). (Reprinted from Ref. 12 with permission from NACE International.)...

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Let us consider, for instance, the response mechanism of a polycation-selective galvanostatically controlled sensor. The polymeric membrane is in contact with a NaCl solution. The membrane of the sensor is formulated with a lipophilic salt, for instance, tetradodecylammonium dinonylnaphthalenesulfonate (TDDA-DNNS), which has a relatively high affinity to protamine. Even though protamine is presented in the sample, spontaneous extraction does not take place due to the high lipophilicity of TDDA-DNNS, thus the initial concentration of protamine or sodium cations in the membrane is close to zero. 

The behavior of potentiometric and pulsed galvanostatic polyion sensors can be directly compared. Figure 4.11 shows the time trace for the resulting protamine calibration curve in 0.1 M NaCl, obtained with this method (a) and with a potentiometric protamine membrane electrode (b) analogous to that described in [42, 43], Because of the effective renewal of the electrode surface between measuring pulses, the polyion response in (a) is free of any potential drift, and the signal fully returns to baseline after the calibration run. In contrast, the response of the potentiometric protamine electrode (b) exhibits very strong potential drifts. 

Within the accuracy of the experimental data the galvanostat-ic transient response of AV is identical to the transient rate response Ari andAr2, i.e. t = x where x is the relaxation time constant for the two rates (17. This s shown in figure 5 for two different reactors under similar operating conditions and also in figure 6 where the transient and the steady state Ar values from four reactors are plotted vs. the cell overvoltage AV. In view of the fact that r. is proportional to the surface area Q it follows from figure 6 tftat for constant gas phase composition... 

Most galvanostatic transients followed a first order response with reasonable accuracy, in agreement with (25) ... 

Electrochemical Equipment. Electrochemical experiments were performed using either a PAR Model 175 universal programmer and a PAR Model 363 potentiostat/galvanostat, or a Pine Instruments RDE-4 bipotentiostat, coupled with a Kipp and Zonen BD 91 X-y-y recorder. The current-time response for the chronoamperometry experiments was recorded with a Nicolet 4094 digital oscilloscope. All potentials were measured vs. a Ag/10"2 M Ag+ reference electrode. 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

The transition time in the galvanostatic mode is listed in Table El. The concentration of electroactive species is 0.1 M and the diffusion coefficient is 10-5 cm2/s. Find the number of electrons transferred and draw a current-time response in a potentiostatic mode. 

Typical plots of AE vs. a dimensionless function of tV2 in Fig. 7 are reproduced from a discussion of the potentialities of the galvanostatic step method given by Kooijman and Sluyters [32], It is seen that, at sufficiently large times, AE becomes a linear function of t1/2. At first [31], analysis procedures of the complex AE vs. tvl relation were based on extrapolation of this linear section to tyl = 0, yielding the intercepts indicated in Fig. 7. However, it has been shown that, in this way, the content of information about the kinetic parameters, k and a, is not optimally utilized [32], Therefore, numerical analysis of the complete AE vs. t response with the aid of suitable computer programs has to be advocated. In principle, such an analysis yields the values of X, R , and Cd as well as a check on the validity of eqn. (30). 

The experiment can be done using a stationary technique, but the well-known large-amplitude step techniques (either galvanostatic or potentio-static) can also be applied. The latter have the advantage that the response vs. time function can be extrapolated to t = 0, thus approaching more closely the situation where the surface concentrations have not been changed (see Sects. 2.1.2 and 2.2.2). 

Radicals are generated at a tubular electrode and are then transported by laminar flow into the ESR cavity which, as a downstream detector, is analogous to a second electrode. The theoretical response for the cases where the radicals are stable or decompose by first- or second-order kinetics has been derived and experimentally confirmed [126, 301, 302]. The flow-rate dependence is different for each of the three situations which provides a diagnostic for the type of kinetics. Further information may be obtained from galvanostatic transients which allow the elucidation of electrode and radical surface processes [303]. Very recently, an in situ channel tube electrode has been described for electrochemical ESR which also allows shorter-lived species to be observed and smaller surface coverages to be analysed [304—306]. 

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

Furthermore, it is also not necessary to discuss different excitations in detail as long as we restrict ourselves to the linear response regime. There it holds that the response to any excitation allows the calculation of the response to other excitations via the convolution theorem of cybernetics.213 In the galvanostatic mode, e.g., we switch the current on from zero to /p (or switch it off from 7p to zero) and follow IKj) as a response to the current step. The response to a sinusoidal excitation then is determined through the complex impedance which is given by the Laplace transform of the response to the step function multiplied with jm (j = V-I,w = angular frequency). 

Figure 38. The voltage response on a galvanostatic polarization of a mixed conductor with ion-blocking electrodes ( eqc. (G3)3,15). Reprinted from J. Maier, Evaluation of Electrochemical Methods in Solid State Research and Their Generalization for Defects with Variable Charges , Z. Physik. Chemie N.F., 191-215, Copyright 1984 with permission from Oldenbourg Verlagsgruppe.

The galvanostatic intermittent titration technique (GITT) has been first proposed by Weppner and Huggins in 1977 [22], This method is of particular interest for the measurement of ion transport properties in solid intercalation electrodes, used in lithium-ion batteries, for instance [18]. The determination of the diffusion constants relies on Fick s law. The GITT method records the transient potential response of a system to a perturbation signal a current step (/s) is applied for a set time xs, and the change of the potential (E) versus time (0 is recorded (Figure 1.11) [18,22],... 

---