Current Interrupt Measurement

The latter equals IRwc where RWc is the ohmic resistance between the working and counter electrode. Experimentally it is rather easy to measure the riohmic.wc term using the current interruption technique as shown in Figure 4.9. Upon current interruption the ohmic overpotential r 0i,mjCtwc vanishes within less than 1 ps and the remaining part of the overpotential which vanishes much slower is t w+T c (Eq. 4.9). 

Figure 4.9. Use of the current interruption technique to measure the ohmic overpotential, r ohmic,wc> between the working (W) and counter (C) electrode.

The voltage drop I R should be estimated and eliminated from the measured potential. It can be directly determined by fast current interruption measurements about 1 xs after shutting down the current, the potential is already decreased by all ohmic voltage drops while all other... 

Figure 5. Measurement and analysis of steady-state i— V characteristics, (a) Following subtraction of ohmic losses (determined from impedance or current-interrupt measurements), the electrode overpotential rj is plotted vs ln(i). For systems governed by classic electrochemical kinetics, the slope at high overpotential yields anodic and cathodic transfer coefficients (Ua and aj while the intercept yields the exchange current density (i o). These parameters can be used in an empirical rate expression for the kinetics (Butler—Volmer equation) or related to more specific parameters associated with individual reaction steps.(b) Example of Mn(IV) reduction to Mn(III) at a Pt electrode in 7.5 M H2SO4 solution at 25 Below limiting current the system obeys Tafel kinetics with Ua 1/4. Data are from ref 363. (Reprinted with permission from ref 362. Copyright 2001 John Wiley Sons.)...

Figure 7. Steady-state cathodic current—overpotential characteristics of porous Pt electrodes on Ca-doped ceria, measured at 600 °C in air using current-interruption. (Reprinted with permission from ref 51. Copyright 1979 Electrochemical Society, Inc.)...

The two main techniques for measuring electrode losses are current interrupt and impedance spectroscopy. When applied between cathode and anode, these techniques allow one to separate the electrode losses from the electrolyte losses due to the fact that most of the electrode losses are time dependent, while the electrolyte loss is purely ohmic. The instantaneous change in cell potential when the load is removed, measured using current interrupt, can therefore be associated with the electrolyte. Alternatively, the electrolyte resistance is essentially equal to the impedance at high frequency, measured in impedance spectroscopy. Because current-interrupt is simply the pulse analogue to impedance spectroscopy, the two techniques, in theory, provide exactly the same information. However, because it is difficult to make a perfect step change in the load, we have found impedance spectroscopy much easier to use and interpret. 

A typical arrangement of components in a tensimetric titration is presented in Fig. 9.5, which shows the previously discussed tensimeter and a calibrated bulb attached to a vacuum line.2 The sample container on the tensimeter is fitted with a small reciprocating stirrer which consists of a thin glass rod connected to a glass-encased headless nail or glass-encased bundle of soft iron wire. This stirrer is driven by an external solenoid, the field of which is switched on and off by a current-interrupting device, the details of which are laid out in Fig 9.6. The size of the calibrated bulb is chosen so that it will contain the desired amount of gas for each addition at a pressure which is convenient and accurately measured (e.g., 100-500 torr). The calibration procedure and steps used dispensing gas from such a bulb are described in Section 5.3.G. 

The particular production case of the DLC-type BCAP0350E250 from Maxwell Technologies has been analyzed. Table 11.1 shows the data of a production sample of 2800 cells. The capacitance data are collected with a 10A charge between 0 and 0.6VDC. The series resistance is measured during the current interruption at 0.6 VDC. The cell datasheet gives a nominal capacitance of 350F ( 20% or 280/420 F) and an ESR of 3.2 mQ ( 25% or 2.4/4.0mQ). 

This set of experiments has focused on the use of two nondestructive electrochemical techniques to measure polarization resistance and thereby estimate the corrosion rate. In addition, the effects of scan rate and uncompensated ohmic resistance were studied. Three main points should have been made by this lab (1) Uncompensated ohmic resistance is always present and must be measured and taken into account before Rp values can be converted into corrosion rates, otherwise an overestimation of Rv will result. This overestimate of Rp leads to an underestimate of corrosion rate, with the severity of this effect dependent upon the ratio Rp/Ra. (2) Finite scan rates result in current shunted through the interfacial capacitance, thereby decreasing the observed impedance and overestimating the corrosion rate. (3) Both of these errors can be taken into account by measuring Ra via EIS or current interruption and by using a low enough scan rate as indicated by an EIS measurement in order to force the interfacial capacitance to take on very large impedance values in comparison to Rp. 

For potential measurements without any current flow, the IR drop is zero. In all other cases, beside the electrode placement mentioned above, IR drop should be estimated and eliminated from the measured potential It can be either compensated during potential control, or one can correct for it to obtain the real electrode potential. The IR drop can be straightaway determined by fast current interruption measurements Shortly, e.g., 1 ps, after a current shut-down, the potential is already decreased by all ohmic voltage drop while other overvoltages remain at their stationary values. 

The current interruption method is used to measure the internal resistance of an electrochemical system. In a fuel cell, it is usually used to measure the membrane resistance. 

In general, the current interruption method is used to measure ohmic losses (i.e., cell resistance) in a PEM fuel cell. The principle of the technique is that the ohmic losses vanish much more quickly than the electrochemical overpotentials when the current is interrupted [26],... 

Abe T, Shima H, Watanabe K, Ito Y (2004) Study of PEFCs by AC impedance, current interrupt, and dew point measurements I. Effect of humidity in oxygen gas. J Electrochem Soc 151(l) A101-5... 

Potential vs. Ag QRE, corrected according to the ohmic resistance measured by current interruption. 

Our laboratory experience, however, has shown that, when measurements are performed axlopting the galvanostatic mode, the use of alternating current is a very satisfactory means of determining the value of the resistance R, accurately. In fact, unlike the direct current interruption technique, the AC technique can be utilized for a very large class of electrochemical systems. 

Based on this consideration and on the certainty that this technique is more effective than the current interruption method, the codes SOFTCOR-AC-GS [71] and SOFTCOR-AC-GE [72] have been developed for determining the value of the resistance R, through sruface impedance measurements over the interval [7, 13] kHz when the capacitance values are rather small. 

Once the cell had reached run temperature, conductivity across the cell was measured by the current interrupt method. The equilibrium potentials at the cathode and anode were measured with respect to the reference electrode. Baseline exit cathode gas compositions were also measure at this point. Current was then applied to the cell in a step-wise fashion and the cell was allowed to equilibrate after each current step. Once stabilized, potentials with respect to the reference electrode and the exit gas compositions were measured. 

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