Current-interrupt method

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. 

Figure 1.12. Current interruption method showing the voltage change with time [3], (From Larminie J, Dicks A. Fuel cell systems explained. 2003 John Wiley Sons Limited. Reproduced with permission of the publisher and the authors.)...

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],... 

The application of the current interruption method is very simple in eJl the cases in which the values of the solution resistance are sufficiently high. 

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. 

Fig. 7. Cell resistance vs. temperature during DMFC operation (current interrupter method).

In the current-interruption method, the flow of current through the cell is periodically stopped. During this time, there is no ohmic drop in the cell (IR = 0) therefore, the true potential of the working electrode... 

Because of the relatively simple implementation, positive feedback is a popular method for reducing ohmic drop. Positive feedback can also operate more rapidly than current-interruption methods and so is useful in analytical or physical measurements. 

The studies of the effect of temperature and humidity (or water uptake) are abundant, beyond those discussed in the above mentioned reviews [305, 306]. Lufrano et al. [310] studied the proton conductivity of Nafion 117 between 25 °C and 90 °C during in-situ fuel cells experiments by measuring the cell resistance by the current interruption method. Yang et al. [311] measured the conductivity of Nafion 115 membranes kept at 100 % RH between 50 °C and 100 °C. Damay and Klein extended the range of temperature of conductivity of Nafion 115 up to 130 °C, including the effect of RH [312]. More recently, Ochi et al. [313] studied the conductivity of Nafion 117 between 31 °C and 118 C as a function of water uptake, and Wu et al. [314] measured the conductivity of Nafion 115 at 60 °C and 80 °C, as a function of RH or water activity, — RH/100. 

The current interrupt method can be used to determine the ohmic resistance of a fuel cell. A resistor is used to close the circuit, which enables the cell to give a stable potential and current output. Subsequently, the external resistor is removed and the instantaneous potential change is used for the calculation of the ohmic resistance by means of Ra = A V/I [36]. EIS is a more sophisticated technique to characterize BES [40]. It entails applying an alternating potential with set amplitude on a set cell potential. The results are analyzed by fitting the data to an equivalent circuit with the potentiostat software and internal resistance is determined from a Nyquist plot. However, the use of EIS has not been widely applied in BES research and therefore no consensus yet exists on frequency range, amplitude, and interpretation of the data with the equivalent circuit [10, 12, 40, 73-75]. 

Compared to other methods, the current interrupt method has the advantage of relatively straightforward data analysis. However, one of the weaknesses of this method is that the information obtained for a single cell or stack is limited. Another issue with this method is the difficulty in determining the exact point at which the voltage jumps instantaneously thus, a fast oscilloscope should be used to record the voltage changes. 

Among the methods for in situ investigating and diagnosing fuel cell stacks, electrochemical methods must also be mentioned, such as current-voltage measurement, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and current-interrupt methods. Among them, EIS is considered one of the most powerful techniques, since it can distinguish kinetically different reaction processes. 

The current interruption (or current interrupt) method is generally used for measuring ohmic losses caused mainly by proton or anion transport resistance in batteries, fuel cells, and other electrochemical cells. The principle of this technique is that the ohmic losses vanish much faster than do the electrochemical overpotentials on the electrodes when the current is interrupted [42]. As shown in Figs 5.5 and 5.6, the cell is operated at a constant current (i) it is... 

To conduct proton conductivity measurements, Buchi et al. [3] designed a current interruption device that used an auxiliary current pulse method and an instrument for generating fast current pulses (i.e. currents > 10 A), and determined the time resolution for the appropriate required voltage acquisition by considering the relaxation processes in the membrane of a PEM fuel cell [3]. They estimated that the dielectric relaxation time, or the time constant for the spontaneous discharge of the double-layer capacitor, t, is about 1.4 x 10 ° s. They found that the potential of a dielectric relaxation process decreased to <1% of the initial value after 4.6r (6.4 x 10 s) and that the ohmic losses almost vanished about half a nanosecond after the current changes. Because there is presently no theory about the fastest electrochemical relaxation processes in PEM fuel cells, the authors assumed a conservative limit of 10 s, based on observations of water electrolysis membranes. They concluded that the time window for accurate current interruption measurements on a membrane is between 0.5 and 10 ns. Another typical application of the current interruption method was demonstrated by Mennola et al. [1], who used a PEM fuel cell stack and identified a poorly performing individual cell in the stack. 

To evaluate the fuel cell performance in detail, the ohmic drop and the cathode overpotential (rj ) were determined and are shown in Fig. 16.8. The ohmic drop was measured using an in situ current interruption method during the H -O PEFC operation and rj was calculated using the equation ... 

The current interrupt technique is the most widely used method of ohmic drop and ohmic resistance evaluation of various electrochemical systems including fuel cells. The principle behind the current interrupt method is the performance of the voltage response of the fuel cell for a given step change of current flow. 

Large current, interruption method, where the voltage is measured as a function of time. 

Further techniques that are exploited for full BES and individual electrode studies include electrochemical impedance spectroscopy [77], current interrupt method [78], Tafel plots [79], and others. 

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