Total operational impedance

The impedance spectra of the DMFC cathode electrodes are obtained by subtracting the anode impedance from the total cell impedance. The cell impedance, ZDMFC, was obtained from normal operation of the DMFC (i.e., the cathode side was fed with air or 02 and the anode side was fed with methanol solution). The anode impedance was measured by supplying H2 to the cathode compartment, which was used as a dynamic hydrogen reference electrode. Since the impedance of the H2 electrode is negligible, the measured impedance is considered to be the anode impedance, Zanode. The cathode impedance is therefore... 

Typical impedance spectra of a DMFC cathode operating on air and pure 02 were shown in Chapter 5, Figure 5.39. Two arcs were observed when air was used as the oxidant, while only one arc was observed for 02. According to the previous equation, the membrane resistance (or the arc at high frequencies caused by the membrane) was not present in the cathode impedance spectra. Since the membrane impedance was included in both the total cell impedance ZDMFC and the anode impedance subtracting the two cancelled out the membrane impedance. 

Quite often, in complex antenna systems, it is found that one or more of the elements has a negative operating impedance that is, the total of the coupled Impedance from all other elements exceeds the self-impedance of that element, and the element actually returns power to the transmitter. 

There is another important factor that strongly affects the performance or rate capability of a cell, the internal impedance of the cell. It causes a voltage drop during operation, which also consumes part of the useful energy as waste heat. The voltage drop due to internal impedance is usually referred to as ohmic polarization or IR drop and is proportional to the current drawn from the system The total internal impedance of a cell is the sum of the ionic resistance of the electrolyte (within the separator and the porous electrodes), the electronic resistances of the active mass, the current collectors and electrical tabs of both elec-... 

The total earth loop impedance for ready operation of the largest rated excess current protective device relied upon for earth leakage protection. 

In pressure-driven operation, considerable band broadening was observed at high linear velocity, although the separation impedance was much lower than that of a particle-packed column owing to the much lower flow resistance. The separation impedance (E = AP to / r N2 = (AP / N) (to / N) (l/r )) expresses the total column performance in terms of the reciprocal number of theoretical plates per unit time and pressure drop. Because the contributions of the B- and C-terms are expected to be similar for a pressure-driven mode and an electro-driven mode, the difference in performance can be attributed to the greater contribution of the A-terms in Eqn. 5.2 in the pressure-driven mode. The contribution of the A-term is known to be less in CEC than in HPLC [6],... 

EIS has played an important role in fuel cell technology development, as one of the most important research tools for fuel cell diagnosis. EIS can help to identify the contributions from different components or processes to the total impedance of a PEM fuel cell. Such information is very helpful for understanding the fundamental processes within the fuel cell, the performance-structure relationships, and the contributions of various components to performance loss, as well as the associated failure modes and mechanisms EIS thus assists with fuel cell design optimization and selection of the most appropriate fuel cell operating conditions. In this chapter, we will present some typical examples of the applications of EIS in PEM fuel cell research, and an overview of EIS spectra analysis. 

Figure 6.44. Nyquist plot of AC impedance spectra of the segmented cell measured during different operating conditions. The plotted data of the total cell and segment Seg03 is uncorrected for the employed voltage and current gains [43], (Reprinted from Journal of Power Sources, 123(2), Bender G, Wilson MS, Zawodzinski TA. Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells, 163— 71, 2003, with permission from Elsevier and the authors.)...

Transformers. Three of the 460- to 15,000-volt transformers shorted and burned out in the first year and a half of operation. The manufacturers concluded that the built-in impedance of 2% was not enough. They recommended the addition of 180 ohms resistance on the high voltage side, to equal total impedance of 4%. This was done and only one transformer has failed in the past 6 years. 

Miura, N., Nakatou, M. and Zhuiykov, S. (2002) Impedance-based total-NOx sensor using stabilized zirconia and ZnCr2O4 sensing electrode operating at high temperature. Electrochem. Commun., 4 (4), 284-7. 

Further development of new impedance-based gas sensors is likely to allow their introduction to niche applications in the field of zirconia-based, solid-state gas sensors. Impedance spectroscopy has the potential to measure changes not detectable with simple current or voltage measurements. Although it is not practical to implement complete impedance spectroscopy in an operating sensor, an optimized frequency suitable for a specific gas sensor may be used. So far, these sensors are still more complex and expensive as compared to the potentiometric zirconia-based gas sensors. However, they have two important advantages (1) measurement of total NOx concentration, regardless of the NO/NO2 ratio in exhausts and (2) near equal sensitivity to NO and NO2 at 700°C. These are essential prerequisites to their practical implementation in vehicle exhausts. Therefore, further investigation is... 

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