Resistance high-frequency

Broudy, R., and H. Levinstein High-frequency resistance of thin films. 

High-frequency resistance of photoconducting films. Physic. Rev. [2]... 

Current, Species, and High-Frequency Resistance Distribution Measurements... 

Figure 26. Current (a) and high-frequency resistance (b) distributions at cell potentials of 0.8 and 0.6 V as measured by Brett et al. ...

Figure 27. Distribution of high-frequency resistances measured at 1.5 kHz in a low humidity PEFC using 30 / m membrane (EW < 1000).

Figure 28. Schematic diagram of the experimental setup for simultaneous measurements of anode/cathode species, current, and high-frequency resistance (HFR) distributions in an operating cell. ...

Figure 5.32. Impedance plots for single cells at ambient temperature, a Nation 117. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A) 0.70 V (0.76 V) ( ) 0.6 V (0.74 V) ( ) 0.5 V (0.74 V). b Nafion 112. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A ) 0.70 V (0.73 V) ( ) 0.6 V (0.67 V) ( ) 0.5 V (0.61 V). Plots were corrected for the high-frequency resistances. Left detail of the high-frequency regions [29]. (Reprinted from Journal of Electroanalytical Chemistry, 503, Freire TJP, Gonzalez ER. Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells, 57-68, 2001, with permission from Elsevier.)...

Ammonia, produced due to the coexistence of H2 and N2 at high temperatures in the presence of catalyst, was estimated to be in the concentration range of 30 to 90 ppm [37, 38], Uribe et al. [39] examined the effects of ammonia trace on PEM fuel cell anode performance and reported that a trace in the order of tens of parts per million could lead to considerable performance loss. They also used EIS in their work. By measuring the high-frequency resistance (HFR, mainly contributed by membrane resistance) with an operation mode of H2 + NH3/air (feeding the anode with hydrogen and ammonia), they obtained some information related to membrane conductivity, and found that conductivity reduction due to ammonia contamination is the major cause of fuel cell degradation. 

Figure 5.36 shows the high-frequency resistances (HFRs) obtained at different current densities [39], It can be seen that after 1 hour of exposure to NH3, the HFRs were nearly the same. However, when NH3 was kept flowing for longer periods of time, the HFR was affected. After 15 hours of continued H2/NH3 flow, the HFR values were much larger than those obtained in the absence of NH3. This performance loss was irreversible when the flow of NH3 was stopped and the cell was allowed to run on pure H2 for several days, the membrane conductivity did not return to its initial value. 

Figure 5.36. Effects that long-term NH3 exposure has on H2-air fuel cell high-frequency resistance at 80°C. 30 ppm NH3 (g) was injected into the anode feed stream [39], (Reproduced by permission of ECS—The Electrochemical Society, from Uribe FA, Gottesfeld S, Zawodzinski Jr. TA. Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance.)...

By using the impedance method, Freire et al. also demonstrated that thinner membranes not only show better performance but also are much less sensitive to humidification conditions, cell temperature, and current density. The dependence of the real resistance at high-frequency intercepts on membrane thickness at different current densities is illustrated in Figure 6.14. Linear dependence of the high-frequency resistance, RhJ. on the membrane thickness was observed at 80°C, whereas non-linear dependence of RhJ on membrane thickness was shown at 40°C and 60°C. They explained that this was due to better hydration of the membrane at higher temperatures. It is also observable that Rhf almost does not depend on current density at 80°C and at low current densities rather, Rhf dependence on membrane thickness is almost linear, which indicates that for thicker membranes at high current densities it no longer behaves as a pure resistor due to a capacitive effect caused by less effective back transport of water [9],... 

Figure 6.39. Progression of the high-frequency resistances over time [38], (Reprinted from Journal of Power Sources, 154(2), Hakenjos A, Zobel M, Clausnitzer J, Hebling C. Simultaneous electrochemical impedance spectroscopy of single cells in a PEM fuel cell stack, 360-3, 2006, with permission from Elsevier and the authors.)...

Any electrochemical cell can be represented in terms of an equivalent electrical circuit that comprises a combination of resistances, capacitances or inductances as well as mathematical components. At least the circuit should contain the doublelayer capacity, the impedance of the faradaic or non-faradaic process and the high-frequency resistance. The equivalent circuit has the character of a model, which more or less precisely reflects the reality. The equivalent circuit should not involve too many elements because then the standard errors of the corresponding parameters become too large (see Sect. II.5.7), and the model considered has to be assessed as not determined, i.e. it is not valid. 

Let us consider a set of data Z/ and Z,". The measurements were performed at the angular frequencies > ( = 1 . The theoretical values are denoted by Zit (Oi,Pi,P2... Pm), where Pi, P2- Pm are parameters to be determined and m is the number of parameters. Such parameters can be rate constants, the charge transfer resistance, the double-layer capacity or the high-frequency resistance. The aim of the complex least-squares analysis consists of minimising the sum S ... 

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