Fuel Nyquist plot

Figure 1.17. Nyquist plot for a fuel cell operated at OCV, 80°C, 3.0 atm, and 100% RH [20], (Reproduced from Electrochimica Acta, 52, Song C, Tang Y, Zhang J, Zhang J, Wang H, Shen J, et al., PEM fuel cell reaction kinetics in the temperature range of 23-120°C, 2552-61. 2007, with permission from Elsevier.)...

Some typical Nyquist plots for an electrochemical system are shown in Figure 2.38. The usual result is a semicircle, with the high-frequency part giving the solution resistance (for a fuel cell, mainly the membrane resistance) and the width of the semicircle giving the charge-transfer resistance. 

Figure 5.4. Nyquist plots obtained at a DC bias potential of 0.2 V versus Ag/AgCl for electrodes with various amounts of catalyst ink applied [2], (Reprinted from Electrochimica Acta, 50(12), Easton EB, Pickup PG. An electrochemical impedance spectroscopy study of fuel cell electrodes, 2469-7, 2005, with permission from Elsevier.)...

In a H2/02 (air) fuel cell, in general, the spectra, i.e., the Nyquist plots obtained by EIS measurements, very often have three features, which are denoted as high-frequency, medium-frequency, and low-frequency. The high-frequency region of an impedance spectrum is associated with the internal ohmic resistance and the contact capacitance in the granular electrode structure of the membrane electrode assemblies, whereas the medium- and low-frequency regions represent the charge-... 

Figure 5.20. Calculated Nyquist plot for a single cell with elimination of the transport barrier in the backing [18], (Reproduced with modifications by permission of ECS—The Electrochemical Society, from Springer TE, Zawodzinski TA, Wilson MS, Gottesfeld S. Characterization of polymer electrolyte fuel cells using AC impedance spectroscopy.)...

The diffusion layer inside the MEA can cause the low-frequency end of the impedance to bend over the real axis, giving rise to a double semicircle. These two arcs for the fuel cell cathode process are defined as a medium-frequency feature and a low-frequency feature. Typical two-arc Nyquist plots from simulation (calculated by Springer et al. [18]) and from experiments are presented in Figure 5.23 and Figure 5.24, respectively. 

Figure 5.29. Impedance spectra at several cathode potentials for an H2/02 PEM fuel cell, a Nyquist plots of the spectra b high-frequency view of the AC impedance spectra [33], (Reprinted from Journal of Power Sources, 118, Romero-Castanon T, Arriaga LG, Cano-Castillo U. Impedance spectroscopy as a tool in the evaluation of MEA s, 179-82, 2003, with permission from Elsevier and from the authors.)...

Figure 5.39. Nyquist plots of a typical DMFC cathode impedance spectrum ( ), operation on air ( ), operation on oxygen [43], (Reprinted from Journal of Power Sources, 75, Muller JT, Urban PM. Characterization of direct methanol fuel cells by AC impedance spectroscopy, 139M3, 1998, with permission from Elsevier and the authors.)...

Guo et al. [7], as shown by the Nyquist plots in Figure 6.10. In their impedance measurements, different amounts of Nafion ionomer in the catalyst layer, ranging from 0.33 to 1.13 mg/cm2 (dry weight) were examined. The active area of their fuel cells was 1.0 cm2. The fuel cells were operated in H2/air gas feeding mode with a flow rate of 220 cm3/min (at standard temperature and pressure) for both sides. The cell temperature as well as the humidification temperature for both electrodes were controlled at 70°C. The cell s AC impedance was measured using a Gamry PC4/750-DHC2 potentiostat. The perturbation amplitude was set at 5 mV in potentiostatic mode, and the frequency was scanned from 0.01 Hz to 100 kHz with 10 points per decade. 

Figure 6.10. Comparison of Nyquist plots of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode [7], (Reproduced by permission of ECS—The Electrochemical Society, from Guo Q, Cayetano M, Tsou Y, De-Castro ES, White RE. Study of ionic conductivity profiles of the air cathode of a PEMFC by AC impedance spectroscopy.)...

Figure 6.18. Nyquist plots of the sprayed electrode (0.1/0.1 mg Ptcnf2) at different voltages [18], (Reproduced from Abaoud HA, Ghouse M, Lovell KV, Al-Motairy GN, Alternative formulation for proton exchange membrane fuel cell (PEMFC) electrode preparation, Journal of New Materials for Electrochemical Systems 2003 6(3) 149-55, with permission from JNMES.)...

Figure 6.22. Nyquist plots for (A) a composite electrode at -450 mV vs. SCE and (B) a Prototech electrode at 450 mV vs. SCE [22], (Reproduced from Ahn S, Tatarchuk BJ, Composite electrode structures for fuel cell apphcations. Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, 1990 3 287-92, with permission from the American Institute of Chemical Engineers.)...

Figure 3.5. Overall impedance response of a proton exchange membrane (PEM) fuel cell for different cell temperatures, depicted as corresponding values of the real and imaginary parts of the complex impedance (sometimes denoted a Nyquist plot). Each sequence of points represents frequencies ranging from 10 to 10 Hz, with the highest values corresponding to the leftmost points. From M. Ciureanu, S. Mik-hailenko, S. Kaliaguine (2003). PEM fuel cells as membrane reactors kinetic cinalysis by impedance spectroscopy. Catalysis Today 82, 195-206. Used with permission from Elsevier).

For illustration, in Figure 4.5.47 a simulated impedance spectra, presented as a Nyquist plot (a) and a Bode plot (b) with an inductive behavior in the low frequency range is shown, which looks similar to the impedance spectra measured at PEFC with H2+ 100 ppm CO as fuel gas. 

Representative time-dependent impedance spectra of the series, i.e. time resolved impedance spectra, are depicted in Figure 4.5.65 for the fuel cell with Pt anode and in Figure 4.5.66 for the fuel cell with PtRu anode as Nyquist plots. 

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]. 

The relationship of the impedance as a complex function is also what leads to the most common data representation in EIS, known as the Nyquist plot (Fig. 8.3). In the Nyquist plot, -Im(Z) is plotted versus Re(Z) over the entire frequency range of the EIS measurement. One of the main shortcomings of the Nyquist plot is that the frequency is not shown there is only an implicit understanding that high frequencies are at the lower Re(Z) values and decrease in the positive X-direction. In the example provided in Figure 8.3, which is the same as the system represented in the Bode plot in Figure 8.2, three separate resistances are apparent as intercepts on the X-axis. Sueh a response is often observed in fuel cells where reactants and products are supplied in excess, and the only resistances governing potential losses are the Ohmic resistance and the activation losses at the anode and the cathode. 

Figure 11.20. Nyquist plots measured at different times diuring poisoning of a Pt/C anode with 100 ppm CO in H2. Cell T = 80 °C current density = 0.217 A cm anode Pt loading = 0.4 mg cm Nation 117 membrane [55]. (Reprinted from Electrochimica Acta, 48(25-26), Wagner N. and Schulze M., Change of electrochemical impedance spectra during CO poisoning of the Pt and Pt-Ru anodes in a membrane fuel cell (PEFC), 3899-907, 2003, with permission from Elsevier.)...

Figure 11.21. Nyquist plot of a H2/air phosphoric acid fuel cell operated at 0.2 A cm T ...

Fig. 17S (a) Nyquist plot at different current densities, (b) Polarization curve, where EIS recorded from (a) have beat highlighted black dots) and correlatirai between fuel cell impedance and polarization curve. 

The ohmic resistance (/ ohm) function of contact pressure cycling is shown in Fig. 17.19 for Dapozol -G55 and Celtec PllOOW MEAs. The values have been extracted from the Nyquist plots at 0.3 A/cm shown in [21]. As previously defined, ohmic or high frequency resistances are used to differentiate the resistance of conduction of protons and electrons and resistances of wires and MEA contact interfaces as BBP/GDL and GDL/CL. If one assumes a constant or nearly constant contribution from proton conductivity resistance through the membrane as the membrane thickness is nearly constant in comparison with GDL thickness changes with increasing the contact pressure [17] and resistance related to fuel cell wires and connections is constant [40, 41], the observed changes in the ohmic resistance will be mainly due to variation of contact between BPP/GDL and GDL/CL interfaces. / ohm decreases in the first cycle of contact pressure as shown in Fig. 17.19 for both the MEAs so contact between BPP/GDL and GDL/CL interfaces has been enhanced. This... 

FIGURE 6.23 Nyquist plots of 430 SS and graphite bipolar plate fuel cell corresponding to increased operation time (a) and change of impedance parameters (b). (Adapted from Kumagai, M. et al. 2010. Journal of Power Sources 195 5501-5507.)... 

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