Cathode impedance rise

The CPE appears to arise solely from roughening of the surface by the corrosion process. This was verified with IS experiments on iron and several steels in 15% HC1 at 25°C. The electrodes were polished with alumina and maintained at 150 mV cathodic of the rest potential. Complex plane plots of the impedance responses were nearperfect semi-circles centered on the V axis. Analyses via EQIVCT using the Rq+P/R circuit, gave rise to n-values of the CPE in excess of 0.93 in all cases and remained constant throughout the tests. 

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. 

The major limitation of photoelectric recording is what can be thought of as the nanosecond barrier. This limitation arises because of the intrinsic time response of the electronic devices that must be used to acquire and process the photongenerated cathode current of the PMT or photodiode. All such devices have impedance, and even the best-designed circuitry has stray capacitance of typically 20 pF which, when combined with the 50-Q industry/standard of electronic amplifiers, yields a RC time constant of 1 ns. Hence, instruments that are built up from conventional electronic units will have minimum rise times in the ns region and therefore chemical changes that have lifetimes less than 5 ns, say, will be severely deformed. Of course, other reasons may intervene (e.g. 10-ns-wide laser pulses) that make the instrument response even poorer than implied by the nanosecond barrier. 

During the processes described by equations 1 and 2, dissolution from the coating layer gives rise to the deposition of a hydroxide layer impeding the cathodic oxygen reduction reaction. [8] This reaction involves the rapid hydrolysis of water via equations 3 and 4. 

For a given voltage, as the resistance rises, current flow is impeded and insufficient electrons reach the cathode and surrounding tissues to allow depolarization. 

Two common approaches are illustrated in Fig. 5.30 and Fig. 5.31. If the screen power supply impedance is excessively high in the direction of reverse electron flow, the screen voltage will attempt to rise to the plate voltage. Note the emphasis on low impedance in the reverse electron flow direction. Most regulated power supplies are low impedance in the forward electron flow direction only. If the supply is not well bled, the reverse electrons will try to flow from anode to cathode in the regulator series pass element. As the screen voltage rises, the secondary... 

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