High-frequency cell resistance

Figure 2.13 High-frequency cell resistance of the individual cells in a 30-cell DMFC stack operated at 60°Cwith a 0.5 M methanol solution feed at 125 ml min at the anode and with 0.76 atm d air feed at 7.35 SLPM at the cathode.

It is important to understand the accelerating factors in RH cycling test. In-cell RH cycling tests are typically carried out by alternating an inert feed gas (N2) of controlled dew points. The water content of the membrane can be monitored by high-frequency cell resistance measurements. The result of the RH cycling test of a Nation based MEA cycled from 0 to 100% RH at 80°C is shown in Fig. 13. It can be seen that with the chosen cycling period ( 30 min)... 

Two kinds of conductivity detector are distinguished contact detectors and contactless detectors. Both types were originally developed for isotachophoresis in 0.2-0.5-mm-inner diameter (i.d.) PTFE tubes. Contactless detectors are based on the measurement of high-frequency cell resistance and, as such, inversely proportional to the conductivity. The advantage is that electrodes do not make contact with the buffer solution and are, therefore, outside the electric field. As these types of detectors are difficult to miniaturize down to the usual 50-75-jU.m capillar inner diameter, their actual application in capillary electrophoresis (CE) is limited. 

Fig. 11 Parallel in situ aging test of MEAs comprising an FEP (25nm)-based radiation-grtifted (RG) membrane (crosslinked) and Nafion 112 in a two-ceU stack of 100-cm active area Reactants were and with a stoichiometry of 1.5 each, pressure 2.5 bar. (a, c, e) simulated application profile temperature 71-76°C, load profile with current density ranging from open circuit voltage (OCV) to 0.7 A cm, gas inlet dew points 40°C. (b, d, f) accelerated aging conditions temperature 90°C, OCV, gas inlet dew points 78°C. (a, b) cell voltage (c, d) high-frequency (HF) resistance measurement at 2 kHz and a stack temperature of 75°C and gas inlet dew points of 40°C as a metisure for chemical membrane integrity, (e, f) electrochemical crossover measurement at a stack temperature of 75°C and gas inlet dew points of 40°C as a measure for mechanical membrane integrity...

In enamels for chemical plant such as autoclaves it is not only the degree of acid resistance which is important but also the freedom of the finish from minute flaws detectable by high frequency spark testing or chemical methods. The chemical methods depend upon a colour change when the reagent such as ammonium thiocyanate reacts with the iron exposed at the bottom of the pinhole or flaw in the finish. Alternatively, an electric cell can be formed via the exposed iron in the flaw and detected chemically. 

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

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 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. ...

The two main techniques for measuring electrode losses are current interrupt and impedance spectroscopy. When applied between cathode and anode, these techniques allow one to separate the electrode losses from the electrolyte losses due to the fact that most of the electrode losses are time dependent, while the electrolyte loss is purely ohmic. The instantaneous change in cell potential when the load is removed, measured using current interrupt, can therefore be associated with the electrolyte. Alternatively, the electrolyte resistance is essentially equal to the impedance at high frequency, measured in impedance spectroscopy. Because current-interrupt is simply the pulse analogue to impedance spectroscopy, the two techniques, in theory, provide exactly the same information. However, because it is difficult to make a perfect step change in the load, we have found impedance spectroscopy much easier to use and interpret. 

A double metal process can be used to reduce the cell pitch and minimize parasitic resistance and capacitance, which can be critical for 4H-SiC BJTs intended for high-frequency operations. The finished structure in Figure 6.12(e) can be covered with an intermetallic dielectric layer. Via holes are then opened, and a thick metal... 

A high-frequency limit for the applied potential is encountered above several kilohertz where the impedance of the conductance cell again begins to deviate from the resistance R. Since the solution medium itself is a dielectric situated between two parallel charged surfaces, it can assume the characteristics of a capacitor placed in parallel across the solution resistance as shown in Figure 8.9a. The magnitude of this capacitance is given by... 

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