Membrane resistance, measurement

An excellent review of experimental techniques for measuring electrical resistivity in aqueous solutions is available [34], Separators used in nonaqueous systems can be characterized by wetting them with a surfactant and measuring the electrical resistivity in an aqueous solution. Then the resistivity in a nonaqueous membrane can be estimated from Eq. (2). 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Step 4 Capsule Mechanical Stability. The mechanical stability of the membranes was assessed semi-quantitatively by applying a compressional force via a micrometer. While this method is not precise, it did permit us to assess if the capsules could withstand deformations and if they ruptured in a controlled or catastrophic manner. Another test which was selectively employed was to place capsules between microscope slides and measure the force required to compromise the integrity of the membrane. These tests measured the resistance of the weakest point of the membrane. For certain capsules a needle was used to probe the breaking strength of a local region of the membrane. 

The thermal resistance will be temperature-dependent as canbe seen in Eq. (3.24), which is not only a consequence of the temperature dependence of the thermal heat conduction coefficients. The measured membrane temperature, Tm, is related to the location of the temperature sensor, so that the temperature distribution across the heated area will also influence the thermal resistance value. The nonlinearity in Eq. (3.24) is, nevertheless, small. The expression thermal resistance consequently often refers to the coefficient t]o only, which is used as a figure of merit and corresponds, according to Eqs. (3.24) and (3.25), to the thermal resistance or thermal efficiency of the microhotplate at ambient temperature, Tq. The temperature Tm can be determined from simulations with distinct heating powers. The thermal resistance then can be extracted from these data. 

Tortuosity is a long-range property of a porous medium, which qualitatively describes the average pore conductivity of the solid. It is usual to define x by electrical conductivity measurements. With knowledge of the specific resistance of the electrolyte and from a measurement of the sample membrane resistance, thickness, area, and porosity, the membrane tortuosity can be calculated from eq 3. 

Localized AC impedance and current distributions were measured by Brett et al." on a single linear channel with the segmented cathode current collector plate. Figure 26 shows their measured current density and membrane resistance distributions along the... 

In this situation, cell lines are shown to be resistant to colchicine, doxorubicin, vinblastine, and actinomycin D. This syndrome is accompanied by an increase in measurable membrane glycoprotein (the P-170 or permeability glycoprotein). It is believed that this protein transports hydrophobic chemicals out of cells and thereby prevents drug action. Current efforts to inhibit this efferent transport protein are currently underway but, sadly, have to date been largely unsuccessful (i5). 

The value of the parameters can be determined with only a cursory consideration of design. The area resistance, r, is composed of the membrane resistance and the stream resistance, its square root appearing in the smallest cost expression. The membrane resistance quoted by manufacturers is a static value, measured while the membrane still has its minority carriers and consequently is not yet markedly permselective. In operation the membrane has a resistance nearly twice the values quoted. A value of 25 ohms per sq. cm. per pair, measured for some thin membranes, is used in the following calculations. Because the resistance of the concentrate stream can be made arbitrarily small by an increase of concentration, a value /4 of dilute stream resistance has been used for the fluid resistance in the preparation of Figure 1. 

Fig. 7 Dependence of the membrane resistance on concentration of trichloroacetic acid (TCAA) in distilled water at 3 kHz, at room temperature, for (1) MIP chemosensor and (2) reference NIP chemosensor. Each point represents the average of three independent measurements (adapted from [128])...

The standard electrode potential and its temperature coefficient are found in the literature.36 Kinetic parameter values were measured in-house for HOR,33 ORR,34 OER,35 and COR.12 22 Table 2 gives cell component materials and transport properties. The membrane and electrode proton conductivity in Table 2 are based on the measured membrane and electrode resistance,42,43 which is a strong function of relative humidity (RH). In what follows next, we will describe the... 

The cool-down process of the cold-start experiment also provides an opportunity to obtain the membrane proton conductivity as a function of temperature at a known water content. Note that the temperature dependence of proton conductivity with low membrane water content is of particular interest here as PEFC cold start rarely involves fully hydrated membranes after gas purge. In addition, unlike PEFCs operated under normal temperatures, the membrane resistance under low water content and low temperature typical of cold start conditions is much greater than the contact resistance, making in-situ measurements of the membrane proton conductivity in a PEFC a simple but accurate method. 

The current interruption method is used to measure the internal resistance of an electrochemical system. In a fuel cell, it is usually used to measure the membrane resistance. 

Bixchi FN, Marek A, Scherer GG (1995) In situ membrane resistance measurements in polymer electrolyte fuel cells by fast auxiliary current pulses. J Electrochem Soc 142(6)4895-901... 

Typical impedance spectra of PEMs measured using the two-probe cell are shown in Figure 5.9, in which a semicircle followed by a 45° line can be observed. The membrane resistance is extracted from the low-frequency intercept of the semicircle. At low frequencies the 45° line is dominated by the blocking behaviour of the Pt/Nafion interface. 

In principle, the four-probe method is more accurate than the two-probe, because in the former the interface impedance can be eliminated. However, it was reported that in different frequency ranges the membrane resistance and contact resistance could be easily separated. For example, in the two-probe method, the favourite frequency range is 100 to 500 kHz, while in the four-probe method the range becomes 1 to 100 kHz [11], Thus, both methods can yield reliable results, and the probe distance can affect measurement accuracy. Our experiments using both methods indicated that a large probe distance is required to obtain accurate results. 

For in-plane membrane resistance measurements, the major error arises from small membrane thickness, as the thickness depends greatly on the relative humidity (RH) of the membrane. For example, the thickness of Nafion 117 changes from 165 pm at 34% RH to 200 pm at 100% RH at room temperature... 

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 6.47. Extracted high-frequency resistivity of the impedance measurement shown in Figure 6.46, compared with the simulated membrane resistivity of the 300 seem simulation [40]. (Reprinted from Journal of Power Sources, 145(2), Hakenjos A, Hebling C. Spatially resolved measurement of PEM fuel cells, 307-11, 2005, with permission from Elsevier and the authors.)...

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

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