Membrane area-specific resistance

For an ionic conducting solid electrolyte to be seriously considered for use in a practical electrochemical device, which operates at a given temperature T, the maximum value for the area-specific resistance f as should be about 0.5 Q-cm. J as is the product of the electrolyte resistivity p at T in ohm-centimeters and the membrane thickness t (cm) in the direction of current flow. Table 2 lists maximum limits on electrolyte resistivity for various electrolyte membrane thicknesses. 

The data presented in Table 1 permit several conclusion to be drawn on the potential use of these materials as electrolytes in electrochemical devices with practical values for the area-specific resistance. Only the j0"-alumina and NASICON electrolytes possess sufficient conductivities for use as membranes with thicknesses of 1 mm or greater. The sodium ion conductivities of polycrystalline NASICON and /l"-alumina are comparable. The glassy electrolytes must be used in the form of thin films ( < 50 pm and possibly under 10-15 pm) or as capillary tubes with very thin walls (10-50 pm). 

For potentiostats that do not include a specific method for Ohmic drop determination, a full EIS scan over a range of frequencies from high to low can be used. In the resulting Nyquist plot, the first intercept on the X-axis is the Ohmic resistance. In case this is not directly apparent, an equivalent circuit including an Ohmic resistor can be used to fit the data and obtain the Ohmic resistance. Once the resistance is determined, it should be reported in the form of area specific resistance (ASR), as Ohm square centimeter. Thus, the area of the membrane used should be multiplied with the subtracted value from the measurement with and without membrane. 

US DOE (Department of Energy) has set technical targets in PEMs for transportation apphcations (Table 7.1) [1]. The targets are for gas crossover (permeability), area-specific resistance, operating temperature, cost, and durability. In 2015, car companies will make a decision whether they continue their endeavor to commercialize fuel cell vehicles. The membrane scientists are facing a big challenge in order to help them go further with fuel cells. 

Here a is the conductivity or the reciprocal of the resistivity, having a unit of ohm cm or more commonly S cm In electrochemical devices, the area-specific resistance (ASR, ohm cm ) of a flat sheet membrane electrolyte is of engineering importance and can be expressed as the product of the resistance and the surface area, or as the ratio between the thickness and the conductivity. The ASR is directly proportional to the voltage loss (V) of the electrolyte at the current density i (A cm ), because V = ASR x i. The expression of the thickness to conductivity ratio indicates that high conductivity (electrolyte thickness (L) lead to a low cell resistance. 

ADLs and thinner membranes which gives reduced area-specific resistance with maintained mechanical strength [12, 117-119], However, it imposes challenges with respect to interfacial adhesion and surface treatment of the FIFE seems necessary to avoid delamination. 

Nafion-112, and Nafion-212, use the thicker membrane Nafion-117 in DMFCs. The use of crosslinked PVA electrospun nano-fiber film supported Nafion composite membranes (Nafion/ PVA-fiber, thickness 50 pm) in DMFCs has been reported to exhibit a much better DMFC performance than Nafion-117 and Nafion/PVA blended PEMs [26-31]. Several researchers blended the Nafion PEMs with low methanol compatible PVA to reduce the methanol crossover in the PEMs [32-35]. However, these modified Nafion membranes had thicknesses greater than 175 pm, which were similar to (or higher than) that of the neat Nafion-117 membrane. Although there was a decrease in the methanol crossover from these Nafion/PVA blended membranes, the proton transfer resistance of these membranes increased, resulting in a lower DMFC performance. The advantage of applying the thin Nafion/PVA-fiber PEMs to the DMFCs is that the methanol crossover can be reduced without increasing the area specific resistance (i.e., Lla) because of low membrane thickness. Table 12.1 summarizes the thickness, proton conductivity, and Lja of the fiber reinforced Nafion composite membranes obtained from literature reports. The mechanical properties of the composite membranes reported in literature are also listed in Table 12.2. 

Table 12.3 Membrane thickness (L), phosphoric acid dop level (PA op), proton conductivities (tr), and area specific resistances (L/tr) of PBI/fiber reinforced composite membranes...

Compared with Nafion/porous PTFE and Nalion/ electrospun nano-fiber composite membranes, there are few reports of PBl/porous PTFE and PBl/electrospun nano-fiber composite membrane for high temperature PEMFC appUcatiOTi. The PBl/porous PTFE composite membrane was shown to exhibit excellent mechanical strength and good durability, which allowed researchers to reduce the membrane thickness and thus reduce the area specific resistance and ultimately improve fuel cell performance. After the report of PBl/porous PTFE composite membrane, two... 

With the trend to higher temperature of fuel cell operation, as is needed for both automotive and stationary applications, and the requirement for high performance, recent developments have tended towards the use not only of low-EW PFSA polymer membranes, but also in the employment of membranes of thickness only 25-30 pm (compared with the use of films of ca. 175 pm ten years ago) for their lower area specific resistance and increased water permeation rate, and both of these factors impact the membrane s mechartical strength. The difficulty lies in... 

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. 

Note A , area of unblocked membrane (m ) Aq, initial area of unblocked membrane (m ) Cb, bulk concentration (g-L ) /, fractional amount of total foulant contributing to deposit growth 7b> filtrate flux within the blocked area (m s ) Q, volumetric flow rate (m s ) Tp, radius of membrane pore (m) Rm, resistance of the clean membrane (m ) Rp, resistance of the deposit (m ) R, specific protein layer resistance (mkg ) t, filtration time (s). Greek letters a, pore blockage parameter (m kg ) J3, pore contriction parameter (kg) 5, membrane thickness (m). 

There has been significant progress made recently in the fabrication of FO membranes. While these were not specifically tested for PRO operation (e.g. permeation rates under pressurized conditions), their estimated potential performance may be calculated based on the experimentally determined characteristics, namely, the water and salt permeabilities of the filtering layer and the mass transfer resistance of the support to diffusive transport. Membrane permeability is expressed in m/s-Pa and measures the water volume flow rate permeated across a membrane area unit under a 1 Pa osmotic pressure difference between the membrane sides. High values of water permeability denote the membrane capability of avoiding resistance to water flux. 

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