Membranes ionic resistance

In approach 2, the membrane ionic resistance is given by integrating over the membrane thickness as... 

Benzoic acid derivatives also altered the electrical potential across the cell membrane in neurons of the marine mollusk Navanax lnermls (46). Salicylic acid (1-30 mM) caused a depolarization very rapidly (1-2 min) and decreased the ionic resistance across the membrane. As pH was decreased, more salicylic acid was required to reverse the effect of pH on the membrane potential (47). This result is contradictory to the influence of pH on the amount of salicylic acid required to affect mineral absorption in roots (32). The ability of a series of salicylic and benzoic acid derivatives to increase PD correlated with their octanol/water partition coefficients and pKa values (48). The authors proposed that the organic acid anions bound directly to membranes to produce the observed results. 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

The typical properties of some commercial microporous membranes are summarized in Table 4. Celgard 2730 and Celgard 2400 are single layer PE and PP separators, respectively, while Celgard 2320 and 2325 are trilayer separators of 20 and 25 fim thickness. Asahi and Tonen separators are single layer PE separators made by the wet process. Basic properties, such as thickness, gurley, porosity, melt temperature, and ionic resistivity are reported in Table 4. These properties are defined in section 6.1.3. 

The ideal battery separator would be infinitesimally thin, offer no resistance to ionic transport in electrolytes, provide infinite resistance to electronic conductivity for isolation of electrodes, be highly tortuous to prevent dendritic growths, and be inert to chemical reactions. Unfortunately, in the real world the ideal case does not exist. Real world separators are electronically insulating membranes whose ionic resistivity is brought to the desired range by manipulating the membranes thickness and porosity. 

As mentioned, the reaction distribution is the main effect on the catalyst-layer scale. Because of the facile kinetics (i.e., low charge-transfer resistance) compared to the ionic resistance of proton movement for the HOR, the reaction distribution in the anode is a relatively sharp front next to the membrane. This can be seen in analyzing Figure 10, and it means that the catalyst layer should be relatively thin in order to utilize the most catalyst and increase the efficiency of the electrode. It also means that treating the anode catalyst layer as an interface is valid. On the other hand, the charge-transfer resistance for the ORR is relatively high, and thus, the reaction distribution is basically uniform across the cathode. This means... 

The all-inclusive costs of hydrogen from PEM and KOH systems today are roughly comparable. Reaction efficiency tends to be higher for KOH systems because the ionic resistance of the liquid electrolyte is lower then the resistance of current PEM membranes. But the reaction efficiency advantage of KOH systems over PEM systems is offset by higher purification and compression requirements, especially at small scale (1 to 5 kilograms per hour). Further details are provided in Appendix G. 

To calculate the impedance of the electric circuit shown in Figure 4.33, an iterative equation should be used. In the calculation process, the membrane resistance is not considered because it only shifts the impedance spectra at the real axis with an Rmembrane value. The electronic resistance of the carbon support is significantly smaller than the ionic resistance. Thus, the electric circuit could be simplified to a transmission line with capacitance and ionic resistance, as shown in Figure 4.34. 

The ionic resistance of a polymer electrolyte membrane is an important parameter in determining the mobility of protons through the membrane and the corresponding voltage loss across the membrane. Currently, the most commonly used membranes in PEM fuel cells are Nafion membranes produced by DuPont. However, these membranes are limited to low-temperature uses (usually below 80°C) because membrane dehydration at high temperatures can lead to reduced water content and then a lower proton transfer rate, resulting in a significant decrease in conductivity. The relationship between conductivity and the diffusion coefficient of protons can be expressed by the Nemst-Einstein equation ... 

MEA performance is mainly limited by ORR kinetics, as well as oxygen transport to the cathode catalyst. Another major loss is due to proton conduction, in both the membrane and the cathode catalyst layer (CL). Characterization of the ionic resistance of fuel cell electrodes helps provide important information on electrode structure optimization, and quantification of the ionomer degradation in the electrodes [23],... 

For the Nafion -based PEM fuel cells, the membrane conductivity decreased with increasing operating temperatures. The resistance points on the Re axis at the high-frequency end in Figure 6.49 can be treated approximately as the membrane through-plane resistance in PEM fuel cells other resistances, including the ionic resistance of the Nation ionomer and the cell contact resistance, may have made... 

We can conclude from the calculations summarized in Table 2 that electrochemical devices, which use solid electrolytes in bulk form with relatively thick walls (1-2 mm), require ionic resistivities on the order of 3-5 ii cm at operating temperature (a very stringent requirement, which excludes many of the materials in Table 1). Devices that involve thin membranes ( 500 pm) or thin films (< 100 pm) can tolerate higher electrolyte resistivities (i.e., 10-250 fi cm, depending on the membrane or film thickness) and thus permit a wider selection of the materials given in Table 1. 

Because the ionic resistance of the PEM is proportional to its thickness, this parameter is extremely important for cell performance [8]. It is necessary to minimize membrane thickness, while maintaining an acceptable mechanical strength. 

Thus, each side of the anode is divided into three compartments separated by the ion-selective membranes. Tie bolts keep the stack tightly sealed. The membrane separating the catholyte from the electrolyte is an Ionics CR-6170 alkali-resistant membrane. The membrane separating the anolyte from the electrolyte is an Ionics CR-61-CZL-183 acid-resistant membrane (Ionics, Inc., Water-town, Mass). The two membranes are separated by a I in. thick plastic spacer. Effective area of the membranes is one square foot. 

Depending on the current density a rather non uniform utilization of the catalyst layer can be expected. While at low current density negligible effects are caused by the ionic resistance in the catalyst layer and almost all platinum particles can be used, the reaction concentrates close to the membrane interface at high current densities causing underutilization of the platinum present in the electrode [54, 55]. Optimization of electrode performance can be expected from microstructural optimization for example by designing catalyst layers having gradients in noble metal concentration and porosity. 

The measurement of separator resistance is very important to the art of battery manufacture because of the influence the separator has on electrical performance. Electrical resistance is a more comprehensive measure of permeability than the Gurley number in that the measurement is carried out in the actual electrolyte solution. The ionic resistivity of the porous membrane is essentially the resistivity of the electrolyte that is embedded in the pores of the separator. Typically, a micropo-rous separator, immersed in an electrolyte, has an electrical resistivity about six to seven times that of a comparable volume of electrolyte, which it displaces. It is a function of the membrane s porosity, tortuosity, the resistivity of the electrolyte, the thickness of the membrane, and the extent to which the electrolyte wets the pores of the membrane.The ER of the separator is the true performance indicator of the cell. It describes a predictable voltage loss within the ceU during discharge and allows one to estimate rate limitations. 

---