Electrolyte ionic resistivity

In the high-frequency range (/>10Hz), the series resistance variation with temperature can be neglected. In the low-frequency range, the ESR increases when the temperature decreases [54], This is caused by the electrolyte ionic resistance RT which is strongly influenced by the temperature. Above 0°C Rt varies slowly with the temperature. Below 0°C, the temperature dependency is more... 

Electrolyte Ionic Resistivity (G-cm) Maximum Membrance Thickness (mm)... 

Electronic insulator Minimal electrolyte (ionic) resistance Mechanical and dimensional stability Sufficient mechanical strength to allow easy handling... 

The flux of matter through a fuel cell or electrolyser is limited by the electrolyte (ionic) resistance, the electrode kinetics, and the external electronic load resistance. We commonly express the steady state situation and the fact that the current is the same through the entire closed circuit in terms of the voltage drops around the circuit ... 

Ohmic losses are mainly attributed to the electrolyte ionic resistivity (/ eiectro-lyte) and the external resistance of electrodes and connections (/ external) when current ( ) is drawn from the cell ... 

The equivalent circuit of a section of this SEI is presented in Fig. 13(b). It was recently found [123, 124] that at temperatures lower than 90 °C, the grain-boundary resistance of composite polymer electrolytes and composite solid electrolytes based on Lil-A Ojis many times larger than their ionic resistance. At 30 °C / GB is several orders of magnitude larger than (the ionic resistance) and for 100 pm-thick CPE foils or Lil-A Oj pellets it reaches [125] 105-106Qcm2 (depending on CPE composition). 

Fabrication techniques, especially the preparation of thin films of functional materials, have made major progress in recent years. Thin-film solid electrolytes in the range of several nanometers up to several micrometers have been prepared successfully. The most important reason for the development of thin-film electrolytes is the reduction in the ionic resistance, but there is also the advantage of the formation of amorphous materials with stoichiometries which cannot be achieved by conventional techniques of forming crystalline compounds. It has often been observed that thin-film electrolytes produced by vacuum evaporation or sputtering provide a struc-... 

Influence on Electrolyte Conductivity In porous separators the ionic current passes through the liquid electrolyte present in the separator pores. Therefore, the electrolyte s resistance in the pores has to be calculated for known values of porosity of the separator and of conductivity, o, of the free liquid electrolyte. Such a calculation is highly complex in the general case. Consider the very simple model where a separator of thickness d has cylindrical pores of radius r which are parallel and completely electrolyte-filled (Fig. 18.2). Let / be the pore length and N the number of pores (all calculations refer to the unit surface area of the separator). The ratio p = Ud (where P = cos a > 1) characterizes the tilt of the pores and is called the tortuosity factor of the pores. The total pore volume is given by NnrH, the porosity by... 

Ohmic Polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials. The dominant ohmic losses, through the electrolyte, are reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey Ohm s law, the ohmic losses can be expressed by the equation... 

The lithium-ion cells have demonstrated power loss when aged and/or cycled at high temperatures. Norin et al." demonstrated that the separator is at least partly responsible for the power loss due to the intrinsic increase in its ionic resistance. They showed that impedance increased significantly upon cycling and/or aging of lithium-ion cells at elevated temperatures and that separators accounts for 15% of the total cell impedance rise. They later reported that the loss in ionic conductivity of the separator was due to blocking of the separator pores with the products formed due to electrolyte decomposition, which was significantly accelerated at elevated temperatures. 

Lithium polymer electrolytes formed by dissolving a lithium salt LiX (where X is preferably a large soft anion) in poly(ethylene oxide) PEO can find useful application as separators in lithium rechargeable polymer batteries.Thin films must be used due to the relatively high ionic resistivity of these polymers. For example, the lithium-ion conductivity of PEO—Li salt complexes at 100 °C is still only about Viooth the conductivity of a typical aqueous solution. 

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. 

The initial decrease in ionic film resistance and Increase in capacitance can be associated with either NaCl electrolyte or water entry into the film. From ER measurements this period is associated with a metal loss process at the substrate surface. However, between 9 to 23 days the ionic film resistance increases, which is associated with an arrest in metal loss at the substrate surface in ER measurements. It appears, therefore, that with the knowledge of an underfilm darkening phenomenon occurring at the substrate/coating interface, a film of a protective (ie passive or high ionic resistance) nature is produced during exposure. 

Most of the models available in the literature are axial symmetric. A second simplification refers to the discretization adopted for the electrodes and electrolyte. Some of the models consider the cathode, electrolyte and anode as a whole and adopt an axial discretization. Electronic/ionic resistivity is computed as the average value of the single resistivites, calculated at the local temperature (Campanari and Iora, 2004). Using this approach means to simplify the solution of mass transfer in the porous media and the conservation of current. Authors have shown that about 200 elements are sufficient to describe the behaviour of a cell 1.5 m long using a finite volume approach (Campanari and Iora, 2004). 

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. 

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 ... 

To increase fundamental knowledge about ionic resistance, it is important to develop a methodology to experimentally isolate the contributions of the various cell components. Electrochemical impedance spectroscopy has been widely used by Pickup s research group to study the capacitance and ion conductivity of fuel cell catalyst layers [24-27] they performed impedance experiments under a nitrogen atmosphere, which simplified the impedance response of the electrode. Saab et al. [28] also presented a method to extract ohmic resistance, CL electrolyte resistance, and double-layer capacitance from impedance spectra using both the H2/02 and H2/N2 feed gases. In this section, we will focus on the work by Pickup et al. on using EIS to obtain ionic conductivity information from operational catalyst layers. 

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. 

Lithium ion conductors are very much desired in commercial applications because of the relatively high open circuit voltages (up to 4 V) that can be achieved in electrochemical devices employing lithium-based anodes with high chemical activities (or chemical potentials). Many of the polycrystalline lithium-based solid electrolytes, that have been studied to date have ionic resistivities at 300°C in the range between 20 and 200 fl cm. While thin-film applications for these materials are possible, the biggest drawback associated with lithium ion conductors is their chemical and electrochemical instability over time at temperatures of interest in environments very high in lithium chemical activity. 

Matrix Particle growth Stability and crack formation Dissolution of y-LiA102 in the electrolyte Phase transformation from y to a variety Changes in the microstructure Increase in the ionic resistivity Decrease in the cell voltage Decrease in the cell life... 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

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