Permeability liquid electrolytes

Besides silicon, other materials have also been used in micro fuel cells. Cha et al. [79] made micro-FF channels on SU8 sheets—a photosensitive polymer that is flexible, easy to fabricate, thin, and cheaper than silicon wafers. On top of fhe flow channels, for both the anode and cathode, a paste of carbon black and PTFE is deposited in order to form the actual diffusion layers of the fuel cell. Mifrovski, Elliott, and Nuzzo [80] used a gas-permeable elastomer, such as poly(dimethylsiloxane) (PDMS), as a diffusion layer (with platinum electrodes embedded in it) for liquid-electrolyte-based micro-PEM fuel cells. 

Liquid electrolytes display significant differences in permeability values. For instance, permeation of hydrogen chloride through polyethylene film when diffusing from concentrated hydrochloric acid is detected in a few minutes, whereas permeation of potassium chloride is not recorded even after three months. Permeation of nitric acid through fluoroplastic film is recorded in a few tens of minutes, while it is necessary to wait more than a year for sulfuric acid to be detected [22]. With increasing concentration of the electrolyte its permeation rises. Pol nners whose electrolyte diffusion factor is D < fO m /s are considered to be practically impermeable. Polyolefins, fluoroplasts and polyesters are easily permeable to hydrochloric, hydrofluoric, nitric, acetic and fluorosilicic acids, and ammonium diffusing form aqua solutes. They are less permeable for sulfuric and phosphoric acids, salts and caustic alkali. Phosphoric acid easily diffuses into PVC as well. In this case [23], diffusion of the acid is conditioned by the presence of a plasticizer in the polymer. 

The separator is also a critical component in the liquid electrolyte batteries. Its structure and properties considerably affect battery performance and safety. The requirements in terms of chemical stability, porosity, pore size, permeability of the separators and the... 

The membranes used in ion-selective electrodes separate two different electrolytes and are not equally permeable to all kinds of ions. At the interface between the two electrolytes, different events contribnte to the measured membrane potential. First, a diffusion potential arises from differences in mobility and concentration of ions in contact at the interface, as seen in liquid junctions. Second, a Donnan potential arises when the membrane completely prevents the diffnsion of at least one species from solution to the other. Third, the exchange equilibria between the electrolyte and the membrane interface must also be considered to adequately describe the membrane potential of ion-selective electrodes with solid or liquid electrolyte manbranes. 

Liquid electrolyte penetrates the carlxMi cathode blocks through permeable pores, sodium intercalates in the lattice of carlxMi, and amorphous carbon undergoes graphitization. All these processes increase the electrical and thermal conductivities. 

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

A special case of interfaces between electrolytes are those involving membranes. A membrane is a thin, ion-conducting interlayer (most often solid but sometimes also a solution in an immiscible electrolyte) separating two similar liquid phases and exhibiting selectivity (Fig. 5.1). Nonselective interlayers, interlayers uniformly permeable for all components, are called diaphragms. Completely selective membranes (i.e., membranes that are permeable for some and impermeable for other substances) are called permselective membranes. 

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

In the discussion on the liquid-liquid potentials it has been shown that the interposition of a layer of oil between two aqueous solutions of an electrolyte may give rise to a difference of potential between the two aqueous layers. Such cells may be reversible with respect to either ion, and may therefore be regarded as cells permeable to one ion. 

Step 2 is usually limited by the permeability of the membrane. In certain sensor designs, the membrane is eliminated to avoid this step. Step 4 refers to the diffusion of the solvated gas in the electrolyte to the electrode-electrolyte interface. Diffusion in liquids is often considerably slower than diffusion across a membrane. If the sensing electrode is flooded with electrolyte, the response is slow because the gas must diffuse through the electrolyte before reaching the reaction surface. 

A possible explanation for this effect can be found in the 3D structure of textile electrodes and its permeability for liquids. While slowly soaking electrolyte solution, the contact surface between textile electrode and electrolyte increases. The latter effect gives rise to a decrease in the resistance because of a larger value for A. As this process is occurring reasonably... 

Figure 5.41 Selective-ion electrodes (a) glass membrane (b) liquid ion exchange (c) homogeneous solid membrane (d) heterogeneous solid membrane (e) solid membrane without reference electrode (/) gas-permeable membrane 1, sensing electrode 2, electrolyte, 2(e) ohmic contact, 2(f) gas-permeable membrane 3, membrane sur-port 4, reference electrode, 4(f) outer electrode body, 5(b) liquid ion exchanger 5(f) electrode body 6(b) reference electrode body, 6(f) electrolyte 7, liquid junction.

Note that Eq. (21.78) corresponds to the limit of 2 —> cxd, but still assumes that electrolyte ions can penetrate the polyelectrolyte layer. Therefore, the particle in this limiting case is neither a perfectly hard particle nor a perfectly soft particle. We term a particle of this type a semisoft particle. The polyelectrolyte layer coating a semisoft particle is ion permeable but there is no liquid flow inside the polyelectrolyte layer. 

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