Electrolyte immobilization

Fig. 3.8 Various types of reference electrode vessels (A) laboratory-type calomel electrode (B), (C) portable calomel electrodes, (D) a Ag/AgCl micropipette with a KC1 electrolyte immobilized with agar in the tip of the capillary... 

Xhs U+ loo. 1 a relatively hi mobility, and the transference number is also high, t+ = 0.7. Quaternary ammonium ions exhibit ion association, which decreases with increasing ion size [464]. Admixing of pyrogenic silica such as Aerosil with liquid organic Li" " electrolytes immobilizes these systems and enhances the conductivity somewhat [465]. 

Bauer (1921) developed the first high-temperature MCFC based on a molten (Na/K)2C03 electrolyte, immobilized in a MgO matrix.The MCFC in its present form was developed by G. H. J. Broers in 1951. Small laboratory cells were constructed using a non-sintered MgO + molten carbonate paste electrolyte. The durability of these cells was tested up to 6 mo. Common features of today s cells and the early Broers cells are the nickel-based electrodes with planar and bipolar construction and the alkali carbonate electrolyte in inert matrix filler. The basic operating principles are still the same and the cell is represented below. 

There are two alternative designs which provide the gas space in VRLA cells. One design has the electrolyte immobilized as a gel, the other has the electrolyte held in an AGM separator. Gas passes through fissures in the gel, or through channels in the AGM (Fig. 1.4). 

The above discussion has shown how the separator-electrolyte-immobilization... 

Fig. 12.9. AGM-type automotive battery electrolyte immobilized in glass-fibre mat.

Numerous electrolytes are used in commercial electrolytic cells. Aqueous solutions of sodium hydroxide, sodium chloride, hydrochloric acid and electrolytes immobilized in polymers are used. The most common electrolyte is a 25-36 wt.Vo potassium hydroxide solution because of its superior conductivity. 

Batteries that require a liquid electrolyte are called wet batteries. Corrosive battery fluid refers to either acid electrolytes syn. battery acid, like the common lead-acid automobile battery which uses a solution of sulphuric acid, or alkali electrolytes syn. alkaline corrosive battery fluid, like potassium hydroxide (1310-58-3) solutions in nickel-cadmium and other alkaline battery systems. Dry batteries or dry cells, like all primary batteries, use electrolytes immobilized in pastes, gels, or absorbed into separator materials. Some batteries are loaded with a dry, solid chemical (e.g., potassium hydroxide) which is diluted with water to become a liquid electrolyte. The hazards associated with handling and transportation prior to use are thereby reduced. 

An example of polymer additive to electrolyte is FORAFAC 1033D (polyfluoroalkyl sulfonic acid). Addition of FORAFAC 1033D in a concentration of 0.1 wt% to the electrolyte immobilized in AGM VRLA batteries leads to a major improvement of battery cycle life [49]. Standby batteries containing FORAFAC have improved their service bfe by a factor of 1.5, suffering smaller water loss and reduced self-discharge. 

In this chapter, we will focus our attention mostly on VRLAB with AGM separator. In the 1970s, McClelland and Devitt invented the first valve-regulated lead—acid cell with electrolyte immobilized in microporous glass mat with gas channels in the AGM and a safety valve mounted on the cell lid, and thus forced the COC to proceed within the active block [13]. Figure 14.7 presents a schematic diagram of the transport of oxygen, H ions and water between the two... 

Most lithium cells available in the market utilize nonaqueous electrolyte solutions, where lithium salts are dissolved in organic solvents. The gelled electrolytes used in lithium-ion polymer batteries are also regarded as an organic electrolyte immobilized with a high molecular weight polymer. 

A dry cell has the electrolyte immobilized as a paste, with only enough moisture... 

If the materials and corrosion problems are solved high temperature fuel cells could be used in the framework of a hydrogen economy for the conversion of hydrogen into electric energy. Cells with molten salt electrolytes immobilized in a ceramic matrix could be applied at 850-950 K and with solid oxygen ion conducting electrolytes at 1200-1300 K. 

Among new materials suggested for the porous electrolyte matrix in PAFCs, we mention a mixture of silicon carbide (SiC) and PTFE (Mori et ah, 1998). A suspension of the components is mixed in a ball mill for a long time, then spread onto the surfaces of the cathode and anode. This assures good contact between the electrodes and the electrolyte immobilized in the matrix. 

The principle is well known from sealed lead acid batteries, which contain their electrolyte immobilized, by glass mats or silica gel. For lithium rechargeable batteries it is called SPE (solid polymer electrolyte) or gel technology. 

Kounaves and coworkers [70] have studied the Ag AgCl systems covered by membrane made of Nafion or polyurethane. These polymers were used to protect the studied solution from a NaCl leakage from the electrode solution. The studies on the stability of potentials of such electrodes have shown that the potentials of the electrodes protected by Nafion were significantly less stable than those covered by the polyurethane membrane. The drift of the potential, in the initial stage, could result from a slow equilibration between the Ag AgCl phase and KCl immobilized in the internal membrane. Further drift could be a consequence of the hydration of the external Nafion membrane. In the case of chloride solutions electrodes of both types exhibited decrease of the potential (about —41 ( 1) mV dec ) with the increase of chloride ions concentrations (from 10 mol dm to 1 mol dm ), showing a behavior similar to that of an indicator electrode. This change could result from the diffusion of more concentrated solution of chloride ions to the electrolyte immobilized in the poly(vinyl chloride) membrane situated under the external protective layer made of Nafion or polyurethane. 

The lithium iron sulfide battery operates at about 400 to 500°C using a fused halide eutectic electrolyte immobilized in the pores of a suitable separator. This battery displays a number of attractive featmes compared to the Na-S battery, including prismatic flat-plate constraction, ability to withstand numerous freeze-thaw cycles, cell failures in short-circuit conditions, ability to withstand overcharge, and low-cost materials and construction techniques. The major disadvantage is a somewhat lower performance. Although this battery is suitable for both EV and load-leveling applications, recent attention has focused on battery designs suitable for EV propulsion. ... 

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