Acid and alkaline cells

In general, because PAFCs are proton conductors like the proton exchange membrane (PEM) and the subcategory direct methanol fuel cells (sections 3.5 and 3.6), there is a continuous conceptual transition between them. The polymers used in PEM cells usually contain weakly acidic components such as HSO3, but may be reinforced with a stronger acid in order to increase conductivity or allow operation at higher temperatures. 

Alkaline fuel cells (AFCs) use an aqueous potassium hydroxide (KOH) solution (around 30%) as electrolyte and have electrode reactions of the form 

Alkaline fuel cells have been used extensively on early spacecraft imtil they were superseded by more reliable solar cells. The high cost of the space cells and the use of corrosive compoxmds requiring special care in handling have been held against AFCs. Current AFC development employs multi-component electrodes using Ni for structural stability and as catalyst, carbon black as electron conductor and polytetrafluoroethylene (PTFE) pore-forming 

General objections to liquid electrolytes include corrosion and difficulty in minimising physical volume requirements for applications such as vehicle systems. The lifetime of AFCs are of the order of 5000 h with a large spread in experience (McLean et ah, 2002). The electrolyte penetrates into the electrode pores and degrades functionality by increasing diffusion paths (Cifrain and Kordesch, 2004). Extending the lifetime requires that KOH be drained out of the system when not in use. 

The individual components used in an AFC are not necessarily expensive compared to those of other fuel cell t3q es under development. Use of Ft catalysts can be avoided, while the bipolar plates collecting the electron flows typically have to be made of fairly expensive black carbon to avoid corrosion. The peripherals needed for water management and electrolyte draining add to the cost, but do not necessarily lead to drawbacks such as long start-up 

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To date, the catalysts for low-temperature fuel cell electrodes (phosphoric acid and alkaline cells) have been the precious metal blacks and, more recently, precious metals on carbon supports. Development of fuel cell catalysts using precious metals remains very active. Also, some work is being done on systems that may be substituted for the noble metals. For example, tungsten carbide based anode catalysts have been shown to have good durability over long periods, but they are not as active as platinum. 

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

For hydrogen production from water, pure water (pH=7.0) is seldom used as an electrolyte. Water is a poor ionic conductor and hence it presents a high Ohmic overpotential. For the water splitting reaction to proceed at a realistically acceptable cell voltage the conductivity of the water is necessarily increased by the addition of acids or alkalis. Aqueous acidic and alkaline media offer high ionic (hydrogen and hydroxyl) concentrations and mobilities and therefore possess low electrical resistance. Basic electrolytes are generally preferred since corrosion problems are severe with acidic electrolytes. Based on the type of electrolytes used electrolyzers are... 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

Exopolyphosphatases of S. cerevisiae were optimal at neutral pH, although the profiles of pH dependence had their own peculiarities for each enzyme. While the cell envelope and cytosolic exopolyphosphatases were able to hydrolyse substrates at acid and alkaline pH,... 

The best-known window materials for liquid cells are listed below with the spectra accessibility range (in cm-1) given in parenthesis (1) NaCl (40000-590), (2) KBr (40000-340), (3) Csl (40000-200), (4) polyethylene (625-50), (5) ZnS, Irtran-2 (17000-835), (6) ZnSe (20000-600), (7) Ge (5500-600), (8) Si (8300-660), and (9) sapphire (50000-1780). Materials (l)-(3) are not suitable for aqueous solutions because of their high hygroscopicity. ZnS and ZnSe are suitable for use with acidic and alkaline solutions. The path length spacers are made of lead and are available with 0.025, 0.05, 0.1, 0.2, 0.5, and 1.0mm path lengths. Permanent cells have an amalgamated lead spacer between the... 

Two familiar kinds of dry cells use different electrol5des. The electrolytes make the cell on the left acidic and the cell on the right alkaline. 

Combined ammonium and/or potassium oxalate does not cause shrinkage of erythrocytes. However, other oxalates can cause shrinkage by drawing water into the plasma. Reduction in hematocrit may be as much as 10%, causing a reduction in the concentration of plasma constituents of 5%. As fluid is lost from the cells, an exchange of electrolytes and other constituents across the cell membrane occurs. Oxalate inhibits several enzymes, including acid and alkaline phosphatases, amylase, and lactate dehydrogenase, and may cause precipitation of calcium as the oxalate salt. 

When hydrogen ions are added to a solution, the pH decreases when they are removed (or hydroxyl ions added), the pH rises. But when certain substances are present in the solution, they will tend to combine with any H+ or OH" ions added, and, by removing them, they act so as to prevent the change in pH that would otherwise occur. Substances that act in this way are called buffers. In their ability to mop up hydrogen or hydroxyl ions, buffers act as regulators of pH. Such regulation is extremely important to the delicately balanced living cell, where sharp fluctuation in acidity and alkalinity can easily spell disaster. 

Extracellular, acidic, and alkaline phosphatases differ not only in their pH optima, but also in their requirements for Mg2+ and Zn2+, in their inhibition by chelators such as EDTA, and in their sensitivity to fluoride (Cembella et al., 1984). These enzymes, known to occur simultaneously in certain algal species, appear to be common among algae and bacteria, although in some, such as Dunaliella tertiolecta, they are lacking. Some phosphatases, particularly the intracellular ones are always present and active in cells, but the activity of their extracellular counterparts is dependent on phosphorus availability. Like the extracellular deaminases, the synthesis and activity of phosphatases is regulated by the nutritional state of the organisms. 

This brief description of the mercury cells in the caustic soda industry reveals how rubber can play a vital role as an anti-corrosive protective material in all the critical equipment and connected piping systems handling acidic and alkaline solutions gases and fumes. 

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