Oxygen cathode

Convert P, Countanceau C, Cromgneau P, Gloaguen F, Lamy C (2001) Electrodes modified by electrodeposition of CoTAA complexes as selective oxygen cathodes in a direct methanol fuel cell. J Appl Electrochem 31 945-952... 

A direct methanol fuel cell consists of two electrodes—a catalytic methanol anode and a catalytic oxygen cathode—separated by an ionic conduc-... 

BetteUieim A, Parash R, Ozer D. 1982. Catalysis of oxygen cathodic reduction by adsorbed iron(in)-tetra(A,A,A-trimethylanilinium)porphyrin on glassy carbon electrodes. J Electrochem Soc 129 2247. 

Boulatov R. 2006. Billion-years old oxygen cathode that actually works Respiratory oxygen reduction and its biomimetic analogs. In Zagal JH, Bedioui F, Dodelet J-P, editors. N4-Macrocyclic Metal Complexes. New York Springer, p. 1. 

This approach was coupled to a system of three NAD+-dependent enzymes comprised of alcohol dehydrogenase (EC 1.1.1.1), aldehyde dehydrogenase (EC 1.2.1.3), and formate dehydrogenase (EC 1.2.1.2) to create an electrode theoretically capable of complete oxidation of methanol to carbon dioxide, as shown in Eigure 5. The anode was, in turn, coupled to a platinum-catalyzed oxygen cathode to produce a complete fuel cell operating at pH 7.5. With no externally applied convection, the cell produced power densities of 0.67 mW/cm at 0.49 V for periods of less than 1 min, before the onset of concentration polarization. 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

Table 5. Activities of some metal chelates in the catalytic decomposition of hydrogen peroxide and in electrocatalysis with an oxygen cathode 38>...

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

The electrolyte for zinc-based cells is always caustic alkali. Calcium hydroxide is sometimes added to remove zinc ions as insoluble CaZn2O3.5H20. A caustic alkali electrolyte is effectively buffered against OHion production by the oxygen cathode, so that OH concentration... 

Aluminium-air cells were first developed for portable applications such as mooring lights, and for recharging nickel-cadmium and lead-acid storage batteries. They have been fabricated in many unusual designs, e.g. the concentric rope battery which has an aluminium core surrounded by a separator and then the oxygen cathode. The rope may be several hundred metres long and can provide 0.03 W/m for a period of 6 months on immersion in the sea. 

Under most usual oxygenated and alkaline-operating conditions for cooling water, the oxygen cathode provides the rate-determining step, and depolarization by O2 fthe removal of hydrogen ion) will increase the rate of corrosion and hence metal wastage, as below ... 

The MEA consists of a thin (10-200 pm) solid polymer electrolyte (a protonic membrane, such as Nation) on both sides of which are pasted the electrode structures (fuel anode and oxygen cathode) (Figure 9.5). The electrode structure comprises several layers the first layer made of carbon paper (or cloth) to strength the structure, on which are coated the GDL, and then the catalyst layer (CL), directly in contact with the protonic membrane (usually Nafion). 

A better way to do it is to use an oxygen cathode and reduce oxygen to water. This greatly decreases die cell potential and hence die electricity costs. The water used up at die cathode is replaced. If one burns H2 to do this, one loses the heat energy produced. 

The electrochemistry of fuel cells is discussed in Chapter 13. Superior catalysts for the oxygen cathode are at the frontier of research. 

Some potential uses of the direct approach30 are worth mentioning. The present process would be advantageous in recreational vehicles and boats, ships, and planes. Stand-alone sets for isolated communities could be powered by photovoltaics. In earth-based use, electricity costs could be reduced by the use of oxygen cathodes, thus also eliminating the evolution of hydrogen. 

The HTC coal was then employed in an anodic half-cell coupled to a model V02+/V02+ cathode, making the procedure more suitable to run on a laboratory scale than using the standard oxygen cathode (note that the redox potential of V02+/V02+ and oxygen/Fe2+ is very similar). Figure 2.4.4 displays the obtained results. 

Sodium tungsten bronzes, Na W03, with the perovskite-like structure [384 386] have been investigated as oxygen cathode materials in acid solutions, but they have to be promoted by traces of platinum on the surface [386-389],... 

For 1300K a switch to 0 ions is involved. Superheated steam is produced at the anode. The O ion concentration gradient climbs from the oxygen cathode to hydrogen anode, to achieve migration/diffusion balance. Thus... 

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