Cathodes in fuel cells

Alloys of noble metals have been tested for O2 reduction [31] and some should even be more efficient than Pt [63]. These metals are expensive so that other materials have been tested, like bronze, some being even slightly better electrocatalysts than platinum [45] however, they do not seem to be stable enough. The factors governing oxygen reduction on oxides used as cathodes in fuel cells have been reviewed recently [92]. 

It has proved much more difficult to select satisfactory electrode materials for the oxidising environments prevailing at the cathode in fuel cells and at the anode in electrolysers. The noble metals are too expensive and it is generally agreed that the only practical class of materials to consider for the relevant electrodes are electronically conducting oxides. 

Kretzschmar C, Wiesiner K. Study of the operation of tungsten carbide anode and iron phthalocyanine cathodes in fuel cells with a sulphuric acid electrol3rtc. Elektrokhimiya 1978 14 1330-4. 

The reduction of molecular oxygen that is supplied either directly from containers or in a diluted form as air constitutes the reaction at the cathode in fuel cells. The use of air is preferable for economic reasons. Platinum metals and alloys of platinum metals are electrocatalysts for acid and alkaline electrolytes. Silver, silver alloys, nickel, carbon, and intermetallic compounds represent less expensive electrocatalysts for the oxygen electrode in alkaline solutions. In contrast to the hydrogen electrode, the overvoltage of the oxygen electrode is large at temperatures below 100 °C when a reasonable current is drawn. 

The product of this reaction is water that is formed at the cathode in fuel cells with proton-conducting membranes. It can be formed at the anode, if an oxygen ion (or carbonate)-conducting electrolyte is used instead, as is the case for high-temperature fuel cells. 

The ordn uses for polypropylene are varied. It is used in the fabrication of personnel body armor (Refs 6 7) in slurry-type expls for the demolition of concrete structures (Ref 11) as a microporous hydrazine-air (cathode) separator in fuel cells (Ref 9) as a propint binder matl, particularly in caseless ammo, (Refs 5 8) and as a candidate to act as a proplnt aging inhibitor for the 155mm RAP round (Ref 10) Refs 1) Beil 1, 196, (82), [167], 677 and (725) 2) A.V. Topchiev V.A. Krentsel,... 

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Solid alkaline membrane fuel cells (SAMECs) can be a good alternative to PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occurring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003 Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the improved electrode kinetics. Eor example, Ag/C catalytic powder can be used as an efficient cathode material [Demarconnay et al., 2004 Lamy et al., 2006]. It has also... 

Ionomer membranes are used in fuel cells in order to separate the anode and cathode compartment and to allow the transport of protons from the anode to the cathode. The typical membrane is Nation , which consists of a perfluorinated backbone and side chains terminated by sulfonic groups. In the oxidizing environment of fuel cells, Nation , as well as other membranes, is attacked by reactive oxygen radicals, which reduce the membrane stability. Direct ESR was used recently in our laboratory to detect and identify oxygen radicals as well as radical intermediates formed in perfluorinated membranes upon exposure to oxygen radicals [73,74]. The three methods used to produce oxygen radicals in the laboratory and the corresponding main reactions are shown below. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

If the flow of oxygen falters, e.g. when the surface of the cathode is covered with water, then no gaseous 02 can reach the platinum outer layer. In response, firstly no electrons are consumed to yield oxide ions, and secondly the right-hand side of the cell floods with the excess protons that have traversed the polymer membrane and not yet reacted with O2-. Furthermore, without the reduction of oxygen, there is no redox couple at the cathode. The fuel cell ceases to operate, and can produce no more electrical energy. 

Section IV emphasizes on nanoparticle catalysts for fuel cell applications. Fuel cell is a clean and desired future energy source. It is interesting to see that nanoparticle electrocatalysts play an important role in fuel cell development. Chapters 14 and 15 explore how nanoparticle catalysts can efficiently catalyze the reactions at anode and cathode of the fuel cells. 

In fuel cells, the combustion energy of hydrocarbons can be converted directly into electrical energy. At the fuel cell anode, the hydrocarbon is in most cases converted to carbon dioxide because the intermediates are more easily oxidized than the starting hydrocarbon (Eq. 9a) at the fuel cell cathode oxygen is reduced to water (Eq. 9b). Most fuel cell research has involved the use of hydrogen as fuel. However, solid oxide fuels cells (SOCFs) can operate at higher temperature and can... 

Summing of Electrode Polarization Activation and concentration polarization can exist at both the positive (cathode) and negative (anode) electrodes in fuel cells. The total polarization at these electrodes is the sum of r act and riconc, or... 

T. Ito, K. Kato, S. Kamitomai, M. Kamiya, "Organization of Platinum Loading Amount of Carbon-Supported Alloy Cathode for Advanced Phosphoric Acid Fuel Cell," in Fuel Cell Seminar Abstracts, 1990 Fuel Cell Seminar, Phoenix, AZ, November 25-28, 1990. J.S. Buchanan, G.A. Hards, L. Keck, R.J. Potter, "Investigation into the Superior Oxygen Reduction Activity of Platinum Alloy Phosphoric Acid Fuel Cell Catalysts," in Fuel Cell Seminar Abstracts, Tucson, AZ, November 29-December 2, 1992. 

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