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Energies 2010, 3 1505 electrochemical step as rate determining under a Temkin isotherm, i.e. the adsorption of reaction intermediates Oads, OHads and HO2ads [43]. The rate expression under Temkin conditions of adsorption is: 1 F 1 F jkO2H2 exp 3.74kO2OH 2 exp (10) RT RT where k is the rate constant, η is the overpotential. In the high current density region, the Tafel slope is −120 mV dec−1 and the reaction order is 0 with respect to pH in alkaline solutions, with H2O as a reacting species. The adsorption of intermediate species changes to a Langmuir isotherm and Temkin conditions no longer hold. The rate expression under Langmuir conditions is: jkO expF (11) 2 where β is the symmetry factor. The reaction is first order with respect to O2 in solutions. It was found that the amount of H2O2 formed is greater in alkaline than in acid media [40,44]. In alkaline solutions, about 80% of the reduction current is via the direct reduction and the remainder of the current goes to the formation of H2O2, which suggests a more complicated mechanism. 3. Catalysts for DAAFCs As in proton exchange membrane fuel cells, platinum and other precious metals are still the most common catalysts used in DAAFCs. Low cost platinum-free and non-precious metal catalysts have been investigated for use in alkaline environment. 3.1. Alcohol Oxidation Catalysts For alcohol oxidation under alkaline solutions, the reaction current densities obtained during alcohol oxidation at certain potentials are at least an order of magnitude greater than in acidic electrolytes. The application of alkali electrolytes could lead to a reduction in catalyst loadings, as well as allowing less expensive, non-precious metal catalysts. In this case, the cost of electrocatalysts and thus fuel cells may be reduced. Methanol oxidation in alkaline media on platinum single crystal and polycrystalline electrodes has been investigated by several research groups [27–29,45]. Unlike in acid, the electrode does not suffer severely from poisoning in alkali because bonding of the chemisorbed intermediates on platinum is weak and the amount of the suggested poisoning species COads, is smaller than in an acidic medium [22]. 3.1.1. Precious Metal Catalysts Uniform platinum nanotubes have been synthesised by directly mixing Ag nanowires and H2PtCl6 in saturated NaI solutions. The Pt nanotubes were tested for methanol oxidation in alkaline. They demonstrated high active areas and higher activity compared to conventional Pt/C catalysts [46]. In acid solution, Pt/Ru is the most effective bimetallic catalyst used for methanol oxidation. It has been applied for more than 30 years and no significantly better system has yet been found. Ru dissolved in a RTPDF Image | Aspects of Direct Alkaline Alcohol Fuel Cells
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