Platinum metal catalysts, cathodic oxygen reduction

Fuel Cells, Non-Precious Metal Catalysts for Oxygen Reduction Reaction Platinum-Based Cathode Catalysts for Polymer Electrolyte Fuel Cells... 

Anodic hydrogen oxidation and even more cathodic oxygen reduction is kinetically hampered at low temperature, so that anodic hydrogen oxidation in AFCs, PEMFCs, and PAFCs demands catalysts of highest activity, that is, platinum metals and platinum in particular. Also Raney nickel is used in... 

Decrease the platinum content of the oxygen reduction reaction catalysts in fuel cell cathodes to meet the DOE 2010 precious-metal-loading goals of 0.2 g/rated kW. 

The catalyst is platinum-based for both the anode and cathode. To promote hydrogen oxidation, the anode uses either pure platinum metal catalyst or, as is common in most modem PEFC catalysts, a supported platinum catalyst, typically on carbon or graphite for pure hydrogen feed streams. For other fuels, such as reformate (containing H2, CO2, CO, and N2), the desired catalyst is an alloy of platinum containing mthenium. Oxygen reduction at the cathode may use either the platinum metal or the supported catalyst. 

An important factor affecting the performance of DMFCs is the kinetics of catalyst. Platinum (Pt/C) is the most effective catalyst for oxygen reduction reaction but it is not selective towards ORR in presence of methanol. The addition of yttrium to Pt increases the ORR activity and are promising ORR electrocatalyst [207]. Carbon supported PtY(OH)3 hybrid catalyst are developed with dynamic spillover of metal oxide [208]. Recently, catalyst for DMFC Pt Pd/C NP was prepared by the galvanic displacement reaction between Pt and Pd. A simple synthesis strategy was followed to prepare carbon based [209] and carbon-supported Pd nanostructure [190]. A higher methanol tolerance of Pt Pd/C with less Pt content than Pt/C suggests that it is potential alternative cathode electrocatalyst for DMFCs [190]. 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

Noble metals applied as electrocatalysts for the oxygen reduction have been largely utilized because of their high electrocatalytic activity and stability. Investigations are concentrated on platinum, palladium, silver and gold. The application of noble metal catalysts is limited by two fundamental disadvantages high cost and low availability. Thus, it is important to construct cathodes with small amounts of the noble metal which are obtained, for example, by dispersed platinum on an appropriate support. 

Fig. 13.1 (a) Trend of number of the research paper deals with the non-platinum cathode catalysts from 1964. (b) Percentage of the paper about the non-precious metal oxygen reduction catalyst in an acid electrolyte... 

However, for technical use of AFC, the long-term behavior of AFC components is important, especially that of the electrodes. Nickel can be used for the hydrogen oxidation reaction (catalyst in the anode) and on the cathode silver can be used as catalyst (see next section), no expensive noble metal (platinum) is necessary, because the oxygen reduction reaction kinetics are more rapid in alkaline electrolytes than in acids and the alkaline electrochanical environment in AFC is less corrosive compared to acid fuel cell conditions. Both catalysts and electrolyte represents a big cost advantage. The advantages of AFC are not restricted only to the cheaper components, as shown by Giilzow [1996]. 

In spite of the success in the optimization of platinum catalysts, a major breakthrough in the field of fuel cells is yet to be achieved. Especially, the desire for a significant cost reduction by the replacement of platinum motivates international research activities investigating new catalyst concepts for cathodes. Thereby, the cathodes have to be sufficiently stable under fuel cell conditions the alternative non-noble metal catalysts (NNMC) need to have a high selectivity for direct reduction of oxygen to water. The US department of energy (DOE) defined 25% of the achievable current density of a commercial platinum catalyst as target value for 2015. For fuel cell application, the catalysts should be producible in such a nano-structured form that suitable gas diffusion structures can be built. 

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