Platinum-free noble metal catalysts

Apart from platinum s intermediate nature on bonding, another point in platinum s favor is availability platinum can be purchased in various suitable forms at a reasonable price some noble metals are difficult to find and purchase. The word noble means here stable and of course that is a first point one wants in an electrocatalyst. It must be a catalyst, not enter into the reaction. It is meant to accelerate the reaction. It must itself be stable, thermally and electrochemically. On the last point, platinum is only fairly good because oxide-free platinum does start itself to dissolve around 1.0 V on the normal hydrogen scale. By using it in anodic reactions in a potential range anodic to 1.0 V, Pt(II) is likely to get into the solution and may be deposited on the cathode. 

As reported so far, one of the best platinum-free ORR catalysts of chalcogenide-type structure is a selenium-modified mthenium catalyst (RuScx/C) [9-20], State-of-the-art catalysts are composed of carbon-supported nano-scaled ruthenium particles whose surface was modified with selenium [9-14], The modification leads to 10 times higher ORR activity, protects the ruthenium particles against electrooxidation, and suppresses the H2O2 formation. As RuSe /C is insensitive to methanol, it might be particularly suitable as an alternative cathode material in direct methanol fuel cells (DMFC) where platinum shows potential losses due to the methanol crossover [15-18]. However, ruthenium is still a costly and rare noble metal and seems not to be a feasible alternative to platinum. Therefore, readers who are interested in this type of catalyst are referred to the cited literature. 

Besides pure platinum-based catalysts, a variety of replacements for platinum, the traditional catalyst metal, have been investigated for mass applications requiring a much lower final system price (compared to the DoE estimates noted above). The research is mainly focused on two aspects, either a reduction of the platinum content or platinum-free compounds based either on substitute noble metals (which might, unfortunately, not necessarily reduce the price framework) or alloyed transitional metals. The main criterion, next to the price, is the long-term stability of the new materials and the synthesis of these compounds. The synthesis should include a fast, efficient reaction scheme avoiding multi-step reactions as well as polluting solvents that, in the end, increase processing costs. 

Examples of both approaches will be discussed below, starting with the so-called core shell catalysts, which mostly consist of non-noble metal cores covered by a noble metal such as platinum. Platinum-free materials, especially when based on non-noble components, have to fulfill the criterion of stability in acidic media. Recently, electrocatalysts including cobalt and iron proved then-suitability in fuel cell applications where the metal ion is incorporated in a nitrogen macrocycle comparable to the natural porphyrin ring system. 

Platinum-free catalysts usually feature two general problems stability in acidic enviromnents, and that none of the alternatives has so far reached the activity level of platinum. The first criterion excludes almost all pure non-noble metals as well as their alloys, unless an extraordinary stability combined with a ehange of the non-noble properties can be observed. The leaching of metal ions from the catalyst catalyses the formation of hydrogen peroxide, which leads to a self-destroying oxidation of the catalyst and to membrane degradation. " ... 

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