Platinum-free materials

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

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. 

Bert, P. and Bianchini, C. (2006) Platinum-free electrocatalysts materials, European Patent EP 1 556 916 Bl. 

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. 

Eor their exploitation at the anode of microbial BBSs, these compounds have to be oxidized at an electrocatalytic electrode surface. This anode concept has been used for the oxidation of hydrogen produced during glucose fermentation on a platinum polymer-based sandwich electrode. In a subsequent step, these noble metal-free materials were replaced by noble metal-free electrode electrocatalysts allowing the oxidation of not only H2 but also low-molecular organic acids such as formate and lactate [33-35]. furthermore, the exploitation of sulfur species [36-38] can be classified within this electron transfer concept, although it needs to be noted that sulfur species can be reversibly cycled over sulfide/sulfur in BESs [39]. 

Method 1. From ammonium chloroplatinate. Place 3 0 g. of ammonium chloroplatinate and 30 g. of A.R. sodium nitrate (1) in Pyrex beaker or porcelain casserole and heat gently at first until the rapid evolution of gas slackens, and then more strongly until a temperature of about 300° is reached. This operation occupies about 15 minutes, and there is no spattering. Maintain the fluid mass at 500-530° for 30 minutes, and allow the mixture to cool. Treat the sohd mass with 50 ml. of water. The brown precipitate of platinum oxide (PtOj.HjO) settles to the bottom. Wash it once or twice by decantation, filter througha hardened filter paper on a Gooch crucible, and wash on the filter until practically free from nitrates. Stop the washing process immediately the precipitate tends to become colloidal (2) traces of sodium nitrate do not affect the efficiency of the catalyst. Dry the oxide in a desiccator, and weigh out portions of the dried material as required. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Perchlorates are also produced electrochemicaUy. The oxidation of chlorate to perchlorate ions occurs at a higher positive potential (above 2.0 V vs. SHE) than chloride ion oxidation. The current yield of perchlorate is lower when chloride ions are present in the solution hence, in perchlorate production concentrated pure chlorate solutions free of chlorides are used. Materials stable in this potential range are used as the anodes primarily, these include smooth platinum, platinum on titanium, and lead dioxide. 

This paper identifies alumina, rare earths, platinum, and magnesia as important SOx capture materials. Alumina is either incorporated directly into the matrix of a cracking catalyst or added as a separate particle. Cerium is shown to promote the capture of SO2 on high alumina cracking catalyst, alumina, and magnesia. Other rare earths are ranked by their effectiveness. The promotional effect of platinum is shown between 1200 and 1400 F for SO2 capture on alumina. Silica, from free silica or silica-alumina in the matrix of cracking catalyst, acts as a poison by migrating to the additive. 

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