Fuel cell electrocatalysis platinum

In this chapter, we provide a succinct review of some of the advances in the development and application of ab initio methods toward understanding the intrinsic reactivity of the metal and the influence of the reactive site and its environment. We draw predominantly from some of our own recent efforts. More specifically we describe (a) the chemistry of the aqueous-phase on transition metal surfaces and its influence on the kinetics and thermodynamics within example reaction mechanisms, and (b) computational models of the electrode interface that are able to account for a referenced and tunable surface potential and the role of the surface potential in controlling electro-catalytic reactions. These properties are discussed in detail for an example reaction of importance to fuel cell electrocatalysis methanol dehydrogenation over platinum(ll 1) interfaces [24,25]. 

Adachi H, Ahmed S, Lee SHD, Papadias D, Ahluwaia RK, Bendert JC, Adzic RR et al (2007) Platinum monolayer fuel cell electrocatalysis. Top Catal 46 249... 

Muketjee S, Srinivasan S. 1993. Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells. J Electroanal Chem 357 201-224. 

Mukeijee S, Srinivasan S. 1993. Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells. J Electroanal Chem 357 201-224. Mukeijee S, Srinivasan S, Soriaga M, McBreen J. 1995. Role of structural and electronic properties of Pt and Pt alloys on electrocatalysis of oxygen reduction. J Electrochem Soc 142 1409-1422. 

Antolini E, Passos RR, Ticianelh EA. 2002. Electrocatalysis of oxygen reduction on a carbon supported platinum-vanadium alloy in polymer electrol3de fuel cells. Electrochim Acta 48 263-270. 

Binder H, Kohling A, Sandstede G. 1972. Effect of alloying components on the catalytic activity of platinum in the case of carbonaceous fuels. In Sandstede G, ed. From Electrocatalysis to Fuel Cells. Seattle University of Washington Press, p. 43. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

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 electrocatalysis 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]. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

For example, work in the fuel cell area at the beginning of the sixties has emphasized the interest of mixing small particles of noble metals, such as platinum and palladium, in conducting materials, e.g., carbon powders, in order to promote microheteroge-neous electrocatalysis. Later on, after the development of redox polymers as electrode materials, inclusion of particles into these electrodes has also been performed electrochemically, by the electroreduction of a metal complex ion, such as PtCl , Cu " or Co " in the... 

Starting point for the emergence of electrocatalysis was the discovery that hydrocarbons could be oxidized at low temperatures (this fact had not been a part of the Ostwald scenario). Then it was discovered that synergistic effects were operative in the use of ruthenium-platinum catalysts for methanol oxidation, and that compounds such as platinum-free metalloporphyrins were useful catalysts for certain electrochemical reactions in fuel cells. Hopes were expressed that in the future expensive platinum catalyst could be replaced. Again, in the attempts of commercial realization of these discoveries considerable difficulties were encountered, which led to a period of disenchantment and pessimism in 1970s and 1980s. It had been demonstrated beyond doubt that, fundamentally, hydrocarbons could be oxidized at low temperatures, but practical rates that could be achieved were unrealistically low. It had also been demonstrated that fuel cells could be made to work without... 

Considering that the electrocatalysis of the oxidation of low molecular weight alcohols in acid media on platinum and platinum group metals has been studied for almost a century it may be said that dramatic improvements in this area should not be expected. However, further smdies are necessary because even incremental improvements may have a significant impact on the pursue of making direct alcohol fuel cells a practical reality. 

Methanol is the typical fuel with one carbon (Cl) atom for fuel cells. Methanol was one of the first small molecules chosen to study the oxidation on platinum group metals in the very early beginning of electrocatalysis. In that time, the oxidation of other Cl molecules such as formic acid and formaldehyde (interest in CO oxidation came latter with the oxidation of reformatted gases) were investigated as a model oxidation because their elementary steps were supposedly present in the mechanism of methanol oxidation. From the point of view of CO2 emission, methanol has, among the other small molecules, the highest energy production per unit of produced... 

In spite of a very significant progress achieved with heat-treated macrocyclic compounds as ORR catalysts since the early 1970s, the activity and durability of that family of catalysts are stiU insufficient for replacing platinum at the fuel cell cathode and in other applications. Furthermore, the complex structure of macrocyclic compounds makes their synthesis expensive and potentially noncompetitive with precious-metal-based catalysts also from the materials cost point of view. For those reasons, much effort has been invested by the electrocatalysis research community in recent years into finding less expensive and catalytically more active non-precious metal ORR catalysts that would not rely on macrocylic compounds as either catalysts or catalyst precursors. In the past decade, there has been a significant improvement both in the activity and of non-macrocyclic catalysts, expected to be manufactured at a fraction of the cost of their macrocyclic counterparts. In this section, we review the precursors, synthesis routes, and applications of this relatively new family of catalysts. 

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