Metal catalysis, cathodic oxygen

Fuel cells (continued) metal catalysis, cathodic oxygen reduction, 40 127... 

F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, et al.. Recent advances in non-predous metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ. Sd. 4 (2011) 114-130. R. Jasinski, A new fuel cell cathode catalyst. Nature 201 (1964) 1212-1213. 

This is a major reason why the majority of non-noble metal catalysis research has focused on the cathode part of the fuel cell system this work began about 1964 with a paper in Nature by Jasinski indicating that N4-metal chelates had electrochemical oxygen reduction capacity, and specifically with the discovery that C0N4 phthalocyanine is an oxygen reduction catalyst in alkaline solution [102,... 

Binary systems of ruthenium sulfide or selenide nanoparticles (RujcSy, RujcSey) are considered as the state-of-the-art ORR electrocatalysts in the class of non-Chevrel amorphous transition metal chalcogenides. Notably, in contrast to pyrite-type MS2 varieties (typically RUS2) utilized in industrial catalysis as effective cathodes for the molecular oxygen reduction in acid medium, these Ru-based cluster materials exhibit a fairly robust activity even in high methanol content environments of fuel cells. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

Transition metal surfaces enriched with S, Se and Te, have been considered as candidates for DAFC cathode catalysts [112-115], For example, ruthenium selenium (RuSe) is a weU-studied electro-catalyst for the ORR [116, 117]. The ORR catalysis on pure Ru surfaces depends on the formation of a Ru oxide-like phase [118]. Ru is also an active catalyst for methanol oxidation. On the other hand, the activity of the ORR on RuSe is found not be affected by methanol [116]. RuS, has also been reported insensitive to methanol [119-122], DPT studies of model transition metal surfaces have provided with atomistic insights into different classes of reactions relevant to fuel cells operation, such as the hydrogen evolution [123], the oxygen reduction [124], and the methanol oxidation [125] reaction. Tritsaris, et al. [126] recently used DPT calculatimis to study the ORR and methanol activation on selenium and sulfur-containing transition metal surfaces of Ru, Rh, Ir, Pd, Co and W (Fig. 8.9). With RuSe as a starting point, the authors studied the effect of the Se on... 

It was noted above that one should seek electrode materials where the overpotential for chlorine evolution is as low as possible but where there is a considerable oxygen overpotential. In addition, for diaphragm and membrane cells there is the requirement for cathodes with a low hydrogen overpotential. Catalysis of hydrogen evolution was discussed in Chapter 1 and it was shown that strong catalysis is promoted by metals where the free energy of adsorption of hydrogen atoms has an intermediate value. This allows mechanisms such as... 

Oxygen reduction can be catalyzed by enzymes, and air-breathing cathodes with laccase and bUimbin oxidase as enzymatic catalysts, for example, have been demonstrated [10-15]. Enzymatically catalyzed cathodes avoid many of the problems discussed for platinum and other precious metal catalysts, but they often provide lower power density and Umited stability. Specifically with air-breathing cathodes, enzymes that catalyze oxygen reduction must be immobilized at the tripoint between hydrophilic (H" conductivity), hydrophobic (O2 supply), and conductive (e conductivity) interfaces for effective catalysis (Figure 16.1) (see Chapter 3). 

The oxygen reduction reaction (ORR) is the major process at fuel cell cathodes. Typically the cathode electrocatalyst is Pt dispersed on high surface area carbon however, the performance of Pt is not ideal. The standard potential for the oxygen electrode (H2O/O2) in acid solution at 298 K is 1.23 V (RHE). The cathode of the Pt-activated acid fuel cell operating at a moderate current density and an O2 pressure of 1.0 atm is in the region of 0.8 V—clearly there is considerable scope for improved performance and cost reduction in the fuel cell cathode area. The catalysis of the oxygen reduction reaction, largely at metal surfaces in aqueous media, was surveyed recently by Adzic [57]. 

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