Cathode catalyst cells, catalysis

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... 

With respect to fuel cell catalysis, most research has been focused on cathode ORR catalysts development, because the ORR kinetics are much slower than flic anodic HOR kinetics in other words, the fuel cell voltage drop polarized by load is due mainly to the cathode ORR overpotential [7, 8]. However, in some cases the overpotential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop [8]. Therefore, the catalytic HOR on the fuel cell anode catalyst is also worth examining. 

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. 

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. 

In most cases, the end of the cell life is neither related to catalysis nor to the state of the membrane, but is caused by a major hydrophobicity loss of the cathode GDL (backing). Such loss leads to catastrophic flooding of the cathode catalyst and prevents oxygen from reaching catalytic sites. 

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... 

The held to which the specific features of CNTs and CNFs could bring the most significant advancements is perhaps that of fuel cell electrocatalysis [125,187]. The main uses of CNTs or CNFs as catalyst support for anode or cathode catalysis in direct methanol fuel cells (DMFCs) or proton-exchange membrane fuel cells (PEMFCs) are covered in Chapter 12. In this section we summarize the main advantages linked to the use of nanotubes or nauofibers for these applications. 

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,... 

Figure 13.6. Durability of Pt/C and Pt Coi. /C-based MEAs tested iu a short stack (active area = 465 cm ) under H2-air at a ceU temperature of 80 °C and total reactant pressures of 150 kPUabs, with both anode and cathode humidities at 100% and anode and cathode reactant stoichiometries of s = 2/2. Data are shown with the stack under a constant load of 0.20 A/cm over 1000 h. Data were averaged over four cells of each type of MEA [1], (Reprinted from Applied Catalysis B Environmental, 56, Gasteiger HA, Kocha SS, Sompalh B, Wagner FT. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs, 9-35, 2005, with permission from Elsevier.)...

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