Development of CO-tolerant Catalysts

Ternary catalyst systems, typieally based on a PtRu alloy, have also been investigated and their performance has been eompared with that of pure Pt/C or PtRu/C [129, 223-227], Specifically, PtRu alloys with Ni, Pd, Co, Rh, Ir, Mn, Cr, W, Zr, and Nb have been investigated. Nevertheless, there remain associated problems with the preparation method and the enhaneement of eleetrochemical performance. 

Many papers in recent years have presented details of new preparation methods and the performance of Pt-based eleetroeatalysts such as PtSn/C, PtMo/C, PtRuMo/C, PtRu-HxMoOa/C, and PtRu/(carbon nanotubes). In addition, efforts to develop R-free electrocatalysts such as PdAu/C have been undertaken. 

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Ruth, K., Vogt, M., and Zuber, R., Development of CO-tolerant catalysts, in Handbook of Fuel Cells Fundamentals, Technology, and Applications, 1st ed., Vielstich, W., Lamm, A., and Gasteiger, H.A., Eds., John Wiley Sons, West Sussex, England, 2003, p. 489. 

Nevertheless, the development of CO tolerant catalysts for PEMFCs is an important issue that needs to be addressed. The impurities in the hydrogen fuel remain a topic for research, although modern reformers typically produce a hydrogen stream with trace amounts of CO, for instance 1-50 ppm. However, even low CO concentrations (< 5 ppm) can poison the catalytic Pt surface of the anode, and reduce the cell performance. ... 

As in the development of CO-tolerant catalysts for PEFC anodes, the main challenge for the development of catalysts for the oxidation of alcohols is to reduce or to avoid the formation of strongly adsorbed poisoning species (i.e., CO) or to favor their oxidation at low overpotentials. 

This review discusses the meehanism of CO tolerance and the development of CO-tolerant catalysts. The development of Pt-based binary/temaiy metallic clcctrocatalysts and Pt-ffee electrocatalysts is discussed. Useful information is also provided on characterization methods for the understanding of the mechanism of CO tolerance and the evaluation of anode electrocatalysts. 

Studies of this type, eoupled with different surface characterization methods, enabled development of several elasses of CO tolerant anode catalysts [77], Markovic and Ross [77] provided eomprehensive description of the strategy of development of CO-tolerant catalyst based on extrapolation of fundamental electrochemisty of massive bimetallic surfaces to real-life supported eleetroeatalysts. 

Alloy surfaces are of substantial importance in catalysis, such as in the hydrogenation of unsaturated hydrocarbons, Fisher-Tropsch synthesis, steam reforming of methane, and many other processes [41]. In electrocatalysis, they have recently received attention in relation to the development of CO-tolerant fuelcell catalysts [42]. In many of these processes, atomic hydrogen and carbon monoxide are the most important intermediates or poisons therefore, these two adsorbates have received a great deal of attention in theoretical and computational studies. 

Wee, J.H., Lee, K.Y. 2006. Overview of the development of CO-tolerant anode electro- catalysts for proton exchange membrane fuel cells. /. Power Sources 157 128-35. 

CO is easily oxidized on platinum, and the recovery of CO-poisoned anodes has been discussed in detail elsewhere (Gottesfeld, 1992). The CO may also be removed by fuel starvation methods discussed in the patent literature. Development of CO-tolerant electro catalysts is also an active area of research (Huang et al., 2007). Operation of FCs at >120°C also improves their CO tolerance. 

For DMFC, methanol crossover is one of the main obstacles to its development Several efforts have been made to avoid or reduce the effect of methanol crossover on the DMFC s cathode performance [50-55], including the development of methanol-tolerant catalysts such as macrocycles or chalcogenides [34-37] and modification of Pt catalyst by adding another metal such as Fe, Co, Ni, and... 

V. Jalan, J. Poirier, M. Desai, B. Morrisean, "Development of CO and H2S Tolerant PAFC Anode Catalysts," in Proceedings of the Second Annual Fuel Cell Contractors Review Meeting, 1990. 

The definition of reformate tolerance is that, compared to running on pure H2, a fuel cell stack can run on reformate and show no change in performance, apart from that expected for dilution effects (of H2 due to CO2, N2, H2O). This requires the development of reformate-tolerant anode catalysts capable of tolerating the remaining levels of CO and CO2 in the fuel feed. 

Another possible catalyst poison, sulfur, was studied by Pope et al., who examined the oxidation of CO and some odorous organic compounds over supported and unsupported COjO. In CO oxidation, they showed that the addition of dimethyl sulfide, even in trace quantities, retarded CO oxidation significantly and irreversibly while the sulfur became incorporated in the catalyst. However, one objective of this research was to develop a sulfur-tolerant catalyst. Thus, by choosing a solid metal oxide catalyst, the authors claim that the CO3O4 surface is renewed as sulfur becomes incorporated [into the catalyst] by diffusion to the interior." Thus, the catalyst acts as both a sorbent (for sulfur) and a catalyst (for CO oxidation). This is an interesting principle because, if the catalyst can either be cheaply regenerated or disposed, any undesired oxidized sulfur products would not be produced, as would be the case for most noble metal deep oxidation catalysis, for example. Questions remain, however, as to whether the slightly lower activity of the catalyst in the presence of sulfur is economically acceptable. The authors also found that for butyric acid oxidation, the effect of (CH3)2S addition was reversible, in contrast to the results for CO oxidation. 

Figure 3-6 shows that performance equivalent to that obtained on pure hydrogen can be achieved using this approach. It is assumed that this approach would also apply to reformed natural gas that incorporate water gas shift to obtain CO levels of 1% entering the fuel cell. This approach results in a loss of fuel, that should not exceed 4 percent provided the reformed fuel gas can be limited to 1 percent CO(l). Another approach is to develop a CO-tolerant anode catalyst such as the platinum/ruthenium electrodes currently under consideration. Platinum/ruthenium anodes have allowed cells to operate, with a low-level air bleed, for over 3,000 continuous hours on reformate fuel containing 10 ppm CO (27). 

It is generally recognized that the development of Pt-based alloys is one feasible strategy for Pt load reduction and activity enhancement in fuel cells, including both hydrogen-fuelled PEMFCs and DMFCs. In this chapter, we introduce the research and development of Pt-based alloy catalysts for both the ORR and the MOR. The alloying effect and corresponding mechanism as well as the stability of Pt-based alloy catalysts towards the ORR and MOR will be described in detail. The topics of CO- and methanol-tolerant Pt-based alloy catalysts will not be discussed in this chapter (please see instead Chapter 16, CO-Tolerant Catalysts ). 

Electrochemical impedance spectroscopy (EIS) technique has been used for the experimental assessment of CO tolerance on different Pt-alloy catalysts and at different temperatures [187]. Hsing et al. [187] proposed that the critical potential at which pseudo-inductive behavior occurs could be used as a criterion for the evaluation of CO tolerance. A mathematical impedance model based on two state-variables (Pt-H and Pt-CO) was also developed to elucidate the reaction kinetics and mechanism of the H2/CO oxidation on a Pt/C catalyst [188]. In fact, this study has given better insight into explicitly understanding the impedance patterns and the quantitative assessment of the effect of applied potentials upon the oxidation reaction kinetics in a broad range of applied potentials. Nevertheless, with the consideration of only two adsorbed species, Pt-H and Pt-CO, the impedance model based on two state-variables was not able to explain the experimental observation... 

However, the Pt anode is seriously poisoned by trace amounts of carbon monoxide in reformates (fuel gas reformed from hydrocarbon), because CO molecules strongly adsorb on the active sites and block the HOR [Lemons, 1990 Igarashi et ah, 1993]. Therefore, extensive efforts have been made to develop CO-tolerant anode catalysts and cell operating strategies to suppress CO poisoning, such as anode air-bleeding or pulsed discharging. 

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