CO tolerance

Several activities, if successful, would strongly boost the prospects for fuel ceU technology. These include the development of (/) an active electrocatalyst for the direct electrochemical oxidation of methanol (2) improved electrocatalysts for oxygen reduction and (2) a more CO-tolerant electrocatalyst for hydrogen. A comprehensive assessment of the research needs for advancing fuel ceU technologies, conducted in the 1980s, is available (22). 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Brankovic SR, Wang JX, Adzic RR. 2001b. Pt submonolayers on Ru nanoparticles—A novel low Pt loading, high CO tolerance fuel cell electrocatalyst. Electrochem Solid State Lett 4 A217-A220. 

Igarashi H, Fujino T, Zhu Y, Uchida H, Watanabe M. 2001. CO tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism. Phys Chem Chem Phys 3 306-314. 

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. 

To evaluate the catalytic activity or to investigate the reaction mechanism, planar electrodes with well-defined characteristics such as surface area, surface and bulk compositions, and crystalline structure have often been examined in acidic electrolyte solutions. An appreciable improvement in CO tolerance has been found at Pt with adatoms such as Ru, Sn, and As [Watanabe and Motoo, 1975a, 1976 Motoo and Watanabe, 1980 Motoo et al., 1980 Watanabe et al., 1985], Pt-based alloys Pt-M (M = Ru, Rh, Os, Sn, etc.) [Ross et al., 1975a, b Gasteiger et al., 1994, 1995 Grgur et al., 1997 Ley et al., 1997 Mukeijee et al., 2004], and Pt with oxides (RuO cHy) [Gonzalez and Ticianelli, 2005 Sughnoto et al., 2006]. 

We have found new CO-tolerant catalysts by alloying Pt with a second, nonprecious, metal (Pt-Fe, Pt-Co, Pt-Ni, etc.) [Fujino, 1996 Watanabe et al., 1999 Igarashi et al., 2001]. In this section, we demonstrate the properties of these new alloy catalysts together with Pt-Ru alloy, based on voltammetric measurements, electrochemical quartz crystal microbalance (EQCM), electrochemical scanning tunneling microscopy (EC-STM), in situ Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). 

Figure 10.1 shows 4 at various electrodes as a function of CO poisoning time at 26 °C. For the pure Pt electrode, the value of 4 decreases and reaches nearly zero after 30 minutes. In contrast, the Pt-Fe, Pt-Ni, Pt-Co, and Pt-Mo alloys retain high HOR activity for a prolonged period of time the reduction in 4 is negligibly small. Such CO tolerance of these alloys was found to be almost independent of the composition for example, alloying Pt with only 5 at% Fe resulted in excellent tolerance. However, Pt alloys with Ti, Cr, Cu, Ge, Nb, Pd, In, Sb, W, An, Pb, or Bi showed complete CO poisoning after a short time, while the combination of Pt with Mn, Zn, Ag, or Sn exhibited only limited CO tolerance. 

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

However, the CO tolerance at Pt-Co degraded at 70 °C. As seen in Fig. 10.9, the HOR activity of Pt-Co at a given dco is close to that of pure Pt, although the deceleration effect on the CO adsorption rate was still observed to some extent at 70 °C. Such a deactivated electrode cannot recover the original CO tolerance. This can certainly be ascribed to a severe dealloying of the nonprecious metal component (Co) in hot acid solution. We will discuss this in Section 10.3.2. 

Gonzalez ER, TicianeUi EA. 2005. Studies of CO tolerance on modified gas diffusion electrodes containing ruthenium dispersed on carbon. J Electroanal Chem 575 53-60. 

Mukeijee S, Urian RC, Lee SJ, TicianeUi EA, McBeen J. 2004. Electrocatalysis of CO tolerance by carbon-supported PtMo electrocatalysts in PEMFCs. J Electrochem Soc 151 A1094-A1103. 

Uchida H, Ozuka H, Watanahe M. 2002. Electrochemical quartz crystal microhalance analysis of CO-tolerance at Pt-Fe alloy electrodes. Electrochim Acta 47 3629-3636. 

Uchida H, Izumi K, Watanabe M. 2006. Temperature dependence of CO-tolerant hydrogen oxidation reaction activity at Pt, Pt-Co, and Pt-Ru electrodes. J Phys Chem B 110 21924-21930. 

Wakisaka M, Mitsui S, Hirose H, Kawashima K, Uchida H, Watanabe M. 2006. Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrol3de fuel cells studied by EC-XPS. J Phys Chem B 110 23489-23496. 

Watanabe M, Igarashi H, Fujino T. 1999. Design of CO tolerant anode catalysts for polymer electrolyte fuel cell. Electrochemistry 67 1194-1196. 

Diemant T, Hager T, Hosier HE, Rauscher H, Behm RJ. 2003. Hydrogen adsorption and coadsorption with CO on well-defined himetallic PtRu surfaces—A model study on the CO tolerance of himetallic PtRu anode catalysts in low temperature polymer electrolyte fuel cells. Surf Sci 541 137. 

Koper MTM, Shubina TE, van Santen RA. 2002. Periodic density functional study of CO and OH adsorption on Pt-Ru alloy surfaces Implications for CO-tolerant fuel cell catalysts. J Phys Chem 106 686. 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

Schmidt TJ, Gasteiger HA, Behm RJ. 1999. Rotating disk electrode measurements on the CO tolerance of a high-surface area Pt/Vulcan carbon fuel cell catalysts. J Electrochem Soc 146 1296-1304. 

Schmidt TJ, Jusys Z, Gasteiger HA, Behm RJ, Endruschat U, Boennemann H. 2001. On the CO tolerance of novel colloidal PdAu/carbon electrocatalysts. J Electroanal Chem 501 132-140. 

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