Methanol tolerance

Initially, at least, methanol vehicles should be capable of operating on either M85, gasoline, or any mixture of the two. These vehicles are caHed flexible fueled (EEV) or variable fueled vehicles (VEV). It is expected that methanol could be sold in existing service stations out of tanks constmcted of methanol-tolerant material such as carbon steel or certain fiber glass formulations. EventuaHy, if enough EEV/VEVs are sold and methanol becomes widely avaHable, dedicated vehicles would likely be buHt and sold. Methanol has been used for years as a racing fuel. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

Reeve RW, Christensen PA, Hamnett A, Haydock SA, Roy SC (1998) Methanol tolerant oxygen reduction catalysts based on transition metal sulfides. J Electrochem Soc 145 3463-... 

Ozenler SS, Kadirgan F (2006) The effect of the matrix on the electro-catalytic properties of methanol tolerant oxygen reduction catalysts based on ruthenium-chalcogenides. J Power Sources 154 364-369... 

Methanol tolerance is a very important property of Pd-M alloys. In particular, methanol tolerance was demonstrated for PdsFe/C and for Pd-Co based alloys [Mustain et al., 2007 Raghuveer et al., 2005 Shao et al., 2006c Zhang L et al., 2007]. The high ORR activity in the presence of a high concentration of methanol indicates that the Pd-Co and Pd-Fe electrocatalysts are not active for methanol oxidation. 

Lee K, Savadogo O, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. 2006. Methanol-tolerant oxygen reduction electrocatalysts based on Pd-3D transition metal alloys for direct methanol fuel cells. J Electrochem Soc 153 A20-A24. 

Koffi RC, Coutanceau C, Gamier E, Leger JM, Lamy C. 2005. Synthesis, characterization and electrocatalytic behaviour of non-alloyed PtCr methanol tolerant nanoelectrocatalysts for the oxygen reduction reaction (ORR). Electrochim Acta 50 4117-4127. 

Selvarani, G., Maheswari, S., Sridhar, P., Pitchumani, S., and Shukla, A.K. (2009) Carbon-supported Pt-Ti02 as a methanol-tolerant oxygen-reduction catalyst for DMFCs. Journal of the Electrochemical Society, 156 (11),... 

Xiong, L. and Manthiram, A. (2004) Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells. Electrochimica Acta, 49 (24), 4163-4170. 

In addition to the slow methanol oxidation kinetics, methanol that crosses over from the anode to the cathode side through the membrane can react with 02 at the cathode catalyst, leading to a mixed potential at the cathode side and thereby reducing cell performance. To solve this problem, methanol-tolerant catalysts as well as membranes with low methanol permeability have been investigated. However, these materials are still in the research stages and commercial applications have not been developed. 

Jefferson MC, Aguirre M. 1980. Methanol tolerances and the effects of methanol on longevity and oviposition behavior in... 

Sun, Y.H. and Barton, S.C. (2006) Methanol tolerance of a mediated, biocatalytic oxygen cathode. Journal of Eiectroanaiytical Chemistry, 590 (1),... 

Meeting of the of the Electrochemical Society, Philadelphia, Pennsylvania, May 12-17, 2002. Title DMFC Cathode Catalyst with Improved Methanol Tolerance Y. Zhu, ... 

The methanol-tolerant properties of the PtNx/C catalysts in terms of polarization in N2-saturated 0.5 mol H2SO4 solution, containing 0.1 mol methanol, is presented in Fig. 5.3b [23]. As can be observed, the methanol oxidatimi peak for the PtNx/C is more than ten times smaller than that obtained for the commercial Pt/C catalyst. These curves show that the PtNx/C materials are less active for methanol oxidation and this might result in a high methanol tolerance during the ORR. With increasing N content, the methanol oxidation is further inhibited and shows almost constant methanol tolerance when the molar ratio of N to Pt is higher than 4. 

Many ORR experiments have been made on electrocatalysts composed by Pt and Pd with the addition of non-noble metals, such as Co, Fe, and Ni. However, under electrochemical conditions these non-noble metals might leach out from the electrocatalyst, as demonstrated in previous investigations [25]. In order to avoid this problem, Yang and co-authors [26] have investigated PdPt-based electrocatalysts for the ORR in absence and in the presence of methanol, because the long-term stability of Pd in acidic solution is comparable to that of Pt (but this depends on the potential - additionally, Pt may stabilize Pd atoms in the alloy). It was report a novel strategy for surface and structure-controlled synthesis of carbon-supported Pd3Pti nanoparticles for the ORR as well as for methanol-tolerant ORR electrocatalyst. The influence of the surface composition and structure of the PdsPti/C on the ORR activity in the absence and presence of methanol was also reported. 

In another work, Shukla and co-authors [27] studied the electrocatalytic activity of carbon-supported Pt-Au alloy catalysts, with different atomic ratios, to improve the oxygen reduction reaction (ORR) kinetics and methanol tolerance, in a direct methanol fuel ceU. The electrocatalysts were prepared by codeposition of Pt and Au nanoparticles onto a carbon support. 

In general, non-noble metal alloy nanoparticles have shown some methanol tolerance effects, but their activity towards ORR is lower than that of Pt/C. Furthermore, it has been found that the non-precious metal catalysts do not present the required stability in the acidic environment, even in the case of Pd (at high potential). On the other hand, some works have shown that this instability (dissolution), mainly of the non-noble metal, can be overcome by the addition of small amounts of stabilizers like Au. Based on this, Mathiyarasu and Phani [30] examined the effect of the addition of Au, Ag and Pt on the activity and stability of several Pd-Co/C electrocatalysts. Results showed higher ORR activities for Pd-Co-Pt/C, equal to that of a commercial Pt/C electrocatalyst. 

The high methanol tolerance of PtPd/C alloy catalysts is attributed to the weak competitive reaction of methanol oxidation, which could be induced by composition effects associated with the presence of Pd atoms. The methanol adsorption-dehydrogenation process requires at least three neighboring Pt atoms with appropriate crystallographic arrangement, so, in the case of Pt-Au/C materials, the probability this arrangement in the surface decreases for increasing Au contents. 

This explains the higher methanol tolerance of the alloy material in relation to that of pure Pt/C. For Pt-free electrocatalysts, PCI4C01/C showed to be very active for the ORR even at a high concentration of methanol. The addition of noble metal such as Au, Ag and Pt onto the PdCo material, in order to increase their stability in acid electrolyte, conducts to a lowered MOR activity and high ORR kinetics. For the RuSe/C and RhS/C materials, the former presents a considerable tolerance to the presence of methanol. However, the observed loss of selenium from the surface, observed upon exposure to potentials greater than 0.85 V, indicates a detrimental effect on the implementation of RuSe/C as a cathode material in fuel cell applications. The commercially available rhodium sulphide underperforms and exhibits higher susceptibility to methanol compared to RuSe/C, but it is more stable under similar testing conditions. 

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