Methanol oxidation PEMFC

Until recently (i.e., till early 1990s), most of the efforts to develop DMFCs has been with sulfuric acid as the electrolyte. The recent success with a proton conducting membrane (perfluorosulfonic acid membrane) in PEMFCs has steered DMFC research toward the use of this electrolyte. The positive feature of a liquid feed to a DMFC is that it eliminates the humidification subsystem, as required for a PEMFC with gaseous reactants. Another positive point is that the DMFC does not require the heavy and bulky fuel processor. Two problems continue to be nerve-wracking in the projects to develop DMFCs (1) the exchange current density for methanol oxidation, even on the... 

Methanol oxidation in a DMFC is more difficult than H2 oxidation in a PEMFC, and the kinetics is slow, even using state-of-the-art PtRu catalysts. The role of Ru in methanol oxidation is to provide oxygenated species to oxidize the CO formed on Pt catalytic sites at low potentials. The mechanism can be written as follows ... 

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

More than a hundred articles have been published on the use of CNTs or CNFs as catalyst supports for DMFC and PEMFC. The most studied reaction is methanol oxidation (anode catalyst), followed by oxygen reduction (cathode catalyst) and to a lesser extent, hydrogen oxidation (anode catalyst). Platinum is... 

Recently, utilization of CNFs and CNTs in the CLs of PEMFCs and DMFCs has become an active research area. Numerous studies have focused on the stabilization of Pt and PtRu particles on the surface of CNTs and CNFs (see the review articles [153 and 230] and references therein). Adhesion of Pt to the basal plane of graphite is weak thus various approaches have been proposed to activate CNTs, including oxidation, grafting with various functional groups [153], and wrapping with a polymer [231], in order to stabilize highly dispersed Pt on their surfaces. Pt and Pt-Ru particles supported on CNTs and CNFs were tested in the oxygen reduction reaction and in the methanol oxidation reaction. 

Fuel Cell Reactions. Low temperature fuel cells such as proton exchange membrane fuel cells (PEMFC) or direct methanol fuel cells (DMFC) employ large amounts of noble metals such as Pt and Ru. There has been extensive research to replace these expensive metals with more available materials. A few studies considered transition metal nitrides as a potential candidate. In an anode reaction of DMFC, Pt/TiN displayed the electroactivity for methanol oxidation (53). Pt/TiN deposited on stainless steel substrate showed the high CO tolerance in voltammogram performed with a scan rate of 20 mV/s and 0.5 M CH3OH - - 0.5 M H2SO4 electrolyte. The bifunctional effect of Pt and TiN for CO oxidation was mentioned as observed between Pt and Ru in commercial PtRu/C catalysts. 

DMFCs are PEMFCs fed with methanol as fuel. The technologies required by DMFCs are similar to those of PEMFCs. What differ between DMFCs and PEMFCs are in the following two aspects The proton exchange membrane used for DMFCs must possess low methanol permeability or crossover, and the anode catalyst must possess high activity toward the oxidation of methanol and high tolerance to CO and other intermediates from methanol oxidation. In this section, the applications of NMR techniques for the development of DMCFs as well as essential materials are going to be briefly reviewed. 

A DMFC is quife similar to a proton exchange membrane fuel (PEMFC) in stack structure and components. They both use a PEM for transporting the protons and Pt-based catalysts at the cathode. The anode catalyst for a DMPC is typically a Pt-Ru alloy that has higher CO tolerance than Pt alone, and this is similar to the PEMFC when H2 contains trace amounts of CO. In fhe infer-mediate sfeps during methanol oxidation, some CO-like species will form, which can seriously poison the anode catalyst. The presence of Ru helps fhe removal of fhe CO-like species from fhe Pt surface trough Reaction 7.6. 

The archetypical direct fuel cell is the DMFC. like the PEMFC, the DMFC uses a proton-conducting membrane to separate the anode and cathode, and protons liberated during electrocatalytic methanol oxidation [Eq. (15.6)] at the anode are involved in oxygen reduction at the cathode. However, whereas in the hydrogen fuel cell the anode reaction is straightforward, the methanol oxidation is comparably sluggish, which is mainly attributed to poisoning effects. 

Anodic Reactions in Electrocatalysis -Methanol Oxidation, Fig. 2 Current density versus electrode potential curves for electrochemical reactions involved in a PEMFC and in a DMFC... 

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. 

The kinetics of methanol oxidation is not as fast as the kinetics of hydrogen oxidation in hydrogen/air PEMFC. In order to make DMFCs viable, the kinetics of methanol electrooxidation must be enhanced. 

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