Direct methanol fuel cell pathway

The electrooxidation of methanol has attracted tremendous attention over the last decades due to its potential use as the anode reaction in direct methanol fuel cells (DMFCs). A large body of literature exists and has been periodically reviewed [130,131,156], [173-199]. Unlike for formic acid, a generally accepted consensus on the specific mechanistic pathways of methanol electrooxidation is still elusive. 

We have seen that water present in the aqueous solution phase can considerably alter the ehemieal behavior of water adsorbed at an electrode surface by stabilizing particular charge transfer reactions. In addition, water can significantly influence the pathways and energetics of different electrocatalytic reaetions via direet participation. Herein, we examine methanol dehydrogenation over Pt(lll) as a case study since it is a central reaction in the electrocatalytic direct methanol fuel cell. 

The utility of carbide and nitride catalysts has prompted numerous studies of their reactivity that use carbide and nitride overlayers as the catalyst rather than bulk carbides or nitrides. This approach permits careful manipulation of the surface metal/nonmetal stoichiometry, which is crucial to probing reactivity. These studies consistently reveal the catalytic activity of carbide and nitride overlayers and, in several cases, the similarities between their behavior and that of noble metal catalysts. For example, the same benzene yield and reaction pathway for the dehydrogenation of cyclohexane was observed for both p(4x4)-C/Mo(110) and Pt(l 11) surfaces. Furthermore, carbon-modified tungsten may be a more desirable catalyst for direct methanol fuel cells than Pt or Ru surfaces because the transition metal carbide exhibits higher activity toward methanol and water dissociation and is more CO-tolerant. ... 

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. 

Polymer electrolyte membrane (PEM) is the heart of the direct methanol fuel cell (DMFC) system, which acts as an electrolyte for proton transfer from anode to cathode, as well as providing a barrier to the pathway of electrons between the electrodes [1]. Perfluorinated (PFl) proton exchange membranes (PEMs) from DuPont, Dow Chemical, Asahi Glass, and Asahi Chemical companies, which have been used as electrolyte manbranes in PEM fuel cell (PEMEC), possess outstanding chemical and mechanical stabilities. They perform excellently not only in PEMEC but also in DMFC [2,3]. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

Understanding the nature of this overpotential is a key feature to the development of better fuel-cell catalysts, and also involves understanding the reactivity of intermediates in methanol oxidation. Spectroscopic studies have shown that the electrooxidation of CH3OH (Eq. 9.10) on Pt is thought to follow a dual path mechanism at sufficiently high potentials that involves both indirect and direct pathways (Fig. 9.4). The indirect path, which proceeds through the formation of CO, is shown in the center of Fig. 9.4... 

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