Methanol and CO electrooxidation

The methanol oxidation reaction has been the subject of a large number of studies in the past.[1, 2] Early work revealed a complex reaction mechanism,[3–6] indicating the electrocatalysis of methanol oxidation as the most difficult task in the realization of a direct methanol fuel cell (DMFC). Several metal catalysts were proposed for the reaction, most of them based on modifications of Pt with some other metal.[7–10] For about 15 years, research on the DMFC has been receiving great attention (see Direct methanol fuel cells (DMFC), Volume 1). Considerable effort has been devoted to the technical realization of the methanol fuel cell and a wealth of basic research has been directed to elucidate the mechanistic aspects of methanol oxidation. In fact, methanol and other small organic molecules have been studied for more than 70 years, but a greater understanding was achieved by the development of ex situ and in situ spectroscopic and microscopic methods for application in electrochemistry,[11–14] together with the use of welldefined monocrystalline electrode surfaces.[15] This chapter presents a state-of-the-art review on the electrocatalysis of methanol oxidation. Research on methanol, as well as on the parent compounds formaldehyde and formic acid, has been reviewed several times (see for example, Refs. [16–18]). Therefore, this discussion is centered on those aspects of the electrocatalysis of methanol oxidation which are at present well established or which may need further investigation. The purpose of this contribution is to present our current understanding of the methanol system and provide a basis for future investigations. It is with this objective in mind, that the references in this chapter were selected. Nevertheless, as in any paper of this kind, it is unavoidable that the criteria of the author prevail and therefore, for thorough literature research, the reader is directed to the excellent reviews in Refs. [16–18]. Thermodynamic data for methanol, carbon monoxide and other small organic molecules are given in Table 1.[1] Accordingly, their thermodynamic open circuit potentials are of the same order as that of hydrogen and these substances can be considered good candidates to operate as anode material in a fuel cell. However, while hydrogen oxidation occurs at relatively high rates near its reversible potential, the oxidation of small organic molecules presents serious kinetic limitations. As an example, in Figure 1, the response of a Pt electrode to a constant current step of 5 mA in a methanol-containing solution is compared with that of hydrogen (1 bar) in the same base electrolyte (0.5 M H2SO4). Whilst at such high rates hydrogen oxidation exhibits a small overpotential, for methanol oxidation the potential is shifted to very high values (0.55 V) only a few seconds after applying the potential step. The results shown in Figure 1, illustrating the behavior of pure Pt towards methanol oxidation, indicate that this pure metal could never be a good catalyst for continuous fuel cell (FC) operation at room temperature or even higher (e.g., 60 ◦C). However, in spite of the problem of being strongly poisoned by adsorption products, no better catalyst than Pt for breaking C–H and O–H bonds in alcohol molecules is known at present. Therefore, methanol electrooxidation at reasonable rates in acid media is only conceivable on Pt-based catalysts. Research in electrocatalysis using modified Pt electrodes has devoted considerable