Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes

The oxidation pathways of methanol (MeOH) have been the subject of intense research due to its possible application as a liquid fuel in polyelectrolyte membrane (PEM) fuel cells. The design of improved catalysts for MeOH oxidation requires a deep understanding of these complex oxidation pathways. This paper will provide a discussion of the literature concerning the extensive research carried out in acidic and alkaline electrolytes. It will highlight techniques that have proven useful in the determination of product ratios, analysis of surface poisoning, anion adsorption, and oxide formation processes, in addition to the effects of temperature on the MeOH oxidation pathways at bulk polycrystalline platinum (Pt_poly) electrodes. This discussion will provide a framework with which to begin the analysis of activation energy (E_a) values. This kinetic parameter may prove useful in characterizing the rate-limiting step of the MeOH oxidation at an electrode surface. This paper will present a procedure for the determination of E_a values for MeOH oxidation at a Pt_poly electrode in acidic and alkaline media. Values from 24–76 kJ mol⁻¹ in acidic media and from 36–86 kJ mol⁻¹ in alkaline media were calculated and found to be a function of applied potential and direction of the potential sweep in a voltammetric experiment. Factors that influence the magnitude of the calculated E_a include surface poisoning from MeOH oxidation intermediates, anion adsorption from the electrolyte, pH effects, and oxide formation processes. These factors are all potential, and temperature, dependent and must clearly be addressed when citing E_a values in the literature. Comparison of E_a values must be between systems of comparable electrochemical environment and at the same potential. E_a values obtained on bulk Pt_poly, compared with other catalysts, may give insight into the superiority of other Pt-based catalysts for MeOH oxidation and lead to the development of new catalysts which lower the E_a barrier at a given potential, thus driving MeOH oxidation to completion.