Stability and electrocatalytical properties of nanostructured Pt / glassy carbon model electrodes

Abstract

The results presented in this thesis mediate mesoscopic transport effects induced by the electrode morphology and reaction characteristics such as activity and selectivity in electrocatalytic reactions. The application of nanostructured Pt/glassy carbon surfaces representing simplified, well-defined and reliable two dimensional model systems of the commonly used real carbon-supported Pt/C catalysts, offered the possibility to vary the transport conditions arbitrarily and to correlate the results, respectively. It is demonstrated that these model systems are ideally suited for studying mass transport processes and their effect on the kinetics of electrocatalytic reactions. Transport of reactants to the electrode not only affects the reaction rate, by means of a "mass transport limited current", but may also alter the overall reaction behaviour, in particular its selectivity and hence the product distribution in reactions leading to more than one product. The measurements reveal a distinct variation in the selectivity of electrocatalytic reactions such as the oxygen reduction or the methanol oxidation reaction with (i) increasing separation of the Pt nanostructures and therefore active Pt sides and (ii) with increasing electrolyte flow rate. Based on these findings, the "desorption-readsorption-reaction" concept was introduced. This reaction model explains the observations by the readsorption probability of reactive incomplete reaction products on neighbouring active Pt sites after desorption into the electrolyte and therefore the chance of further reaction towards the final reaction products. The readsorption probability strongly depends on (i) the separation of the Pt nanostructures and (ii) the electrolyte flow rate and hence the thickness of the stagnant diffusion layer above the electrode surface. Hence, the readsorption probability decreases with increasing interparticle distances or with increasing electrolyte flow rate resulting in a thinner diffusion layer.