Sol–gel-modified boron-doped diamond surfaces for methanol and ethanol electro-oxidation in acid medium

Studies of the methanol and ethanol electro-oxidation reactions on boron-doped diamond (BDD) electrode surfaces modified with Pt, Pt-RuO2 and Pt-RuO2-RhO2 by the sol–gel method are reported here. The materials were initially characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The XRD analyses indicate that the sol–gel method produces nano-sized deposits on the BDD surfaces. These deposits also form nano-clusters with a size of ca. 100 nm as observed by SEM and AFM. The EDX maps showed that the metals are homogeneously distributed on the BDD surface and have a composition close to the expected one. Cyclic voltammetry experiments in acid medium revealed that the CO poisoning effect for the methanol and the ethanol oxidation reactions is largely inhibited on the Pt-RuO2-RhO2/BDD electrode showing the positive contribution of the rhodium oxide to the electrocatalysts performance in these reactions. Potentiostatic polarization curves and the corresponding Tafel plots showed that the addition of RuO2 and RhO2 to Pt/BDD produces a more reactive electrocatalyst that adsorbs methanol and ethanol more efficiently and changes the reactions onsets by 120 or 180 mV towards less positive potentials, respectively. Moreover, the stationary current density measured at a fixed potential for ethanol oxidation on the Pt-RuO2-RhO2/BDD composite electrode is more than one order of magnitude larger than on a Pt/BDD surface. In addition, chronoamperometric experiments indicate that on those composite electrodes the effect of CO poisoning only appears after a considerable amount of charge has passed through the interface. Consequently, the catalyst containing Pt, RuO2 and RhO2 deposited on BDD by the sol–gel method is a very promising composite material to be used in fuel cell anodes.