Fundamental studies of electrochemical oxide formation on platinum single crystal electrodes

Platinum single crystal electrodes were used to investigate electrochemical oxides and related surface species and their impact upon important catalytic reactions such as the oxygen reduction reaction (ORR). Notions of perchlorate anions being “non-specifically adsorbed” were re-evaluated and challenged. For example, the voltammetry of Pt single crystal electrodes as a function of perchloric acid concentration (0.05–2.00 M) was studied in order to test the assertion by Watanabe et al. that perchlorate anions specifically adsorb on polycrystalline platinum. Specific adsorption of perchlorate anions was found in varying degrees for Pt(hkl) surfaces. By flame-annealing and cooling a series of Pt n(110)x(111) and Pt n(110)x(100) single crystal electrodes in a CO ambient, new insights into the nature of the electrosorption processes associated with Pt{110} voltammetry in aqueous acidic media were elucidated. For Pt n(110)x(111) electrodes, a systematic change in the intensities of voltammetric peaks indicated a lack of surface reconstruction (in contrast to hydrogen cooled analogues). Pt n(110)x(100) stepped electrodes displayed a marked tendency towards surface reconstruction irrespective of cooling environment. Pt(110) terrace sites were found to afford a specific affinity for sulphonate groups contained within a Nafion adlayer. Pt n(100)x(110) surfaces showed rapid quenching of the Nafion ‘spike’ as a function of increasing step density. Reactivity measurements involving oxygen reduction and hydrogen peroxide oxidation/reduction largely revealed the importance of adsorbed oxide/OH in regulating activity. Kinetic studies suggested that for Pt(100) terraces, oxide formation was also accompanied by rapid surface reconstruction. Fast potential cycling of all electrode surfaces confirmed the likelihood of structural changes occurring in real fuel cells. It is deduced that roughened catalyst particles should actually exhibit an enhanced ORR activity, even in the presence of Nafion