Electrocatalysis at the nanoscale : from polycrystalline platinum to nanoparticles

The influence of the electrode surface structure on its activity is a major theme in electrochemical research, especially in electrocatalysis. To obtain a truly fundamental understanding of the structure-activity relationship requires probing electrochemistry locally, as most electrode surfaces are inherently heterogeneous. The main concern of this thesis is studying the effect of the electrode surface structure on the activity of electrocatalytic reactions. This is attained by utilising scanning probe techniques as well as traditional macroscopic electrochemical methods, coupled with a number of structure characterisation techniques. A new approach, termed the ‘pseudo-single-crystal approach’, was introduced. It utilises scanning electrochemical cell microscopy and electron backscatter diffraction to obtain electrochemical activity and crystallographic orientation information across a surface of interest, providing a novel platform for understanding structure-reactivity relationship of complex electrode materials. Platinum electrodes are the main focus. The principle of the approach was first demonstrated by studying a simple one-electron transfer reaction, Fe2+ oxidation to Fe3+ in aqueous acid solution. Results showed clear grain dependent activity for Fe2+ oxidation in perchloric acid but grain boundary controlled activity in sulfuric acid, showing how electrolyte species can significantly alter the patterns of reactivity. These results are consistent with data obtained by single-crystal approaches. Then, this approach was employed to study the oxygen reduction reaction. Significantly, for this reaction, SECCM provides a three-phase boundary configuration, which allows the ORR to be studied under high mass transport conditions. Clear grain-dependent activity of the ORR on high-index Pt surfaces was visualised and the possible effect of differential mass transport of the reactants and products was discussed. This feature of SECCM (high mass transport of oxygen) allowed us to study the effect of oxygen on the electrochemical oxidation of hydrazine. Hydrazine oxidation in air and in deaerated environment showed activity that was strongly grain dependent, but the magnitude of the current decreased dramatically in air. For this study, CVs at each pixel of a scan were recorded, providing potentiodynamic information at high resolution. Electrochemistry at single nanoparticles and their interaction with support electrode is a topic of considerable interest. In the last results chapter, the electron transfer properties of single Au nanoparticles and the interaction of these particles with self-assembled monolayer-modified gold substrates was investigated. Results showed that surface functionality plays an important role in this interaction. By comparing the results with atomic force microscopy measurements, it is shown that SECCM can detect subtle variations in the interactions, confirming it be a good tool for detecting surface chemistry at the nanoscale.