Copper Single Crystal Electrocatalysts: Interfacial Structure and Reactivity
A very important parameter for the electrocatalytic performance of a material is its potential of zero free charge (pzfc). For Cu(111) at pH 13 it was identified at −0.73 VSHE in the apparent double layer region. It shifts by (88 ± 4) mV to more positive potentials per decreasing pH unit. At the pzfc, Cu(111) starts to restructure. At higher potentials, full reconstruction and electric field dependent OH adsorption occur, causing a remarkable decrease in the atomic density of the first Cu layer.
It is this restructuring that leads to the ability of Cu(111) to efficiently oxidize CO and to reduce water. Therefore, knowledge of the surface structure and the position of the pzfc is of paramount importance for the understanding of copper’s catalytic properties and for the rational design of electrocatalysts, in general.
CO is a key intermediate in the electro-oxidation of energy carrying fuels and known to act as a catalyst poison. Single-crystal Cu(111) model catalysts can efficiently electro-oxidize CO in alkaline media, where strong surface structural changes are observed under reaction conditions with electrochemical scanning tunneling microscopy (EC-STM). Supported by first-principles microkinetic modelling, we have shown that the concomitant presence of high-energy undercoordinated Cu structures at the surface is a prerequisite for the high activity. In water electrolyzers, it is possible to produce H2 in the course of the hydrogen evolution reaction (HER), which was studied with Ni(OH)2 modified Cu(111) electrodes in alkaline media. Strong morphological changes upon adatom modification lead to a significant HER rate enhancement. Intriguingly, this is induced through a decrease of the electric field strength negative of the potential of zero charge. This implies an easier reorganization of the interfacial water molecules facilitating charge transfer through the double layer, and thus enhancing the efficiency of electrocatalytic reactions.