Oxygen Electroreduction at High-Index Pt Electrodes in Alkaline Electrolytes: A Decisive Role of the Alkali Metal Cations

Abstract

Currently, platinum group metals play a central role in the electrocatalysis of the oxygen reduction reaction (ORR). Successful design and synthesis of new highly active materials for this process mainly rely on understanding of the so-called electrified electrode/electrolyte interface. It is widely accepted that the catalytic properties of this interface are only dependent on the electrode surface composition and structure. Therefore, there are limited studies about the effects of the electrolyte components on electrocatalytic activity. By now, however, several key points related to the electrolyte composition have become important for many electrocatalytic reactions, including the ORR. It is essential to understand how certain "spectator ions" (e.g., alkali metal cations) influence the electrocatalytic activity and what is the contribution of the electrode surface structure when, for instance, changing the pH of the electrolyte. In this work, the ORR activity of model stepped Pt [n(111) × (111)] surfaces (where n is equal to either 3 or 4 and denotes the atomic width of the (111) terraces of the Pt electrodes) was explored in various alkali metal (Li+, Na+, K+, Rb+, and Cs+) hydroxide solutions. The activity of these electrodes was unexpectedly strongly dependent not only on the surface structure but also on the type of the alkali metal cation in the solutions with the same pH, being the highest in potassium hydroxide solutions (i.e., K+ Na+ > Cs+ > Rb+ ≈ Li+). A possible reason for the observed ORR activity of Pt [n(111) × (111)] electrodes is discussed as an interplay between structural effects and noncovalent interactions between alkali metal cations and reaction intermediates adsorbed at active catalytic sites.