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

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.

Why conclusions from platinum model surfaces do not necessarily lead to enhanced nanoparticle catalysts for the oxygen reduction reaction.

Experiments on model surfaces commonly help in identifying the structural sensitivity of catalytic reactions. Nevertheless, their conclusions do not frequently lead to devising superior "real-world" catalysts. For instance, this is true for single-crystal platinum electrodes and the oxygen reduction reaction (ORR), an important reaction for sustainable energy conversion. Pt(111) is substantially enhanced by steps, reaching a maximum at short terrace lengths of 3-4 atoms. Conversely, regular platinum nanoparticles with similar undercoordinated defects are less active than Pt(111) and their activity increases alongside the terrace-to-defect ratio. We show here that a model to design ORR active sites on extended surfaces can also be used to solve this apparent contradiction and provide accurate design rules for nanoparticles. Essentially, only surfaces and nanostructures with concave defects can surpass the activity of Pt(111), whereas convex defects are inactive. Importantly, only the latter are present in regular nanoparticles, which is why we design various concave nanoparticles with high activities.