Surface spectators and their role in relationships between activity and selectivity of the oxygen reduction reaction in acid environments

Abstract We use the rotating ring disk (RRDE) method to study activity–selectivity relationships for the oxygen reduction reaction (ORR) on Pt(111) modified by various surface coverages of adsorbed CN ad (Θ CNad ). The results demonstrate that small variations in Θ CNad have dramatic effect on the ORR activity and peroxide production, resulting in “volcano-like” dependence with an optimal surface coverage of Θ CNad  = 0.3 ML. These relationships can be simply explained by balancing electronic and ensemble effects of co-adsorbed CN ad and adsorbed spectator species from the supporting electrolytes, without the need for intermediate adsorption energy arguments. Although this study has focused on the Pt(111)–CN ad /H 2 SO 4 interface, the results and insight gained here are invaluable for controlling another dimension in the properties of electrochemical interfaces.