Transient and Persistent Effects of Air Exposure on Electrocatalytic Activity of Sub-Nano Pt Clusters on Different Electrode Supports

Electrodes are exposed to air prior to use in most electrochemistry experiments, but the resulting adventitious contaminants are easily removed by cycling the electrode potential to drive reactions or desorption. In most cases, the resulting changes to the electrode surface are probably inconsequential, however, for electrodes with catalytic centers consisting of sub-nanometer clusters or single atoms, potential cycling may drive dissolution, migration, or other processes that significantly alter the catalyst properties. We have developed the capability to prepare clean electrodes in ultrahigh vacuum, decorated with precisely controlled clusters or atoms, and to study their electrochemistry within the vacuum system after controlled exposures to pure gases or to air. Here, we examine the effects air exposure on the activity for HER, ORR, and oxidation of ethanol and propanol, catalyzed by Pt4 clusters deposited on both ITO and nitrogen-implanted HOPG electrodes. Air exposure was found to have no significant effects on HER but did have substantial effects on the other reactions. Some effects were transient, i.e., they decreased over the course of a few dozen CV cycles, but there were also effects such as increased overpotentials and changes in peak currents or potentials, that persisted after extensive potential cycling.