Regulating OH Adsorption Equilibrium via I _ad Adlayer: An Efficient Strategy to Boost ORR on Pt Electrodes

Electrochemical energy and substance conversion devices involve complex electrode processes characterized by multiple charge transfer steps, competing pathways, and various intermediates. Such complexity makes it challenging to enhance the electrocatalytic activity. The prevailing strategy typically focuses on optimizing the geometric and electronic structures of the electrocatalysts to align the adsorption energies of reaction intermediates with the peak of the activity Volcano curve. In this study, we demonstrate that surface decoration can effectively shape the micro reaction environment for the model system of the oxygen reduction reaction (ORR) on Pt electrodes. By applying a partial hydrophobic I* adlayer on the Pt surface, we can shift the equilibrium of OH* reduction and weaken H 2 O* adsorption, which significantly enhances ORR kinetics. With in situ scan tunneling microscopy (STM) and theoretical calculations, our study reveals the formation of isolated Pt 2 surface units situated in a hydrophobic valley surrounded by adsorbed iodine atoms. This minimalist Pt 2 active unit exhibits significantly greater activity for the ORR compared to an extended Pt surface. Our results overturn the previous consensus that the I* adlayer always poisons the electrocatalytic reaction. Our systematic studies also reveal that whether I* acts as a poison or a promoter, as well as the extent of its promotional effect, depends on its coverage. This finding could pave the way for developing highly efficient catalysts with potential applications in fuel cell technology and metal air batteries and extension to other electrochemical conversion reactions such as ammonia synthesis and CO 2 reduction.