Monday, 14 May 2018: 11:40
Room 602 (Washington State Convention Center)
The conversion of oxygen into water is crucial to the operation of polymer electrolyte fuel cells and other emerging electrochemical energy technologies. Chemisorbed oxygen species play a central role in this reaction, since they necessarily act as intermediates but also as site-blockers. Any attempt to decipher the oxygen reduction reaction must relate the formation of oxygen intermediates to basic electronic and electrostatic properties of the catalyst surface and, on the other hand, link it to effective parameters of the electrode activity. An approach that accomplishes this feat will be of great utility in guiding design and development of catalyst materials and developing predictive models of electrode operation. Here, we present a theoretical framework for the multiple interrelated surface phenomena and processes involved. It correlates the formation of chemisorbed oxygen intermediates, metal surface charging phenomena, ion density and potential distribution in electrolyte, field-dependent ordering of interfacial water molecules, and effective kinetic parameters of the ORR. Parameterized with density functional theory results and rotating disk electrode data, the model sheds light on the double-edged roles of oxygen intermediates; it produces as output the Tafel slope and exchange current density as continuous functions of electrode potential and presents a new perspective on the volcano relation the ORR. The optimal oxide coverage is a result of two oppositely-headed trends upon increasing coverage by oxygen intermediates, viz, the intuitive site-blocking effect and the increasing protophilicity of the surface.