Kinetic and Mechanistic Studies of the Oxygen Reduction Reaction on Platinum Electrocatalysts

Hydrogen fuel cells are promising devices that can be utilized as a carbon-neutral mobile energy source. These devices can produce electric power from pressurized hydrogen gas and oxygen from the air with only water as a by-product. However, the adoption of this technology has been hampered by its poor technoeconomic performance, especially at scale. The high costs arise from the expensive platinum group metal catalysts needed to speed up the rate of the oxygen reduction reaction (ORR) at the fuel cell cathode. Efforts to improve the catalytic performance focus on novel electrocatalyst design (through metal alloying, nanostructuring, etc.) and understanding the mechanism of ORR at the elementary step level under relevant reaction conditions. The former relies on accurate measures of catalytic activity but is hampered by incorrect measurements of active catalytic area and mass transport limitations. The latter has proven challenging as mechanistic analyses (in particular: spectroscopic analysis of relevant surface intermediates) are confounded by solvent and electric field effects. The first scope of this thesis is to establish fundamental tools and methods to accurately quantify catalyst activity. A modeling example shows the impact on the catalyst film construction and how a thin, conductive film is required to eliminate internal mass transport limitations. Different surface area measurements are discussed, with examples demonstrating the effects of improper surface area characterization. The accurate measurement of catalyst activity is essential for any novel catalyst design studies and mechanistic studies vying to explain observed catalyst performance from elementary steps. The second scope of this thesis pertains to mechanistic analyses of electrochemical reactions and the ORR specifically. Microkinetic modeling is one tool that can be used to explain macroscopic observables (such as reaction orders and Tafel slopes) by solving a system of elementary rate equations with certain initial conditions based on the chemical environment. The accuracy of these models is heavily impacted by the inherent assumptions such as the mean-field approximation and the treatment of the electrochemical potential, solvent effects, electric field effects, and ion/electrolyte effects. The accuracy of these models can be improved by advanced modeling techniques such as kinetic Monte Carlo and through in-situ/operando spectroscopy. To demonstrate, this work presents an in-operando study of the ORR that sheds light on the potential- dependent mechanism. A unique spectroscopic platform to probe the electrified Pt surface at realistic reaction conditions is created to accomplish this. A non-model Ag-Pt core-shell nanoparticle is synthesized and characterized with UV-vis spectroscopy, and STEM with elemental mapping showing a distinct core-shell architecture. Localized surface plasmons resonance enables plasmon-enhanced Raman spectroscopy (PERS) measurements at the reactive and electrified Pt/electrolyte interface. Data analysis shows how experimental spectra matches with density functional theory (DFT) derived vibrational modes to shed light on the reaction mechanism. The vibrational model assignments are further verified and validated by isotopic labeling studies. This analysis reveals critical insights on the ORR mechanism, showing a crowded network of OH/H2O at low overpotentials We also present evidence of an OOH species adsorbed on a cleaner surface at high overpotentials. These insights can be adopted as design criteria for future electrocatalyst optimizations and increase the accuracy of microkinetic models. Furthermore, the spectroscopic platform can be expanded to other chemistries to yield fundamental insights into electrochemical reaction mechanisms.PhDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/171383/1/sdix_1.pd