Atomistic Modeling Of The Cathode/electrolyte Interface In Proton Exchange Membrane Fuel Cells

Open Access
Author:
Yeh, Kuan-yu
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
None
Committee Members:
  • Michael John Janik, Dissertation Advisor
  • Janna Kay Maranas, Committee Member
  • Darrell Velegol, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • fuel cells
  • oxygen reduction reaction
  • electrochemical interface
  • DFT models
  • MD simulations
Abstract:
Proton exchange membrane fuel cells (PEMFCs) are promising alternative vehicle power sources because of their high efficiency and low emissions. PEMFC efficiencies are limited by the slow oxygen reduction reaction (ORR) at the cathode, which uses active but high cost Pt catalysts. The ORR and possible adsorption of electrolyte anions occur at the electrolyte/cathode interface, and are controlled by the complex interactions between molecules and ions with the solvated electrode under applied electrode potentials. The dielectric and transport properties of water near the electrode are different from those in bulk water, and water facilitates proton transfer to the cathode. However, interfacial water structure and dynamics are not well defined, and the influence of interfacial water on electrochemical reactions is poorly understood at the molecular level. The electrode potential provides a driving force for electrochemical processes by changing the electron chemical potential, and affects interfacial water structure, dynamics, and the binding strengths of reaction intermediates. The presence of water and electrode potential creates substantial challenges in atomistic modeling of the ORR and ion adsorption at the electrolyte/cathode interface. This dissertation develops and applies a series of atomistic modeling approaches to link macroscopic electrochemical measurements with atomistic-level processes at the electrolyte/electrode interface. Various density functional theory (DFT) based electrochemical models were utilized to investigate the influence of water structure and electrode potential on the ORR and ion adsorption reactions at Pt(111) electrodes. We performed a detailed study of aqueous sulfuric acid electrochemistry on the Pt(111) surface, and assign three characteristic voltammetric peaks to different bisulfate/sulfate adsorption processes. This modeling work resolves the adlayer composition and structure, a longstanding goal of previous experimental studies. DFT models are also applied to investigate the elementary reaction kinetics of the ORR over Pt(11) electrodes. For this PEMFC cathode reaction, we observed that the ORR reaction path and the rate-limiting step depend on the electrode potential. This result is consistent with potential dependent ORR reactivity in experiments and provides possible ORR mechanisms. The use of a static water structure in DFT models limits the robustness of a pure DFT analysis. The length and time scales associated with the dynamic interface are beyond DFT methods. The influence of model choices on reaction energetics and reaction path determinations is demonstrated in this dissertation. The water structure dependence of ORR reaction energetics motivated us to develop a molecular dynamics (MD) model to probe water structure fluctuation and dynamics at the electrified water/Pt interface. This is the first MD model that combines a potential controlled electrode with a reactive water force field, and thus allows us to study proton transfer dynamics at the water/Pt interface. Our simulations suggest a connection between electrolyte dynamics and surface reactions, where the slower charge transfer of OH- compared to H3O+ may account for less efficient hydrogen oxidation in an alkaline electrolyte compared to an acid electrolyte.