Unraveling the elusive oxygen reduction reaction electrokinetics and energetics in PEM fuel cells

Unraveling the elusive electrokinetics and energetics of the sluggish Oxygen Reduction Reaction (ORR) is critical for the development of efficient Proton Exchange Membrane Fuel Cells (PEMFCs). Physical microkinetic modeling, Transition State Theory (TST), Transmission Line Modeling and Degree of Rate Control (DRC) analysis was employed to simulate analytically both the experimental Electrochemical Impedance Spectra (EIS) and the polarization curve data of a High Temperature PEMFC. The excellent fitting results revealed that: i) O2(g) dissociative adsorption is the main limiting step responsible for the high ORR overpotentials, ii) both EIS and polarization resistance are dominated by the intrinsic ORR kinetic inertia due to the competitive nature of the elementary reaction steps on the coverages of the adsorbed species (Oad and OHad). Finally, from TST the activation energies (kinetics) and the reaction steps’ free energies (thermodynamics) were extracted, which can serve as a guide for catalyst design optimizing the oxygen binding strength.