Pressure and bias dependence of the rate-limiting steps of the oxygen reduction reaction.

The oxygen reduction reaction limits the energy efficiency of H2 fuel cells and Li-air batteries, yet, it remains poorly understood within popular kinetic frameworks. Here, we study the oxygen reduction reaction on Pt/C, Ir/C, Ru/C and Rh/C nanoparticles as a function of electrochemical bias, temperature and O2 pressure at industrially-relevant conditions in membrane electrode assemblies. Bias-, and pressure-dependent Arrhenius analysis reveals distinct changes in the (apparent) activation energy and pre-exponential factor that we relate to kinetics that cascade through a series of rate-limiting steps and transition states. Further, while the kinetics are accelerated by the pressure and bias, they remain pinned to pseudo-capacitive reduction processes and structural changes at the water-solid interface. Collectively, our study informs on how the free energy driving force and pressure tune the degree of rate control of rate-limiting steps and transition states of (electro)catalytic multi-step reactions and how this is related to structural and chemical changes at the interface. This is at the very heart of catalysis.