Effect of temperature and pressure on the kinetics of the oxygen reduction reaction

Fundamental understanding of the oxygen reduction reaction in aqueous medium at temperatures above 100 °C is lacking due to the practical limitations related to the harsh experimental conditions. In this work, the challenge to suppress water from boiling was overcome by conducting the electrochemical investigation under pressurized conditions. A striking improvement in the kinetics of the electrocatalytic reduction of O₂ by about 150 fold relative to room temperature and pressure was recorded under an O₂ pressure of 3.4 MPa at 200°C in basic aqueous environment. To deconvolute the combined effect of temperature and pressure, the underlying variables that dictate the observed O₂ reduction kinetics of Pt and carbon electrodes were examined individually. O₂ availability at the electrode-solution interface was controlled by the interplay between the diffusion coefficient and concentration of O₂. Accurate knowledge of the temperature and pressure dependence of O₂ availability at the electrode surface, the Tafel slope, the transfer coefficient, and the electrochemical active surface area was required to correctly account for the enhanced O₂ reduction kinetics.