Oxygen Reduction Reaction Kinetics at Elevated Temperatures and Pressures

Fundamental understanding of the oxygen reduction reaction (ORR) in aqueous medium at temperatures between 100 and 200 °C is challenging due to practical limitations related to the harsh experimental conditions. By using a high-temperature, pressurized electrochemical cell, we obtain a 150-fold improvement in the kinetics of the electrocatalytic reduction of O₂ on Pt relative to room temperature and pressure under an O₂ pressure of 3.4 MPa at 200 °C in basic solution.

Figure 1 demonstrates that the ORR kinetics is a function of both temperature and pressure. To deconvolute their combined effect, we examine the underlying variables that dictate the observed ORR kinetics individually. In the Temkin region, we observe temperature-insensitive Tafel slopes, suggesting that the transfer coefficient (α) is temperature-dependent. Another factor contributing to this temperature dependence is a change in the surface morphology of Pt at 200 °C, likely reflecting instability of Pt at elevated temperatures during ORR. O₂ availability at the electrode-solution interface is controlled by the interplay between the diffusion coefficient and the concentration of O₂ in bulk solution.

ORR kinetics measured on a glassy carbon (GC) electrode yields similar temperature-dependent behavior of the Tafel slopes and α values. GC electrode reduces O₂ via a 2 e^– process even at elevated temperatures. We conclude that correctly accounting for the many temperature- and pressure-dependent variables is necessary to accurately describe the enhanced ORR kinetics observed at intermediate temperatures under pressurized conditions.

 

Acknowledgements. E.C.M.T. acknowledges a Croucher Foundation Scholarship. We thank the US Department of Energy (DE-FG02-95ER46260) for support of this research. This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, which are partially supported by the US Department of Energy (DE-FG02-07ER46453 and DE-FG02-07ER46471).