14. Kinetic Activity in Electrochemical Cells

The oxygen reduction reaction (ORR) is typically the rate-limiting reaction in electrocatalytic energy systems wherein the oxidizer is air, such as proton exchange membrane fuel cells (PEMFCs) or metal–air batteries. The effectiveness of these air-breathing electrochemical technologies hinges on the use of highly active electrocatalysts that can convert O₂ to H₂O (four-electron reaction) rather than the undesirable product H₂O₂ (two-electron reaction). The evaluation of new electrocatalysts in full cells is not practical due to competing contributions of mass transport and \(I^{2}R\) losses in the electrodes to measurements of kinetic activity. This chapter describes characterization of the electrocatalyst kinetic activity in electrochemical cells by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) methodologies and describes specifically how to apply the methodology to the porous electrodes used in energy systems. When the experimental procedures are executed properly, RDE and RRDE methodologies can be used to determine electrocatalyst ORR activities as a screening tool prior to full cell testing. Detailed methods are given for the accurate determination of the ORR of two standard materials: platinum/carbon in acid for PEMFC cathode catalysts, and MnO₂ in base for metal–air batteries.