Transient Kinetic QXAFS Approach for Understanding the RDE-MEA Gap in Fuel Cell (Oxygen Reduction Reaction) Performances of Pt-Based Electrocatalysts.

Abstract

There is a large gap between the performances indicated by rotating disk electrode (RDE) results in acidic media and the actual performances obtained in membrane-electrode assemblies (MEAs) composed of the same electrocatalysts. It is unclear whether the intrinsic kinetic reactivity of the available surface Pt sites of Pt-based cathode electrocatalysts is similar or different at RDE and in MEA. To address this, we used an operando element-selective time-resolved Pt L_III-edge quick X-ray absorption fine structure (QXAFS) technique to determine transient response profiles and rate constants, k _d(WL), k _d(CNPt-O), and k _d(CNPt-Pt), corresponding to changes in the oxidation states [white line (WL) intensity] and local structures (coordination numbers of Pt-O and Pt-Pt bonds) at Pt sites for nine representative Pt-based cathode electrocatalysts under transient voltage operations, aiming to understand the oxygen reduction reaction (ORR) performance gap between RDE and MEA. For the first time, the transient kinetics and reactivity of electrocatalyst themselves in MEA, characterized by the operando QXAFS analysis technique, were systematically compared with the electrochemical activity [mass activity (I _mass) and surface specific activity (I _specific)] of the electrocatalysts in MEA and at RDE. The operando time-resolved QXAFS analysis revealed that the ORR activities of available surface Pt sites at RDEs of the electrocatalysts, including notably structured electrocatalysts (concave octahedral PtNi _x /C and Pt nanowire/C), were kinetically reflected at good levels of k _d(WL) and k _d(CNPt-O) in MEA performances, despite large RDE-MEA gaps observed in the electrochemically determined I _mass and I _specific. As the I _mass and I _specific of MEA increased, the relaxation time k _d(CNPt-Pt) ^-1, which indicates long-term durability, decreased, reflecting a dilemma in the development of remarkable Pt-based electrocatalysts, while the k _d(CNPt-Pt) ^-1 was almost independent of ECSA. The differences and similarities in the kinetic reactivity and durability of the Pt surface between RDE and MEA were examined using operando QXAFS transient kinetics and electrochemical performance measurements to elucidate the underlying factors contributing to the performance gap between RDE and MEA. The insights gained aim to support the development of next-generation polymer electrolyte fuel cells with enhanced performance and durability by leveraging the operando time-resolved QXAFS technique under the transient kinetic-response operation.