Key Structural Kinetics for Carbon Effects on the Performance and Durability of Pt/Carbon Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by In Situ Time-Resolved X-ray Absorption Fine Structure

摘要

The structural kinetics (rate constants for electronic and structural transformations) of the Pt charging/discharging, Pt Pt bond dissociation/re-formation, and Pt-O bond formation/dissociation of Pt/Ketjenblack, Pt/acetylene black, and Pt/multiwalled carbon nanotube cathode catalysts in polymer electrolyte fuel cell (PEFC) membrane electrode assemblies (MEAs) under transient potential operations (0.4 V-RHE -> 1.4 V-RHE -> 0.4 V-RHE) has been studied by in situ/operando time-resolved quick X-ray absorption fine structure (QXAFS; 100 ms/spectrum), while measuring electrochemical currents/charges in the MEAs under the potential operations. From the systematic QXAFS analysis for potential-dependent surface structures and rate constants (k and k') for the transformations of Pt nanoparticles under the operations (0.4 V-RHE -> 1.4 V-RHE and 1.4 V-RHE -> 0.4 V-RHE), respectively, we have found the structural kinetics (k(Pt-O)' and k(valence)') controlling the oxygen reduction reaction (ORR) activity and also the structural kinetics (k(pt-pt/kpt-pt)') reflecting the durability of the cathode catalysts. The relaxation time of the Pt Pt bond re-formation and Pt-O bond dissociation processes in the activated MEAs was also suggested to predict the relative durability of similar kinds of cathode catalysts. The in situ time-resolved XAFS analysis provided direct information on the key structural kinetics of the Pt/C catalysts themselves for thorough understanding of the cathode catalysis toward PEFC improvement.