The Relationship between the Active Pt Fraction in a PEFC Pt/C Catalyst and the ECSA and Mass Activity during Start-Up/Shut-Down Degradation by in Situ Time-Resolved XAFS Technique

Transient-response kinetics of the transformations (six elementary steps) of the Pt valence, coordination number of Pt–Pt bonds, and coordination number of Pt–O bonds of a Pt/C cathode catalyst in a polymer electrolyte fuel cell (PEFC) under cyclic voltage operations (0.4 → 1.4 → 0.4 V RHE) during anode–gas exchange (AGEX) treatments (start-up/shut-down) has been studied by in situ time-resolved quick X-ray absorption fine structure (QXAFS, 100 ms/spectrum). The transient-response analysis identified the existence and fractions of three different kinds (active, less active, and inactive) of Pt nanoparticles in the Pt/C cathode. The active Pt nanoparticles degraded to less active and inactive Pt nanoparticles by the AGEX cycles. The degradation probability and mechanism were clarified by the transient-response kinetics. The electrochemical surface area (ECSA) and mass activity (MA) of the Pt/C cathode catalyst also decreased with increasing AGEX cycles. It was found that the change in the sum of the fractions of the active and less active Pt nanoparticles correlates with the change in the ECSA and MA during the AGEX treatments. The in situ time-resolved QXAFS analysis provides direct information on the dynamic behavior of the Pt/C catalyst relevant to the electrochemical performance and property under the operando conditions for thorough understanding of the degradation process toward PEFC improvement.