Kinetics and Mechanism of Redox Processes of Pt/C and Pt₃Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test

Abstract

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt3Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt LIII-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt3Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of the Pt electrocatalyst. On the other hand, in the case of Pt3Co/C, the rate constants of the redox reactions of the cathode electrocatalyst gradually reduced as the number of ADT cycles increased. The differences in the kinetics for the redox processes would be differences in the degradation mechanism of these cathode electrocatalysts.

Synthesis of catalysts with fine platinum particles supported by high-surface-area activated carbons and optimization of their catalytic activities for polymer electrolyte fuel cells

“…The detailed experimental procedures for XAFS measurements can be found elsewhere. 34 The emitted X-rays were monochromatized by a standard Si (111) channel-cut crystal monochromator. Two vertical mirrors between the monochromator and incident ion chamber were used to deny the higher harmonics.…”

PtCo Cathode Catalyst Morphological and Compositional Changes after PEM Fuel Cell Accelerated Stress Testing

“…Despite the exceptional initial activity of these alloy catalysts, they degrade rapidly compared to pure Pt catalysts due to rapid dissolution of both Pt 3,4 and the transition metal component, 5,6 which in turn results in a decay of mass activity and loss of electrochemically active surface area as well as a concomitant change in the catalyst nanoparticle morphology.…”

Pt–Co/C Cathode Catalyst Degradation in a Polymer Electrolyte Fuel Cell Investigated by an Infographic Approach Combining Three-Dimensional Spectroimaging and Unsupervised Learning

“…A similar decrease in the Co/Pt ratio in the cathode catalyst layer was also indicated by the SEM–EDX results (0.3 → 0.2). The dissolution, segregation, and dealloying of Co from Pt 3 Co catalysts have been reported as the results of electrochemical analysis, ex situ TEM, theoretical calculations, and so forth. ,,− The operando XANES–CT imaging carried out in the current work provided actual 3D images of the Pt–Co/C catalyst degradation in the MEA under practical PEFC operating conditions for the first time.…”

“…Pt 3 Co/C catalysts are more durable than Pt/C catalysts, but still degrade at cathode to an unacceptable degree under PEFC operating conditions. , Although the dissolution and aggregation of Pt in Pt–Co/C catalysts are more suppressed than they are in Pt/C catalysts, Co gradually dissolves from the bimetallic catalyst, and hence the alloy benefit gradually disappears. There are many reports on the preparation of durable cathode catalysts with bimetallic structures, but to our knowledge, there are no reports yet on PEFC cathode catalysts without degradation at cathode in MEAs.…”