|dc.description.abstract||The main challenges of polymer electrolyte membrane fuel cells are Pt scarcity, cost, and the sluggish kinetics of oxygen reduction reaction (ORR). Therefore, the development of cost-effective and high performance cathode catalysts have become one of the most popular research topics in recent years. In this study, carbon-supported CoPtAg nanoparticles (NPs) with different atomic ratios and different structures are prepared for the ORR. We have tried to find the appropriate amount of Ag addition into CoPt catalysts and then the correlation between the fine structures and the electrochemical properties of CoPtAg catalysts is elucidated. The phases and structures, surface compositions, morphologies, electrochemical properties, and the number of unoccupied d-state of prepared catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy (HRTEM), rotating ring-disk electrode technique and X-ray absorption spectroscopy (XAS), respectively.
This study is divided into two parts. In the first part, carbon-supported CoPtAg NPs with different atomic ratios are reduced in CO atmosphere. The electrochemical surface area (ECSA) and ORR activity of CoPt is enhanced by the Ag addition. Among these CoPt samples, Co90 with Co/Pt/Ag atomic ratio of 90/5/5 displays the best ORR performance and stability attributed to the highest ECSA and appropriate amount of Ag addition. The mass activity at 0.85 V of Co90 is 8.4 times higher than that of commercial pure Pt. Thus, we have demonstrated that adding appropriate amount of Ag into CoPt/C catalyst can effectively promote ORR performance. However, Co50 with 45 at. % of Ag addition has a decay of 75 % after ADT of 1000 cycles, caused by significant Ag dissolution, leading to structure collapse.
In the second part, carbon-supported Co90Pt5Ag5 NPs with different atomic arrangements have been prepared by different reduction agents. These samples with low Pt loading has superior ORR activity and stability to Pt/C. Based on the analysis of the HRTEM, XAS, and electrochemical measurements, the relationship between atomic arrangement and ORR performance of Co90Pt5Ag5 is systematically elucidated. The coordination numbers of Pt-Co and Co-Co (CNPt-Co and CNCo-Co) of catalysts are related to their ORR activity and stability, respectively. Co90 with Pt and Ag randomly distributing within the Co matrix has the highest CNCo-Co, which does not have positive effect on the ORR performance. On the other hand, Co90-1 sample with Pt and Ag cores within the Co matrix has the highest CNPt-Co and ORR activity ascribed to the electronic modification effect. However, it losses 32 % ORR activity after accelerated durability test (ADT) of 1000 cycles due to high CNCo-Co. Co90-2 with Pt in the inner core and Ag on the outer shell structure has moderate CNPt-Co, very low CNCo-Co, and un-filled d-states, which benefit the ORR stability and the decay rate is 21 % after ADT of 1000 cycles.||en_US|