Examining the Structure-Activity Relationship for Acid Treated Ordered PtCo with Different Ordering Degree Using PDF and XAS Analysis for Oxygen Reduction Reaction
Authors: Kumar Mukesh , Gao Yunfei , Thakur Neha , Cao Weijie , Watanabe Toshiki , Tominaka Satoshi , Sonobe Kazutaka , Sato Ryota , Teranishi Toshiharu , Imai Hideto , Sakurai Yoshiharu , Uchimoto Yoshiharu 
Published: December 2024

Proton exchange membrane fuel cells (PEMFCs) have garnered significant attention to fulfill the "carbon neutrality" goal. The ever-growing demand for PEMFCs necessitates the development of effective catalysts to maximize the utilization of platinum (Pt), given its rocketing price and inadequate availability.1 The formation of Pt alloys with earth-abundant transition metals such as Co, Fe, Cu, and Zn is recognized as one of the most promising approach to reduce the Pt content and increase the overall activity.2 Particularly, ordered PtCo intermetallic alloys that balance high catalytic activity and stability through ligand and strain effects due to the smaller Pt-Pt bond distance, which weaken the binding of oxygen-containing intermediate and boost the overall oxygen reduction reaction (ORR) performance.3 Despite numerous research efforts, there is still a lack of experimental evidence regarding the impact of an internally ordered phase on surface structure, especially after establishing a stable Pt-rich shell. This study comprehensively examines two essential aspects of ordered PtCo catalysts by integrating electrochemical investigations with advanced in-situ and ex-situ techniques. The first aspect involves quantifying the transition from disordered to ordered phases and its consequence on catalytic structure, while the second aspect aims to comprehend the role of the ordered structure in catalytic efficiency. Herein, we fabricated a series of PtCo catalysts with different ordering degrees by regulating the thermal treatment time and temperatures followed by acid treatment, aiming to investigate the surface structure and the activity relationship. Traditional powder X-ray diffraction (XRD) coupled with high-resolution scanning transmission electron microscopy (HR-STEM) to ascertain the degree of atomic ordering. The comprehensive electrochemical performance showed higher ORR activity for the PtCo alloy, which had a higher ordering degree. To establish a quantitative relationship between ordering degree and structure, PDF and XRD analyses were combined together. The comprehensive PDF analysis discloses the existence of different phases pFCC-Pt, FCC-Pt, FCC-PtCo, pBCT, and BCT phases post-acid treatment and higher ordering degree would contribute to a higher Co retention and minimize structure changes, which ultimately results in higher ORR activity (Figure 1). Additionally, operando XAS measurements indicated that catalysts with a higher order degree could suppress the formation of Pt-OH species compared to disordered alloys during ORR. These findings offer valuable insights into how ordering degree enhances ORR activity through strain effects and can be instrumental in designing next-generation catalysts.

Acknowledgment

This work is supported by the PEFC and the NEDO FC-Platform commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

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Figure 1