Effect of Crystal Structure on Dealloying Behavior in PtCo Alloy and Its Impact on the Oxygen Reduction Reaction

The proton exchange membrane fuel cell (PEMFC) is considered ideal for power generation due to its high energy conversion efficiency, low operating temperature, and environmentally friendly features. However, the performance of PEMFC is limited by the slow oxygen reduction reaction (ORR) at the cathode, necessitating the use of catalysts such as Pt to enhance the reaction. The Pt catalyst, although effective, poses the challenge of high cost. Consequently, numerous studies have been undertaken to develop more cost-effective catalysts that can match the excellent activity of platinum while maintaining high stability. In particular, the development of alloy nano-catalysts has garnered attention, and promising results have been reported for enhancing catalyst performance through dealloying on the surface of the alloy catalyst. Alloy catalysts prepared by the dealloying method offer the advantage of improving performance by selectively dissolving less-noble metal surfaces and constructing a Pt-rich shell layer. In this study, we synthesized two PtCo alloy catalysts with distinct crystal structures: face-centered cubic (FCC) and face-centered tetragonal (FCT). To prevent particle growth during the transformation from FCC to FCT when subjected to high-temperature heat treatment for an extended duration, we employed the carbon shell encapsulation technique. Subsequently, the synthesized catalyst underwent acid treatment for dealloying. We investigated the variations in dealloying characteristics based on crystal structures and examined their impact on electrochemical performance. Various characterizations, including X-ray diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Electron Microscopy (SEM), Thermo-Gravimetric Analysis (TGA), and Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), were conducted to validate the structure of the synthesized PtCo alloy nano-catalyst. Half-cell and unit-cell experiments were carried out to verify electrochemical properties and performance, following the testing protocol suggested by the Department of Energy (DOE) in 2020. Finally, we explored the relationship between the dealloying feature of FCC and FCT and their impact on electrochemical activity and stability.