Platinum single atom alloy-based catalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) are essential for transitioning to clean energy technologies. The oxygen reduction reaction (ORR) at the cathode is a slow and energy-demanding four-electron process, while Pt significantly enhances the ORR rate, but its scarcity and high costs pose substantial challenges to the development of PEMFCs. This review presents Pt single-atom alloy catalysts (Pt SAACs) as innovative solutions to dramatically reduce Pt usage, while maintaining a high performance. Pt SAACs combine the advantages of both alloy and single-atom catalysts, featuring a unique electronic structure, well-defined active sites, and optimal atomic utilization, thereby offering a sustainable pathway for future advancements in PEMFC technology. The review discusses properties, synthesis methods, and characterization techniques associated with Pt SAACs. Various synthetic strategies are discussed, including electrochemical substitution, wet chemistry, and atomic layer deposition as well as other practical methods to ensure precise control over Pt atomic dispersion. These techniques serve as benchmarks for the design of efficient electrocatalysts. Advanced characterization methods such as high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine-structure spectroscopy are highlighted to elucidate the structure and function of these Pt SAACs. The mechanisms by which Pt SAACs modulate ORR activity demonstrate their significant advantages over the conventional Pt-based catalysts in PEMFC applications. Finally, challenges to enhance stability and reduce catalyst degradation under PEMFC operating conditions are discussed to propose future research directions.