Platinum (Pt) is the most active single-element catalyst known to date. However, Pt is expensive and scarce, so the amount of Pt used must be reduced. Additionally, to achieve the high efficiency and power density required in mobility applications, it is necessary to further increase catalytic activity while maintaining durability. Among the various methods reported to achieve high activity, alloying Pt with Co or Ni is considered the most established approach. The high activation of Pt through alloying was originally discovered in the research and development of phosphoric acid fuel cells (PAFCs). Subsequently, there has been fundamental and applied researches and developments to utilize similar alloys in PEM-FCs. These studies have revealed that monolayer Pt skins on Pt alloys enhances activity.