High vacancy formation energy boosts the stability of structurally ordered PtMg in hydrogen fuel cells.

Alloys of platinum with alkaline earth metals promise to be active and highly stable for fuel cell applications, yet their synthesis in nanoparticles remains a challenge due to their high negative reduction potentials. Herein, we report a strategy that overcomes this challenge by preparing platinum-magnesium (PtMg) alloy nanoparticles in the solution phase. The PtMg nanoparticles exhibit a distinctive structure with a structurally ordered intermetallic core and a Pt-rich shell. The PtMg/C as a cathode catalyst in a hydrogen-oxygen fuel cell exhibits a mass activity of 0.50 A mg Pt -1 at 0.9 V with a marginal decrease to 0.48 A mg Pt -1 after 30,000 cycles, exceeding the US Department of Energy 2025 beginning-of-life and end-of-life mass activity targets, respectively. Theoretical studies show that the activity stems from a combination of ligand and strain effects between the intermetallic core and the Pt-rich shell, while the stability originates from the high vacancy formation energy of Mg in the alloy.