Platinum nanoparticle-supported boron-doped diamond powder (Pt/BDDP) was prepared and investigated as a durable polymer electrolyte fuel cell (PEFC) cathode catalyst. The use of the nanocapsule method enabled dense deposition of Pt nanoparticles (25 nm in size) on a boron-doped diamond (BDD) powder <500 nm in size. The Pt/BDDP cathode catalyst showed oxygen reduction reaction activity comparable to Pt-supported carbon black (Pt/C), indicating sufficient conductivity of the BDDP as a catalyst support. Potential cycling in a highly positive potential region (+1.0-+1.5 V vs. NHE) that simulates the start-stop operations of the PEFC was performed to investigate the durability of the Pt/BDDP catalyst. Decreases in the electrochemically active surface area of the Pt/BDDP were suppressed compared to that of Pt/C. Corrosion resistance of BDD against potential cycling was demonstrated by testing a BDD thin-film electrode. The corrosion resistance should be responsible for the improved durability of the BDDP support, possibly by lowering the Pt nanoparticle association process (e.g., agglomeration). (C) The Author(s) 2018. Published by ECS.