LAUSR

To reveal how the size effect and strain effect synergistically regulate the mass activity (MA) and specific activity (SA) of Pt alloy nanocrystal catalysts in oxygen reduction reaction (ORR), remains to be difficult due to the highly entangled factors. In this work, six ternary PtCoCu catalysts with sequentially changed composition, size, and compression strain are prepared. It is found that the smaller the alloy particles, the higher the electrochemical active surface area (ECSA) and MA values, that is, the particle size plays a decisive role in the size of the ECSA and MA. While, along alloy size decrease, the intrinsic activity SA first increases, then remains unchanged, and finally rapidly increases again. This detailed analysis shows that for the alloys above 4 nm, it is the surface coordination number that decides the SA, while for those below 4 nm, it is the well-regulated compression strain that determines the SA. Particularly, Pt47 Co26 Cu27 demonstrates the MA of 1.19 A mgPt -1 and SA of 1.48 mA cm-2 , being 7.9 and 6.4 times those of commercial Pt/C respectively, representing an especially superior ORR catalyst.