Core@shell nanostructured Au-d@NimPtm for electrochemical oxygen reduction reaction: effect of the core size and shell thickness

We showed earlier that using Au nanoparticles of ca. 3.5 nm for the cores to construct core-shell Au@NimPt2 nanostructures can be promising for developing high performance Pt catalysts for the oxygen reduction reaction (ORR) in acidic electrolytes (L.-L. Shen, G.-R. Zhang, S. Miao, J. Liu and B.-Q. Xu, ACS Catal., 2016, 6, 1680-1690). By varying the nanosize of Au cores (d = 2.2-10.3 nm) and the atomic Pt/Au (or Ni/Au) ratio (m = 0.5-6), serial Au-d@NimPtm nanostructures are herein constructed and characterized comprehensively to investigate effects of the Au-core size and NiPt-shell thickness on the Pt activity for the ORR. Maximization of the Pt intrinsic activity (IA(Pt)) to as high as ca. 1.06 mA cm(Pt)(-2) at 0.9 V (vs. RHE) is observed on Au-d@Ni2Pt2 of d = 7.7 and 10.3 nm, 4-fold higher than that of conventional Pt/C (E-TEK). Construction of Au-2.2@NimPtm samples by variation of m demonstrates that the formation of an integrated NiPt-shell of suitable thickness is essential for improving IA(Pt), whose number appears to correlate with the reduction potential of oxidized surface Pt (or surface Pt-O). The integrated NiPt-shell is also shown to be crucial to the catalytic durability of Pt during the ORR. These observations are discussed in terms of surface faceting, strain and electronic effects in association with the Au cores.