Supplementary Information for

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1 Supplementary Information for One-Nanometer-Thick PtNiRh Trimetallic Nanowires with Enhanced Oxygen Reduction Electrocatalysis in Acid Media: Integrating Multiple Advantages into One Catalyst Kan Li,, Xingxing Li,, Hongwen Huang,*,, Laihao Luo, Xu Li, Xupeng Yan, Chao Ma,, Rui Si, Jinlong Yang, and Jie Zeng*, Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui , P. R. China. College of Materials Science and Engineering, Hunan province key laboratory for advanced carbon materials and applied technology, Hunan University, Changsha, Hunan, , P. R. China. Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai , P. R. China. These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.Z. ( or to H.H. (

2 Supplementary Figure S1. The XPS survey spectrum of the PtNiRh trimetallic NWs. S2

3 Supplementary Figure S2. XRD patterns of the PtNiRh trimetallic NWs, PtNi bimetallic NWs and Pt NWs. S3

4 Supplementary Figure S3. a Approximate geometrical models for five-atomic-layer-thick PtNiRh NWs and the corresponding subunit extracted along <110> direction of the NW. b Approximate geometrical models for six-atomic-layer-thick PtNiRh NWs and the corresponding subunit extracted along <110> direction of the NW. Red spheres: Pt atoms; green spheres: Ni atoms; blue spheres: Rh atoms. The utilization efficiency (UE) of Pt atoms can be calculated based on the relation: UE = Number of surface Pt atoms/total number of Pt atoms. Because the UE of Pt atoms in a NW is identical to that in a subunit extracted along the <110> direction of NW, the UE of Pt atoms can be worked out by counting the UE in an equivalent subunit. Noted that both of the cases showed the UEs more than 50%. S4

5 Supplementary Figure S4. Structural characterizations for PtNi bimetallic NWs. a low-magnification HAADF-STEM image. Scale bar, 50 nm. The inset shows the histogram of diameter and length distributions. b Atomic-resolution HAADF-STEM images. Scale bars, 5 nm. c EDX elemental mapping images of Pt and Ni. d EDX line-scanning profile. e High-resolution XPS spectra of Pt 4f. f XPS spectra of Ni 2p. S5

6 Supplementary Figure S5. Structural characterizations for Pt NWs. a low-magnification TEM image. Scale bar, 50 nm. b The histogram of diameter and length distributions. c Atomic-resolution HAADF-STEM images. Scale bars, 2 nm. d High-resolution XPS spectra of Pt 4f. S6

7 Supplementary Figure S6. The CO stripping voltammogram collected from the Pt NWs. S7

8 Supplementary Figure S7. TEM images of the samples obtained at different reaction stages using the standard procedure: (a) 30 s, (b) 2 min, (c) 4, and (d) 8 h. Scale bars, 50 nm. S8

9 Supplementary Figure S8. The plot showing the time-dependent compositional evolution for the formation of PtNiRh trimetallic NWs. S9

10 Supplementary Figure S9. FT-IR spectra recorded from CTAB, Pt NWs/C, PtNi NWs/C and PtNiRh NWs/C catalysts, respectively. S10

11 Supplementary Figure S10. a-d The TEM images of PtNiRh NWs/C (a), PtNi NWs/C (b), Pt NWs/C (c) and commercial Pt/C (d) catalysts before ADT. e-h The TEM images of PtNiRh NWs/C (e), PtNi NWs/C (f), Pt NWs/C (g) and commercial Pt/C (h) catalysts after 10,000 cycles of ADT. Scale bars, 50 nm. S11

12 Supplementary Figure S11. The positive-going polarization curves and corresponding Tafel plots of PtNi NWs/C (a), Pt NWs/C (b) and commercial Pt/C (c). S12

13 Supplementary Figure S12. The acidic ORR performance of representative Pt-based electrocatalysts. The X-axis refers to the percentage of remaining mass activity after ADTs. Noted that the standard condition for the performance is at at 0.9 VRHE ( for Y-aixs) and cycled for cycles (for X-axis). The superscripts mean the different test conditions. a, our measured data; b, 5000 cycles of ADTs; c, 9000 cycles of ADTs; d, cycles of ADTs; e, 8000 cycles of ADTs; f, cycles of ADT; g, at 0.85 VRHE. S13

14 Supplementary Figure S13. CVs in N2-saturated electrolyte (black line) and CO stripping curves (red lines) to estimate the ECSAs for PtNi bimetallic NWs/C and PtNiRh trimetallic NWs/C catalysts. PtNi bimetallic NWs/C catalyst before ADTs (a) and after ADTs (b). PtNiRh trimetallic NWs/C catalyst before ADTs (c) and after ADTs (d). Noted that the very slightly different values of ECSA(HUPD) estimated in this figure and that shown in the main text for the catalysts before ADTs are due to the different batch of catalysts, confirming excellent reproducibility. In principle, the value of ECSA(CO stripping)/ecsa(hupd) is 1.5 for Pt-skin structure. 13 On the basis of this judgment, we can analyze the surface structure of Pt-Based alloy catalysts. For PtNi bimetallic NWs/C catalyst, it was found that the Pt-skin structure was formed before the ADTs, supporting the initially high specific activity. After ADTs, the value of ECSA(CO stripping)/ecsa(hupd) decreased to 1.0, suggesting the loss of Pt-skin structure. By contrast, the PtNiRh trimetallic NWs/C catalyst could maintain their Pt-skin structure after ADTs, supporting the remarkable catalytic durability. S14

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