Supporting Information. Dendrite-Embedded Platinum-Nickel Multiframes as. Highly Active and Durable Electrocatalyst toward the

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1 Supporting Information Dendrite-Embedded Platinum-Nickel Multiframes as Highly Active and Durable Electrocatalyst toward the Oxygen Reduction Reaction Hyukbu Kwon, Mrinal Kanti Kabiraz,, Jongsik Park, Aram Oh, Hionsuck Baik, Sang-Il Choi,*, and Kwangyeol Lee*,, Department of Chemistry, Korea University, Seoul 02841, Korea Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Korea Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Korea Korea Basic Science Institute (KBSI), Seoul 02841, Korea These authors contributed equally to this work. *Corresponding authors: (S.-I.C. for electrochemical measurements); (K.L. for synthesis and characterization) S1

2 Materials and Methods Reagents. Platinum acetylacetonate (Pt(acac) 2, 97%), nickel acetylacetonate (Ni(acac) 2, 95%), and oleylamine (98%) were purchased from Sigma-Aldrich. Hexadecyltrimethylammonium chloride (CTAC) (98%) was purchased from Alfa Aesar. All reagents were used as received without further purification. Material characterization. TEM and HRTEM studies were carried out in a TECNAI G2 F30ST microscope and Tecnai G2 20 S-twin microscope. Aberration-corrected imaging and high spatial resolution EDS were performed at FEI Nanoport in Eindhoven using a Titan Probe Cs TEM 300kV with Chemi-STEM technology. EDS elemental mapping data were collected using a higher efficiency detection system (Super-X detector with XFEG); it integrates 4 FEI-designed Silicon Drift Detectors (SDDs) very close to the sample area. Compared to conventional EDX detector with Schottky FEG systems, ChemiSTEM produces up to 5 times the X-ray generation with the X-FEG, and up to 10 times the X-ray collection with the Super-X detector. X-ray diffraction (XRD) patterns were collected to understand the crystal structures of the Pt-Ni multiframes with a Rigaku Ultima III diffractometer system using a graphite-monochromatized Cu-Kα radiation at 40 kv and 40 ma. Metal contents in the Pt-Ni multiframe/c catalyst were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Preparation of the Pt-Ni multiframes. A slurry of Pt(acac) 2 (0.010 mmol), Ni(acac) 2 (0.045 mmol), CTAC (0.045 mmol), and oleylamine (15 mmol) was prepared in a 100 ml Schlenk tube with magnetic stirring. After mixing the solution at 50 o C for 7 min, the Schlenk tube was directly placed in a hot oil bath, which was preheated to 270 o C. After heating at the same S2

3 temperature for 30 min, the reaction mixture was cooled down to room temperature with magnetic stirring. For precipitation of a product, 15 ml toluene and 25 ml ethanol were added to the reaction mixture, which was then centrifuged at 4000 rpm for 5 min. The resulting precipitates were further purified 2 times by washing with ethanol/toluene (v/v = 10 /5 ml). Then the resulting precipitates were dispersed in a mixture of 2 ml of toluene, 2 ml of ethanol, and 2 ml of 3 M HCl solution. The mixture was placed in a preheated oil bath at 60 C for 1 h. Finally, the precipitated Pt-Ni multiframes were centrifuged and washed with 10 ml of ethanol for two times, then dried under vacuum. Preparation of the Pt-Ni multiframes/c catalyst. A suspension of 10 mg of the Pt-Ni multiframes and 90 mg of Ketjen black carbon was dispersed in 20 ml of chloroform, and the mixture was then magnetically stirred and ultrasonicated for 5 min. After centrifugation, the resulting catalyst was re-dispersed in 20 ml of acetic acid and then heated at 60 for 1 h to clean the residual surfactants. The Pt-Ni multiframes/c catalyst was washed with ethanol for three times and dried under vacuum. Pt loading of the Pt-Ni multiframes/c was 5 wt% obtained by ICP-AES. Preparation of [MTBD][NTf2] ionic-liquid (IL) 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5- ene [MTBD] and lithium salt of bis(trifluoromethane)sulfonimide [NTf2] were purchased from Sigma-Aldrich. [MTBD] was neutralized by drop wise addition of 10.6 M HNO 3 solution at 0 o C and then dissolved in DI water. An aqueous solution of [NTf2] and the neutralized [MTBD] was mixed with a molar ratio of 1:1 at 0 o C. The precipitated viscous [MTBD][NTf2] IL was washed S3

4 several times with DI water. Residual water was removed from the [MTBD][NTf2] IL by drying in oven at 70 C for 12 h. Electrochemical characterization. Electrochemical measurements of the samples were performed in a three-compartment electrochemical cell connected to a CHI600E potentiostat (CH Instruments, USA). A Pt mesh (1 cm 2 ) and a Ag/AgCl electrode were used as a counter and a reference electrode, respectively. Applied potential was converted to reversible hydrogen electrode (RHE). All electrochemical measurements were performed at room temperature. For the oxygen reduction reaction (ORR) measurements, a rotating disk electrode (RDE, Pine Research Instrumentation) with a glassy carbon disk (GC, 5 mm in diameter) was used as the working electrode (E3TPK, Model. No. AFE3T050GCPK). Prior to measurements, RDE was polished with 0.05 μm Al 2 O 3 suspension and washed several times with DI water. State-of-theart Pt/C (20 wt%) was purchased from Alfa Aesar. Catalyst ink was prepared by adding a catalyst powder (2.5 and 5.0 mg for commercial Pt/C and Pt-Ni multiframes/c, respectively), 5 wt% Nafion (10 μl, Aldrich), deionized water (2.0 ml), and isopropyl alcohol (0.50 ml, >99.99%, Aldrich) and then ultrasonicating for 30 min μl of the ink was cast on GC electrode and dried in oven. The total Pt loadings on freshly polished GC electrode were 20.4 and 10.2 μg cm -2 for commercial Pt/C and Pt-Ni multiframes/c catalysts, respectively. Electrochemical dealloying (cleaning) process as cyclic voltammetry (CV) was used to clean the catalyst from residual surfactants as well as to remove Ni to make Pt rich surface. CV was carried out in a potential cycle ranging from 0.08 to 1.10 V RHE in an Ar-saturated 0.1 M HClO 4 solution for 50 cycles at a scan rate of 100 mv s -1. The final CV was recorded at a scan rate of 50 mv s -1 for 2 cycles under the same condition. S4

5 From the CV curve, hydrogen underpotential (H upd ) based electrochemical surface area (ECSA) was calculated from H upd adsorption/desorption peak areas located between 0.08 and 0.45 V RHE. The charges (Q H ) generated from averaging adsorption/desorption peak areas were normalized with a reference value of 210 μc cm -2 and then again normalized by the total mass of Pt loaded on GC electrode for each catalyst. 1,2 CO stripping measurement was carried out in a catalyst-loaded GC where potential was held at 0.05 V RHE for 10 min in a CO-saturated 0.1 M HClO 4 solution. Then, Ar was purged for 30 min to remove CO from solution and the CO stripping curve was taken with a scan rate of 50 mvs -1. The first CV curve was for CO stripping peak and the second curve was used as baseline for the first cycle. From CO stripping peak, charge (Q CO ) was measured, and normalized with a reference value of 420 μc cm -2, and further with the mass of Pt loaded on GC. 3 ORR polarization curve was measured in an O 2 -saturated 0.1 M HClO 4 solution with a scan rate of 10 mv s -1 and a rotation speed of 1600 rpm. All ORR results were ir compensated. The Koutecky-Levich equation was applied to derive kinetic current density (j k ): 1 = j j k j d j k = (j j d) j d j Where, j and j d are the measured current density and the diffusion limiting current, respectively. For electrochemical accelerated durability test, potential was swept for 5000 and cycles between 0.6 and 1.0 V RHE at a rate of 100 mv s -1 in an O 2 -saturated 0.1 M HClO 4 solution. After 5000 and cycles, the CV and ORR activity were recorded in a fresh 0.1 M HClO 4 solution. CVs, ORR polarization curves, and durability measurements in the presence of [MTBD][NTf2] IL were performed similarly as outlined above. First, a drop of IL was cast onto GC electrode covered with dried commercial Pt/C or Pt-Ni multiframes/c. In the case of Pt-Ni multiframes/c, S5

6 capillary force pulled the [MTBD][NTf2] IL inside of the frames. After 3h, excess [MTBD][NTf2] IL was removed from GC electrode using a micro pipette and further electrochemical cleaning was done through potential cycling. Figure S1. (a) HRTEM image of Pt-Ni dendritic intermediates at 2.5 min. (b) Enlarged HRTEM image of white squared section in panel (a) and (c) the corresponding FFT pattern. White dotted lines in (b) represent the grain boundaries. S6

7 Figure S2. EDS spectra and atomic compositions of products in different reaction stages; (a) 2.5, (b) 5, (c) 30 min, and (d) after acid etching. Peaks near 8 and 9 kev represent the Cu element from Cu TEM grid. S7

8 Figure S3. Elemental mapping analyses, HAADF-STEM image, and line profile analysis of Pt- Ni complex at 5 min of reaction. Figure S4. TEM image of chemically etched Pt-Ni intermediates of 5 min of reaction. S8

9 Figure S5. Elemental mapping analyses, HAADF-STEM image, and line profile analysis of Pt- Ni complex after 30 min of reaction. Figure S6. Size distributions of the products at 30 min (a) before and (b) after the chemical etching process. S9

10 Figure S7. PXRD patterns of the different Pt-Ni multiframes obtained by adding different equivalents of Ni precursor compared to that in the standard protocol (a) before and (b) after chemical etching process. References: Pt for PDF# and Ni for PDF# S10

11 Figure S8. EDS spectra of the chemically etched Pt-Ni multiframes with the use of (a) 0.25, (b) 0.5, and (c) 2 equiv. of Ni precursor in the reaction. Peaks near 8 and 9 kev represent the Cu element from Cu TEM grid. Figure S9. Effect of the amount of CTAC on the morphology of product. TEM images of etched Pt-Ni nanocrystals obtained by adding (a) 0, (b) 0.25, (c) 0.5, and (d) 2 equiv. of CTAC compared to that in the standard protocol. S11

12 Figure S10. Effect of the different halide sources on the morphology of product. TEM images of etched Pt-Ni nanocrystals obtained by adding (a) K 2 PtCl 4 or (b) NiCl 2 instead of Pt(acac) 2 or Ni(acac) 2 with 2 equiv. of CTAC, and by adding (c) cetyltrimethylammonium bromide (CTAB) instead of CTAC in the standard protocol. Figure S11. TEM image of the Pt-Ni multiframes loaded on carbon support. Enlarged TEM image in the inset shows the morphology of the Pt-Ni multiframes. S12

13 Figure S12. HAADF-STEM image and corresponding elemental mapping analysis of Pt-Ni multiframes (a, c) before and (b, d) after cyclic voltammetry test. S13

14 Figure S13. CO stripping voltammograms of (a) the state-of-the-art Pt/C and (b) the Pt-Ni multiframes/c performed in a 0.1 M HClO 4 solution at 50 mv s -1. (Pt loadings for Pt/C and Pt- Ni multiframes/c are 20.4 and 10.2 μg cm -2, respectively). (c) The comparison of CO oxidation voltammograms of the Pt/C and the Pt-Ni multiframes/c. (d) Comparison of ECSAs of the catalysts obtained by H upd and CO stripping methods. S14

15 Figure S14. Cyclic voltammograms (CVs) of (a) the state-of-the-art Pt/C and (b) the Pt-Ni multiframes/c before and after stability test for 5000 and cycles in an O 2 -saturated 0.1 M HClO 4 solution. (c) Tafel plots of specific activities given as kinetic current densities (j k ) normalized against the ECSA Hupd of the catalyst. (d) Comparison of the specific activities of the catalysts recorded at 0.9 V RHE S15

16 Figure S15. ORR polarization curves of the state-of-the-art Pt/C before and after stability test for 5000 and cycles in an O 2 -saturated 0.1 M HClO 4 solution. Figure S16. TEM images of (a) the state-of-the-art Pt/C and (b) the Pt-Ni multiframes/c after cycles of the ORR. S16

17 Figure S17. (a) CVs and (b) ORR polarization curves of the Pt-Ni intermediates/c obtained at different reaction times. (c) Comparison of ECSA Hupd of the catalysts indicating the Pt migration to the surface of nanostructure with the prolonged reaction time. (d) Comparison of ORR mass activities of the catalysts measured before and after 5000 cycles in an O 2 -saturated 0.1 M HClO 4 solution. S17

18 Figure S18. (a) CVs and (b) ORR polarization curves of the state-of-the-art Pt/C and the Pt-Ni multiframes/c in the presence or absence of [MTBD][NTf2] IL. (c) Comparison of mass activities of the catalysts in terms of kinetic-current density j k at 0.93 V RHE. (d) ORR polarization curves of the Pt-Ni multiframes/c/il before and after stability test for 5000 cycles in an O 2 - saturated 0.1 M HClO 4 solution. S18

19 Table S1. Comparison of the mass ORR activities of the catalysts at 0.9 and 0.93 V RHE. Catalyst Mass activity at 0.9 V RHE (A mg -1 Pt) Mass activity at 0.93 V RHE (A mg -1 Pt) Pt/C (Initial) Pt/C (After 5000 cycles) Pt/C (After cycles) Pt-Ni multiframes/c (Initial) Pt-Ni multiframes/c (After 5000 cycles) Pt-Ni multiframes/c (After cycles) Pt-Ni intermediate/c 2.5 min (Initial) Pt-Ni intermediate/c 2.5 min (After 5000 cycles) Pt-Ni intermediate/c 5.0 min (Initial) Pt-Ni intermediate/c 5.0 min (After 5000 cycles) Pt-Ni intermediate/c 30 min (Initial) Pt-Ni intermediate/c 30 min (After 5000 cycles) S19

20 Table S2. Comparison of ECSAs and specific activities (SAs) of the catalysts before and after the stability test. Catalyst ECSA Hupd (m 2 g -1 Pt) SA Hupd at 0.9 V RHE (ma cm -2 Pt) ECSA CO (m 2 g -1 Pt) SA CO at 0.9 V RHE (ma cm -2 Pt) Pt/C (Initial) Pt/C (After 5000 cycles) Pt/C (After cycles) Pt-Ni multiframes/c (Initial) Pt-Ni multiframes/c (After 5000 cycles) Pt-Ni multiframes/c (After cycles) Pt-Ni intermediate/c 2.5 min (Initial) Pt-Ni intermediate/c 2.5 min (After 5000 cycles) Pt-Ni intermediate/c 5.0 min (Initial) Pt-Ni intermediate/c 5.0 min (After 5000 cycles) Pt-Ni intermediate/c 30 min (Initial) Pt-Ni intermediate/c 30 min (After 5000 cycles) S20

21 Table S3. Performance of Pt-Ni multiframes/c in this work and several representative results from recent literatures. Catalyst ECSA (m 2 g -1 ) Mass Activity Specific Activity H upd CO at 0.9 V vs RHE at 0.9 V vs RHE Stripping (A mg -1 Pt ) (ma cm -2 ) Pt-Ni multiframe Ref This work Pt 3 Ni RDH nanoframe S4 PtNi THH nanoframe S5 PtCu octopod nanoframe S6 PtCu@PtCuNi dendrite@frame S7 PtCu RDH nanoframe S8 Pt icosahedral nanocage S9 Pt octahedral nanocage S10 Co-PtCu nanoframe S11 PtNi THH nanoframe S12 PtNi skeletal nanoframe S13 PtPb@Pt nanoplate S14 Mo-doped Pt 3 Ni octahedron S15 Ultrafine jagged Pt-Ni nanowire S16 S21

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