Supporting Information Surface-Engineered PtNi-O Nanostructure with Record-High Performance for Electrocatalytic Hydrogen Evolution Reaction Zipeng Zhao, # Haotian Liu, # Wenpei Gao, Wang Xue, Zeyan Liu, Jin Huang, Xiaoqing Pan, Yu Huang * Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States Department of Chemical Engineering and Materials Science, University of California, Irvine, California 92697, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States Department of Physics and Astronomy, University of California, Irvine, California 92697, United States California NanoSystems Institute, University of California, Los Angeles, California 90095, United States # These authors contribute equally to this work Supporting Information Materials and Methods Chemicals and Materials. Platinum(II) acetylacetonate [Pt(acac)2, Pt 48.0% min], nickel(ii) acetylacetonate [Ni(acac)2, 95%], nickel acetate tetrahydrate [Ni(Ac)2 4H2O, 99%], benzoic acid (> 99.5%), perchloric acid (HClO4) were purchased from Sigma Aldrich. Commercial Pt/C catalyst (20 wt% Pt, and particle size 2 to 5 nm) were purchased from Alfa Aesar. Ethanol (200 proof) were obtained from Decon Labs, Inc. Potassium hydroxide (KOH), N, N-dimethylformamide (DMF, 99.8%), acetone ( 99.5) and isopropanol ( 99.5%) were purchased from Fisher Chemical. All reagents were used as received without further purification. Carbon black (Vulcan XC-72) was received from Cabot Corporation, and annealed 2 h at 400 under Ar gas environment before used. The deionized water (18 MΩ/cm) was obtained from an ultra-pure purification system (Milli-Q advantage A10). Synthesis of Pt-Ni Octahedra. 18 mg of carbon black was suspended in 9 ml DMF in a 25 ml glass vial and underwent ultrasonication for at least 30 min. 9 mg of Pt(acac)2, 7.2 mg of Ni(acac)2 and 85 mg of benzoic acid were dissolved in 1 ml DMF to attain a clear solution, then were added to the carbon black suspended DMF solution. The vial was then capped and ultrasonic processed for 5 min. The vial was then heated with a magnetic stirring rate of 300 rpm in an oil bath to 140 C and kept for 1.5 hr, then heated to 150 C and kept for 48 hrs. After being cooled to room temperature, the carbon supported dispersive Pt-Ni alloy octahedra were obtained by centrifugation. Then the precipitate was washed with isopropanol/acetone mixture for at least 3 times to remove the organic impurities and precursor residues. The obtained catalyst was then dried under vacuum. S1
Catalyst Annealing. About 20 mg of catalyst was loaded in a quartz boat and the quartz boat was placed within the center area of a quartz tube. Then, the quartz tube was heated to 200 C in the air for 2 hours. Characterization Transmission electron microscopy (TEM) images were taken on a FEI T12 operated at 120 kv. Energydispersive X-ray spectroscopy (EDS) were taken on a FEI TITAN operated at 300 kv. Atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF STEM) images and EDS mapping were also taken on a JEOL Grand ARM300CF TEM/STEM operated at 300 kv. Samples for TEM measurements were prepared by dropping about 10-20 µl Pt-based octahedra isopropanol dispersion onto a carbon-coated copper grid (Ladd Research, Williston, VT) using a pipette and then drying under ambient conditions. Al grids and Au grids (Ted Pella, Redding, CA) were used for the EDS sample preparation. Powder X-ray diffraction patterns (PXRD) were collected on a Panalytical X'Pert Pro X-ray Powder Diffractometer with Cu-Kα radiation. The composition of catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Shimadzu ICPE- 9000) as well as EDS. X-ray photoelectron spectroscopy (XPS) tests were done with Kratos AXIS Ultra DLD spectrometer Electrode Preparation and Electrochemical Test To obtain a homogeneous catalyst ink, 0.71 mg of dried PtNi/C was mixed with 1 ml ethanol and ultrasonic processed for 5 min. Then 16 µl of Nafion (5 wt%) was added to each solution. Then, 10 µl of the homogeneous ink was dropped onto a 5 mm diameter glassy-carbon electrode (Pine Research Instrumentation) using a pipette. Pt loading was about 1.0 µg, which was estimated by ICP-AES for all Ptbased alloy samples. The ink was dried under ambient air, then was ready for electrochemical test. The procedure for obtaining PtNi-O/C catalyst ink is similar but changed PtNi/C to PtNi-O/C. For the baseline commercial Pt/C (20 wt%), the procedure was still similar but changed 0.71 mg of PtNi/C to 0.50 mg of Pt/C. Pt loading was still about 1.0 µg for the commercial Pt/C catalyst. All electrochemical tests were carried out on a three-electrode cell from Pine Research Instrumentation. The working electrode was a glassy carbon rotating disk electrode (RDE) coated with corresponding catalyst. The reference electrode was a Hg/HgO electrode from CH Instrument. A graphite rod was used as the counter electrode. The alkaline electrolyte was 1.0 M KOH and 0.1 M KOH were saturated by N2 respectively. Cyclic voltammetry (CV) was conducted in each solution between 50 mv to 1100mV vs. Reverse Hydrogen Electrode (RHE) at a sweep rate of 100mV/s. Hydrogen Evolution Reaction (HER) was tested between -200mV to 100mV vs. RHE in each solution at a sweep rate of 5 mv/s for 1 M KOH and -500 mv to 100 mv vs. RHE for 0.1 M KOH. The impedance of each solution was tested on a Princeton VersaSTAT 4 electrochemistry workstation. The solution resistances measured via impedance test are 4.5 Ω and 39.2 Ω for 1 M and 0.1 M KOH, respectively. The above values are used for S2
post-test ir correction. Electrochemical surface area (ECSA) was tested through hydrogen underpotential deposition (Hupd) in nitrogen saturated 0.1 M HClO4. Figure S1. TEM images of before annealing (A) PtNi/C and after annealing (B) PtNi-O/C. S3
Figure S2. EDS mapping images of elemental components in octahedral PtNi/C (A) Ni, (B)Pt, (C) Pt and Ni. The insert image is the STEM image of mapped octahedron. S4
Figure S3. EDS mapping images of elemental components in octahedral PtNi-O/C (A) Ni, (B)Pt, (C) Pt and Ni. The insert image is the STEM image of mapped octahedron. S5
Table S1. Composition comparison of octahedral PtNi/C, PtNi-O/C nanostructures before and after the HER durability test. Sample Pt Ni PtNi/C 60.5% 39.5% PtNi-O/C 60.5% 39.5% PtNi/C After HER Stability 66.4 % 33.5 % PtNi-O/C After HER stability 66.1% 33.9% Table S2. XRD comparison of octahedral PtNi/C, PtNi-O/C nanostructures Sample (111) peak at 2θ (degree) Approximate Pt ratio according to Vegard s law PtNi/C 41.16 0.70 PtNi-O/C 40.63 0.81 S6
Table S3. XPS comparison of octahedral PtNi/C, PtNi-O/C nanostructures. The atomic ratio is based on Pt 4f and Ni 2p peaks. Sample Pt Ni PtNi/C 49.1% 50.9% PtNi-O/C 39.8% 60.2% S7
Figure S4. Tafel plot of (A) Pt/C, (B) PtNi/C, (C) PtNi-O, (D) overlap of fitted Tafel plot for Pt/C, PtNi/C, PtNi-O/C (PtNi/C and PtNi-O/C are noted as PtNi, PtNi-O correspondingly in graph for saving space and consistency with manuscript). S8
Table S4. Comparison of exchange current densities. In this work, the exchange current densities were calculated by extrapolation of the corresponding Tafel plots, and all the values were normalized to electrochemical surface area of loaded nanocatalyst. Material Exchange Current Density (ma/cm 2 ) Electrolyte Temperature Reference Pt/C 0.42 1M KOH 20 C This Work PtNi/C 1.85 1M KOH 20 C This Work PtNi-O/C 2.18 1M KOH 20 C This Work Pt/C 0.46 0.1 M KOH 20 C Ref. 1 fcc PtNi 0.68 0.1 M KOH 20 C Ref. 1 hcp PtNi 1.65 0.1 M KOH 20 C Ref. 1 Pt(100) 0.4 0.1M KOH 25 C Ref. 2 Pt(110) 0.7 0.1 M KOH 25 C Ref. 2 Pt (pc) 0.69 0.1M KOH 21 C Ref. 3 Pt/C 0.57 0.1M KOH 21 C Ref. 3 Pt(pc) 0.7 0.1M KOH 20 C Ref. 4 Pt(pc) 0.5 1M KOH 20 C Ref. 4 pc: polycrystalline S9
Figure S5. CV curves of Pt/C, PtNi/C, PtNi-O/C for Hupd estimation and related ECSA. Figure S6. HER comparison of Pt/C, octahedral PtNi/C, PtNi-O/C (A) polarization curve in 0.1M KOH, (B) mass activities at 0.07V vs. RHE. S10
Figure S7. HER comparison of polarization curves in 1M KOH about Pt/C, octahedral PtNi/C, PtNi- O/C (without ir compensation). S11
Table S5. Comprehensive comparison of octahedral PtNi/C and PtNi-O/C in this work and state of art literature. Sample Loading µg/cm 2 Overpotential at 10 ma/cm 2 (mv) Overpotential at 4 ma/cm 2 (mv) 0.1 M 1 M 0.1 M 1 M 0.1 M KOH KOH KOH KOH KOH Current density at -0.07V (ma/cm 2 ) 1M 0.1 M KOH KOH Mass activity at -0.07V (ma/µg) 1M 0.1 M KOH KOH Specific activity at -0.07V (ma/cm 2 ) 1M KOH Reference 57.4 PtNi/C 5.1 41.7 30.9 26.3 14.2 27.3 2.78 5.35 5.63 10.83 This Work 50.1 PtNi-O/C 5.1 39.8 30.3 25.6 21.9 36.9 4.29 7.23 8.79 14.8 This Work Pt NWs/ ~48 16 ~70 SL-Ni(OH)2 NA NA 25.6 NiO x /Pt 3 Ni Pt 3 Ni 3 -NWs 15.3 45 40 NA NA 23 10.9 39.7 1.59 0.679 NA NA Ref. 5 1.50 2.59 NA NA Ref. 6 Pt 3 Ni 2 -NWs/SC 15 45 42 NA NA 20.2 37.2 1.35 2.48 NA NA Ref. 7 hcp excavated Pt- Ni nano-multipods 7.65 65 NA 38 NA 22.5 3.03 NA NA 11.1 NA Ref. 1 : result with ir compensation. NA: not available. S12
Figure S8. TEM images of materials before HER durability test (A) Pt/C, (B) octahedral PtNi/C (C) octahedral PtNi-O/C, after HER stability test (D) Pt/C, (E) octahedral PtNi/C, (F) octahedral PtNi-O/C. S13
Figure S9. EDS mapping images of elemental components in octahedral PtNi/C after HER stability test (A) Ni, (B)Pt, (C) Pt and Ni. The insert image is the STEM image of mapped octahedron. S14
Figure S10. EDS mapping images of elemental components in octahedral PtNi -O/C after HER stability test (A) Ni, (B)Pt, (C) Pt and Ni. The insert image is the STEM image of mapped octahedron. Reference: (1) Cao, Z.; Chen, Q.; Zhang, J.; Li, H.; Jiang, Y.; Shen, S.; Fu, G.; Lu, B.-a.; Xie, Z.; Zheng, L. Nat. Commun. 2017, 8, 15131. (2) Markovica, N. M.; Sarraf, S. T.; Gasteiger, H. A.; Ross, P. N. J. Chem. Soc., Faraday Trans. 1996, 92, 3719. (3) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. J. Electrochem. Soc. 2010, 157, B1529. (4) Rheinländer, P.; Henning, S.; Herranz, J.; Gasteiger, H. A. ECS Transactions 2013, 50, 2163. (5) Yin, H.; Zhao, S.; Zhao, K.; Muqsit, A.; Tang, H.; Chang, L.; Zhao, H.; Gao, Y.; Tang, Z. Nat. Commun. 2015, 6, 6430. (6) Wang, P.; Jiang, K.; Wang, G.; Yao, J.; Huang, X. Angew. Chem. Int. Ed. 2016, 55, 12859. (7) Wang, P.; Zhang, X.; Zhang, J.; Wan, S.; Guo, S.; Lu, G.; Yao, J.; Huang, X. Nat. Commun. 2017, 8, 14580. S15