Supporting Information. Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable. Electrocatalysts

Transcription

1 Supporting Information Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts Lei Wang 1, Zhenhua Zeng 2, Cheng Ma 3, Yifan Liu 1, Michael Giroux 1, Miaofang Chi 3, Jian Jin 4, Jeffrey Greeley 2, Chao Wang 1, * 1 Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA; 2 Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA; 3 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA; 4 Nano-Bionics Division and i-lab, Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu , China; * chaowang@jhu.edu 1

2 S1. More details about synthesis and characterization Materials. Dioctylamine (97%, Sigma Aldrich), 1,2-dichlorobenzene (99%, Sigma Aldrich), oleic acid (90%, Sigma Aldrich), oleylamine (70%, Sigma Aldrich), Dicobalt octacarbonyl (Co2(CO)8, with 1-5% hexane, 90%, Alfa Aesar), Nafion (5% in lower aliphatic alcohols and water (15-20%), Sigma Aldrich), Platinum(II) acetylacetonate (Pt(acac)2, 99.8% trace metals basis, Sigma Aldrich), and Perchloric acid (70%, % trace metals basis, Sigma Aldrich) were used as received. High purity (99.999%) argon (Ar), oxygen (O2), and carbon monoxide (CO) were purchased from Air Gas. Growth of nanoparticles. Co seeds were first synthesized by injection of 0.1 g of Co2(CO)8 dissolved in 3 ml of DCB into the mixture of 15 ml of DCB, 160 ul of DOA, and 100 ul of OAc at 180 o C under Ar sphere. After 15 min, the temperature was decreased to 150 o C. After another 15 min, the temperature was allowed to further drop to 120 o C by removing the heating mantle. At this temperature, Ar flow was used to blow out the extra CO left in the reaction. After 1 hour, the solution was heated to 160 o C, where 0.27 g of Pt(acac)2 dispersed in 3 ml of OAm was injected into the solution. The solution was then kept at this temperature for another 3 hours for the overgrowth of Pt. After that, the solution was cooled down to the room temperature and ethanol was added to precipitate the nanoparticles. The final product was collected by centrifugation at 8,000 rpm, and the obtained nanoparticles were re-suspended in 15 ml of hexane for further use. Besides CO (Figures S6 and S7), we found the successful growth of Pt shell on Co was also dependent on the conditions for injection of the Pt precursor. Uniform Co@Pt nanoparticles were only obtained by the injection of Pt(acac)2 dissolved in oleylamine at 160 C. Free Pt and heterodimer-like Co-Pt nanoparticles were obtained when the injection temperature was altered to 120 (Figure S8), 140 (Figure S8), and 180 C (Figure S10), respectively. Similar results were also 2

3 observed when other solvents instead of oleylamine were used to dissolve the Pt precursor (see, e.g., the case of dichlorobenzene shown in Figures S11 S14). Characterization. TEM images were acquired on an FEI Tecnai 12 microscope, and highresolution S/TEM images and EDS analysis were performed on a JEOL 2200FS aberrationcorrected microscope which is equipped with a BrukerAXS X-Flash Si drift detector (SDD) spectrometer. ICP-AES was performed on a Varian 710-ES. X-ray diffraction (XRD) patterns were collected on a PAN analytical X Pert 3 Powder X-Ray Diffractometer equipped with a Cu Kα radiation source. 3

4 Figure S1. Supplemental TEM images of the Co nanoparticles collected before Pt shell growth. 4

5 Figure S2. Supplemental HRTEM image of Co nanoparticles collected before Pt shell growth. 5

6 Figure S3. Supplemental TEM images of nanoparticles. 6

7 Figure S4. HAADF-STEM and corresponding elemental mapping images of as-prepared nanoparticles. Figure S5. XRD patterns of Pt and as-prepared nanoparticles. It is noticed the position of (111) peak has a positive shift for the Co@Pt nanoparticles (~40.4 o ) versus pure Pt (~39.8 o ). This peak shift could be indicative of the compressive strain in the Pt shell, but this correlation could be undermined by considering that the Co@Pt nanoparticles are polycrystalline and the corresponding (111) peak is rather weak and broad. 7

8 Figure S6. TEM images of obtained sample when injected Pt(acac)2 dissolved in olylamine at 120 o C. Figure S7. TEM images of obtained sample when injected Pt(acac)2 dissolved in olylamine at 140 o C. 8

9 Figure S8. TEM images of obtained sample when injected Pt(acac)2 dissolved in olylamine at 180 o C. Figure S9. TEM images of obtained sample when injected Pt(acac)2 dissolved in DCB at 120 o C. 9

10 Figure S10. TEM images of obtained sample when injected Pt(acac)2 dissolved in DCB at 140 o C. Figure S11. TEM images of obtained sample when injected Pt(acac)2 dissolved in DCB at 160 o C. 10

11 Figure S12. TEM images of obtained sample when injected Pt(acac)2 dissolved in DCB at 180 o C. 11

12 S2. More details about electrochemical studies Preparation of catalysts. 20 mg of as-prepared core-shell nanoparticles in toluene and 20 mg of high-surface carbon (Ketjenblack, ~900 m 2 /g) were mixed together and ultra-sonication for 30 min in order to obtain homogeneous distribution of Co@Pt nanoparticles on carbon. The Co@Pt/C was precipitated by centrifuge at 10,000 rpm for 10 min. To get rid of excessive ligand, the Co@Pt/C was further re-disperse in hexane and following by centrifuge at rpm for 10 min by 2 more times. After that, the washed Co@Pt/C was dried and annealed at 185 C in an oven under air atmosphere for overnight. Total Pt loading was controlled to be ~20%. ICP-MS was used to determine the actual loading of Pt. Electrode preparation. The catalysts were prepared to catalyst inks by adding water containing 10% isopropanol and 0.05% Nafion (1 ml catalysts/ml liquid), followed by an ultrasonication for 1 h. After that, 20 μl inks were deposited on the glassy carbon electrode (5 mm in diameter) and dried to form a uniform thin film that was further characterized in electrochemical cell. The actual loading of Pt on the glass carbon disk is around 0.02 mg/cm 2. 12

13 Figure S13. CO stripping curves recorded for the and Pt/C catalysts. The ECSA of estimated from CO stripping (ca. 60 m 2 /gpt) is slightly higher than by using Hupd (54 m 2 /gpt). This difference can be ascribed to the suppression of hydrogen adsorption on the core/shell catalyst as compared to pure Pt, which is consistent with the previous report on Pt-bimetallic catalysts. 1 13

14 Figure S14. TEM (a), HRTEM (b), and HADDF-STEM (c) images of before stability test. 14

15 Figure S15. TEM (a), HRTEM (b), and HADDF-STEM (c) images of after 5,000 cycles. 15

16 Figure S16. Polarization curves for the ORR recorded after 5,000 and 10,000 potential cycles between 0.6 and 1.0 V. Figure S17. Element composition of the catalysts in terms of weight percentage (a) and molar ratios (b). 16

17 S3. DFT calculations Periodic Density Functional Theory (DFT) calculations were performed with the projected augmented wave (PAW) method. 2, 3 and PBE exchange-correlation functionals, 4 as implemented in the Vienna Ab-initio Simulation Package (VASP). 5 Pt and Co@Pt catalysts were modeled with a 8ML Pt(111)-(2x2) slab and 1-4 ML Pt(111)-(2x2) skin supported on a Pt-Co alloy substrate (with Pt:Co=3:1) with a total thickness of 8ML, respectively. The choice of this Pt:Co ratio of the alloy substrate is motivated by the fact that there is a compositional gradient at the interface of Co core and Pt shell, which leads to an Pt-rich alloy region, rather than Co-rich alloy region or pure Co directly underneath the Pt shell (see Figure 1f). In addition, although this model is clearly not an exact representation of the nanoparticles, we expect that the trends in adsorption energies (weakening because of the compressive strain) will not change. 6-8 ORR intermediates were adsorbed on Pt side of the slabs. The slab and the images were separated by at least 14 Å vacuum, and dipole corrections were employed to screen the artificial interaction through the vacuum region. A cutoff energy of 400 ev and a k-point grid of (7 7 1) were used for the plane wave expansion and Brillouin zone integration, respectively. An orthorhombic box ( ) Å and a single k-point (0.25, 0.25, 0.25) were employed for gas phase species. The equilibrium geometries were obtained when the maximum atomic forces are smaller than 0.02 ev/å and by employing a total energy convergence of 10 5 ev for the electronic self-consistent field loop. The above computational setups and parameters have been verified to be well converged. 9 For the oxygen reduction reaction, a well-established association mechanism is considered, for which oxygen reduction go through 4 steps, O2+4H + +4e - +* OOH*+3H + +3e - (1) OOH*+3H + +3e - O*+H2O+2H + +2e - (2) 17

18 O*+H2O+2H + +2e - OH*+H2O+H + +e - (3) OH*+H2O+H + +e - 2H2O+* (4) The adsorption energy of intermediates (OOH*, O* and OH*) are calculated using a water reference scheme, i.e. 2H2O+* OOH*+1.5H2 (5) H2O+* O*+H2 (6) H2O+* OH*+0.5H2 (7) where the asterisk (*) denotes a site on the surface. The calculated adsorption energies are given in Table S1 and S2. The free energies of adsorption (Gad) under electrochemical environments are obtained by adding zero point energy (ZPE), integrated heat capacities (δh), entropy (S) and solvation energy to the DFT total energy: G=EDFT+ZPE+ δh+ts+esolvation (8) For gas phase species, ZPE, δh and S are obtained from standard thermodynamic tables (Chase, Malcolm W. NIST-JANAF Thermochemical Tables, 4th Ed., American Chemical Society, 1998). For adsorbates, the corresponding data are calculated from vibrational frequencies within the harmonic approximation. The vibrational contributions of atoms in the slab are neglected since they do not change appreciably during adsorption, desorption, and reaction. The solvation energy of OH* is calculated based on the adsorption energy difference between an isolated OH and the OH in (H2O-OH) overlayer. 9 The solvation energy of OOH* is evaluated through ab initio molecular dynamics simulation using a Pt(111) slab with OOH adsorbed on one side and the vacuum filled by water. For O* and OH*, the energetics are further corrected based on the 18

19 experimental data on Pt(111). The correction of OOH* is scaled based on the formation energy of H2O2 and H2O. The corresponding data are summarized in Error! Reference source not found.table S3. The electrode potential-dependent free energy of adsorption (Gad(U)) is calculated by introducing the computational hydrogen electrode concept, 10 where 0.5H2 H + +e - (9) is in equilibrium at the reduction potential U 0 V vs standard hydrogen electrode (SHE) under standard conditions (α(h + )=1 mol/l, i.e. ph=0). The reaction free energies of ORR at 0.9 V on Pt(111) with different strain are given in Figure S18(a), and the free energies for the most favourable sites on Pt-Co bimetallic alloy with different skin thickness are given in Figure S19(a). Then for reaction (1)-(4), the reaction free energy at potential U can be written as (10) 0 (11) 0 (12) 0 (13) 0 is the reaction free energy at U=0V. For a specific reaction step, the rate constant at potential U can be written as / (14) 19

20 is the reaction free energy of reaction (1), (2). (3) or (4). If we define the, as the maximum potential and, as the minimum overpotential (,, 1.23 ) at which the forward reaction no thermodynamic barrier (, 0). can be re-written as 0,,,,, can be expressed as, / (16) In units of current density, the rate constant is,,, / (17) where A is the surface area per site., can be obtained by fitting experimental data. Alternatively, we can circumvent the fitting process by focussing the relative current density versus that on Pt(111)., /, / (18) As,,, equation 18 also can be further expressed as, /, / (19) Where min,,,,,,, is the maximum potential at which the forward reactions are no thermodynamic barrier, max,,,,,,, is the minimum overpotential. In figure S18(b), the relative current of Pt with various amount of strain are plotted with respective to the relative OH binding energy. As seen from figure 18(a), reaction 1 and 4 are rate limiting steps. Thus, we only include the relative current of these two steps, plus reaction 3. As (15) 20

21 confirmed in figure S18(b), in which reaction 1 and 4 give rise to a volcano shape current, while the currents from reaction 3 are always higher than that from reaction 1 or reaction 4. For Pt-Co bimetallic alloys with different skin thickness, the averages of the relative current density from different adsorption site of versus OH bonding energy are given in Figure S19(b), where / / or / / We note that the OH binding energies have been linearly weighted based the calculated current density of each site. It has previously been reported that the ORR on Pt is limited by the oxidative desorption of OHad due to the rather strong binding of OH to Pt, and weakening of the Pt-OH binding by ~0.1 ev is desired to reach the maximum ORR activity (Figure S18a). 9, 11 For the bimetallic Pt-Co catalysts with Pt-skin surfaces 12, 13, the reduction of OH binding energy in comparison to Pt varies from ~0.05 to ~0.2 V as the skin thickness varies from 4 to 1 monolayers, which is primarily a result of the strain effect 14 (Although some contribution from ligand effects for the one-monolayer skin). This trend gives rise to a volcano-type of dependence for the ORR activity on the Pt-skin thickness, with the two-monolayer skin reaching the peak and having the highest activity (Figure S18b). Thereby, the enhanced catalytic activity of the Co@Pt nanoparticles can be ascribed to the formation of Pt-skin type of surfaces, while the further activation may originate from the evolution of Pt-skin thickness during potential cycling (see Figure 4 in the main text). 21

22 Figure S18. The predicted strain dependent of oxygen reduction reaction on Pt(111). (a) Calculated free energy profile of ORR intermediates at 0.9 V on Pt(111) with strain -5% to 0. (b) Relative current of reaction 1, 3 and 4 with respective to the strain and the OH adsorption energy. The results of reaction 2 is not given here because of too high current. 22

23 Figure S19. The predicted skin-thickness dependent of oxygen reduction reaction on Pt-Co alloys. (a) Calculated potential profiles of ORR on Pt(111) and Pt-skin surfaces at 0.9 V; (b) volcano plot of the ORR activity depending on the binding energy of hydroxide (OH) for the different types of surface, with the dash line showing the predictions using Pt(111) surface with varying extents of strain (see figure S18). 23

24 Table S1 Adsorption energy (in ev) of OOH, O and OH on Pt(111) with different strain. Strain (%) adsorbate OOH* O* OH* Table S2. Adsorption energies (in ev) of OOH, O and OH on Pt(111) and Pt-Co bimetallics with various skin thickness and subsurface composition (for 1 ML skin). adsorbate skin thickness (ML) OOH 1 (sub50%co) 1 sites (sub25%co) Pt(111) 3.86 O (sub50%co) (sub25%co) Pt(111) 1.25 OH (sub50%co) (sub25%co) Pt(111)

25 Table S3 Thermodynamic data used in the free energy analyses. ZPE (ev) (ev) (ev) E solvation (ev) E correction (ev) H 2 O H OOH* O* OH*

26 Supplemental References 1. van der Vliet, D. F.; Wang, C.; Li, D. G.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Angew Chem Int Edit 2012, 51, (13), Blochl, P. E. Phys. Rev. B 1994, 50, (24), Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, (3), Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (18), Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, (1), Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Phys. Rev. Lett. 1998, 81, (13), Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, (14), Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Nat Chem 2010, 2, (6), Zeng, Z.; Greeley, J. Nano Energy 2016, 29, Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, (46), Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Nat Chem 2009, 1, (7), Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J Am Chem Soc 2006, 128, (27), Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, (5811), Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C. F.; Liu, Z. C.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Nat Chem 2010, 2, (6),