Core-Shell Nanostructured Cobalt-Platinum Electrocatalysts

Transcription

1 Supporting Information Core-Shell Nanostructured Cobalt-Platinum Electrocatalysts with Enhanced Durability Lei Wang 1, Wenpei Gao 2, Zhenyu Liu 3, Zhenhua Zeng 4, Yifan Liu 1, Michael Giroux 1, Miaofang Chi 5, Guofeng Wang 3, Jeffrey Greeley 4, Xiaoqing Pan 2,6, Chao Wang 1, * 1 Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA; 2 Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, USA; 3 Department of Mechanical Engineering & Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA; 4 Davison School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA; 5 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA; 6 Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA * chaowang@jhu.edu S1

2 S1. Supplemental Results from Characterization and Electrochemical Studies Figure S1. Supplemental TEM images of the nanoparticles. S2

3 Figure S2. (a-c) TEM (a), HRTEM (b), and HADDF-STEM (c) images of the as-synthesized catalyst. (d) XRD patterns showing the shift of (111) peak between Pt and (e, f) Intensity line profile analysis for the STEM image of showing the compressive strain (g) present in the Pt shell. S3

4 Figure S3. (a-d) TEM images of the commercial Pt/C catalyst (Alfa Asear) before (a, b) and after (c, d) stability tests. (e, f) CVs and ORR polarization curves of Pt/C at initial and 5,000th cycles. S4

5 Figure S4. (a-d) TEM images of the commercial Pt 3 Co/C catalyst (Sigma Aldrich) before (a, b) and after (c, d) stability tests. (e, f) CVs and ORR polarization curves of Pt 3 Co/C at initial and 5,000th cycles. EDS analysis shows the cycled Pt 3 Co/C catalyst has a composition of Pt/Co = 5/1, corresponding to the decrease of Co content from 25% to ~17%. S5

6 Figure S5. Compositions tracked for the nanoparticles during the potential cycling: (a) weight and (b) molar percentages. S6

7 Figure S6. TEM (a), HRTEM (b), and HADDF-STEM (c) images of after 30,000 cycles. S7

8 Figure S7. CO stripping patterns recorded for (a) Pt/C and (b) Pt 3 Co/C before and after 5,000 potential cycles. (c) Comparison of the CO stripping patterns for the Pt/C, Pt 3 Co/C and Co@Pt/C catalysts before potential cycling. S8

9 S2. Comparison of ORR activities and durabilities The catalysts reported in this study are among the most active ones after extensive potential cycling. The figure below (Figure S24) summarizes the comparison of mass activities to several reported Pt-based ORR electrocatalysts after various numbers of potential cycles. It is noticed that the previous report on coreshell catalysts (Pt shell and Pt-Co core) formed by dealloying of Pt-Co bimetallic catalysts did not present the mass activities before and after potential cycling, but stating After 5,000 potential cycles, the Pt 3 Co/C- 400 catalyst showed a degradation of more than 30 mv in its half-wave potential, while the degradation of the Pt 3 Co/C-700 catalyst was much lower, with less than a 10 mv negative shift in the half-wave potential..." (Wang et al., Nature Mater. 2013, 12, 81-87). The Co@Pt catalyst reported here showed nearly negligible degradation (about 2 mv of negative shift in its half-wave potential) after 30,000 cycles, albeit the activity fluctuation as presented in Figure 3 in the main text. S9

10 Figure S8. Comparison of the durabilities of different Pt-based ORR catalysts. S10

11 S3. Atomistic Modeling for Durability The structural evolution processes in a Co@Pt core-shell nanoparticle and a CoPt homogeneous alloy nanoparticle under cyclic voltammetry in solution were simulated using the grand canonical Monte Carlo (GCMC) method. The energy changes associated with the structural evolution of the CoPt nanoparticles were calculated using the empirical modified embedded atom method (MEAM). In their initial structures, both CoPt nanoparticles had cuboctahedral shape, 3884 Co atoms and 9050 Pt atoms, and particle size of about 7.66 nm, but differed in the distribution of Co and Pt atoms in the particles. In the GCMC simulations, the surface Pt and Co atoms of the nanoparticles were allowed to leach out and the Pt atoms in solution were allowed to redeposit back onto the nanoparticle surface, based on the predicted energy changes associated with these structural evolutions and externally applied electrode potentials. The temperature of the MC simulations was chosen as room temperature (298K). It notes that the diffusion of Pt and Co atoms through the nanoparticles were not included in our current MC simulations because the kinetic rate of diffusion is very low at 298K. In this study, the electrode potential was cycled in the range of between 0.6 V to 1.0 V, with step size of V. At each electrode potential, we dissolved all the surface Co atoms, and attempted 10 Monte Carlo (MC) trials to dissolve randomly selected surface Pt atoms and/or deposit Pt atoms to randomly selected surface sites. Among the MC trial steps, 95% of the attempts were for Pt dissolution whereas 5% of the attempts were for Pt deposition. In each GCMC simulation, we performed 2000 cycles of electrode potential variations and reached the thermodynamic equilibrium states of these CoPt nanoparticles. The temperature of the MC simulations was chosen as room temperature (298K). In the grand canonical Monte Carlo (GCMC) simulation, Co-Pt nanoparticles were assumed to be in equilibrium with an aqueous solution containing Pt 2+ /Co 2+ ions. Consequently, the chemical potential change (Δμ) of Pt during its dissolution/deposition process (Pt 2+ (in solution)+2e - Pt(in particle)) can be computed as: Δ Δ ln Pt2+ 2 Δ (1) S11

12 where Δ is the standard Gibbs free energy change for reduction reaction Pt 2+ +2e - Pt(metal) at standard condition, C Pt2+ is the concentration of Pt 2+ in solution and taken as 10-6 M, is the chemical potential of a Pt atom in metal Pt, and is the potential energy change associated with Pt dissolution/deposition. K B is the Boltzmann constant, T is the temperature, and U is the electrode potential. In our GCMC simulations, we varied the electrode potential between 0.6 V to 1.0 V in cycles and with step size of V. At each step of the electrode potential change, we first dissolved all the Co atoms exposed from the nanoparticle surface and then attempted two kinds of configuration changes (dissolution of a Pt atom from the particle surface and deposition of a Pt atom to the particle surface) for the CoPt nanoparticles. In this study, we assume that the dissolution of Pt from the particle is a free-energy biased process and hence its occurrence probability is determined by P N N 1 min 1, exp βδ (2) N N 1 (3) Here, β 1, Λ 2 is the thermal de Broglie wavelength (h is Plank's constant and m is the particle mass) and V is the volume of surface atoms. E(N) and E(N-1) represent the particle energies before and after Pt dissolution. Moreover, we assume that the deposition of Pt to the particle is an energy unbiased process and its occurrence probability is determined by P N N 1 min 1, exp βδ (3) (4) Here, E s is a constant energy and has a value equal to the average energy of surface Pt atoms regardless of the location of Pt deposition. We performed the GCMC simulations to model the structural evolution of two types of CoPt nanoparticles under cyclic electrode potentials in electrochemical conditions. In their initial structures, both CoPt nanoparticles have a cuboctahedral shape truncated by {111} and {100} planes of the face-centered S12

13 cubic lattice, a total of atoms, and an overall composition of about Co 30 Pt 70. However, the Pt and Co atoms were randomly distributed over the whole volume in the CoPt homogeneous alloy nanoparticle (shown in Figure 5 a, b) whereas the Co@Pt core-shell nanoparticle was composed of a pure Co core of 1.95nm radius, a diffusion layer (containing both Co and Pt) of 1nm thick, and pure Pt shell (shown in Figure 5 d, e). Starting these initial structures of the two Co-Pt nanoparticles, our GCMC simulations generated a series of configurations in proportion to the possibility of a configuration change occurring in the equilibrium ensemble (Eq. (2) and (3)). Our GCMC simulations showed that the Pt dissolution took place preferably at the sharp corners and edges of the CoPt nanoparticles. As a result, the two Co-Pt nanoparticles gradually became round and evolved to stable pseudospherical shapes (shown in Figure 5 c, f) with the help of uniform Pt re-deposition process in our simulations. In this study, we modeled the structural evolution of the CoPt nanoparticles up to 2000 electrode potential cycles. Figure S9 depicts that the total atom numbers in the CoPt nanoparticles do not vary appreciably in the last 1000 electrode potential cycles in our GCMC simulations, suggesting the stability of the attained pseudospherical nanoparticles under the modelled cyclic electrode potentials in electrochemical conditions. Further analysis revealed that the stable pseudospherical nanoparticles would have a Pt shell (with 4 to 5 atomic layers) enclosed the CoPt core (shown in Figure 5 c, f). In this work, we found that the final pseudospherical nanoparticle derived from the CoPt homogeneous alloy nanoparticle had a composition of Co 24 Pt 76, lower Co content than the initial nanoparticle. In contrast, the final pseudospherical nanoparticle derived from the Co@Pt core-shell nanoparticle had a composition of Co 40 Pt 60, higher Co content than the initial nanoparticle. Consequently, our GCMC simulation predicted that the Co@Pt core-shell nanoparticles would preserve more transition metal Co contents inside the particles than the CoPt homogeneous alloy nanoparticle in electrochemical environment. S13

14 Figure S9. Changes in atom numbers of our modeled (a) CoPt homogeneous alloy nanoparticle and (b) core-shell nanoparticle during the last 1000 cycles of electrode potential variation in our GCMC simulations. S14

15 S4. DFT Calculations As the shells of the nanoparticles are Pt-rich with thickness about to 1nm, and the nanoparticles are with a diameter of ~10 nm for which fcc(111) and fcc(100) are the dominant surface facets planes, we use PtCo alloy (with Pt:Co=3:1) with Pt skin thickness up to 4ML in the modelling. As a comparison, we use Pt(111) and Pt(100) as the representatives of Pt particle terraces. Figures S10 shows the models and adsorption sites considered in the DFT calculations. For CO, COOH, OOH and OH, top site adsorption is studied, while for O, fcc site adsorption is calculated. 8,9 The typical adsorption geometries are given in Figure S11. The adsorption energies (E ad ) of intermediates are calculated with CO, H 2 O and H 2 as reference states through the reactions below and are summarized in Table S1: CO+* CO* (1) CO+H 2 O+* COOH*+0.5H 2 (2) 2H 2 O+* OOH*+1.5H 2 (3) H2O+* O*+H 2 (4) H2O+* OH*+0.5H 2 (5) where the asterisk (*) denotes a site on the surface. The free energies of adsorption (G ad ) 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=E DFT +ZPE+ δh+ts+e solvation (6) 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. For CO* and COOH*, the solvation energies are S15

16 not considered as they do not influence the trends in the thermodynamic barrier of CO* COOH. For OH*, solvation energy is calculated based on the adsorption energy difference between an isolated OH and the OH in (H 2 O-OH) overlayer. 8 For OOH*, solvation energy 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 experimental data on Pt(111). For OOH*, the correction is scaled based on the formation energy of H 2 O 2 and H 2 O. The assumption here is that the errors of bonds in DFT calculation are proportional to the bong strength: the stronger bond (larger formation energy and adsorption energy), the larger possible error. Thus, the errors of OH-Pt bond and OOH-Pt bond are proportional to the bond strengths of OH-Pt and OOH-Pt, which are further are proportional to OH-H and OOH-H bond strengths (formation energy). The formation energy of H 2 O and H 2 O 2 are ev and ev. The error of OH-Pt bond is 0.12 ev by comparing with the experimental data (i.e. the correction is 0.12 ev). 8 Thus, the error (and correction) of OOH-Pt is deduced to be 0.07 ev. The corresponding data are summarized in Tables S2 and S3. The electrode potential-dependent free energy of adsorption (G ad (U)) is calculated by introducing the computational hydrogen electrode concept, where 0.5H 2 H + +e - (7) is in equilibrium at the reduction potential U 0 V vs SHE under standard conditions (α(h + )=1 mol/l, i.e. ph=0). 8,10 For CO stripping, because of strong CO adsorption but weak COOH adsorption, the reaction CO*+H 2 O COOH*+H + +e - largely determines the thermodynamic barrier and onset potential of CO oxidation (see Figure 6). Thus, we use the corresponding reaction free energy at the equilibrium potential of CO oxidation (-0.1 V) as a trends-based descriptor of CO oxidation on Co@Pt with various Pt skin thickness. Figure 6d shows the relative adsorption energies of CO and COOH on PtCo alloy surfaces versus that on Pt(111). For both species, the adsorptions on PtCo alloy are universally weakened in comparison to S16

17 those on Pt(111), with the weakest adsorption and the largest variation on 1ML Pt skins induced by the Corich subsurface environment. Because of the different bond orders of CO* and COOH* on the surface, i.e. 2 and 1, respectively, 11 the weakening of CO adsorption is generally larger than that of COOH*. Consequently, the CO* COOH* reaction free energies, and the thermodynamic barriers of CO oxidation, on the skins of PtCo alloy are smaller than that on Pt(111), and decrease with decreasing skin thickness (the reaction free energy is 1.04 ev on Pt(111) at the equilibrium potential -0.1 V, while the smallest reaction free energies are 0.86 ev on 1ML skin, 0.89 ev on 2ML skin, 0.95 ev on thicker skins, respectively). This trend is consistent well with lower CO stripping potential on Co@Pt than that on Pt. Also, based on the reaction free energies, we can further assign the stripping peaks at 0.78 V and 0.72 V to CO oxidation on Pt shells with >=3ML Pt and with >=2ML Pt, respectively; and the stripping peak at <=0.7 V to CO oxidation on Pt shells containing 1ML shell with various subsurface environment. We note that these assignments are also consistent with that using the energy difference between relative adsorption energies of CO* and OH* as the descriptor, as OH* theoretically has same binding order as COOH*. 12 S17

18 Figure S10. Models and adsorption sites considered in the calculations for Pt 3 Co(111) (a,b,c,d) and Pt 3 Co(100) (e,f). (a) 1ML Pt skin with 50% subsurface Co of Pt 3 Co(111), (b) 1ML Pt skin with 25% subsurface Co of Pt 3 Co(111), (c) 2ML Pt skin of Pt 3 Co(111), (d) multilayer ( 3ML) Pt skin of Pt 3 Co(111) and Pt(111), (e) 1ML Pt skin with 50% subsurface Co of Pt 3 Co(100), (f) 2ML and multilayer Pt skin of Pt 3 Co(100) and Pt(100). Black numbers indicates the sites of CO, COOH, OOH and OH adsorption. Red numbers are the sites of O adsorption. S18

19 Figure S11. The adsorption geometries of (a) CO, (b) COOH, (c) OOH, (d) O and (e) OH on the site No. 4 of Pt 3 Co(111) with 1ML skin. Similar adsorption geometries are observed on the other sites of Pt 3 Co(111) and Pt 3 Co(100) with various skin thickness, and on Pt(111). S19

20 dg(co-->cooh) (ev) U=-0.1 V Pt(100) Pt skin thickness (ML) Figure S12. Thermodynamic free energy barrier for CO electrooxidation on Pt 3 Co(100) with Pt thickness of 1-4 ML at an applied potential of -0.1 V. The dashed line is the energetics on Pt(100). S20

21 Table S1. Adsorption energies (in ev) of CO, COOH, OOH, O and OH on Pt(111) and Pt 3 Co(111) with various skin thickness and subsurface composition (for 1 ML skin). See Figures S10-S11 for the notation of the models, sites and subsurface compositions. sites adsorbate skin thickness (ML) CO 1 (sub50%co) (sub25%co) Pt(111) COOH 1 (sub50%co) (sub25%co) Pt(111) OOH 1 (sub50%co) (sub25%co) Pt(111) 3.89 O 1 (sub50%co) (sub25%co) Pt(111) 1.25 OH 1 (sub50%co) (sub25%co) Pt(111) 0.86 S21

22 Table S2. Adsorption energies (in ev) of CO and COOH on Pt 3 Co(1p00) with various skin thicknesses. See Figures S10-S11 for the notation of the models, sites and subsurface compositions. sites adsorbate skin thickness (ML) 1 2 CO Pt(100) COOH Pt(100) Table S3. Thermodynamic data used in the free energy analyses. ZPE (ev) δh@298k (ev) TS@298K (ev) E solvation (ev) E correction (ev) CO CO* COOH* H 2 O H OOH* O* OH* S22

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