Mechanisms of the Oxygen Reduction Reaction on Defective Graphene- Supported Pt Nanoparticles from First-Principles

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1 Supporting Information Mechanisms of the Oxygen Reduction Reaction on Defective Graphene- Supported Pt Nanoparticles from First-Principles Dong-Hee Lim and Jennifer Wilcox * Department of Energy Resources Engineering, Stanford University 367 Panama Street, Green Earth Sciences, Stanford, CA * Corresponding author: wilcoxj@stanford.edu, Phone: (650) , Fax: (650) Journal Name: The Journal of Physical Chemistry C Author Address: Dong-Hee Lim (limkr@stanford.edu), Jennifer Wilcox (wilcoxj@stanford.edu) S1

2 Computational Methodology 1. Neutrally charged oxygen adsorbates on Pt 13 surfaces Oxygen adsorbates on Pt 13 surfaces were considered as neutrally charged oxygen adsorbates, instead of superoxide O 2 or peroxide O 2 2, as studied by Nørskov et al., 1 showing reasonable agreement with experiments in terms of ORR overpotential calculations. Adsorbed oxygen on Pt(111) is identified as superoxide O 2 and peroxide O 2 2, which would make the charge assignment crucial in the oxygen adsorption calculations. The charge assignment would induce an electric dipole moment upon O 2 adsorption. However, negligible induced dipole moments for adsorbed O 2 and O have also been reported. 2 Due to this contradiction as discussed in Qi et al. 3, neutrally charged oxygen adsorbates have also been studied by Nørskov et al., 1 showing reasonable agreement with experiments. We also employed the same methods as Nørskov et al. 1 and based on these methods, our study examines the effect of the defective graphene support on oxygen reduction along with a comparison of the Pt nanoparticle reactions with the Pt(111) reactions of Nørskov et al. 1 In addition, many previous DFT studies 1,4-7 have assumed charge neutrality for the O 2 molecule, not only for the simplicity of the model but also since it suits the specific applications. 2. Justification of fixing the graphene surface The boundary comprising of 83 carbon atoms out of 127 carbon atoms of defective graphene were frozen as shown in Figure S1. Upon O 2 adsorption, the change in atomic position of the carbon atoms (δ P in units of Å) is defined as: ( x z x1 ) + ( y2 y1) + ( z2 1) where subscripts 1 and 2 represent the atomic positions before and after adsorption, respectively. The area of fixed carbon atoms in Figure S1(b) is determined based on Figure S2 showing that most of the geometric change occurs near the carbon vacancy site of graphene (i.e., less than δ P = 0.1 Å). a) Pt 13 -defective graphene b) Fixed and relaxed carbon atoms Figure S1. Side view of adsorbed Pt 13 nanoparticle on defective graphene (a) and fixed and relaxed carbon atoms of defective graphene (b). S2

3 EO2 ( 2.30) δp (0.08) EO2 ( 1.96) δp (0.10) EO2 ( 1.80) δp (0.05) EO2 ( 0.78) δp (0.03) Figure S2. O2 adsorption energies (EO2 in units of ev) on Pt13-defective graphene and their geometries (left)8 and change in atomic position of carbon atoms (δp in units of Å). S3

4 3. Free energy calculations The free energy change ( G) is, for example, calculated for the following reaction. HO* + H + + e H 2 O + * where the asterisk (*) denotes an adsorbed species on Pt 13 -defective graphene systems. At zero-potential (i.e., U = 0 V), G = μ(h 2 O) + μ(*) μ(ho*) μ(h + + e ) = μ(h 2 O) + μ(*) μ(ho*) μ(1/2 H 2(g) ) where μ(ho*) = E(HO*) + ZPE(HO*) TS(HO*), E is the total energy directly obtained from DFT calculations, ZPE is zero-point energy, T is temperature (i.e., K), and S is entropy. At an applied potential of U = 1.23 V, G = μ(h 2 O) + μ(*) μ(ho*) μ(h + + e ) eu = μ(h 2 O) + μ(*) μ(ho*) μ(1/2 H 2(g) ) eu where e = 1 since there is only one (H + + e ). Thus, eu = 1.23 ev. If there are two (H + + e ), e = 2. S4

5 Table S1. Electronic DFT total energies (E), zero-point energies (ZPE), entropies multiplied by temperature (= K) (TS), and free energies (G) of the ORR intermediates used for Figure 3 and 4 in the main paper. Gas-phase E (ev) ZPE (ev) TS (ev) G (ev) H 2 O H /2 O Free Pt Pt 13 -defective graphene Direct on Pt 13 -defective graphene Step (2) O 2 * Step (3) 2O* Step (4) HO*+O* Step (5) O* Step (6) HO* TS Step (2) (3) Alt. Step (5) O* Alt. Step (6) HO* Series on Pt 13 -defective graphene Step (2) O 2 * Step (3) HOO* Step (4) HO*+O* Step (5) O* Step (6) HO* Alt. Step (3) HOO* Alt. Step (3) HOO* Alt. Step (3) HOO* Direct on free Pt 13 Step (2) O 2 * Step (3) 2O* Step (4) HO*+O* Step (5) O* Step (6) HO* TS Step (2) (3) where Direct: a direct four-electron pathway, Series: a series two-electron pathway, TS: transition states for O 2 dissociation, Alt.: alternative configurations for a given step (less stable configurations), ZPE and TS in bold are obtained from thermodynamics database 1 S5

6 Figure S3. Free energy diagram for the simple oxygen reduction model over bridge (red) and atop (blue) sites of free Pt 13 nanoparticles. Solid and dashed lines represent reactions at zero cell potential (U = 0 V) and the equilibrium potential (U = 1.23 V), respectively. S6

7 H O Pt (a) Initial configuration 1 (b) Optimized configuration 1 (c) Initial configuration 2 (d) Optimized configuration 2 Figure S4. Geometry optimization of H + HOO* adsorbed on free Pt 13 nanoparticles. (a) and (c) represent initial configurations and (b) and (d) represent optimized configurations. S7

8 References (1) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Journal of Physical Chemistry B 2004, 108, (2) Hyman, M. P.; Medlin, J. W. Journal of Physical Chemistry B 2005, 109, (3) Qi, L.; Qian, X. F.; Li, J. Physical Review Letters 2008, 101, (4) Miller, D. J.; Öberg, H.; Näslund, L. Å.; Anniyev, T.; Ogasawara, H.; Pettersson, L. G. M.; Nilsson, A. Journal of Chemical Physics 2010, 133, (5) Shan, B.; Kapur, N.; Hyun, J.; Wang, L.; Nicholas, J. B.; Cho, K. Journal of Physical Chemistry C 2009, 113, 710. (6) Escaño, M. C. S.; Nakanishi, H.; Kasai, H. Journal of the Physical Society of Japan 2009, 78, (7) Tripković, V.; Skúlason, E.; Siahrostami, S.; Nørskov, J. K.; Rossmeisl, J. Electrochimica Acta 2010, 55, (8) Lim, D.-H.; Wilcox, J. Journal of Physical Chemistry C 2011, 115, S8