Supporting Information

Transcription

1 Supporting Information Atomic Mechanism of Electrocatalytically Active Co-N Complexes in Graphene Basal Plane for Oxygen Reduction Reaction Feng Li, Haibo Shu,,* Chenli Hu, Zhaoyi Shi, Xintong Liu, Pei Liang, and Xiaoshuang Chen College of Optical and Electronic Technology, China Jiliang University, Hangzhou, China, National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai, China Corresponding author. Haibo Shu, Phone: , S-1

2 S1. Models and energetics of N-doped, Co-doped, and Co-N codoped graphene The models of N-doped, Co-doped, and Co-N codoped graphene surfaces are shown in Figure S1-S3, respectively. For N-doped graphene, we have considered three doping configurations (Figure S1), including graphitic N (N G ), pyridinic N (N Pyrid ), and pyrrolic N (N Pyrro ), respectively. For Co-doped graphene, we have also considered three doping configurations (Figure S2), including Co-embedded into a carbon single-vacancy (Co-G-I), Co-embedded into a carbon divcancy (Co-G-II), and Co adatom (Co-G-III), respectively. For Co-N x codoping (x =1-4) in graphene, ten possible configurations have been considered (Figure S3), three Co-N 1 complexes (Co-N 1 -G), four Co-N 2 complexes (Co-N 2 -G), two Co-N 3 complexes, and one Co-N 4 complex in graphene basal plane, respectively. The stability of various doping configurations is evaluated by calculating their formation energies, as listed in Table S1. The graphite N, Co-G-III, and Co-N 4 -G are the most stable configurations for N-doped, Co-doped, and Co-N codoped graphene surfaces, respectively. Figure S1 Doping configurations of N-doped graphene, including graphitic N (N G ), pyridinic N (N Pyrid ), and pyrrolic N (N Pyrro ), respectively. The gray and blue balls represent C and N atoms, respectively. S-2

3 Figure S2 The doping configurations of Co-doped graphene, including Co-embedded into a carbon single-vacancy (Co-G-I), Co-embedded into a carbon divcancy (Co-G-II), and Co adatom (Co-G-III), respectively. The small gray and large pink balls denote C and Co atoms, respectively. Figure S3 The potential configurations of Co-N codoped graphene, including (a) three Co-N 1 complexes (Co-N 1 -G), (b) four Co-N 2 complexes (Co-N 2 -G), (c) two Co-N 3 complexes, and (d) one Co-N 4 complex in graphene basal plane, respectively. The gray, blue, and pink balls represent C, N, and Co atoms, respectively. S-3

4 Table S1. Formation energies (in ev) of all potential configurations shown in Figure S1-S3 for N-doped, Co-doped, Co-N codoped graphene surfaces. Model Configuration Formation energy (ev) N G 0.62 N-G N Pyrid 3.32 N Pyrro Co-G-I 5.25 Co-G Co-G-II 6.46 Co-G-III Co-N 1 -G Co-N 2 -G Co-N 3 -G Co-N 4 -G S-4

5 following reaction, 1,2 HO* + H + +e - H 2 O + * S2 Details and the related data of free-energy diagram calculations In the oxygen reduction reaction process, the free energy change G of each step involves an electrochemical proton-electron transfer. Taking the oxygen reduction from HO* to H 2 O as an example, the free energy change G is calculated by the where the asterisk (*) denotes an adsorbed species on doped graphene systems. Based on the computational hydrogen electrode (CHE) model, 3 the free energy change G is a function of the applied electrical potential (U). 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(1/2h 2 (g)) where µ is chemical potential. 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(1/2h 2 (g)) eu where e = 1 since there is only one proton-electron transfer (H + +e - ). Thus, eu is equal to 1.23 ev. If there are two (H + +e - ), e = 2. Here chemical potentials of each adsorbed species were calculated by standard DFT techniques. For example, µ(ho*) = E(HO*) + ZPE(HO*) TS(HO*), where 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. We have listed DFT total energies, ZPE, TS, and Gibbs free energies of gas molecules and the ORR intermediates in Table S2 and Table S3, respectively. Based on the calculated data, the free energy change G of each step for the ORR on graphene doping surfaces can be obtained. S-5

6 Table S2. DFT total energies (E), zero-point energies (ZPE), entropies multiplied by temperature (= K) (TS), free energies (G) of gas molecules and the total energies of catalytic surfaces. E (ev) ZPE (ev) TS (ev) G (ev) Gas-phase H 2 O H O H 2 O N-G Co-G Co-N 4 -G Table S3. DFT total energies (E), zero-point energies (ZPE), entropies multiplied by T (= K) (TS), free energies (G) and relative free energies ( G, at 1.23 V) of ORR intermediates used in the main text. Path I (I 1 and I 2 ) and II denote four-electron and two-electron reduction pathways respectively, as demonstrated in Figure 1. E (ev) ZPE (ev) TS (ev) G (ev) G (ev) N-G Path II Step (2) O 2 * Step (3) HOO* Step (4) H 2 O Path I 2 Step (4) O* Step (5) HO* Co-G Path I 1 Step (2) O 2 * Step (3)2O* Step (4) O*+HO* Step (5) HO* Path II Step(4) H 2 O Co-N 4 -G Path I 2 Step (2) O 2 * Step (3) HOO* Step (4) O* Step (5) HO* Path I 1 Step(3) 2O* Path II Step(4) H 2 O S-6

7 S3 Tests of Hubbard U value for the Co-3d electrons To obtain a reasonable Hubbard U eff (U eff = U J) value for the Co-3d electrons, we have firstly carried out the calculations for total magnetic moment of Co-N 4 -G as a function of U value. As shown in Figure S4, it can be found that the total magnetic moment of Co-N 4 -G gradually increases with increasing U value but it reaches an equilibrium value when the U value is beyond 5 ev. Therefore, we choose U eff of 5 ev to calculate electronic structures of graphene doping structures with the Co dopant. Figure S5 shows partial density of states (PDOS) of Co-3d electrons on Co-N 4 -G surface using GGA and GGA+U (U = 5 ev) methods. It is found that both spin-up and spin-down Co-3d states calculated by GGA mainly locate the range from -2 ev to the Fermi level and the range from Fermi level to 3 ev at the conduction band. In contrast, the spin-up Co-3d states indicate highly occupied and mainly locate in the range of -6.5~-2 ev and the spin-down Co-3d states are partially occupied and locate mainly in the range of -2.5~1 ev, using the GGA+U method. 1.1 Magnetic moment (µ B ) Hubbard U (ev) Figure S4 The total magnetic moment of Co-N 4 -G as a function of Hubbard U value. The dash line displays that system magnetic moment is nearly convergent when the U value is beyond 5 ev. S-7

8 (a) DOS (arb.units) GGA (b) DOS (arb.units) GGA+U Energy (ev) Figure S5 The comparison of density of states for the Co-3d electrons of Co-N 4 -G using (a) GGA and (b) GGA+U (U = 5 ev) methods. The upper and lower panels denote spin-up and spin-down electronic states, respectively. S-8

9 S4 Four-electron reduction pathway of ORR on N-G surface Figure S6 Pathway of oxygen reduction on N-G with the 4e - reduction process. The images of 1-6 indicate optimized atomic structures of various reaction steps. S-9

10 S5 The d-band center of Co-G and Co-N 4 -G surfaces The d-band centers of Co-3d states in Co-G and Co-N 4 -G can be calculated by the following equation, E d = PDOS ( E) ( E E ) de d PDOS ( E) de where the PDOS d is the DOS projected to the 3d oribitals of Co atoms in graphene doping structures. The PDOS of Co-3d states in Co-G and Co-N 4 -G are shown in Figure S7. Based on the definition, the calculated d-band centers of Co-3d states in Co-G and Co-N 4 -G surfaces are and -1.84, respectively. d F DOS (arb.units) Co-G Co-N 4 -G Energy (ev) Figure S7 The Co-3d electronic states of Co-G (dash lines) and Co-N 4 -G (solid lines). The Fermi level is set to energy zero. S6 Movie: oxygen reduction on Co-N 4 -G surface The movie records the oxygen reduction process on Co-N 4 -G surface within 5 ps. S-10

11 REFERENCES AND NOTES (1) Lim, D.-H.; Wilcox, J. Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles. J. Phys. Chem. C 2012, 116, (2) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, (3) Nørskov, J.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin,J.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, S-11