Supporting Information Design of Efficient Catalysts with Double Transition Metal Atoms on C 2 N Layer Xiyu Li, 1, Wenhui Zhong, 2, Peng Cui, 1 Jun Li, 1 Jun Jiang 1, * 1 Hefei National Laboratory for Physical Sciences? at the Microscale, ichem (Collaborative Innovation Center of Chemistry for Energy Materials), School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China 2 Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Institute of Applied Physics, Guizhou Synerget-ic Innovation Center of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Normal College, Gaoxin Road 115, Guiyang, Guizhou 550018, P. R. China *Corresponding author. E-mail: jiangj1@ustc.edu.cn These authors contributed equally. S1
The GGA+U method was applied to describe partially filled d-orbitals by considering coulomb and exchange corrections. 1 Referring to reported theoretical studies of system containing 3d transition metal, we used 4 ev as correlation energy (U) and 1 ev as the exchange energy (J). 22 The empirical correction method (DFT-D2) was played to describe the long-range van der Waals (vdw) interactions. 1 The periodic boundary condition was set with a 15 Å vacuum region above the plane of one C 2 N unit cell. All geometric structures were fully relaxed until energy and forces were converged to 10-5 ev and 0.01 ev Å -1, respectively. The Brillouin zone was sampled with 5 5 1 Monkhorst-Pack k-meshes for the geometry optimization, and the kinetic energy cutoff is set to be 550 ev in the plane-wave expansion. The HSE06 functional 2,3 was utilized to examine and validate the simulated band structure of monolayer C 2 N. The method of climbing image nudged elastic band (CI-NEB) was used for transition state search. 13 Three images were inserted into initial and final states. The spring force between adjacent images was 5.0 ev Å -1, and images were optimized until the forces on each atom are less than 0.03 ev Å -1. All calculations were performed with the PBE+vdW+U method. An implicit solvation model (VASPsol) 4 was employed to examined the water-solvent effect on O 2 dissociations for TM 2 @C 2 N and ORR pathways for Co 2 @C 2 N. S2
Figure S1. (a) Top and side view of atomic structure of monolayer C 2 N-h2D in the unit cell. Energy band structures of monolayer C 2 N-h2D computed with the functional of PBE (b) and HSE06 (c). S3
Figure S2. Top view of optimized geometries of Pt-N in Pt@C 2 N (a), Co-N in Co@C 2 N (b), Ni-N in Ni@C 2 N (c), Cu-N in Cu@C 2 N (d). With two TM-N bonds formed in TM@C 2 N, there are only negligible changes of lattice and planarity of C 2 N induced by anchoring one TM into the N-hole of C 2 N. The bond lengths are given in Å. S4
Figure S3. Top view of optimized geometries of Pt-N/Pt-Pt in Pt 2 @C 2 N (a), Co-N/Co-Co in Co 2 @C 2 N (b), Ni-N/Ni-Ni in Ni 2 @C 2 N (c), Cu-N/Cu-Cu in Cu 2 @C 2 N (d). With four TM-N bonds formed in TM@C 2 N, there are slight distortion of C 2 N plane induced by anchoring two TM into the N-hole of C 2 N. The bond lengths are given in Å. S5
Table S1. The computed binding energies (E b ) of TM clusters on C 2 N, TM polarization positive charges extracted from the C 2 N plane in TM@C 2 N and TM 2 @C 2 N, and the bulk cohesive energy (ev/atom) of four TMs. E b (ev) TM charge (e + ) Bulk cohesive TM@C 2 N TM 2 @C 2 N TM to TM@C 2 N TM@C 2 N TM 2 @C 2 N energy(ev/atom) Pt 4.69 7.02 2.33 0.47 0.64 5.99 Co 6.21 11.30 5.09 0.96 1.50 5.36 Ni 4.85 7.48 2.63 0.74 1.14 4.38 Cu 3.85 5.76 1.91 0.68 1.10 3.38 All of the polarization charges were obtained from Bader charge analysis. For TM 1-2 @C 2 N, it is calculated by subtracting the Bader charges of the individual C 2 N monolayer and TM atoms from that of the hybrid system TM 1-2 @C 2 N. The same protocol applies to calculate the polarization charges between O 2 and TM 1-2 @C 2 N. S6
Table S2. The computed diffusion barriers (ev) of TM atoms on the perfect monolayer of C 2 N, graphene and h-bn. C 2 N (ev) graphene (ev) h-bn (ev) Pt 2.97 0.69 0.53 Co 3.91 0.11 0.03 Ni 3.04 0.14 0.02 Cu 3.33 0.08 0.02 S7
Figure S4. Computed energy band structures of TM@C 2 N (a) and TM 2 @C 2 N (b) (From left to right: TM=Pt, Co, Ni, Cu), showing the metallic nature. Here Cu@C 2 N, Pt 2 @C 2 N, Ni 2 @C 2 N, Cu 2 @C 2 N contain no magnetic moment (see Table S2 below), while the others use black and red curves to represent spin up and down electrons. S8
Table S3. Computed magnetic moment per TM atom (M TM ), total magnetic moment per unit cell (M U ) of TM@C 2 N and TM 2 @C 2 N, and the exchange energies (E ex ) for 2 2 supercell of TM@C 2 N and one unit cell of TM 2 @C 2 N. TM@C 2 N TM 2 @C 2 N Pt Co Ni Cu Pt Co Ni Cu M (µ TM B ) 1 2 1 0 1 2 1 0 M (µ U B ) 1 3 1 0 0 0 0 0 E ex (mev) -1 36-4 0 0 109 45 0 A 2 2 supercell of TM@C 2 N was employed to investigate magnetic states with the exchange energy (E ex ). The exchange energy is defined as E ex = E FM E AFM, in which E FM and E AFM represent the energy of ferromagnetic state and antiferromagnetic state, respectively. Negative values of E ex indicate that the ferromagnetic state is the ground state, otherwise antiferromagnetic state is the ground one. The E ex and magnetic moment per unit cell are listed in Table S3. Here Pt/Ni atoms interact ferromagnetically with each other, while Co atoms interact antiferromagneticly. Cu@C 2 N is a nonmagnetic system. For the case of TM 2 @C 2 N, we investigated the magnetic coupling between two TM atoms in a unit cell. The ground state of Pt 2 /Cu 2 @C 2 N is the nonmagnetic state, while Co 2 /Ni 2 @C 2 N prefers antiferromagnetic state. The total magnetic moment per unit cell (M U ) of TM 2 @C 2 N is zero for all double TM systems. Besides, the adsorption and dissociation of O 2 were simulated on TM 2 @C 2 N with ground state. It is noted that the magnetic coupling of TMs in TM/TM 1-2 @C 2 N can only slightly affect the adsorption and dissociation of O 2. S9
Figure S5. The atomic structure of O 2 physically adsorbed on pure C 2 N monolayer. The bond length is in Å. S10
Figure S6. From left to right: the optimized geometries of initial state (IS), transition state (TS), final state (FS), and the potential energy profile of one adsorbed O 2 molecule dissociated on Pt@C 2 N (a), Co@C 2 N (b), Ni@C 2 N (c), Cu@C 2 N (d). The bond lengths are in Å. The O 2 adsorbed on Pt@C 2 N is side-on configuration; the end-on configuration of O 2 is preferred for system of Co/Ni/Cu@C 2 N. S11
Figure S7. From left to right: the optimized geometries of initial state (IS), transition state (TS), final state (FS), and the potential energy profile of one adsorbed O 2 molecule dissociated on Pt 2 @C 2 N (a), Co 2 @C 2 N (b), Ni 2 @C 2 N (c), Cu 2 @C 2 N (d). The bond lengths are in Å. The O 2 adsorption are all side-on configuration. S12
Table S4. The computed charges trapped by the adsorbed O 2 after adding one extra electron to TM@C 2 N and TM 2 @C 2 N to model the external field effect (normally induced by electrochemistry or photocatalysis process). O 2 charge after one additional electron (e - ) TM@C 2 N TM 2 @C 2 N Pt -0.24-0.16 Co -0.22-0.25 Ni -0.21-0.25 Cu -0.24-0.24 S13
Figure S8. The dependence of reaction barrier on O 2 polarization negative charge induced by external field of one additional electron in TM@C 2 N and TM 2 @C 2 N. S14
Table S5. The computed charges donated from the C 2 N plane to O 2 induced by its adsorption to TM@C 2 N and TM 2 @C 2 N. charge donated from C 2 N to O 2 (e - ) TM@C 2 N TM 2 @C 2 N Pt -0.22-0.16 Co -0.33-0.42 Ni -0.14-0.29 Cu -0.17-0.26 S15
Figure S9. Atomic structures of relaxed geometries for various ORR chemical species adsorbed on the Co 2 @C 2 N. (a d) H, O, OH, and OOH adsorption on the central Co-Co of Co 2 @C 2 N, respectively. S16
Figure S10. The reaction pathway from O 2 dissociation, and the two subsequent hydrogenation of the atomic O to generate H 2 O molecular. Here, the transition states (TS) are marked by the red rectangular box, and the corresponding reaction and activation energy are presented above the TS in the form of ( E, E a ). In general, the reaction energies should be negative (exothermic) along the reaction pathways. While, the second OH hydrogenation reaction is endothermic with a reaction energy of 0.35 ev along the O 2 dissociation pathway and OOH dissociation pathway. It should be noted that ORR often involves many reaction steps, in which some endothermic elementary steps could occur to achieve overall exothermic ORR. For instance, the reaction energies of OH hydrogenation reaction of ORR on layered SiC sheet is also endothermic with of 0.46~0.54 ev. 5 In our work, the overall ORR reaction energy along the O 2 dissociation pathway and OOH dissociation pathway on Co 2 @C 2 N is -13.82 ev and -6.62 ev, respectively (Figure 4c). In addition, the product H 2 O of OH hydrogenation reaction can be easily removed due to the weak adsorption energy and O 2 competitive adsorption. These suggests that the ORR could proceed on Co 2 @C 2 N. S17
Figure S11. The hydrogenation process of the adsorbed O 2 and the formation and subsequent dissociation of the OOH species: O atom and OH molecular. After the hydrogenation of the atomic O, the configuration with two OH molecular transform into that result from the hydrogenation of two dissociated O atom on the pathway of O 2 dissociation. And the remaining steps are the same as those along the O 2 dissociation pathway. Here, the transition states (TS) are marked by the red rectangular box, and the corresponding reaction and activation energy are presented above the TS in the form of ( E, E a ). The adsorbed structures of reaction species on Co 2 @C 2 N, such as H, O, OH, OOH, were displayed in Figure S9. There are two nitrogen atoms with lone pair electrons in one hole of the configuration of Co 2 @C 2 N, with which the hydrogen atoms tend to form H-N bonds in the plane (Figure S9a). For the O 2 dissociation pathway, following the O 2 dissociation with no barrier (exothermic with reaction energy of 1.25 ev), the formed oxygen atom can easily take the near H from N due to its higher electronegativity to form OH, shown in Figure S10. Subsequently, the two lone-pair-electron N atoms can capture another two H atoms, respectively (Figure S18
S10). And the two following hydrogenation reactions will proceed to two H 2 O molecules of the final ORR product with activation barriers of 0.11 ev (endothermic with reaction energy of 0.07 ev ) and 0.39 ev (endothermic with reaction energy of 0.35 ev), respectively. In addition, the pathway of OOH dissociation (Figure S11), initiated with a hydrogenation reaction to form an adsorbed OOH with an activation barrier of 0.28 ev (exothermic with reaction energy of 0.02 ev). Subsequently, the OOH is decomposed to an O and an OH by the break of O-O bond with activation energy of 0.09 ev (endothermic reaction energy of 0.14 ev). The formed O atom will undergo hydrogenation reaction to generate two OH. Overcoming a barrier of 0.31 ev, this configuration transforms into the 2OH configuration in O 2 dissociation pathway. And the remaining steps are the same as those in the O 2 dissociation pathway. Considering the extremely low dissociation barrier of OOH (0.09 ev), it is not necessary to study the hydrogenation reaction of OOH and the reaction pathway of HOOH dissociation. Hence, we investigated the reaction pathways for ORR on Co 2 @C 2 N using first-principles DFT calculations. Our DFT results indicate that the O 2 would be chemisorbed to the two central Co atoms, and a four-electron O 2 dissociation pathway would be kinetically favorable, in which the H 2 O formation from OH hydrogenation is the rate-determining step with an activation energy of 0.39 ev. S19
Figure S12. Calculated free energy diagrams for ORR on Co 2 @C 2 N along O 2 dissociation pathway. U is the applied electrode potential and the limiting potential for ORR is as high as 0.30 ev, which indicate that the O2 dissociation ORR pathway is thermodynamically viable for some critical electrode potentials. The change of Gibbs free energy ( G) for all ORR step was calculated by 35 G = E + E zpe - T S + G ph -1/2G H2 + neu The reaction energy ( E) can be obtained by analyzing the DFT total energies. The harmonic vibrational frequency calculations were performed to determine the zero point energy E zpe. S is the entropy difference between the adsorbed state ant the gas phase, and T is of room temperature 298.15 K. G ph = 2.303k B T ph is the free energy contribution depending on the variations of H concentration, and the value of ph was assumed to be zero for acidic medium in this work. The contribution of potential was computed assuming (H + + e ) =1/2G H2 neu (ph = 0), where G H2 is the free energy of H 2 molecule, n is the number of transferred electrons, e represents the transferred electron, and U is the operating electrochemical potential relative to the reversible hydrogen electrode (RHE). The entropies of the free molecules (such as O 2 ) can be taken from the NIST database 6 and the energy contribution from the configuration entropy in the adsorbed state was not included. S20
Figure S13. (a) The schematic arrangement of two TM atom embedded into two adjacent hole vacancies (Hole1 and Hole2) in TM@C 2 N. Here we have tried six kinds of configurations as labeled with number 1-6 in the Hole2 vacancy. (b) The schematic arrangement of two double-tm embedded into two adjacent hole vacancies (Hole1 and Hole2) in TM 2 @C 2 N. Here we have tried three kinds of configurations as labeled with number 1-3 in the Hole2 vacancy. Here we built a model of 2 1 C 2 N supercell with one or two TM atoms deposited in each hole. The optimized geometries for these configurations exhibit nearly the same C-N/TM-N bond lengths in two adjacent holes. S21
Table S6. The Bader charge (e + ) of TM in different hole vacancy sites in TM@C 2 N (structure in Figure S12). Pt Co Ni Cu TM@C 2 N Hole1 Hole2 Hole1 Hole2 Hole1 Hole2 Hole1 Hole2 1 0.47 0.47 0.96 0.96 0.74 0.74 0.68 0.68 2 0.47 0.47 0.96 0.96 0.74 0.74 0.68 0.68 3 0.47 0.46 0.96 0.97 0.74 0.75 0.68 0.68 4 0.47 0.47 0.96 0.96 0.74 0.74 0.68 0.68 5 0.46 0.47 0.96 0.96 0.74 0.74 0.68 0.68 6 0.47 0.47 0.96 0.96 0.74 0.74 0.68 0.68 S22
Table S7. The Bader charge (e + ) of TM in different hole vacancy sites in TM 2 @C 2 N (structure in Figure S12). Pt Co Ni Cu TM 2 @C 2 N Hole1 Hole2 Hole1 Hole2 Hole1 Hole2 Hole1 Hole2 1 0.63 0.64 1.46 1.46 1.14 1.14 1.10 1.10 2 0.68 0.69 1.46 1.47 1.16 1.16 1.09 1.09 3 0.64 0.66 1.46 1.46 1.15 1.15 1.08 1.09 S23
Table S8. The dissociation barriers (E a ) without solvent effect, in comparing to those (E a solv ) in the water-solvated phase for O 2 adsorbed on TM 2 @C 2 N. TM 2 @C 2 N Pt Co Ni Cu E a (ev) 0.63 0.00 0.11 0.56 solv E a (ev) 0.68 0.00 0.10 0.64 S24
Figure S14. The O 2 and OOH dissociation initiated pathways for ORR catalyzed by Co 2 @C 2 N in the water-solvated phase, the reaction energy ( E solv ) and activation energy (E solv a ) for all steps are given in parentheses with the form of ( E solv, E solv a ). We have investigated the reaction pathways for ORR on Co 2 @C 2 N in the water-solvated phase. The results indicate that the solvent effect on the Co 2 @C 2 N-catalyzed ORR along the favorable O 2 dissociation pathway is insignificant, when the H 2 O formation from OH hydrogenation is the rate-determining step with an activation energy of 0.39 ev. S25
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