C Electrocatalyst for Oxygen

Article pubs.acs.org/JPCC

Density Functional Theory Study of Ni−Nx/C Electrocatalyst for Oxygen Reduction in Alkaline and Acidic Media Shyam Kattel,† Plamen Atanassov,‡ and Boris Kiefer*,† †

Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003, United States Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, United States

‡

ABSTRACT: Graphitic Ni−Nx (x = 2, 4) and Ni−N2 edge defect motifs in Ni−Nx/C electrocatalyst and their ORR activity are studied using density functional theory. The results show that the formation of graphitic Ni−Nx and Ni−N2 edge defects is energetically favorable. Furthermore, the computations predict that O2 and peroxide both chemisorb to Ni−N2 edge site but not to graphitic Ni−N2 and Ni−N4 sites. This indicates that ORR in Ni−Nx/C electrocatalyst occurs predominantly on edge sites via a sequential 2 × 2e− process in alkaline and acidic media. The free energy diagram for O2 reduction on Ni−N2 edge defect shows fewer uphill processes in alkaline medium than in acidic medium, predicting that Ni−Nx/C ORR electrocatalyst is more active in alkaline medium, especially at high potentials. We find that the presence of magnetism in Ni−N2 edge site decreases the adsorption energy of H2O2 by ∼37% as compared to the nonmagnetic case. This significant effect of magnetism on the O2 adsorption energy suggests that the magnetic state of the catalytic sites may provide an additional degree of freedom in designing efficient non-PGM ORR electrocatalysts.

■

INTRODUCTION The production of useful energy will likely be one of the most significant challenges in the current century. Fuel cells are a very promising technology for addressing the increased future need for power generation especially for nonstationary applications. However, a remaining challenge is that the most effective electrocatalysts depend on platinum group metals (PGM).1,2 Currently, active research explores the design of efficient non-PGM electrocatalysts to reduce or eliminate the dependence on rare and expensive PGM.3 Significant attention is given to the oxygen reduction reaction (ORR) at the cathode (overall ORR: eq 1 in alkaline medium and eq 2 in acidic medium), which at present limits the overall performance of a fuel cell.4 O2 + 2H 2O + 4e− → 4(OH−)

(1)

O2 + 4(H+ + e−) → 2H 2O

(2)

activity. Moreover, TM−Nx (TM = Co, Fe and x = 2, 4) sites have been identified by XPS and linked to ORR activity in selfassembled pyrolyzed TM−Nx electrocatalysts.15−19 Similarly, ORR activity of Ni- and N-containing carbon-supported (Ni− Nx/C) electrocatalysts has been reported in both alkaline and acidic media.21,22 Previous experimental observations show that Ni−Nx/C ORR electrocatalyst is less active as compared to Co−Nx/C and Fe−Nx/C ORR electrocatalysts.21,22 However, the cause of this difference in activity and how it correlates to chemical nature, stability, geometry, and location of active sites remains unknown. Interestingly, the TMs in TM−Nx/C electrocatalysts are often ferromagnetic transition metals. This implies that the magnetic state of the catalyst may provide an additional degree of freedom for catalyst design. Conversely, if the catalytic sites are magnetic, additional probes such as magnetic Mössbauer measurements may facilitate the unique identification of the geometry, chemistry, and location of catalytically active sites. Previous experimental studies show that magnetism in PGM electrocatalysts improves ORR kinetics.23−25 However, the effect of magnetism on ORR activity in non-PGM TM−Nx/C ORR catalysts remains unexplored. The present density functional theory (DFT) study explores the stability, ORR activity, and the role of magnetism on the ORR activity of graphitic Ni−Nx (x = 2, 4), N edge, and Ni−N2 edge defects in carbon-supported electrocatalysts. We have studied the interaction of O2, OOH−, and H2O2 with graphitic

Nitrogen motifs with and without transition metal (TM) in self-assembled carbon-supported materials have gained significant interest as an alternative design to PGM ORR electrocatalysts.5−20 Previous work has attributed the increased ORR activity of N-containing carbon-supported (N/C) catalysts to various N moieties (N-doped, pyrrolic, and pyridinic) as identified by XPS.5−7,10−13 However, the N moiety responsible for the ORR activity of these catalysts has not been unambiguously identified. This identification is hindered by the frequent presence of TMs such as Co, Fe, or Ni during the synthesis of the catalysts.10−13 Yet it remains unclear if the TMs can be completely removed during acid wash or if small remaining amounts of TMs may affect the overall catalytic © 2012 American Chemical Society

Received: May 8, 2012 Revised: June 26, 2012 Published: July 20, 2012 17378

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383

The Journal of Physical Chemistry C

Article

incorporated into pyridinic-N2 and pyridinic-N4 defects (Figure 2d, e) to form graphitic Ni−N2 and Ni−N4 defects. N2 and

Ni−Nx, N edge, and Ni−N2 edge sites to identify the active sites in Ni−Nx/C-based ORR electrocatalyst.

■

COMPUTATIONAL METHODS All calculations were performed within the framework of density functional theory (DFT)26 as implemented in the Vienna ab-initio simulation package (VASP).27,28 PAW potentials29,30 were used to describe nuclei−electron interactions. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) as given by Perdew, Burke, and Ernzerhof.31 All systems were modeled as three-dimensional periodic structures. Monolayer graphene served as a model system for the carbon support (Figure 1) in self-assembled Ni−Nx/C ORR electroFigure 2. Nitrogen-only defect motifs: (a) N-doped, (b) pyrrolic N, (c) pyridinic N3, (d) pyridinic N2, and (e) pyridinic N4. Gray, C; blue, N atoms.

Ni−N2 edge defects were created by replacing edge carbon atoms with N atoms and by incorporating Ni to N2 edge defect, respectively. The formation energies of N and Ni−Nx defects were calculated relative to pristine graphene using eq 3:

Figure 1. (a) 4 × 4 graphene supercell and (b) graphene edge modeled with 7.95 Å channel. Gray, C; and white, H atoms.

ΔE = Egraphene + N/(Ni − Nx) + yμC − (Egraphene + xμ N + E Ni)

catalyst, similar to previous studies.20,32 The chosen orthorhombic arrangement of graphene is equivalent to the usual hexagonal unit cell but affords treating graphitic (nonedge) defects and edge defects within the same geometrical framework, which reduces/eliminates computational biases due to different geometries. The dimensions of the simulation cells for the graphitic defects (Figure 1a) and edge defects (Figure 1b) were a = 9.843 Å, b = 8.524 Å, and c = 14.0 Å (32 C atoms) and a = 19.685 Å, b = 8.524 Å, and c = 14 Å (36 C atoms), respectively. The large vacuum region perpendicular to the sheet was chosen to reduce artificial interactions between the modeled system and its periodic images in the z-direction. The remaining artifacts were minimized by applying a dipole correction normal to the graphene sheet.33,34 In addition, for the edge simulations, neighboring edges were separated by a 7.95 Å wide vacuum channel, and all carbon dangling electrons were saturated with hydrogen atoms (Figure 1b). The Brillouin zone was sampled on regular 4 × 4 × 1 and 2 × 4 × 1 Monkhorst−Pack grids35 for graphitic and edge defects, respectively. A plane wave energy cutoff of Ecut = 800 eV was used throughout the computations, and the Fermi level was slightly broadened using a Fermi−Dirac smearing of 25 meV. The optimized pristine graphene sheet (Figure 1a) had an inplane C−C distance of 1.42 Å, which agrees well with previous computations36,37 and the in-plane C−C distance in graphite.38 All computations were spin-polarized. Initial magnetic moments were specified as high-spin Ni (m = 2 μB) and N (m = 3 μB). Nonmagnetic carbon atoms were chosen on the basis of test calculations that showed that this setting does not affect the magnetic state of the relaxed structure. All electronic and structural degrees of freedom of the atoms in the simulation cell were allowed to relax simultaneously, while the shape of the simulation cell was held fixed at the values as obtained from the DFT optimized graphene structure. Energies of gas-phase Ni, N2, O, O2, OH, OOH, H2O, and H2O2 were each optimized as isolated species in an asymmetric orthorhombic cell of dimensions 12 Å × 13 Å × 14 Å. Charged species such as OH− and OOH− were optimized in the same unit cell in the presence of one additional explicit electron. Ni was

(3)

Here, Egraphene+N/(Ni−Nx) and Egraphene are the energies for optimized graphene with N/Ni−Nx defects and graphene, respectively. μC is the chemical potential of carbon defined as the total energy of graphene per carbon atom,39−41 μN is the chemical potential of N taken as one-half the total energy of the N2 molecule,40,42,43 and ENi is the energy of isolated Ni atom. Ni atomic energy has been used as reference energy to calculate the formation energy of Ni−Nx defects following previous work.40,44 x is the number of N atoms introduced, and y is the number of carbon atoms that have been removed from the perfect graphene sheet during defect formation. Ni binding energies on pyridinic-N2 and pyridinic-N4 defects were evaluated as: BE(Ni) = Egraphene + (Ni − Nx) − (Egraphene + N + E Ni)

(4)

For adsorption of molecules on N and on Ni−Nx defects, we tested several different initial positions/orientations of each molecule, and the reported results refer to the most stable/ favorable configurations unless noted otherwise. We also tested the coadsorption of O2 and H2O on single graphitic Ni−Nx and Ni−N2 edge defect motifs. The binding energy (BE) of molecules on defects was calculated by using eq 5: BE(molecules) = Edefect + molecule − (Edefect + Emolecule) (5)

where Edefect+molecule is the energy of the molecule adsorbed defective graphene configuration, Edefect is the energy of defective graphene configuration, and Emolecule is the energy of the isolated molecule. We have adopted the sign convention that ΔE or BE < 0 corresponds to an exothermic process. Bader charge analysis45 was performed to calculate the charge transfer from Ni to O2 molecule adsorbed on the Ni−N2 edge defect. ORR involves charge transfer, proton (H+) in acidic medium, and hydroxyls (OH−) in alkaline medium. Previously, it has been shown that gas-phase reactions can be used to model charge transfer reaction on the catalyst surface by linking these reactions with equilibrium electrocatalytic quantities.46,47 Here, 17379

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383

The Journal of Physical Chemistry C

Article

we have used the approach developed by Nørskov et al.46 to calculate the free energy diagrams of the reaction. The reaction free energy (ΔG) is defined as the difference between free energies of the initial and final states and is given by the expression:

Table 2. Formation Energy (ΔE in eV) of Graphitic Ni−Nx (x = 2, 4) Defects, N2 and Ni−N2 Edge Defects, and Binding Energy (BE in eV) of Ni and ORR Reactant Molecules O2 and H2O on These Defects defects

ΔG = ΔE + ΔZPE − T ΔS + ΔG U + ΔGpH + ΔGfield (6)

ΔE BE(Ni) BE(O2) BE(H2O)

where ΔE is reaction energy of reactant and product molecule adsorbed on catalyst surface obtained from DFT calculations, ZPE is the zero point energy, and S is entropy. ΔGU = −eU, where U is the potential at the electrode, and e is charge transferred. ΔGpH = kBT ln 10 × pH, where kB is the Boltzmann constant, and T = 300 K. Here, we assume pH = 0 for acidic medium and pH = 14 for alkaline medium.8,47 The free energy correction due to the electrochemical double layer is neglected as in previous studies.46,47 The free energy of O2 is obtained from the reaction O2 + 2H2 → 2H2O for which the free energy change is 4.92 eV at 298 K temperature and a pressure of 0.035 bar.8,46 The entropies and vibrational frequencies of molecules in the gas-phase are taken from the NIST database.48 The vibrational frequencies of adsorbed species (*O, *O2, *OH, and *OOH) were calculated to obtain ZPE contribution in the free energy expression. Only adsorbate vibrational modes were calculated explicitly, while the graphene sheet remained fixed. Vibrational frequencies were computed by displacing each adsorbate atom by 0.01 Å in each of the three cartesian directions and diagonalizing the resulting dynamical matrix.

Table 1. Formation Energy (ΔE in eV) of N Defects, Binding Energy (BE in eV) of O2, and Shortest O−N Distance (XO−N in Å) for O2 Interaction on Various N Defects defects ΔE BE(O2) XO−N a

P−N3

P−N4

P−N2

0.87a −0.12 3.18

5.88a −0.03 3.39

3.53a −0.03 3.42

3.99a −0.06 3.48

7.05a −0.01 3.24

edge Ni−N2

edge N2

−1.27 −8.32 −0.14 −0.10

−7.22 −2.86 −1.85 −0.76

−4.36 NA −0.01 −0.11

(BEs) of −8.32 and −7.44 eV (Table 2) in Ni−N2 and Ni−N4, respectively. These BEs are significantly higher than those of Ni on pristine graphene of −1.26 eV (ref 50) and −1.55 eV (ref 51). The large difference in BEs of Ni in graphitic Ni-Nx defects and in pristine graphene shows that the minimum Ni migration barrier (difference of BE’s) onto the graphene sheet is high, indicating the stability of dispersed graphitic Ni−Nx sites. The ORR activity of various defect motifs in the present study is explored by their interaction with O2. During the geometry optimization, the distance between O2 and candidate graphitic N-defects increases, indicating the presence of a weak interaction. We find a very low binding energy BE(O2) = −0.12 eV in N-doped graphene in agreement with previous computations.52 For pyridinic and pyrrolic N defects, our computations show even lower O2 binding energies ranging from −0.01 to −0.06 eV (Table 1). The weak interactions are consistent with the very small change of the OO bond length (less than an ∼2% increase) of physisorbed O2 as compared to the gas-phase OO bond length of 1.22 Å and the large N−O distance (Table 1, XO−N) as determined from the relaxed geometries, which is at least twice the sum of covalent radii 1.48 Å (N = 0.75 Å and O = 0.73 Å, ref 53). Furthermore, we predict physisorption of O2 on graphitic Ni−Nx (x = 2, 4) defect motifs (Table 2). The physisorption of O2 on graphitic Ni−Nx defect motifs is corroborated by the small increase of the OO bond length (∼2% on Ni−N4 and ∼3% on Ni−N2). Thus, neither graphitic N-only nor Ni−Nx defects are likely sites for the first step in ORR. This contrasts with findings for graphitic Co−Nx (x = 2, 4) and Fe−Nx (x = 2, 4) defects, which have been linked to this step.15−17,19,20 Thus, Ni−Nx/C and Co, Fe analogue catalysts show very different catalytic activities. To further explore the origin of the experimentally observed ORR activity in Ni−Nx/C electrocatalyst, we have investigated the stability and ORR activity of N2 and Ni−N2 edge defects (Figure 4a, b). Our computations show that formation of N2 and Ni−N2 edge defects is energetically favorable (Table 2). Comparison of the formation energies further demonstrates that Ni stabilizes the N2 edge defect by ∼2.9 eV, similar to the stabilization of graphitic N defects but lower in magnitude. This lower stabilization energy as well as the lower Ni BE imply that

RESULTS AND DISCUSSION Previous work shows that N-only graphitic (N-doped: N-Sub; pyrrolic N: Py−N; pyridinic N3: P−N3; pyridinic N2: P−N2; and pyridinic N4: P−N4) defects (Figure 2) possess high formation energies (Table 1).49 However, the present results

Py−N

graphitic Ni−N2

−3.45 −7.44 −0.06 −0.02

Figure 3. Graphitic Ni−Nx defect motifs: (a) x = 4 and (b) x = 2. Gray, C; blue, N; and yellow, Ni atoms.

■

N-Sub

graphitic Ni−N4

Reference 49.

demonstrate that the addition of Ni into pyridinic-N2 and pyridinic-N4 defects leads to the formation of graphitic Ni−Nx (x = 2, 4) defects with significantly more favorable formation energies (Tables 1 and 2). For graphitic Ni−N2 and Ni−N4 defects (Figure 3), we find a reduction of the formation energies by 8.32 and 7.44 eV, respectively. The formation energies of graphitic Ni−Nx defects are similar to the previously reported values of formation energies for topologically similar Co−Nx and Fe−Nx defects,49 supporting the presence of graphitic TM−Nx defects in TM−Nx/C (TM = Fe, Co, Ni) electrocatalysts. Furthermore, the formation energy of Ni−N4 is found to be 2.2 eV lower than that of Ni−N2, suggesting that Ni−N4 is the dominant graphitic defect motif, consistent with previous computations for the Co and Fe analogue defect motifs.49 Correspondingly, we find high Ni binding energies 17380

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383

The Journal of Physical Chemistry C

Article

distance dO−O = 2.91 Å is approximately twice the value of dO−O = 1.48 Å in gas-phase H2O2, which indicates the breaking of the O−O bond. The BE is −1.78 eV/OH. H+ is highly mobile in acidic medium,55 which facilitates the reaction H+ + *OH + e− → H2O and the removal of the two OH groups from the Ni− N2 edge sites.56 Therefore, the overall ORR is predicted to occur via a single site sequential 2 × 2e− mechanism on the Ni−N2 edge defect. This result highlights the difference from the previously reported dual site mechanism in Co−Nx/C electrocatalysts.16,17 The experimental observation of peroxide formation in Ni−Nx/C electocatalyst requires a second site that promotes peroxide formation. A likely site is the previously described graphitic N at the exposed zigzag edge that has been linked to the promotion of the first step in ORR from O2 to H2O2.54 Free energy diagrams for the complete reduction of O2 in alkaline and acidic media were computed using eq 6 to explore the presence of uphill and downhill processes along the proposed reaction pathways (alkaline medium, reactions 7−10; and acidic medium, reactions 11−15). We tested for Langmuir−Hinshelwood mechanism by coadsorbing O2 and H2O on the Ni−N2 edge defect. The optimized geometry shows that O2 binds to Ni−N2 edge defect, while H2O desorbs as evidenced by the increased H2O−Ni distance from initially 1.85 to 2.42 Å consistent with |BE(O2)| > |BE(H2O)|. Therefore, the reaction free energy is calculated for sequential partial reactions where only one molecule is adsorbed to the Ni−N2 edge site, following previous work.47,57

Figure 4. (a) N edge defect, (b) Ni−N2 edge defect, (c) O2 on Ni−N2 edge defect, (d) OOH− on Ni−N2 edge defect, and (e) H2O2 decomposed on Ni−N2 edge defect. Gray, C; blue, N; yellow, Ni; red, O; and white, H atoms.

O2 + 2H 2O + 4e− → *O2 + 2H 2O + 4e− −

(7)

−

*O2 + 2H 2O + 4e → *OOH + OH + H 2O + 3e

−

(8) −

*OOH + OH + H 2O + 3e

Ni−N2 edge defects are likely less stable than graphitic Ni−Nx defects. We find very weak interactions of O2 and H2O on the N-only edge defect (Table 2), which suggest that this type of defect is unlikely to enhance ORR activity. These conclusions are consistent with previous computations of similar edge defect topologies, which show that N in armchair edge position does not activate ORR.54 In contrast, chemisorption of O2 occurs on the Ni−N2 edge defect (Table 2). Therefore, the ORR inactive edge N2 defect can be activated by Ni incorporation. The chemisorption of O2 on the Ni−N2 edge (Figure 4c) leads to a ∼13% increase of the O−O bond length relative to the corresponding gas-phase value (dOO = 1.22 Å). This bond length increase follows from the interactions between the Ni dorbitals and the antibonding π* orbital of O2 molecule. The Bader charge analysis shows a ∼0.70e− charge transfer from the Ni d-orbitals to the π* orbital of the O2 molecule in support of predicted strong interaction. H2O binds more weakly to this defect than O2 and is easily displaced by O2. Thus, we predict that the Ni−N2 edge sites promote complete ORR. These findings can account for the experimentally observed lower ORR activity of Ni−Nx/C electrocatalyst as compared to Co− Nx/C and Fe−Nx/C electrocatalysts21,22 where graphitic defect motifs contribute to the overall ORR activity.16−20 We further studied the interaction of peroxide (OOH− in alkaline medium and H2O2 in acidic medium) with Ni−N2 edge defect. In contrast to the molecular chemisorption of OOH− (BE = −2.22 eV and Figure 4d), the interaction of H2O2 on Ni−N2 edge defect leads to the decomposition of H2O2 into two OH groups to form a Ni(OH)2 complex (Figure 4e). The O−O

−

→ *OOH− + OH− + H 2O + 2e−

(9)

*OOH− + OH− + H 2O + 2e− → 4(OH−)

(10)

O2 + 4(H+ + e−) → *O2 + 4(H+ + e−)

(11)

*O2 + 4(H+ + e−) → *OOH + 3(H+ + e−) +

−

+

(12) −

*OOH + 3(H + e ) → *O + H 2O + 2(H + e ) (13) +

−

+

*O + H 2O + 2(H + e ) → *OH + H 2O + (H + e−) (14) +

−

*OH + H 2O + (H + e ) → 2H 2O

(15)

Here, *() denotes adsorbed species on Ni−N2 edge defect. In alkaline medium, O2 reduction is predicted to be a complete downhill for potentials (U) higher than 0.8 V (Figure 5). In acidic medium, H2O2 decomposes to form a Ni(OH)2 complex. We find that subsequent OH reduction remains an uphill process for all potentials (Figure 6). Thus, the Ni−N2 edge defect is predicted to be more ORR active in alkaline medium consistent with experimental observations.21 The detailed analysis of the electronic and magnetic structure of graphitic Ni−Nx, N2 edge, and Ni−N2 edge defects shows that only the Ni−N2 edge defect possesses a finite magnetic moment of m = 0.9 μB. The absence of magnetic moments in the graphitic Ni−N2 and Ni−N4 defects can be rationalized by considering the bond length of Ni to coordinating atoms and 17381

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383

The Journal of Physical Chemistry C

Article

density plot in Figure 7 shows that the magnetic moment of the Ni−N2 edge defect is localized on the Ni atom. Our spin-

Figure 7. Spin density plot for the magnetic Ni−N2 edge defect. Gray, C; blue, N; yellow, Ni; and white, H atoms. Isosurface value = 0.01 e Å−3 (following ref 58).

polarized computations show topologically identical absorption geometries for H2O2 on Ni−N2 edge defect independent of magnetism. However, the |BE(H2O2)| = 4.89 eV on nonmagnetic (nm) Ni−N2 edge defect is predicted to be 1.33 eV higher as compared to the |BE(H2O2)| = 3.56 eV on the corresponding energetically more favorable magnetic defect. Estimating the relative desorption probabilities at T = 300 K, we obtain exp(ΔE/kBT) = ∼1023, where ΔE = 1.33 eV, favoring the magnetic defect. Therefore, magnetism of the Ni−N2 edge defect and more generally of TM-Nx defects may significantly affect the catalytic performance of non-PGM ORR electrocatalysts. This prediction is supported by experimental observations, which show that magnetism in PGM electrocatalysts improves ORR kinetics.23−25 Thus, magnetic measurements may be used as a complementary tool to XPS and FTIR to determine the geometry, chemistry, and location of the preferred catalytic active site at least in Ni−Nx/C ORR electrocatalysts.

Figure 5. Free energy diagram for O2 reduction on Ni−N2 edge defect in alkaline medium.

■

CONCLUSIONS The DFT computations show that the formation of graphitic Ni−Nx (x = 2, 4) and Ni−N2 edge defect is energetically favorable. O2 and H2O interact very weakly with graphitic Ni− Nx defects, indicating that these defect motifs are unlikely catalytic sites for ORR in Ni−Nx/C electrocatalyst. The chemisorption of O2 and OOH− and the decomposition of H2O2 on Ni−N2 edge defect show that ORR occurs via a 2 × 2e− single site mechanism in alkaline as well as in acidic medium. The free energy diagram for O2 reduction on Ni−N2 edge defect is completely downhill in alkaline medium at high potentials, U > 0.8 V, while OH reduction remains uphill in acidic medium for the same range of potentials. Thus, Ni−Nx/ C-based electrocatalysts are predicted to perform better in alkaline medium as compared to acidic medium. Furthermore, our results provide a rationale for the previously observed but poorly understood better performance of this class of electrocatalysts in alkaline medium. The prediction of a catalytically active magnetic Ni−N2 edge site implies that the use of magnetic measurements may provide new and complementary insights into the location, chemistry, and geometry of ORR active sites at least in carbon-supported Ni−Nx electrocatalyst. Finally, magnetic electrodes may be beneficial for the design of non-PGM electrocatalysts with improved ORR kinetics.

Figure 6. Free energy diagram for O2 reduction on Ni−N2 edge defect in acidic medium.

electron counting. The Ni−N bond lengths in graphitic Ni−N4 and Ni−N2 defects are very similar (Ni−N4, dNi−N = 1.88 Å; Ni−N2, dNi−N = 1.90 Å). On the other hand, Ni−C bond length in graphitic Ni−N2 is 1.87 Å, which is 6% shorter than the sum of covalent radii of Ni and C (Ni−C = 1.98 Å). These observations support the formation of σ bonds between Ni and coordinating N/C atoms. Ni has 10 valence electrons, four of which form σ bonds with coordinating N/C atoms, and the remaining electrons are paired up in graphitic Ni−Nx defects. The presence of the finite magnetic moment in Ni−N2 edge defect is due to the formation of Ni−N σ bonds as supported by the Ni−N bond lengths (1.90 Å), which are ∼3% shorter than the sum of the covalent radii (Ni−N: 1.96 Å, ref 53). In the case of the N edge defect, N forms 2σ bonds with 2 C atoms, contributes one electron to the π electron system of C5N hexagon, and the remaining two electrons are paired up resulting in zero magnetic moment, while during the formation of Ni−N2 edge defect, Ni forms 2σ bonds with 2 N, contributes one electron to the π-electron system in the C2N2Ni pentagon, and one electron remains unpaired, as computed. The spin

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 17382

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383

The Journal of Physical Chemistry C

Article

Notes

(30) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758−1775. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (32) Calle-Vallejo, F.; Martinez, J. I.; Rossmeisl, J. Phys. Chem. Chem. Phys. 2011, 13, 15639−15643. (33) Neugebauer, J.; Scheffler, M. Phys. Rev. B 1992, 46, 16067− 16080. (34) Makov, G.; Payne, M. C. Phys. Rev. B 1995, 51, 4014−4022. (35) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (36) Jiang, D. E.; Sumpter, B. G.; Dai, S. J. Chem. Phys. 2007, 126, 134701. (37) Rajesh, C.; Majumder, C.; Mizuseki, H.; Kawazoe, Y. J. Chem. Phys. 2009, 130, 124911. (38) Trucano, P.; Chen, R. Nature 1975, 258, 136−137. (39) Garcia, A. G.; Baltazar, S. E.; Castro, A. H. R.; Robles, J. F. P.; Rubio, A. J. Comput. Theor. Nanosci. 2008, 5, 2221−2229. (40) Shang, Y.; Zhao, J. X.; Wu, H.; Cai, Q. H.; Wang, X. G.; Wang, X. Z. Theor. Chem. Acc. 2010, 127, 727−733. (41) Faccio, R.; Fernandez-Werner, L.; Pardo, H.; Goyenola, C.; Ventura, O. N.; Mombru, A. W. J. Phys. Chem. C 2010, 114, 18961− 18971. (42) Finazzi, E.; Di Valentin, C.; Selloni, A.; Pacchioni, G. J. Phys. Chem. C 2007, 111, 9275−9282. (43) Yang, K. S.; Dai, Y.; Huang, B. B.; Han, S. H. J. Phys. Chem. B 2006, 110, 24011−24014. (44) Krasheninnikov, A. V.; Lehtinen, P. O.; Foster, A. S.; Pyykko, P.; Nieminen, R. M. Phys. Rev. Lett. 2009, 102, 126807. (45) Tang, W.; Sanville, E.; Henkelman, G. J. Phys.: Condens. Matter 2009, 21, 084204. (46) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886−17892. (47) Zuluaga, S.; Stolbov, S. J. Chem. Phys. 2011, 135, 134702. (48) http://cccbdb.nist.gov/. (49) Kattel, S.; Atanassov, P.; Kiefer, B. J. Phys. Chem. C 2012, 116, 8161−8166. (50) Hu, L. B.; Hu, X. R.; Wu, X. B.; Du, C. L.; Dai, Y. C.; Deng, J. B. Phys. B: Condens. Matter 2010, 405, 3337−3341. (51) Valencia, H.; Gil, A.; Frapper, G. J. Phys. Chem. C 2010, 114, 14141−14153. (52) Sidik, R. A.; Anderson, A. B.; Subramanian, N. P.; Kumaraguru, S. P.; Popov, B. N. J. Phys. Chem. B 2006, 110, 1787−1793. (53) Huheey, J. E. Inorganic Chemistry Principles of Structures and Reactivity; Harper & Row Publishers: New York, 1978; p 232. (54) Ikeda, T.; Boero, M.; Huang, S. F.; Terakura, K.; Oshima, M.; Ozaki, J. J. Phys. Chem. C 2008, 112, 14706−14709. (55) Ramaswamy, N.; Mukerjee, S. J. Phys. Chem. C 2011, 115, 18015−18026. (56) Zhang, L. P.; Xia, Z. H. J. Phys. Chem. C 2011, 115, 11170− 11176. (57) Lee, D. H.; Lee, W. J.; Lee, W. J.; Kim, S. O.; Kim, Y. H. Phys. Rev. Lett. 2011, 106, 175502. (58) Wu, M.; Cao, C.; Jiang, J. Z. Nanotechnology 2010, 21, 505202.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by the DOE-EPSCoR Implementation Program: Materials for Energy Conversion. Computing resources were provided by the New Mexico Computing Applications Center (NMCAC) and by the National Science Foundation through TeraGrid resources provided by NCSA and LONI under grant number DMR100075.

■

REFERENCES

(1) 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, 552−556. (2) Neergat, M.; Shukla, A. K.; Gandhi, K. S. J. Appl. Electrochem. 2001, 31, 373−378. (3) Othman, R.; Dicks, A. L.; Zhu, Z. H. Int. J. Hydrogen Energy 2012, 37, 357−372. (4) Wang, B. J. Power Sources 2005, 152, 1−15. (5) Qu, L. T.; Liu, Y.; Baek, J. B.; Dai, L. M. ACS Nano 2010, 4, 1321−1326. (6) Shao, Y. Y.; Zhang, S.; Engelhard, M. H.; Li, G. S.; Shao, G. C.; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y. H. J. Mater. Chem. 2010, 20, 7491−7496. (7) Geng, D. S.; Chen, Y.; Chen, Y. G.; Li, Y. L.; Li, R. Y.; Sun, X. L.; Ye, S. Y.; Knights, S. Energy Environ. Sci. 2011, 4, 760−764. (8) Yu, L.; Pan, X. L.; Cao, X. M.; Hu, P.; Bao, X. H. J. Catal. 2011, 282, 183−190. (9) Zhang, L. P.; Xia, Z. H. J. Phys. Chem. C 2011, 115, 11170− 11176. (10) Matter, P. H.; Wang, E.; Ozkan, U. S. J. Catal. 2006, 243, 395− 403. (11) Matter, P. H.; Zhang, L.; Ozkan, U. S. J. Catal. 2006, 239, 83− 96. (12) Matter, P. H.; Wang, E.; Millet, J. M. M.; Ozkan, U. S. J. Phys. Chem. C 2007, 111, 1444−1450. (13) Matter, P. H.; Ozkan, U. S. Catal. Lett. 2006, 109, 115−123. (14) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Science 2009, 324, 71−74. (15) Medard, C.; Lefevre, M.; Dodelet, J. P.; Jaouen, F.; Lindbergh, G. Electrochim. Acta 2006, 51, 3202−3213. (16) Olson, T. S.; Pylypenko, S.; Fulghum, J. E.; Atanassov, P. J. Electrochem. Soc. 2010, 157, B54−B63. (17) Olson, T. S.; Pylypenko, S.; Atanassov, P.; Asazawa, K.; Yamada, K.; Tanaka, H. J. Phys. Chem. C 2010, 114, 5049−5059. (18) Artyushkova, K.; Levendosky, S.; Atanassov, P.; Fulghum, J. Top. Catal. 2007, 46, 263−275. (19) Ziegelbauer, J. M.; Olson, T. S.; Pylypenko, S.; Alamgir, F.; Jaye, C.; Atanassov, P.; Mukerjee, S. J. Phys. Chem. C 2008, 112, 8839− 8849. (20) Titov, A.; Zapol, P.; Kral, P.; Liu, D. J.; Iddir, H.; Baishya, K.; Curtiss, L. A. J. Phys. Chem. C 2009, 113, 21629−21634. (21) Yang, R. Z.; Stevens, K.; Dahn, J. R. J. Electrochem. Soc. 2008, 155, B79−B91. (22) He, P.; Lefevre, M.; Faubert, G.; Dodelet, J. P. J. New Mater. Electrochem. Syst. 1999, 2, 243−251. (23) Wakayama, N. I.; Okada, T.; Okano, J.; Ozawa, T. Jpn. J. Appl. Phys., Part 2 2001, 40, L269−L271. (24) Chaure, N. B.; Rhen, F. M. F.; Hilton, J.; Coey, J. M. D. Electrochem. Commun. 2007, 9, 155−158. (25) Hult, B.; Gulla, L.; Mukerjee, S. Meet. Abstr. Electrochem. Soc. 2008, 801, 1177. (26) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, B864. (27) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169−11186. (28) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15−50. (29) Blochl, P. E. Phys. Rev. B 1994, 50, 17953−17979. 17383

dx.doi.org/10.1021/jp3044708 | J. Phys. Chem. C 2012, 116, 17378−17383