Nitrogen-Doped Graphene on Transition Metal Substrates as Efficient

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Nitrogen doped graphene on transition metal substrates as efficient bifunctional catalysts for oxygen reduction and oxygen evolution reactions Si Zhou, Nanshu Liu, Zhiyu Wang, and Jijun Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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ACS Applied Materials & Interfaces

Nitrogen doped graphene on transition metal substrates as efficient bifunctional catalysts for oxygen reduction and oxygen evolution reactions Si Zhou a, Nanshu Liu a, Zhiyu Wang b, Jijun Zhao a,c* a

Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China b

State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology, Dalian 116024, China c

Beijing Computational Science Research Center, Beijing 100094, China

Abstract Composites of transition metal and carbon based materials are promising bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and are widely used in rechargeable metal-air batteries. However, the mechanism of their enhanced bi-catalytic activities remains elusive. Herein we construct N-doped graphene supported by Co(111) and Fe(110) substrates as bifunctional catalysts for ORR and OER in alkaline media. First-principles calculations show that these heterostructures possess a large number of active sites for ORR and OER with overpotentials comparable to those of noble metal benchmark catalysts. The catalytic activity is modulated by the coupling strength between graphene and metal substrates, as well as the charge distribution in the graphitic sheet delicately mediated by N dopants. These theoretical results uncover the key parameters that govern the bi-catalytic properties of hybrid materials, and help prescribe the principles for designing multifunctional electrocatalysts of high performance. Keywords: oxygen reduction, oxygen evolution, overpotential, rechargeable metal-air battery, graphene, transition metal

*

Corresponding author. Email: [email protected] 1

ACS Paragon Plus Environment


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1. Introduction Rechargeable metal-air battery is one of the most promising energy conversion devices to replace the matured Li-ion battery owing to their higher energy density and lower cost.1-4 So far, Li-air battery outperforms any other rechargeable batteries with theoretical specific energy density of 3500 Wh/kg, about 10 times of that of Li-ion batteries (387 Wh/kg).5 On the cathode of a metal-air battery, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occur during the discharging and charging processes, respectively. The sluggish kinetics of both reactions is the major source of cell efficiency loss. Thus, developing highly active, durable and affordable bifunctional catalysts is crucial for commercialization of this renewable energy technology. Potential bifunctional catalysts are mainly categorized into the non-precious metal based materials, i.e., transition metal oxides and derivatives, and carbon based materials such as heteroatom-doped graphitic carbon nanostructures.6-8 However, their practical applications are impeded by the unsatisfactory ORR activity and poor electrical conductivity of most transition metal oxides, as well as by the lack of OER activity and instability during the oxidative OER conditions for carbon based catalysts.7 Taking the advantage of synergic effects, the combination of transition metal and carbon based materials may give rise to enhanced bi-catalytic activity and durability than the individual components.7,8 A variety of such hybrid catalysts have been synthesized, for instance, Co3O4 nanocrystals supported by N-doped graphene,9 noble metal nanoparticles (Pd, Ru, Pt, Au) encapsulated by carbon nanotubes,10 and spinel FeCo2O4 nanoparticles inside hollow structured reduced graphene oxide.11 Their performance can be comparable to that of the noble metal benchmark catalysts, i.e. Pt and IrO2 for ORR and OER, respectively. Moreover, the large surface area of carbon materials help optimize the morphologies and distributions of discharge product (metal oxides), which alleviates the electrode polarization problem and facilitates the OER process.10 On the other side, the mechanisms for ORR and OER catalysis have been intensively investigated from both experimental and theoretical aspects.12-18 It is 2


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generally accepted that ORR proceeds through sequential electron-transfer processes in which gaseous O2 molecule dissociates into oxygenated intermediates, and the opposite processes apply for OER.19 Using an ingenious sophisticated model, Nøskov et al. established volcano relations of ORR and OER activities with the oxygen binding energies for various transition metals and their oxides.20-23 The relative stabilities of intermediates on catalysts and the involved reaction barriers dictate the overall reaction rate. Furthermore, Nøskov et al. suggested that the oxygen–metal bond strength is determined by the occupancy of antibonding state formed between the adsorbates valence state and transition metal d state.24-26 This so-called “d-band model” offers a direct way to engineer the surface reactivity of transition metals via their d band center and has been verified by a series of experiments.27,28 For graphene-based electrocatalysts, Qiao et al. compared the roles of B, N, O, P and S doping on the ORR activity in experiment.29 Their observed higher activity for B- and N-doped graphene; the B atoms and the C atoms bonded to N dopants serve as the active sites, respectively. Dai et al. synthesized N and P co-doped carbon foams as bifunctional catalysts for ORR and OER; the C atoms close to both N and P atoms at the edges of graphitic sheet are active for OER catalysis.30 The catalytic activity can be further related to charge density,31,32 spin density,33,34 and valence orbital levels of C atoms in the graphitic sheet.29 Despite of the aforementioned successes, the catalytic mechanisms for bifunctional hybrid catalysts remains elusive. How to understand the synergic effect from the atomic level? How does the catalytic activity of a hybrid system correlate with the electronic band structure? How do the N atoms of different configurations in carbon materials affect the catalytic properties? These obstacles hinder the rational composition of hybrid materials as efficient bifunctional catalysts. To this end, herein we propose to use N-doped graphene supported by transition metals — Co(111) and Fe(110) — as bifunctional catalysts for ORR and OER in alkaline media. By first-principles calculations, the overpotentials at standard conditions are predicted, and the rate-limit steps and active sites are identified. The electronic density of states and charge distributions are analyzed to uncover the key parameters that govern the 3



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catalytic activity of the graphene−transition metal heterostructures, and to clarify the roles of graphitic and pyridinic N dopants on ORR and OER catalysis. These theoretical results provide vital insights into the mechanism of the enhanced bi-catalytic

activities

of

hybrid

materials,

and

help

screen

non-precious

multifunctional electrocatalysts of high performance.

2. Methods Spin-polarized density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP).35 We adopted the planewave basis with energy cutoff of 500 eV, the projector augmented wave (PAW) potentials,36 and the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) for the exchange-correlation functional.37 For the transition metal substrates, we considered the Co(111) surface of fcc phase and Fe(110) surface of bcc phase, respectively, both of which are commonly used for the growth of graphene in experiment.38,39 To model graphene on the metal surfaces, we used a supercell consisting of 6 × 6 graphene unit cells and a three-layer slab for the metal substrates with a vacuum region of 16 Å in the vertical direction. A 6 × 6 unit cells was used for Co(111) with lattice mismatch of 1.99%. For Fe(110) substrate, we adopted a supercell with lattice vectors of b1 = 5a1 − a2 and b2 = −a1 + 6a2 (a1, a2 are the primitive vectors) giving mismatch for the lattice parameters and included angle below 3.39%. In both cases, the in-plane lattice of metal substrates was slightly deformed to fit that of graphene. Within the simulation supercell, one graphitic N atom and three pyridinic N atoms were then substituted into the graphene sheet, corresponding to N:C ratio of 5.97%, a typical doping level for N-doped graphitic carbon catalysts.40,41 The Brillouin zone of each supercell was sampled by 2 × 2 × 1 uniform k point mesh. The model structures, hereafter denoted as NG−Co(111) and NG−Fe(110), with fixed supercell were optimized by only ionic and electronic degrees of freedom using thresholds for the total energy of 10−4 eV and force of 0.02 eV/Å. During the structural relaxation, the bottom layer metal atoms were constrained to mimic a 4


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semi-infinite metal solid. The Grimme’s semiempirical DFT-D3 scheme of dispersion correction was adopted to describe the van der Waals (vdW) interactions in these layered systems.42 Based on the equilibrium configurations from VASP calculations, Mulliken charge analysis43 was performed by CASTEP code,44 using the planewave basis with energy cutoff of 1000 eV and norm-conserving pseudopotentials. The overpotentials that characterize the catalytic activities for ORR and OER at various sites were calculated by the standard hydrogen electrode (SHE) method developed by Nøskov et al.22 Specifically, we considered the four-electron reaction pathway for ORR in alkaline media, which is the dominant mechanism for N-doped graphene catalysis:15,30 * + O2 (g) + H2O (l) + e− → OOH* + OH−

(1)

OOH* + e− → O* + OH−

(2)

O* + H2O (l) + e− → OH* + OH−

(3)

OH* + e− → * + OH−

(4)

where * represents an adsorption site on the catalyst surface; OOH*, O* and OH* are the oxygenated intermediates; (g) and (l) indicate the gas phase and liquid phase, respectively. Thus, the overall ORR reaction is: O2 (g) + 2H2O (l) + 4e− → 4OH−

(5)

For each step of ORR, the Gibbs free energy of formation is given by ∆G = ∆EDFT + ∆ZPE – T∆S – eU

(6)

where ∆EDFT, ∆ZPE and ∆S are the changes of DFT total energy, zero-point energy, and entropy from the initial state to the final state, respectively; T is temperature; U is the electrode potential; e is the charge transfer. ∆ZPE and ∆S can be obtained by the NIST-JANAF thermodynamics table for gaseous molecules45 and by calculating the vibrational frequencies for the oxygenated intermediates (see Table S1 of Supporting Information), respectively. Within the SHE method, the difference of Gibbs free energies of OH− and e− can be related to those of H2 and H2O via22,30 OH− – e− → H2O – 1/2H2 G(OH−) – G(e−) = G(H2O) – 1/2G(H2) + kBT·ln10·PH 5


(7)


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where the last term accounts for a correction to the Gibbs free energy of OH− anion at a certain PH value; kB is the Boltzmann constant. Here we consider PH = 14 and T = 298 K. It is easy to show that the calculated overpotential is independent on the choice of PH value.46 Based on eq 6 and eq 7, to obtain ∆G for each ORR step is actually a matter of calculation of DFT total energy (or binding energy) of oxygenated intermediates adsorbed on various sites of the catalyst surface. The ORR overpotential is then determined by ηORR = ∆Gmax/e + U0

(8)

where ∆Gmax is the maximum Gibbs free energy of formation of the four reaction steps given by eq 1-4; U0 = 0.40 V is the equilibrium potential for PH = 14 at T = 298 K and it gives a zero Gibbs free energy of formation for the overall reaction by eq 5. The OER in alkaline media follows the opposite processes from eq 4 to eq 1, and the corresponding overpotential equals to ηOER = ∆Gmax/e – U0

(9)

The binding energies of oxygenated intermediates are all referred to the energies of H2 and H2O as follows: ∆EOH* = E(OH*) – E(*) – [E(H2O) – 1/2E(H2)]

(10)

∆EOOH* = E(OOH*) – E(*) – [2E(H2O) – 3/2E(H2)]

(11)

∆EO* = E(O*) – E(*) – [E(H2O) – E(H2)]

(12)

where E(*), E(OH*), E(OOH*) and E(O*) are the DFT total energies of a clean catalyst surface, and that adsorbed by a OH*, OOH* and O* species, respectively; E(H2O) and E(H2) are the energies of a H2O and H2 molecule in vacuum, respectively. The Gibbs free energy of O2 is derived from H2O →1/2O2 + H2 using the experimental reaction energy of 2.46 eV to avoid the well-known errors by DFT calculation in describing the high-spin ground state of O2 molecule.22 As a remark, the SHE method only considers the potential energy differences between each elemental step, without taking the activation energies into account. Nevertheless, the theoretical overpotential is a prerequisite parameter that characterizes the electrocatalytic properties of materials. To date, the SHE method has successfully predicted the trends of activities for a variety of catalytic materials.23,29 6


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3. Results and discussions 3.1. N-doped graphene on Co(111) Graphene and Co(111) surface have commensurate in-plane lattice structure with experimental lattice parameters of 2.46 and 2.51 Å, respectively.47,48 Previous experimental studies revealed that graphene exhibits an on-top registry with respect to the Co substrate: one of the two C atoms in the graphene unit cell sits directly above the underlying Co atom, while the other C atom is located on the hollow site of the substrate.39,47,49-51 Thus, we consider two models with the N dopants on the top site and hollow site, respectively, as shown in Figure 1 and Figure S1a of Supporting Information; and we will demonstrate later that they have similar catalytic properties. We also tested models with the C(N) atoms lying on the intermediate positions relative to the Co substrate as the initial configurations; however, after structure optimization, the C(N) atoms return to the top site or hollow site, consistent with the experimental observations. The two models we consider both have interlayer distance of 2.17 Å between graphene and Co(111) surface, with interfacial binding energy of −0.10 eV per C atom. In the heterostructure, the graphene sheet keeps the planar honeycomb structure with vertical buckling height of only 0.03 Å. The differential charge density shows prominent electron accumulation between the top site C (or N) atoms and the underneath Co atoms, signifying the formation of appreciable C–Co bond (see Figure S2a of Supporting Information). Figure 2 displays the binding energies of oxygenated intermediates on various sites of the two model structures. The binding strength of OH* and OOH* species shows a scaling relation of ∆EOH* = ∆EOOH* – 3.18 eV very similar to that observed for transition metals (oxides).21,23 For O* species, the binding energies follow a linear relation of ∆EOH* = 0.26∆EO* + 0.09 eV with reduced slope compared to the value of 0.5 of transition metals (oxides).23 This is ascribed to the inhomogeneity of surface C atoms in NG–Co(111), that is, the adsorbed O* species involves two C–O bonds with one C atom on the top site and the other on the hollow site, respectively, resulting in different bonding characters. Thanks to that, it is possible to eliminate the scaling 7



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relations between oxygenated intermediates and achieve the optimum oxygen binding for the highest catalytic activity, as will be discussed below. Figure 3a shows the volcano plot of ORR overpotentials using ∆EOOH* as the descriptor. The scaling constant ∆EOOH* – ∆EOH* = 3.18 eV gives a lower limit of 0.38 V for the ORR overpotential and the corresponding optimum oxygen binding is ∆EOOH* = 3.74 eV, as indicated by the black dashed lines in Figure 3a (the details are provided in Supporting Information S3). Above this optimum binding energy, i.e. the weak binding regime, the ORR process is limited by the dissociation of O2 molecule into OOH* species given by eq 1; the overpotential increases with the OOH* binding strength. In the strong binding regime with ∆EOOH* < 3.74 eV, the formation of OH– anion from OH* species given by eq 4 limits the reaction rate, and the overpotential decreases as the OOH* binding strength increases. In our NG–Co(111) heterostructures, the ORR overpotential highly relies on the environment of C atoms as demonstrated by Table 1. As revealed by Figure 1a, we classify surface C atoms into three types: (1) the C atoms bonded to graphitic or pyridinic N dopants (hereafter denoted as “C–N1” and “C–N2”, respectively); (2) the C atoms on the hollow site in the para-positions of graphitic or pyridinic N dopants (hereafter denoted as “H–N1” and “H–N2”, respectively) or far from N (hereafter denoted as “H”); (3) the C atoms on the top site (hereafter denoted as “T”). The lowest overpotentials are achieved on the “C–N2” site and “H–N1” site with ηORR = 0.41 and 0.48 V, respectively, comparable to the value of 0.45 V for Pt(111) calculated by the SHE method.20 These two C sites provide moderate oxygen binding with ∆EOOH* = 3.77 and 3.84 eV, respectively, slightly weaker than the optimum binding strength of ∆EOOH* = 3.74 eV; thus the rate limit step is the formation of OOH* species from O2 molecule. The “H–N2” and “H” sites have weaker OOH* binding (∆EOOH* = 4.00 eV) and larger overpotentials of ηORR = 0.64 V. For the “T” site either close to or far from N atoms, the OOH* binding is rather weak (∆EOOH* = 4.41 eV), giving poor ORR activities with ηORR ≥ 0.78 V. In contrast, the “C–N1” site provide too strong oxygen binding with ∆EOOH* = 3.54 eV and large ηORR = 0.81 V; the ORR process is limited by the dissociation of OH* species to the formation of OH– ions. 8


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For comparison, we also calculate the ORR overpotentials for freestanding N-doped graphene. The graphitic basal plane shows much weaker oxygen binding and larger ORR overpotentials (see Table 1 and Supporting Information S4). The most active site is the C atom bonded to graphitic N (denoted as “C–N1”) having ∆EOOH* = 4.60 eV and ηORR =1.24 V, even inferior to the top site C atoms in NG–Co(111). Clearly, hybridizing graphene and transition metals can significantly enhance the oxygen binding strength of the surface C atoms and improve the catalytic activities for ORR. For OER, the scaling constant also gives a lower limit of 0.38 V for the overpotential with optimum oxygen binding of ∆EOOH* – ∆EO* = 1.30 eV, as illustrated by Figure 3b (the details are provided in Supporting Information S3). In the energy regime with ∆EOOH* – ∆EO* > 1.30 eV, the OOH* binding is too weak with regard to the O* binding. The formation of OOH* from O* species (the opposite process of eq 2) limits the reaction rate, and the overpotential increases with (∆EOOH* – ∆EO*). When the OOH* (or OH*) binding is too strong relative to O* species (∆EOOH* – ∆EO* < 1.30 eV), formation of OH* from O* species (the opposite process of eq 3) becomes the rate limit step, and the overpotential decreases as (∆EOOH* – ∆EO*) increases. In our NG–Co(111) heterostructures, the optimum oxygen binding can be achieved on the “H–N2” and “H–N1” sites with ηOER = 0.38 and 0.39 V, respectively, even lower than the value of 0.42 V for RuO2 evaluated by the SHE method.23 The “C–N2” and “H” sites lie on different shoulders of the activity volcano, giving ∆EOOH* – ∆EO* = 1.18 and 1.37 eV and larger overpotentials of ηOER = 0.41 and 0.44 V, respectively. For the “C–N1” and “T” sites, the OOH* binding is too weak compared to O* species, resulting in poor OER activities with ηOER ≥ 0.94 V. In comparison, freestanding N-doped graphene has the lowest ηOER = 0.56 V, obtained from the C atoms in the meta-positions of two pyridinic N dopants (denoted as “Cm– 2N2”). The other C sites provide even weaker OOH* binding relative to O* species and endow large overpotentials of ηOER ≥ 0.80 V (see Supporting Information S4). Note that another NG–Co(111) model with N atoms in different relative positions exhibits ORR and OER activities very similar to those of the model in Figure 1a, 9



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revealing that the NG–Co(111) heterostructures are inherently active for catalysis of oxygen reactions. The detailed results can be found in Supporting Information S5.

3.2. N-doped graphene on Fe(110) Fe(110) surface has a rhombic lattice with lattice constant of 2.48 Å and included angle of 109.47º from experimental measurement,38 which is not commensurate with the graphene lattice. As a result, the constructed NG–Fe(110) heterostructure exhibits a more complicated surface structure than that of NG–Co(111), that is, the C and N atoms can be in principle in any position with respect to the underlying Fe atoms. Regarding this issue, we consider two NG–Fe(110) models with the N dopants in different relative positions of the Fe substrate (see Figure 1b and Figure S1b of Supporting Information), and we will demonstrate later that the catalytic properties of these heterostructures are not model sensitive. Both models have interlayer spacing of 2.10 Å and interfacial binding energy of −0.16 eV per C atom, signifying stronger interaction between graphene and the Fe substrate than that of NG–Co(111). The graphene sheet shows slightly larger vertical buckling height of 0.09 Å and the C atoms form noticeable chemical bonds with the Fe atoms underneath (see Figure S2b of Supporting Information for a plot of differential charge density). Similar to that of NG–Co(111), the binding energies of OH* and OOH* species follow a scaling relation as displayed by Figure 2a. However, the entire linear curve for NG–Fe(110) shifts to the lower energy values. This can be understood by the fact that Fe (3d64s2) is chemically more reactive than Co (3d74s2). As demonstrated by the interfacial binding energy and differential charge density, the Fe substrate induces stronger interactions with the adsorbed graphitic sheet, leading to enhanced oxygen binding strength compared to that of the NG–Co(111) systems. On the other aspect, the O* binding energy is correlated with that of OH* species via ∆EOH* = 0.44∆EO* – 0.50 eV. The slope close to 0.5 may be attributed to the incommensurate lattice of graphene and Fe metal that gives rise to a relatively homogenous C surface in terms of oxygen bonding strength. According to the above discussions, the differences in both crystal structures and chemical reactivity between Fe and Co substrates may 10


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have distinct impacts on the catalytic properties of the graphene–transition metal heterostructures. These results give insightful implication to optimize the performance of hybrid catalysts by selecting proper metal substrates. Figure 4a shows the volcano plot of ORR overpotentials as a function of ∆EOOH* for the two NG–Fe(110) models. A large portion of data points fall into the strong binding regime (∆EOOH* < 3.74 eV) attributed to the enhanced reactivity of the graphitic sheet by Fe substrate. The overpotential varies in a wide range depending on the relative positions of C atoms. To help identify the active sites, we categorize the C atoms into four types (see Figure 1b): (1) the C atoms bonded to N dopants (hereafter denoted as “C–N”); (2) the C atoms close to the hollow site in the meta-/para-position of N or far from N atoms (hereafter denoted as “H–N” and “H”, respectively); (3) the C atoms in the intermediate positions (hereafter denoted as “M”); (4) the C atoms close to the top site (hereafter denoted as “T”). As revealed by Table 2, the “M” sites provide moderate oxygen binding (∆EOOH* = 3.61~3.74 eV) and exhibit high activities for ORR with ηORR = 0.38~0.48 V. In particular, optimum binding strength (∆EOOH* = 3.74 eV) can be achieved on the “M” site C atom in the para-position of pyridinic N atoms, attaining the lower limit ηORR = 0.38 V. For the “T” sites, the oxygen binding strength is weaker (∆EOOH* = 3.80~4.08 eV); the ORR process is limited by the formation of OOH* species, yielding larger ηORR = 0.44~0.72 V. On the contrary, the “H” and “C–N” sites provide too strong oxygen binding with ∆EOOH* = 3.35~3.61 and 3.07~3.27 eV, respectively. Desorption of OH* to OH– anion becomes difficult on those sites, resulting in lower activities with ηORR = 0.47~0.78 and 0.88~1.04 V, respectively. For OER, the “M” and “H” sites have moderate binding strength of OOH* relative to O* species, and are close to the top of activity volcano with ηOER = 0.37~0.71 V, as shown in Figure 4b. The optimum binding can be achieved on the “M” site C atom in the para-position of pyridinic N with ηOER = 0.37 V and the “H” site C atom in the para-position of graphitic N with ηOER = 0.39 V, respectively. The former reaction site even breaks the lower limit constrained by the scaling constant, thanks to the roughness of the graphitic surface hybridized with the Fe substrate. For the “T” 11



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and “C–N” sites, the OOH* binding is too weak compared to O* species; thus formation of OOH* from O* is difficult, giving poor OER activities with ηOER ≥ 0.63 V. For another NG–Fe(110) model with N dopants in different relative positions, the catalytic properties for ORR and OER are very similar to those of the model in Figure 1b (see Supporting Information S6). The origin of overpotential can be better understood from the free energy diagrams at the equilibrium potential U0 = 0.40 V as shown in Figure 5. For an ideal catalyst, the Gibbs free energy does not change throughout the ORR (OER) steps, i.e. the oxygen reaction can occur spontaneously from the thermodynamic point of view and overpotential is zero. For our NG–Co(111) and NG–Fe(110) heterostructures, the most active C sites show uphill at the 1st and 4th steps of ORR, both having larger free energy gain at the 1st step (rate limit step) equal to 0.41 and 0.38 eV, resulting in overpotentials of 0.41 and 0.38 V, respectively. For the OER process, the 2nd and 3rd steps are uphill; the 3rd step (rate limit step) involves larger free energy gain of 0.37 and 0.38 eV for NG–Co(111) and NG–Fe(110), giving overpotentials of 0.37 and 0.38 V, respectively.

3.3. Electronic origin of catalytic activity The mechanism of catalytic activity of the graphene–transition metal heterostructures is illustrated by Figure 6. Density of states (DOS) reveals that both NG–Co(111) and NG–Fe(110) exhibit metallic behaviors and the graphitic sheet is strongly hybridized with the metal substrates (see Figure 6a,b). Such electron coupling entirely destructs the π conjugation of the pristine graphene. For NG– Co(111), the C atoms on the top site form C–Co bonds with the underlying Co atoms by overlapping their partially filled pz orbital and d orbital (see Figure 6c). Mulliken charge analysis suggests that the top layer Co atoms lose 0.26 e each to the graphitic sheet — about 0.15 e and 0.10 e for top site and hollow site C atoms, respectively. The hollow site C with partially filled pz orbital is thus highly reactive and can provide strong binding with oxygenate intermediates. The top site C atoms involved in the p-d hybridization, which are still partially occupied, can retain some catalytic 12


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activity. For NG–Fe(110), the d band center of Fe substrate is higher than Co metal,26 resulting in stronger coupling with the C pz orbital and more reactive graphitic surface. In comparison, freestanding N-doped graphene shows finite DOS at the Fermi level, however, these states are localized around the N atoms in contrast to the delocalized electrons in the graphene–transition metal heterostructures (see Supporting Information S7). Hence, most C atoms on the basal plane keeping the sp2 configuration are still inactive; only the C atoms bonded or close to N dopants show some catalytic activity. In addition, the N atoms in the graphene–transition metal heterostructures induce further in-plane charge redistribution, which delicately mediates the catalytic activities of C atoms in various local environments. For both NG–Co(111) and NG– Fe(110), the oxygen binding strength is correlated to the occupancy of the C pz orbital, i.e. the OOH* binding energy increases with the partial charge on the C atom as demonstrated by Figure 6d. Generally speaking, the C atoms bonded to N are positively charged, while the other types of C atoms are negatively charged with partial charge density following the order: “H–N” site < “H” site (< “M” site) < “T” site (see Table 1,2). In NG–Co(111), the “H–N” sites with less charge can provide moderate oxygen binding for ORR catalysis, while for NG–Fe(110) with a more reactive surface, the “M” sites carrying more charge are active sites for ORR. It is the coupling strength between the graphitic sheet and the metal substrates as well as the surface charge distribution that determine the trends of oxygen binding energies and catalytic activities shown in Figure 3,4. The N dopants in the graphitic and pyridinic forms can induce different charge redistributions due to their different chemical natures (see Table 1). They both play important roles on creation of active sites for ORR and OER. In particular, the N-doping results in diverse environments for the C atoms, which is beneficial for comprising the binding strength between OOH* and O* species and gives rise to OER activity. Our proposed graphene−transition metal heterostructures possess a large number of active sites — at least 10% of the C atoms have both ORR and OER overpotentials below 0.5 V. These C atoms are located on the graphene basal plane and do not rely 13



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on the edges. The catalytic performance of these heterostructures can be further improved with the guidance of the computed activity volcanos. For instance, surface reactivity between that of NG–Co(111) and NG–Fe(110) may be achieved by using Fe−Co or Fe−Ni alloys as the substrate, which may offer more adsorption sites that close to the top of the volcano. The in-plane charge distribution can be fine-tuned by doping other heteroatoms into the graphitic sheet, allowing the modulation of oxygen binding strength in a wide range. Similar results were reported for graphene on Cu(111) surface proposed as the cathode of rechargeable Li-air batteries.52 It was revealed that the Cu substrate greatly enhances the adsorption of discharge products (Li2O and Li2O2) on the graphene sheet and reduces the overpotentials for both ORR and OER. Our studies, consistent with these results, suggest that compositing graphitic carbon materials with proper transition metals is an effective strategy for tuning the surface activity of hybrid systems. Furthermore, we unveil the origin of activity of hybrid catalysts from the viewpoint of interlayer and intralayer charge distributions. Previous studies showed that the activity of transition metal and carbon based catalysts can also be related to the band edge of the material and frontier orbital levels of the active centers.29,53 Actually, the electron densities and frontier orbitals are closely correlated with each other. These key parameters govern the activity of materials and are to be considered in the design of catalysts for ORR and OER by band structure engineering.

4. Conclusion We construct N-doped graphene supported by Co(111) and Fe(110) substrates as bifunctional electrocatalysts for ORR and OER in alkaline media. First-principles calculations show that these heterostructures possess a large number of active sites for ORR and OER with theoretical overpotentials comparable to or even lower than those of noble metal benchmark catalysts. These models carry structural features and N doping concentrations that commonly observed in experiment. Moreover, the catalytic properties of these composites do not rely on specific locations of N atoms with respect to the metal substrate, and thus they are inherently active for ORR and OER 14


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catalysis. The catalytic activity originates from the electron coupling between graphene and transition metals, which destructs the π conjugation and significantly enhances the oxygen binding strength of the graphitic sheet. The N-doping further induces charge redistribution that delicately mediates the catalytic activities of the graphitic surface. Both pyridinic and graphitic N atoms play pivot roles by creating diverse environments for C atoms and offering active sites for ORR and OER catalysis. Our theoretical results provide vital insights into the mechanism of enhanced bi-catalytic activities of the hybrids of graphitic carbon and transition metal based materials, and help prescribe the principles for compositing hybrid materials as efficient multifunctional electrocatalysts.

ASSOCIATED CONTENTS Supporting Information Zero-point energy and entropy correction (Table S1), model structures of NG–Co(111) and NG–Fe(110) and differential charge densities (Figure S1, S2), scaling relation of oxygen binding energies, overpotentials, detailed structures of oxygen intermediates and band decomposed charge densities of freestanding N-doped graphene, NG– Co(111) and NG–Fe(110) (Table S2-S4, Figure S3-S7), coordinates of all the models

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11504041, 11574040), the China Postdoctoral Science Foundation (2015M570243, 2016T90216), the Fundamental Research Funds for the Central Universities of China (DUT16-LAB01), and the Supercomputing Center of Dalian University of Technology.

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Table 1. Binding energies of oxygenated intermediates (∆EOH*, ∆EOOH* and ∆EO*), ORR and OER overpotentials (ηORR and ηOER) and partial charge for various types of C sites in N-doped graphene on Co(111) substrate as shown in Figure 1a. The values of selected C sites in freestanding N-doped graphene (denoted as “free”) are also listed for comparison. The atomic structures of oxygenated intermediates are shown in Supporting Information S4,5. site

∆EOH* (eV)

∆EOOH* (eV)

∆EO* (eV)

ηORR (V)

ηOER (V)

charge (e)

1 (C−N1)

0.37

3.59

1.11

0.81

1.56

0.06

1 (C−N2)

0.63

3.77

2.59

0.41

0.41

0.03

2 (H−N1)

0.68

3.84

2.53

0.48

0.39

−0.09

2 (H−N2)

0.78

4.00

2.69

0.64

0.38

−0.10

2 (H)

0.83

4.00

2.63

0.64

0.44

−0.11

3 (T)

1.21

4.41

2.56

1.05

0.94

−0.15

free (C−N1)

1.34

4.60

2.88

1.24

0.80

0.15

free (Cm−2N2)

1.45

4.63

3.17

1.27

0.56

−0.02

free (C−C)

2.28

5.44

3.37

2.07

1.39

0

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Page 20 of 27

Table 2. Binding energies of oxygenated intermediates (∆EOH*, ∆EOOH* and ∆EO*), ORR and OER overpotentials (ηORR and ηOER) and partial charge for various types of C sites in N-doped graphene on Fe(110) substrate as shown in Figure 1b. The values of specific C sites and corresponding atomic structures of oxygenated intermediates are shown in Supporting Information S6. site

∆EOH* (eV)

∆EOOH* (eV)

∆EO* (eV)

ηORR (V)

ηOER (V)

charge (e)

1 (C−N)

−0.15~0.01

3.07~3.27

0.66~1.61

0.88~1.04

0.74~1.53

0.02~0.04

2 (H)

0.11~0.42

3.35~3.61

1.81~2.30

0.47~0.78

0.39~0.62

−0.12~−0.10

3 (M)

0.41~0.56

3.61~3.74

2.04~2.36

0.38~0.48

0.37~0.71

−0.14~−0.13

4 (T)

0.59~0.89

3.80~4.08

1.74~3.10

0.44~0.72

0.63~1.20

−0.16~−0.15

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Figure 1. Model structures of N-doped graphene on (a) Co(111) and (b) Fe(110) substrates. The C, N, Co and Fe atoms are shown in silver, blue, green and pink colors, respectively. The bottom layers of Co and Fe atoms are displayed in medium and dark blue, respectively. The red and cyan numbers indicate various types of C sites.

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Figure 2. Binding energies of (a) OH* vs. OOH* and (b) OH* vs. O* species on various sites of N-doped graphene on Co(111) (red circles) and Fe(110) substrate (blue squares), respectively. The dashed lines show the linear fitting of the data points.

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Figure 3. Volcano plots of (a) ORR overpotential vs. binding energy of OOH* and (b) OER overpotential vs. binding energy difference between OOH* and O* species on various sites of N-doped graphene on Co(111) substrate in the alkaline media. The blue, red and green symbols are the data points calculated from the type 1, 2 and 3 C sites as illustrated in Figure 1a, respectively. The colored backgrounds indicate the binding energy regimes with different rate limit steps. The atomic structures (top and side views) and adsorption energies of oxygenated intermediates having the lowest overpotentials are shown below each volcano plot. The H, C, N, O and Co atoms are shown in white, silver, blue, red and green colors, respectively. Only the topmost layer of Co atoms in the metal substrate are displayed for clear view.

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Figure 4. Volcano plots of (a) ORR overpotential vs. binding energy of OOH* and (b) OER overpotential vs. binding energy difference between OOH* and O* species on various sites of N-doped graphene on Fe(110) substrate in the alkaline media. The blue, red, light blue and green symbols are the data points calculated from the type 1, 2, 3 and 4 C sites as illustrated in Figure 1b, respectively. The colored backgrounds indicate the binding energy regimes with different rate limit steps. The atomic structures (top and side views) and adsorption energies of oxygenated intermediates having the lowest overpotentials are shown below each volcano plot. The H, C, N, O and Fe atoms are shown in white, silver, blue, red and pink colors, respectively. Only the topmost layer of Fe atoms in the metal substrate are displayed for clear view.

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Figure 5. Free energy diagrams of (a) ORR and (b) OER in the alkaline media (PH = 14 and T = 298 K) at the equilibrium potential (U0 = 0.402 V) for ideal catalyst (black dashed lines), and the most active sites of N-doped graphene on Co(111) (red solid lines) and Fe(110) substrate (blue solid lines). The colored arrows and the numbers next to them indicate the rate limit steps and the overpotentials for the latter two systems, respectively.

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Figure 6. (a) Spin-polarized density of states (DOS) of N-doped graphene on Co(111) (top panel) and Fe(110) substrate (bottom panel). The colored solid lines show the total DOS and projected DOS from s, p, and d orbitals, respectively. The black dashed lines indicate the d band center for each system. (b) DOS from the pz orbital of all C atoms in N-doped graphene without substrate, and that on the Co(111) and Fe(110) substrate. (c) Schematic illustration of charge transfer (CT) in the graphene/transition metal heterostructure. (d) Partial charge on various C sites vs. binding energy of OOH* species for N-doped graphene on Co(111) and Fe(110) substrates. The black solid lines are guide for eyes.

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Graphitic Abstract

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Nitrogen-Doped Graphene on Transition Metal Substrates as Efficient

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