Journal of Catalysis 314 (2014) 66–72
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
N-doped graphene as catalysts for oxygen reduction and
oxygen evolution reactions: Theoretical considerations
Mingtao Li a,b, Lipeng Zhang a, Quan Xu a, Jianbing Niu a, Zhenhai Xia a,c,⇑
a
Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China
c
Department of Chemistry, University of North Texas, Denton, TX 76203, USA
b
a r t i c l e
i n f o
Article history:
Received 7 December 2013
Revised 11 March 2014
Accepted 19 March 2014
Available online 26 April 2014
Keywords:
N-doped graphene
Oxygen evolution reaction
Oxygen reduction reaction
Catalysts
Fuel cells
First principles calculation
a b s t r a c t
Electrocatalysts are essential to two key electrochemical reactions, oxygen evolution reaction (OER) and
oxygen reduction reaction (ORR) in renewable energy conversion and storage technologies such as regenerative fuel cells and rechargeable metal–air batteries. Here, we explored N-doped graphene as costeffective electrocatalysts for these key reactions by employing density functional theory (DFT). The
results show that the substitution of carbon at graphene edge by nitrogen results in the best performance
in terms of overpotentials. For armchair nanoribbons, the lowest OER and ORR overpotentials were estimated to be 0.405 V and 0.445 V, respectively, which are comparable to those for Pt-containing catalysts.
OER and ORR with the minimum overpotentials can occur near the edge on the same structure but different sites. These calculations suggest that engineering the edge structures of the graphene can increase
the efficiency of the N-doped graphene as efficient OER/ORR electrocatalysts for metal–air batteries,
water splitting, and regenerative fuel cells.
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
The oxygen reduction reaction (ORR) and oxygen evolution
reaction (OER) play pivotal roles in various renewable energy conversion and storage technologies, such as fuel cells [1], direct solar
driven water splitting [2] hydrogen production from water electrolysis [3], rechargeable metal–air batteries [4], and regenerative
fuel cells [5]. The current bottleneck of fuel cells lies in the sluggish
ORR on the cathode side, while OER, a reverse reaction of ORR,
plays an important role in efficiency of energy storage such as
direct solar driven water splitting systems, Li–air batteries and
regenerative fuel cells. Extensive efforts have been devoted to the
development and understanding of electrocatalysts for OER/ORR.
To date, platinum and its alloy have been used as catalysts for
ORR [6] while platinum and metal oxides/hydroxides of iridium,
ruthenium, nickel and other metals for OER [7]. However, the
OER/ORR is sluggish, even facilitated by these catalysts. Furthermore, the limited resources and high cost of the platinum catalysts
has been shown to be the major ‘‘showstopper’’ to mass market
fuel cells for commercial applications. Even though the amount
of platinum needed for desired catalytic effect could be reduced,
⇑ Corresponding author at: Department of Materials Science and Engineering,
University of North Texas, Denton, TX 76203, USA. Email: [email protected]
E-mail address: [email protected] (Z. Xia).
http://dx.doi.org/10.1016/j.jcat.2014.03.011
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.
the commercial mass production will still require large amount
of platinum. Therefore, it is highly desirable to develop low-cost
OER and ORR catalysts. In particular, bifunctional OER/ORR catalysts are needed for rechargeable metal–air batteries and regenerative fuel cells.
Graphene, a two-dimensional one-atom-thick single layer of
graphite, has attracted intense scientific and technological interest
since its freestanding form was isolated in 2004 [8]. With its
unique properties such as huge surface area [9], high mobility of
charge carriers [10], excellent mechanical properties [11], and
superior thermal conductivity [12], graphene could be an excellent
materials candidate for energy conversion and storage such as batteries [13], supercapacitors [14], and fuel cells [15,16]. Nitrogendoped graphene has been demonstrated as an excellent catalyst
for electrochemical reactions, such as oxygen reduction [17,18]
and hydroperoxide oxidation [19,20] due to their unique electronic
properties derived from the conjugation between the nitrogen
lone-pair electrons and the graphene p system. N-doped graphene
as bifunctional catalysts for OER and ORR has also been demonstrated recently [21,22]. These carbon-based electrocatalysts have
the potential to replace noble metals for energy applications, particularly for Li–air batteries and regenerative fuel cells [15].
Understanding of the OER/ORR mechanisms of N-doped
graphene could provide design guidelines for materials and process development, and discovery of new catalysts. Many
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M. Li et al. / Journal of Catalysis 314 (2014) 66–72
of 50 meV. All calculations were spin polarized and were done until
the force of the system converged to about 0.02 eV/Å. The lattice a
was optimized previously by fitting the energy-volume relationship with Murnaghan Equation of State for further absorption
and reaction free energy calculations. All structures were fully
relaxed. Bader charge analysis was performed to calculate the
effective charge for these N-doped graphene nanoribbons [31].
The formation energy for a nitrogen dopant, Ef, was calculated
by
Ef ¼ ENGNR þ mlC ðEGNR þ mlN Þ
Fig. 1. (a) Armchair and (b) Zigzag N-doped graphene structures used in the
calculations. The numbers denote substitutional sites and reaction sites. Symbols a,
b, c, d, e, and f denote reaction sites apart from 1, 2, 3, 4, 5, and 6.
researchers have been devoted to theoretical studies to understand
the high ORR activity in N-doped carbon-based materials. Based on
DFT investigations, Okamoto suggested that the complete fourelectron ORR activation can be achieved with an adequate binding
energy of the oxygen atoms on multiple N-doped graphitic sites
[23]. Zhang et al. found that the ORR over N-doped graphene is a
four-electron pathway but pure graphene does not have such catalytic activities. They also concluded that the catalytic active sites
on the N-doped graphene depend on spin density distribution and
atomic charge distribution on the neighboring carbon atoms to the
doping sites [17,24]. Kim et al. proposed that the ORR activation
initiates at the outermost graphitic nitrogen site with the first
reduction step, the latter graphitic nitrogen becomes pyridinic-like
in the next reduction steps via the ring-opening of a cyclic C–N
bond [25]. However, to our knowledge, little work is done on single
OER or bifunctional OER/ORR over N-doped graphene.
In this work, we studied OER/ORR mechanism of N-doped armchair and zigzag graphene nanoribbons by using density functional
theory (DFT) calculations, in an effort to understand how ORR/OER
occur over N-doped graphene, and provide design principles how
the doping structure can be translated into enhanced performance
of electrocatalysts for renewable energy conversion and storage.
2. Computational models and methods
First-principle calculations were performed within the framework of density functional theory (DFT) as implemented in the
plane wave set Vienna ab initio Simulation Package (VASP) code
[26,27]. Nuclei–electron interactions were described by the projector augmented wave (PAW) pseudo-potentials [28,29], while the
electronic exchange and correlation effects were described within
the generalized gradient approximation (GGA) as parameterized
by Perdew, Burke, and Ernzerhof [30]. The C-2s22p2, O-2s22p4,
H-1s1 were treated as valence electrons.
All nanoribbons were modeled as three-dimensional periodic
structures, where a rectangle periodic box was represented with
a dashed line in Fig. 1 and vacuum layers were set around 14
and 18 Å in the y- and z-directions, respectively to avoid interaction between slabs. The k-point sampling of the Brillioun zone
was obtained using a 4 1 1 grid centered at the gamma (C)
point using Monkhorst Pack Scheme. The plane wave basis set with
a high cut off energy of 450 eV was used throughout the computations. The k point meshes and energy cut off were chosen to ensure
that the energies were converged within 1 meV/per atom. The
Fermi level was slightly broadened using a Fermi–Dirac smearing
ð1Þ
where ENGNR is the total energy of the supercell with N-doped
graphene nanoribbon, and EGNR is the total energy of the pristine
graphene nanoribbon. lC is the chemical potential of carbon defined
as the total energy of graphene per carbon atom. lN is the chemical
potential of N taken as one-half the total energy of N2 molecule. And
m is the number of nitrogen atoms in the model.
3. Results and discussion
We developed a series of models for graphene nanoribbons
with a single nitrogen dopant at different sites, as shown in
Fig. 1. For each N-doped structure, we consider all the sites at
which ORR/OER could occur. These models are labeled as Ax–y,
or Zx–y, where A and Z refer to armchair and zigzag graphene,
respectively, and symbols x and y denote substitutional sites of
nitrogen atoms and reaction sites of ORR/OER, respectively. Also,
we use x = h to denote pristine graphene nanoribbons. For example, Model A1–2 (Fig. 1a) represents an armchair graphene nanoribbon with a nitrogen dopant on site 1, and reactions occur on
site 2.
The location of nitrogen dopants is important to OER/ORR activities. In order to identify efficiently catalytic sites of N-doped
graphene for OER/ORR, we began with calculations of formation
energies Ef of nitrogen dopants. The calculated formation energy
and minimum C–N bond length are plotted as a function of the distance (de) from the dopants to the edge, as shown in Fig. 2. It can be
seen that the formation energy is relatively low when the dopant
locates at the edge, but it increases and becomes constant with
increasing the distance de. Thus, nitrogen atoms prefer to substitute the carbon atoms near the edge. Among all the models, Model
Z1 is the most stable one from the formation energy viewpoint,
which is consistent with the results reported in Ref. [32]. The minC—N
imum C–N bond length dmin also varies with regard to the distance
de. Besides, the N doping changes the C–C bond length due to larger
electronegativity of nitrogen atoms. From the general trends of the
formation energy and bond length, shown in Fig. 2, the changes in
C—N
the formation energy Ef and minimum C–N bond length dmin are
C—N
Fig. 2. Formation energy Ef and minimum C–N bond length dmin as a function of the
distance de from the nitrogen dopants to the edge of the graphene nanoribbons.
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M. Li et al. / Journal of Catalysis 314 (2014) 66–72
correlated with each other, and an edge effect exists within the
range of 2.5 Å from the edge. Such edge effect may have pronounced influence on catalytic activities of the N-doped graphene.
The ORR and OER activities on active sites of N-doped graphene
were studied in detail. In acidic environment, OER could occur over
N-doped graphene in the following four electron reaction paths,
adsorption free energy of O, OH and OOH. The absorption energies were calculated as follows [35],
H2 OðlÞ þ ! OH þ ðHþ þ e Þ
DEO ¼ EðO Þ EðÞ EH2 O EH2
ð2Þ
OH ! O þ ðHþ þ e Þ
ð3Þ
O þ H2 OðlÞ ! OOH þ ðHþ þ e Þ
ð4Þ
OOH ! þ O2 ðgÞ þ ðHþ þ e Þ
ð5Þ
where stands for an active site on the graphene surface, (l) and (g)
refer to gas and liquid phases, respectively, and O, OH and OOH
are adsorbed intermediates. The ORR can proceed incompletely
through a two-step two-electron pathway that reduces O2 to hydrogen peroxide, H2O2, or completely via a direct four-electron process
in which O2 is reduced directly to water, H2O, without involvement
of hydrogen peroxide. Here, we study the complete reduction cycle
because the previous results showed that the ORR proceeds on Ndoped graphene through the four-electron mechanism [17]. The
ORR mechanism is summarized using the following elementary
steps [17],
O2 þ ðHþ þ e Þ ! OOH
ð6Þ
OOH þ ðHþ þ e Þ ! O þ H2 OðlÞ
ð7aÞ
OOH þ ðHþ þ e Þ ! OH þ OH
ð7bÞ
O þ ðHþ þ e Þ ! OH
ð8aÞ
OH þ OH þ ðHþ þ e Þ ! OH þ H2 OðlÞ
ð8bÞ
OH þ ðHþ þ e Þ ! þ H2 OðlÞ
ð9Þ
There are two branching paths for the 2nd and 3rd steps, where
OOH is reduced to O and H2O in path (a) defined in Eq. (7a), or to
2OH in path (b) defined in Eq. (7b). Both paths lead to the same
final products in the third step as shown in Eqs. (8a) and (8b).
These reactions (6), (7a), (8a), and (9) for ORR are inversed from
the reactions (2)–(5) for OER.
In OER, the potential-determining steps can either be the formation of O from OH (Eq. (3)) or the transformation of O to OOH
(Eq. (4)) [33]. However, in ORR, it was reported that the rate determining steps can either be the adsorption of O2 as OOH (Eq. (6)) or
the desorption of OH as water (Eq. (9)) [33]. Here, we took reactions (2)–(5) to derive the thermochemistry of both OER and
ORR, because the reactions (6), (7a), (8a), and (9) are inversed from
reactions (2)–(5). The overpotentials of the ORR/OER processes can
be determined by examining the reaction free-energies of the different elementary steps. The thermochemistry of these electrochemical reactions was obtained by using DFT calculations in
conjunction with SHE model developed by Nørskov and co-workers [34,35]. This thermodynamic approach establishes a minimum
set of requirements for the reactions based on the binding of the
intermediates and the assumption that there are no extra barriers
from adsorption/dissociation of O2 or proton/electron transfer
reactions. In our calculations, the OER and ORR were analyzed
using intermediate species associated with one electron transfer
at a time, which is energetically more favorable than the simultaneous transfer of more than one electron.
In order to obtain the rate limiting step of OER and ORR on different sites for different model structures, we calculated the
DEOH ¼ EðOH Þ EðÞ EH2 O 1=2EH2
DEOOH ¼ EðOOH Þ EðÞ 2EH2 O 3=2EH2
ð10Þ
ð11Þ
ð12Þ
in which, E( ), E(OH ), E(O ), and E(OOH ) are the ground state energies of a clean surface and surfaces adsorbed with OH, O, and
OOH, respectively. EH2 O and EH2 are the calculated DFT energies of
H2O and H2 molecules in the gas phase using the approaches outlined by Nørskov et al. [35]. Also, we considered the ZPE and
entropy corrections here. These calculations transform DFT binding
energies, DEDFT
ads , into free energies of adsorption, DGads, by the following equation [35],
DGads ¼ DEDFT
ads þ DZPE T DS
ð13Þ
where T is the temperature and DS is the entropy change. For the
zero-point energy (ZPE), the vibrational frequencies of adsorbed
species (O, OH, and OOH) were calculated with the N-doped
graphene nanoribbons fixed to obtain ZPE contribution in the free
energy expression. Moreover, only vibration entropy contributions
were considered for adsorbates and total entropies for solvent molecules were taken from standard thermodynamic tables (see the
Supporting information). In present study, we did not use any solvent corrections to the adsorbed species.
Fig. 3(a) shows the adsorption free energies of OOH and OH for
various N-doped structures. The free energies of OOH are linearly
related to that of OH by y = x + 3.177 with a constant of approximate 3.177 eV, independent of the binding strength to the surface.
The slope of unity in the linear fit is motivated by the single bond
between O and the carbon of N-doped graphene nanoribbons for
both OH and OOH, which is very similar to that on the surface
of ABO3 perovskite [29]. The constant energy difference between
the binding energies of OH and OOH implies that there is a scaling
relation between OH and OOH. Fig. 3(b) shows the adsorption free
energies of OH and O for various N-doped structures. These is also
a scaling relation with a slope of 0.5 between OH and O except
some sites where oxygen atom is adsorbed in bridge-on mode, such
as A2–1, Z1–2, Z2–1 and Ah–1. Due to the scaling relation between
OH and OOH, the total reaction enthalpy differences for reaction
(3), (4), (7a), and (8a) should be a constant, which results in a lower
limit over potential of OER and ORR as described below.
For each step, the reaction free energy DG is defined as the difference between free energies of the initial and final states and is
given by the expression [36],
DG ¼ DE þ DZPE T DS þ DGU þ DGpH
ð14Þ
where DE is the reaction energy of reactant and product molecules
adsorbed on catalyst surface, obtained from DFT calculations,
DGU = eU, where U is the potential at the electrode, and e is the
charge transferred. DGpH is the correction of the H+ free energy by
the concentration dependence of the entropy:
DGpH ¼ kB T ln½Hþ ð15Þ
The free energy of reaction (2)–(5) can be calculated using Eq.
(14). For the OER reactions, Nørskov et al. developed a method to
determine the overpotentials [35],
GOER ¼ maxfDG1 ; DG2 ; DG3 ; DG4 g
ð16Þ
gOER ¼ GOER =e-1:23 V
ð17Þ
where DG1, DG2, DG3, and DG4 are the free energy of reaction (2)–
(5), respectively. An ideal catalyst should be able to facilitate water
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M. Li et al. / Journal of Catalysis 314 (2014) 66–72
(a)
(b)
Fig. 3. (a) Adsorption energies of OOH versus adsorption energies of OH and (b) adsorption energies of OH versus adsorption energies of O on different sites of armchair
and zigzag graphene nanoribbons.
oxidation just above the equilibrium potential, but requires all the
four charge transfer steps to have reaction free energies of the same
magnitude at zero potential (i.e., 4.92 eV/4 = 1.23 eV). This is equivalent to all the reaction free energies being zero at the equilibrium
potential, 1.23 V. Since there is a scaling relation between OH
and OOH, this set a constraint on DG2 and DG3, i.e., DG2 + DG3 =
DGOOH* DGOH* = 3.177 eV, resulting a lower limit of OER
overpotential. When DG2 = DG3 = (GOOH* GOH*)/2 = 1.589 eV, the
OER
overpotential
has
the
minimum
value,
gOER
limit ¼ 1:589—1:23 ¼ 0:359 V. For ORR, there is also a lower limit
of the overpotential. The overall free energy of reaction (2)–(5) is
4.92 eV, leading to DG1 + DG2 + DG3 + DG4 = 4.92 eV. Since DG2 + DG3 = constant, we derived that DG1 + DG4 = constant. When
DG1 = DG4 = 0.871 eV, the ORR overpotential has its minimum
value, gOER
limit ¼ 1:23—0:871 ¼ 0:359 V.
To derive the minimum overpotential in N-doped graphene systems, we calculated the overpotenials for different reaction sites on
different structures employing a descriptor DG0O DG0HO : Fig. 4(a)
shows the volcano plot, i.e., overpotential gOER versus the descriptor for various reaction sites on armchair and zigzag graphene
structures. From this theoretical analysis, Model A2–3 is identified
to have a minimum OER overpotential (gOER
min ¼ 0:405 V). For the
ORR activity, previous results showed that the pathway (6), (7a),
(8a), and (9) had close reaction enthypl distribution for each steps
[17], which would result in small overall ORR overpotentail. Using
similar methods described above, we calculated the overpotenials
of this ORR pathway gORR for various reaction sites on armchair/
zigzag graphene structures. A volcano plot was made using the
descriptor DG0OH . As shown in Fig. 4(b), among the structures studied, Model A2–1 has the lowest ORR overpotential, which was estimated to be 0.445 V. These values of the overpotential for ORR and
OER are comparable to those of Pt containing catalysts (0.4 V for
OER on PtO2-rutile and 0.45 V for ORR on Pt [34]), indicating that
(a)
N-doped graphene as bifunctional catalysts may have as good performance as its counterparts. To determine at what condition the
OER or ORR can spontaneously occur, we calculated the free energy
under different electrode potentials U. Fig. 5 shows the diagrams of
OER substeps on reaction site A2–3 and ORR substeps on reaction
site A2–1. For sites on A2–3, the OER is uphill when the electrode
potential is 0 V. At U = 1.23 V, an ideal water splitting potential, the
transformation of OOH to O2 becomes downhill, but the reactions
(2)–(4) are still uphill. Only when the potential increases to 1.635 V
(i.e., 0.405 V in overpotentail), can all the elementary reaction steps
become downhill. So, 1.635–1.23 = 0.405 V is the overpotentail for
this reaction site, and the transformation of O to OOH is the rate
determination step. Since the OER overpotential is reduced by
nitrogen doping, the above OER is facilitated overall by the Ndoped graphene.
For ORR on A2–1, when the electrode potential is 0 V, the ORR
substeps (6), (7a), (8a), and (9) are all downhill, corresponding to
a short circuit condition of fuel cells. As the electrode potential
increased to 1.23 V, the reaction steps (7a), (8a), and (9) are all
uphill, corresponding to an open circuit condition of fuel cells.
Since the reaction (6), adsorption of O2 as OOH becomes uphill
while other subreactions still keep downhill at U = 0.785 V, the
adsorption of O2 as OOH must be the rate determination step in
ORR. Thus, the minimum ORR overpotential is 1.23–
0.785 = 0.445 V.
The most active sites identified above are attributed to the
redistribution of surface charge induced by the incorporation of
nitrogen atoms into carbon lattice. As shown in Fig. 6, some carbon
atoms become positively charged while others are negative after a
nitrogen atom is doped on the graphene. Those carbon atoms with
positive effective charge will facilitate the adsorption of some species with negative charges [17]. However, if the adsorption free
energy of O is too high due to high positive charge or edge effect,
(b)
Fig. 4. Volcano plots for (a) OER and (b) ORR on different sites of armchair and zigzag graphene nanoribbons.
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M. Li et al. / Journal of Catalysis 314 (2014) 66–72
(b)
(a)
Fig. 5. Free energy diagram for (a) the OER on site 3 for A2 structure (A2–3), and (b) the ORR on site 1 for Z2 structure (Z2–1) at different electrode potential U.
Fig. 6. Bader effective charges of (a) A2 and (b) Z2 structures.
it in turn becomes a barrier for O to transfer to OOH in OER. For
example, sites A1–a, A1–2, A2–1 and Z2–1 have relatively large
positive charge (>0.21–0.38) or edge effect (de = 0–1.2 Å), leading
to high adsorption energy, and consequently high overpotentials.
For site 3 of structure A2 (Fig. 6(a)), with moderate charge (0.27)
and edge effect (de = 2.4 Å), the adsorption free energies of O
and OOH both have moderate values, which result in low OER
overpotentail (Fig. 4). It was found that there is a belt region near
the edge (de 2.4 Å) but within the edge effect range identified in
Fig. 2, where active sites are generated by nitrogen dopants, resulting in high OER activity because of the moderate edge and charge
effect.
Similar phenomena were observed in ORR, but the reaction
active sites were quite different from those in OER. For most of
armchair nanoribbons, the reaction (9) has moderate free energy,
but the reaction (6) has the largest free energy, making this the rate
determination step. An example is site A2–1, which has relatively
high positive charge (0.209) and strong edge effect (de = 0), resulting in the lowest OER overpotential, 0.445 V. Since reaction (6)
involves two substeps, the adsorption of O2 on graphene and
reaction with a proton, the adsorption of O2 may also be the critical
step. We calculated the adsorption O2 on different sites of model
A2 and found that O2 can be adsorbed on the edge in side-on mode
and nitrogen doping increases the adsorption energy, which means
nitrogen doping can promote the adsorption process. An exception
is site A1–a. Although the adsorption of O2 is relative easy on this
site (in side-on mode), the overall potential is still larger than that
of A2–1, due to a very small free energy of the reaction (9), making
it the rate determination step. For zigzag graphene, although from
the overpotential viewpoint, the site Zh-1 has a relatively low
overpotential 0.468 V, a small adsorption energy of O2 makes
adsorption O2 inefficient. For site Z2–1 with large positive charge
and de = 0 (Fig. 6(b)), the adsorption of O2 and the subsequent
transformation of OH to H2O are easy on this site and the calculated overpotential is relative lower than those of other zigzag
graphene nanoribbon cases. However, the reaction (7a), transformation OOH to O, involves a ring-opening of a C–N bond on this
site, which is consistent with the results of Ref. [25]. Even in disso-
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M. Li et al. / Journal of Catalysis 314 (2014) 66–72
(a)
(b)
Fig. 7. (a) OER and (b) ORR overpotential verse the distance from edge to N atom.
ciative reaction pathway, the dissociation of O2 as 2O involves a
ring-opening of a C–N bond, which makes this site inefficient. In
all cases, these most active sites for ORR are located at a distance
of de = 0. These results suggest that while ORR takes place at the
edge of the graphene, OER usually occurs near the edge but within
the range of edge effect.
Fig. 7 shows the plot of ORR and OER overpotentials versus the
distance of N atom from the edge. Generally speaking, the ORR and
OER overpotentials for zigzag graphene are larger than those for
armchair graphene nanoribbons, but in some cases, the potential
for the zigzag can be as small as that for the armchair. In most
cases, the low OER/ORR overpotentails can be achieved by doping
nitrogen atom near the edge within the distance of edge effect. For
ORR, near-edge doping of N makes the adsorption O2 as OOH easier, except some structures such as A1–2, which has a high OER
overpotential up to 1.96 V. It should be noted that the large OER
overpotential for the armchair graphene nanoribbons with nitrogen atom near edge originates from different oxygen adsorption
configuration. For example, the O atom adsorbed near A2–1 in
bridge mode rather than end-on mode is unfavorable to the efficient transformation of O to OOH. Overall, edge doping plays an
important role in reducing the overpotentail and enhancing OER/
ORR catalytic capability of graphene. Also, under sufficiently high
potentials, the nitrogen present in the graphene nanoribbons could
be removed as NH3. From the formation energy viewpoint, the
structure with the substituting nitrogen atom near the edge is
more stable. Since the graphene edge structures could be controlled by various methods [37–41], engineering the edge structure
of the graphene could significantly increase the efficiency of the Ndoped graphene as OER/ORR electrocatalysts for energy conversion
and storage.
4. Conclusion
Oxygen reduction reaction and oxygen evolution reaction on
nitrogen doped graphene nanoribbons were analyzed by density
functional theory calculations. It was found that there is a linear
relation between the binding energy of OOH and OH for these
structures. The OER active sites were identified on the armchair
nanoribbons at the carbon atoms near the nitrogen atom, while
the ORR active sites are those on the edge carbon near the nitrogen
atom. The armchair nanoribbons with nitrogen dopants near the
edge have the minimum theoretical OER overpotential, which
was estimated to be 0.405 V; while the minimum overpotential
for ORR was calculated to be 0.445 V. Those values of ORR and
OER overpotentials are comparable to those of Pt containing
catalysts. These theoretic calculations suggest that N doped
graphene nanoribbons are a highly promising OER/ORR catalyst
for metal–air batteries, water splitting systems, and regenerative
fuel cells.
Acknowledgments
We thank Air Forces MURI program for the support of this
research under the contract #FA9550-12-1-0037, and National Science Foundation (NSF) for the suppoet under the contract: #IIP1343270. Computational resources were provided by UNT high
performance computing initiative, a project of academic computing and user services within the UNT computing and information
technology center.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.03.011.
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