O2 reduction on graphite and nitrogen-doped graphite: Experiment and theory

Article abstract of DOI:10.1021/jp055150g

An experimental and theoretical study of electroreduction of oxygen to hydrogen peroxide is presented. The experimental measurements of nitrided Ketjenblack indicated an onset potential for reduction of approximately 0.5 V (SHE) compared to the onset pote

Full text of DOI:10.1021/jp055150g

J. Phys. Chem. B 2006, 110, 1787-1793
1787
O2 Reduction on Graphite and Nitrogen-Doped Graphite: Experiment and Theory
Reyimjan A. Sidik and Alfred B. Anderson*
Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106
Nalini P. Subramanian, Swaminatha P. Kumaraguru, and Branko N. Popov
Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208
ReceiVed: September 12, 2005; In Final Form: NoVember 22, 2005
An experimental and theoretical study of electroreduction of oxygen to hydrogen peroxide is presented. The
experimental measurements of nitrided Ketjenblack indicated an onset potential for reduction of approximately
0.5 V (SHE) compared to the onset potential of 0.2 V observed for untreated carbon. Quantum calculations
on cluster models of nitrided and un-nitrided graphite sheets show that carbon radical sites formed adjacent
to substitutional N in graphite are active for O2 electroreduction to H2O2 via and adsorbed OOH intermediate.
The weak catalytic effect of untreated carbon is attributed to weaker bonding of OOH to the H atom-terminated
graphite edges. Substitutional N atoms that are far from graphite sheet edges will be active, and those that are
close to the edges will be less active. Interference from electrochemical reduction of H atoms on the reactive
sites is considered, and it is shown that in the potential range of H2O2 formation the reactive sites are not
blocked by adsorbed H atoms.
Introduction
the compositions and structures of the active sites are not yet
known.
Catalyst support materials such as Vulcan XC-72R, Black
Pearl 2000, Ketjenblack, and others are used in low-temperature
fuel cells because of their high surface area, good chemical and
mechanical stability, and good electrical conductivity,1 but not
because of catalytic properties.2 The supports are known to be
only weakly active toward the electroreduction of oxygen.
However, Wang et al. reported making nitrogenated carbon
particles that were active toward oxygen reduction.3 They
oxidized Vulcan XC-72R (surface area ∼250 m2/g) with 30%
HNO3 under refluxing condition, reacted the product with NH3
at 600 °C, and then heat-treated it in Ar at 900 °C. The
nitrogenated product exhibited greater catalytic activity toward
oxygen reduction in sulfuric acid than the untreated carbon, and
there was a 210 mV anodic shift in the oxygen reduction peak.3
Carbon nitrides (CNx) in thin film forms have been studied
over the past decade. They possess high hardness, low coef-
ficients of friction, chemical inertness, and low work functions.11
Recently, Hellgren et al.12 studied the electronic structures of
carbon nitride thin films by using a combination of experimental
electronic structure probes and density functional theory. From
comparisons of experimental spectra with calculated electronic
properties for different model systems, they identified three
bonding structures for the nitrogen: nitrile bonds (CtN),
pyridine-like nitrogen where N bonded to two C atoms, and
graphite-like N, i.e., N bonded to three C atoms, N substituting
into the graphitic network. In the present paper, the last of these
is referred to as “nitrided graphite” or “graphite nitride”. They
also reported that thin films made by dc magnetron sputtering
at low temperatures, <150 °C, were amorphous. For tempera-
tures above 200 °C, when the N concentration was greater than
5%, the structures were fullerene-like, but for N concentrations
below 5%, graphite nitride formed.13 The graphitic CNx thin
films exhibited better thermal stability than the films grown at
low temperature.14
The mechanisms for oxygen reduction, both in the presence
and in the absence of substitutional N in the graphite, and the
cause of the anodic shift are not known. The catalytic effect of
nitriding is a worthwhile issue to explain. To understand the
mechanisms and catalytic effect, we have undertaken a quantum
chemical study of nitrided graphite using a small and a large
cluster model, both terminated on the edge by bonding to
hydrogen atoms.
This paper reports new oxygen reduction results for nitrided
Ketjen Black EC 300 J. The surface area of pristine ketjen black
is 886 m2/g. A comparison of the experimental and theoretical
results will be made. The comparison will indicate that two-
electron oxygen reduction to hydrogen peroxide takes place on
the edges of the graphite sheets or on nitrided graphite basal
Carbon-nitrogen materials prepared in other ways can be
inactive toward oxygen reduction. Lalande et al.4 reported that
CNx particles prepared by pyrolysis of nitrogen-containing
organic compounds in Ar at 1000 °C or by pyrolysis of a
hydrocarbon, followed by heat treatment in NH3 at 1000 °C,
did not show any activity toward oxygen reduction.
Carbon has a long history as a support in the heterogeneous
catalysis literature,5 and only recently have the effects of adding
N been studied. For example, compared to their nondoped
counterparts, some N-doped carbon particles show more oxida-
tion resistance.6 Some show catalytic activities toward NOx
reduction.7,8 Others are oxidation catalysts.9 In the case of NOx
-
oxidation and O2 reduction, the O2 superoxide radical was
proposed to explain the enhanced activity.9,10 These properties
have been attributed to electronic or structural changes caused
by the nitrogen incorporation into the graphite layers. However,
* Corresponding author. E-mail aba@case.edu. Phone 216-368-5044.
Fax 216-368-3006.
10.1021/jp055150g CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/07/2006

1788 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Sidik et al.
planes, where it is activated by radical carbon sites adjacent to
substitutional N atoms in the graphite sheet. The idea that a
carbon radical is involved was motivated by a previous study
of hydrogen evolution over radical sites on boron-doped
diamond electrodes.15 A subsurface substitutional boron atom
gave a second-layer carbon atom radical character, allowing it
to chemisorb an H atom weakly. This atom then participated in
electrochemical H2 evolution.15 The same is found for the OOH
intermediate formed in the first step in oxygen reduction: it is
stabilized by bonding to a radical carbon site that is created by
the substitutional N, and its reduction forms H2O2. It is found
that, at potentials below the OOH(ads) formation potential, the
carbon radical has H bonded to it, but the H is predicted to be
oxidized off at higher potentials.
Experimental Approach
Synthesis of Nitrogen-Doped Carbon. Commercially avail-
able carbon black, namely Ketjen Black EC 300 J, was treated
in HCl and HNO3 to remove the metallic impurities.16,17 The
as-received Ketjen Black EC 300 J was pre-washed twice with
6 M HCl for 24 h. Next, the chloride impurities present in the
sample were washed out with distilled water. The pre-washed
carbon was subjected to oxidation in 70% HNO3 for 7 h under
reflux conditions. It was then washed in distilled water and dried
in an oven at 75 °C. For brevity, as-received Ketjen Black EC-
300J is denoted as K, HCl-treated carbons as K1, and HNO3-
treated carbons as K2. The HNO3-treated carbon was subjected
to heat treatment in NH3 at 900 °C to obtain nitrogenated carbon
(K3).
Electrochemical Characterization. Electrochemical char-
acterizations were performed in a single-compartment three-
electrode cell. The electrocatalytic activity of the modified
carbon was studied using the rotating ring disk electrode
(RRDE) technique.18-21 Briefly, a rotating ring disk electrode
from Pine Instruments employing a glassy carbon disk (5 mm
o.d.) and Pt ring (5.52 mm i.d., 7.16 o.d.) was used as the
working electrode. Platinum wire was used as the counter
electrode, and a standard Hg/HgSO4 electrode was used as the
reference electrode. The potentials presented in this study are
referred with respect to standard hydrogen electrode (SHE). A
catalytic film was produced at the disk electrode according to
the following procedure: 8 mg of the modified carbon was
suspended in isopropyl alcohol and ultrasonically blended for
10 min. 15 µL of this suspension was applied as an ink on the
disk electrode in steps of 5 µL. A 5 µL Nafion solution [1(5 wt
% Nafion)/10(water)/10(isopropyl alcohol)] was added on top
of the film. 0.5 M H2SO4 was the electrolyte in all of the studies,
and prior to measurements, the solution was purged with
nitrogen. Cyclic voltammograms (CVs) were recorded by
scanning the disk potential from 1.04 to 0.04 V vs SHE at a
scan rate of 5 mV s-1. For all electrochemical measurements,
the ring potential was maintained at 1.2 V vs SHE in order to
oxidize any hydrogen peroxide produced.19 First, CVs were
recorded at 5 mV s-1 using nitrogen atmosphere to obtain the
background capacitive currents. Next, the CVs were recorded
using the oxygen-saturated electrolyte. The electrolyte solution
was purged with oxygen for 15 min before commencing oxygen
reduction on the disk electrode. Linear sweep voltammograms
for disk potentials were recorded from 1.04 to 0.04 V vs SHE
at a scan rate of 5 mV s-1 for different rotation rates of the
RRDE. The linear sweep voltammograms shown in this paper
were obtained at 900 rpm unless specified otherwise. The net
faradic current was obtained by subtracting the background
capacitive current from the N2 CV.20 All measurements were
Figure 1. (a) Disk currents and (b) ring currents for modified carbons
K, K1, K2, and K3 in 0.5 M H2SO4 solution saturated with oxygen.
The linear sweep voltammograms were recorded at 5 mV/s scan rate
and 900 rpm rotation rate.
obtained with a bipotentiostat (model AFRDE from Pine
Instruments).
Experimental Results and Discussion
The as-received carbon (K), HCl-treated carbon (K1), HNO3-
treated carbon (K2), and nitrogenated carbon (K3) were tested
for activity toward oxygen reduction. Figure 1a shows typical
polarization curves for oxygen reduction by these carbons on a
rotating disk electrode in 0.5 M H2SO4. The highest activity is
for the HNO3-treated carbon that was subjected to heat treatment
in NH3. A performance trend of K3 > K2 > K > K1 is clearly
seen in Figure 1a. The as-received carbon support shows greater
activity than the HCl-treated carbon, and this is attributed to
the presence of metallic impurities. The HCl treatment evidently
removes the metallic impurities, and K1 represents the activity
of bare carbon. Oxidized carbon K2 shows about 100 mV less
activation overpotential for oxygen reduction reaction compared
to K1. Electrochemical analysis indicated that oxidation of
carbon in nitric acid introduces oxygen, probably as quinone
groups, on the carbon surface.3 The 0.55 V vs SHE peak in the
linear sweep voltammograms of K2 is characteristic of the
quinone-hydroquinone redox couple. The onset potential for
oxygen reduction reaction is 0.3 V vs SHE. Further introduction
of nitrogen, by heat treatment at 900 °C in a NH3 atmosphere,
increases the activity of the catalyst by nearly 200 mV in the
anodic direction. The onset potential is as high as 0.510 V vs
SHE.

Electroreduction of Oxygen
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1789
Figure 3. Optimized structure for the N-substituted graphite model,
C41NH16. The larger gray circles are carbon atoms, and the smaller
white circles are hydrogen atoms.
electron at higher overpotentials, leading to the production of
hydrogen peroxide. At still higher overpotentials, the decrease
in percentage hydrogen peroxide produced is attributed to the
reduction of oxygen to water through a four-electron process
or through two two-electron processes. The high OdO bond
strength of 494 kJ/mol has been discussed in this context.22 The
series pathway, through peroxide production and its subsequent
reduction to water, and the direct four-electron pathway can
occur in parallel on some surfaces, though at high overpotentials
to the four-electron process. The formation of peroxide does
not require the breaking of the OdO bond, but does weaken it
to an OsO single bond. Further reduction to water requires
that this bond be broken. Thus, on less reactive surfaces, such
as the synthesized graphite and nitrided carbon, hydrogen
peroxide is the reduction product.
Figure 2. (a) Number (No) of electrons produced and (b) % H2O2
produced as calculated from eq 1 and eq 2 for the nitrogenated carbon
K3 whose disk and ring currents are shown in Figure 1a,b.
Theoretical Method
Oxygen electroreduction in acid can be either a four-electron
discharge to form water or a two-electron reduction to form
peroxide. Figure 1b gives the ring currents of the carbons K,
K1, K2, and K3. The ring currents were measured to estimate
the amount of generated hydrogen peroxide. It can be observed
that the ring currents initiate at the same potentials as the onset
potentials for oxygen reduction in the disk, indicating that the
generation of hydrogen peroxide is taking place. For nitrided
carbon, the ring current initiates at the onset potential of 0.51
V vs SHE. Also, the bell-shaped curve of the ring current is
characteristic of hydrogen peroxide produced on non-noble metal
catalysts and activated carbon.20 The number of electrons
transferred (n) and the percentage of hydrogen peroxide
produced (% H2O2) can be determined by the following
equations:20,21
A. Cluster Calculations. In this study, a cluster approach
was used with the B3LYP hybrid density functional theory in
Gaussian 03.23 The basis sets used for O, N, and H were
6-31G**. Graphite models used in this work contained four and
fourteen hexagonal rings with delocalized π electrons and
terminated with C-H bonds. For modeling the nitrided basal
plane sheet of graphite, a nitrogen atom was placed substitution-
ally as shown in Figure 3. Adsorption of reaction intermediates
on sites a-d and others of the graphite and nitrided graphite
models was examined. The bond strength values were calculated
using the formula
BS ) E(cluster) + E(adsorbate) - E(adsorbate on cluster)
(3)
n ) 4ID/(ID + IR/N)
(1)
(2)
B. Calculating Reversible Potentials for Forming Ad-
sorbed Reaction Intermediates. The reversible potentials were
calculated by a Gibbs linear free energy relationship.24,25 The
essence of this free energy relationship is that, if one has the
reversible potential for a redox reaction in solution bulk, then
the electrode surface value can be approximated as a perturbation
to this because of bonding to the surface. It is assumed that
when the intermediates are bonded to the surface the solvation
interactions of the reactants and the products of the electron
transfer are equally reduced. Thus, the adsorption bond strengths
may be used to make predictions of reversible potentials. As
% H2O2 ) 100(4 - n)/2
where N, ID, and IR are the collection efficiency, disk current,
and ring current, respectively. We assumed the collection
efficiency to be a constant at an estimated value of 0.25 for all
the catalysts studied. Figure 2 shows the number of electrons
and % H2O2 generated for the nitrogenated carbon K3. The
obtained values clearly indicate that oxygen reduction on the
nitrogenated carbon surface becomes predominantly two-

1790 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Sidik et al.
Figure 5. H adsorption bond strengths (eV) to C41NH16 as a function
of position. Note that the cluster symmetry makes the H bond strength
distribution symmetric.
Figure 4. Spin and charge distributions for C41NH16 in its optimized
structure. Both values are shown along the symmetry axis. The first
number in each pair refers to the spin density, and the underlined
numbers refer to the charge density. The cluster is symmetric, so spin
densities are given on the top half and charge densities are given on
the bottom half. For the edge H atoms, the spin and charge densities
are, on average, <0.001 and ∼0.085, respectively, and they are not
shown.
an example, consider OH reduction in solution and on the Pt
surface. The reversible potential for forming the intermediate
on the electrode surface is
Us°urface ) U° + (product adsorption bond strength -
reactant adsorption bond strength)/F (4)
where F is the Faraday constant.
The bulk and surface reactions can be related in a reaction
cycle
Figure 6. OOH adsorption bond strengths (eV) to C41NH16 as a
function of position. The “/” means that the calculation for that site
did not converge.
The reversible potential for the onset of the surface process is
then given by
the charge densities are shown. The N atom is quite negative,
with a charge of -0.916. This is a consequence of N being a
radical and more electronegative than carbon. Most of the
compensating positive charge is distributed on the three
neighboring carbon atoms. The spin density due to the unpaired
electron is seen in Figure 4 to delocalize onto adjacent C atoms
mostly on the left side of the N atom, with the highest spin
densities found on the edge C atom c (defined in Figure 3) and
its symmetric counterpart.
Us°urface ) 2.81 + -2.50 + 0.42 ) 0.73
(6)
where 2.81 V is the experimental reversible potential for the
water oxidation and OH reduction in acid solution, and 2.50
and 0.42 eV are the respective measured adsorption bond
strengths for OH26 and H2O27 at low coverage. The predicted
water oxidation onset potential of 0.73 V on Pt is within 0.2 V
of the observed value of 0.55 V in acid in the absence of anion
adsorption.28
As expected, the optimized geometry of the pure graphene
sheet, C42H16, is perfectly planar. A spin-unrestricted calculation
revealed no spin polarization.
Theoretical Results and Discussion
The adsorption bond strengths for H and OOH on C41NH16
used in this study are shown for several adsorption sites in Figure
5 and Figure 6, respectively. In Figure 7, it is shown that the
adsorption bond strengths of adsorbate radicals, H and OOH,
exhibit a correlation with the spin density only when the net
spin is >0.1. The highest bond strengths within the cluster for
H and OOH are 2.238 and 0.891 eV, respectively, on the carbon
atom that is directly bonded to N. Structures are shown in Figure
8a,b. When a radical adsorbs on the radical sheet, the changes
in the hybridization of the adsorption site atoms distort the planar
geometry, raising the adsorption site carbon out of the plane
A. Adsorption Energies for H, O2, OOH, and H2O2 on
the Radical Sheet Cluster C41NH16 and on C42H16. Figure 3
shows the fully optimized geometry for the 14-ring C41NH16
sheet radical in the absence of an adsorbate and in the doublet
state. It has a C2V symmetry along the Ca-N axis and is flat
with no more than (0.001 Å variations from planarity. Because
of the symmetric distribution of spin and charge densities along
the Ca-N axis, the display may be simplified: The top half of
Figure 4 shows only spin densities, while both spin and charges
are shown along the Ca-N axis, and at the bottom half, only

Electroreduction of Oxygen
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1791
Figure 7. Bond strengths as functions of spin density for H and OOH
bonded to various sites on C41NH16.
Figure 9. The H (first number) and OOH (second number) radical
adsorption bond strengths (eV) on the pure graphite sheet, C42H16. The
“/” means that the calculation for that site did not converge.
TABLE 1: Reversible Potentials for Reactions Used in This
Work
reactions
U° (V/SHE)
H+(aq) + e- (U°) T H(aq)
-2.11a
-0.046b
1.436b
O2(g) + H+(aq) + e- (U°) T OOH(aq)
OOH(aq) + H+(aq) + e- (U°) T H2O2(aq)
a ref 32. b ref 30.
graphite sheet. These bond strengths will next be used to explain
the catalytic abilities of the doped and undoped graphite.
B. Reversible Potentials for Oxidation of H and O2
Reduction Intermediates. Graphite. For all the reversible
potential calculations in this paper, the values for bulk reactions
in Table 1 were used in eq 6. The adsorption energies of
nonradical reactants and products were taken as zero. The
resulting predicted reversible potentials for forming H on the
undoped C42H16 cluster model are -1.254 V, -0.636 V, -0.084
V, and -0.414 V for H on sites a-d, respectively. The value
0.311 V is predicted for OOH formation on the edge site c, and
this appears to be the cause of the observed current onset
potential on Ketjenblack in Figure 1a. The reversible potential
for the OOH reduction step will be 1.097 V, which is much
greater than 0.311 V. This value is based on assigning values
of zero to the adsorption bond strengths of the O2 reactant and
the H2O product molecules and the realization that any errors
in the adsorption bond strengths of the intermediates cancel
out.29 In this case, OOH is the adsorbed intermediate for the
first and second reduction steps. The standard reversible
potential for hydrogen peroxide formation is 0.695 V.30 Given
these conditions, the sum of the two one-electron reversible
potentials divided by 2 will be 0.695 V, which is the reversible
potential for the overall two-electron process. The second step
can take place at any potential less than 1.079 V, and so the
first step determines the overpotential in this case according to
this simple picture. It is obvious that the ideal situation for a
reaction where several electrons are transferred is for the
reversible potential for each electron-transfer step to equal the
reversible potential for the overall multielectron reaction.29 The
electron-transfer activation energies will also contribute to the
overpotential, as will be evident in the Tafel plot.31 It is
important that edge sites be available for reaction at this
potential, and not blocked by adsorbed H. They are available,
Figure 8. Optimized structures for OOH (a) and H (b) when bonded
adjacent to the substitutional N in C41NH16. Upon adsorption of OOH
or H, the flat 14-ring sheet adopts a concave geometry.
toward a tetrahedral structure. If the adsorbate is bonded to an
edge carbon atom, it is able to relax to a nearly tetrahedral
structure. The adsorption bond strengths of the neutral molecules
O2 and H2O2 on the radical sheet are very weak, and the planar
geometry of the sheet radical is not significantly affected.
Bond strengths to carbon atoms on the edges of C41NH16 took
a range of values. For sites labeled b, c, and d, the respective
bond strengths of H are 1.445, 2.780, and 1.484 eV, and for
OOH, they are 0.051, 1.197, and 0.089 eV. When H is bonded
to edge site c, it forms a C-H bond that is stronger than the
bond it forms to the carbon atom at site a, adjacent to
substitutional N. OOH bonds by 1.197 eV to edge site c, which
is also stronger than its bonding to site a. The strong bonding
at the edge of the nitrided cluster is caused in part by the
flexibility of edge sites to adopt the tetrahedral structure and,
as will be shown below, the spin density on the site c carbon in
the nitrided cluster, which is absent in the case of the undoped
cluster. The two H atoms in the methylene group are sym-
metrically distributed above and below the sheet plane, while
the neighboring C and H atoms are nearly unmoved. A similar
structure was adopted in the case of OOH adsorption on the
edge site c.
To understand better the effect of the substitutional N on the
adsorption bond strength of H and OOH radicals, we calculated
adsorption bond strengths for sites a-d on the undoped graphite
sheet, C42H16. The results are shown in Figure 9. For both
radicals, the undoped graphite sheet has stronger adsorption
bonds at the edge than at the center, just like the nitrogenated
counterpart. However, the bonding is weaker on the undoped

1792 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Sidik et al.
TABLE 2: Predicted Reversible Potentials for O2 Reduction
on the 14-Ring Radical Cluster, C41NH16
U° (V/SHE)
on C
on edge C adjacent
reaction
(site c)
to N
O2(g) + H+(aq) + e- (U°) T OOH (ads)
1.151
0.239
0.845
0.545
OOH (ads) + H+(aq) + e- (U°) T H2O2(aq)
overall reaction
O2(g) + 2H+(aq) + e- T H2O2(aq)
0.695
0.695
TABLE 3: Adsorption Bond Strengths, De (eV), for H and
OOH on the Adjacent Site Carbon and the Highest Bond
Strength Edge Site on the 4-Ring and 14-Ring Model
Systems with N in the Center
adjacent site
highest De edge site
a
a
adsorbate 4 ring 14 ring
∆
4 ring 14 ring (no N)
∆
H
OOH
2.315 2.238 -0.077 3.000 2.780 (2.026) -0.220
0.903 0.891 -0.039 1.566 1.197 (0.357) -0.369
a ∆ ) De(14 ring) - De(4 ring).
Figure 10. Optimized structures for O2 (a) and H2O2 (b) when bonded
adjacent to the substitutional N in C41NH16.
because according to the calculated reversible potentials, H(ads)
will be unstable to oxidative removal in the potential range of
interest.
is too low to account for the experimental current onset potential
for H2O2 formation in Figure 1a. However, using the highest
bond strength, 0.891 eV, which is for OOH on the site adjacent
to N, respective reversible potentials 0.845 and 0.545 V are
predicted for the first and second electron-transfer steps. The
second matches the observed current onset potentials for H2O2
generation on nitrogenated Ketjenblack quite well, as shown
by the curve labeled K3 in Figure 1a.
The spatial extent of the influence of the substitutional N
atom is of interest. Calculations of H and OOH bond strengths
to a smaller four-ring cluster were made. As is evident from
Table 3, bond strengths to C adjacent to N in a 4-ring C15NH10
cluster increased very little, but the bond strengths at the edge
sites increased somewhat more. The bond energies to the edge
of the undoped 14-ring cluster approximates the limit for large
clusters and the limit of substitutional N being far from an edge.
These values are in the column labeled “no N”. The bond
energies indicate that the biggest perturbation effect of N is local,
but when the N is near an edge, it magnifies the reactivity of
edge sites toward bonding radical molecules to the extent that
they will be passivated against oxygen reduction activity at the
potentials shown in Figure 1a, curve K3.
Graphite Nitride. As can be seen from Figure 5, the highest
bond strength site for H adsorption on the nitrided cluster lies
at the edge site c, where the bond strength is 2.780 eV. The
reversible potential for the oxidation of this H is 0.67 V, based
the reversible potential for the reduction of H+(aq), -2.11 V.32
This potential is too high to account for the observed current
onset potential in Figure 1a, where O2 reduction occurs below
0.67 V. This means that in the 14-ring model the N is too close
to the edge, and hence, we conclude that N near a graphite edge
will not generate activity. However, when there is no edge site
nearby, a C atom adjacent to N is a candidate for being an active
site. Of the nonedge sites, it has the highest bond strength for
H, and the reversible potential for its oxidation is 0.128 V, which
is a good potential for maintaining an active radical site for O2
reduction.
For the centrally located substitutional N, the spin density is
highest on the site a carbon neighboring the N, but other atoms
have nonzero spin densities, so adsorption bond strength for
reaction intermediates O2, OOH, and H2O2 were calculated on
some of the other sites too. The interactions of O2 and H2O2
with all sites are very weak, with the highest adsorption bond
strengths on the nonedge sites being 0.056 and 0.076 eV,
respectively. For the edge sites, the respective results are 0.036
and 0.179 eV. The structure perturbations to the cluster caused
by the weak bonds are small, as shown in Figure 10. Bond
strengths for OOH on the nitride cluster, as may be seen in
Figure 6, range from 0.02 to 1.197 eV, depending on the
adsorption site. The most stable edge site is c, just as for H
adsorption. By using the highest adsorption bond strength values
for OOH, the reversible potentials in Table 2 were obtained for
the formation of H2O2 from O2 reduction in acidic solution.
The calculated reversible potential for the overall reaction is
the same as the reported standard experimental values of 0.695
V, because as mentioned earlier, (i) the reversible potential for
the overall reaction does not depend on the adsorption bond
strength of the OOH reaction intermediate, and (ii) the adsorp-
tion energies of the O2 reactant and H2O2 product are taken as
zero. As shown in Table 2, for the edge site c, the second
electron-transfer step has a reversible potential of 0.239 V, which
Conclusions
Nitrided carbon obtained from oxidized carbon that is heat-
treated in NH3 at 900 °C shows a high 0.510 V vs SHE onset
potential toward oxygen reduction. Oxygen reduction is pre-
dominantly by the two-electron process to hydrogen peroxide,
as observed from RRDE studies. Quantum calculations show
that carbon radical sites formed adjacent to substitutional N in
graphite are active for O2 electroreduction to H2O2 in acid
electrolyte, and this may explain the catalytic effect observed
for nitrided carbon. Furthermore, the weak catalytic effect of
untreated carbon can be attributed to the weaker bonding of
the OOH(ads) reaction intermediate to H atom-terminated
graphite edge sites. Substitutional N atoms that are far from
the graphite sheet edges will be active, and those that are close
to the edges will be less active.
Acknowledgment. This work was supported by the Depart-
ment of Energy through grant no. DE-FC36-03GO13108.

Electroreduction of Oxygen
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1793
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