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An Ultra-Thin Cobalt-Oxide Overlayer Promotes Catalytic
Ac-tivity of Cobalt Nitride for Oxygen Reduction Reaction Hadi
Abroshan,† Pallavi Bothra,† Seoin Back,† Ambarish Kulkarni,† Jens
K. Nørskov,‡,† and Samira Si-ahrostami† * †SUNCAT Center for
Interface Science and Catalysis, Department of Chemical
Engineering, Stanford University, 443 Via Ortega, Stanford,
California 94305, USA ‡SUNCAT Center for Interface Science and
Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill
Road, Menlo Park, California 94305, USA
ABSTRACT: The oxygen reduction reaction (ORR) plays a crucial
role in various energy devices such as proton-exchange mem-brane
fuel cells (PEMFC) and metal-air batteries. Owing to the scarcity
of the current state-of-the-art Pt-based catalysts, cost-effective
Pt-free materials such as transition metal nitrides, and their
derivatives have gained overwhelming interest as alternatives. In
particular, cobalt nitride (CoN) has demonstrated a reasonably high
ORR activity. However, the nature of its active phase still remains
elusive. Here, we employ density functional theory (DFT)
calculations to study the surface reactivity of rocksalt (RS) and
zincblend (ZB) cobalt nitride. The performance of the catalysts
terminated by the facets of (100), (110), and (111) are studied for
the ORR. We demonstrate that the cobalt nitride surface is highly
susceptible to oxidation under ORR conditions. The as-formed oxide
overlayer on the facets of CoNRS(100) and CoNZB(110) presents a
significant promotional effect in reducing the ORR overpo-tential
and thereby increasing the activity in comparison with those of the
pure CoNs. The results of this work rationalize a number of
experimental reports in the literature and disclose the nature of
the active phase of cobalt nitrides for ORR. Moreover, they offer
guidelines for understanding the activity of other transition metal
nitrides and designing efficient catalysts for future generation of
PEMFCs.
■ INTRODUCTION Low temperature proton-exchange membrane fuel
cells (PEMFC) stand out as one of the most promising classes of
energy conversion devices owing to their high energy efficien-cy,
ease of operation, and low/zero emissions.1-6 The operating
principle of fuel cells involves four-electron reduction of O2 (at
the cathode) and oxidation of a fuel, e.g., H2 or methanol (at the
anode). Generally, kinetics of oxygen reduction reac-tion (ORR) at
the cathode is sluggish, which diminishes the power density of fuel
cells.7,8 Platinum (Pt) based materials are known as the most
efficient catalysts for ORR.9-17 However, a wide-spread
implementation of Pt-based catalysts in fuel cells is limited due
to high cost and scarce supply of Pt as well as their low
durability under operating condition.18-21 In this re-gard,
rational design and synthesis of cheap and stable cata-lysts with
low or zero content of Pt that show similar or supe-rior activity
to Pt-based catalysts are subjects to continuing interest from many
research groups worldwide.22-39 Over the past decades, a tremendous
effort has been made to examine a wide range of earth-abundant
based materials as alternative ORR catalysts. Extensive experiments
have been carried out to investigate catalytic ORR performance of
metal-free carbon-based materials,22-25 transition metal
chalcogeni-des,26,27 non-Pt transition metal intermetallics28-30,
as well as
transition metal oxides,31,32 nitrides,33-35 and
oxynitrides.36,37 In particular, transition metal nitrides (TMN)
have attracted par-ticular attention as competitive catalysts
because of their unique properties, such as high electrical and
thermal conduc-tivities, tailorable electronic features, high
melting points, exceptional hardness, and chemical resistance to
corrosion in aqueous media.40-44 Indeed, TMNs have been applied as
sup-ports to enhance the activity and durability of precious metals
(e.g., Pt and Pd) in comparison to commercial Pt/C catalyst.45-49
Subsequent studies have shown that TMNs not only increase the
corrosion resistance and electrochemical stability of the precious
materials but can also act as the active sites for ORR.34,50-52
Therefore, many transition-metal nitrides, such as TiN, MoN, ZrN,
CrN and CoN, have been experimentally scrutinized for ORR
activity.11,53-55 Previous studies show that considerable ORR
performance using cost-effective TMNs with high tolerance towards
methanol in alkaline pH is achievable. The catalytic activity of
transition metal nitrides for ORR is assumed to be attributed to
the presence of nitro-gen atoms, leading to d-band shrinkage of
metals. This in turn increases the electron density of the TMNs
near the Fermi level, which allows the nitrides to pass electrons
to adsorbates, thereby facilitating the reduction of
oxygen.56-58
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Cobalt is one of the non-precious elements that is widely tested
as electrocatalysts for oxygen and hydrogen reactions.59-62
However, issues such as poor electrical conductivity of co-balt
oxide,63-65 and low catalytic activity of metallic cobalt66,67 have
hampered the electrochemical reactions’ kinetics; thus, limiting Co
applications in electrochemical devices. Therefore, electronic
properties of the cobalt-based materials are required to be tuned
for promotion of their catalytic behaviors. For instance, a study
by Yeo et al. shows cobalt oxides supported by gold exhibit a
superior electrocatalytic activity due to an increase in CoIV
population on surface oxide that is mediated by the metallic Au
support.68 Further, N-doped graphene sup-ported cobalt oxide is
shown to have high electrocatalytic ac-tivity that has been
attributed to the synergistic coupling ef-fects between the
wide-bandgap oxide and the conductive doped graphene.69 These
results indicate improvement in the catalytic activity of cobalt
oxide is achievable through doping certain heteroatoms or
incorporating other functional materi-als.70-72 Recently, cobalt
nitride (CoN) has gained particular interest as a metallic
interstitial system. Structural analysis of Co ni-tride revealed
that nitrogen atoms are bonded to cobalt atoms, emerging metallic
properties through the nitride framework; therefore, providing an
ideal catalyst platform for the electro-catalytic reactions.73,74
For example, Xia et al. have recently shown carbon-supported CoN
exhibits a high ORR activity and stability, with an onset potential
of 0.85 V (vs. RHE).75 Furthermore, the ORR activity of several
TMNs (e.g., NbN, TiN, MoN) are shown to be enhanced if they are
doped or co-synthesized with CoN.57,58,76-79 Nevertheless, a lack
of in-depth understanding of the intrinsic electrocatalytic
activity of the cobalt-based nitrides makes it difficult to
establish definitive structure–activity relationships for CoN
catalyzed reactions, thereby posing a serious challenge to
systematic development and applications of CoN. Given the lower
stability of nitrides than oxides from the solid-state chemistry
point of view, it would be prudent to consider that surface metal
nitrides easily become oxidized to form thin films of corresponding
metal oxides.80,81 For exam-ple, it is previously shown that 2p
orbital of surface nitrogen atoms in Tantalum nitrides prone to
oxidative decomposition to molecular nitrogen (N2) to form Ta2O5.82
Similar surface oxidation is reported for Co0.6Mo1.4N2, MoN, n- or
p-GaN, and InxGa1-xN.83-88 Although the oxide formation has been
evi-denced and reported in the literature, there is clearly the
lack of careful experimental characterization and future
experi-mental studies are required to get further details and
insight on
the nature of active phase. Nevertheless, the surface oxidation
is especially highly possible under harsh electrochemical
con-ditions and operating potentials of the ORR. With these issues
in mind, a controversial question on the nature of the CoN
catalysts’ active sites is raised. Therefore, it is both
worth-while and desirable to investigate possible active sites on
the surface of cobalt nitrides at the atomic level. Here we pursue
this survey and consider two different phases of cobalt nitride,
namely rocksalt (CoNRS) and zincblend (CoNZB) (Figures 1A and B,
respectively). Catalytic activity of different facets of these
phases are evaluated via density functional theory (DFT)
calculations. Further, oxidations of the CoN systems are
sys-tematically studied. We show that both nitrides are highly
prone to form surface oxide films under operating potentials. The
as formed oxide layers significantly promote the ORR activity of
the cobalt nitrides. Even though this work is fo-cused on
Co-systems, it is very likely that similar phenome-na/active sites
may be responsible for the high catalytic activi-ty of the other
transition metal nitride systems. ■ COMPUTATIONAL DETAILS Periodic
DFT calculations are performed to investigate ad-sorption energies
of ORR species which are formed during the electrocatalytic process
on the cathode surface. We use the computational hydrogen electrode
(CHE) approach which exploits that the chemical potential of a
proton-electron pair is equal to that of gas-phase H2, at Uelec =
0.0 V vs. the reversible hydrogen electrode (RHE).67 The effect of
the electrode poten-tial on the free energy of the intermediates is
taken into ac-count through shifting the electron energy by –eUelec
where e and Uelec are the elementary charge and the electrode
potential, respectively. We consider the associative mechanism with
OOH*, O* and OH* as ORR intermediates. The catalytic ac-tivity is
evaluated on the basis of calculated limiting potential, defined as
the highest potential at which all the reaction steps are downhill
in free energy diagram scheme.We investigate all different possible
adsorption modes for different adsorbates at surface, and the
lowest adsorption energies are taken into account for calculating
the limiting potential. It is worth men-tioning that our DFT
results in the gas phase on the perfect surfaces only offer
qualitative insights rather than accurately quantifying the actual
bonding free energy in the solution phase under operating
conditions. The presented theoretical results in this work are
based on thermodynamic analysis which have played an essential role
in providing insights on the nature of active sites and guiding the
design and optimiza-tion of various catalysts.89,90 Thermodynamic
predicted activi-ty volcano for transition metals has been shown to
be in close agreement with the predicted kinetic activity volcano.
There-fore, there is a close connection between the kinetic and
ther-modynamic formulations for four-electron oxygen reduction
reaction.91,92 We consider zincblend and rocksalt cobalt nitrides
(Figure 1) with (100), (110), and (111) facets. Each system is
modeled using a 2 ´ 2 slab extended in eight layers. Each
computation-al unit cell contains 32 atoms, i.e., 16 Co and 16 N
atoms. The top three layers of the slabs and the adsorbates were
allowed to relax during the geometry optimizations (Figures S1 and
S2, and Table S1 in the Supporting Information). To study the
effect of a thin oxide layer on the catalytic ac-tivity, we replace
the first to fourth layers of the CoN with CoO. In this case, CoO
layer(s) as well as two CoN layers
Figure 1. Unit cell of bulk cobalt nitride in the forms of (A)
rocksalt and (B) zincblend. Of note, in a periodic view of the
systems, coordination number of Co and N atoms is similar and equal
to 6 and 4 for rocksalt and zincblend structures, respectively.
Color code: Co, pink; N, blue.
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below the deepest CoO layer are set to move during geometry
optimizations. The rest of atoms of the systems were kept fixed at
their optimized bulk positions. The Hubbard correc-tion (DFT+U with
U = 3.32 eV)93 is applied and results are compared with those of
DFT. In a periodic view, separation of slabs in the z-direction is
set to at least 15 Å of vacuum to reduce the effects of the dipole
moment formed by molecular adsorption on the surface. The
Vanderbilt method (GBRV pseudopotential library)94 and a revised
version of the Perdew-Burke-Ernzerhof func-tional (RPBE)95 are used
to describe the electrons-nuclei inter-action, and electron
exchange and correlation, respectively. The kinetic energy cutoff
was chosen to be 500 eV and inte-gration in the reciprocal space
was carried out at 8 ´ 8 ´ 1 k-point sampling. Spin polarization is
carried out for all systems. All calculations are performed with
the Quantum Espresso package.96
■ RESULTS AND DISCUSSION Figures 2A and 2B display the free
energy diagrams for the four-electron reduction of oxygen at Uelec
= 0.0 V on different facets of CoNRS and CoNZB. Surface coverage of
0.25 mono-layer (ML) adsorbate is considered.
The general trend in the adsorption energies of the ORR
intermediates on different facets of CoNRS is (111) > (110) >
(100) with the exception of O* intermediate on (100) facet (Figure
2A). This arises from a strong interaction of a surface nitrogen
atom with O*, while in the case of the other facets O* binds to the
surface cobalt atom (Figure S3 in the Supporting
Information). We also note the nitrogen atom of the facet (100)
is pulled out of the surface upon interacting with the O*, turning
its coordination number to 3. Such a strong interaction of O* with
the facet (100) makes O* + (H+ + e−) à OH* as the limiting step
with limiting potential of 0.14 V, i.e., overpo-tential = (1.23 -
0.14) V = 1.09 V. The coordination chemistry at the surface of
CoNRS(110) and CoNRS(111) allows more than one surface cobalt atom
to interact with O* and OH* (Figures S3 and S4 in the Supporting
Information). This in turn strengthens the interaction energies of
the intermediates on these facets, lowering the ORR activity of
(110) and (111) facets. Of note, OH* + (H+ + e−) à H2O is found to
be the limiting step with the calculated overpotential of 1.07, and
1.73 V for (110) and (111) facets, respectively. We also note the
interaction of the intermediates OOH* and OH* exerts a considerable
influence on the atomic surface arrangement of CoNRS(111), i.e.,
two nitrogen atoms of the second layers are pulled up to the
surface (Figures S4 and S5 in the Supporting Information). These
results, in particular for OH*, indicate very high overpotential
(1.73 V) for the facet (111). For the CoNZB (Figure 2B), the
calculated overpotentials are found to be 1.4, 0.96, and 0.64 V for
the facets of (100), (110) and (111), respectively. The general
trend in adsorption ener-gies of the ORR adsorbates is in the order
of (100) > (111) > (110). In particular, the OOH* interaction
with the facets of (100) and (111) is significantly strong that
leads to a sponta-neous O-O bond dissociation, leaving O* and OH*
at the sur-face (Figure S6 in the Supporting Information). While Co
is the interacting surface atom on CoNZB(100) and CoNZB(111),
nitrogen interacts with O* in the case of CoNZB(110) (Figure S7).
It is worth noting that more than one surface cobalt atom of the
(100) and (111) facets interacts with O* and OH*, re-sulting in
stronger adsorption free energies and hence lower
Figure 2. Free energy diagrams for the four-electron reduc-tion
of oxygen at Uelec = 0.0 V on different facets of (A) CoNRS and (B)
CoNZB.
Figure 3. Calculated ORR limiting potentials for rocksalt (RS)
and zincblend (ZB) CoN with different facets. Presence of a thin
cobalt oxide layer considerably increases the limit-ing potential.
Of note, “L” in figure legend is an abbreviation for “Layer”. For
example, 1L-CoO/7L-CoN represents sys-tems with 1 layer of CoO over
7 layers of CoN.
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activities (Figures S8). For the (110) and (111) facets, O* +
(H+ + e−) à OH* is the limiting step. For the (100) facet, OH* +
(H+ + e−) à H2O is found to be the limiting step. Figure 3
summarizes the calculated limiting potential for different studied
configurations. A more positive limiting po-tential is desirable
for catalysis on cathodes in fuel cells. Based on our DFT results
(red bars, Figure 3) the CoNZB(111) is the most active facet for
the ORR, while the other facets contribute minimally to the
activity of the cobalt nitrides for the reaction. Nevertheless, the
highest limiting potential calcu-lated in this study, 0.59 V for
CoNZB(111), is lower than the experimentally reported onset
potential (0.85 V)75 which again raises the question on the actual
active site in these materials. Next, we examine the relative
stability of different facets of the CoNs and their formation
probability, we compare surface energies for each facet based on
energy per atom of the sys-tems with respect to the bulk state. Of
note, to reduce the finite size effect of the slabs, we increase
layers of the systems so that thickens of each slab is ~ 14 Å. On
the basis of these analysis, we predict the (100) and (111) facets
to have the lowest surface energy for CoNRS and CoNZB, respectively
(Figure S9 in the Supporting Information). We also note, the energy
difference between CoNZB(111) and CoNZB(110) is about 0.001
eV/atom, indicating a comparable formation pos-sibility of both
facets. In search for catalytic active sites that show limiting
potential closed enough to the reported experi-
mental results,75 we next consider the possibility of oxide
for-mation over the nitride structures under ORR conditions.
In the view of solid-state chemistry, it is known that nitrides
are thermodynamically less stable than oxides, especially in the
aqueous and strongly oxidative conditions of the ORR.80,81 Hence it
is crucially important to evaluate the presence of the oxide that
may form over the nitride surfaces under relevant ORR conditions.
To further elaborate this, we investigate con-version of one layer
of CoNRS(100) and CoNZB(110) to the corresponding oxides though the
following reaction:
𝐶𝑜𝑁 + 𝐻&𝑂 → 𝐶𝑜𝑂 +*&𝑁& + 𝐻& (1)
The pH and potential dependence of the reaction (1) is
esti-mated via the method introduced by Rossmeisl et al.97, and the
calculated phase diagrams are shown in Figure 4. We predict that
for both the rocksalt and zincblend phases, the oxide for-mation
starts in early oxidation potentials of ~ 0.2 V at pH = 0
(Figure 3). An increase in pH decreases the oxidation potential
required for the conversion of the nitrides to oxides. These
results propose the surface nitrides are vulnerable under ORR
conditions to form oxide layers. In particular, we note the CoN
systems are mainly used in alkaline pH for the ORR catalysis,
Figure 5. Free energy diagram for ORR over (A) CoNRS(100) and
(B) CoNZB(110) in the presence and absence of an overlay-er of CoO.
The oxy-overlayer significantly affects the bonding free energy of
the O* for both CoNs systems. The O* is bond-ed to a surface
nitrogen atom in the absence of the CoO layer, while it is bonded
to Co atom if the oxy-overlayer is present (insets). Color code:
Co, pink; N, blue; O, red. Figure 4. Pourbiax diagrams as a
function of pH and poten-
tial (vs SHE) for conversion of a layer of (A) CoNRS(100) and
(B) CoNZB(110) to the corresponding oxides. For the oxides, DFT+U
is employed.
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where a low potential is sufficient to convert surface CoN to
CoO. These results may indicate the active sites are predomi-nately
on the CoO layers that are formed due to the surface oxidation of
the CoNs. Additionally, we consider the possibil-ity of nitrogen
constituent oxidation resulting in NO formation during ORR
reaction, leaving behind the metallic Co. As shown in Figure S10 in
the Supporting Information, the for-mation of metallic Co is
considerably unlikely in comparison to the oxides formation. Given
that the complete conversion of the bulk CoN to bulk CoO may be
limited by diffusion kinetics, it is important to consider a
situation where only a few top layers of CoN are oxidized. For
these calculations, we use a computational mod-el system where the
top n layers (n = 1 – 4) of 8-layer CoN are converted to CoO. We
then investigate the effects of a thin oxide layer on the activity
of the CoN systems. Figure 3 shows the presence of a CoO layer
drastically alters the calculated ORR limiting potential. The CoO
overlayers of CoNRS(100) and CoNZB(110) show considerably higher
limiting potentials in comparison with that of the corresponding
pure CoNs (Fig-ure 3, blue and red bars). The calculated limiting
potential for CoNRS(100) and CoNZB(110) with one oxy-overlayer is
found to be 0.88 and 0.77 V, in very good agreement with the
exper-imental result (0.85 V).75 Additionally, DFT with the Hubbard
correction on the Co sites of oxy-overlayer is performed. Note, CoN
is known to be metallic with no band-gap; therefore, the Hubbard-U
correction is not applied on the Co sites of the nitrides. The
inclusion of U correction on oxy-overlayer struc-tures decreases
the limiting potentials by ~0.1 V (Figure S11 in the Supporting
Information). Nevertheless, the limiting potential of the
CoNRS(100) and CoNZB(110) with an oxy-overlayer remains
significantly higher than that of the pure nitrides, ensuring the
remarkable promotional effect of an ultra-thin layer of CoO on the
ORR. We further examine mul-tilayer of CoO over CoNRS(100) and
CoNZB(110) to explore the depth of promotional effect on the ORR
activity. Based on the DFT results, the first oxide layer plays the
key role to promote the ORR activity of the nitrides (Figure 3,
blue bars). Addition of the subsequent layers of CoO is found to
decrease the ORR activity of CoNRS(100) relatively, while has
negligi-ble effect on CoNZB(110). Nevertheless, care should be
taken in expecting the formation of more oxide layers as it might
be kinetically hindered. Other facets of the cobalt nitrides are
predicted to exhibit a lower limiting potential (Figure 3) upon
formation of top oxide layer. In particular, replacement of the
topmost nitrogen atoms of the CoNRS(111) is found to result in a
relative detachment of the oxide layer from the rest of the slab;
thus, a stable CoO layer over CoNRS(111) is unlikely. To further
elucidate the effects of CoO overlayers on the ORR activity, we
construct the free energy diagrams over the CoNRS(100) and
CoNZB(110) in the presence and absence of the oxy-overlayer in
comparison to their corresponding pure oxides (Figures 5A and 5B).
While replacement of the topmost nitrogen atoms with oxygen
moderately affects interaction energy of OOH* and OH* with the
surface, a significant in-fluence on the adsorption of the O* is
observed. As shown in the insets of Figures 5A and 5B, the presence
of the oxy-overlayer of the CoNRS(100) and CoNZB(110) forces the
sur-face Co atoms to be the active sites for the adsorption of the
O* rather than the surface N atoms as for the pure CoN sys-tems.
Such a change in the adsorption pattern decreases the bonding
energy of O* in comparison with that of the pure CoNs. This in turn
lowers the endothermicity of the reaction
O* + H+ + e− à OH* when electrode potential (Uelec) is ap-plied,
leading to a higher limiting potential. Figures S12-S14 in the
Supporting Information shows the effect of an oxy-overlayer on ORR
free energy diagram for CoNRS(110), CoNZB(100), and CoNZB(111). Of
note, bonding patterns of the ORR intermediates with the surfaces
are found to be similar to those of the corresponding pure CoNs. In
general, the presence of a CoO overlayer strengthens the bonding
energy of the in-termediates with the surfaces. Therefore, the
formation of the oxy-overlayer is expected to lead to a lower
limiting potential, as shown in Figure 3. Previous studies on
rutile oxide structures have shown that the solvent can slightly
affect the free energy of individual intermediates and in some
cases the limiting potential, howev-er, the overall trend in
activity across the same class of materi-als remains
unchanged.98,99 As our goal in this study is to in-vestigate the
trends in activity across different phases and fac-ets of CoN, we
do not consider solvation correction for all the systems. However,
to evaluate the solvation effect on the ac-tivity of oxy-overlayer
of the nitrides, as an example, we cal-culated the limiting
potential for the CoNRS(100) with one layer of CoO using
VASPsol.100,101 The calculated limiting potentials for such systems
are 0.94 and 0.81 V in the absence and presence of implicit solvent
(water). These results confirm our hypothesis and agrees with our
previous observations98 on negligible effect of solvation
corrections on the overall activity of oxides. Aforementioned
results establish that formation of CoO overlayer is responsible
for the changes in the binding sites, and explains the
experimentally observed activity of CoN ma-terials. However, it is
useful to examine the underlying reason of the differences in
binding energies. The bulk state of the CoN and CoO systems has
different size of unit cell (a). Our DFT calculations show a =
4.08, 4.28, 4.30, and 4.55 Å for the CoNRS, CoNZB, CoORS, and
CoOZB, respectively, in agreement with previous studies.102,103
Therefore, the formation of a thin oxy-layer over the CoN systems
requires compressive strain in CoO with respect to those of the
corresponding bulks. To shed light on the effects of the strain
induced by the CoN platforms on the catalytic activity, we select
the CoNRS(100) and CoNZB(110) systems in which four layers of CoO
are loaded on four layers of CoN (4L-CoO/4L-CoN, Figure 2). The CoN
layers are simply removed, and DFT are applied to calculate the
limiting potential of the remaining CoO slabs using two different
cell parameters imposed by corresponding bulk CoN and CoO. Of note,
all atoms are allowed to relax during the DFT optimization. On the
basis of these analysis we predict the strain applied to the CoOZB
increases ORR limiting poten-tial significantly, while has a less
effect on the case of CoORS (Figures S15 and S16 in the Supporting
Information). It is worth mentioning the strain imposed by the
nitride supports is bigger for the CoOZB (~6%) than that for the
CoORS (~5%); thus, a stronger effect for the CoOZB is reasonable.
We briefly pause here for perspective. With an increase in
thickness of CoO shell over CoN, it is expected that CoO overlayer
eventually adopts lattice parameters that correspond to bulk CoO.
With this issue in mind, our DFT results predict an increase in
thickness of CoOZB overlayer, that can be pru-dent to form during
the synthesis of CoO/CoN nanoparticles, can lower the limiting
potential significantly. It is also worth mentioning that our
theoretical results offer qualitative insights on the trends in
activity across several
-
studied structures and provide a hint for understanding the
observed experimental activity rather than predicting the
ex-perimentally measured onset potential. ■ CONCLUSION DFT
calculations are carried out to investigate catalytic activity of
cobalt nitrides for the oxygen reduction reaction. Two different
CoNs with the structures of rocksalt and zincblend are considered.
The interaction energy of ORR in-termediates with the CoNs
terminated by the facets of (100), (110), and (111) are calculated.
Our results show CoNZB(111) is the most active facet for the ORR.
Nevertheless, pure CoNs most probably undergo surface oxidation
under ORR condi-tion, leading to the formation of a thin CoO layer
over the nitride systems. Such an oxide layer is found to
significantly improve the ORR performance. These results
rationalize a wide range of reports in the literature on the
activity of CoN and provides guidelines for understanding the
active phase in other nitrides. In particular, our results explain
how ‘insulat-ing’ and inactive oxides can be modified by supporting
them on a ‘conducting’ nitride, leading to strained active oxide.
This design paradigm offers guidelines to disentangle/de-convolute
the conductivity and activity properties, leading to design of
catalysts that are active and stable under reaction conditions.
ASSOCIATED CONTENT Supporting Information The Supporting
Information is available free of charge on the ACS Publications
website. Free energy diagrams for oxygen reduction reaction over
different facets of cobalt nitrides in the absence and presence of
a CoO overlayer. Optimized structures of the ORR adsorbates over
the surface of CoNs. The energy per atom of the CoNs’ slabs with
respect to the bulk states. Effect of strain on ORR limiting
poten-tials of CoO. Pourbiax diagrams for the bulk metallic and
nitrides of Co (PDF).
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Notes The authors declare no competing
financial interest.
ACKNOWLEDGMENT The authors gratefully acknowledge the support by
Toyota Re-search Institute.
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