Ultrathin Cobalt Oxide Overlayer Promotes Catalytic Activity of Cobalt

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An Ultra-Thin Cobalt-Oxide Overlayer Promotes Catalytic Activity of Cobalt Nitride for Oxygen Reduction Reaction Hadi Abroshan, Pallavi Bothra, Seoin Back, Ambarish R. Kulkarni, Jens K. Norskov, and Samira Siahrostami J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12643 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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The Journal of Physical Chemistry

An Ultra-Thin Cobalt-Oxide Overlayer Promotes Catalytic Activity of Cobalt Nitride for Oxygen Reduction Reaction Hadi Abroshan,† Pallavi Bothra,† Seoin Back,† Ambarish Kulkarni,† Jens K. Nørskov,‡,† and Samira Siahrostami† * †

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 membrane fuel cells (PEMFC) and metal-air batteries. Owing to the scarcity of the current state-of-the-art Pt-based catalysts, costeffective 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 overpotential 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 efficiency, 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 reaction (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 regard, rational design and synthesis of cheap and stable catalysts with low or zero content of Pt that show similar or superior 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 metalfree carbon-based materials,22-25 transition metal chalcogenides,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 particular attention as competitive catalysts because of their unique properties, such as high electrical and thermal conductivities, tailorable electronic features, high melting points, exceptional hardness, and chemical resistance to corrosion in aqueous media.40-44 Indeed, TMNs have been applied as supports to enhance the activity and durability of precious metals (e.g., Pt and Pd) in comparison to commercial Pt/C catalyst.4549 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 nitrogen 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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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.

Cobalt is one of the non-precious elements that is widely tested as electrocatalysts for oxygen and hydrogen reactions.5962 However, issues such as poor electrical conductivity of cobalt 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 supported cobalt oxide is shown to have high electrocatalytic activity that has been attributed to the synergistic coupling effects 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 materials.70-72 Recently, cobalt nitride (CoN) has gained particular interest as a metallic interstitial system. Structural analysis of Co nitride revealed that nitrogen atoms are bonded to cobalt atoms, emerging metallic properties through the nitride framework; therefore, providing an ideal catalyst platform for the electrocatalytic 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 cosynthesized 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 example, 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 evidenced and reported in the literature, there is clearly the lack of careful experimental characterization and future experimental 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 conditions 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 worthwhile 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 systematically 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 focused on Co-systems, it is very likely that similar phenomena/active sites may be responsible for the high catalytic activity of the other transition metal nitride systems. ■ COMPUTATIONAL DETAILS Periodic DFT calculations are performed to investigate adsorption 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 potential on the free energy of the intermediates is taken into account 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 activity 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 mentioning 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 optimization of various catalysts.89,90 Thermodynamic predicted activity volcano for transition metals has been shown to be in close agreement with the predicted kinetic activity volcano. Therefore, there is a close connection between the kinetic and thermodynamic 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 computational 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 activity, we replace the first to fourth layers of the CoN with CoO. In this case, CoO layer(s) as well as two CoN layers


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The Journal of Physical Chemistry 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 correction (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 functional (RPBE)95 are used to describe the electrons-nuclei interaction, and electron exchange and correlation, respectively. The kinetic energy cutoff was chosen to be 500 eV and integration in the reciprocal space was carried out at 8 × 8 × 1 kpoint sampling. Spin polarization is carried out for all systems. All calculations are performed with the Quantum Espresso package.96

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

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 limiting potential. Of note, “L” in figure legend is an abbreviation for “Layer”. For example, 1L-CoO/7L-CoN represents systems with 1 layer of CoO over 7 layers of CoN.

Figure 2. Free energy diagrams for the four-electron reduction of oxygen at Uelec = 0.0 V on different facets of (A) CoNRS and (B) CoNZB.

■ 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 monolayer (ML) adsorbate is considered.

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−) OH* as the limiting step with limiting potential of 0.14 V, i.e., overpotential = (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−) 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 energies 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 spontaneous O-O bond dissociation, leaving O* and OH* at the surface (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*, resulting in stronger adsorption free energies and hence lower



activities (Figures S8). For the (110) and (111) facets, O* + (H+ + e−) OH* is the limiting step. For the (100) facet, OH* + (H+ + e−) H2O is found to be the limiting step. Figure 3 summarizes the calculated limiting potential for different studied configurations. A more positive limiting potential 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 calculated 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 systems 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 possibility of both facets. In search for catalytic active sites that show limiting potential closed enough to the reported experi-

Figure 4. Pourbiax diagrams as a function of pH and potential (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.

mental results,75 we next consider the possibility of oxide formation over the nitride structures under ORR conditions.

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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 conversion of one layer of CoNRS(100) and CoNZB(110) to the corresponding oxides though the following reaction:

→

(1)

The pH and potential dependence of the reaction (1) is estimated 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 formation starts in early oxidation potentials of ~ 0.2 V at pH = 0

Figure 5. Free energy diagram for ORR over (A) CoNRS(100) and (B) CoNZB(110) in the presence and absence of an overlayer of CoO. The oxy-overlayer significantly affects the bonding free energy of the O* for both CoNs systems. The O* is bonded 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 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,


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The Journal of Physical Chemistry where a low potential is sufficient to convert surface CoN to CoO. These results may indicate the active sites are predominately on the CoO layers that are formed due to the surface oxidation of the CoNs. Additionally, we consider the possibility 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 formation 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 model 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 (Figure 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 experimental 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 structures 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 oxyoverlayer 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 multilayer 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 negligible 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 influence on the adsorption of the O* is observed. As shown in the insets of Figures 5A and 5B, the presence of the oxyoverlayer of the CoNRS(100) and CoNZB(110) forces the surface Co atoms to be the active sites for the adsorption of the O* rather than the surface N atoms as for the pure CoN systems. 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− OH* when electrode potential (Uelec) is applied, leading to a higher limiting potential. Figures S12-S14 in the Supporting Information shows the effect of an oxyoverlayer 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 intermediates 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, however, the overall trend in activity across the same class of materials remains unchanged.98,99 As our goal in this study is to investigate the trends in activity across different phases and facets of CoN, we do not consider solvation correction for all the systems. However, to evaluate the solvation effect on the activity of oxy-overlayer of the nitrides, as an example, we calculated 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 materials. 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 potential 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 prudent 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 experimentally 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 intermediates 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 condition, 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 ‘insulating’ 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 potentials 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 Research Institute.

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Ultrathin Cobalt Oxide Overlayer Promotes Catalytic Activity of Cobalt

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