Reaction Pathway for Oxygen Reduction on FeN4 Embedded Graphene

Jan 14, 2014 - OH, OOH, HOOH, and H2O) on the FeN4 embedded graphene is a ..... (10) Gasteiger, H. A.; Markovic, N. M. Just a Dream-or Future. Reality...
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Reaction Pathway for Oxygen Reduction on FeN4 Embedded Graphene Shyam Kattel and Guofeng Wang* Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: The detailed reaction pathways for oxygen reduction on FeN4 embedded graphene have been investigated using density functional theory transition-state calculations. Our first-principles calculation results show that all of the possible ORR elementary reactions could take place within a small region around the embedded FeN4 complex. It is predicted that the kinetically most favorable reaction pathway for ORR on the FeN4 embedded graphene would be a four-electron OOH dissociation pathway, in which the rate-determining step is found to be the OOH dissociation reaction with an activation energy of 0.56 eV. Consequently, our theoretical study suggests that nonprecious FeN4 embedded graphene could possess catalytic activity for ORR comparable to that of precious Pt catalysts.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis catalysts are rather lacking. In this Letter, we report the firstprinciples DFT calculation results of the binding energies for all possible ORR chemical species and the activation energies for all possible elementary chemical reactions in ORR on a FeN4 embedded graphene catalyst. Furthermore, we identify the most favorable pathway for ORR to adopt on the FeN4 embedded graphene from our DFT-calculated activation energies. Our DFT results in this study not only show that the catalytic activity of the FeN4 embedded graphene is comparable to that of Pt catalyst but also demonstrate that the ORR on this class of catalyst progresses most possibly via an OOH dissociation reaction pathway. Our spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP) code.19,20 Projector augmented wave potentials were used to describe the core electrons.21 Generalized gradient approximation of the Perdew, Burke, and Ernzernhof functional was used to describe the electronic exchange and correlation.22 The Kohn−Sham one-electron wave functions were expanded by using a plane wave basis set with a kinetic energy cutoff of 400 eV. The Brillouin zone was sampled using a 4 × 4 × 1 k-point grid in the Monkhorst−Pack scheme.23 During geometry optimization, the atomic positions were optimized until the forces were below 0.02 eV/Å. The transition states of chemical reactions were located through the climbing image nudged elastic band (NEB) method,24 in which all of the force components

t is of great societal benefit to find efficient, durable, yet lowcost electrocatalysts for commercial deployment of polymer electrolyte membrane fuel cells (PEMFCs), especially in automotive transportation.1−3 In this regard, pyrolyzed carbon-supported nonprecious transition-metal and nitrogencontaining catalysts have demonstrated promise as abundant and cost-effective electrocatalysts,3−8 alternative to precious Pt alloys,9−11 to electrocatalyze the oxygen reduction reaction (ORR) occurring at the cathode of a PEMFC. For example, Lefevre et al.4 reported that the current density of an iron-based catalyst for ORR was equal to that of a Pt catalyst at a cell voltage of ≥0.9 V. Moreover, a nonprecious metal catalyst derived from polyaniline, iron, and cobalt was found to catalyze the ORR at onset potentials within ∼60 mV of that delivered by a state-of-the-art Pt/C catalyst.3 The synthesis approach of these nonprecious metal ORR catalysts involves pyrolysis of nitrogen, transition metals, and carbon precursors.3−5,12,13 It is believed that nitrogen-coordinated transition-metal complexes (such as FeN4) embedded into carbon templates are formed during the pyrolysis process4,12−15 and are conjectured as the active sites in these nonprecious metal catalysts for ORR.4,12,13,15 Bridging the performance gap between these nonprecious metal catalysts and Pt-based catalysts requires fundamental understanding about the underlying catalytic mechanisms of ORR on both types of electrocatalysts. For Pt-based catalysts, first-principles density functional theory (DFT) calculations have been successfully applied to predict the energetics, evaluate the activity, and elaborate on the catalytic mechanisms of ORR on their surfaces.16−18 However, theoretical mechanistic studies of the ORR progression on nonprecious metal

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© 2014 American Chemical Society

Received: December 17, 2013 Accepted: January 14, 2014 Published: January 14, 2014 452

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Figure 1. Atomic structures of relaxed geometries for various ORR chemical species adsorbed on the FeN4 embedded graphene. (a−g) H, O, OH, O2, OOH, H2O, and HOOH adsorption on the central Fe atom of the FeN4 complex, respectively. (h−j) H, O, and OH adsorption on adjacent graphene surface. In the figure, the gray, blue, yellow, red, and white balls represent C, N, Fe, O, and H atoms, respectively.

Considering the entropy of the gas O2 molecule at T = 298.15 K (0.63 eV27), we predicted a free energy of −0.35 eV for the O2 adsorption on the FeN4 embedded graphene, with the most stable configuration shown in Figure 1d. This prediction agrees qualitatively with the experimental value of −15.7 kJ mol−1 (i.e., −0.16 eV) obtained from model fitting on the reduction charge of adsorbed O2 on heat-treated iron and polyaniline-based catalysts.28 Hence, both our DFT study and the previous measurement suggest that the O2 molecule could interact strongly with the nonprecious ORR catalysts. In contrast, we found a relatively weak binding (Eb = −0.18 eV) of the ORR product H2O molecule on the FeN4 embedded graphene. Interestingly, our DFT calculations suggest that the ORR intermediate HOOH molecule cannot maintain its chemical structure on the FeN4 complex; instead, it immediately decomposes into 2(OH) molecules during the adsorption process (as shown in Figure 1g). The chemisorption of an O2 molecule on the FeN 4 embedded graphene is considered to be the first necessary step to initialize the ORR on the catalyst. Following the O2 adsorption, the four-electron ORR on the FeN4 embedded graphene could proceed through three possible reaction pathways, as shown in Figure 2b (i.e., (i) O2 dissociation pathway, (ii) OOH dissociation pathway, and (iii) HOOH dissociation pathway).16,29 In this study, we hypothesize that the involved ORR chemical species would coadsorb onto the catalyst with their stable adsorption configurations (shown in Figure 1) at the reactant and product states of the ORR reactions. Furthermore, we employed the DFT method to find the geometric configurations of the transition states (Figures 3 and S2, Supporting Information) and calculated the reaction energetics (heats of reactions (ΔH0) and activation energies (Ea)) for all of the possible elementary ORR reactions on the FeN4 embedded graphene. In pathway (i) of Figure 2b, we depict the progression of ORR on the FeN4 embedded graphene via an O2 dissociation pathway. In this ORR pathway, the chemisorbed O2 molecule immediately undergoes the O−O bond scission reaction to

perpendicular to the tangent of the reaction path were relaxed to be less than 0.05 eV/Å. In this work, we have included zeropoint energy corrections in all of the calculated reaction energies. The FeN4 embedded graphene catalyst was modeled with a 32-atom graphene super cell containing an FeN4 complex in the graphene plane. Our choice of FeN4 embedded graphene is consistent with the recent experimental evidence4,12−15,25,26 that the FeN4 complexes in carbon exhibit high activity for ORR. Additionally, our DFT calculation results indicated that the FeN4 embedded graphene possessed the best thermodynamic stability among various FeNx (x = 0−4) complexes embedded in a monolayer graphene (Figure S1 in the Supporting Information). The favorable adsorption of various ORR species (which directly participate in the ORR process and include O2, H, O, OH, OOH, HOOH, and H2O) on the FeN4 embedded graphene is a prerequisite for ORR to proceed on the catalyst. Consequently, we calculated the binding energies of these ORR species on the FeN4 embedded graphene using the DFT method. The binding energy (Eb) of an adsorbate is calculated as the energy difference between the adsorbate−catalyst system and the assembly of the isolated FeN4−graphene complex and the adsorbate in the gas phase. For each ORR species, we examined its various possible adsorption configurations and energies on different adsorption sites on the FeN4 embedded graphene. Particularly, we determined the stable adsorption configurations for H, O, and OH both on the central transition metal of the FeN4 complex and on the surrounding graphene sheet. In Figure 1, we present the optimized geometries and calculated binding energies of the ORR species adsorbed on the FeN4 embedded graphene. Our DFT results show that all of the ORR species would bind favorably to the central transition metal Fe of the FeN4 complex. It is worth mentioning that the ORR reactant O2 molecule was predicted to stably bind (Eb = −0.98 eV) only to the FeN4 complex with an end-on adsorption configuration, whereas another ORR reactant H could bind favorably to both the FeN4 complex (Eb = −2.20 eV) and the adjacent graphene surface (Eb = −1.40 eV). 453

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Figure 2. (a) Various adsorption sites on the FeN4 embedded graphene (T1 and T2 refer to two top sites; B1 and B2 refer to two bridge sites) involved in ORR reactions, which occur within a reactive region (shown as the light green shaded region). In the figure, the gray, blue, and yellow balls represent C, N, and Fe atoms, respectively. (b) Three possible reaction pathways for ORR on the FeN4 embedded graphene: (i) O2 dissociation pathway, (ii) OOH dissociation pathway, and (iii) HOOH dissociation pathway. In the figure, zero-point energy corrected heats of reaction and activation energies (in units of eV) are given in parentheses in the form of (ΔH0, Ea).

Figure 3. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for (a) O2 dissociation, (b) O2 hydrogenation, (c) OOH dissociation, (d) OOH hydrogenation, (e) O hydrogenation, and (f) OH hydrogenation reactions on the FeN4 embedded graphene. In the figure, the gray, blue, yellow, red, and white balls represent C, N, Fe, O, and H atoms, respectively.

form two separated O atoms. At the product state (shown in Figure 3a), one O atom is adsorbed on top of the Fe atom (denoted as site T1), and the other O atom is adsorbed between two neighboring C atoms (denoted as site B2). Our DFT calculation predicts that this O2 dissociation reaction on the FeN4 embedded graphene is endothermic with ΔH0 of +0.60 eV and requires an activation energy of 1.19 eV. Following the O2 dissociation reaction, the O atom at site T1 will proceed with two sequential hydrogenation reactions to form OH (Figure S2a (Supporting Information)) and the final ORR product H2O molecule (Figure S2b (Supporting Information)), whereas the other O atom is chemisorbed at site B2. Once the produced H2O molecule is removed from site T1, the O atom at site B2 could diffuse through site B1 (bridging between two T2 sites) to site T1. Subsequently, this O atom undergoes two hydrogenation reactions (Figures 3e and 3f) and is reduced to the second H2O molecule at site T1. Our DFT results reveal that the rate-determining step in the O2 dissociation ORR pathway on the FeN4 embedded graphene is the O2 dissociation step converting *O2(T1) to *O(T1) + *O(B2) and has an Ea of 1.19 eV.

In pathway (ii) of Figure 2b, we depict the progression of ORR on the FeN4 embedded graphene via an OOH dissociation pathway in which the chemisorbed O2(T1) first undergoes a hydrogenation reaction to form an adsorbed OOH(T1) molecule, as shown in Figure 3b. Then, the O−O bond of the OOH is split (shown in Figure 3c) to produce an O atom adsorbed at site T1 and an OH molecule adsorbed on the top of a nearby C atom (denoted as site T2). Subsequently, the O atom at site T1 undergoes sequential hydrogenation reactions to form the first H2O molecule, whereas the OH molecule stays at site T2. After the product H2O molecule is released from site T1, the OH molecule at site T2 will migrate to site T1 and be reduced to generate the second H2O molecule through a hydrogenation reaction. Our DFT calculations indicate that the rate-determining step in the OOH dissociation ORR pathway on the FeN4 embedded graphene is the OOH 454

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dissociation step, in which *OOH(T1) dissociates to form *O(T1) + *OH(T2) and has an Ea of 0.56 eV. In pathway (iii) of Figure 2b, we present the progression of ORR on the FeN4 embedded graphene via a HOOH dissociation pathway. The steps that follow the O2 adsorption in this pathway are a hydrogenation reaction to form OOH at site T1 and a subsequent hydrogenation reaction (shown in Figure 3d) to form a HOOH molecule, which dissociates spontaneously to generate two OH molecules both adsorbed on the central transition metal Fe atom (site T1). For the next hydrogenation reaction to occur at T1, one of the two OH molecules at site T1 must migrate to the adjacent site T2. It is found from our DFT calculations that this migration process of the OH molecule required a rather high activation energy of 1.15 eV. Starting from the coadsorption configuration of *OH(T1) + *OH(T2), the remaining steps of ORR along this HOOH dissociation pathway are the same as those in the OOH dissociation pathway. In this study, we predict that the rate-determining step in the HOOH dissociation ORR pathway on the FeN4 embedded graphene is the reorganization step in which a coadsorption configuration 2*OH(T1) transforms to a configuration *OH(T1) + *OH(T2) and has an Ea of 1.15 eV. Summarizing our calculated activation energies for all of the ORR elementary steps in Figure 2b, we conclude that the OOH dissociation pathway is the most probable pathway for ORR to proceed on the FeN4 embedded graphene. Our reasoning process is as follows. The O2 molecule adsorption on the catalyst is the first step in all three possible ORR pathways. Following this step, the adsorbed O2 molecule could be either dissociated into two O atoms or be hydrogenated to form an OOH molecule. Our DFT calculations predicted the activation energy for the O2 dissociation reaction to be 1.19 eV, whereas the activation energy for the O2 hydrogenation reaction is 0.00 eV. It can be inferred that the ORR on the FeN4 embedded graphene would be more favorable to proceed from *O2(T1) to OOH(T1) than to *O(T1) + *O(B2) immediately following O2 adsorption. Hence, the O2 dissociation pathway is not believed as a kinetically favorable one for ORR on the FeN4 embedded graphene. After the OOH is formed at site T1, it can be either dissociated into O and OH species by breaking the O−O bond in the OOH or further hydrogenated to form a HOOH molecule that spontaneously breaks its O−O bond on the catalyst and dissociates into two OH molecules. Our DFT results suggested that on the FeN4 embedded graphene catalyst, the OOH dissociation reaction with an activation energy of 0.56 eV would be more favorable than the OOH hydrogenation reaction with an activation energy of 0.74 eV. Consequently, the OOH dissociation pathway is predicted to be the most preferred ORR pathway on the FeN4 embedded graphene under zero electrode potential. Our finding here is consistent with previous propositions that the ORR pathway on nonprecious Fe/N/C catalysts involves the formation and dissociation of OOH intermediate.30,31 Moreover, our DFT results gave the activation energy for the rate-determining step of the ORR on the FeN4 embedded graphene to be 0.56 eV following the OOH dissociation pathway. Our previous study predicted the activation energy of the rate-determining step of the ORR to be 0.79 eV on the Pt(111)16 and 0.80 eV on the Pt(100) surfaces.32 Therefore, our DFT calculations suggested that the FeN4 embedded graphene could promote a fourelectron ORR via an OOH dissociation pathway and with catalytic activity for ORR comparable to (may even higher) that of Pt catalysts. Our calculated free energy diagrams in Figure S3

(Supporting Information) indicate that the OOH dissociation ORR pathway is thermodynamically feasible up to some critical electrode potential. In conclusion, we investigated the reaction pathways for ORR on a nonprecious FeN4 embedded graphene catalyst using first-principles DFT calculations. Specifically, we calculated the adsorption energy of ORR species and the energetics (heats of reaction and activation energies) of all of the possible ORR elementary reactions on the modeled FeN4 embedded graphene. Our DFT results indicate that the O2 molecule is only chemisorbed to the central transition metal Fe of the FeN4 complex embedded in the graphene layer, implying that the existence of an N-chelated transition metal plays a pivotal role to initializing the ORR in this class of catalysts. Moreover, we show that for ORR on the FeN4 embedded graphene, the rate-determining step is the O2 dissociation reaction with an activation energy of 1.19 eV in the O2 dissociation pathway, the OOH dissociation reaction with an activation energy of 0.56 eV in the OOH dissociation pathway, and the OH diffusion step with an activation energy of 1.15 eV in the HOOH dissociation pathway. Furthermore, our analysis suggests that the OOH dissociation pathway is the kinetically most favorable one for the ORR to follow on the FeN4 embedded graphene. Hence, our current study reveals the details of ORR progression on the FeN4 embedded graphene and predicts a comparable catalytic activity of the FeN4 embedded graphene for ORR as compared to that of Pt.

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ASSOCIATED CONTENT

* Supporting Information S

Optimized geometries and formation energies of Nx and FeNx complexes embedded in graphene, atomic structures of the initial state, transition state, and final state for some ORR elementary reactions, and DFT-calculated free energy diagrams for ORR on the FeN4 embedded graphene following the OOH dissociation ORR pathway. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant No. DE-FG02-09ER16093). The authors gratefully acknowledge the computational resources provided by the computer facility at Center for Simulation and Modeling of the University of Pittsburgh.

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