Silicon-Doped Nitrogen-Coordinated Graphene as Electrocatalyst for

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C: Energy Conversion and Storage; Energy and Charge Transport

Silicon Doped Nitrogen Coordinated Graphene as Electrocatalyst for Oxygen Reduction Reaction Chandra Chowdhury, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09203 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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The Journal of Physical Chemistry

Silicon Doped Nitrogen Coordinated Graphene as Electrocatalyst for Oxygen Reduction Reaction Chandra Chowdhury and Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur – 700032, Kolkata, WestBengal, India.

Abstract: For large scale commercial applications of fuel cells, it is necessary to develop carbon based metal-free electrocatalysts which are highly durable, cost-effective and environmentally benign for oxygen reduction reaction (ORR). Here using first principles simulations, we have explored the potentials of silicon doped nitrogen coordinated graphene (SiGN4) system as efficient electrocatalyst for ORR in fuel cell in acidic environment. Introduction of different electronegative atoms (Si, N) on graphene surface facilitates the activation of O2 and desorption of H2O from the surface which are two key steps for good ORR catalyst. The plausible reaction pathways are studied and it is revealed that the reaction mainly occurs via 4ereduction pathway following associative approach. Least stabilization of HOOH on Si-GN4 surface ruled out the possibility of 2e- reduction pathway. Hydrogenation of oxygen (O2) is found to be the kinetically rate determining step (RDS). Our computational study reveals that SiGN4 surface is quite a promising catalyst with high efficiency for ORR in fuel cells.

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Introduction: Due to the increasing energy demands, resources available from fossil fuels are getting depleted rapidly which has generated interests for highly efficient and renewable energy technologies. Fuel cell which is a replacement for combustion based energy sources, has created considerable attractions for its high efficiency and clean power source.1 But the sluggish kinetics of cathodic oxygen reduction reaction (ORR) is the main obstacle for its global usage. So, selecting appropriate cathode catalyst to accelerate ORR kinetics has become a key. Till date Pt based materials are the most efficient catalysts for ORR but their high cost and scarcity hinder commercial applications for fuel cells.2-4 Therefore, it is an important research area to develop affordable and easily available electrocatalysts with high catalytic activity which can be used as a replacement for Pt-based fuel cell catalysts. Heteroatom doped carbon based nanomaterials namely, mesoporous carbon5, carbon aerogels6, carbon nanoparticles7, carbon nanotubes8, graphene9,10 have been used widely as a replacement of Pt-based catalysts due to their high abundance, low cost, high durability and good CO tolerance. Amongst them, demand for graphene based systems are highest due to their high surface to volume ratio, high carrier mobility, high mechanical flexibility and superior electrical conductivity which lead the catalyst to its optimum efficiency.11 As defect free graphene and other carbon based systems are chemically inert, one requires the introduction of additional defects or doping by heteroatoms.12 Extensive research has been done on heteroatom doped graphene based systems for exploration of their ORR catalytic activity such as nitrogen (N), boron (B), phosphorus (P), sulfur (S), antimony (Sb), tin (Sn) and their mixtures.13-17 Xia et al observed that B-doped graphene showed excellent catalytic activity towards ORR and also it has better CO tolerance capability than Pt/C catalyst in alkaline medium.18 Among the heteroatom 2


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doped surfaces considered previously, N-doped carbon surfaces have been studied in great details both experimentally and computationally. Due to an electronegativity difference between N and C resulting in removal of electroneutrality of graphene which enhances the catalytic activity of N-doped graphene. There are different types of N containing graphene systems namely, graphitic, pyridinic and pyrrolic systems. Wu and coworkers suggested the enhanced catalytic performances of pyridinic N-doped graphene arises from the introduction of positive spin density and asymmetry in atomic charge density.19,20 Besides N-doped graphene, there have been extensive researches on metal embedded N-doped graphene systems. N-doped graphene generally stabilize the transition metal atoms which lead to transition metal coordinated N-doped graphene (MNx-G) making accelerated activity and stability towards ORR activity. It has been already reported both experimentally and theoretically that introducing nonprecious transition metal atoms (Mn, Fe, Co, Ru) can enhance the activity, selectivity and durability of these catalysts.21-24 Several studies revealed that the ORR catalytic activity of these M-GN4 systems are comparable with that of Pt-based catalysts which mainly arises as a result of the incorporated metal atom which activates the bonded nitrogen atoms as well as neighboring carbon atoms that causes good binding of adsorbed O2 molecule. Research in the area of non-metal (NM) coordinated GN4 sheet (NM-GN4) is still at infancy. NM atoms are more inclined to the GN4 surfaces and as a result they are able to transfer more electrons towards the coordinated nitrogens and the neighboring carbon atoms and therefore are expected to activate O2 more strongly than M-GN4 systems. Recently, Dai et al revealed that silicon doped GN4 system (Si-GN4) is a highly efficient catalyst for CO oxidation reaction and are in fact, comparable to that of noble metal based catalysts.25 In their study they found that this system has strong affinity to bind with O2 molecule which in turn results in preferred ER 3



pathway for CO oxidation reaction. So it becomes important to examine the catalytic activity of this Si-GN4 system for as an ORR catalyst. Inspired by the above studies, in the present manuscript, we examined the detailed kinetic behavior of Si-GN4 system for ORR catalytic activity in acidic medium. The activation barriers and reaction energies were studied for each elementary step for the diagnosis of the kinetics and thermodynamics of the process. The solvent effect is considered for the energy calculations. To the best of our knowledge this is the first report for the catalytic activity of non-metal doped nitrogen coordinated graphene system.

Computational Details: All the calculations are done using density functional theory (DFT) approach using the projector augmented wave method implemented in Vienna Ab Initio Simulation Package (VASP).26 The exchange-correlation energy has been accounted within the generalized gradient approximation method (GGA) parameterized by the Perdew-Burke-Ernzerhof (PBE) functional and the van der Waals interactions have been incorporated by using the empirical correction method proposed by Grimme (DFT-D2).27,28 The kinetic energy cut off for all calculations is set to be 500 eV using the plane wave expansion. The relaxations were done using the conjugant gradient algorithm. The convergence threshold for all calculations is set to be 10-4 eV for energy. To avoid periodic image interactions we have considered a 15 Å vacuum distance along the c-direction. The Brillouin zone has been sampled using 5×5×1 Monkhorst-Pack k-point grid for structural relaxations and 7×7×1 for density of states (DOS) calculations.29 The charge transfer between surface and adsorbate is calculated using Bader charge population analysis method.30 All calculations were performed with spin-polarization effect. The climbing image nudge elastic 4


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band method (CI-NEB) was used to calculate activation barriers and six intermediate images were considered along the minimum energy path (MEP) to search for transition states for the ORR steps.31 Frequency calculation at the optimized transition state verifies the nature of vibrational mode of the TS along MEP which is done through density functional perturbation theory (DFTP). For all the pathways, the activation barriers (∆E‡) and the reaction energies (∆E) were evaluated as follows: ∆E‡ = ETS – EIS and ∆E = EFS – EIS; where IS, TS and FS correspond to initial state, transition state and final state, respectively. Adsorption energies of the intermediate species were calculated using the following relation: ∆𝐸!"# = 𝐸!"#$%&!'(/!"!!"! − 𝐸!"#$%&!'( − 𝐸!"!!"!

(1)

For calculating the change in free energies for the elementary reactions (∆𝐺), we have considered CHE model originated by Norskov et al and this is written as: ∆𝐺 = ∆𝐸 + ∆𝑍𝑃𝐸 − 𝑇∆𝑆 + ∆𝐺! + ∆𝐺!"

(2)

where ∆𝐸, ∆𝑍𝑃𝐸, and ∆𝑆 represent reaction energy, change in zero point energy during reaction and entropy change, respectively.32,33 Entropy of gas molecules were taken from NIST database. ∆𝐺! = -eU where U is the electrode potential and ∆𝐺!" corresponds to free energy correction due to concentration of H+ ion in solution calculated as 2.303kBT×pH where kB is Boltzmann constant and T is the temperature of the medium (298 K) and pH=0 is considered for acidic pH. The free energy of (H+ + e-) was considered as that of ½ (H2). We have also considered the solvation effect for calculation of free energy change with respect to electrode potential using VASPsol code which considers an implicit solvation model.34

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Results and Discussions: Properties of Si-GN4 system: In this study we have considered a 5×5 supercell of graphene of which four carbon atoms are substituted by four nitrogen atoms and in the middle of this structure one silicon atom is placed which stabilizes the whole Si-GN4 type of structure shown in Figure 1a.

Figure 1: (a) Top view of optimized geometry of Si-GN4 surface, (b) charge density difference plot (cyan and olive color represent positive and negative charge density, respectively), (c) partial density of states (PDOS) of the elements C, N and Si on Si-GN4 system. To check the size dependence, we have taken the larger supercell (6 × 6) and calculated the O2 adsorption energy and it is seen that there is negligible difference between these two surfaces and 6


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for this reason we have taken (5 × 5) for the rest calculations. To check the stability of Si adatom on GN4 surface, we have calculated the binding energy of Si on GN4 surface. It shows an adsorption energy of -3.27 eV where the energy of single Si atom is chosen from the cohesive energy per Si atom of the Si bulk (= -4. 63 eV).35 It is seen that the substitution of carbon atoms with four nitrogen atoms produces two types of bonds: d1 and d2 with the bond-lengths ~1.43 Å and 1.39 Å, respectively which are higher than that of normal C-N double bond length (~1.37 Å) indicating activation of C-N bond. The Si-N bond lengths are of order ~1.82 Å. From Bader charge distribution analysis, it is seen that a large amount of charge transfer occurs from silicon to nitrogen (3.58 |e|) which causes the Si atom to be positively charged and from neighboring carbon atoms to nitrogen atoms of ~0.62-0.90 |e| resulting four nitrogen atoms negatively charged and the neighboring carbon atoms positively charged. The other carbon atoms of SiGN4 remain nearly neutral. The charge transfer is directly correlated with the electronegativity differences between C, Si and N. From Fig. 1b, it is seen that there is charge accumulation (olive color) near four N centers and charge depletion (cyan color) near Si and neighboring C centers. As a consequence of such strong CT interactions, the reactivity of the system increases which supports its capability to serve as a good supportive material for catalysis. Therefore, the active region of the system is the charged Si, four N atoms and the carbon atoms directly attached to the N atoms. For further understanding we have also proposed PDOS analysis (Fig. 1c) of neighboring C atoms, N and Si atoms in Si-GN4 system. From the figure it is clear that there is strong overlapping of density of states of C atoms with that of N atoms which strengthen the above discussion and confirms that there is strong interaction between the neighboring C, Si atoms with that of substituted N.

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Adsorption of O2 molecule: The chemisorption ability of molecular O2 on the catalyst surface is the prerequisite step for reduction of O2 in fuel cell. So we first investigated the adsorption behavior of O2 on Si-GN4 surface. O2 molecule mainly adsorbs with end-on and side-on configuration in which side-on configuration is the most stable adsorption site as shown in Figure 2a which agrees well with previous report.25 In this side-on configuration, O2 exhibits adsorption energy of -2.70 eV which indicates strong chemisorptions behavior of O2 molecule on this surface.

Figure 2: (a) Optimized configuration of O2 adsorbed on Si-GN4, (b) PDOS plot of O2 in gas phase and adsorbed state and of N and Si elements in Si-GN4 surface.

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From figure 2(a), it is seen that O2 chemisorbs above the Si center. The bond length of adsorbed O2 is ~ 1.55 Å which is significantly elongated with respect to molecular O2 (1.23 Å) which further elongates upto ~ 2.36 Å resulting in O2 dissociation. This bond elongation in adsorbed state occurs due to the large amount of charge transfer (~1.88 |e|) from the surface to the O2 moiety and these transferred electrons occupy the 2𝜋 ∗ molecular orbital and hence reduces the bond orders. This large amount of charge transfer mainly occurs due to the involvement of Si, N and neighboring C atoms which are directly attached to the N atoms. Mainly, C and Si atoms donate electrons to the O2 moiety and part of the charge of C and Si atoms goes to the N atoms which are negatively charged of ~ 0.80 e and this is used to saturate the dangling bonds of N atoms implying a covalent interaction between substrate and adsorbates. To get deeper insight in the bonding nature of O2 with that of substrate we have calculated the projected density of states (PDOS) of adsorbed O2 state, O2 gas phase and of N and Si atoms in the Si-GN4 system. From the Figure 2b, it is seen that the 2𝜋 ∗ antibonding orbital of spin-down channel of adsorbed O2 gets significantly stabilized and moves to the VB with respect to Fermi level which was located above Fermi level for free O2 molecule, illustrating that the adsorption of O2 occurs by accepting electrons from surface to the LUMO 2𝜋 ∗ antibonding orbital.36 Due to this occupation in 2𝜋 ∗ state the bond order becomes smaller resulting in bond elongation. However, the 1π orbital of the adsorbed O2 split into two peaks near -2.5 eV showing observable downshifting. The 5σ peak of adsorbed O2 is now significantly broadened over a large energy (~-2.5 to ~ -7.5 eV) region and merges with the 1π peak of adsorbed O2. Due to this splitting of 1π orbital and broadening of 5σ orbital, they overlap in an efficient way resulting orbital hybridization between adsorbate O2 and substrate resulting strong interaction between them. Due to this large electron transfer the 6σ* orbital now gets downshifted towards Fermi level and this now becomes LUMO for the new

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system. From this PDOS analysis it is also clear that peroxo type of state arises due to almost completely filling of 2𝜋 ∗ orbital resulting in no considerable spin polarization of O2 orbitals and the system becomes highly reactive one. This type of interaction is also known to occur during interaction of O2 with that of Au8 cluster.37 This overlaps with the density of states arise from N and Si atom. So, it can be concluded that due to the electron transfer and orbital overlap between the adsorbed O2 and that substrate is responsible for the strong interaction between them for which O2 shows strong chemisorptions. From Figure 2b, it can also be said that the system becomes magnetically inactive as seen from the symmetric spin-up and spin-down channels in the PDOS plot. Adsorption of other ORR intermediates on Si-GN4 surface: We have now investigated the most stable adsorption configurations of various ORR intermediates namely, O, OOH, OH, H2O and CO. The energetically most favorable configurations are shown in Figure 3.

Table 1: Binding energies of adsorbates on the Si-GN4 surface considering solvent effect. Adsorbate OOH OH O H2 O CO

∆𝐸!"# (𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑠𝑜𝑙𝑣𝑒𝑛𝑡) ∆𝐸!"# (𝑤𝑖𝑡ℎ 𝑠𝑜𝑙𝑣𝑒𝑛𝑡) (eV) (eV) -3.40 -3.37 -4.85 -4.81 -6.56 -6.84 -0.64 -----0.59 -----

It is seen that the adsorption intermediates prefer to bond with the positively charged Si atom on surface. The adsorption energies of all the intermediates are shown in Table 1. For OOH, the O10


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O bond distance is 1.48 Å which is significantly larger than its gas phase configuration (~1.35 Å) showing considerable activation of this conformer. Similarly the other adsorbates also show high activation which is confirmed by their adsorption energy and from the charge transfer analyses (see Table S1). We have also considered the solvation effect and it is seen that the adsorption energies get altered by a small extent due to solvated which is clear from the Table 1. Also, it is seen that H2O shows a rather weak adsorption of -0.64 eV which makes it easier for the desorption of the final product from the surface. We have also observed that the H2O2 species is not stable on the Si-GN4 surface as it spontaneously dissociates into two ·OH radicals which finally forms O+H2O (See Figure S4 in Supp. Info. for final optimized geometry of H2O2 on SiGN4 surface). Therefore, we can exclude the 2e- reduction pathway and believe that the Si-GN4 surface prefers to proceed via 4e- reduction pathway in the acidic environment.

Figure 3: Most stable adsorption configurations of (a) OOH, (b) O, (c) OH and (d) H2O on SiGN4 surface.

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Another important factor for a good ORR fuel cell catalyst is its CO tolerance limit. It is seen that CO adsorption energy on this surface is -0.59 eV which indicates only a weak adsorption configuration on the surface and therefore, does not alter the adsorption performances of other ORR intermediates. Clearly, Si-GN4 is better than the traditional Pt-based catalysts (-1.89 eV) due to better CO tolerance limit.38 As the experimentally measured current densities depend on the reaction rate, the rate obtained from kinetic measurement is more specific for comparison with that of experimental measurement and in the next section we discuss the detailed ORR pathway and identify the kinetically bottleneck structures (rate-determining step).39,40 The ORR Catalytic Pathway: After initial adsorption of O2, the subsequent reactions may occur via two pathways namely dissociative and associative pathway. In dissociative pathway, adsorbed O2 molecule directly dissociates into 2O molecule which sequentially hydrogenates to give the final product H2O. On the other hand in associative pathway, first O2 hydrogenation occurs to produce OOH and then it can be either hydrogenated or dissociated to reach the final product. The overall possible reaction pathways are shown in Scheme 1 where the individual microsteps of the dissociative pathway and associative pathway are shown.

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Scheme 1: Overall oxygen reduction pathway (ORR) considered here. Direct dissociation of O2: It is seen that though direct dissociation of O2 is endothermic by an amount of +0.04 eV but it requires an activation barrier of 0.53 eV which is not unattainable at the working temperature ~ 80°C of a fuel cell. In the final structure, O-O bond length changes from 1.55 Å to 2.36 Å as shown in Figure 4a. One O is bonded to the Si atom while the other one is in between the Si-N bond. O2 hydrogenation: O2 hydrogenation to OOH is highly exothermic in nature and the barrier for this transformation is 0.61 eV which is nearly equal to that of direct dissociation of O2 (0.53 eV) indicating that O2 hydrogenation is favorable kinetically for ORR on this Si-GN4 surface. The initial, final and the corresponding TS geometries of this transformation are shown in Figure 4b. So, between the two considered pathways, we first describe dissociative path followed by associative path in the next section, respectively.

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Figure 4: (a) O2 dissociation and (b) O2 hydrogenation pathway. The intermediate structure shown in dotted box represents TS structures which are confirmed by the presence of an imaginary frequency.

Dissociative Pathway: As shown in Scheme 1, after dissociation of O2 molecule into two O atoms on the Si-GN4 surface, it hydrogenates to form O+OH wherein the H-atom was situated above one neighboring carbon atom (shown in Figure 5a) and in the final state H atom attaches to the O atom situated between Si-N bond and the O-O bond length turns out to be ~2.44 Å and O-H bond length becomes 0.97 Å which is similar with the O-H bond distance in free H2O molecule. This transformation requires an energy barrier of 0.47 eV and the reaction energy for this process is highly favorable (~ -2.45 eV). The initial, final and the TS geometries of CI-NEB calculations are shown in Figure S1 (a). But after easy formation of O+OH, next step namely, the

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formation of O+H2O becomes energetically unfavorable as it takes a high activation barrier of ~1.13 eV to reach this step (see Figure S1 (b)). So, we eliminate the possibility of considering the dissociative pathway and we consider only associative one which is discussed elaborately in the next section.

Associative Pathway: Following the formation of OOH there are three possible ways to reach the final product namely, direct dissociation of OOH, direct hydrogenation of OOH into OH+OH or O+H2O. Direct dissociation of OOH: Direct dissociation of OOH forms O+OH where O is situated above Si centre and the OH is situated above neighboring C atom. Though this transformation is exothermic (~ -0.68 eV), but the activation barrier of this transformation is very high of 3.06 eV making this path highly improbable for the reaction. This path is shown in Supp. Info. file Fig.S2. This is different from other cases like MnP codoped graphene where the direct dissociation is favorable.41 OOH hydrogenation: The hydrogenation process of OOH may result in two different products namely, O+H2O and OH+OH. For O+H2O, where H2O desorbs from the surface occur rather efficiently in this associative pathway with an activation barrier of 0.44 eV from OOH whereas it has a high activation barrier in the dissociative pathway from O+OH structure. In the second path, OOH hydrogenates to produce OH+OH and here one OH resides above Si centre and the other one resides above one neighboring carbon centre. The energy required to overcome the barrier is 0.16 eV which implies that both the processes can undergo at room temperature. Now, the first product namely, O+H2O consumes one proton and forms OH and it is seen that this reaction step is very much favorable requiring a negligible energy of activation (0.03 eV). It is 15



seen from Table 1 that the solvent effect is most prominent in the adsorption of O species and this is why we have checked the activation barrier of this transformation taking into account the solvent effect and it is seen that the activation barrier for this transformation is ~ 0.08 eV taking into account the solvent effect implying negligible difference in the barrier height. In the final step, OH resides above Si centre where the O-H bond distance becomes ~ 0.97 Å. This adsorbed OH then accepts one proton and converts to H2O which then desorbs from the surface retaining the surface as intact one. The activation energy for this process is 0.33 eV which is quite reasonable thereby making this process kinetically feasible. The initial and final geometries for this transformation pathway are shown in Figure 5b and 5c, respectively. On the other hand, OH+OH take one proton arising an intramolecular pathway to form O+H2O. Here, the OH resides above neighboring C-atom, takes H from another OH resides above Si and finally forms the final structure (shown in Figure 5e). The H2O molecule formed in this case shows less binding with the C atom with a distance of ~ 2.92 Å indicating its desorption from the surface. This is an exothermic process with reaction energy of -0.91 eV and an activation barrier of 0.19 eV. Then it follows the previous pathway resulting in final product H2O. The initial, final and TS geometries are shown in Figure 5.

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Figure 5: ORR pathway in associative approach.

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Overall it can be said that the most favorable pathway would be the O2 hydrogenation. From this hydrogenation step, it can follow two pathways. The possible pathways followed by the Si-GN4 surface for ORR catalytic activity is shown in Scheme 2.

Scheme 2: ORR pathway followed by Si-GN4 surface. Examining all possible pathways it can be concluded that rate determining step is the hydrogenation of O2 giving rise to OOH which has activation barrier of 0.61 eV. This value is much lower than that of other surfaces predicted to be good ORR catalysts.42-45 Thus, it is expected that the Si-GN4 structure can promote the ORR to proceed along a 4e- reduction pathway and this finally generates H2O by successive hydrogenation reactions.

Conclusion: In conclusion, we have investigated the detailed reaction pathways for ORR on non-precious silicon doped nitrogen coordinated graphene system using first principles calculations. From 18


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charge distribution analyses, it is seen that due to the electronegativity difference between neighboring Si, N and C atoms, the central Si and neighboring C atoms are positively charged whereas four N atoms are negatively charged. Due to this type of differential charge distribution in the system, Si-GN4 can act as a good catalyst for ORR. Chemisorptions of O2 molecule occurs mainly on central Si atom and this adsorption of O2 molecule creates a peroxide type behavior which is confirmed by estimating the amount of charge transfer and PDOS analysis. By studying the detailed kinetic pathway, it is revealed that ORR follows 4e- reduction pathway utilizing associative path. Due to the least stabilization of HOOH on Si-GN4 surface, the possibility of following a 2e- reduction pathway can be safely ruled out. It is seen that rate determining step is the hydrogenation of O2 which requires an activation barrier of 0.61 eV which is affordable under fuel cell working conditions and therefore, Si-GN4 is a suitable for an ORR. ASSOCIATED CONTENT Supporting Information. Optimized lattice constants value of (5 × 5) supercell, charge transfer analysis for the adsorbates, dependence on size of O2 adsorption, initial, TS and final state geometries of the transformations of O2 to O+OH, O+OH to O+H2O, OOH to O+OH, free energy diagram, final structure of H2O2 on Si-GN4 surface. AUTHOR INFORMATION Corresponding Author: [email protected] ACKNOWLEDGMENT C.C. thanks CSIR India for SRF. AD thanks INSA, DST and BRNS for partial funding. We thank CRAY supercomputer for computational facility. 19



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