Theoretical insights on the reaction pathways for oxygen reduction reaction on phosphorus doped graphene

Accepted Manuscript Theoretical insights on the reaction pathways for oxygen reduction reaction on phosphorus doped graphene Xiaowan Bai, Erjun Zhao, Kai Li, Ying Wang, Menggai Jiao, Feng He, Xiaoxu Sun, He Sun, Zhijian Wu PII:

S0008-6223(16)30294-9

DOI:

10.1016/j.carbon.2016.04.033

Reference:

CARBON 10914

To appear in:

Carbon

Received Date: 29 January 2016 Revised Date:

6 April 2016

Accepted Date: 14 April 2016

Please cite this article as: X. Bai, E. Zhao, K. Li, Y. Wang, M. Jiao, F. He, X. Sun, H. Sun, Z. Wu, Theoretical insights on the reaction pathways for oxygen reduction reaction on phosphorus doped graphene, Carbon (2016), doi: 10.1016/j.carbon.2016.04.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Theoretical insights on the reaction pathways for oxygen reduction reaction on

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phosphorus doped graphene

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Xiaowan Bai,a,b Erjun Zhao,a,* Kai Li,b Ying Wang,b Menggai Jiao,b Feng He,b Xiaoxu

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Sun,b He Sun,c Zhijian Wub,*

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a

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China

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College of Science, Inner Mongolia University of Technology, Hohhot 010051, P. R.

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b

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Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

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c

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State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of

The Computing Center of Jilin Province, Changchun 130012, P. R. China

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Abstract

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The reaction mechanisms for oxygen reduction reaction (ORR) on phosphorus doped

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divacancy graphene (P-GDV) are investigated by using the density functional theory

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method. Our results showed that all of the possible ORR elementary reactions could

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take place within a small region around the P atom and its adjacent four carbon atoms.

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The hydrogenation of O2 molecule which forms OOH and hydrogenation of OOH

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which forms H2O+O have negligible energy barrier. This reaction pathway is also the

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kinetically most favorable. The rate-determining step is the final step in the pathway,

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i.e., the hydrogenation of OH into H2O with an energy barrier of 0.85 eV. Therefore,

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ORR mechanism on P-GDV would be a four electron process. The free energy diagram

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of the ORR predicted that for the most favorable pathway, the working potential is

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0.27 V. Consequently, our theoretical study suggests that P doped graphene with

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intrinsic carbon defects could possess good catalytic activity for ORR.

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* Corresponding authors. E-mail: [email protected] (EJZ); [email protected] (ZJW).

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1. Introduction Fuel cells (FCs), as highly efficient and environmentally friendly energy

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converting devices, have been studied extensively in the past decades. For FCs, the

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electrons from the cathode to anode which makes oxygen reduction reaction (ORR)

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play a crucial role in energy conversion efficiency [1,2]. Since ORR is a kinetically

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sluggish process on the cathode [3,4], the cathode catalysts are important for the

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energy conversion efficiency, operating cost, durability of the FCs, etc.. Up to now,

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the best performing cathode catalysts for ORR are Pt or Pt-based alloy electrocatalysts

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[5-8]. However, the high cost, poor durability and stability of Pt hinder its widespread

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commercial applications. Therefore, it is indispensable to search for low cost and high

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efficiency electrocatalysts, aiming to substitute Pt or Pt-based catalysts. Recently,

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heteroatom-doped graphene catalysts, such as non-noble metal [9-12] and metal-free

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[13-18] doped graphene, attract a great deal of attention due to their low cost and

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good performance for ORR.

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Among the heteroatom-doped graphene, N-doping is the most popular choice as the catalytic substrate [19-21] or direct catalyst [22-24]. For instance, Xiong et al. [20]

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found that N-doped graphene as substrate could improve the durability of Pt catalyst

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and exhibit higher catalytic activity for CO oxidation. As a direct catalyst, Chai et al.

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[22] considered the various structures in nitrogen doped carbon catalysts (CACs). The

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results indicated that N-doped Stone-Wales (SW) defect graphene structure provides

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good active sites for ORR. Recently, most of the theoretical [10-12] and experimental

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[25-27] investigations have shown that nitrogen-coordinated transition metals (e.g.,

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Mn, Fe, and Co) in graphene exhibit also good ORR catalytic performance. These

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results demonstrated that N-doped graphene can be acted as the alternatives for Pt in

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ORR. Besides N-doped graphene, researches on graphene doped by other heteroatoms

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are also available, such as boron (B) [28], sulfur (S) [29,30], selenium (Se) [31] and

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their mixtures [32,33], etc.. In particular, due to the similarity of nitrogen and

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phosphorus in structure and chemical properties, phosphorus (P) doped graphene

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received a great deal of research interests. Both the experimental [34-36] and

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theoretical studies [37] confirmed that P-doped graphene could be used as a high

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ACCEPTED MANUSCRIPT efficient metal-free ORR electrocatalyst. On experimental aspect, Liu et al. [34]

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synthesized P-doped graphite layers through the thermolysis method and pointed out

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that P-doped graphite could exhibit high electrocatalytic activity, long-term stability

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and strong durability in ORR. Li et al. [35] found that doping by P atoms could create

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new active sites on graphene and the activity is comparable to the commercial

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benchmark Pt/C catalyst for ORR. On the theoretical aspect, Kaukonen et al. [37] has

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studied the formation energies for P atoms embedded in graphene with monovacancy

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and divacancy and adsorption energies of O2 and H2O on the P-doped graphene. The

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results showed that single P doped graphene with divacancy could be a good

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candidate for ORR and the weak O2 binding is a main criteria for its good

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performance [37]. For ORR mechanism on P-doped graphene with monovacancy,

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Zhang et al. [38] demonstrated that P dopants are active sites and OOH formation and

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dissociation is the most favorable pathway with the rate-limiting step of OH

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hydrogenation to H2O. Recently, experimental researches suggested that significant

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contribution of intrinsic carbon defects could break the symmetry of the carbon

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framework and promote ORR activity [18,39,40].

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In this work, we have studied the graphene doped by single P atom. By replacing

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one C atom with P, we obtained three configurations, i.e., P-doped monovacancy

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graphene (P-GMV) , P-doped divacancy graphene (P-GDV) and P-doped graphene with

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Stone-Wales defect (P-GSW) (Fig. S1, Supporting Information). The calculated

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formation energy indicated that P-GDV is the most stable with the formation energy

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-4.58 eV. While for P-GMV and P-GSW, the formation energy is positive, indicating

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that they are unstable. Therefore, P-GDV is selected for further study in this work.

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Besides ORR mechanism, we have also investigated that the effects of the electrode

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potentials on the activity in acid environment. The calculated results revealed that the

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phosphorus coordinated intrinsic defect could improve the catalytic activity of

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graphene for ORR.

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2. Computational details All the geometrical and energetic calculations are performed using Vienna ab 3

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exchange-correlation functional within the generalized gradient approximation (GGA)

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is employed [45], while the Blöchl’s all-electron-like projector augmented wave

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(PAW) method [46,47] is adopted to describe the interactions between ion cores and

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valence electrons. The wave functions at each k-point are expanded with a plane wave

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basis set and the kinetic cutoff energy is set to be 400 eV. The electron occupancies

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are determined according to the Fermi scheme with an energy smearing of 0.1 eV. The

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Brillouin zone integration is approximated by a sum over special selected k-points

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generated from 5×5×1 Monkhorst–Pack method [48]. The convergence tolerance of

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energy of 1.0×10-5 eV/atom and maximum force of 0.05 eV/Å are employed in all the

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geometry optimizations. The reaction pathways and the transition state (TS) structures

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are located using the climbing image nudged elastic band (CI-NEB) method [49]. The

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minimum energy path is optimized using a force-based conjugate-gradient method [43]

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until the maximum force is less than 0.05 eV/Å. In order to describe the

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van-der-Waals (vdW) interactions between the reactants and the substrate, a

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semiempirical dispersion-corrected density functional theory (DFT-D2) force-field

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approach [50,51] is used in our calculations.

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A 4×4 graphene hexagonal supercell (containing 32 atoms) with lattice parameters

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a= b = 9.84 Å are chosen to model the P-doped graphene. A vacuum layer of 12 Å is

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chosen along the C axis direction normal to the sheet to avoid periodic interactions.

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For P-GDV, P atom prefers to be anchored at the center of divacancy with the four

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nearest carbon atoms having the same P-C bond distance (1.85 Å). This is in

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agreement with the previous study [37].

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The formation energy (∆Ef) is calculated as:

(

∆E f = EP−GDV + µC − EGDV + µP

)

(1)

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where E P −GDV and EGDV are the total energies for optimized with P-doped and

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without P-doped divacancy graphene structures, respectively. µC is the chemical

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potential of carbon in graphene [52,53]. µP is the chemical potential of the 4

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phosphorus in the bulk phase of black phosphorus. The negative formation energy

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means that the P-doped graphene is stable.

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The adsorption energy (∆Eads) is calculated as:

(

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∆Eads = Eadsorbate/ P−GDV − Eadsorbate + EP −GDV

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where Eadsorbate/ P −GDV , Eadsorbate and EP −GDV are the total energies of the P-GDV

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structure with adsorbate, free adsorbate and P-GDV structure, respectively. All

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energies are calculated by using the same periodic box and parameter setting.

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Since the cathode electrocatalysts work under a positive electricpotential in reality,

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the free energy changes of the ORR are studied under different electrode potentials.

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The change in free energy (∆G) of per reaction step is calculated as [54]:

∆G = ∆E + ∆ZPE − T ∆S + ∆GU + ∆G pH + ∆G field

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where ∆E is the change of the total reaction energy obtained from DFT calculations,

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∆ZPE is the change of the zero-point energy, T is the temperature (298.15 K), and

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∆S is the change of the entropy. ∆GU = -eU, here, U is the potential at the

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electrode and e is the transferred charge. ∆G pH = kBT×ln10×pH where kB is the

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Boltzmann constant and T=300 K. We define pH = 0 for acidic medium [55,56].

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∆G field = 0 where ∆G field is the free energy correction due to the electrochemical

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double layer and is neglected as in previous study [54]. The free energy of O2 is

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obtained from the reaction O2 + 2H2→2H2O, which is 4.92 eV at temperature of

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298.15 K and pressure of 0.035 bar. The entropies and vibrational frequencies of

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molecules (including O2, H2, H2O, etc.) in the gas phase are taken from the NIST

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database. The zero-point energy and the entropies of the possible adsorbed species are

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calculated from the vibrational frequencies.

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Fig. 1 Geometrical structure of P-GDV and possible adsorption sites.

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3. Results and discussion

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3.1 The adsorption of reaction species

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The possible adsorption configurations of the various reaction species on P-GDV

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involved in ORR mechanism, including O2, O+O, O, H, OH, OOH, O+OH, OH+OH,

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and H2O have been studied first. The surface structure with possible adsorption sites

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on P-GDV is shown in Fig. 1. After optimization of the reaction species on these

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possible sites, the obtained stable structures are shown in Fig. S2, Supporting

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Information. The most stable configurations and the calculated adsorption energies

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(∆Eads) for each species are listed in Fig. 2. Generally, adsorption of O2 molecule has

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two configurations, i.e., end-on and side-on. In our work, the end-on configuration

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(Fig. 2a) is more stable than side-on configuration (with positive ∆Eads = 0.34 eV, Fig.

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S2). Thus, the end-on O2 is selected as the initial state in ORR in the following

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calculations. This is different from the case in P-GMV, in which side-on configuration

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is found to be the most stable [38]. This indicates that the graphene defect type has

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great influence on the adsorption behavior of O2 molecule. For the two separated O

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atoms, the most stable configuration is that one O is on the top of P atom, while the

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other one sits on the bridge site of P-C bond, similar to the case in P-GMV [38]. For

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the single O atom, the bridge site of P-C is the most stable site, while the site on the

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top of P atom is slightly unstable (by about 0.33 eV, Fig. S2, Supporting Information).

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For H, OH and OOH, the most stable site is on the top of P atom (Fig. 2d-f). For H,

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the second most stable site is on the C atoms adjacent to P atom, which is slightly

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Fig. 2 Atomic structures of the most stable geometries for various ORR chemical

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species adsorbed. ∆Eads is the adsorption energy. (a) end-on O2, (b) two O atoms, (c)

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atomic O, (d) atomic H, (e) OH, (f) OOH, (g) atomic O and OH co-adsorption, (h)

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two OH co-adsorption, and (i) H2O. In the figure, the gray, pink, red, and white balls

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represent C, P, O, and H atoms, respectively.

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lower by 0.08 eV (Fig. S2, Supporting Information). Therefore, the C atoms adjacent

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to P atom are the important active site for the hydrogenation in study of ORR

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mechanism. In the searching of HOOH, our calculations indicate that the

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hydrogenation of OOH always gives H2O+O. The product of both OH+OH and H2O2 7

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can not be found. Thus, the ORR mechanism on P-GDV could be mainly a four

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electron process, in agreement with the experimental observations [35,57]. From the calculated adsorption energy, it is seen that O2 molecule only adsorbs at

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P top site with a relatively small adsorption energy (∆Eads = -0.19 eV). This result is

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similar to the pervious theoretical result for P-GDV (< 0.1 eV) [37]. The smaller

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adsorption energy for O2 molecule is also found on Pt/Cu(111) surface (-0.14 eV, Ref.

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8). According to the Sabatier principle, the adsorption energy of O2 on an ideal

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catalyst for ORR should be as small as possible to avoid strong adsorption of O2 [58].

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Therefore, we expect that P-GDV will be a suitable candidate as a good catalyst for

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ORR due to its weak adsorption of O2 [37,58].

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Fig. 3 Possible reaction pathways for ORR. The numbers in parentheses are the 8

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energy barrier and reaction heat in units of eV. For the details of labels from a1 to f1,

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see Fig. 4; a1' to c1' see Fig. 5; a1" to b1" see Fig. 6. * denotes that the ORR species is

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adsorbed on the catalyst surface.

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3.2 The ORR mechanism In ORR mechanism, the chemisorption of O2 on P-GDV is the first necessary step

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to initialize the reaction. Following the adsorption of O2 molecule on P-GDV, there are

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two possible reaction pathways, i.e., the dissociation of O2 into two separated O atoms

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and hydrogenation of O2 into OOH species. During our research, we have found

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several possible pathways for ORR as shown in Fig. 3. In the following, we shall

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describe these reaction pathways in detail.

O2 dissociation. In the first step, the O2 molecule adsorbs on the surface with

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end-on configuration (Fig. 4). The O-O bond distance is 1.31 Å, slightly larger than

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its bond distance 1.22 Å in free gas O2 molecule. In transition state, the O-O distance

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enlarges to 1.86 Å. At the final state, one O atom is adsorbed on top site of the P atom

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and the other O atom sits on bridge site of P-C bond with O-O bond distance 2.44 Å.

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The O2→O+O reaction is an exothermic process with an energy of -2.49 eV and an

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energy barrier of 0.81 eV. This energy barrier is larger than 0.38 eV for O2

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dissociation on P-GMV [38]. Following the O2 dissociation, the O atom at P top site

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will be hydrogenated to form O+OH. The energy barrier is 0.64 eV with an

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exothermic energy of -1.37 eV. The O-H bond distance is 1.46 Å in transition state.

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After the formation of O+OH, its hydrogenation will give either H2O+O or OH+OH.

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exothermic by -0.71 eV (Fig. 4). Once the weak adsorbed H2O molecule is released

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from P top site, the remaining O at P-C bridge site could diffuse to P top site with a

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diffusion barrier of 0.77 eV. In transition state, the O-P bond distance is 1.61 Å.

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Subsequently, it will be hydrogenated to give OH, then the further hydrogenation of

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OH forms H2O. After the release of the H2O molecule, the catalyst will be refreshed

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and another cycle of reaction would begin. The reaction of O+H→OH is exothermic

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by -0.77 eV and the energy barrier is 0.54 eV. The hydrogenation of OH gives H2O

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with a slightly higher barrier of 0.85 eV and less exothermic energy of -0.29 eV. In 9

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Although the hydrogenation of OOH does not produce OH+OH as mentioned earlier, OH+OH species can be generated from the hydrogenation of O+OH. To form

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transition state, the H-O bond distance is 1.24 Å.

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Fig. 4 Atomic structures of the initial state (left panel), transition state (middle panel),

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and final state (right panel) for O2 dissociation (a1), atomic O hydrogenation (b1), first

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H2O formation (c1), the diffusion of O atom from P bridge site to top site (d1), second

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OH formation (e1), and second H2O formation (f1).

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OH+OH, the very large reaction barrier is required (1.95 eV) with less exothermic by

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-0.10 eV (Fig. 5), indicating that this reaction pathway is difficult. In OH+OH, the

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most stable configuration is that one OH is on the top of P atom, while the other OH

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on the top of C atom (adjacent to P atom) (Fig. 2h). After OH+OH is generated, there

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are two reaction pathways. The first one is the hydrogenation of OH+OH gives

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OH+H2O with a relatively lower energy barrier of 0.58 eV. This process has large

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exothermic energy of -1.95 eV. After the release of H2O, the hydrogenation of OH, i.e.,

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H+OH, will produce another H2O molecule. The reaction has a barrier of 0.85 eV as

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mentioned above (Fig. 4). For the second reaction OH+OH→H2O+O, the energy

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barrier is 0.37 and the reaction is exothermic by -0.27 eV (Fig. 5). This means that

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this reaction is relatively easy compared with OH+OH→OH+H2O. After the

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desorption of H2O, the two sequential hydrogenations of O give water as discussed in

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Fig. 4.

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Fig. 5 Atomic structures of the initial state (left panel), transition state (middle panel),

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and final state (right panel) for two OH formation (a1'), OH hydrogenation to H2O (b1')

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and two OH disproportionation (c1').

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O2 hydrogenation. In this reaction pathway, the hydrogenation of O2 is very easy

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due to the negligible reaction barrier (Fig. 6). Similar behavior is also observed for

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FeN4 doped graphene [10]. This is in agreement with the experimental observation

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that peroxide has been formed [34,59]. The conclusion is also different from the result

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on P-GMV, where the hydrogenation of O2 molecule needs a barrier of 0.45 eV [38].

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After the formation of OOH, the reaction will proceed in two pathways (Fig. 3). The

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first reaction is the dissociation of OOH. After OOH dissociation, OH is located on

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the top of P atom, while O is on the bridge site of P-C bond (Fig. 6). The reaction is 12

ACCEPTED MANUSCRIPT exothermic by -2.58 eV with an energy barrier of 0.83 eV. This barrier is close to the

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one for O2 dissociation (0.81 eV) (Fig. 4). The following reactions from O+OH is the

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same as mentioned in O2 dissociation (Figs. 3, 4). The second reaction is the

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hydrogenation of OOH. The product is H2O+O, in which O atom is on the bridge site

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of P-C bond (Fig. 6). The calculated energy barrier is 0.04 eV. This means that the

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hydrogenation of OOH is a spontaneous process. After the release of H2O, the

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hydrogenation of the remaining O produces the second water, as mentioned in Fig. 4.

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Fig. 6 Atomic structures of the initial state (left panel), transition state (middle panel),

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and final state (right panel) for OOH dissociation (a1") and OOH hydrogenation to

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H2O (b1").

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In a word, it is seen from the reaction pathways shown in Fig. 3 that for O2

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dissociation, the favorable pathway is the process a1→b1→c1→d1→e1→f1 with the

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rate-determining step of H2O+O formation (energy barrier 0.86 eV). Nonetheless, we

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also noticed that O2 dissociation and OH hydrogenation to form H2O have also similar

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energy barriers (i.e., 0.81 and 0.85 eV, respectively). On the other hand, the formation

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of OH+OH is not preferred due to the very large energy barrier (1.95 eV). For the O2 13

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hydrogenation,

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a1"→c1→d1→e1→f1,

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a1"→c1→d1→e1→f1, the dissociation of OOH has a barrier of 0.83 eV. The

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rate-determining step is the same as a1→b1→c1→d1→e1→f1 for H2O+O formation

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with an energy barrier 0.86 eV. For the pathway b1"→d1→e1→f1, however, the

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formation of OOH and H2O+O is a spontaneous process for ORR with negligible

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barrier, suggesting that OOH hydrogenation is more easily than OOH dissociation. In

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this pathway, the rate-determining step is the hydrogenation of OH with a barrier of

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0.85 eV. The similar energy barriers for a1"→c1→d1→e1→f1 (0.86 eV) and

are

favorable

the

second

pathways. one

The

is

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one

b1"→d1→e1→f1.

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two

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b1"→d1→e1→f1 (0.85 eV) indicated that they are competitive pathways. The above results indicated that the O2 hydrogenation is much easier than O2

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dissociation. On the whole, the most favorable reaction pathway would be

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b1"→d1→e1→f1. The energy barrier of 0.85 eV is similar to that of 0.88 eV on P-GMV

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[38] and 0.86 eV on Pt (111) surface [8]. Atomic O will be the main species on the

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catalyst surface. Therefore, our study revealed that the doped P atom could improve

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the catalytic activity of graphene for the ORR. Recalling that for P-GMV, the most

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favorable pathway is the dissociation of OOH [38], different from our conclusion.

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This demonstrated that the defect type in graphene has great influence on reaction

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pathways due to the different geometries and the active sites.

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3.3 Effect of electrode potentials on ORR

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Since the above results are from zero electrode potential, while in reality, the

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electrochemical systems are operated under positive electrode potentials. Therefore,

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we have studied the reaction pathways under different electrode potentials. We only

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give

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b1"→d1→e1→f1 in Fig. 7, the remaining pathways are shown in Fig. S3, Supporting

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Information. From Fig. 7, it is seen that at zero potential, all the reactions are

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exothermic since all the elementary reaction steps are downhill. With the increase of

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the electrode potentials, some intermediate reactions become less exothermic and

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there exists the highest electrode potential to keep all the elementary reactions to be

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exothermic, which is defined as the working potential of the electrocatalyst. For

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pathways

a1→b1→c1→d1→e1→f1,

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a1"→c1→d1→e1→f1

and

ACCEPTED MANUSCRIPT instance, in the pathway a1→b1→c1→d1→e1→f1, the working potential is 0.10 V. For

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potentials higher than 0.10 V, endothermic reaction is found, e.g., the first H2O

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formation is endothermic at V=0.20 V, while at V=1.23 V, all the reactions are

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endothermic after the formation of O+OH. Similarly for the pathways

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a1"→c1→d1→e1→f1 and b1"→d1→e1→f1, the working potentials are 0.10 V and

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0.27 V, respectively. For the remaining pathways (Fig. S3), the reaction step is uphill

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for the formation of OH+OH, consistent with its large energy barrier (Fig. 3).

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Fig. 7 The free energy diagrams for the reduction of O2 to H2O at different electrode

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potential

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a1"→c1→d1→e1→f1 and b1"→d1→e1→f1; see also Fig. 3 for the detailed reaction

4

pathways. * denotes that the ORR species is adsorbed on the catalyst surface.

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4. Conclusions

O2

dissociation,

a1→b1→c1→d1→e1→f1;

O2

hydrogenation,

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DFT computations show that the formation of P-doped graphene with divacancy

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is energetically favorable compared with monovacancy and Stone Wales defect. By

8

using P-doped graphene with divacancy as catalyst, we performed systematic

9

investigations on possible reaction mechanisms for ORR. We first calculated the

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adsorption energy of the ORR species and found the most stable configuration for

11

each species. Then, the energy barriers and free energies of all the possible ORR

12

elementary steps are calculated. We have indentified three competitive reaction

13

pathways, in which the energy barrier is around 0.85~0.86 eV for the rate-determining

14

step. Among them, the kinetically most favorable one would be the hydrogenation of

15

O2 molecule to form OOH, then the hydrogenation of OOH gives H2O+O. The

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rate-determining step for this pathway is the final step, i.e., the hydrogenation of OH

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to produce H2O with an energy barrier of 0.85 eV. Since the hydrogenation of O2

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molecule to form OOH and the hydrogenation of OOH to form H2O+O can be

19

happened spontaneously, this implies that atomic O would be the main species on the

20

catalyst surface. The free energy diagram of the ORR predicts that the working

21

potential for P-GDV catalyst is 0.27 V for the most favorable pathway. Therefore, we

22

expect that P cooperating with intrinsic carbon defect could lead to a quite promising

23

alternative non-Pt ORR catalyst and is helpful for designing novel high efficiency

24

catalysts for fuel cells.

25

Acknowledgements

26

This work is supported by the National Natural Science Foundation of China

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(21261013, 21503210, 21521092), the program for Young Talents of Science and

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Technology in Universities of Inner Mongolia Autonomous Region (NJYT-15-B16),

29

the Natural Science Foundation of Inner Mongolia Autonomous Region (Grant Nos.

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ACCEPTED MANUSCRIPT 2015MS0120 and 2011BS0104), the Key Science Research Project of Inner Mongolia

2

University of Technology (Grant No. ZD201517 and ZD201117), Jilin Province

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Youth Fund (20130522141JH) and Jilin Province Natural Science Foundation

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(20150101012JC). The computing time is supported by the Performance Computing

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Center of Jilin University and Special Program for Applied Research on Super

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Computation of the NSFC-Guangdong Joint Fund (the second phase).

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Supporting Information

SC

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Fig. S1 is the geometrical structures and formation energy of the three P doped

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graphenes. Fig. S2 is possible configurations for each adsorbed species (*O2, *2O,

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*OOH, *O+*OH, *2OH, *O, *OH and H2O) involved in the ORR. Fig. S3 shows the

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free energy diagrams for the reduction of O2 to H2O at different electrode potentials.

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