Graphyne As a Promising Metal-Free Electrocatalyst for Oxygen

Sep 6, 2012 - hybridized carbon atoms, has been shown in recent studies to have the potential .... plane at a distance of 3 (for OOH+) or 1.5 Å (for ...
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Graphyne As a Promising Metal-Free Electrocatalyst for Oxygen Reduction Reactions in Acidic Fuel Cells: A DFT Study Ping Wu,† Pan Du,†,‡ Hui Zhang,† and Chenxin Cai*,† †

Jiangsu Key Laboratory of New Power Batteries, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, P. R. China ‡ Department of Chemistry, Jiangsu Institute of Education, Nanjing 210013, P. R. China S Supporting Information *

ABSTRACT: Graphyne, a new two-dimensional periodic carbon allotrope with a one-atom-thick sheet of carbon built from triple- and double-bonded units of two sp- and sp2hybridized carbon atoms, has been shown in recent studies to have the potential for high-density hydrogen and lithium storage. We report here a density functional theory (DFT) study of an oxygen reduction reaction (ORR) involving graphyne and demonstrate that graphyne is a good, metal-free electrocatalyst for ORRs in acidic fuel cells. We optimized the geometrical structure, calculated the charge densities on each carbon atom in the graphyne, and simulated each step of the ORR reaction involving graphyne. The simulation results indicate that the distribution of the charge density at each carbon atom on the graphyne plane is not uniform and that a large number of positively charged carbon atoms, which are beneficial to the adsorption of O2 and OOH+ molecules, can behave as catalytic sites to facilitate ORRs. When H+ is introduced into the system, a series of reactions can occur including the formation of an O−C chemical bond between oxygen and graphyne, breakage of the O−O bond, and the creation of water molecules. The results also indicate a decrease in the energy of the system and a positive value of the reversible potential for each reaction step on the graphyne surface. In addition, a spontaneous electron transformation process occurs during the ORR along a four-electron pathway. The results presented here should lead to an improvement in the catalytic efficiency of carbon nanomaterials and provide a theoretical framework for the analysis of their catalytic activity. This paper highlights the urgent need for new experimental syntheses for graphyne.

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phene) (PEDOT),17 nitrogen-doped carbon nanotubes (NCNTs),5 boron-doped carbon nanotubes (BCNTs),18 nitrogen-doped graphene,19,20 nitrogen-doped carbon nanotube cups,21 and g−C3N4@carbon (incorporating the graphiticcarbon nitride catalysts into the framework of a highly ordered mesoporous carbon).22 Among these catalysts, carbon-based materials have attracted particular attention as Pt-free electrocatalysts due to their low cost and high electrocatalytic activity. For example, the vertically aligned NCNTs (VA-NCNTs) could actively catalyze ORRs via a high efficiency four-electron process without crossover and CO poisoning effects with a 3time higher electrocatalytic activity and better long-term durability than that of the commercially available Pt-based electrocatalysts (20% Pt on Vulcan XC-72R, E-TEK).5 Similar ORR electrocatalytic activity was observed for nitrogen-doped graphene.19,20 In addition to these experimental studies, the mechanism of the enhanced electrocatalytic ORR activities of these carbon materials was studied using density functional theory (DFT)

INTRODUCTION Fuel cells, such as direct methanol fuel cells (DMFCs) and proton exchange membrane fuel cells (PEMFCs), have received much attention because of their high energy conversion efficiency, low pollution, low operating temperature, high power density, and wide range of applications.1−4 Oxygen reduction reactions (ORRs), or cathodic reactions in fuel cells, are dominant processes and play an essential role in producing electricity. However, the rate of an ORR is considerably slower than that of an anodic reaction and is a major limiting factor on the performance of fuel cells.5−8 Many studies have focused on improving the ORR activity using various catalysts. Pt and Ptbased materials are the most common catalysts used to promote ORRs,9−12 but several problems exist with Pt catalysts including limited natural resources, high cost, toxic CO-like intermediate species, and sluggish ORR kinetics. These problems have prevented the development of fuel cells for large-scale commercial applications.13,14 Thus, the exploration of non-noble metals or metal-free catalysts with high ORR activity and durability is critical to the development of fuel cells. Good electrocatalytic performance for ORRs has been reported with non-noble metal cobalt-polypyrrole15 and Fe− N complexes16 and with metal-free poly(3,4-ethylenedioxythio© 2012 American Chemical Society

Received: July 27, 2012 Revised: September 1, 2012 Published: September 6, 2012 20472

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terials to improve their catalytic efficiency and provides a theoretical framework to analyze their catalytic activity.

calculations. The improved ORR catalytic activities of the nitrogen-doped carbon materials was attributed to the electronaccepting ability of the nitrogen atoms, which creates a net positive charge (via intramolecular charge-transfer) on the adjacent carbon atoms to readily facilitate O2 adsorption and the ORR process.5,23 This conclusion was supported by a study of ORRs catalyzed by poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene, which showed significantly enhanced activity toward ORRs24 because PDDA, a positively charged polyelectrolyte with an electron-withdrawing ability, can create a net positive charge on the carbon atoms in a graphene plane via an intramolecular charge transfer. In contrast with the nitrogen-doped carbon materials in which O2 is adsorbed onto the carbon atoms adjacent to the nitrogen dopant, in BCNTs, O2 is adsorbed onto the boron dopant itself because carbon has a larger electronegativity than boron inducing a significant positive charge on the boron atom. This positively charged boron atom can favorably capture the O2 molecule, which gains a slight negative charge as it approaches the CNTs.18 Therefore, in both the nitrogen- and boron-doped carbon materials, the O2 adsorption favors the positively charged sites: the carbon connected to the nitrogen dopant in nitrogen-doped carbon materials and the boron dopant in the BCNTs. For pristine carbon materials, this process cannot be achieved as no positively charged site exists. Thus, the positively charged sites in the carbon materials play a significant role in enhancing the ORR activity. Therefore, graphyne, a new two-dimensional periodic carbon allotrope with a one-atom-thick layer of carbon built from triple- and double-bonded units of two sp- and sp2-hybridized carbon atoms,25 is a good candidate for metal-free catalysis of ORRs because Bader charge density analysis demonstrated that the charge distributions on the graphyne sheet are not even some carbon atoms have a net positive charge caused by charge transfer at the graphyne sheet.26 These positively charged sites can improve the interaction between graphyne and the O2 molecules and facilitate the ORR process. Graphyne maintains the intrinsic character of carbon materials including good chemical stability, large surface area, and excellent electronic conductivity, which are essential for a good catalyst. In addition, graphyne has a high possibility of synthesis. Finite building blocks and cutouts have already been created, and the initial steps toward the preparation of extended graphyne sheets have been proposed and developed.27−31 Large area (∼3.6 cm2) films of graphdiyne, which belong to the same family as graphyne but have two acetylenic (diacetylenic) linkages between the carbon hexagons, have been successfully prepared via a cross-coupling reaction using hexaethynylbenzene on a copper surface.32 However, single-sheet graphyne is still unavailable. Whether attempts to synthesize extended graphyne materials are worthwhile depends on the properties and applications of such materials. A detailed study of the properties of graphyne places increased urgency on the experimental synthesis of this novel carbon material. We report the investigation of graphyne as a possible metalfree electrocatalyst for ORRs based on DFT calculations. The simulation results demonstrated that OOH+ and O2 can bind with graphyne through the chemisorption mode, and the electron transformation process of ORRs on a graphyne surface occurs through a four-electron pathway. Therefore, graphyne can be good electrocatalyst for fuel cells in an acidic environment. This work inspires the study of carbon nanoma-

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CALCULATION METHODS The ORR on graphyne in an acidic environment was theoretically calculated based on DFT using Gaussian 03 (revision B.03; Gaussian Inc., Pittsburgh, PA, 2003).33 The geometrical optimizations were performed using a DFT method with Becke’s hybrid three-parameter nonlocal exchange functional combined with the Lee−Yang−Parr gradientcorrected correlation functional (B3LYP). The 6-31G (d,p) basis set was used for all elements. The spin multiplicity corresponding to the lowest energy state was adopted for the calculations. We constructed an α-graphyne (also called 18,18,18-graphyne) as the graphyne model, which is the most analogous to graphene. Periodic boundary conditions with a supercell of 3 × 3 unit cells composed of 106 carbon atoms were built into our calculation (C106H20). The charge densities on each carbon atom depended on the number of carbon atoms selected for calculation. Our calculation results indicated that a graphyne sheet with 3 × 3 unit cells (106 carbon atoms) was a reasonable model structure to depict the distribution characteristics of the charge densities on each carbon atom. Therefore, graphyne sheets with 3 × 3 unit cells were selected for calculation. The carbon atoms on the edge of the graphyne were terminated with hydrogen atoms. We optimized the geometrical structure of the graphyne and calculated the charge densities on each atom in the optimized structure via Mulliken charge density analysis. The HOMO and LUMO energy levels of the graphyne were also evaluated based on the calculations. We simulated the ORR process beginning with the first electron transmission following Wang’s work on Pt(111),34 which suggested that, in an acidic environment, a decomposition is primarily driven by the chemisorption of hydroxyl according to Yeager’s dissociative chemisorption proposal for the first step of ORRs. In this step, we set OOH+ (OOH+ was presumably formed in solution as the acidic environment allows O2 to adsorb an H+ to form OOH+) or O2 near the graphyne plane at a distance of 3 (for OOH+) or 1.5 Å (for O2). The optimized structures for the adsorption of OOH+ or O2 onto graphyne were obtained through structural optimization calculations. ORRs are known proceed by a two-step, twoelectron pathway forming hydrogen peroxide or by a more efficient four-electron process to directly combine oxygen with electrons and protons in water. Hence, the ORRs on graphyne could follow either a two-electron pathway or a four-electron process, which will be analyzed using the simulation of subsequent electron transforming reactions. The subsequent electron transformation reactions were simulated by introducing H+ into the system. At each step, we obtained the optimized structure and calculated the adsorption energy (Ead) for these molecules on the graphyne using frequency analysis after the initial structure optimization.35 The adsorption energy is defined as the energy difference between the total system including the adsorbed molecules and the isolated system. The energy of the isolated system is the sum of the energies of graphyne and the individual isolated adsorbed molecules. Thus, a negative adsorption energy indicates that the adsorbate molecules are energetically favored for adsorption onto the graphyne surface. We also calculated the reversible potential of each reaction step occurring on the graphyne surface following the procedures described by Anderson.35,36 The reversible potential 20473

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Figure 1. (a) 3 × 3 supercell of the optimized structure of α-graphyne. (b) The hybridized state of the carbon atoms and the bond lengths. Atomic color code: gray, carbon; white, hydrogen.

Figure 2. (a) Calculated charge distribution on each carbon atom graphyne sheet. The red and green colors represent negative and positive charges, respectively. (b) The detailed charge density value on each carbon atom in a unit cell of graphyne.

Baughman et al.25 The vibration analysis confirmed that the optimized structure is stable. Next, we calculated the charge densities on each carbon atom in the graphyne using Mulliken charge density analysis because the charge distribution on the carbon atoms is important for analyzing the ORR process on the graphyne sheet. The distribution of the charge density on the graphyne plane is not uniform because of the localization of the π electrons, which is due to the presence of the CC groups in the construction units of the graphyne (Figure 2a, the red and green colors represent negative and positive charges, respectively). An electron transfer occurs from the sp hybridized carbon atoms to the sp2 hybridized carbon atoms leading to the formation of sp2 carbon atoms in a hexagonal ring with a negative charge and sp carbon atoms with a positive charge (Figure 2b). The six vertex sp2 hybridized carbon atoms have negative charges of −0.447, −0.445, −0.445, −0.447, −0.446, and −0.445 au. The sp hybridized carbon atoms around these vertices have positive charges; the neighboring carbon atoms in the butatriene-like rods have the largest positive values. This feature is in contrast to that of graphene. Due to the delocalization of the π electrons in graphene, the carbon atoms except those near the H-terminated ends exhibit a weak negative charge due to hydrogen bonding; the carbon atoms a few hexagons away from the H-terminated ends are almost neutral.38

(Urev(surface)) for forming adsorbed surface intermediates (surface redox reactions) was estimated from the adsorption bond strengths and the standard reversible reduction potential (U°) for the reactions in bulk solutions based on the following equation:37 Urev(surface) = U ° + ΔE /nF

(1)

in which ΔE is the adsorption bond strength of the product minus the adsorption bond strength of the reactant. The bond strength is equal in magnitude to the value of the adsorption energy but carries the opposite sign.

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RESULTS AND DISCUSSION We first optimized the geometrical structure of graphyne using vibration analysis. Having a planar geometry with the same symmetry as graphene, α-graphyne (18,18,18-graphyne) is periodically built of hexagons consisting of 18 carbon atoms (Figure 1). These 18 carbon atoms are interconnected via C− CC−C acetylenic and CCCC butatriene-like rods in C(sp2)C(sp)C(sp)C(sp2) hybridized states (Figure 1a). The mean bond lengths of the C(sp)C(sp) and C(sp2)−C(sp) in acetylenic rods are 1.221 and 1.417 Å, respectively, and the mean bond lengths in the butatriene-like rods of C(sp2) C(sp) and C(sp)C(sp) are 1.372 and 1.239 Å, respectively (Figure 1b). These values are consistent with those reported by 20474

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Figure 3. Optimized structure for the adsorption of OOH+ and O2 molecules onto the graphyne sheet. Panels a and a′ illustrate the initial position and final optimization structure for the adsorption of the OOH+ molecule onto the graphyne, respectively; panels b and b′ illustrate the initial position and final optimization structure for the adsorption of the O2 molecule onto the graphyne, respectively. Atomic color code: gray, carbon; red, oxygen; and white, hydrogen.

on the carbon atoms around the C68 atom. This charge rearrangement is beneficial for the adsorption of OOH+ onto the graphyne because the OOH+ molecule requires an increasing negative charge as the adsorption distance decreases to strengthen the interaction between the positively charged carbon atoms and OOH+. An elongation of the O−O bond length accompanied this electron transfer from 1.320 Å in the isolated state to 1.473 Å in the adsorbed state indicating a weakening of the O−O bond upon adsorption. The O−O bond length is larger than the C−O (1.323 Å) bond length implying that the O−O bond can be easily broken. The bond strength of the O−O bond in its native state is too high to be broken, thereby limiting the ORR kinetics. The ease of dissociating an O−O bond in adsorbed OOH on graphyne suggests that graphyne can function as an effective catalyst to facilitate ORRs. The adsorption of O2 on graphyne was simulated using the same procedure as the OOH+ adsorption. The simulated results indicate that the O2 molecule, which was placed in the region for bond formation (∼1.5 Å, note that if O2 molecule was placed 3 Å far from the surface of graphyne, the O2 can only be physically adsorbed the graphyne surface, the C−O bond cannot be formed between O2 molecule and graphyne), can move to the graphyne and be adsorbed on its surface at two carbon atoms, the C68 and C70, in a side-on mode (known as Yeager’s model). Following the adsorption, the distances between the C68 and O128 and the C70 and O127 are 1.393 and 1.489 Å, respectively, indicating the formation of two new C−O bonds and the chemisorption of O2 on the graphyne. However, the O2 adsorption energy is −1.48 eV, which is smaller than that for OOH+ (−4.28 eV), implying that OOH+ adsorption is a favorable pathway to the first electron transmission process.39 Previous reports have shown that the chemisorption of both O2 and OOH+ could not take place on the surfaces of graphene and pristine CNTs,18,23,39 which suggests the graphyne has higher electrocatalytic activity for ORRs than graphene and CNTs. After the OOH+ was adsorbed onto the graphyne (forming G−OOH in which G represents graphyne), we added an H+ to the system. This H+ may move to a position near the oxygen atom (O127) that is bonded to the C68 carbon atom (known as reaction path I), or it may move close to the oxygen atom (O128) that is bonded to the H atom (known as reaction path

These positively charged carbon atoms on graphyne will be benefit to the O2 adsorption and can be catalytic sites for ORRs. In comparison with the nitrogen-graphene and the NCNTs, the graphyne will have a higher electrocatalytic activity for ORRs because it has a large number of positively charged carbon atoms (catalytic sites), whereas the number of catalytic sites in the nitrogen-graphene and NCNTs is limited because the nitrogen content in these nitrogen-doped carbon materials is relatively low (2−5%).22 Having optimized the geometrical structure and calculated the charge densities on each carbon atom of graphyne, we studied the catalytic pathways of graphyne toward ORRs starting by simulating the adsorption mode of OOH+ and O2 on graphyne (e.g., the first electron transmission process). Two possible reaction pathways exist for the first electron transfer: In one, O2 interacts with H+ in an acidic environment to form an intermediate OOH+ which then adsorbs onto the graphyne. In the other pathway, O2 directly adsorbs onto the graphyne and further interacts with H+ to form an adsorbed OOH. First, we simulated the adsorption of OOH+ onto graphyne and demonstrated that OOH+ can move from its initial position far from the graphyne surface (∼3 Å, Figure 3a) to the graphyne surface and be adsorbed at the C68 carbon atom with a positive charge of 0.198 au (Figure 3a′, note that the charge at this carbon atom changes from 0.198 to 0.152 au after the adsorption of the OOH+). Following the adsorption, the graphyne plane distorted into a saddle-shaped surface while the C68 rose out of the plane to form a tetrahedral structure (Figure 3a′), which is indicative of the formation of a C−O chemical bond. The distance between the C68 and the O128 was reduced to 1.323 Å from an initial value of 3 Å, further confirming the formation of a chemical bond between the OOH+ and the graphyne. This step is important to the catalytic activity of the graphyne because adsorption and formation of the chemical bond is necessary for the subsequent reactions, and the decomposition of O2 is primarily driven by this step. The C68 atom is not the only the absorption site for OOH+; other positively charged carbon atoms are able to act as absorption sites for the OOH+ molecule. Careful analysis of the charge distribution during the adsorption process indicated that a 1.269-au negative charge transferred from the graphyne to OOH+ via the C68, which served as a bridge leading to a decrease in the negative charge 20475

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Figure 4. Side (a−f) and top (a′−f′) views of the optimized configurations of steps 2, 3, and 4 for the four-electron transformations in the ORR process for pathway I (a−c and a′−c′) and II (d−f and d′−f′). (a, a′) Cleavage of the O−O bond and formation of two adsorbed OHs; (b, b′) formation of the first H2O molecule; (c, c′) formation of the second H2O molecule; (d, d′) cleavage of O−O bond with the formation of a bonded O atom and one H2O molecule; and (e, e′) formation of an adsorbed OH and the drift of an H2O molecule. Atomic color code: gray, carbon; red, oxygen; and white, hydrogen.

II). We first simulated reaction path I. After optimization, we found that one of oxygen atoms (O127) still bonds to the graphyne at C68 in the form of OH, whereas the other (O128) with a hydrogen drifts away and adsorbs onto another carbon atom (C123) forming a C−O bond (Figure 4, panels a and a′). During this process, the distance between the two O atoms (O127 and O128) increased from an initial value of 1.473 to 4.267 Å, indicating a break in the O−O bond. Thus, the introduction of an H+ into the system near the OOH leads to the decomposition of OOH into two OH molecules. The breaking of the O−O bond is an important step that represents a four-electron transformation pathway in an ORR. We added the second H+ to the system near the O128 oxygen causing a

cleavage of the C−O bond (the C123−O128 bond) and the formation of the first water molecule (Figure 4, panels b and b′). Another adsorbed OH molecule is simultaneously stretched by the newly formed water molecule (the bond length of the C68−O127 bond increases from 1.369 to 1.377 Å). As the third H+ is added into the system, a second water molecule is formed (Figure 4, panels c and c′). After the water molecules drift away, the saddle-shaped graphyne recovers its original shape and is ready for the next reaction cycle. The ORR processes depicted for reaction path I can be represented as follows: G + OOH+ → G−OOH 20476

(2)

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G−OOH + H+ + e− → G−OH + G−OH

(3a)

G−OH + H+ + e− → G + H 2O

(4a)

G−OH + H+ + e− → G + H 2O

(5)

For the case in which the first added H+ is close to the oxygen atom (O128) that is bonded to an H atom (reaction path II), a four-electron transformation also occurs, but the subreaction paths are different (Figures 4d−f, and 4d′−f′). Instead of producing two OH molecules after the first H+ is introduced, one water molecule is generated while only the oxygen atom (O127) adsorbs onto the graphyne. Consequently, when the next two H+ ions are added near the oxygen (O127), another water molecule is generated. The reaction processes for reaction path II can be represented as follows: G + OOH+ → G−OOH

G−OOH + H+ + e− → G−O + H 2O

(3b)

G−O + H+ + e− → G−OH

(4b)

G−OH + H+ + e− → G + H 2O

Figure 5. Illustration of the relative energy differences and configurations for the reaction intermediates (showing only part of the graphyne) in paths I and II of the ORR process on graphyne. The values are the relative energies for each step of the ORR process on graphyne. For the first step, the reference energy state is the total energy of the optimized graphyne and isolated OOH molecules. For the other reaction steps, the reference energy states are the total energy of the product of the previous reaction and H+ + e−. Note that G−OH in paths I and II was produced in a different way. In path I, the G−OH was produced via eq 3a, whereas the G−OH was produced via eqs 3b and 4b in path II. The energies of the G−OH in paths I and II are different. Therefore, the energies drop for eq 5 are different in paths I and II. Atomic color code: gray, carbon; red, oxygen; and white, hydrogen.

(2)

(5)

We calculated the adsorption energy difference between the reactants and products for each step in both reaction paths I and II using the procedures suggested by Zhang et al.39 For the first step, the reference energy state is the total energy of the optimized graphyne and OOH+ molecules. For the other reduction steps, the reference energy states are the total energy of the products of the preceding reaction step and H+ + e−. In the first step of the reaction in this simulation, the energy decreases by 4.28 eV when the OOH+ adsorbs onto the graphyne. When the H+ is subsequently introduced into the system, the adsorption energies for the reaction steps of reaction path I are −24.10, −18.87, and −13.54 eV as shown in Figure 5. For reaction path II, the adsorption energies for the reaction steps as shown in Figure 5 are −22.82, −17.50, and −16.00 eV. For the second reaction step, the decreasing energy of reaction path I (−24.10 eV) is slightly greater than that of reaction path II (−22.82 eV), suggesting that the reaction favors the production of two OH molecules when the O−O bond is broken. In each step of the electron transformation, the energy becomes more negative, driving the system to a more stable state. Therefore, the four-electron ORR on a graphyne surface is energetically favorable, i.e., graphyne exhibits high electrocatalytic activity toward ORRs and can be utilized as a metal-free electrocatalyst for ORRs. We also calculated the reversible potential Urev(surface) for each step in the ORR process on the graphyne surface (Table 1, note that only the values of Urev(surface) for reaction path I are listed). For each step of the electron transformation, the reversible potential is positive, suggesting that the system moves to a more stable state during the reactions. Therefore, the four-electron reaction can spontaneously take place on the graphyne, which is in good agreement with the values obtained from the adsorption energy analysis. The reversible potential for the overall reaction is calculated to be ∼1.226 V (versus SHE), almost same as the standard reversible potential for the ORR (1.229 V versus SHE). The reaction step having the lowest reversible potential value can be considered the ratelimiting step, and its potential may control the onset potential of the overall reaction because the reversible potentials are related to the electrocatalyst efficiency and activity. Thus, we

can estimate from Table 1 that the onset potential for the ORR on graphyne is ∼0.35 V, which is higher than those obtained on graphene (−0.45 V versus Ag/AgCl)20 and CNT (−0.1 V versus SHE)40 but similar to those obtained on nitrogen-doped graphene and NCNTs (−0.25 V versus SCE),5,41,42 which further suggests that graphyne has high activity toward catalytic ORR processes. We studied the chemical reactivity of graphyne by calculating the energy gap between the HOMO and LUMO and the spatial distributions of α and β electrons to explain the high catalytic activity of graphyne toward ORRs.44 The HOMO−LUMO energy gap can be used as a simple indicator of kinetic stability. A small HOMO−LUMO gap implies low kinetic stability and high chemical reactivity because it is energetically favorable to add electrons to a high LUMO and to extract electrons from a low HOMO to form the activated complex of any potential reaction. The calculated HOMO−LUMO gap of graphyne is ∼0.97 eV, which is lower than those of graphene (2.43 eV for C30H44 with the same periodic units as the constructed graphyne, and 2.77 eV for C46H2038), nitrogen-doped graphene (1.40, 1.20, and 1.39 eV for pyridinic, pyrrolic, and graphitic nitrogen-doped graphene, respectively),38,39 and graphene doped with 1−4 nitrogen atoms (1.1−1.7 eV).45 The small gap may represent the metallic character of the graphyne and ease the electron transfer from graphyne to OOH+.46 Hence, the graphyne should have higher chemical reactivity because the electrons are easily excited from the valence bands to the conduction bands. We also obtained the HOMO and LUMO spatial distributions of the α and β electrons for graphyne as depicted in Figure 6, which shows that both the HOMO and LUMO spatial distributions are localized with the same spatial distribution for the α and β electrons. This feature is similar 20477

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Table 1. Standard Reversible Potential and Reversible Potential for Each ORR Step on Graphynea step no.

standard reversible potential U° (V/SHE)b

reversible potential Urev (surface) (V/SHE)

−0.046

0.347

−0.664

3.196 0.751

O2 + H+ + e− → G−OOH

1

+

2

a

reaction

−

G−OOH + H + e → G−OH + G−OH +

−

3

G−OH + H + e → G−H 2O

2.813

4

G−OH + H+ + e− → G−H 2O

2.813

0.610

overall

O2 + 4H+ + 4e− → 2H 2O

1.229

1.226

b

Only the values of Urev(surface) for reaction path I are listed. The standard reversible potentials were acquired from ref 43.

Figure 6. HOMO (a) and LUMO (b) spatial distribution of electrons in graphyne.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (20905036, 21175067, and 21273117), the Research Fund for the Doctoral Program of Higher Education of China (20103207110004), the Natural Science Foundation of Jiangsu Province (BK2011779), the Foundation of the Jiangsu Education Committee (09KJA150001 and 10KJB150009), the Foundation of Jiangsu Provincial Key Laboratory of Palygorskite Science and Applied Technology (HPK201102), the Program for Outstanding Innovation Research Team of Universities in Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

to that of nitrogen-doped graphene, but is in contrast with that of undoped graphene in which both α and β electrons are delocalized (not shown here). The localized distribution of the electrons in graphyne causes a nonuniform distribution in the charge density at each carbon atom, leading to the formation of a large quantity of positive charge centers, which behave as catalytic sites for ORRs.

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CONCLUSIONS We have used DFT calculations to demonstrate that the novel carbon material, graphyne, has good catalytic activity for ORRs in acidic fuel cells. The simulation results indicate that the distribution of the charge density on the graphyne plane is not uniform, and a large number of positively charged carbon atoms exist on the graphyne, which are beneficial to the adsorption of O2 and OOH+ molecules and act as catalytic sites to facilitate ORRs. When an H+ is introduced into the system, the ORR electron transformation process occurs spontaneously along a four-electron pathway that includes the formation of an O−C chemical bond between oxygen and the graphyne, an O−O bond breakage, and the creation of water molecules. This work should inspire the study of carbon nanomaterials to improve their catalytic efficiency and provides a theoretical framework for the analysis of their catalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

Full list authors of refs 22 and 33. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 20478

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