Density Functional Theory Study of the Oxygen Reduction Reaction on

Article pubs.acs.org/JPCC

Density Functional Theory Study of the Oxygen Reduction Reaction on a Cobalt−Polypyrrole Composite Catalyst Xin Chen,† Fan Li,† Xiayan Wang,† Shaorui Sun,*,† and Dingguo Xia*,‡ †

College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China College of Engineering, Peking University, Beijing 100871, China

‡

ABSTRACT: A theoretical study of the oxygen reduction mechanism catalyzed by cobalt−polypyrrole is investigated in detail by means of density functional theory method using the BLYP/DZP basis set. The calculations suggest that the cobalt−polypyrrole has a platinum-like catalytic behavior based on the adsorption energetics of the reaction intermediates. The dicobalt−polypyrrole catalyst exhibits a higher catalytic activity than that of mono-cobalt−polypyrrole, due to the fact that the PPy chains in di-cobalt− polypyrrole have a regular structure.

1. INTRODUCTION In recent years, the polymer electrolyte fuel cell (PEFC), a device for the direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions, has been considered as the key enabling technology for a transition to a hydrogen-based economy.1−3 The PEFC has high energy conversion, a low operating temperature, and environmental benefits, so it has been considered as the most promising system for applications in automotive transportation, distributed stationary power, portable electronics, and military use.4−6 However, Pt and Pt-based catalysts, the best and most frequently used in the PEFC cathode for the oxygen reduction reaction (ORR) until now, have been little commercialized due to their price and scarcity.7 Thus, partial or complete replacement of Pt and Pt-based materials for ORR has attracted considerable interest, in order to reduce costs. Recently, heterocyclic conjugated polymers such as polypyrrole, polyaniline and polythiophene have been the subject of much research owing to their wide applications in biosensors, electrochemistry, and electrocatalysis.8,9 Polypyrrole (PPy) is a chemical compound formed from a number of connected pyrrole rings and has a high electronic conductivity. It can also be employed as a matrix for incorporating metallic catalysts for the reduction of oxygen due to its high surface area.10 Bashyam and Zelenay11 developed a cobalt polypyrrole carbon (Co− PPy−C) composite which had high ORR activity without any noticeable loss of performance during operation of the PEFC. It is also expected that the Co−PPy−C composite would work well as a cathode catalyst in a direct hydrazine fuel cell (DHFC)12 and direct borohydride fuel cell (DBFC).13 However, an understanding of the origin of its catalytic activity is far from complete. For instance, the reported catalysis of the Co−PPy−C composite has been arbitrarily attributed to the presence of different active sites.11 The lack of fundamental understanding of the active site structure and the mechanism of ORR hinders the development of commercially viable nonprecious metal catalysts for fuel cells. © 2012 American Chemical Society

In order to understand oxygen reduced processes catalyzed by Co−PPy, two kinds of PPy fragments have been constructed and their ORR mechanisms were investigated using density functional theory (DFT).

2. COMPUTATIONAL METHODS The calculations are carried out with the Amsterdam Density Functional (ADF) program package.14−16 The electronic interaction is described using the Becke (exchange) and the Lee−Yang−Parr (correlation)17 functions (BLYP). The cobalt atom(s) was calculated with a triple-ζ polarized (TZP) Slatertype basis set and other atoms with double-ζ polarized (DZP) set. The inner core orbitals, 1s for C, N, and O and (1s-3p) for Co are kept frozen. The atomic charges are taken from the multipole derived charge analysis (MDC-q),18 which gives charges that reproduce by construction both the atomic and molecular multipoles. All atoms are completely relaxed. For all stationary states, every possible spin multiplicity is carefully checked. The chemical potential (the free energy per H) for the reaction (H+ + e−) can be related to that of 1/2H2 in the gas phase by use of the standard hydrogen electrode.19−21 Therefore, under standard conditions (U = 0, pH = 0, p = 1 bar, T = 298 K), the free energy difference of a reaction *AH → A + H+ + e− can be calculated as the free energy for the reaction *AH → A + 1/2H2. The free energy change for a reaction is calculated as follows: ΔG = ΔE + ΔZPE − TΔS where ΔE is the reaction energy, ΔZPE is the difference in zero-point energy, T is the temperature, and ΔS is the change in entropy. In this work, the values of ΔE and ΔZPE were calculated using DFT, and the value of entropy was taken from a chemical database.22 Received: January 18, 2012 Revised: May 17, 2012 Published: May 23, 2012 12553

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−12558

The Journal of Physical Chemistry C

Article

Two selected cobalt−polypyrrole models have been studied. One contains ten pyrrole rings with only one Co atom, and the other contains ten pyrrole rings with two Co atoms (Figure1).

Figure 3. Structures of dioxygen adsorbed on mono-Co−PPy with the end-on mode (a) and the side-on mode (b).

Table 2. Calculated Key Bond Lengths, R (Å), for the Four Steps in the O2 Reduction Catalyzed by the mono-Co−PPy Model

Figure 1. Optimized structures of the two selected cobalt−polypyrrole models. The pink circles are cobalt atoms, the blue circles are nitrogen atoms, the gray circles are carbon atoms, and the light white circles are hydrogen atoms.

The design of the structural models is based on the fact that the chemical valence of Co is +2 in Co−PPy according to the 2p1/2 and 2p2/3 electron binding energies of cobalt.23 Therefore, it is reasonable to consider that Co might be connected to nitrogen in the pyrrole ring to form a metallo-organic coordination compound and thus generate the Co−N active sites. During the preparation of our paper, we became aware of a report24 which also proves Co2+ only accommodates two polypyrrole chains.

Table 1. Calculated Reaction Energy Changes structure = = = = = =

1, 3, 3, 4, 4, 5,

n n n n n n

= = = = = =

1 3 4 4 5 5

system

spin multiplicity

ΔE (eV)

PPy−Co−PPy PPy3−Co−PPy3 PPy3−Co−PPy4 PPy4−Co−PPy4 PPy4−Co−PPy5 PPy5−Co−PPy5

4 4 4 4 4 4

−0.88 −1.28 −1.35 −1.43 −1.58 −1.70

RCo(1)−N(2)

RCo(1)−N(3)

RCo(1)−O(4)

RO(4)−O(5)

A0 A1 A2 A3 A4 A5

1.955 1.996 2.024 1.963 2.060 1.981

1.950 2.005 2.014 1.962 2.043 1.959

1.790 1.844 1.642 1.873 2.297

1.329 1.520

Table 3. Calculated Key Atomic Charges for the Four Steps in the O2 Reduction Catalyzed by the mono-Co−PPy Model

3. RESULTS AND DISCUSSION 3.1. Stability Study of the Selected Co−PPy Models. In order to estimate the stability of the selected model structures,

m m m m m m

state

state

Co(1)

N(2)

N(3)

O(4)

O(5)

A0 A1 A2 A3 A4 A5

0.630 0.664 0.721 0.762 0.712 0.653

−0.378 −0.357 −0.371 −0.345 −0.370 −0.382

−0.389 −0.351 −0.361 −0.345 −0.364 −0.385

−0.116 −0.302 −0.405 −0.697 −0.529

−0.231 −0.404

Figure 4. RO−O dependence of electronic energy at the A3 state for mono-Co−PPy.

water molecules in solution,22 forming an octahedral structure with a high−spin state. The calculated results are listed in Table 1.

Figure 2. Reaction energy changes calculated with eq 1.

H − PPym + H − PPyn + [Co(H 2O)6 ]2 +

we calculated a series of energy changes (ΔE) for the reaction between the hydrated cobalt ions and polypyrrole, as shown below. As is well-known, the Co(II) ion is coordinated to six

→ PPym − Co − PPyn + 2[H3O+(H 2O)2 ] 12554

(1)



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In state A1, the calculated bond lengths for Co(1)−N(2) and Co(1)−N(3) (1.996 and 2.005 Å, respectively) become elongated after the adsorption of O2, as observed in Table 2. The O(4)−O(5) bond length is stretched from an equilibrium value of 1.245−1.329 Å, indicating that the molecular oxygen has been activated by the catalyst. The positive MDC-q charge of Co(1) atom is increased from 0.630 to 0.664, and the negative charges of O(4) and O(5) are −0.116 and −0.231, respectively, indicating electron flow from the metal atom to the π* orbital of the adsorbate, as shown in Table 3. In state A2, the adsorbed oxygen seizes a hydrogen, and the electrode reaction is

Figure 5. Optimized structures of state A3 and B3.

A 0 − O − O + H+ + e−(U) → A 0 − OOH(A 2) (3)

ΔG = −0.40 eV

From A1 to A2, the Co(1)−O(4) bond length has been elongated from 1.790 to 1.844 Å, and at the same time, the distance between O(4)−O(5) is stretched to an enormous 1.520 Å and nearly broken. Hydrogen peroxide, H2O2, is usually proposed as either a product or an intermediate of the ORR on Pt surfaces.27−29 When −OOH combines with a hydrogen, one finds that the energy decreases with increasing distance between the two oxygen atoms, as shown in Figure 4. The fully optimized structure is shown in Figure 5 and shows that peroxide is not formed, and the replacement is −O (the adsorbed oxygen) and H2O. In this step, the electrode reaction is

Figure 6. Dioxygen adsorbed on di-Co−PPy.

where m or n is the number of pyrrole rings in the PPy chain. From Table 1, it can be seen that each reaction has a negative ΔE value, indicating that all of the calculated reactions are exothermic. In other words, the selected cobalt polypyrrole structures are more stable than hydrated cobalt ions. It is interesting that with the sum of m and n increasing, the ΔE decreases almost linearly, as shown in Figure 2. Accordingly, Co−PPy with long chains should have good stability. 3.2. Oxygen Reduction Reaction on the mono-Co− PPy Model. Generally, there are two dioxygen adsorption modes: side-on and end-on. In this work, calculations are performed for both modes and both structures are fully optimized, as shown in Figure 3a (end-on) and Figure 3b (sideon). In the end-on mode, the two PPy chains are still almost parallel to each other, whereas in the side-on mode, the adsorbed dioxygen needs more space than that for the end-on mode,25,26 so the chains are significantly rotated and almost perpendicular to each other. The dioxygen adsorption energies of the two modes are close to each other, which implies that they are both possible. However, for PPy with a long chain, a large rotation would be impeded by the complex environment. In addition, if more than one cobalt atom is between the two chains, a large rotation is impossible. Consequently, in this work, the side-on adsorption mode is abandoned. For convenience of discussion, the initial state of the monoCo−PPy model is specified as A0, and A1 is the state when O2 is adsorbed on the A0 surface. The electrode reaction at the first step is A 0 + O2 → A 0 − O(4) − O(5)(A1)

A 0 − OOH + H+ + e−(U) → A 0 − O(A3) + H 2O (4)

ΔG = −2.28 eV

The results above may lead to a lower activation energy in the following reduction step since, on Pt surfaces, the Pt−O that forms is very reactive, with zero or very small activation energy for reduction to Pt−OH.30 The bond length of Co(1)−O(4) has now been shortened to 1.642 Å, indicating a very strong adsorption interaction. The chemical adsorption energy of adsorbed −O is −3.81 eV, which is very close to its adsorption energy on the Pt3 cluster (−3.86 eV).31 The generated H2O molecule would be removed from the reaction system and be associated with an energy release of −0.29 eV. From A3 to A4, the absorbed −O captures a hydrogen forming −OH in the active site. The reaction is A 0 − O + H+ + e−(U) → A 0 − OH(A4) (5)

ΔG = −0.78 eV

The greatest structural change for state A4 is the value of the Co(1)−O(4) bond. The distance between Co(1) and O(4) is significantly elongated to 1.873 Å, which is about 0.231 Å longer than in state A3. The adsorption energy of the adsorbed −OH is −2.61 eV, which is close to the adsorption energy on

ΔG = −0.031 eV (2)

Table 4. Calculated Key Bond Lengths, R (Å), for the Four Steps in the O2 Reduction Catalyzed by the di-Co−PPy Model state

RCo(1)−N(2)

RCo(1)−N(3)

B0 B1 B2 B3 B4 B5

1.965 1.998 2.023 1.952 1.963 1.982

1.959 2.026 2.078 1.991 1.992 1.970

RCo(1)−O(9)

RCo(6)−N(7)

RCo(6)−N(8)

1.790 1.849 1.638 1.862 2.458

1.957 1.959 1.965 1.963 1.965 1.956

1.969 1.964 1.983 1.960 1.955 1.954

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RO(9)−O(10) 1.339 1.531



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Table 5. Calculated Key Atomic Charges for the Four Steps in the O2 Reduction Catalyzed by the di-Co−PPy Model state

Co(1)

N(2)

N(3)

N(4)

N(5)

Co(6)

N(7)

N(8)

O(9)

O(10)

B0 B1 B2 B3 B4 B5

0.494 0.694 0.702 0.775 0.527 0.495

−0.322 −0.326 −0.326 −0.324 −0.280 −0.332

−0.349 −0.366 −0.353 −0.355 −0.279 −0.342

−0.396 −0.390 −0.402 −0.389 −0.400 −0.395

−0.403 −0.377 −0.340 −0.395 −0.222 −0.221

0.494 0.509 0.472 0.493 0.465 0.464

−0.318 −0.337 −0.340 −0.335 −0.333 −0.327

−0.351 −0.359 −0.361 −0.358 −0.305 −0.318

−0.127 −0.306 −0.409 −0.653 −0.615

−0.233 −0.449

Figure 7. Potential energy surface profile for the O2 reduction of the two selected reaction systems.

Figure 8. Ratio of charge changing of the whole reaction steps from A0−A5 for mono-Co−PPy.

Figure 9. Ratio of charge changing of the whole reaction steps from B0−B5 for di-Co−PPy.

32

the nanometer Pt cluster. This implies that the cobalt− polypyrrole catalyst presents a platinum-like behavior in the oxygen reduction process. In the last step, the adsorbed −OH captures another hydrogen, forming the second H2O molecule. The reaction is

Table 6. Calculated Dihedral Angles between the Pyrrole Units for the Structures of mono-Co−PPy and di-Co−PPy Py1−Py2 Py1−Py3 Py1−Py4 Py1−Py5

A 0 − O − H + H+ + e−(U) → A 0 − OH 2(A 5) (6)

ΔG = −0.76 eV

Therefore, the distance between Co(1) and O(4) has been greatly elongated, from 1.873 to 2.297 Å. The free energy change suggests that the removal of the adsorbed H2O molecule is a spontaneous process. A 0 − OH 2 → A 0 + H 2O

ΔG = − 0.41 eV

mono-Co−PPy

di-Co−PPy

37.4 12.5 42.6 32.3

22.4 2.3 22.6 2.5

between two neighboring long PPys. The purpose of designing the di-Co−PPy model is to evaluate the synergistic effect between two Co atoms. Here we consider only the situation when the ORR is on one of the cobalts. The optimized structure with an adsorption dioxygen is presented in Figure 6.

(7)

3.3. Oxygen Reduction Reaction on the di-Co−PPy Model. It is possible that more than one cobalt atom can fit 12556



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indicates that the change of charge for Co(6) is less than 10%, but those of some nitrogen atoms (especially N(5) and N(11)) are obvious. This means that Co(6) does not donate electron in the ORR, but some nitrogen atoms do. Then the problem becomes why the nitrogen atoms in mono-Co−PPy are not active, and those in di-Co−PPy are active. The dihedral angles between the pyrrole units of mono- and di-Co−PPy are listed in Table 6. For mono-Co−PPy, the angles have no regular pattern, while for di-Co−PPy, the angles appear to show a perfect arrangement. This implies that the structure of PPy chains is significantly improved, and almost becomes a periodic structure. This kind of structure is helpful for electron transfer in the chain, and makes the nitrogen atoms become active and donate electrons to the ORR process. It is reasonable to conclude that a Co−PPy containing many cobalt atoms between two long PPy chains with periodic structure should show good catalytic ability for ORR.

The treatment for di-Co−PPy is similar to that for mono-Co− PPy, and each step is listed as follows: B0 + O2 → B0−O(9) − O(10)(B1)

ΔG = 0.016 eV (8)

B0 − O − O + H+ + e−(U) → B0 − OOH(B2) (9)

ΔG = −0.46 eV

B0 − OOH + H+ + e−(U) → B0 − O(B3) + H 2O (10)

ΔG = −2.36 eV

B0 − O + H+ + e−(U) → B0 − OH(B4 ) (11)

ΔG = −1.18 eV

B0 − OH + H+ + e−(U) → B0 − OH 2(B5)

B0 − OH 2 → B0 + H 2O

4. CONCLUSIONS In this paper, the oxygen reduction mechanism catalyzed by cobalt−polypyrrole catalysts has been investigated in detail by means of DFT. The calculations suggest that the cobalt− polypyrrole has a platinum-like catalytic behavior based on the adsorption energetics of the reaction intermediates. The diCo−PPy model exhibits a higher catalytic activity than that of mono-Co−PPy, which is due to the fact that the structure of the PPy chains are regular, as rearranged by the two cobalt atoms. It suggests that the Co−PPy with a periodic structure should have a good ORR catalytic ability.

(12)

ΔG = −0.39 eV ΔG = −0.29 eV

(13)

The calculated key bond lengths and atomic charges are listed in Tables 4 and 5. In those steps, for step B3 (eq 10), the adsorption energy of −O is about −3.86 eV; in the next step (eq 11), that of −OH is −2.99 eV, which are all close to the nanometer Pt cluster.32 If the free energy of A0 + O2 + 4(H+ + e−) and B0 + O2 + 4(H+ + e−) are specified as 0 eV, the relative free energies of A1(B1) + 4(H+ + e−), A2(B2) + 3(H+ + e−), A3(B3) + H2O + 2(H+ + e−), A4(B4) + H2O + (H+ + e−), and A5(B5) + H2O can all be illustrated in the potential energy surface profile (Figure 7). As shown in Figure 7, the relative total energy of each oxygen reduction step of mono-Co−PPy is higher than that for di-Co−PPy. The largest energy difference between these two systems is in the third reduction step, with a difference of 0.49 eV. Therefore, oxygen reduction steps catalyzed by di-Co−PPy are energy favorable compared with reduction catalyzed by mono-Co−PPy. 3.4. Synergistic Effect. The two selected models both exhibit a higher catalytic activity for ORR, and there are three differences between the mono- and di-Co−PPy. First, the adsorption energy of −O on di-Co−PPy is −3.86 eV, which is stronger than that of mono-Co−PPy (−3.81 eV). The calculations by Xu et al. have revealed that the stronger a material binds atomic oxygen, the more effective it will be in breaking apart molecular oxygen, which could be used to identify the efficiency of a catalyst.33 Second, the adsorption energy of −OH on di-Co−PPy (−2.99 eV) is larger than that of mono-Co−PPy (−2.61 eV).34 Third, as shown in Figure 7, for most steps in the oxygen reduction reaction, that catalyzed by di-Co−PPy is more energetically favorable than that catalyzed by mono-Co−PPy. Therefore, di-Co−PPy may have a better catalytic activity than that of mono−Co−PPy. When the electron-withdrawing group, −OO, −OOH, −O, and −OH, is adsorbed on the cobalt, the electron flows from the catalyst molecule to one of these groups. For mono-Co− PPy, in the ORR, as shown in Figure 8, the maximum charge changing of cobalt is about 23%, and that of each nitrogen is less than 10%. This implies that the electron withdrawn by the adsorbed groups is mainly contributed by the cobalt. For di-Co−PPy, as shown in Figure 9, the maximum change of charge for Co(1) is about 56%, which is obviously larger than that of cobalt in mono-Co−PPy. Further charge analysis

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 67396158. Fax: +86 10 67391983. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed with the financial supports from the Fundamental Science Research Foundation of Beijing University of Technology (X4005012201101), the major program of Beijing Municipal Natural Science Foundation (No. 20110001), and National Natural Science Foundation of China (No. 11179001).

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Density Functional Theory Study of the Oxygen Reduction Reaction on

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