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 Metalloporphyrins and Metallophthalocyanines Shaorui Sun, Ning Jiang, and Dingguo Xia* Environment and Energy College, Beijing University of Technology, Beijing 100124, China, and College of Engineering, Peking University, Beijing 100085, China ABSTRACT: In this paper, density functional theory is applied to study the electrochemical reduction of oxygen on iron phthalocyanine (FePc), iron porphyrin (FeP), cobalt phthalocyanine (CoPc), and cobalt porphyrin (CoP). According to the calculation results, for the four metal
macrocyclic complexes, O2 will not directly be cleaved without the cooperation of hydrogen. In the reduction process, on FePc or FeP, H2O2 does not form as an intermediate, and O2 is reduced to H2O, while on CoPc or CoP, O2 is just reduced to H2O2. The reason why the oxygen reduction ability of FePc or FeP is higher than that of CoPc or CoP, respectively, is that the energy level of the highest-occupied 3d orbital of the former is higher than that of the later. The high energy level of the metal 3d orbital leads to the strong ability of oxygen reduction.

1. INTRODUCTION One of the major challenges for proton exchange membrane (PEM) fuel cell commercialization is the high cost of platinum (Pt)-based catalysts. Thus, it is essential to develop alternative, cost-effective catalysts to reduce or eliminate the need for Ptbased ones. Since Jasinki1 first reported that cobalt phthalocyanine (CoPc) is active in electrocatalyzing reduction of oxygen in 1967, cobalt and iron macrocyclic complexes have been studied extensively as catalysts for oxygen reduction.1 Numerous reviews of the oxygen reduction reaction (ORR) on a metal
macrocyclic complex and its mechanisms have been published.2
7 Despite these investigations, our understanding of the ORR mechanism on metal
macrocyclic complexes and the factor to determine their oxygen reduction activity is far from complete. Tsuda et al.8 reported that O2 is directly cleaved on metal porphyrins, and the macrocycle takes part in the O2 dissociation (the macrocycle picks up one oxygen atom, and the central metal picks up the other one). Anderson et al.9 used an Fe(NH2)4 model to replace FeP and study the oxygen reduction process, and they reported that O2 does not directly dissociate, as mentioned by Tsuda et al.,8 and H2O2 formed as an intermediate, which bonded on an NH2 ligand and further reduced to H2O. It is a typical four-electron reduction process. Wang et al.10 mentioned that the two- or four-electron reduction process is dependent on the oxygen adsorption configuration; a side-on adsorption configuration leads to four-electron reduction, and an end-on adsorption leads to two-electron reduction. As for the factor to determine the oxygen reduction ability of a metal
macrocyclic complex, Shi et al.11 reported that the catalyst’s oxygen reduction activity could be related to the ionization potentials of the metal
macroyclic complex. Tsuda et al.8 attributed the abilities of metalloporphyrins to r 2011 American Chemical Society

dissociate O2 into the lowest unoccupied molecular orbital and highest occupied molecular orbital (LUMO
HOMO) characters. In this paper, DFT calculations are applied to study the mechanism of ORR on four metal
macrocyclic complexes, FePc, FeP, CoPc, and CoP, whose structure are presented in Figure 1. We come to new insights as follows: (i) ORR only takes place on the top of the central metal, that is, there is no bond forming between O2 (or intermediate) and the macrocycle; (ii) in the ORR process on FePc or FeP, H2O2 does not form as an intermediate, as mentioned in ref 9, and O2 is directly reduced to H2O; on CoPc or CoP, O2 is just reduced to H2O2; (iii) the oxygen reduction ability is determined by the 3d-electron configuration of the center metal.

2. CALCULATION METHOD All calculations are carried out using the Amsterdam Density Functional (ADF) program package developed by Baerends and co-workers.12 The exchange and correlation energy is described with BLYP.13,14 A triple-ζ plus polarization basis (TZP) set is employed for all elements. The criterion of the self-consistent convergence of the total energy is set as 0.001 eV/atom. To save calculation time, the frozen core technology is implemented in the work.12 Due to the fact that iron and cobalt are transition metals, the relativistic effect is considered. For the open-shell states, the unrestricted Hartee
Fock (UHF) spin density functional12 approach is used. In order to evaluate the reliability of the calculation method, the structures of FePc, CoPc, FeP, and CoP (as shown in Received: February 3, 2010 Revised: February 26, 2011 Published: April 26, 2011 9511

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Figure 1. Molecular structure of (a) metal phtalocyanine (MPc, M = Fe, Co) and (b) metal porphyrin (MP, M = Fe, Co).

Figure 2. Optimization structure of oxygen molecular adsorption for MPc (MP).

Table 1. Calculated Properties of Transition-Metal Macrocyclic Systems RM
N(Å) this work CoP

other work 1.987c

2.000

CoPc

1.950

FeP

2.014

FePc a

expt a

1.999c

1.958 b

1.92d

1.91

1.928 c

b

d

1.923e e

Reference 17. Reference 18. Reference 8. Reference 19. Reference 11.

Table 2. Bond Length RO
O of O2 Adsorbed at the Transition-Metal Macrocyclic Systems RO
O/Å

a

this work

other worka

CoP

1.289

1.264

CoPc

1.282

1.261

FeP

1.304

1.267

FePc

1.300

1.269

Reference 11.

Figure 1) are all fully optimized. The calculated distances (RM
N) between the central metal atom and the nearestneighboring N atom are listed in Table 1. The oxygen adsorption configurations on the four metal
macrocyclic complexes are also investigated, and for each of them, only the end-on type, as shown in Figure 2, is stable, which agrees with the previous results.11 The calculated O
O bond lengths of the absorbed

Figure 3. Free-energy profiles of O2 reduction on (a) FePc, (b) CoPc, (c) FeP, and (d) CoP.

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Figure 4. (a) Optimization structure of oxygen cleaved on FePc. (b) Optimization structure of two
OH absorbed on FePc.

dioxygen are listed in Table 2. The calculated RM
N and O
O bond lengths are slightly longer than the experimental values and other theoretical values,15
17 which is mainly due to the fact that BLYP, a kind of GGA, overestimated the bond length.18,19 The standard hydrogen electrode is hypothesized to be used in the work, of which the chemical potential for (Hþ þ e
) could be related to that of 1/2H2 in the gas phase. Under standard condition (U = 0, pH = 0, p = 1 bar, T = 298 K), the reaction energy of20,21 AH f A þ H þ e þ




FePc
OO þ 2Hþ ðaqÞ þ 2e
ðUÞ f FePc
ðOHÞðOHÞ ð5Þ Under the standard condition, its reaction energy is calculated as FePc
OO þ H2 f FePc
ðOHÞðOHÞ

ð1Þ

could be calculated as that of AH f A þ 1=2H2

Figure 5. Optimization structure of
OOH adsorbed on FePc.

ð2Þ

ΔE ¼
2:837 eV

ð6Þ

From A3 to A4, the two
OH are reduced to H2O, and the reaction is FePc
ðOHÞðOHÞ þ 2Hþ ðaqÞ þ 2e
ðUÞ f FePc þ 2H2 O ΔE ¼
1:881 eV

3. RESULTS AND DISCUSSION 3.1. Oxygen Reduction Path on FePc. For oxygen reduction on FePc, there are some possible reaction paths, such as those reported by Tsuda et al.8 or by Anderson et al.9 An effective method to identify the favorable path is to compare the potential energy of different reaction paths. The path in which the dioxygen is directly cleaved (mentioned by Tsuda et al.8) is specified as A, and A0 is the initial state (before reactions). A1 is oxygen adsorption on FePc, and the electrode reaction at this step is

FePc þ O2 f FePc
OO

ΔE ¼
0:454 eV

ð3Þ

where ΔE is the reaction energy (the change of the total energy between products and reactants). In state A 2 , the absorbed dioxygen is completely cleaved. The fully optimized structure is presented in Figure 4a, in which one oxygen atom is on the central metal and the other is on a carbon atom in the macrocylce, and it is consist with Tsuda et al.’s calculation. 8 The reaction from A 1 to A 2 is FePc
OO f FePc
OO

ΔE ¼ 0:326 eV

ð4Þ

where O* is the absorbed oxygen atom on FePc. From A2 to A3, each oxygen grasps a hydrogen and the two
OH absorbed on FePc, as shown in Figure 4b. In the step, the reaction is

ð7Þ

Its reaction energy is calculated in the same way as eq 5. For each step, the change of the free energy could be calculated as follows20,21 ΔG ¼ ΔE þ ΔZPE
TΔS
neU þ kT ln 10  ΔpH ð8Þ where ΔE is the reaction energy, ZPE is the zero-point energy, T is temperature, and S is entropy. The forth term is the effect of a bias involving electrons in the electrode, n is the electron number, and U is the electrode potential relative to the standard hydrogen potential. The fifth term is the change of free energy contributed from the change of pH value (the Hþ concentration) . Here, we only consider the situation with U = 0 (in eq 8, it is clear that the value of U dose not alter the free-energy difference between two paths) and pH = 0, and then, the forth and fifth terms are both zero. The value of ZPE is calculated with DFT in this work, and the value of entropy is taken from chemical data base.22 If the free energy of A0 (FePc þ O2 þ 4(Hþ þ e
)) is specified as 0 eV, the relative free energies of A1 (FePc
OO þ 4(Hþ þ e
)), A2 (FePc
O*O* þ 4(Hþ þ e
)), A3 (FePc
(OH)*(OH)* þ 2(Hþ þ e
)), and A4 (FePc þ 2H2O) are all specified in Figure 3a. In path B, the B0 and B1 state are also the A0 and A1 state in path A, respectively. From B1 to B2, the absorbed dioxygen does not cleave but seizes a hydrogen. The optimized structure is 9513

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presented in Figure 5. In the step, the electrode reactions are FePc
OO þ Hþ ðaqÞ þ e
ðUÞ f FePc
OOH ΔE ¼
0:951 eV

ð9Þ

From B2 to B3,
OOH is combined with a hydrogen. The change of the total energy versus the O
O distance is presented in Figure 7a, which means that when the distance between the two oxygen atoms (RO
O) increases, the total energy decreases at the same time. The final simulated structure, as shown in Figure 6a, shows that H2O2 is not formed. The parameters of the inside structure formed by one oxygen and two hydrogens are listed in Table 3, which means that H2O is formed. Then, at this step, the electrode reactions are FePc
OOH þ Hþ ðaqÞ þ e
ðUÞ f FePc
O þ H2 O ΔE ¼
2:221 eV ð10Þ From B3 to B4, the absorbed oxygen catches a hydrogen and forms
OH on iron; the reactions are FePc
O þ Hþ ðaqÞ þ e
ðUÞ f FePc
OH ΔE ¼
0:656 eV

ð11Þ

From B4 to B5,
OH gets another hydrogen and forms
OH2 on the central iron; the reactions are FePc
OH þ Hþ ðaqÞ þ e
ðUÞ f FePc
OH2 ΔE ¼
0:787 eV

ð12Þ

H2O þ (Hþ þ e
)), B5 (FePc
OH2 þ H2O), and B6 (FePc þ 2H2O) are all shown in Figure 3a. As shown in Figure 3a is that the relative free energy of each step in path A is higher than that in path B. The key difference between the two paths, A and B, is the dissociation of molecular oxygen (A2 state) or the formation of
OOH (B2 state). As presented in Figure 3a, the free energy of A2 is higher than that of B2 by about 1.14 eV, which is a relatively large value. Then, B is the energy-favorable path for oxygen reduced on FePc (U = 0), and in this way, as in the above discussion, H2O2 is not formed as an intermediate, which agrees well with the experimental result reported by Zagal et al.23 Furthermore, due to the fact that the energies of B3 and B4 are both lower than that of A3, even for H2O2 reduced on FePc, the energy-favorable intermediates are B3 and B4, not A3. 3.2. Oxygen Reduction Path on CoPc. The treatment for CoPc is similar to that for FePc. The first step (from A0 to A1) is dioxygen adsorption, and the reaction is CoPc þ O2 f CoPc
OO

ΔE ¼ 0:220 eV

CoPc
OO f CoPc
OO

ΔE ¼ 1:777 eV

ð15Þ

If the free energy of A0 (CoPc þ O2 þ 2(Hþ þ e
)) is specified as 0 eV, as presented in Figure 3b, the relative free energy of A2 (CoPc
O*O* þ 2(Hþ þ e
)) is too high (compared with path B, which is discussed as follows), and then, dioxygen cannot be reduced in this way. Table 3. Calculated Properties of the Optimization Reduction Produced Structures for FeP and FePc RO(2)
H(1)/Åa RO(2)
H(4)/Åa band angle/° ΔQO(2)a ΔQO(3)a

ð13Þ

The relative free energies of B0 (FePc þ O2 þ 4(Hþ þ e
)), B1 (FePc
OO þ 4(Hþ þ e
)), B2 (FePc
OOH þ 3(Hþ þ e
)), B3 (FePc
O* þ H2O þ 2(Hþ þ e
)), B4 (FePc
OH þ

ð14Þ

From A1 to A2, the dioxygen directly dissociates as that on FePc, and the reaction is

From B5 to B6, the absorbed H2O moves away from FePc FePc
OH2 f FePc þ H2 O

ΔE ¼
0:287 eV

FeP FePc

0.975 0.975

0.986 0.983

102.9 103.2

H2O*

0.976

0.976

103.7


0.686
0.677


0.545
0.534

a

The numbers in brackets are the atom order numbers, which are displayed in Figure 6.

Figure 6. Optimization structure of the reduction products at the B3 state for (a) FePc, (b) CoPc, (c) FeP, and (d) CoP. 9514

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For another pathway, from B1 to B2, the dioxygen on cobalt grasps a hydrogen, and the reaction is CoPc
OO þ Hþ ðaqÞ þ e
ðUÞ f CoPc
OOH ΔE ¼
0:536 eV

ð16Þ

For B3, the optimized structure is presented in Figure 6b, and the change of the total energy in the dioxygen dissociation is presented in Figure 7b, which implies that the energy is optimized to the minimum value when the O
O distance is close to that in H2O2. The parameters of the inside structure formed by two oxygens and two hydrogens are listed Table 4; it demonstrates that H2O2 is formed. Then, the reaction is CoPc
OOH þ Hþ ðaqÞ þ e
ðUÞ f CoPc
H2 O2 ΔE ¼
0:681 eV

ð17Þ

The adsorption energy of H2O2 on CoPc is very small, and it moves away from CoPc at B4 CoPc
H2 O2 f CoPc þ H2 O2

Figure 7. RO
O dependence of relative total energy of the step from B2 to B3 on (a) FePc, (b) CoPc, (c) FeP, and (d) CoP.

ΔE ¼ 0:075 eV ð18Þ

The relative free energies of B0 (A0), B1 (A1), A2 (CoPc
O*O* þ 2(Hþ þ e
)), B2 (CoPc
OOH þ (Hþ þ e
)), B3 (CoPc
H2O2), and B4 (CoPc þ H2O2) are presented in Figure 3b. The energy of A3 is higher than that of B3 by about 2.37 eV. Then, for O2 reduced on CoPc, the favorable path is B, and the final product is H2O2. 3.3. Oxygen Reduction Path on FeP and CoP. The calculation method for oxygen reduction on FeP is similar to that on FePc; the relative free energy of each step in path B is presented in Figure 3c. The free energies of path A are not calculated, and only the relative potential energy of A2, reported by Tsuda et al.8 (corrected with the zero-point energy here), is specified in Figure 3c. From A1 to A2, the relative free energy largely increases, while in path B, the energy keeps on decreasing until oxygen is completely reduced to H2O. Similar to that on FePc, path B is the energy-favorable pathway for oxygen reduction on FeP. In path B, from B2 to B3, the absorbed
OOH combines with hydrogen, and as shown in Figure 7c, the total energy decreases with an O
O distance increase. The optimized structure of B3 is presented in Figure 6c, and the geometry parameters listed in Table 3 demonstrate that H2O is produced at the step, which is the same as that on FePc. For CoP, the relative energy of A2, reported by Tsuda et al.8 (corrected with the zero-point energy here), is specified in Figure 3d, which is largely higher than that of any state in path B, and it implies that path B is favorable. From B2 to B3, the absorbed
OOH is cohensive with hydrogen, and the O
O distance dependence of total energy, which is presented in Figure 7d, demonstrates that the total energy decreases until the O
O bond length is very close to that in H2O2. The optimized structure of B3 is shown in Figure 6d, and the simulated geometry parameters of the structure formed by the four atoms (two oxygen and two hydrogen), which are list in Table 4, show that the H2O2 is produced. In step B4, H2O2 could easily move away from CoP due to the fact that the combination energy between H2O2 and CoP is very small. 3.4. Oxygen Reduction Ability. The oxygen reduction abilities of metallophthalocyanines and metalloporphyrins are 9515

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Table 4. Calculated Properties of the Optimization Reduction Produced Structures for CoP and CoPc RO(2)
H(1)/Åa

a

RO(3)
H(4)/Åa

RO
O/Åa

dihedral/°

ΔQO(2)a

ΔQO(3)a

Ead(H2O2)/eV

CoP

0.981

0.982

1.537

128.8


0.400


0.337

0.0996

CoPc

0.982

0.982

1.516

124.6


0.380


0.336

0.0751

H2O2*

0.981

0.981

1.503

108.6

The numbers in parentheses are the atom order numbers, which are displayed in Figure 6.

Figure 8. Energy levels of MPc and MP (M = Fe, Co; R = spin-down orbital; β = spin-up orbital).

mainly determined by their highest occupied metal 3d orbital. The orbital energy levels of FePc and CoPc are shown in Figure 8a and b, respectively, in which the orbitals are all labeled according to the irreducible representation of D4h. Two orbital levels of the ligand shell (the macrocycle formed by carbon, nitrogen, and hydrogen atoms) are specifiedl one is a1u24 (occupied), and the other is 2eg (unoccupied). The five 3d orbitals of the central metal are represented as a1g (dz2), b1g (dx2
y2), 1eg (dzx, dyz), and b2g (dxy). For FePc, the eg (HOMO) level in the β state of FePc is higher than that of CoPc by about 0.40 eV. Then, when O2 or H2O2 is absorbed, compared with CoPc, FePc’s metal 3d electron is more easily transferred to the π* orbital of the O
O bond. It could be demonstrated with the Mullikan charges (ΔQ) listed in Tables 3 and 4 and the charges of the two oxygens of H2O2 on FePc (Table 3) are obviously larger than those of H2O2 on CoPc (Table 4). Then, on FePc, the O
O bond is more weakened

than that on CoPc, that is, the oxygen reduction ability of FePc is more powerful than that of CoPc. It is the reason why FePc leads to a four-electron reduction and CoPc leads to a two-electron reduction. Similarly, as presented in Figure 8c and d, the highest occupied metal 3d orbitals of the β states of FeP and CoP are both eg, and the former is higher than the later by about 0.84 eV, which is evidence that FeP’s oxygen reduction ability is more powerful than that of CoP.

4. CONCLUSIONS In this paper, ORR on FePc, CoPc, FeP, and CoP is studied with DFT calculation. For the four metal
macrocyclic complexes, ORR takes place on the central metal. For ORR on FePc or FeP, H2O2 does not form as an intermediate, and O2 is reduced to H2O; for that on CoPc or CoP, O2 is just reduced to 9516

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H2O2. The reason why FePc has a four-electron ORR and CoPc has a two-electron ORR is that the energy level of the eg orbital of FePc is obviously higher than that of CoPc. The reason is also proper for FeP and CoP. This may be helpful for designing new non-noble electrocatalyst materials of fuel cell.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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