Density Functional Theory Study of Oxygen Reduction Reaction on Non-PGM Fe-Nx-C Electrocatalyst

Density Functional Theory Study of the Oxygen Reduction Reactionon a Cobalt−Polypyrrole Composite CatalystXin 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 mechanismcatalyzed by cobalt−polypyrrole is investigated in detail by means of densityfunctional theory method using the BLYP/DZP basis set. The calculationssuggest that the cobalt−polypyrrole has a platinum-like catalytic behaviorbased on the adsorption energetics of the reaction intermediates. The di-cobalt−polypyrrole catalyst exhibits a higher catalytic activity than that ofmono-cobalt−polypyrrole, due to the fact that the PPy chains in di-cobalt−polypyrrole have a regular structure.

1. INTRODUCTIONIn recent years, the polymer electrolyte fuel cell (PEFC), adevice for the direct conversion of the chemical energy of a fuelinto electricity by electrochemical reactions, has beenconsidered as the key enabling technology for a transition toa hydrogen-based economy.1−3 The PEFC has high energyconversion, a low operating temperature, and environmentalbenefits, so it has been considered as the most promisingsystem for applications in automotive transportation, distrib-uted stationary power, portable electronics, and military use.4−6

However, Pt and Pt-based catalysts, the best and mostfrequently used in the PEFC cathode for the oxygen reductionreaction (ORR) until now, have been little commercialized dueto their price and scarcity.7 Thus, partial or completereplacement of Pt and Pt-based materials for ORR has attractedconsiderable interest, in order to reduce costs.Recently, heterocyclic conjugated polymers such as poly-

pyrrole, polyaniline and polythiophene have been the subject ofmuch research owing to their wide applications in biosensors,electrochemistry, and electrocatalysis.8,9 Polypyrrole (PPy) is achemical compound formed from a number of connectedpyrrole rings and has a high electronic conductivity. It can alsobe employed as a matrix for incorporating metallic catalysts forthe reduction of oxygen due to its high surface area.10 Bashyamand Zelenay11 developed a cobalt polypyrrole carbon (Co−PPy−C) composite which had high ORR activity without anynoticeable loss of performance during operation of the PEFC. Itis also expected that the Co−PPy−C composite would workwell 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 activityis far from complete. For instance, the reported catalysis of theCo−PPy−C composite has been arbitrarily attributed to thepresence of different active sites.11 The lack of fundamentalunderstanding of the active site structure and the mechanism ofORR hinders the development of commercially viable non-precious metal catalysts for fuel cells.

In order to understand oxygen reduced processes catalyzedby Co−PPy, two kinds of PPy fragments have been constructedand their ORR mechanisms were investigated using densityfunctional theory (DFT).

2. COMPUTATIONAL METHODSThe calculations are carried out with the Amsterdam DensityFunctional (ADF) program package.14−16 The electronicinteraction is described using the Becke (exchange) and theLee−Yang−Parr (correlation)17 functions (BLYP). The cobaltatom(s) was calculated with a triple-ζ polarized (TZP) Slater-type basis set and other atoms with double-ζ polarized (DZP)set. The inner core orbitals, 1s for C, N, and O and (1s-3p) forCo are kept frozen. The atomic charges are taken from themultipole derived charge analysis (MDC-q),18 which givescharges that reproduce by construction both the atomic andmolecular multipoles. All atoms are completely relaxed. For allstationary states, every possible spin multiplicity is carefullychecked.The chemical potential (the free energy per H) for the

reaction (H+ + e−) can be related to that of 1/2H2 in the gasphase by use of the standard hydrogen electrode.19−21

Therefore, under standard conditions (U = 0, pH = 0, p = 1bar, T = 298 K), the free energy difference of a reaction *AH→A + H+ + e− can be calculated as the free energy for thereaction *AH → A + 1/2H2. The free energy change for areaction is calculated as follows: ΔG = ΔE + ΔZPE − TΔSwhere ΔE is the reaction energy, ΔZPE is the difference inzero-point energy, T is the temperature, and ΔS is the changein entropy. In this work, the values of ΔE and ΔZPE werecalculated using DFT, and the value of entropy was taken froma chemical database.22

Received: January 18, 2012Revised: May 17, 2012Published: May 23, 2012

Article

pubs.acs.org/JPCC

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

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

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

3. RESULTS AND DISCUSSION3.1. Stability Study of the Selected Co−PPy Models. In

order to estimate the stability of the selected model structures,

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

water molecules in solution,22 forming an octahedral structurewith a high−spin state. The calculated results are listed in Table1.

− + − +

→ − − +

+

+

H PPy H PPy [Co(H O) ]

PPy Co PPy 2[H O (H O) ]m n

m n

2 62

3 2 2 (1)

Figure 1. Optimized structures of the two selected cobalt−polypyrrolemodels. The pink circles are cobalt atoms, the blue circles are nitrogenatoms, the gray circles are carbon atoms, and the light white circles arehydrogen atoms.

Table 1. Calculated Reaction Energy Changes

structure system spin multiplicity ΔE (eV)

m = 1, n = 1 PPy−Co−PPy 4 −0.88m = 3, n = 3 PPy3−Co−PPy3 4 −1.28m = 3, n = 4 PPy3−Co−PPy4 4 −1.35m = 4, n = 4 PPy4−Co−PPy4 4 −1.43m = 4, n = 5 PPy4−Co−PPy5 4 −1.58m = 5, n = 5 PPy5−Co−PPy5 4 −1.70

Figure 2. Reaction energy changes calculated with eq 1.

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

Table 2. Calculated Key Bond Lengths, R (Å), for the FourSteps in the O2 Reduction Catalyzed by the mono-Co−PPyModel

state RCo(1)−N(2) RCo(1)−N(3) RCo(1)−O(4) RO(4)−O(5)

A0 1.955 1.950A1 1.996 2.005 1.790 1.329A2 2.024 2.014 1.844 1.520A3 1.963 1.962 1.642A4 2.060 2.043 1.873A5 1.981 1.959 2.297

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

state Co(1) N(2) N(3) O(4) O(5)

A0 0.630 −0.378 −0.389A1 0.664 −0.357 −0.351 −0.116 −0.231A2 0.721 −0.371 −0.361 −0.302 −0.404A3 0.762 −0.345 −0.345 −0.405A4 0.712 −0.370 −0.364 −0.697A5 0.653 −0.382 −0.385 −0.529

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

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−1255812554

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 areexothermic. In other words, the selected cobalt polypyrrolestructures are more stable than hydrated cobalt ions. It isinteresting that with the sum of m and n increasing, the ΔEdecreases 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 adsorptionmodes: side-on and end-on. In this work, calculations areperformed for both modes and both structures are fullyoptimized, as shown in Figure 3a (end-on) and Figure 3b (side-on). In the end-on mode, the two PPy chains are still almostparallel to each other, whereas in the side-on mode, theadsorbed dioxygen needs more space than that for the end-onmode,25,26 so the chains are significantly rotated and almostperpendicular to each other. The dioxygen adsorption energiesof the two modes are close to each other, which implies thatthey are both possible. However, for PPy with a long chain, alarge rotation would be impeded by the complex environment.In addition, if more than one cobalt atom is between the twochains, a large rotation is impossible. Consequently, in thiswork, the side-on adsorption mode is abandoned.For convenience of discussion, the initial state of the mono-

Co−PPy model is specified as A0, and A1 is the state when O2 isadsorbed on the A0 surface. The electrode reaction at the firststep is

+ → − − Δ = −GA O A O(4) O(5)(A ) 0.031 eV0 2 0 1(2)

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

electrode reaction is

− − + + → −

Δ = −

+ −

G

A O O H e (U) A OOH(A )

0.40 eV0 0 2

(3)

From A1 to A2, the Co(1)−O(4) bond length has beenelongated from 1.790 to 1.844 Å, and at the same time, thedistance between O(4)−O(5) is stretched to an enormous1.520 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 theenergy decreases with increasing distance between the twooxygen atoms, as shown in Figure 4. The fully optimizedstructure is shown in Figure 5 and shows that peroxide is notformed, and the replacement is −O (the adsorbed oxygen) andH2O. In this step, the electrode reaction is

− + + → − +

Δ = −

+ −

G

A OOH H e (U) A O(A ) H O

2.28 eV0 0 3 2

(4)

The results above may lead to a lower activation energy in thefollowing reduction step since, on Pt surfaces, the Pt−O thatforms is very reactive, with zero or very small activation energyfor reduction to Pt−OH.30 The bond length of Co(1)−O(4)has now been shortened to 1.642 Å, indicating a very strongadsorption interaction. The chemical adsorption energy ofadsorbed −O is −3.81 eV, which is very close to its adsorptionenergy on the Pt3 cluster (−3.86 eV).31 The generated H2Omolecule would be removed from the reaction system and beassociated 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

− + + → −

Δ = −

+ −

G

A O H e (U) A OH(A )

0.78 eV0 0 4

(5)

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

Figure 5. Optimized structures of state A3 and B3.

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

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) RCo(1)−O(9) RCo(6)−N(7) RCo(6)−N(8) RO(9)−O(10)

B0 1.965 1.959 1.957 1.969B1 1.998 2.026 1.790 1.959 1.964 1.339B2 2.023 2.078 1.849 1.965 1.983 1.531B3 1.952 1.991 1.638 1.963 1.960B4 1.963 1.992 1.862 1.965 1.955B5 1.982 1.970 2.458 1.956 1.954

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−1255812555

the nanometer Pt cluster.32 This implies that the cobalt−polypyrrole catalyst presents a platinum-like behavior in theoxygen reduction process.In the last step, the adsorbed −OH captures another

hydrogen, forming the second H2O molecule. The reaction is

− − + + → −

Δ = −

+ −

G

A O H H e (U) A OH (A )

0.76 eV0 0 2 5

(6)

Therefore, the distance between Co(1) and O(4) has beengreatly elongated, from 1.873 to 2.297 Å. The free energychange suggests that the removal of the adsorbed H2Omolecule is a spontaneous process.

− → + Δ = −GA OH A H O 0.41 eV0 2 0 2 (7)

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

between two neighboring long PPys. The purpose of designingthe di-Co−PPy model is to evaluate the synergistic effectbetween two Co atoms. Here we consider only the situationwhen the ORR is on one of the cobalts. The optimizedstructure with an adsorption dioxygen is presented in Figure 6.

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 0.494 −0.322 −0.349 −0.396 −0.403 0.494 −0.318 −0.351B1 0.694 −0.326 −0.366 −0.390 −0.377 0.509 −0.337 −0.359 −0.127 −0.233B2 0.702 −0.326 −0.353 −0.402 −0.340 0.472 −0.340 −0.361 −0.306 −0.449B3 0.775 −0.324 −0.355 −0.389 −0.395 0.493 −0.335 −0.358 −0.409B4 0.527 −0.280 −0.279 −0.400 −0.222 0.465 −0.333 −0.305 −0.653B5 0.495 −0.332 −0.342 −0.395 −0.221 0.464 −0.327 −0.318 −0.615

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 fromA0−A5 for mono-Co−PPy. Figure 9. Ratio of charge changing of the whole reaction steps from

B0−B5 for di-Co−PPy.

Table 6. Calculated Dihedral Angles between the PyrroleUnits for the Structures of mono-Co−PPy and di-Co−PPy

mono-Co−PPy di-Co−PPy

Py1−Py2 37.4 22.4Py1−Py3 12.5 2.3Py1−Py4 42.6 22.6Py1−Py5 32.3 2.5

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−1255812556

The treatment for di-Co−PPy is similar to that for mono-Co−PPy, and each step is listed as follows:

+ → − − Δ =GB O B O(9) O(10)(B ) 0.016 eV0 2 0 1(8)

− − + + → −

Δ = −

+ −

G

B O O H e (U) B OOH(B )

0.46 eV0 0 2

(9)

− + + → − +

Δ = −

+ −

G

B OOH H e (U) B O(B ) H O

2.36 eV0 0 3 2

(10)

− + + → −

Δ = −

+ −

G

B O H e (U) B OH(B )

1.18 eV0 0 4

(11)

− + + → −

Δ = −

+ −

G

B OH H e (U) B OH (B )

0.39 eV0 0 2 5

(12)

− → + Δ = −GB OH B H O 0.29 eV0 2 0 2 (13)

The calculated key bond lengths and atomic charges arelisted in Tables 4 and 5. In those steps, for step B3 (eq 10), theadsorption 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 thenanometer 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 ofA1(B1) + 4(H+ + e−), A2(B2) + 3(H+ + e−), A3(B3) + H2O +2(H+ + e−), A4(B4) + H2O + (H+ + e−), and A5(B5) + H2O canall be illustrated in the potential energy surface profile (Figure7). As shown in Figure 7, the relative total energy of eachoxygen reduction step of mono-Co−PPy is higher than that fordi-Co−PPy. The largest energy difference between these twosystems is in the third reduction step, with a difference of 0.49eV. Therefore, oxygen reduction steps catalyzed by di-Co−PPyare energy favorable compared with reduction catalyzed bymono-Co−PPy.3.4. Synergistic Effect. The two selected models both

exhibit a higher catalytic activity for ORR, and there are threedifferences between the mono- and di-Co−PPy. First, theadsorption energy of −O on di-Co−PPy is −3.86 eV, which isstronger than that of mono-Co−PPy (−3.81 eV). Thecalculations by Xu et al. have revealed that the stronger amaterial binds atomic oxygen, the more effective it will be inbreaking apart molecular oxygen, which could be used toidentify the efficiency of a catalyst.33 Second, the adsorptionenergy of −OH on di-Co−PPy (−2.99 eV) is larger than thatof mono-Co−PPy (−2.61 eV).34 Third, as shown in Figure 7,for most steps in the oxygen reduction reaction, that catalyzedby di-Co−PPy is more energetically favorable than thatcatalyzed by mono-Co−PPy. Therefore, di-Co−PPy may havea 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 fromthe catalyst molecule to one of these groups. For mono-Co−PPy, in the ORR, as shown in Figure 8, the maximum chargechanging of cobalt is about 23%, and that of each nitrogen isless than 10%. This implies that the electron withdrawn by theadsorbed 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 largerthan that of cobalt in mono-Co−PPy. Further charge analysis

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 inthe ORR, but some nitrogen atoms do. Then the problembecomes why the nitrogen atoms in mono-Co−PPy are notactive, 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, theangles have no regular pattern, while for di-Co−PPy, the anglesappear to show a perfect arrangement. This implies that thestructure of PPy chains is significantly improved, and almostbecomes a periodic structure. This kind of structure is helpfulfor electron transfer in the chain, and makes the nitrogen atomsbecome active and donate electrons to the ORR process. It isreasonable to conclude that a Co−PPy containing many cobaltatoms between two long PPy chains with periodic structureshould show good catalytic ability for ORR.

4. CONCLUSIONSIn this paper, the oxygen reduction mechanism catalyzed bycobalt−polypyrrole catalysts has been investigated in detail bymeans of DFT. The calculations suggest that the cobalt−polypyrrole has a platinum-like catalytic behavior based on theadsorption energetics of the reaction intermediates. The di-Co−PPy model exhibits a higher catalytic activity than that ofmono-Co−PPy, which is due to the fact that the structure ofthe PPy chains are regular, as rearranged by the two cobaltatoms. It suggests that the Co−PPy with a periodic structureshould have a good ORR catalytic ability.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +86 10 67396158. Fax: +86 10 67391983. E-mail:[email protected]; [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was performed with the financial supports from theFundamental Science Research Foundation of Beijing Uni-versity of Technology (X4005012201101), the major programof Beijing Municipal Natural Science Foundation (No.20110001), and National Natural Science Foundation ofChina (No. 11179001).

■ REFERENCES(1) Jacobson, M. Z.; Colella, W. G.; Golden, D. M. Science 2005, 308,1901−1905.(2) Dusastre, V. Nature 2001, 414, 331−331.(3) Schultz, M. G; Diehl, T.; Brasseur, G. P.; Zittel, W. Science 2003,302, 624−627.(4) Zhang, L.; Zhang, J.; Wilkinson, D. P.; Wang, H. J. Power Sources2006, 156, 171−182.(5) Hogarth, M. P.; Ralph, T. R. Platinum Met. Rev. 2002, 46, 146−164.(6) Carrette, L.; Friedrich, K. A.; Stimming, U. Chem. Phys. Chem.2000, 1, 162−193.(7) Feng, Y.; Alonso−Vante, N. Phys. Stat. Sol. (b) 2008, 245, 1792−1806.(8) Lu, W.; Fadeev, A. G.; Qi, B.; Smela, E.; Mattes, B. R.; Ding, J.;Spinks, G. M.; Mazurkiewicz, J.; Zhou, D.; Wallace, G. G.; et al. Science2002, 297, 983−987.(9) Hepel, M.; Chen, Y. M.; Stephenson, R. J. Electrochem. Soc. 1996,143, 498−505.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−1255812557

(10) Yuasa, M.; Yamaguchi, A.; Itsuki, H.; Tanaka, K.; Yamamoto,M.; Oyaizu, K. Chem. Mater. 2005, 17, 4278−4281.(11) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63−66.(12) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.;Kobayashi, T. Angew. Chem., Int. Ed. 2007, 46, 8024−8027.(13) Qin, H. Y.; Liu, Z. X.; Yin, W. X.; Zhu, J. K.; Li, Z. P. J. PowerSources 2008, 185, 909−912.(14) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; FonsecaGuerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J.Comput. Chem. 2001, 22, 931−967.(15) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J.Theor. Chem. Acc. 1998, 99, 391−403.(16) ADF2009.01, SCM, Theoretical Chemistry; Vrije Universiteit:Amsterdam, The Netherlands; http://www.scm.com.(17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.(18) Swart, M.; van Duijnen, P. Th.; Snijders, J. G. J. Comput. Chem.2001, 22, 79−88.(19) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.;Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108,17886−17892.(20) Karlberg, G. S.; Rossmeisl, J.; Norskov, J. K. Phys. Chem. Chem.Phys. 2007, 9, 5158−5161.(21) Sun, S.; Jiang, N.; Xia, D. J. Phys. Chem. C 2011, 115, 9511−9517.(22) Wang, Z. L.; Zhou, Y. P. Physical Chemistry, 4th ed.; HigherEducation Press: Beijing, China, 2007; in Chinese.(23) Wang, X.; Xu, J.; Zhang, B.; Yu, H.; Wang, J.; Zhang, X.; Yu, J.;Li, Q. Adv. Mater. 2006, 18, 2476−2480.(24) Shi, Z.; Liu, H.; Lee, K.; Dy, E.; Chlistunoff, J.; Blair, M.;Zelenay, P.; Zhang, J.; Liu, Z. S. J. Phys. Chem. C 2011, 115, 16672−16680.(25) Dipojono, H. K.; Saputro, A. G.; Aspera, S. M.; Kasai, H. Jpn. J.Appl. Phys. 2011, 50, 055702.(26) Dipojono, H. K.; Saputro, A. G.; Belkada, R.; Nakanishi, H.;Kasai, H.; David, M.; Dy, E. S. J. Phys. Soc. Jpn. 2009, 78, 094710.(27) Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. J.Phys. Chem. B 2002, 106, 11970−11979.(28) Nakanishi, S.; Mukouyama, Y.; Karasumi, K.; Imanishi, A.;Furuya, N.; Nakato, Y. J. Phys. Chem. B 2000, 104, 4181−4188.(29) Shao, M. −H.; Liu, P.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128,7408−7409.(30) Anderson, A. B.; Albu, T. V. J. Electrochem. Soc. 2000, 147,4229−4238.(31) Wang, Y.; Balbuena, P. B. J. Chem. Theory Comput. 2005, 1,935−943.(32) Han, B. C.; Miranda, C. R.; Ceder, G. Phys. Rev. B 2008, 77,075410.(33) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004,126, 4717−4725.(34) For mono- and di-Co−PPy, at the step, B0−OH + H+ +e−(U)→ B0−OH2, the change of the free energy is negative, which meansthat the adsorbed OH could be easily transformed into H2O. The−OH does not occupy the active site, and does not block the O2adsorption. Then it also could not induce overpotential.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300638e | J. Phys. Chem. C 2012, 116, 12553−1255812558