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DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites
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DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites I.S. Flyagina*, K.J. Hughes, M. Pourkashanian, D.B. Ingham Energy Technology and Innovation Initiative, Faculty of Engineering, The University of Leeds, Leeds LS2 9JT, UK
article info
abstract
Article history:
The best performing non-precious metal based catalysts for polymer electrolyte membrane
Received 10 March 2014
fuel cells are manufactured by incorporation of nitrogen into a carbon structure in the
Accepted 15 September 2014
presence of iron and cobalt. Herein, density functional theory (DFT) calculations have been
Available online xxx
performed to investigate the oxygen reduction reaction on catalyst active sites modelled as transition metal macrocycles with iron, cobalt or manganese central atoms. The effects of
Keywords:
the transition metal and macrocycle structure have been investigated. The structure of the
Density functional theory
most promising active sites has been proposed, and the detailed potential energy profiles of
Transition metal macrocycles
the oxygen reduction reaction have been obtained over the active sites, including all in-
Oxygen reduction reaction
termediate steps with corresponding activation barriers. The efficiency of the active sites
Catalyst active sites
depends primarily on the transition metal nature, and the central iron atom accounts for the higher catalytic activity than cobalt and manganese. The central manganese atom can favour the two-electron oxygen reduction pathway and thus yielding hydrogen peroxide. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Over the recent years, polymer electrolyte membrane (PEM) fuel cells have been considered as one of the most promising technologies for stationary, automotive and portable power generation. The incontestable advantages of PEM fuel cells are high energy output and nearly zero emissions to the environment. However, the widespread use of PEM fuel cells is limited by some factors, in particular by the high and everincreasing cost of the platinum catalysts. Consequently, research and development of alternative non-precious metal based cathode catalysts for PEM fuel cells has been receiving more attention. The most prominent experimental results among these catalysts have been demonstrated by carbon based materials that contain transition metals such as iron and cobalt along with nitrogen [1,2]. However it still remains a
question of debate as to whether the transition metals play a direct role in the ORR catalysis. The problem to be addressed in the development of nonprecious metal based catalysts for PEM fuel cell cathodes is how to identify the structure of the catalytic active sites and determine the mechanism of the oxygen reduction reaction (ORR) on those sites. Since it is difficult to elucidate the structure of active sites experimentally, first principles calculations can be used to model the active sites as well as mechanistic mechanisms of the ORR. Such understanding of the catalytic ORR mechanism on the atomic level can aid in the development of effective cathode catalysts for PEM fuel cells. In recent years, there have been several theoretical studies of possible active sites in carbon based catalysts containing transition metals. In 2004, Anderson and Sidik [3] performed density functional theory (DFT) calculations of some ORR intermediate steps on the chelate complex with an iron atom
* Corresponding author. E-mail address: [email protected] (I.S. Flyagina). http://dx.doi.org/10.1016/j.ijhydene.2014.09.075 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Table 1 e The formulae for calculating the total energies, relative energy levels and elementary reaction energies of the ORR intermediate steps. Reaction/state Reactants Molecular oxygen binding First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Full energy formula E0 E1 E2 E3
¼ ¼ ¼ ¼
E E E E
catalystþEO2þ4·EH
Relative energy level, eV
Elementary reaction energy, eV
0 E1 e E0 E2 e E0 E3 e E0
0 E1 e E0 E2 e E1 E3 e E2
EH2O2desorp. ¼ E catalystþEH2O2 E4 ¼ EH2OO-catalystþ2·EH
EH2O2desorp. e E0 E4 e E0
EH2O2desorp. e E3 E4 e E2
E5 ¼ EH3O2-catalystþEH E6 ¼ E2H2O-catalyst E2H2Odesorp. ¼ EcatalystþE2H2O
E5 e E0 E6 e E0 E2H2Odesorp. e E0
E5 e E4 E6 e E5 E2H2Odesorp. e E6
O2-catalystþ4·EH OOH-catalystþ3·EH H2O2-catalystþ2·EH
coordinated to four nitrogen atoms representing an iron macrocycle. Recently, Calle-Vallejo et al. [4] obtained the ORR volcano plot for two different types of macrocyclic systems with a wide variety of transition metals in the centre. According to them, the most active sites with non precious metals have the structure of iron and manganese porphyrins. Chen et al. [5,6] have performed a DFT study of the ORR mechanism on the models of cobalt polypyrrole and iron and/ or cobalt polyaniline and obtained the potential energy profiles for the reaction. Molecular oxygen adsorption on iron and cobalt phthalocyannines has been studied by Wang et al. [7,8]. Later, He et al. [9] employed DFT calculations to predict the structures and binding energies of O2, OH and H2O2 on iron and cobalt porphyrin and phthalocyannine based molecular systems. A few other theoretical studies report estimations of reaction energies of some of the ORR intermediate steps on transition metal macrocycles [3,4,7,9]. On the experimental side, cathode catalysts synthesized using iron and cobalt salts exhibited the highest activity and stability amongst the other non-precious metal based catalysts tested in PEM fuel cells [2,10,11]. In addition, a considerable amount of literature has been published on the activity of manganese oxides towards the ORR in an alkali media [12e15] and manganese polypyrrole in microbial fuel cells [15,16]. Both the theoretical and experimental reports suggest four main types of transition metal macrocycles that show the ORR catalytic activity: (i) phthalocyannines [7,9,17e19], (ii) porphyrins [20,21], (iii) corrins [22] and (iv) tetraaza-annulenes [23]. However, to our knowledge, there has been no complete investigation on whether the ORR activity is dependent on the macrocycle structure. In addition, there is still uncertainty regarding the role and the nature of transition metal atoms in the active sites of efficient catalysts for the ORR. In this paper, the active sites of PEM fuel cell cathode catalysts have been modelled as macrocycles with iron, cobalt or manganese as the central atoms coordinated with four nitrogen atoms. The DFT study includes the following parts. First, the effect of the macrocycle structure has been investigated by modelling of the molecular oxygen binding on the four types of macrocycles with either Fe, Co or Mn in the centre. Second, the effect of the nature of the central transition metal atom has been investigated in a selected macrocycle structure. The ORR thermodynamic pathways have been obtained as the potential energy profiles, including all the
intermediate steps of the oxygen reaction. Finally, a modified model of the transition metal active sites has been used to obtain the ORR pathways, including the activation barriers of all the elementary intermediate steps.
Theoretical method All calculations have been performed in Gaussian09 [24] using the DFT [25] model chemistry with the B3LYP hybrid functional and the 6-31G(d) split valence double zeta polarised basis set. The effect of the aqueous medium existing at the cathode in PEM fuel cells has been represented by the Polarized Continuum Model (PCM) of the Self-Consistent ReactionField (SCRF) method in which water has been chosen as a model solvent. The structures have been optimised to the local minimum on the potential energy surface. The technique of “relaxed” Potential Energy Surface (PES) scan calculation has been employed in order to evaluate the activation barriers of the elementary ORR steps. The structures with the highest electronic energy have been taken as the transition states (TS). The binding energy of molecular oxygen to the macrocycles has been calculated using the formula: DE ¼ EProduct ECatalyst þ EO2
where DE is the binding energy, EProduct is the energy of the molecular oxygen binding product, ECatalyst is the energy of the model system representing a catalyst's macrocycle-based active sites, and EO2 is the energy of the oxygen molecule in the triplet spin state. The sum of ECatalyst and EO2 is referred to as the reactants energy. The geometries of the catalyst model systems have been optimised with the lowest spin multiplicity, namely M ¼ 1 or 2 depending on the electronic structure. The spin multiplicities of the products of molecular oxygen binding have been allowed to relax, i.e. their structures have been optimized with two spin multiplicities: (i) the lowest, M ¼ 1 or 2, and (ii) the next higher, M ¼ 3 or 4, respectively. The products with the lower electronic energies have been taken as the ground states and used for calculating the binding energies. Since the cathode in PEM fuel cells is a reservoir of electrons, the effect of a negative charge has been accounted for
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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by assigning a (1) charge to the catalyst model systems. The electron affinities have been calculated as follows: EA ¼ ENeutral ECharged where EA is the electron affinity, ENeutral is the energy of the neutral catalyst model system, and E(1)Charged is the energy of the catalyst model system bearing a (1) charge. The addition of the four H atoms in the course of the ORR has been modelled by the consequent introduction of the H atoms to the product of molecular oxygen binding over a model system. The product of the first H atom addition is the OOH chemisorbed on the model system's surface. The second H atom addition can result in the formation of either hydrogen peroxide H2O2, or water oxide H2OeO, or di-hydroxyl OHeOH chemisorbed on the model system. The hydrogen peroxide formation terminates the two-electron ORR pathway and is not desirable since hydrogen peroxide in PEM fuel cells causes destruction of both the catalyst layer and the electrolyte membrane. The formation of the water oxide H2OeO and the di-hydroxyl OHeOH allows for the four-electron ORR pathway. The third H addition results in H3O2 formation. The fourth H addition terminates the four-electron ORR pathway and yields
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two H2O molecules. Thus, investigation of the ORR intermediate steps includes obtaining the optimised structures of model systems bearing the O2, OOH, H2O2, H2OeO, OHeOH, H3O2 and H2O species on the surface. In addition, the O2, H, H2O2, and two H2O molecules have been optimised separately. Table 1 lists the ORR intermediate steps and formulae used to calculate their total energies, relative energy levels and elementary reaction energies. The total energies of each intermediate step were calculated based on the principle of mass conservation: the total sum of nuclei masses remains constant and includes the two O atoms of the oxygen molecule, the four H atoms and the atoms constituting the catalyst model system. To obtain the relative energy levels of the ORR intermediate steps, the reactants total energy has been set as a reference, and its total energy was subtracted from each of the following energies of the ORR intermediate steps. The elementary reaction energies were calculated as the difference between two neighbouring relative energy levels. Relative energy levels of the TS and the corresponding activation energies of the ORR elementary steps have also been calculated similarly, with the energies of products being replaced by the TS energies.
Fig. 1 e Model systems representing the four types of macrocycles. The C, N, H and the central M atoms are designated by the yellow, purple, blue and pink colours, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 1 illustrates the optimised geometries of the four types of the macrocycle model systems representing the active sites with Fe, Co or Mn in the centre: phthalocyannine, porphyrin, corrin and tetraaza-annulene. The optimised structure of the tetraaza-annulene system is almost planar whereas the geometry of the other three macrocycle systems has a semispherical shape. Oxygen molecules can attack the catalyst surface at various angles. In order to account for this, two basic configurations of the oxygen molecule on the model system have been modelled: (i) side-on configuration in which the oxygen molecule is parallel to the model system's plane with the molecule's centre of mass being above the metal atom, and (ii) end-on configuration in which the oxygen molecule is perpendicular to the model system's plane and aligned with the metal atom. Since the geometries of the phthalocyannine, porphyrin and corrin model systems are semi-spherical, the molecular oxygen binding has been investigated on both the convex and concave sides. Fig. 2 shows the molecular oxygen binding energy levels for the four types of Fe macrocycles. The central parts of the optimised structures with the interatomic distances are shown adjacent to the corresponding energy levels. It can be observed that the molecular oxygen binds to the Fe centre both in the side-on and end-on configurations and the calculated binding energies for all the macrocycles vary in the range of 1.3 to 2.3 eV. Further, the calculated binding energies for each of the macrocycle types vary significantly depending on the local geometry of the reaction centre, i.e. the FeeO, FeeN and OeO interatomic distances. The differences in the optimised geometries and calculated binding energies can be due to the different initial input geometries and spin multiplicities.
Fig. 3 illustrates the molecular oxygen binding energies on the four types of Co macrocycles. Co atom has one more electron on its external electron shell than Fe atom, and the lowest spin multiplicity of the Co macrocycle model systems equals two (M ¼ 2). Assigning an additional electron to the Co macrocycles results in M ¼ 1. To model these two electron states, the molecular oxygen binding on the Co macrocycles has been modelled both on the neutral and negative single charged systems. It can be observed that the oxygen molecule binds to the Co atom predominantly in the end-on configuration, and the interatomic distances do not differ significantly for the different types of macrocycles. The binding energies for the neutral systems vary from 0.1 to 0.8 eV, and for the negative single charged systems from 0.9 to 1.6 eV, i.e. the binding energies of the negative single charged systems are significantly more exothermic than the neutral ones. In addition, the calculated electron affinities of the four Co macrocycles are significant, namely between 2.12 and 3.26 eV. This indicates that the molecular oxygen binds to the negative single charged Co macrocycles, rather than to the neutral ones. Fig. 4 illustrates the molecular oxygen binding on the four types of Mn macrocycles. Mn has one electron less than Fe, and, similarly to the Co macrocycle model systems. The lowest multiplicity of the Mn macrocycles has a multiplicity of M ¼ 2, and assigning a single negative charge results in M ¼ 1. Molecular oxygen binds to the Mn centre predominantly in the side-on configuration. For all the four types of macrocycles, the binding energy of the negative single charged systems is significantly more exothermic than that of the neutral systems, with the binding energies of the neutral systems being from 0.7 to 2.5 eV, and those of the negative single charged systems being from 2.4 to 3.4 eV. The calculated electron affinities of the Mn macrocycles are significant, varying in the range from 1.69 to 2.69 eV. As well as in the case of Co, the negatively charged Mn macrocycles bind the molecular oxygen more strongly than the neutral ones.
Fig. 2 e Molecular oxygen binding to the four Fe macrocycles. The C, N, O and Fe atoms are designated by the yellow, purple, red and pink colours, respectively, and ˚ . (For the interatomic distances are shown in A interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3 e Molecular oxygen binding on the four neutral and negative single charged Co macrocycles. The C, N, O and Co atoms are designated by the yellow, purple, red and blue colours, respectively, and the interatomic distances are ˚ . (For interpretation of the references to colour shown in A in this figure legend, the reader is referred to the web version of this article.)
Results and discussion The effect of macrocycle structure
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 4 e Molecular oxygen binding on the four neutral and negative single charged Mn macrocycles. The C, N, O and Mn atoms are designated by the yellow, purple, red and pink colours, respectively, and the interatomic distances ˚ . (For interpretation of the references to are shown in A colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 summarizes the calculated molecular oxygen binding energies illustrated in Figs. 2e4. The dependence of the strength of molecular oxygen binding has been evaluated with respect to the transition metal centre and the type of macrocycle structure. It can be observed that for all the macrocycle structures, the strength of molecular oxygen binding is in the following order: Mn > Fe > Co, i.e Mn centre accounts for the strongest binding of molecular oxygen. The binding strength depending on the macrocycle structures is in the following order: phthalocyannines > tetraazaannulenes > corrins > porphyrins. This correlation has been drawn based on a comparison of the binding energies for each of the four macrocycles over the transition metals both in the neutral and negative single charged state. Thus, it can be concluded that the strength of molecular oxygen binding is determined not only by the nature of the central transition metal atom, but also by the macrocycle structure, i.e. the local geometry of the macrocycle-based active site. This conclusion is in agreement with that derived by He et al. [9].
The ORR thermodynamics on the Fe, Co and Mn tetraazaannulene model systems As mentioned in the previous section, the optimised structures of the transition metal phthalocyannines, corrins and
Table 2 e Molecular oxygen binding energies on the four types of the Fe, Co and Mn macrocycles (neutral and negatively charged). Macrocycle Phthalocyannine Porphyrin Corrin Tetraaza-annulene
Fe
Co0
Co1
Mn0
Mn1
2.21 1.44 1.50 2.22
0.78 0.20 0.18 0.10
1.59 0.96 1.01 1.06
2.52 0.68 1.39 2.33
3.45 2.35 2.76 2.68
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porphyrins are semispherical, and this may cause differences in the calculated energies of the ORR steps on the convex and concave sides. The planar structure of the tetraaza-annulene system implies that both sides are equal with respect to the reaction modelling. Therefore, the tetraaza-annulene model system has been chosen to represent a base case of the catalyst active sites and model the entire ORR mechanism. The high exothermic values of the calculated molecular oxygen binding energies reported in the previous section indicate that the metal centres in all the macrocycle systems bind the molecular oxygen stronger than necessary for this catalytic reaction and thus can potentially impede it. Therefore, in order to decrease the capacity of binding the reaction species on the transition metal macrocycle active sites, it is suggested that the metal centres should be terminated by a functional group at one side, e.g. hydroxyl (OH) group. Thus, modelling of the ORR mechanism has been performed using the Fe, Co or Mn tetraaza-annulene model systems with the metal centre terminated by the OH group at one side. Fig. 5 illustrates a schematic representation of the model system representing the transition metal macrocycle active site and the molecular oxygen binding as a first step of the ORR. The potential energy profiles of the ORR on the OHterminated Fe, Co and Mn tetraaza-annulene model systems are shown in Figs. S1 e S3 of the Supporting Information. Table 3 summarizes the calculated reaction energies of the ORR elementary steps, referenced to the reactants energy. It can be observed that the calculated molecular oxygen binding energy has almost double the exothermicity on the Mn centre than on the Fe and Co centres. This indicates that the Mn centre binds molecular oxygen too strongly and may impede the kinetics of the following steps. The first H addition leads to formation of the chemisorbed OOH group and is found to be the least exothermic in the case of the Fe centre. Introduction of the second H atom to each system in the H2O2 mode (twoelectron pathway) results in formation of the H2O2 molecule in all three cases. The second H addition in the H2OeO mode (four-electron pathway) leads to the formation of the first water molecule and the oxidized metal centre in the case of Fe and Mn central atoms, and two hydroxyl groups in the case of the Co central atom (Figs. S1 e S2). The reactions of the second H addition in both the two- and four-electron pathways and the H2O2 desorption are the most exothermic for the Fe centre. This suggests that both the hydrogen peroxide and the first water molecule are thermodynamically more favourable to form on the Fe centre than on the Co and Mn centres. The third H addition is the most exothermic, while the fourth H addition is the least exothermic for the Mn centre. This indicates that the Mn centre terminated by the second OH group is very stable thermodynamically, and the OH group is more difficult to reduce to water on the Mn centre compared to the Fe or Co centres. The calculated thermodynamics of the ORR steps on the OHterminated Fe, Co and Mn tetraaza-annulene model systems enables us to conclude the following: (i) The ORR thermodynamics strongly depends on the nature of the central transition metal atom. (ii) Both the two- and four-electron ORR pathways are thermodynamically possible on all the three metal centres of the macrocycle model system. (iii) The four-electron ORR pathway is more exothermic in all cases and thus more
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 5 e Schematic of the molecular oxygen binding on the active site modelled by the OH-terminated Fe tetraaza-annulene system.
Table 3 e The calculated reaction energies of the ORR elementary steps on the OH-terminated Fe, Co and Mn tetraaza-annulene model systems. Elementary reaction/state
Reactants Molecular oxygen binding First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Energy of the ORR elementary steps, eV Fe
Co
Mn
0 0.64 2.53 2.59
0 0.73 3.08 2.40
0 1.34 3.50 2.32
0.18 4.90
0.28 3.41
0.22 3.34
3.14 2.80 0.15
4.35 2.95 0.38
4.78 1.46 0.26
thermodynamically favourable regardless of the nature of the transition metal centre. (iv) The high chemisorption energies of the oxygen molecule and the OH group on the Mn centre suggest that it is likely to be blocked by a second OH group.
The ORR energy profiles on the Fe, Co and Mn 4Ncoordinated model systems In order to evaluate the kinetic aspects of the ORR on Fe, Co and Mn macrocycle active sites, it is necessary to obtain the ORR potential energy profiles with activation barriers for each elementary reaction. Calculating the activation barriers on the large tetraaza-annulene model systems is computationally expensive. Therefore, in order to attain a reasonable computational time, the model system size has been reduced. Fig. 6
illustrates the OH-terminated transition metal tetraazaannulene model system, employed in the previous section, and the small-size system representing the transition metal macrocycle active sites. The small-size model system is further referred to as the M4N, with the M being either Fe, Co or Mn. In the subsequent calculations, the M4N model systems have been partially optimised, i.e. all the CeN and CeC bond lengths have been set constant and equal to those in the large model system. First, the energies of the ORR elementary steps on the M4N systems have been obtained in a similar way as on the OH-terminated transition metal tetraaza-annulene systems. The calculated elementary reaction energies of the ORR intermediate steps both for the M4N (small) and the tetraazaannulene (large) systems are listed in Table 4. Although some of the elementary reaction energies for the small and large model systems have differences in value, the proposed M4N model systems have been employed to study activation barriers of the ORR elementary steps. As described in the Theoretical Method section, the activation barriers of the elementary reactions have been obtained using the technique of the “relaxed” PES scan calculations, with the highest energy point being taken as the transition state. The detailed scan calculations for each elementary step of the ORR on all the M4N systems are illustrated in Figs. S4eS6 of the Supporting Information. Fig. 7 illustrates the ORR pathways obtained on the Fee4N model system, showing the relative energy levels of the ORR intermediate steps and the corresponding transition states. The optimised structures of the ORR intermediate products are shown adjacent to the corresponding energy levels. The dashed lines designate that the scan calculations did not reveal activation barriers and therefore the reactions are considered to be barrierless. It has been found that the ORR on
Fig. 6 e Model systems representing the transition metal macrocycle active sites: the large tetraaza-annulene and the small M¡4N model systems. Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Table 4 e The calculated reaction energies of the ORR elementary steps on the Fee, Coe and Mne4N model systems compared to the OH-terminated tetraaza-annulene (large) model systems. Elementary reaction/state
Reactants Molecular oxygen chemisorption First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Energy of the ORR elementary steps, eV Fee4N
Fe large
Coe4N
Co large
Mne4N
Mn large
0 0.35 2.48 3.25 0.15 4.95 3.04 3.49 0.11
0 0.64 2.53 2.59 0.18 4.90 3.14 2.80 0.15
0 0.42 3.07 2.59 0.16 2.98 4.82 3.11 0.20
0 0.73 3.08 2.40 0.28 3.41 4.35 2.95 0.38
0 1.36 3.17 1.68 0.28 3.34 4.81 1.57 0.05
0 1.34 3.50 1.32 0.22 3.34 4.78 1.46 0.26
the Fee4N model system branches into two four-electron pathways, referred to as the H2OO and OHeOH pathways, with all the intermediate steps being barrierless except the third H addition. It should be noted that the two-electron pathway via hydrogen peroxide has not been found. The calculated reaction and activation energies of the ORR elementary steps on the Fee4N model system are illustrated in Table 5. Although the second H addition steps in the two pathways have a difference in exothermicity of 0.45 eV, virtually this is unimportant since the third H addition leads to the same product in the both cases. As observed in Fig. 7, the non-zero activation barriers have been obtained for the third H addition only, and these values are rather small: 0.01 eV in the H2OO pathway, and 0.16 eV in the OHeOH pathway. Given the lower activation energy in the H2OO pathway, it can be concluded that it may have a faster kinetics than the OHeOH pathway. Overall, the non-activated potential energy profiles of the ORR indicate that the active sites modelled by the Fee4N system may be potentially very effective in catalysing the ORR predominantly in the four-electron pathway via the H2OO formation. Fig. 8 illustrates the ORR on the Coe4N model system branching into two four-electron pathways, namely, via the H2OO and OHeOH pathways. Table 6 lists the elementary reaction energies and corresponding activation energies of the two ORR pathways on the Coe4N system. It is observed in Fig. 8 and Table 6 that the molecular oxygen binding on the Coe4N system does not have an activation barrier, similar to
the Fee4N system. However, the first H addition ORR step on the Coe4N system has an activation energy of 0.07 eV. The ORR splits into the two pathways at the second H addition step. As in the case of the Fee4N system, the two-electron pathway via hydrogen peroxide has not been found. Although the intermediate products of the second H addition on the Coe4N model system in the two pathways are structurally similar to those of the Fee4N system, the OHeOH pathway is 1.12 eV more exothermic than the H2OO pathway. The two pathways merge at the step of the third H addition, which is found to have no activation energy in the H2OO pathway, but has significant activation energy of 1.11 eV in the OHeOH pathway. This implies that the reaction may stop at this point, or at least its rate be considerably slowed. Therefore, the ORR on the Coe4N model system can take a fourelectron pathway via the H2OO formation only. The rate limiting step in this case is the second H addition, since it has the highest calculated activation energy of 0.14 eV. The results of modelling the ORR on the Mne4N system are shown in Fig. 9, and the elementary reaction energies and corresponding activation energies are summarized in Table 7. Fig. 9 illustrates that the reaction branches into the twoelectron pathway, terminating with the hydrogen peroxide formation, and the four-electron pathway. The second H addition step in the four-electron pathway is markedly more
Table 5 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Fee4N model system. Reaction/state
Fig. 7 e The ORR pathways on the Fee4N model system.
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Reaction/activation energy, eV H2OO pathway
OHeOH pathway
0 none 0.35 none 2.48 none 4.95 0.01 3.04 none 3.49 0.11
0 none 0.35 none 2.48 none 5.40 0.16 2.58 none 3.49 0.11
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 8 e The ORR pathways on the Coe4N model system.
exothermic than that of the two-electron pathway. Further, the two-electron pathway is found not to have activation barriers, whereas the second H addition in the four-electron pathway is the rate limiting step with the calculated activation energy of 0.14 eV. Thus, both the ORR pathways on the Mne4N model system are thermodynamically possible, but the kinetics of the four-electron pathway should be somewhat slower than that of the two-electron pathway due to the nonzero activation energy of the rate limiting step. This suggests that during the ORR, the Mne4N active sites can potentially yield hydrogen peroxide.
Conclusion In this paper, the oxygen reduction reaction has been studied on various model systems representing transition metal macrocycle active sites in order to identify potentially the most efficient ones. The effect of macrocycle structure has been studied by modelling molecular oxygen binding on phthalocyannine, porphyrin, corrin and tetraazaannulene
Table 6 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Coe4N model system. Reaction/state
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Fig. 9 e The ORR pathways on the Mne4N model system.
model systems either with iron, cobalt or manganese atoms in the centre. The central transition metal atom is the reaction centre, with the molecular oxygen binding strength depending primarily on the metal nature. It has been found that the binding strength increases in the order of cobalt, iron, and manganese. The local structure of macrocycles can also influence molecular oxygen binding and hence the efficiency of the active sites. The highly exothermic calculated values of molecular oxygen binding energies suggest the metal centre in macrocycle active sites should be terminated by a functional group, e.g. hydroxyl group, on one side, in order for the oxygen reduction to occur on the other free side. The effect of the central transition metal atom has been studied with respect to the thermodynamics and kinetics of the oxygen reduction reaction. Modelling of the ORR energy profiles on the iron, cobalt and manganese macrocyle model systems indicates that the iron macrocycle active sites demonstrate the highest catalytic activity towards the ORR, since the reaction proceeds through the four-electron pathway with a negligible calculated activation energy of the rate limiting step (0.01 eV). The cobalt and manganese
Table 7 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Mne4N model system. Reaction/state
Reaction/activation energy, eV H2OO pathway
OHeOH pathway
0 none 0.42 0.07 3.07 0.14 2.98 none 4.82 none 3.11 0.20
0 none 0.42 0.07 3.07 0.06 4.10 1.11 4.82 none 3.11 0.20
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition H2O2 desorption Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Reaction/activation energy, eV Two-electron pathway
Four-electron pathway
0 none 1.36 none 3.17 none 1.68 0.28
0 none 1.36 none 3.17 0.14 3.34 e none 4.81 none 1.57 0.05
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macrocycle active sites are predicted to have similar reaction rates in the four-electron ORR pathway, since the calculated activation energies of the rate limiting steps are equal (0.14 eV). However, the ORR on the cobalt macrocycle active sites can proceed via a pathway that may trap the catalyst in a kinetically stable intermediate at typical PEM fuel cell operating temperatures, or at least substantially reduce the overall rate of the four-electron pathway. The manganese macrocycle system provides an accessible route to the two-electron ORR pathway producing undesirable hydrogen peroxide. Therefore, non-precious catalysts with high concentration of iron macrocycle active sites with the proposed structure are predicted to be the most promising for PEM fuel cells.
[10]
[11]
[12]
[13]
Acknowledgements [14]
The authors would like to thank the Energy Technology and Innovation Initiative of the University of Leeds and the NetScientific Plc. for the financial support.
[15]
Appendix A. Supplementary data [16]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.09.075. [17]
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DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites I.S. Flyagina*, K.J. Hughes, M. Pourkashanian, D.B. Ingham Energy Technology and Innovation Initiative, Faculty of Engineering, The University of Leeds, Leeds LS2 9JT, UK
article info
abstract
Article history:
The best performing non-precious metal based catalysts for polymer electrolyte membrane
Received 10 March 2014
fuel cells are manufactured by incorporation of nitrogen into a carbon structure in the
Accepted 15 September 2014
presence of iron and cobalt. Herein, density functional theory (DFT) calculations have been
Available online xxx
performed to investigate the oxygen reduction reaction on catalyst active sites modelled as transition metal macrocycles with iron, cobalt or manganese central atoms. The effects of
Keywords:
the transition metal and macrocycle structure have been investigated. The structure of the
Density functional theory
most promising active sites has been proposed, and the detailed potential energy profiles of
Transition metal macrocycles
the oxygen reduction reaction have been obtained over the active sites, including all in-
Oxygen reduction reaction
termediate steps with corresponding activation barriers. The efficiency of the active sites
Catalyst active sites
depends primarily on the transition metal nature, and the central iron atom accounts for the higher catalytic activity than cobalt and manganese. The central manganese atom can favour the two-electron oxygen reduction pathway and thus yielding hydrogen peroxide. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Over the recent years, polymer electrolyte membrane (PEM) fuel cells have been considered as one of the most promising technologies for stationary, automotive and portable power generation. The incontestable advantages of PEM fuel cells are high energy output and nearly zero emissions to the environment. However, the widespread use of PEM fuel cells is limited by some factors, in particular by the high and everincreasing cost of the platinum catalysts. Consequently, research and development of alternative non-precious metal based cathode catalysts for PEM fuel cells has been receiving more attention. The most prominent experimental results among these catalysts have been demonstrated by carbon based materials that contain transition metals such as iron and cobalt along with nitrogen [1,2]. However it still remains a
question of debate as to whether the transition metals play a direct role in the ORR catalysis. The problem to be addressed in the development of nonprecious metal based catalysts for PEM fuel cell cathodes is how to identify the structure of the catalytic active sites and determine the mechanism of the oxygen reduction reaction (ORR) on those sites. Since it is difficult to elucidate the structure of active sites experimentally, first principles calculations can be used to model the active sites as well as mechanistic mechanisms of the ORR. Such understanding of the catalytic ORR mechanism on the atomic level can aid in the development of effective cathode catalysts for PEM fuel cells. In recent years, there have been several theoretical studies of possible active sites in carbon based catalysts containing transition metals. In 2004, Anderson and Sidik [3] performed density functional theory (DFT) calculations of some ORR intermediate steps on the chelate complex with an iron atom
* Corresponding author. E-mail address: [email protected] (I.S. Flyagina). http://dx.doi.org/10.1016/j.ijhydene.2014.09.075 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Table 1 e The formulae for calculating the total energies, relative energy levels and elementary reaction energies of the ORR intermediate steps. Reaction/state Reactants Molecular oxygen binding First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Full energy formula E0 E1 E2 E3
¼ ¼ ¼ ¼
E E E E
catalystþEO2þ4·EH
Relative energy level, eV
Elementary reaction energy, eV
0 E1 e E0 E2 e E0 E3 e E0
0 E1 e E0 E2 e E1 E3 e E2
EH2O2desorp. ¼ E catalystþEH2O2 E4 ¼ EH2OO-catalystþ2·EH
EH2O2desorp. e E0 E4 e E0
EH2O2desorp. e E3 E4 e E2
E5 ¼ EH3O2-catalystþEH E6 ¼ E2H2O-catalyst E2H2Odesorp. ¼ EcatalystþE2H2O
E5 e E0 E6 e E0 E2H2Odesorp. e E0
E5 e E4 E6 e E5 E2H2Odesorp. e E6
O2-catalystþ4·EH OOH-catalystþ3·EH H2O2-catalystþ2·EH
coordinated to four nitrogen atoms representing an iron macrocycle. Recently, Calle-Vallejo et al. [4] obtained the ORR volcano plot for two different types of macrocyclic systems with a wide variety of transition metals in the centre. According to them, the most active sites with non precious metals have the structure of iron and manganese porphyrins. Chen et al. [5,6] have performed a DFT study of the ORR mechanism on the models of cobalt polypyrrole and iron and/ or cobalt polyaniline and obtained the potential energy profiles for the reaction. Molecular oxygen adsorption on iron and cobalt phthalocyannines has been studied by Wang et al. [7,8]. Later, He et al. [9] employed DFT calculations to predict the structures and binding energies of O2, OH and H2O2 on iron and cobalt porphyrin and phthalocyannine based molecular systems. A few other theoretical studies report estimations of reaction energies of some of the ORR intermediate steps on transition metal macrocycles [3,4,7,9]. On the experimental side, cathode catalysts synthesized using iron and cobalt salts exhibited the highest activity and stability amongst the other non-precious metal based catalysts tested in PEM fuel cells [2,10,11]. In addition, a considerable amount of literature has been published on the activity of manganese oxides towards the ORR in an alkali media [12e15] and manganese polypyrrole in microbial fuel cells [15,16]. Both the theoretical and experimental reports suggest four main types of transition metal macrocycles that show the ORR catalytic activity: (i) phthalocyannines [7,9,17e19], (ii) porphyrins [20,21], (iii) corrins [22] and (iv) tetraaza-annulenes [23]. However, to our knowledge, there has been no complete investigation on whether the ORR activity is dependent on the macrocycle structure. In addition, there is still uncertainty regarding the role and the nature of transition metal atoms in the active sites of efficient catalysts for the ORR. In this paper, the active sites of PEM fuel cell cathode catalysts have been modelled as macrocycles with iron, cobalt or manganese as the central atoms coordinated with four nitrogen atoms. The DFT study includes the following parts. First, the effect of the macrocycle structure has been investigated by modelling of the molecular oxygen binding on the four types of macrocycles with either Fe, Co or Mn in the centre. Second, the effect of the nature of the central transition metal atom has been investigated in a selected macrocycle structure. The ORR thermodynamic pathways have been obtained as the potential energy profiles, including all the
intermediate steps of the oxygen reaction. Finally, a modified model of the transition metal active sites has been used to obtain the ORR pathways, including the activation barriers of all the elementary intermediate steps.
Theoretical method All calculations have been performed in Gaussian09 [24] using the DFT [25] model chemistry with the B3LYP hybrid functional and the 6-31G(d) split valence double zeta polarised basis set. The effect of the aqueous medium existing at the cathode in PEM fuel cells has been represented by the Polarized Continuum Model (PCM) of the Self-Consistent ReactionField (SCRF) method in which water has been chosen as a model solvent. The structures have been optimised to the local minimum on the potential energy surface. The technique of “relaxed” Potential Energy Surface (PES) scan calculation has been employed in order to evaluate the activation barriers of the elementary ORR steps. The structures with the highest electronic energy have been taken as the transition states (TS). The binding energy of molecular oxygen to the macrocycles has been calculated using the formula: DE ¼ EProduct ECatalyst þ EO2
where DE is the binding energy, EProduct is the energy of the molecular oxygen binding product, ECatalyst is the energy of the model system representing a catalyst's macrocycle-based active sites, and EO2 is the energy of the oxygen molecule in the triplet spin state. The sum of ECatalyst and EO2 is referred to as the reactants energy. The geometries of the catalyst model systems have been optimised with the lowest spin multiplicity, namely M ¼ 1 or 2 depending on the electronic structure. The spin multiplicities of the products of molecular oxygen binding have been allowed to relax, i.e. their structures have been optimized with two spin multiplicities: (i) the lowest, M ¼ 1 or 2, and (ii) the next higher, M ¼ 3 or 4, respectively. The products with the lower electronic energies have been taken as the ground states and used for calculating the binding energies. Since the cathode in PEM fuel cells is a reservoir of electrons, the effect of a negative charge has been accounted for
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by assigning a (1) charge to the catalyst model systems. The electron affinities have been calculated as follows: EA ¼ ENeutral ECharged where EA is the electron affinity, ENeutral is the energy of the neutral catalyst model system, and E(1)Charged is the energy of the catalyst model system bearing a (1) charge. The addition of the four H atoms in the course of the ORR has been modelled by the consequent introduction of the H atoms to the product of molecular oxygen binding over a model system. The product of the first H atom addition is the OOH chemisorbed on the model system's surface. The second H atom addition can result in the formation of either hydrogen peroxide H2O2, or water oxide H2OeO, or di-hydroxyl OHeOH chemisorbed on the model system. The hydrogen peroxide formation terminates the two-electron ORR pathway and is not desirable since hydrogen peroxide in PEM fuel cells causes destruction of both the catalyst layer and the electrolyte membrane. The formation of the water oxide H2OeO and the di-hydroxyl OHeOH allows for the four-electron ORR pathway. The third H addition results in H3O2 formation. The fourth H addition terminates the four-electron ORR pathway and yields
3
two H2O molecules. Thus, investigation of the ORR intermediate steps includes obtaining the optimised structures of model systems bearing the O2, OOH, H2O2, H2OeO, OHeOH, H3O2 and H2O species on the surface. In addition, the O2, H, H2O2, and two H2O molecules have been optimised separately. Table 1 lists the ORR intermediate steps and formulae used to calculate their total energies, relative energy levels and elementary reaction energies. The total energies of each intermediate step were calculated based on the principle of mass conservation: the total sum of nuclei masses remains constant and includes the two O atoms of the oxygen molecule, the four H atoms and the atoms constituting the catalyst model system. To obtain the relative energy levels of the ORR intermediate steps, the reactants total energy has been set as a reference, and its total energy was subtracted from each of the following energies of the ORR intermediate steps. The elementary reaction energies were calculated as the difference between two neighbouring relative energy levels. Relative energy levels of the TS and the corresponding activation energies of the ORR elementary steps have also been calculated similarly, with the energies of products being replaced by the TS energies.
Fig. 1 e Model systems representing the four types of macrocycles. The C, N, H and the central M atoms are designated by the yellow, purple, blue and pink colours, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 1 illustrates the optimised geometries of the four types of the macrocycle model systems representing the active sites with Fe, Co or Mn in the centre: phthalocyannine, porphyrin, corrin and tetraaza-annulene. The optimised structure of the tetraaza-annulene system is almost planar whereas the geometry of the other three macrocycle systems has a semispherical shape. Oxygen molecules can attack the catalyst surface at various angles. In order to account for this, two basic configurations of the oxygen molecule on the model system have been modelled: (i) side-on configuration in which the oxygen molecule is parallel to the model system's plane with the molecule's centre of mass being above the metal atom, and (ii) end-on configuration in which the oxygen molecule is perpendicular to the model system's plane and aligned with the metal atom. Since the geometries of the phthalocyannine, porphyrin and corrin model systems are semi-spherical, the molecular oxygen binding has been investigated on both the convex and concave sides. Fig. 2 shows the molecular oxygen binding energy levels for the four types of Fe macrocycles. The central parts of the optimised structures with the interatomic distances are shown adjacent to the corresponding energy levels. It can be observed that the molecular oxygen binds to the Fe centre both in the side-on and end-on configurations and the calculated binding energies for all the macrocycles vary in the range of 1.3 to 2.3 eV. Further, the calculated binding energies for each of the macrocycle types vary significantly depending on the local geometry of the reaction centre, i.e. the FeeO, FeeN and OeO interatomic distances. The differences in the optimised geometries and calculated binding energies can be due to the different initial input geometries and spin multiplicities.
Fig. 3 illustrates the molecular oxygen binding energies on the four types of Co macrocycles. Co atom has one more electron on its external electron shell than Fe atom, and the lowest spin multiplicity of the Co macrocycle model systems equals two (M ¼ 2). Assigning an additional electron to the Co macrocycles results in M ¼ 1. To model these two electron states, the molecular oxygen binding on the Co macrocycles has been modelled both on the neutral and negative single charged systems. It can be observed that the oxygen molecule binds to the Co atom predominantly in the end-on configuration, and the interatomic distances do not differ significantly for the different types of macrocycles. The binding energies for the neutral systems vary from 0.1 to 0.8 eV, and for the negative single charged systems from 0.9 to 1.6 eV, i.e. the binding energies of the negative single charged systems are significantly more exothermic than the neutral ones. In addition, the calculated electron affinities of the four Co macrocycles are significant, namely between 2.12 and 3.26 eV. This indicates that the molecular oxygen binds to the negative single charged Co macrocycles, rather than to the neutral ones. Fig. 4 illustrates the molecular oxygen binding on the four types of Mn macrocycles. Mn has one electron less than Fe, and, similarly to the Co macrocycle model systems. The lowest multiplicity of the Mn macrocycles has a multiplicity of M ¼ 2, and assigning a single negative charge results in M ¼ 1. Molecular oxygen binds to the Mn centre predominantly in the side-on configuration. For all the four types of macrocycles, the binding energy of the negative single charged systems is significantly more exothermic than that of the neutral systems, with the binding energies of the neutral systems being from 0.7 to 2.5 eV, and those of the negative single charged systems being from 2.4 to 3.4 eV. The calculated electron affinities of the Mn macrocycles are significant, varying in the range from 1.69 to 2.69 eV. As well as in the case of Co, the negatively charged Mn macrocycles bind the molecular oxygen more strongly than the neutral ones.
Fig. 2 e Molecular oxygen binding to the four Fe macrocycles. The C, N, O and Fe atoms are designated by the yellow, purple, red and pink colours, respectively, and ˚ . (For the interatomic distances are shown in A interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3 e Molecular oxygen binding on the four neutral and negative single charged Co macrocycles. The C, N, O and Co atoms are designated by the yellow, purple, red and blue colours, respectively, and the interatomic distances are ˚ . (For interpretation of the references to colour shown in A in this figure legend, the reader is referred to the web version of this article.)
Results and discussion The effect of macrocycle structure
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Fig. 4 e Molecular oxygen binding on the four neutral and negative single charged Mn macrocycles. The C, N, O and Mn atoms are designated by the yellow, purple, red and pink colours, respectively, and the interatomic distances ˚ . (For interpretation of the references to are shown in A colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 summarizes the calculated molecular oxygen binding energies illustrated in Figs. 2e4. The dependence of the strength of molecular oxygen binding has been evaluated with respect to the transition metal centre and the type of macrocycle structure. It can be observed that for all the macrocycle structures, the strength of molecular oxygen binding is in the following order: Mn > Fe > Co, i.e Mn centre accounts for the strongest binding of molecular oxygen. The binding strength depending on the macrocycle structures is in the following order: phthalocyannines > tetraazaannulenes > corrins > porphyrins. This correlation has been drawn based on a comparison of the binding energies for each of the four macrocycles over the transition metals both in the neutral and negative single charged state. Thus, it can be concluded that the strength of molecular oxygen binding is determined not only by the nature of the central transition metal atom, but also by the macrocycle structure, i.e. the local geometry of the macrocycle-based active site. This conclusion is in agreement with that derived by He et al. [9].
The ORR thermodynamics on the Fe, Co and Mn tetraazaannulene model systems As mentioned in the previous section, the optimised structures of the transition metal phthalocyannines, corrins and
Table 2 e Molecular oxygen binding energies on the four types of the Fe, Co and Mn macrocycles (neutral and negatively charged). Macrocycle Phthalocyannine Porphyrin Corrin Tetraaza-annulene
Fe
Co0
Co1
Mn0
Mn1
2.21 1.44 1.50 2.22
0.78 0.20 0.18 0.10
1.59 0.96 1.01 1.06
2.52 0.68 1.39 2.33
3.45 2.35 2.76 2.68
5
porphyrins are semispherical, and this may cause differences in the calculated energies of the ORR steps on the convex and concave sides. The planar structure of the tetraaza-annulene system implies that both sides are equal with respect to the reaction modelling. Therefore, the tetraaza-annulene model system has been chosen to represent a base case of the catalyst active sites and model the entire ORR mechanism. The high exothermic values of the calculated molecular oxygen binding energies reported in the previous section indicate that the metal centres in all the macrocycle systems bind the molecular oxygen stronger than necessary for this catalytic reaction and thus can potentially impede it. Therefore, in order to decrease the capacity of binding the reaction species on the transition metal macrocycle active sites, it is suggested that the metal centres should be terminated by a functional group at one side, e.g. hydroxyl (OH) group. Thus, modelling of the ORR mechanism has been performed using the Fe, Co or Mn tetraaza-annulene model systems with the metal centre terminated by the OH group at one side. Fig. 5 illustrates a schematic representation of the model system representing the transition metal macrocycle active site and the molecular oxygen binding as a first step of the ORR. The potential energy profiles of the ORR on the OHterminated Fe, Co and Mn tetraaza-annulene model systems are shown in Figs. S1 e S3 of the Supporting Information. Table 3 summarizes the calculated reaction energies of the ORR elementary steps, referenced to the reactants energy. It can be observed that the calculated molecular oxygen binding energy has almost double the exothermicity on the Mn centre than on the Fe and Co centres. This indicates that the Mn centre binds molecular oxygen too strongly and may impede the kinetics of the following steps. The first H addition leads to formation of the chemisorbed OOH group and is found to be the least exothermic in the case of the Fe centre. Introduction of the second H atom to each system in the H2O2 mode (twoelectron pathway) results in formation of the H2O2 molecule in all three cases. The second H addition in the H2OeO mode (four-electron pathway) leads to the formation of the first water molecule and the oxidized metal centre in the case of Fe and Mn central atoms, and two hydroxyl groups in the case of the Co central atom (Figs. S1 e S2). The reactions of the second H addition in both the two- and four-electron pathways and the H2O2 desorption are the most exothermic for the Fe centre. This suggests that both the hydrogen peroxide and the first water molecule are thermodynamically more favourable to form on the Fe centre than on the Co and Mn centres. The third H addition is the most exothermic, while the fourth H addition is the least exothermic for the Mn centre. This indicates that the Mn centre terminated by the second OH group is very stable thermodynamically, and the OH group is more difficult to reduce to water on the Mn centre compared to the Fe or Co centres. The calculated thermodynamics of the ORR steps on the OHterminated Fe, Co and Mn tetraaza-annulene model systems enables us to conclude the following: (i) The ORR thermodynamics strongly depends on the nature of the central transition metal atom. (ii) Both the two- and four-electron ORR pathways are thermodynamically possible on all the three metal centres of the macrocycle model system. (iii) The four-electron ORR pathway is more exothermic in all cases and thus more
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 5 e Schematic of the molecular oxygen binding on the active site modelled by the OH-terminated Fe tetraaza-annulene system.
Table 3 e The calculated reaction energies of the ORR elementary steps on the OH-terminated Fe, Co and Mn tetraaza-annulene model systems. Elementary reaction/state
Reactants Molecular oxygen binding First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Energy of the ORR elementary steps, eV Fe
Co
Mn
0 0.64 2.53 2.59
0 0.73 3.08 2.40
0 1.34 3.50 2.32
0.18 4.90
0.28 3.41
0.22 3.34
3.14 2.80 0.15
4.35 2.95 0.38
4.78 1.46 0.26
thermodynamically favourable regardless of the nature of the transition metal centre. (iv) The high chemisorption energies of the oxygen molecule and the OH group on the Mn centre suggest that it is likely to be blocked by a second OH group.
The ORR energy profiles on the Fe, Co and Mn 4Ncoordinated model systems In order to evaluate the kinetic aspects of the ORR on Fe, Co and Mn macrocycle active sites, it is necessary to obtain the ORR potential energy profiles with activation barriers for each elementary reaction. Calculating the activation barriers on the large tetraaza-annulene model systems is computationally expensive. Therefore, in order to attain a reasonable computational time, the model system size has been reduced. Fig. 6
illustrates the OH-terminated transition metal tetraazaannulene model system, employed in the previous section, and the small-size system representing the transition metal macrocycle active sites. The small-size model system is further referred to as the M4N, with the M being either Fe, Co or Mn. In the subsequent calculations, the M4N model systems have been partially optimised, i.e. all the CeN and CeC bond lengths have been set constant and equal to those in the large model system. First, the energies of the ORR elementary steps on the M4N systems have been obtained in a similar way as on the OH-terminated transition metal tetraaza-annulene systems. The calculated elementary reaction energies of the ORR intermediate steps both for the M4N (small) and the tetraazaannulene (large) systems are listed in Table 4. Although some of the elementary reaction energies for the small and large model systems have differences in value, the proposed M4N model systems have been employed to study activation barriers of the ORR elementary steps. As described in the Theoretical Method section, the activation barriers of the elementary reactions have been obtained using the technique of the “relaxed” PES scan calculations, with the highest energy point being taken as the transition state. The detailed scan calculations for each elementary step of the ORR on all the M4N systems are illustrated in Figs. S4eS6 of the Supporting Information. Fig. 7 illustrates the ORR pathways obtained on the Fee4N model system, showing the relative energy levels of the ORR intermediate steps and the corresponding transition states. The optimised structures of the ORR intermediate products are shown adjacent to the corresponding energy levels. The dashed lines designate that the scan calculations did not reveal activation barriers and therefore the reactions are considered to be barrierless. It has been found that the ORR on
Fig. 6 e Model systems representing the transition metal macrocycle active sites: the large tetraaza-annulene and the small M¡4N model systems. Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Table 4 e The calculated reaction energies of the ORR elementary steps on the Fee, Coe and Mne4N model systems compared to the OH-terminated tetraaza-annulene (large) model systems. Elementary reaction/state
Reactants Molecular oxygen chemisorption First H addition Second H addition (two-electron pathway) H2O2 desorption Second H addition (four-electron pathway) Third H addition Fourth H addition 2H2O desorption
Energy of the ORR elementary steps, eV Fee4N
Fe large
Coe4N
Co large
Mne4N
Mn large
0 0.35 2.48 3.25 0.15 4.95 3.04 3.49 0.11
0 0.64 2.53 2.59 0.18 4.90 3.14 2.80 0.15
0 0.42 3.07 2.59 0.16 2.98 4.82 3.11 0.20
0 0.73 3.08 2.40 0.28 3.41 4.35 2.95 0.38
0 1.36 3.17 1.68 0.28 3.34 4.81 1.57 0.05
0 1.34 3.50 1.32 0.22 3.34 4.78 1.46 0.26
the Fee4N model system branches into two four-electron pathways, referred to as the H2OO and OHeOH pathways, with all the intermediate steps being barrierless except the third H addition. It should be noted that the two-electron pathway via hydrogen peroxide has not been found. The calculated reaction and activation energies of the ORR elementary steps on the Fee4N model system are illustrated in Table 5. Although the second H addition steps in the two pathways have a difference in exothermicity of 0.45 eV, virtually this is unimportant since the third H addition leads to the same product in the both cases. As observed in Fig. 7, the non-zero activation barriers have been obtained for the third H addition only, and these values are rather small: 0.01 eV in the H2OO pathway, and 0.16 eV in the OHeOH pathway. Given the lower activation energy in the H2OO pathway, it can be concluded that it may have a faster kinetics than the OHeOH pathway. Overall, the non-activated potential energy profiles of the ORR indicate that the active sites modelled by the Fee4N system may be potentially very effective in catalysing the ORR predominantly in the four-electron pathway via the H2OO formation. Fig. 8 illustrates the ORR on the Coe4N model system branching into two four-electron pathways, namely, via the H2OO and OHeOH pathways. Table 6 lists the elementary reaction energies and corresponding activation energies of the two ORR pathways on the Coe4N system. It is observed in Fig. 8 and Table 6 that the molecular oxygen binding on the Coe4N system does not have an activation barrier, similar to
the Fee4N system. However, the first H addition ORR step on the Coe4N system has an activation energy of 0.07 eV. The ORR splits into the two pathways at the second H addition step. As in the case of the Fee4N system, the two-electron pathway via hydrogen peroxide has not been found. Although the intermediate products of the second H addition on the Coe4N model system in the two pathways are structurally similar to those of the Fee4N system, the OHeOH pathway is 1.12 eV more exothermic than the H2OO pathway. The two pathways merge at the step of the third H addition, which is found to have no activation energy in the H2OO pathway, but has significant activation energy of 1.11 eV in the OHeOH pathway. This implies that the reaction may stop at this point, or at least its rate be considerably slowed. Therefore, the ORR on the Coe4N model system can take a fourelectron pathway via the H2OO formation only. The rate limiting step in this case is the second H addition, since it has the highest calculated activation energy of 0.14 eV. The results of modelling the ORR on the Mne4N system are shown in Fig. 9, and the elementary reaction energies and corresponding activation energies are summarized in Table 7. Fig. 9 illustrates that the reaction branches into the twoelectron pathway, terminating with the hydrogen peroxide formation, and the four-electron pathway. The second H addition step in the four-electron pathway is markedly more
Table 5 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Fee4N model system. Reaction/state
Fig. 7 e The ORR pathways on the Fee4N model system.
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Reaction/activation energy, eV H2OO pathway
OHeOH pathway
0 none 0.35 none 2.48 none 4.95 0.01 3.04 none 3.49 0.11
0 none 0.35 none 2.48 none 5.40 0.16 2.58 none 3.49 0.11
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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Fig. 8 e The ORR pathways on the Coe4N model system.
exothermic than that of the two-electron pathway. Further, the two-electron pathway is found not to have activation barriers, whereas the second H addition in the four-electron pathway is the rate limiting step with the calculated activation energy of 0.14 eV. Thus, both the ORR pathways on the Mne4N model system are thermodynamically possible, but the kinetics of the four-electron pathway should be somewhat slower than that of the two-electron pathway due to the nonzero activation energy of the rate limiting step. This suggests that during the ORR, the Mne4N active sites can potentially yield hydrogen peroxide.
Conclusion In this paper, the oxygen reduction reaction has been studied on various model systems representing transition metal macrocycle active sites in order to identify potentially the most efficient ones. The effect of macrocycle structure has been studied by modelling molecular oxygen binding on phthalocyannine, porphyrin, corrin and tetraazaannulene
Table 6 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Coe4N model system. Reaction/state
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Fig. 9 e The ORR pathways on the Mne4N model system.
model systems either with iron, cobalt or manganese atoms in the centre. The central transition metal atom is the reaction centre, with the molecular oxygen binding strength depending primarily on the metal nature. It has been found that the binding strength increases in the order of cobalt, iron, and manganese. The local structure of macrocycles can also influence molecular oxygen binding and hence the efficiency of the active sites. The highly exothermic calculated values of molecular oxygen binding energies suggest the metal centre in macrocycle active sites should be terminated by a functional group, e.g. hydroxyl group, on one side, in order for the oxygen reduction to occur on the other free side. The effect of the central transition metal atom has been studied with respect to the thermodynamics and kinetics of the oxygen reduction reaction. Modelling of the ORR energy profiles on the iron, cobalt and manganese macrocyle model systems indicates that the iron macrocycle active sites demonstrate the highest catalytic activity towards the ORR, since the reaction proceeds through the four-electron pathway with a negligible calculated activation energy of the rate limiting step (0.01 eV). The cobalt and manganese
Table 7 e Elementary reaction energies and activation energies of the ORR intermediate steps on the Mne4N model system. Reaction/state
Reaction/activation energy, eV H2OO pathway
OHeOH pathway
0 none 0.42 0.07 3.07 0.14 2.98 none 4.82 none 3.11 0.20
0 none 0.42 0.07 3.07 0.06 4.10 1.11 4.82 none 3.11 0.20
Reactants O2 binding activation O2 binding First H addition activation First H addition Second H addition activation Second H addition H2O2 desorption Third H addition activation Third H addition Fourth H addition activation Fourth H addition 2H2O desorption
Reaction/activation energy, eV Two-electron pathway
Four-electron pathway
0 none 1.36 none 3.17 none 1.68 0.28
0 none 1.36 none 3.17 0.14 3.34 e none 4.81 none 1.57 0.05
Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075
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macrocycle active sites are predicted to have similar reaction rates in the four-electron ORR pathway, since the calculated activation energies of the rate limiting steps are equal (0.14 eV). However, the ORR on the cobalt macrocycle active sites can proceed via a pathway that may trap the catalyst in a kinetically stable intermediate at typical PEM fuel cell operating temperatures, or at least substantially reduce the overall rate of the four-electron pathway. The manganese macrocycle system provides an accessible route to the two-electron ORR pathway producing undesirable hydrogen peroxide. Therefore, non-precious catalysts with high concentration of iron macrocycle active sites with the proposed structure are predicted to be the most promising for PEM fuel cells.
[10]
[11]
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Acknowledgements [14]
The authors would like to thank the Energy Technology and Innovation Initiative of the University of Leeds and the NetScientific Plc. for the financial support.
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Appendix A. Supplementary data [16]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.09.075. [17]
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Please cite this article in press as: Flyagina IS, et al., DFT study of the oxygen reduction reaction on iron, cobalt and manganese macrocycle active sites, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.075