Theoretical Study of the Electrocatalytic Reduction of Oxygen by

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Theoretical Study of the Electrocatalytic Reduction of Oxygen by Metallocorroles Monica Kosa, Naomi Levy, Lior Elbaz, and Dan Thomas Major J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05831 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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The Journal of Physical Chemistry

Theoretical Study of the Electrocatalytic Reduction of Oxygen by Metallocorroles

Monica Kosa,1 Naomi Levy, 1 Lior Elbaz,1 Dan Thomas Major*,1 1

Chemistry Department, Bar-Ilan University, Ramat Gan 52900, Israel

*Corresponding author E-mail: [email protected] Tel. + 972 3 531 73 92; Fax. + 972 3 738 40 53

1 Environment ACS Paragon Plus

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Abstract Metallocorroles are transition metal complexes showing great promise as oxygen reduction reaction catalysts. The performance of metallocorrole catalysts is highly sensitive to the nature of the transition metal employed, although currently this dependence remains elusive. In the current work, we present a first principles density functional theory (DFT) investigation of the oxygen reduction reaction mechanism in acidic media using several firstrow transition metal corroles. We show that the identity of the metal center, M, determines the relative formation free energies of the reaction intermediates, and thus the potentialdetermining step in the 4-electron reduction process directly to water. For M=Mn, Fe, Co the hydroperoxyl intermediate is a thermodynamic maximum along the reaction path, while for Ni and Cu, the formation of the hydroperoxyl intermediate is a thermodynamic trap, with the following oxo intermediate being highly unstable. The formation of the oxo intermediate was carefully investigated using several flavors of DFT. The calculations suggest that in the oxo intermediate the spin density on the oxygen atom increases from M=Mn to M=Cu indicating a purely metal oxygen double bond for M=Mn, a single metal oxygen bond with an unpaired electron on the oxygen atom for M=Ni and Cu, while for M=Fe and Co, the spin density on the oxygen atom has intermediate values. When plotting the experimentally observed onset potentials as a function of the computed O2 adsorption free energies, a volcano-like plot is observed, indicating that for the best catalyst, [Co(tpfcBr8)], a negative binding free energy is observed. Good correlation between computed limiting and experimental onset potentials indicate that the computationally proposed reaction intermediates are viable states along the reaction coordinate. The current work is expected to be of importance for the future design of efficient metallocorrole-based catalysts.


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Introduction With the constantly growing demand for energy, energy efficiency is one of the major challenges facing civilization.1 Oscillating oil prices, limited fossil fuel reservoirs, and constant climate changes have led global leaders and policy makers to undertake severe measures towards development and implementation of alternative energy conversion technologies. One of the most promising alternative energy solution available today is polymer electrolyte membrane fuel cell (PEMFC).2 However, the limited efficiency and high cost of producing PEMFCs presents a significant challenge, and the quest for improved and cheaper technologies has driven much of the fundamental and applied fuel cell research in recent years. The commercial viability of PEMFCs is predominantly limited by the rate determining oxygen reduction reaction (ORR) at the cathode and the high price of its Ptbased catalyst.3 Alternatives to Pt-based catalysts have been studied extensively over the years with the aim of lowering the price of PEMFCs. In particular, much work has focused on developing non–precious metal catalysts with high ORR activity, such as transition metal complexes.4-8 Inspired by Nature’s catalytic efficiency, well-known biological systems, such as metalloporphyrins (Scheme 1), with four symmetric bi-anionic charged nitrogen atoms, MN4, have been shown to possess desirable ORR catalyst properties in terms of onset potential and turnover frequency. Yet studies conducted with synthetic metalloporphyrins revealed that their activity, and, equally important their stability, are much lower than of state-of-the-art Pt-based catalysts. Alternatively, catalysts in which the MN4 catalytic site is incorporated in graphitic sheets via pyrolysis of transition metal salt together with nitrogen and carbon precursors, are stable. However, it is difficult to systematically improve their properties, due their unknown structure derived from the pyrolytic synthetic conditions.9-10



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Scheme 1. Structures of bare (a) corrole and (b) porphyrin, and the corresponding (c) metallocorrole and (d) metalloporphyrin.

Metallocorroles

are

among

the

state-of-the-art

non-noble,

non-pyrolyzed

organometallic ORR catalysts (Scheme 1).11-13 Because of their slightly contracted metalcoordinating core and the tri-anionic charge of their ligands, the chemistry of the chelated transition-metal ion is greatly affected when compared to other MN4-based molecular catalysts, for example metalloporphyrins.14-21 Therefore, metallocorroles may be expected to display novel features as catalysts for electrocatalytic ORRs. Several reports have confirmed this hypothesis,22-30 but not all the desired properties required for optimal performance were attained. In earlier work, we elucidated the factors that affect both the electrochemical onset potential and the kinetics of ORR using a combined computational-experimental approach.1112, 31

This was successfully achieved by tuning the properties of the metallocorroles by

varying the metal center, leading to a catalyst whose onset potential is almost as positive as that of Pt.


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In the current work, we employ gas phase and implicit water solvent model density functional theory (DFT) calculations to elucidate the direct 4 electron mechanism of ORR for a range of metallocorrole-based

catalysts,

2,3,7,8,12,13,17,18-octabromo-5,10,15-

tris(pentafluorophenyl) corrole [M(tpfcBr8)], where M=Mn, Fe, Co, Ni, Cu (Scheme 1). The hypothetical ORR mechanism leading to formation of plausible reaction intermediates is proposed to proceed as follows: in the first step, the metal center of the free catalyst, designated as *, binds an oxygen molecule and receives the first electron in conjunction with a proton, forming the hydroperoxyl complex, *OOH. The second electron and proton transfers result in the formation of the oxo-complex, *O, and release of the first water molecule. The third electron and proton transfers to *O, result in the formation of hydroxyl, *OH, which is finally transformed to the second water molecule upon the fourth electron and proton transfers, as described in Scheme 2. The reaction intermediates shown in Scheme 2, i.e. *OOH, *O and *OH, are plausible in both concerted and stepwise proton electron transfer mechanisms.32-33



Scheme 2. Possible four electron reduction mechanism for the oxygen reduction reaction catalyzed by metallocorroles.

In the current work, we show how the metal center influences the electronic structure of these metallocorroles, and dictates the stability of the reaction intermediates and hence the ORR mechanism.34 The effect of the corrole organic framework on the formal oxidation state of the transition metal is emphasized and contrasted with that observed for metalloporphyrins.35


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Computational Methods The M(tpfcBr8) complexes were constructed employing the GaussView software (Gaussian, Inc.) and all calculations were performed with the Gaussian 09 program,36 unless otherwise stated. Total energies were computed using the M06L density functional37 with the 6-31G(d,p) basis set.38 The geometries were optimized using the Berny algorithm39 with default tight convergence settings (Opt keyword in Gaussian 09). Stable=Opt option was used to ensure that the lowest state of the wavefunction was obtained. Several possible spin states for all reactants, products and reaction intermediates of the M(tpfcBr8) complexes, M=Mn, Fe, Co, Ni, Cu, were investigated, and the most stable states were chosen for reaction mechanism analysis. The metallocorroles in the various spin states and their total energies are shown in Tables S1a-b. Zero-point energy and thermal corrections to the total energy were computed within the harmonic approximation using standard expressions of statistical mechanics at 298K, as implemented in Gaussian 09 (Freq keyword).40 When computing the free energy using the Freq keyword, only a single configuration of each reaction intermediate was employed. Using a single configuration is not expected to significantly affect the reaction intermediate’s formation free energy trend for the five metallocorroles. To account for the aqueous solution environment, we used the SMD implicit solvent model,41 i.e. for structure optimizations and free energy calculations, denoted as SMD-uM06L/6-31G(d,p). In SMD, the electrostatic part of the solvation free energy is obtained from the IEFPCM continuum solvent model,42-44 whereas the cavity-dispersion-solvent-structure term, which accounts for short-range interactions between the solute and solvent molecules in the first solvation shell, is described by terms proportional to the solvent-accessible surface areas of the solute atom. For the sake of consistency with previous studies, we computed the formation free energies of *OOH and *OH intermediates also by correcting gas phase free



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energies by empirical value of -0.30 eV to account for the stabilizing effect of hydrogen bonds of surrounding water molecules,45-47 see Table S2 and Table S3 for corrected and uncorrected values. This correction accounts for 0.15 eV stabilization energy per hydrogen bond formed between the reaction intermediate and the supposed acidic water environment. A comparison between the gas-phase, H-bond corrected and SMD corrected free energy surfaces is included in the Supporting Information (Fig. S1), showing that the trends are very similar for the different approaches. In the current study the origin of the stabilization of reactive intermediates as a function of the first-row transition metal is discussed. The pH plays a similar role for all complexes under study and is expected to shift the absolute values to a similar extent, hence not expected to change the conclusions of the current work. Vertical electron affinities, EA, were computed as the energy differences between the optimized structure of the neutral metallocorroles and their corresponding anions at the geometry of the neutral species in the gas phase. The reaction intermediate free energies were computed according to the following equations:

* + 2H2O → *OOH + 1.5 H2

∆G*OOH=1.5·G(H2) + G(*OOH) – G(*) – 2·G(H2O)

(1)

* + H2O → *O + H2

∆G*O=G(H2) + G(*O) – G(*) – G(H2O)

(2)

* + H2O → *OH + 0.5 H2

∆G*OH=0.5·G(H2) + G(*OH) – G(*) – G(H2O)

(3)

Results and Discussion I. Electronic structure of the metallocorroles Metallocorroles are close relatives of metalloporphyrins, and have very similar organic macrocycle framework, in which one carbon atom at the meso position is missing and the


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two pyrroles are directly fused (highlighted in yellow in Scheme 1). This missing carbon in corroles induces a different conjugation pattern of the π-system, and thus the number of secondary amino, sp3 hybridized nitrogen atoms, is different. In the neutral macrocyle, the corrole has three secondary amino groups, while the porphyrin has two secondary amino groups. These secondary amino groups, are deprotonated upon metal insertion and hence corroles stabilize metals in a 3+ oxidation state, while porphyrins stabilize metals in a 2+ oxidation state. The electronic structure of corroles resembles that of the porphyrins in terms of Goutermans’ four orbital model with the HOMO-1, HOMO, LUMO and LUMO+1 constituting corrole π orbitals.48-49 Upon insertion of a metal, the 3d metal orbitals interact with the π system of the corrole, resulting in a set of mixed molecular orbitals. This observation is particularly important for the LUMO orbitals of the catalysts, which are the acceptor states for the first electron transfer and participate in binding of the reaction intermediates. Inspection of the computed frontier molecular orbitals, reveals that there is substantial contribution of the metal 3d orbitals to the LUMO for M=Mn, Fe and Co, while for Ni and Cu the LUMO constitutes mainly the corrole π-system (Fig. 1 and S2). These results suggest that the reduction process, i.e. the first electron transfer, will be on the metal atom for M=Mn, Fe and Co, while for M=Ni and Cu the reduction will occur on the corrole ligand. The extra charge localization on a metal atom following reduction, i.e. M=Mn, Fe and Co, which is also the active site of the catalyst complex, is expected to further facilitate the oxygen reduction reaction. The HOMO orbitals of the metallocorroles constitute practically the entire corrole π system for all metals considered herein, except for Cu, where HOMO constitute both metal and the corrole ligand orbitals. This observation suggests that in metallocorroles, oxidation occurs primarily on the ligand.



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The initial step of the ORR mechanism (discussed below) presumably involves a oneelectron reduction followed by oxygen binding and proton transfer. To gain insight into the initial electron transfer, we computed the vertical electron affinities using DFT. The computed trend of the vertical electron affinities correlates fairly well with the experimental reduction potentials, in an oxygen free environment, Mn(-0.37), Fe(0.05), Co(0.36), Ni(0.97), Cu(0.64) V vs. Ag/AgCl in CH3CN (Fig. 1),50-51 reflecting the changes in electronegativity for the first row transition metals.

M

Mn

Fe

Co

Ni

Cu

2.58

3.16

3.51

3.76

3.25

LUMO

HOMO

EA (eV)

Figure 1. Singly occupied spin down HOMO and LUMO orbitals computed using uM06L/631g(d,p). The surface isodensity was 0.02 (e/a.u.3)1/2 and the electron affinities, EA, are given in eV.

II. Reaction Mechanism The full reduction of oxygen to water involves the transfer of four electrons and four protons to obtain two water molecules, according to the hypothetical mechanism depicted in Scheme 2. Several species and reaction intermediates can be envisioned along the reaction path: the free catalyst metallocorrole, denoted as *; the hydroperoxyl complex, *OOH; the oxo


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complex, *O; the hydroxyl complex, *OH. The total catalyzed oxygen reduction reaction can be written as: * + 4(H++ e-) + O2 → * + 2H2O.

(i) Reactive intermediates formation free energies The computed formation free energies for the reactive intermediates are presented in Figure 2a and 2b. The free energy of formation trends indicate a general increase in the interaction free energies for all reactive intermediates with the catalyst, i.e. *OOH, *OH and *O, when moving from Mn to Cu, implying their destabilization. This destabilization of *OOH, *OH and *O can be rationalized in terms of increasing number of 3d-electrons along the first-row transition metal series and the charge repulsion between the binding site of the catalyst and the ligand, i.e. OOH, OH and O. This general trend is similar to the computed intermediates formation free energies for the first row transition metal metalloporphyrins,47 as well as the computed formation free energies for intermediates on functionalized graphitic materials with the catalytic site comprising a transition metal intercalated by four nitrogen atoms, MN4.45-46 Notably, when the same methodology is considered, i.e. gas phase geometry optimized structures and energies augmented with an empirical H-bond correction, the OOH, O and OH adsorbates formation free energies to metallocorroles are in general less positive than those for metalloporphyrins, indicating stronger binding of OOH, O and OH to metallocorroles than to metalloporphyrins. For example, the *OH formation free energies vary from 0.55 and 0.72 eV for M=Mn and Fe, up to 1.05 and 1.99 eV for M=Co and Ni for metalloporphyrin, see Table S3 of this work and Table 2 in reference,47 while *OH formation free energies to metallocorroles vary from 0.21 and -0.04 eV for M=Mn and Fe, up to 0.53 and 1.01 eV for M=Co and Ni (Fig. 2a and Table S3). The increase in the formal oxidation state of the metals in corroles relative to porphyrins, as well as the decrease in the number of 3d electrons, are



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likely responsible for the greater stabilization of reaction intermediates in metallocorroles compared to metalloporphyrins. When using the SMD implicit solvent model, the OOH, O and OH adsorbates formation free energies are considerably higher, Figure 2b, yet the trends for the intermediates *OOH, *O or *OH, are like the hydrogen-bond corrected gas phase computed trends. The absolute change in the adsorbates formation free energies using gas phase and SMD optimized geometries depends on the reference equations, i.e. (1)-(3) in the Computational methods section.

(a) (a)

5.5 5.5 4.5 4.5

∆G, eV 3.5 3. 5

3.22

3.35

3.06

2.5 2.5

1.90

4.09 3.89

3.69 3.07

2.12

1.75

1.52

1.5 1.5

1.01 0.5 0.5

0.53

0.21

-0.04 OOH

-0. -0 .5

Mn

Fe

Co

O

OH

Ni

Cu

(b) (b) ∆G, eV

Figure 2. Computed formation free energies for the metallocorrole intermediates, according to equations 1-3, respectively at (a) uM06L/6-31G(d,p) level of theory using the empirical correction for H-bond stabilization and at (b) SMD-uM06L/6-31G(d,p).


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(ii) The elusive oxo complex, *O The *O oxo formation free energy shows a gradual change for M=Mn, Fe and Co, varying from 1.90, 1.75 and 2.12 eV, respectively (Fig. 2a), using uM06L/6-31G(d,p) gas-phase optimized geometries. For M=Ni and Cu, however, there is a substantial energetic jump, with oxo intermediate formation free energies of 3.07 and 3.89 eV. Similar results are obtained employing implicit solvation (Fig. 2b). The difference in the oxo intermediate formation free energy between the periodic table neighbors, Co and Ni, suggests substantial electronic structure differences between the Co and Ni metallocorrole oxo-complexes. This is seemingly a general feature, which separates between the first group of metallocorroles, M=Mn, Fe, Co and the second group of metallocorroles, M=Ni, Cu. These oxygen adsorption trends can be rationalized in terms of increasing electronegativity and more specifically, in terms of ligand field considerations and specific orbital populations.52-53 This is supported by the experimental observation that many group 3-8 transition metal oxo-complexes were isolated, while only a handful of group 9-11 transition metal oxo-complexes have been isolated. As such, for early transition metals, the dxz and dyz orbitals are located at much higher energies than the oxygen px and py orbitals, and therefore the π* orbitals have predominantly metal character.52 Moving rightward in the periodic table, the energy of the metal d orbitals decreases up to the point where the metal d orbitals can even possess lower energy than that of the oxygen p orbitals. Thus, the early transition metals generally form stable metal-oxo cores, containing high oxidation states of the metal, while the O ligand is dianionic, O2-. In contrast, the late transition metals are expected to stabilize metal-oxyl [M-O·] species, involving lower valent metals and an oxygen-centered radical anion, O-·. This ideal ligand field description, however, is only partially observed in the present study, as seen in both geometric and electronic structure parameters of the oxo adsorbates. Examining the geometry of each oxo-metallocorrole, *O, compared to its hydroxyl



counterpart, *OH, reveals that the first group of metallocorroles forms a relatively short metal-oxygen bond of *O, which elongates, as expected, in *OH (Table 1a). The bond elongation is most significant for M=Mn (17.5%), while less pronounced for M=Fe, Co and Ni, with values of 12.5, 8.6 and 9.5 %, respectively, and very small for M=Cu (1.8%). The computed spin density on the oxygen atom in the *O complexes shows an increase from 0.00 for M=Mn through 0.62 and 1.08 for M=Fe and Co to 1.37 and 1.38 for M=Ni and Cu, indicating accumulation of unpaired electron density and thus a radical character of the oxygen atom in the oxo adsorbates. The computed trend of increasing spin density on the oxygen atom in the *O intermediate is observed for several flavors of DFT, see Tables S4 – S8 in the SI, although individual values vary between M06-L and hybrid functionals54 and among hybrid functionals as well, see Table S9. It can be thus concluded that oxygen forms an oxyl radical in the *O intermediate with M=Ni and Cu, and a partial double bond when M=Mn, Fe and Co. This observation has significant impact on the reaction mechanism as will be discussed in the next section (iii). When optimizing the structures using the implicit solvent model, i.e. SMD-uM06L/6-31G(d,p), the trends in bond length elongation and spin densities are similar to gas phase structures for Mn, Fe and Co (Tables 1a and 1b). For Ni and Cu we observe greater change in the metal oxygen bond length in *O compared to *OH when using SMD-uM06L/6-31G(d,p), pointing to a greater stabilization of charges on the oxygen with implicit solvent optimized geometries. This observation demonstrates that different metallocorrole reaction intermediates accommodate solvent interactions differently and using the same empirical solvent correction for a row of catalysts might not be appropriate.


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Table 1a. Selected geometric parameters for metallocorrole reaction intermediates computed using uM06L/6-31G(d,p). M

Mn

Fe

Co

Ni

Cu

M=O bond in *O, Å

1.542

1.606

1.654

1.696

1.881

M-O bond in *OH, Å

1.811

1.807

1.797

1.858

1.916

% elongation

17.5

12.5

8.6

9.5

1.8

Mulliken spin density

0.00

0.62

1.08

1.37

1.38

on O in *O, a.u.

Table 1b. Selected geometric parameters for metallocorrole reaction intermediates computed using SMD-uM06L/6-31G(d,p). M

Mn

Fe

Co

Ni

Cu

M=O bond in *O, Å

1.546

1.628

1.664

1.701

1.893

M-O bond in *OH, Å

1.827

1.851

1.812

1.883

1.945

% elongation

18.14

13.70

8.87

10.73

2.73

Mulliken spin density

0.00

0.61

1.02

1.29

1.28

on O in *O, a.u.

(iii) Mapping the oxygen reduction reaction free energy surface Free energy diagrams using hydrogen-bond corrected gas-phase values show that at zero cell potential, U=0 V vs. Standard Hydrogen Electrode (SHE), all reaction steps are exergonic (Fig. 3a, solid lines). However, at the thermodynamic limit, U=1.23 V vs. SHE,34 the catalysts with M=Mn, Fe and Co form very stable hydroxyl intermediate, *OH (Fig. 3a, dashed lines). Inspection of the free energy curves shows that the last reduction step, i.e. formation of the second water molecule from *OH, will be the potential determining step.



For M=Ni and Cu, the diagram at U=1.23 V vs. SHE looks qualitatively different since the oxo intermediate is very unstable, relative to the preceding *OOH. It is therefore possible that its formation is avoided, and this will inhibit the 4-electron process and turn the reaction towards competitive routes (e.g. a 2-electron process). This qualitative difference in the free energy diagrams between M=Mn, Fe, Co and M=Ni, Cu is due to the high formation free energy of the oxo, *O, intermediate for M=Ni and Cu. Underlying this difference is the electronic structure of M=Ni and Cu, which suggests an oxyl radical intermediate, as discussed above. At the continuum solvation SMD-uM06L/6-31G(d,p) level of theory the formation free energy profiles are similar to the hydrogen-bond corrected gas-phase values, although some differences exist (Fig. 3a, b). For example, for all metals, the relative formation free energy of *OOH is greater at the SMD-uM06L/6-31G(d,p) than at the hydrogen-bond corrected uM06L/6-31G(d,p), precluding its formation already at low potentials, Fig. 3b. The trends obtained from the free energy profile (Fig. 3b) can be compared with the experimentally observed onset potentials. To this end we compute the theoretical limiting potential55 in the gas-phase, with empirical hydrogen bond corrections, and using the SMD continuum solvation model (Fig. 4a, Table S12). Our results suggest that when including continuum solvation, the theoretical limiting potential trend is in good agreement with experiment (Fig. 4a), whereas gas-phase and hydrogen bond results do not reproduce the experimental trend correctly (Table S12). This further emphasizes the importance of including a rigorous solvent treatment. Scaling relations between ∆G*OH, ∆G*OOH and ∆G*O, Fig.S3a,b and the free energy difference between ∆G*OOH and ∆G*OH, Fig. S4a,b suggest slight breaking of scaling relations, similarly to porphyrins when gas phase calculations are considered. Inclusion of implicit solvent water modifies the scaling between ∆G*OOH and ∆G*O and the


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free energy difference between ∆G*OOH and ∆G*OH, yet still show a slight break in scaling relations relative to that observed in other systems.56 Additionally, the free energy differences between *OH and *OOH are close to the proposed “universal constant” 3.2 eV,56-57 although some deviations are observed (Fig. S4b).

(a) ∆G, eV

(b) ∆G, eV

5.5 5.5 5.0 5.0 4.5 4.5 4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 -0. -0 .5 -1. -1 .0 -1. -1 .5 -2. -2 .0 5.5 5.5 5.0 5.0 4.5 4.5 4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 -0. -0 .5 -1. -1 .0 -1. -1 .5 -2. -2 .0

Cu Ni Co Mn Fe Cu Ni Co Mn Fe

* + O2 + 4(H+ + e-) *OO *OOH + 3(H+ + e-) *O + 2(H+ + e-)

*OH *OH + (H+ + e-)

* + 2H2O

* + O2 + 4(H+ + e-)

*OO *OOH + 3(H+ + e-) *O + 2(H+ + e-)

Cu Ni Co Mn Fe Cu Ni Co Mn Fe

*OH *OH + (H+ + e-)

* + 2H2O

Figure 3. Computed free energy diagrams, ∆G (eV), of the ORR involving 3 possible intermediates, *OOH, *O and *OH at zero cell potential, U=0 V (solid lines) and at the equilibrium potential, U=1.23 V (dashed lines) in the (a) gas phase uM06L/6-31G(d,p) with hydrogen bond corrected energies and (b) using continuum solvent description, SMDuM06L/6-31G(d,p).



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(iv) Dioxygen, O2, adsorption energies and the experimental onset potential of the ORR reaction. While the proposed 4-electron mechanism can partially account for the experimentally observed reactivity of metallocorrole-based catalysts, [M(tpfcBr8)], the exact ORR mechanism, that could account for the excellent experimentally observed performance of these catalysts is unknown.11 The O2 adsorption energies to transition metal macrocyclic complexes were discussed as a viable ORR reaction descriptor.35,

58-59

The computed O2

adsorption free energies to [M(tpfcBr8)] are negative for M=Mn, Fe and Co, while positive for M=Ni and Cu. When plotting the experimentally observed onset potentials as function of the computed O2 adsorption free energies, a volcano-like plot is observed (Fig. 4b), indicating that for the best catalyst, [M(tpfcBr8)] M=Co, a negative binding free energy is observed, yet with an intermediate value between the strong binders, like M=Mn and the weak binders, like M=Cu. Thus, oxygen binding seems to also here be a solid descriptor for the experimentally observed ORR activity, which also shows a volcano plot character as a function of transition metal atomic number.

Figure 4. Experimentally observed onset potential as a function of (a) Theoretical limiting potential (SMD-uM06L/6-31G(d,p)) and (b) the computed O2 adsorption free energies (uM06L/6-31G(d,p)) for [M(tpfcBr8)], M=Mn, Fe, Co, Ni, Cu.


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Conclusions In the current manuscript, we studied the 4-electron ORR mechanism catalyzed by first row transition metal corroles, [M(tpfcBr8)] where M=Mn, Fe, Co, Ni, Cu. The computed electronic structure for these catalysts in their resting state shows a fundamental difference in the LUMO orbitals for M=Mn, Fe, Co metals and the late transition metals, M=Ni and Cu. In the case of the early transition metal complexes, the LUMO orbital is concentrated on the metal, while for the later transition metal group the LUMO is concentrated on the corrole macrocycle. This difference, together with the increasing number of 3d electrons along the first-row transition metals, account for the destabilization of all reaction intermediates along the transition metal row, and especially for the difference in the oxo complex *O stability. The destabilization of the oxo complex for the late transition metals accounts for the change in the potential determining step for M=Ni and Cu metals versus M=Mn, Fe and Co metals. When comparing the theoretical limiting potential with the experimentally observed onset potentials, we obtain good correlation, suggesting that the computationally proposed reaction intermediates are viable states along the reaction coordinate. When plotting the experimentally observed onset potentials as function of the computed O2 adsorption free energies, a volcano-like plot is observed, indicating that for the best catalyst, [M(tpfcBr8)] M=Co, a negative binding free energy is observed. We note that it has previously been demonstrated that the carbon substrate and environmental factors such as the axial ligand of the porphyrin47 and proton relays, for example, greatly affect the selectivity, such as 2 electrons vs. 4 electrons, in metalloporphyrins ORR catalysis.60 We are currently examining the effect of the axial ligand and this study will be presented at a later stage.



Acknowledgements

This work was partially supported by the Israel Science Foundation (ISF) and the Planning & Budgeting Committee / ISRAEL Council for Higher Education (CHE) and Fuel Choice Initiative (Prime Minister Office of ISRAEL), within the framework of “Israel National Research Center for Electrochemical Propulsion (INREP)”. The authors would also like to thank the Israeli Ministry of Energy for their partial support.

ASSOCIATED CONTENT

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

(1) Figures of the frontier orbitals of the studied metallocorrole complexes in their resting state. (2) Tables and graphs detailing absolute complex energies, intermediate formation free energies with and without correction for hydrogen bonding, and selected computed properties of the oxo species using several flavors of DFT. (3) Cartesian coordinates of the optimized geometries of the discussed structures.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions M.K. and D.T.M. designed the research, M.K. performed the studies, and M.K., D.T.M., N.L., Z.G., and L.E. wrote the paper.


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Theoretical Study of the Electrocatalytic Reduction of Oxygen by

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