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Climbing the 3D Volcano for the Oxygen Reduction Reaction Using Porphyrin Motifs Hao Wan, Thomas Mandal Østergaard, Logi Arnarson, and Jan Rossmeisl ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04173 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Sustainable Chemistry & Engineering

Climbing the 3D Volcano for the Oxygen Reduction Reaction Using Porphyrin Motifs Hao Wan, Thomas Mandal Østergaard, Logi Arnarson, and Jan Rossmeisl∗ Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark E-mail: [email protected] Abstract A major contribution to the energy loss in fuel cells originates from poor kinetics of the oxygen reduction reaction (ORR) at the cathode. The ORR mechanism has been understood in descriptor-based approaches, that reveals an activity volcano with a significant overpotential of at least 0.4 V. This energy loss is directly linked to the scaling relation between the binding energy of the ORR intermediates, OH and the OOH. It has become apparent that new catalyst designs are necessary in order to circumvent this scaling relation. One strategy is to stabilize the OOH intermediate in a dissociated state on two active sites, as an O+OH intermediate. Here we demonstrate the feasibility of this strategy in a systematic study of diporphyrin molecular catalysts. This class of catalysts contains two metal sites, whose catalytic chemistry can be influenced by ligands. Using density functional theory (DFT), we study the ORR activity as a function of intermetallic distance, metals and ligands. Several diporphyrin catalysts are identified with a theoretical overpotenial of less than 0.3 V. The enhanced catalytic activity is understood as a combination of a geometric effect from the diporphyrin structure and an electronic effect from the choice of metal center and ligand. We propose a strategy to reduce the energy loss and climb the 3D volcano by appropriate design of

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the geometric and the electronic effects.

Keywords Single site catalysis, electrocatalysis, ORR, Diporphyrins, DFT

Introduction In proton exchange membrane fuel cells the main source of energy loss originates from poor kinetics of the oxygen reduction reaction (ORR) at the cathode. 1–3 The activity of ORR catalysts are limited by a fundamental constraint of the ORR pathway, in the form of a scaling relation between the stability of the adsorbed intermediates. 4 Due to this fundamental constraint, only modest improvements in the overpotential have been achieved in the last decade. 5 It has been made clear in several papers that disruptive changes are required in the functionality of new catalysts, in order to circumvent the limitations of the scaling relation. 5–9 The ORR pathway includes four proton–electron transfers and at least three intermediates. The associative reaction pathway is given by

∗ + O2 + (H+ + e− ) −−→ ∗OOH

(1)

∗OOH + (H+ + e− ) −−→ ∗O + H2 O

(2)

∗O + (H+ + e− ) −−→ ∗OH ∗OH + (H+ + e− ) −−→ ∗ + H2 O

(3) (4)

where ∗ denotes the active site. The mechanism is shown schematically as Pathway 1 in Fig. 1. The overpotential of the ORR originates from a scaling relation between the free energy

2


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of adsorption for OH and OOH 10–12

GOOH ' GOH + 3.2 eV,

(5)

where GOOH and GOH are defined as the formation energies of the adsorbed OOH and OH intermediates respectively (see Computational Details). For the ideal catalyst the ORR occurs close to zero overpotential and all reaction energies are equal. A consequence of the scaling relation in Eq. (5) is that even the best catalysts operate at an overpotential of at least ∼0.4 V. 13 Therefore this scaling relation must be avoided in order to increase the activity of future ORR catalysts and for that, several strategies have been proposed in the literature. 5,6,9,14–16 The activity of catalysts can be perturbed by changing the electronic structure of the binding site, 5,6,9,14–17 such that the binding energy is modified in accordance with the scaling relations. Geometric effects on the other hand, 5,6,15,16 hold the potential to introduce new reaction pathways, or new types of interactions with the ORR intermediates and thereby break the scaling relation. This is shown schematically for a dual site catalyst as Pathway 2 in Fig. 1. In this work, we explore such electronic and geometric effects for a class of bifunctional molecular diporphyrin catalysts, and we derive general principles for lowering the ORR overpotential beyond the limitations that conventional surface catalysts suffer from. The metalloporphyrin catalysts are characterized by having a single metal site coordinated to four nitrogen atoms in a carbon framework where the metal site is generally treated as the most active site. 18–22 This class of materials includes extended 2D graphene-like sheets 20–25 and large molecular structures, 26–33 and the catalytic activity is highly tunable, as the embedded transition metal can be varied. Here, we consider molecular porphyrin motifs with two active metal sites in a three-dimensional structure (see Fig. 2), that may allow for a unique ORR mechanism. The initial motivations for synthesizing these diporphyrin structures (mainly Co diporphyrin) included the idea that two Co metals would stabilize a µ-

3



peroxide intermediate and would then favor the 4 electron ORR, as most monomeric cobalt catalysts produced primarily H2 O2 . 28 Different selectivity has been achieved for cobalt diporphyrins with different intermetallic distance and side groups. 28–30,32 Due to distinctive geometry of diporphyrins, these catalysts may have the potential to entirely avoid the OOH intermediate by possessing two single-metal sites a few angstroms apart. 34,35 This reaction pathway is shown as Pathway 2 in Fig. 1, where O2 is split to O and OH (labeled O+OH) in the first step, binding to separate metal atoms instead of forming OOH according to Eq. 1 (Pathway 1 in Fig. 1). Pathway 1 M1 OOH

ΔG1 O2

M1

H +e

M2

+

-

H ++ O M2

H +e +

H 2O

H 2O

OH

M2

-

H+ +e -

-

M1

M1 O

ΔG4 H 2O

ΔG2

ΔG1 O2

e-

M2

Pathway 2

+ e H+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H++eΔG3

M1 OH

ΔG2 H++eΔG3

M2

M1 O M2

Figure 1: Catalytic cycle for the associative ORR pathway to the left (Pathway 1, black), and the dissociative pathway to the right (Pathway 2, red). Pathway 1 utilizes one of the two active sites of the diporphyrin motif which follows the associative ORR mechanism just like metal surface catalysts or single porphyrin catalysts. Pathway 2 utilizes the two active sites of the diporphyrin motif where OOH splits to O+OH, and consequently circumvents the scaling relation between the OH and OOH intermediates. The top adsorption or bridge adsorption of O2 is indicated through Pathway 1 or Pathway 2 respectively. OOH is omitted in Pathway 2 since it is a thermal process from OOH to O+OH and is not potential dependent. Metal surface catalysts could in principle also dissociate O2 or OOH to O and OH however, metal surface with a small dissociation barrier is prone to too strong binding of OH or O. 36 The potential determining step is in that case reduction of OH (∆G4 in Fig. 1). On Pt there is a barrier for O2 dissociation, however, Pt also needs an overpotential to remove OH from the surface. 4 By weakening the OH binding slightly, the overpotential for OH reduction 4


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is decreased, but on these surfaces oxygen must be reduced to OOH or H2 O2 before it dissociates. This means that metal surfaces are limited by the scaling relation as it is impossible to dissociate O2 without binding OH too strongly. On single porphyrins dissociation is impossible with leaves on alternative of the OOH intermediate. The Fe porphyrin is the standard for a Pt free ORR catalyst. However, on Fe porphyrin, as on Pt, the potential is determined by reduction of OH (∆G4 in Fig. 1) and not OOH (∆G1 of Pathway 1 in Fig. 1). Furthermore, weaker binding porphyrin such as Co has a high selectivity towards 2 electron reaction due to the weaker O binding and a different scaling relation between O and OH than on the metals, 37 which makes it difficult to improve the overpotential on single porphyrin sites. The inside of diporphyrins holds the promise to overcome the challenges on metals as well as on single porphyrin sites. On diporphyrins sites dissociation can be combined with weaker binding. The reason is that O2 in a µ-bridge binding between the two metals in a diporphyrin is structural similar to the dissociated oxygen as the oxygen atoms are not sharing metal site and does not have to move to a hollow site as is the case on metals. Therefore, diporphyrin can in principle dissociate O2 or OOH having a weaker binding of OH. However, the outside of the diporphyrins behaves as single porphyrin which means that the outside can by 2 electrons selective while the inside is 4 electrons selective, 38 therefore adding ligands on the outside can be important to obtain good selectivity. Here, O2 dissociation is not included since it is not potential dependent. At finite overpotentials the driving force for the reduction step increases and therefore at relevant potentials the reduction step is the relevant. Besides, it has been observed that sufficient vertical flexibility to bind and activate O2 substrate for diporphyrin catalysts since it involves a small change in conformational energy. 27–29,39 In this adsorption configuration, the O+OH binding energy can be tuned separately from the OH binding energy. Hence in this way, the scaling relation between OH and OOH can be circumvented and the overpotential be lowered below ∼0.4 V.

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The results are structured as follows: First we review the electrochemical framework for the ORR mechanism and obtain the 3D volcano. We present the results for the diporphyrin structures and a strategy to climb the ORR 3D volcano towards lower overpotentials, by utilizing geometric and electronic effects. We modify the electronic properties by varying the metal atoms as the active sites and by adding ligands. The geometric effect of the catalysts are studied by changing between three different diporphyrin motifs with different metal-metal distance.

Methods Theory The associative ORR path describes the activity of conventional surfaces well where the individual reaction energies, according to Eqs. (1) - (4), are given by

∆G1 = GOOH − GO2

(6)

∆G2 = GO − GOOH

(7)

∆G3 = GOH − GO

(8)

∆G4 = GH2 O − GOH

(9)

Gx is the formation energy of species x, x = [O2 , OOH, O, OH, H2 O] (See Computational Details for definition of adsorption and free energies). The ORR is studied in the framework of the computational hydrogen electrode (CHE). 4,13 For an ORR catalyst, the electrochemical potential is determined by the least exothermic step of the reaction mechanism

UORR = Min(|∆Gn |)/e

6


(10)

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where ∆Gn is the change in free energy of reaction step n, from Eq. (6)-(9) and e is the electron charge. The overpotential can be expressed as a function of only the OH and OOH binding energies, by using that the formation energy of OH is equal to half of the binding energy of O 13 GO ' 2GOH

(11)

and by applying the constraint that the total reaction energy is

∆GORR =

4 X

∆Gn = −4.92 eV

(12)

n=1

which is the standard Gibbs free energy of formation of H2 O with respect to O2 . This leads to the following expression for the overpotential

ηORR =1.23 V − [Min (|∆G1 |, |∆G2 |, |∆G3 |, |∆G4 |)] /e

(13)

=1.23 V − [Min (|GOOH − 4.92 eV|, |2GOH − GOOH |, | − GOH |, |0 eV − GOH |)] /e (14)

where 1.23 V is the thermochemical limit of the ORR, and H2 O is the reference. From Eq. (14), the overpotential can be plotted as a heatmap as a function of the OH and OOH binding energies (see Fig. 3). The heatmap forms a 3D volcano, with the top at ηORR = 0 V where GOH = 1.23 eV and GOOH = 3GOH = 3.69 eV. For two identical surfaces, infinitely separated, and where the OOH intermeditate is assumed to split into O+OH, the formation energy of O+OH can be written as the sum of the formation energy of the two intermediates independently, GO+OH = GO + GOH ' 3GOH using Eq. (11). However, for the molecular porphyrin motifs, the O and OH intermediates in O+OH will repel each other, due to the finite distance between the two metal sites and

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consequently we expect to find GO+OH bound between two extremes (for GOH < 1.6 eV):

GOH + 3.2 eV > GO+OH > 3GOH

(15)

which is shown with grey solid lines in Fig. 3. The 2D volcano 4,13 is obtained by projecting the heatmap onto the OOH–OH scaling relation in Eqn. 5, where the overpotential is constrained to be > 0.4 V. When the UORR is limited by ∆G4 , the catalyst is classified as strong binding and when limited by ∆G1 , it is is weak binding.

Structures Three different diporhyrin motifs are considered in this study, shown in Fig. 2. All the structures posses porphyrin sheets connected by an aromatic pillar. The pillar controls the distance between the two porphyrin sheets, thereby modify the geometry of the active site. The Pacman (Pac) molecule 26 has the shortest distance between the metal sites, ∼4.2 Å. In the diporphyrin anthracene (DPA) molecule 28,29,32 the distance between the two metals sites is ∼4.8 Å, and the diporphyrin dibenzofuran (DPD) molecule 29,32 has the longest metalmetal distance, ∼7.3 Å. The porphyrin sheets of the DPA and the DPD molecules are closer to parallel and more flexible than the sheets of the Pac molecule. The electronic properties of the catalysts can be modified by varying the 3d metal atoms 32 in the porphyrin sheets or by adding ligands to the metal sites. We consider metal combinations of Mn, Fe, Co, Ni and Cu, and ligands of pyridine (Py), 26,40 NH3 , OH, 41,42 F, Cl, 41,42 I and NCS, shown in Fig. 2. In this work we focus on the catalytic activity of the catalysts, and do not consider the stability under operating conditions. One exception however, is the case of OH ligands, which is discussed in the end of the Results and Discussion section.

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Pacman (Pac)

DPA

DPD

Ligands (R) Pyridine (Py)

Top

R

R 4.2 Å

Side R

NH3

R

NCS 4.8 Å

7.3 Å

R

R

OH X = F, Cl, I

Figure 2: The different diporhyrin motifs and ligands that are considered in this work. The Pacman (Pac), Diporphyrin anthracene (DPA) and Diporphyrin dibenzofuran (DPD) motifs are viewed from the top (upper) or the side (bottom). The intermetallic distance increases from Pac to DPA to DPD. R- indicates the coordination of the ligands. Metal atoms are pink, N blue, C grey, O red, H white and S yellow.

Computational Details The computational analysis was carried out using the grid-based projector-augmented wave (GPAW) method, a DFT code based on a projected augmented wave (all-electron frozen core approximation) method integrated with the atomic simulation environment (ASE). 43–45 The revised Perdew–Burke–Ernzerhof (RPBE) functional was used as an exchange-correlation functional. 46 The wavefunctions were represented on a uniform real-spaced grid with 0.20 Å grid-spacing. The electronic spins are treated separately, and a vacuum of minimum 7 Å was employed. The quasi-Newton minimization scheme was employed for the geometry optimizations, and the systems were relaxed until the forces were less than 0.1 eV/ Å. Structures, total energies, scripts to run calculations, and plotting methods are collected in the KatlaDB database available at this link: http://nano.ku.dk/english/research/ theoretical-electrocatalysis/katladb/. The formation energies of the ORR intermediates at 0 K are calculated with respect to

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H2 O(g) and H2 (g) in the following way:

EOH = E∗OH − E∗ − (EH2 O (g) − 1/2EH2 (g) ) EO = E∗O − E∗ − (EH2 O (g) − EH2 (g) )

(16) (17)

EOOH = E∗OOH − E∗ − (2EH2 O (g) − 3/2EH2 (g) )

(18)

EO+OH = E∗O+OH − E∗ − (2EH2 O (g) − 3/2EH2 (g) )

(19)

In order to obtain the Gibbs free energies at room temperature we have added zero point energy correction and entropy contributions according to Ref. 4, where H2 O(g) is considered at the vapor pressure of H2 O(l) at 300K. Furthermore we have accounted for water-induced stabilization of the OH and OOH intermediates corresponding to 0.3 eV per intermediate. 47 At U = 0 V the Gibbs free energies are given by:

GOH = EOH + 0.35 eV − 0.3 eV GO = EO + 0.05 eV

(20) (21)

GOOH = EOOH + 0.40 eV − 0.3 eV

(22)

GO+OH = EO+OH + 0.40 eV − 0.3 eV

(23)

The analysis of GOOH and GO+OH is identical and we therefore introduce a variable that refers to both formation energies, GOOH / O+OH . Following the CHE, 4,13 the Gibbs free energies can obtained for an arbitrary value of the potential according to

Gi (U ) = Gi − neU

where n is the number of electrons involved in the electrochemical reaction and e is the electron charge.

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Results and Discussion We report the ORR overpotential as a function of GOH and GOOH/O+OH for three different molecular diporhyrin catalysts with different combinations of metals and ligands in Fig. 3. The geometric effects move the catalyst to a new scaling relation, on which the catalysts can be tuned by electronic effects by changing metal atoms and ligands. By applying such modifications we obtain catalysts with a predicted ORR activity that is beyond the limitations of the scaling relation. All the results are plotted in Fig. 3 where pathways to climb the 3D volcano are indicated with arrows for different catalysts.

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2.2

Pac DPA DPD

1.8

4.0 H

OH

)

OH

OH

+

1.4 1.2

+F

+ Py

0.8

GO -O H

0.5

FeCu CoCu

CoNi CoCu CoCo CoCo FeNi

CoCo FeFe

FeFe No ligands Py NH3

FeFe

0.0

0.5

1.0

GOH [eV]

DPA CoCo

4.5

1.5

(d) CoNi

CoCo CoCu FeCu FeNi

4.0

FeFe IrCo

FeFe CoCo

3.5 FeCo 3.0 FeFe

X (X=F, Cl, I) Py-X (X=F, Cl, I)

2.5

0.0

1.5

GOH [eV]

FeFe

FeCo FeCo FeFe FeFe FeFe

3.0

(c)

1.0

No ligands Py NH3

FeFe

MnFe

2.5 0.0

X (X=F, Cl, I)

0.5

1.0

1.5

GOH [eV]

DPD

No ligands Py NH3

4.5

GOOH/O + OH [eV]

Pac

4.5

GOOH/O + OH [eV]

(b)

0.2

(O

(2)

0.0

0.4

+O H

(1)

2.5

4.0

0.6

)

DPD

DPA

GOOH/O + OH [eV]

1.0

H

(O

3.0

3.5

1.6

H

3.5

GO

=

GO

+ NH3

V

2e

3.

+O

GOOH/O + OH [eV]

Pac CoCo

Pac FeFe

2.0

DPA

GO

4.5

=3

(a)

ORR (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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X (X=F, Cl, I) Py-X OH NCS

4.0

3.5

FeCo FeCo CoCo CoCu CoCo CoNi FeFe FeFe MnFe FeFe FeNi/FeCu FeCo FeCo FeCo FeCo FeFe MnMn FeCo FeFe FeFe MnFe

MnCo FeFe 3.0

FeCo FeCo

FeFe FeFe MnFe

2.5 0.0

0.5

MnFe 1.0

1.5

GOH [eV]

Figure 3: GOOH / O+OH against GOH for the Pac, DPA and DPD catalysts with different combinations of metal-metal centres (eg. CoCu) and ligands (see Fig. 2), on a heat map of the overpotential corresponding to the pair of adsorption energies (Eq. 14). The solid lines marks the region in which the catalysts are expected to be found, according to Eqn. 15. (a) Scaling relation between GOOH /GO+OH and GOH , are shown in grey dashed lines for 1) the Pac motif and 2) the DPD motif. The white dashed lines indicate the border of what reaction step that determines the overpotential. Catalysts with the FeFe combination are red and catalysts with the CoCo combination are purple, all with different motifs and ligands. The remaining catalysts are shown in grey for clarity. The arrows show the activity change in the FeFe and CoCo catalysts upon moving from Pac to DPA to DPD and upon addition of ligands. Ligands coordination are demonstrated without contour outside the markers. (b), (c) and (d) are the heat maps of Pac, DPA and DPD catalysts respectively with specific labels and ligands are demonstrated with different colors.

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Geometric and Electronic Effect The pristine Pac catalysts i.e. without ligands, (Fig. 3(a), circles with contour) follow the scaling relation of the associative pathway in Eq. (5), as the OOH intermediate is more stable than the splitted O+OH intermediate. For the catalysts on the strong binding side of the volcano (FeFe, FeCo, FeCu, FeNi and CoCo), we observe a destabilization of the OOH intermediate due to steric hindrance from a narrow intermetallic distance. Consequently are the pristine Pac catalysts limited to the 2D volcano and would operate at overpotentials >0.4 V. Upon addition of ligands to the Pac catalysts, the intermetallic distance is increased by 0.2-0.3 Å (see Fig. S1). This allows for a stabilization of the O+OH intermediate, compared to OOH, due to the lowered repulsion between O and OH. This is only observed for the metals on the strong binding side of the 2D volcano. To illustrate the geometric effect of opening the Pac structure, a fitting line is shown in Fig. 3(a) of GO+OH as a function of GOH for the Pac FeFe catalysts with different ligands (Py, NH3 , F, Cl). This is denoted scaling relation (1), and is given by GO+OH = 3GOH + 1 eV (R2 ' 0.8). This allows for a ∼ 0.15 V decrease in the overpotential for the optimal catalyst obeying this scaling relation. As the intermetallic distance is increased further with the DPD motif (∼3.1 Å increase compared to the Pac motif) we obtain scaling relation (2), fitted for the FeFe metal combination with different ligands (Py, NH3 , OH, NCS, F, Cl, I). The scaling relation is GO+OH = 3GOH + 0.7 eV (R2 = 0.8) which allows for ∼0.2 V decrease in the overpotential for the optimal catalyst compared to the 2D volcano. The intercept for the two scaling relations can be interpreted as a measure of the repulsion between O and OH, and has been lowered from 1 eV to 0.7 eV from Pac with ligands to DPD. The O+OH intermediate has consequently been stabilized by increasing the intermetallic distance. For catalysts that include either a Ni or a Cu atom, which are weak binding metals, the ORR will follow the associative pathway and be constrained to the 2D volcano. The reason is that the weak binding metals cannot stabilize the separate O+OH intermediate 13



and they will therefore behave as a single metal porphyrin catalyst with the activity of the most reactive metal in the structure (shown in Fig. S2). Circumventing the scaling relation between OH and OOH is the first step in climbing the 3D volcano. This shifts the catalyst to a new scaling relation where OOH splits into O+OH, when two requirements are satisfied: 1) the intermetallic distance should be larger than in the pristine Pac motif and 2) the metals should bind strong enough. The reactivity of the catalyst can be tuned along the new scaling relation by electronic effects to achieve optimal binding energies.

Climbing the 3D Volcano In this section, we demonstrate that it is possible to climb the 3D activity volcano by applying a combination of geometric and electronic effects to the catalyst. The CoCo and the FeFe metal combinations with different motifs and ligands are used as examples, which are illustrated with arrows in Fig. 3(a). The most straightforward approach to obtain a catalyst with η = 0.0 V is to stabilize the O+OH intermediate with the ideal OH binding energy, GOH = 1.23 eV. In this strategy, the catalyst is initially found on the weak binding side of the volcano. For such catalysts, the O+OH intermediate cannot be stabilized by geometric effects alone, as increasing the intermetallic distance shifts the activity towards that of a single porphyrin motif, following the OOH–OH scaling relation in Eq. 5. The Pac CoCo catalyst is found near the optimum of the 2D volcano on the strong binding side (GOH = 0.71 eV). Following the arrows shown in Fig. 3(a), we find that changing to the DPA geometry (DPA CoCo) only shifts the catalyst along the OOH–OH scaling relation to GOH = 1.09 eV with 0.67 V (vs. SHE) overpotential close to the experimental value (0.63 V vs. SHE) observed for diporpoyrin xanthene. 29 For DPD CoCo, a theoretical reduction potential of 0.67 V is calculated which is very close to the experimental value 0.59 V (vs. SHE). 29 It demonstrates that further increase of the intermetallic distance does not help for the ORR activity. Since GOH of DPA CoCo is close to the optimal value on the weak binding 14


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side, the right ligand has to be applied in order to stabilize the O+OH intermediate. With the NH3 ligand, the DPA CoCo is shifted close to the top of the 3D volcano, by stabilization of GO+OH with 0.56 eV and stabilization of GOH by 0.05 eV. The DPA CoCo-NH3 has an overpotential of η = 0.15 V. We note that the NH3 ligand has a different effect on the DPA motif than the Pac motif, where the ligand weakens both GOH and GOOH along the OOH–OH scaling relation. The Pac FeFe is found on the strong binding side of the OOH–OH scaling relation, binding OH with GOH = 0.0 eV. For this catalyst, the OOH–OH scaling relation is circumvented by adding a ligand or by changing to the DPA structure, which both increases the intermetallic distance. The arrows from Pac FeFe in Fig. 3(a) indicates how the addition of Py and the change to DPA or DPD weakens GOH and stabilizes GO+OH . Upon changing to the DPD motif, the catalyst now follows the new O+OH-OH scaling relation (2) (dashed grey line). DPD FeFe is found at GOH = 0.74 eV and GO+OH = 2.77 eV. In Fig. 4 a free energy diagram for the ORR mechanism on the different motifs with FeFe is shown at an applied potential of U = 1.01 V, where all the steps for the DPD FeFe-F system are exergonic. The free energy diagram reveals how much the energetics of the FeFe system can be modified with geometric and electronic effects, to shift the catalyst to the top of the 3D volcano. We find a significantly lower overpotential than the 2D volcano allows for, with DPD FeFe-F (η = 0.22 V), DPD FeCo-(OH or F) (η = 0.32 V and η = 0.23 V respectivly) and DPD MnMn-I (η = 0.25 V). In all cases GO+OH and GOH is appropriately weakened by strongly electronegative ligands. Until now we have not considered the stability of the catalysts under operating conditions. This may be justified when considering conventional ligands such as Py. 26,40 However, when using OH as a ligand the question of stability must be addressed. Below we report the

15



1.5

+* O

H

ideal Pac FeFe Pac FeFe-Py DPA FeFe DPD FeFe DPD FeFe-F

H

*+

2H

2O

*O

0.5 4.92 - 4*U eV

*O

/*O H*

OH

*O OH /*O

*+ O2 U = 1.01 V

Free energy [eV]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

1.5

Reaction Coordinate

Figure 4: ORR free energy diagram for the different FeFe motifs plotted at U = 1.01 V where all the steps for the DPD FeFe-F catalyst are exergonic. The Pac motif is on the strong binding leg of the 2D volcano as the potential limiting step is either of the last two steps where the desorption is endergonic. The pristine DPA and DPD motifs both bind the O+OH intermediate too strongly, but adding F as a ligand to DPD FeFe results in a catalyst with GO+OH closer to the optimal (η =1.23 V−1.01 V=0.22 V). binding energies for OH as a ligand on the outside of DPD FeCo at UORR = 0 V: Fe + Co + H2 O −−→ Fe −OH + Co + (H+ + e− ) Fe + Co + 2 H2 O −−→ Fe −OH + Co −OH + 2 (H+ + e− )

∆G = 0.84 eV

(24)

∆G = 2.25 eV

(25)

Adding one OH ligand on the Fe atom is stable above 0.84 V whereas adding an OH ligand on both the Fe and the Co atom is stable above 1.13 V. Therefore, we have to consider the ORR energies for the catalyst having a single OH on the Fe atom and nothing on the Co atom (DPD Fe-OH,Co) (see Fig. S8-S9). For DPD Fe-OH,Co, the highest attainable potential is

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UORR = 0.95 V. At this potential, the reaction energy is -0.11 eV for binding one OH on the Fe atom (Eq. (24)) and +0.35 eV for binding two OH on the Fe and Co atoms (Eq. (25)). As a result, the DPD Fe-OH,Co structure is stable at a potential of UORR = 0.95 V and can be considered as a prosmising catalyst under operating conditions with an overpotential of 0.28 V (Fig. S8-S9).

Conclusion Our study shows that a minimal overpotential of the ORR can be achieved by stabilization of the O+OH intermediate over the OOH intermediate. The diporphyrin molecular catalysts posses a geometry that enables this stabilization. The ORR activity of three different diporphyrin motifs (Pac, DPA and DPD) were studied by varying the metal site combinations and the ligands, at different intermetallic distances, in order to circumvent the limitations of the scaling relation between GOH and GOOH . For metal combinations of strong binding metals, the OOH intermediate can be stabilized as O+OH by geometric effects alone. For such catalysts, changing the geometry modifies the catalyst such that it follows a new scaling relation between GOH and GO+OH , indicated by line (1) and (2) in Fig. 3(a). As the intermetallic distance increases, the repulsion between O and OH decreases in the O+OH intermediate and the scaling relation approaches the idealized line GO+OH = 3GOH , where the optimal catalyst will lie. For metal combinations of weak binding metals, ligands are required to stabilize to O+OH intermediate over the OOH intermediate in addition to geometric effects. A catalyst can be shifted along the scaling relation, given by its geometry, by adding ligands or changing the metal combination. By such geometric and electronic modifications, a catalyst can be shifted to the top of the 3D volcano. Specifically the DPA CoCo-NH3 , DPD FeFe-F and DPD FeCo-(OH or F) catalysts were found to reduce the overpotential to 0.15 V, 0.22 V, 0.32 V and 0.23 V respectively. The DPD Fe-OH,Co is particular promising as it could be stable in water at the operating overpotential of 0.28 V. Our findings lead to

17



a design principle for dual site catalysts: The intermetallic distance should be above 4.2 Å, which is the distance of the Pac motif, although it should not be too large as to impede the dissociation of either O2 or OOH. Our findings show that ligands tends to make a catalytic site less reactive. Therefore, diporphyrin catalysts containing combinations of strong binding metals (Mn, Fe or Co) along with the right combination of ligands, have the potential to make catalysts performing beyond the limitation of the 2D volcano. The bimetallic molecular catalysts are promising candidates for highly efficient ORR catalysts, as they posses a flexible and modifiable geometry that allow for new ORR intermediates that circumvent the limitations of traditional catalysts. The catalytic activity of a diporphyrin motif is highly tunable with variable metal centers and ligands, and thus a zero-overpotential catalyst may be engineered from this class of molecular catalysts.

Acknowledgement We acknowledge support from the Danish Council for Independent Research Sapere Aude Program, Grant No. 11-1051390, research grant 9455 from VILLUM FONDEN and Innovation Fund Denmark (grand solution ProActivE 5124-00003A). The Center for Nanostructured Graphene is sponsored by the Danish National Research Foundation, Project DNRF58.

Supporting Information Available The more detailed illustration for the effects from ligands coordination, the tendency of the OH adsorption energy and the reaction path.

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Graphical TOC Entry

A zero-overpotential catalyst for ORR may be engineered from this class of diporphyrin molecules.

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Climbing the 3D Volcano for the Oxygen Reduction ... - ACS Publications

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