Building Pyridinium Molecular Wires as Axial Ligands for Tuning the

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Building Pyridinium Molecular Wires as Axial Ligands for Tuning the Electrocatalytic Activity of Iron Phthalocyanines for the Oxygen Reduction Reaction Ana Pizarro, Gabriel Abarca Anjari, Cristian Gutiérrez-Cerón, Diego Cortés-Arriagada, Fabiano Bernardi, Cristhian Berrios, J. Francisco Silva, Marcos Caroli Rezende, Jose H. Zagal, Ruben Oñate, and Ingrid Ponce ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01479 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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ACS Catalysis

Building Pyridinium Molecular Wires as Axial Ligands for Tuning the Electrocatalytic Activity of Iron Phthalocyanines for the Oxygen Reduction Reaction. Ana Pizarro†, Gabriel Abarca⊥, Cristian Gutiérrez-Cerón†, Diego Cortés-Arriagadaξ, Fabiano Bernardi‡, Cristhian Berrios†, Juan F. Silva†, Marcos C. Rezende†, José H. Zagal*†, Rubén Oñate†*, Ingrid Ponce†*.

AUTHOR ADDRESSES † Facultad de Química y Biología, Universidad de Santiago de Chile. Avenida Libertador Bernardo O’Higgins 3363, Casilla 40, Correo 33, Santiago 9170022, Chile. ⊥Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor, Chile. Camino la Pirámide 5750, Huechuraba 8580745, Santiago, Chile. ξ Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación. Universidad Tecnológica Metropolitana. Ignacio Valdivieso 2409, P.O. Box 8940577, San Joaquín, Santiago, Chile. ‡ Institute of Physics, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, 91501970, RS, Brazil.

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ABSTRACT We have been able to “tune” the electrocatalytic activity of iron phthalocyanine (FePc) and iron hexadodecachlorophthalocyanine (16(Cl)FePc) for the oxygen reduction reaction (ORR) by manipulating the “pull effect” of pyridinium molecules axially bounded to FePc. These axial ligands play both the role of molecular anchors and also of molecular wires. The axial ligands also affect the reactivity of the Fe metal center in the phthalocyanine. The "pull effect" originates from the positive charge located on the pyridinium core. We have explored the influence of the core positions (Up or Down), in two structural pyridiniums isomers on the activity of FePc and 16(Cl)FePc for the ORR. Of all self-assembled catalysts tested, the highest catalytic activity was exhibited by Au(111)/Up/FePc system. XPS measurements and DFT calculations showed that it is possible to tailor the FePc-N(Pyridiniums) Fe-O2 binding energies, by changing the core positions and affecting the “pull effect” of pyridiniums. This affects directly the catalytic activity of FePcs. The plot of activity as (log I)E versus the calculated Fe-O2 binding energies gives an activity volcano correlation indicating that an optimum binding energy of O2 with the Fe center provides the highest activity.

KEYWORDS Iron phthalocyanines, pyridiniums, pull effect, axial ligand, electrocatalytic activity, oxygen reaction reduction, Fe-O2 binding energy, volcano correlations.


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1. INTRODUCTION

The oxygen reduction reaction (ORR) is an essential electrochemical process that plays a key role in energy conversion, especially in fuel cells and air batteries. Besides energy conversion, ORR is also relevant in corrosion, which is essentially an energy dissipation process.1–4 Even though the O2 reduction in energy conversion devices is a spontaneous process its kinetics on most electrode materials are rather slow and to proceed at rates compatible with energy conversion requires catalysts.5–7 Up to know the more active known catalytic materials contain platinum but this metal is scarce and costly, so considerable efforts have been made by several groups over the years to develop ORR non-precious metal catalysts (NPMC).8–16 The lack of practical alternative materials to Pt has prevented in part the widespread use of fuel cells. Thereby, the development of new kind of O2 cathodes based on low cost industrial raw materials with acceptable ORR catalytic activity is necessary. In this way, several research groups have attempted for many years to develop non-precious-metal catalysts (NPMCs) as alternative materials to Pt and their alloys. 8–26 One particular group of NPMCs are macrocyclic MN4 complexes.8–16,24,25 These kinds of molecules mimic bio-catalysts of O2 reduction like cytochrome-c in heme-proteins which contains a central FeN4 center.10 These complexes have been examined as potential catalysts for the ORR, both in acid and alkaline media.27 Most macrocyclic complexes studied have involved transition-metal phthalocyanines (MPcs),8–16,24,25 metal porphyrins (MPs),28–30 and similar molecules21,25,26 supported or adsorbed on carbon and graphitic materials. However, they still lack the necessary long-term stability for fuel operation. In cytochrome-c, which is a Fe porphyrin, the presence of an axial ligand plays a crucial role in activating the Fe center for the reduction of O2.10,25 The key effect of this axial ligand on the electrocatalytic activity for ORR has been investigated by few authors and only rather recently.16,31–34 The axial ligand in self-assembled systems of MN4-


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complexes can serve as an anchoring molecule for MN4 macrocyclic complexes to an electrode surface and plays a key role in modifying the electrocatalytic activity of these complexes for the ORR.16,28,29,31–34 Thus, the axial ligands essentially play three roles: (i) it anchors the complex to the electrode surface, (ii) it acts as a molecular wire facilitating electron transport between the electrode and the metal center and (iii) it modifies the electron density of the metal center and changes its reactivity. The role of both axial ligands and substituents on the ligand in modifying the redox activity of the metal center in phthalocyanines has been discussed by Lever.35,36

The effects of an axial ligand and the substituents on the ligand are both reflected in changes in the Mn+/M(n+1) redox potential which is a very important reactivity predictor.37 This reactivity predictor is directly correlated with M-O2 binding energy,37 a classical well-known reactivity predictor for metals and alloys but less studied for MN4 metal complexes.6,7,9,11,37 In the present case, the axial ligand decreases the electron density of the Fe center, in iron phthalocyanine (FePc), and changes the Fe-O2 binding energy compared to the FePc molecule directly adsorbed on the gold surface, without any axial ligand. Therefore, changes in this parameter will affect the catalytic activity for ORR of the metal chelate directly.37 To the best of our knowledge, the role of the fifth ligand on the metal center on the M-O2 binding energy and subsequently on the catalytic activity has not been quantified before. On the other hand, the catalytic activity of the MN4-complexes, such as iron phthalocyanines (FePcs) involving the formation of charge transfer intermediates, is directly related with the ability of the complex to “bind” extraplanar ligands like O2.11,31,38 Plots of catalytic activity of several metal porphyrins as log(i)E and metal phthalocyanines versus the M-O2 binding energy give volcano correlations.37 Our results, essentially confirm the well-known Sabatier Principle for heterogeneous catalysis that to achieve the highest activity the binding energy of the key intermediate to the active site needs to be not too strong not too weak.37 The


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correlations reported by Zagal and Koper37 have been obtained with those MN4 complexes directly adsorbed on graphite surfaces, and there are no similar studies performed on gold surfaces modified with metal chelates. In this work, the electrocatalytic activity of FePc and the binding of O2 to the active sites (Fe-center) is altered by the presence of a fifth extra-planar ligand positioned on the axial position of the FePc complex, see Chart 1 below.

For example, well-established electrocatalytic molecular platforms involving FePc anchored on single-walled carbon nanotubes by an axial ligand,16,27 or on FePc anchored on gold surface by axial ligation using self-assembled monolayers, the activity for ORR is enhanced by the presence of the axial ligand compared to that for the FePc directly adsorbed on the electrode surface.16,31–33 Moreover, the activity of FePc can be modulated by the effect of peripheral and non-peripheral substituents on the macrocyclic ligand, which allows the possibility of tuning the Fe-O2 donor-acceptor intermolecular hardness and the binding energy.37 Electron-withdrawing groups like chlorine atoms remove electron density from the metal center, resulting in a shift of the Fen+/Fe(n-1)+ redox processes to more positive values. The presence of these substituent groups increases the catalytic activity of the FePc for ORR. A plausible explanation for this phenomenon has been explained by the activity volcano correlation reported recently.37 FePc are located on the strong M-O2 binding leg of the volcano so weakening the M-O2 bond favors the catalysts as the catalyst moves up on that side of the volcano to higher activities. Hypothetically, if the electronwithdrawing effect were too strong, the new catalyst will climb the volcano and fall into the weak binding leg with a lower activity. It is important to point out that similar correlations are observed for pure metals.39 Pt appears on the active binding side of the volcano but close to the top. In contrast, Au appears on the weak binding side and is less catalytic. Alternatively, the catalytic activity of FePc for the ORR can be explained regarding of the intermolecular hardness of the


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system, strictly speaking, less separation between the energies of the frontier orbitals of the donor (FePc) and the acceptor (O2), will result in a higher reactivity.11,36 Hence, electron-withdrawing groups on the ligand decrease the gap between the energy of the frontier orbitals. Cao et al. 16 observed by XPS that for the highly catalytic FePc-Py-CNTs hybrid material a higher oxidation state of iron is observed in comparison with FePc directly adsorbed on the surface. The indicated behaviour can be attributed to the coordination of the electron-withdrawing pyridine axial ligand.16 The pyridine axial ligand then decreases the electron density of the metal center in a similar way to the role of electron-withdrawing groups located on the ligand, for example, chlorine atoms in 16(Cl)FePc. The aforementioned performance favors the donor-acceptor electronic coupling and results in a higher activity for ORR.11,37 Due to the reasons discussed above; we have attempted to quantify the effect of the “axial ligand” in FePc to tune and optimize the electrocatalytic activity of complexes of this type for ORR. We have built 4 self-assembled systems of FePc anchored to an Au(111) electrode surface using pyridinium molecules, denoted as Au(111)/pyridinium/FePc systems. Pyridiniums are functional molecular entities which can exert a “pull effect” due to their electron acceptor properties.40–43 This pull effect is attributed to the fact that pyridiniums are heterocyclic compounds consisting of 6-membered rings, with a positively charged nitrogen, called core pyridinium, Chart 1. To unravel the effect of the axial pull effect on the activity of FePc for ORR in self-assembled systems, we have designed and synthesized two structural pyridiniums isomers, where the only difference in the molecular backbone is the core positions (Up or Down), Chart 1.


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ACS Catalysis

Chart 1. Structure of all molecular systems studied.

2. MATERIALS AND METHODS

Materials and equipment for Synthesis. Melting points were recorded on a capillary microthermal instrument and were not corrected. 1H-NMR and 13C-NMR spectra were recorded on a Bruker Avance equipment (1H-NMR at 400MHz and 13C-NMR at 100MHz). Mass spectra were obtained with a Micromass Q-Tof micro™ spectrometer, UV-Vis were obtained with Scinco3100 spectrophotometer.


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All solvents and BF3-OEt2 were purchased from Merck and used as provided. Acetophenone, 4pyridinecarboxaldehyde, 4-(methylthio)benzaldehyde, 4-(methylthio)aniline and pyridin-4-amine were purchased from Aldrich and were used as received unless otherwise specified.

Synthesis

procedure

and

characterizations

(Scheme

1).

Pyrylium

salt

1

4-(4-

(methylthio)phenyl)-2,6-diphenylpyrylium tetrafluoroborate was prepared according to a published procedure.44 2,6-diphenyl-4-[(1H)-pyridinium-4-yl] pyrylium bis(tetrafluoroborate) 2 and 1-(4-methylthiophenyl)2,6-diphenyl-4-(4-pyridyl) pyridinium tetrafluoroborate Down were prepared previously for us.31

1-(4-pyridyl)-2,6-diphenyl-4-[4(methylthio)phenyl]

pyridinium

tetrafluoroborato,

Up.

Pyrylium salt 1, 0.4852 g, 1 mmol, was added to a stirred solution of freshly prepared pyridin-4amine (0.1393 g, 1 mmol) in 7 mL of ethanol and the mixture was heated at reflux for 8 h. After cooling to room temperature, solution was poured into 50 mL of diethyl ether. The yellow precipitate obtained was filtered off, dried and purified by flash chromatography on silica gel with CH2Cl2, as eluent to give 0.4248 g (82% yield) of compound Up. mp >280°C. ESI-MS [M-BF4-]+ 431.1582 C29H23SN2+, requires 431.1576. 1H RMN (400 MHz, DMSO-d6) δ 8.72 (s, 2H), 8.45 (d, J = 6.2 Hz, 2H), 8.38 (d, J = 8.6 Hz, 2H), 7.57 – 7.38 (m, 14H), 2.61 (s, 3H). 13C RMN (101 MHz, DMSO-d6) δ 155.90, 155.73, 150.95, 146.82, 146.42, 133.04, 130.19, 129.70, 129.23, 128.75, 126.40, 124.53, 123.97, 14.37.


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ACS Catalysis

Scheme 1. Synthesis of Up/Down pyridiniums.

Electrochemical methods. The electrochemical measurements were performed with a BioAnalytical Systems, BasI-Epsilon electrochemical work-station, using a conventional Pyrex glass cell with 5 entries. The glass cell was immersed in a thermostat bath to control the temperature at 25ºC. The working electrode was a thin Au film deposited on glass (12 ×12 mm slides purchased from Arrandee, Germany). The reference electrode was Ag/AgCl (3M), and a platinum wire of 10 cm2 square area served as the counter electrode. For readability, we have aligned the potential to the RHE scale. Tests of the surface characterizations were performed in freshly prepared phosphate buffer (pH=5). Electrocatalytic activities for O2 were measured in a 0.1 M NaOH solutions. Ultrapure water was used for preparing all solutions and was obtained with a MilliporeMilliQBiocel Ultrapure water system fed with distilled water). Before performing electrochemical measurements, solutions were purged with ultrapure N2 or ultrapure O2, depending on the experiment, during 30 min. prior to each measurement. FePc was obtained from Aldrich and 16(Cl)FePc from PorphyChem. All other reagents were of analytical grade and used without further purification unless otherwise specified. Gold slides were annealed using an H2 flame to obtain a preferential Au(111) orientation.


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Au/FePcs and Au(111)/Down(Up)/FePcs electrodes were prepared and characterized (see SI) according to our previous work.31,32 To obtain Au(111)/16(Cl)FePc electrodes, the Au(111) electrodes were placed for 12 h in a freshly prepared 1 mM solution of 16(Cl)FePc in dry THF, under a nitrogen atmosphere. The electrodes were rinsed with pure THF to eliminate any excess of phthalocyanine molecules. The Self-assembled monolayers, SAMs, of Down(Up) on Au(111) surface, Au(111)/Down(Up) systems, were prepared as follows: after annealing, the Au(111) slides were submersed in an ethanol 50 μM solution containing Up or Down pyridinium for 24 h. After this period, the electrodes were further rinsed with ethanol for 20 min and dried using a nitrogen flow. The SAMs were characterized electrochemically by cyclic voltammetry technique, CV33,45,46 in a 0.1 M NaOH solution saturated with N2 at a scan rate of dE/dt= 0.05 V/s. The surface characterization was conducted using tunneling scanning microscopy technique, STM, with an ECM-2 microscope (Veeco, U.S.), employing commercial cut Pt−Ir tips (Veeco probes, 0.25 mm in diameter, stored in O2-free atmosphere before use). An off-line plane-fit and low-pass filter was applied to the STM images. The Au(111)/Down(Up) electrodes were further modified with 16(Cl)FePc by incubating the preformed Au(111)/Down(Up) for 6 h in a 1 mM solution of 16(Cl)FePc in dry THF. After that, the Au(111)/Down(Up)/16(Cl)FePc samples were submersed for 5 min. in dry THF under a smooth flow of pure N2. After all of these procedures, the electrodes were used immediately for the different electrochemical testings. The modified electrodes were characterized using Square Wave Voltammetry Measurements conducted in pH= 5 phosphate buffer solutions saturated with N2, 25 mV and 15 Hz. Polarization curves for measuring the electrocatalytic activity of ORR were performed in a O2 saturated 0.1 M NaOH solution at a scan rate of dE/dt= 0.005 Vs-1.


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Experimental and Details of XPS measurements. For the XPS measurements, the Au(111)/Down(Up)/FePcs samples were introduced into the analysis chamber at the D04A-SXS beamline end station at LNLS.20 The O2 treatment at room temperature (RT) was accomplished in the preparation chamber using a pressure of 20 psi of 5% O2 + 95% He during 1 h. The samples were investigated using the long scan, Fe 2p, Au 4f, O 1s, N 1s and C 1s scan regions. The spectra were collected using an InSb (111) double crystal monochromator a fixed photon energy of 1840 eV. The hemispherical electron analyzer (PHOIBOS HSA500 150 R6) was set at a pass energy of 30 eV, and the energy step was 0.1 eV, with an acquisition time of 100 ms/point. The base pressure used inside the chamber was around 2.0×10-9 mbar. The monochromator photon energy calibration was done at the Si K edge (1839 eV). An additional calibration of the analyzer’s energy was performed using a standard Au foil (Au 4f7/2 peak at 83.8 eV). We also considered the C 1s peak value of 284.6 eV as references to verify possible charging effects. The XPS measurements were obtained at a 15° take-off angle at room temperature. XPS Peak version 4.1 was used to fit the XPS results. All peaks were adjusted using a Shirley type background and an asymmetric Gaussian-Lorentzian sum function (20% Lorentzian contribution). For the fitting procedure, the FWHM value of a given component was kept constant over the adjustment procedure. For the Fe 2p3/2 region, the intensity ratio and energy separation between the main component and the satellite component were kept constant for all the cases. The intensity of each chemical component in the N 1s region was constrained to the value expected and given by the number of equivalent N atoms in the structure (4:4:1:1 for C-N=C: N-Fe (Pc): N-Fe (Py-Pc): N core pyridinium).

Theoretical Calculations. Density functional theory calculations were performed in the ORCA 3.0.3 program;47 the PBE48 functional was used in combination with the all-electron Def2-SVP basis sets for all the atoms.49 The PBE functional was selected due to its application in studies


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related to FePc complexes.50,51 Dispersion force corrections for energies and gradients were included using the DFT-D3 method, in combination with the Becke-Johnson damping function.52,53 Basis set superposition errors were also corrected by the geometrical counterpoise correction.53 Geometry optimizations were performed without symmetry or geometry constraints. Interaction energies (Eint) were obtained as Eint  EA B  ( EA  EB ) EA, EB and EA-B correspond to the total energies of the fragment A, B, and the A-B adduct, respectively; the lower the value of Eint, the more substantial interaction strength. interaction strength. interaction strength.

3. RESULTS AND DISCUSSION

The effect on the electronic properties, of both pyridiniums, due to the structural change of core positions in the molecular backbone was corroborated using UV−vis measurements, Figure 1. The UV-vis spectra of pyridiniums show three bands with very different intensities of absorbance, principally in the medium and lowest energy transitions, 315 and 387 nm. These electronic transitions were assigned using TD-DFT, essentially as intramolecular charge transfer involving aryl substituents at positions 2,6 for lowest energy band and position 4 for the medium band to the pyridinium core.40,41 Highest energy band agrees with π→π* electronic transition of pyridinium core.42,43 The intensity of lower and medium energy transitions are drastically reduced in Down-pyridinium and may be due to its enhanced intramolecular steric hindrance with the N(4-(methylthio)phenyl) fragment and therefore a higher rotation of phenyls at 2,6 positions.43 Therefore, changing the core positions in the molecular backbone is an exciting strategy to modify their electronic features and provide an excellent platform to explore the effect of the axial ligand in the self-assembled system of FePc.22


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ACS Catalysis

Figure 1. UV-Vis spectra of Up and Down pyridiniums, red and blue line, in acetonitrile solution, [c=5x10-5M].

The formation of Up and Down SAMs on Au(111) surface was confirmed through the observation of electro-reductive desorption processes of the SAMs from the Au(111) surfaces by cyclic voltammetry technique, CV, (Figure S4, in SI). The CV technique is usually employed to characterize SAMs with -SH, -SCH3 and thiophene functional linker groups on Au(111) surface in electrolyte solutions by obtaining the electro-reductive desorption processes.27,45,54,55 By the integration of the cathodic voltammetric peak from electro-reductive desorption processes was possible to determine the SAMs coverage of Up and Down SAMs on Au(111), (see Figure S4 and Table S1, in SI). Thus, the charge density (q) can be used to estimate the coverages values from the cathodic peaks corresponding to the electro-reductive desorption curves of the Up and Down pyridiniums27,45,54 (ec. S.1 in SI). The coverages values of both SAMs (Up and Down) are in the order of a monolayer (4.62 x 10-10 mol/cm2 - 4.97 x 10-10 mol/cm2 for Up and Down respectively, Table S1 in SI). The morphological characterization of the new system Au(111)/Up was carried out using tunneling scanning microscopy technique, STM, see Figure S4 c-d in SI. The images of


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Au(111)/Up. The surface structure has a high coverage with holes which represent vacancy islands and the presence of aromatic groups in the structure, which exhibits starry shapes attributed to the interaction of neighboring groups.45 Au(111)/Down and its analogous systems (with functional linker group -SH) have been previously well characterized by STM.31,56

To explore the influence of the “pull effect” of the axial ligand, on the catalytic activity of FePc and 16(Cl)FePc for ORR, self-assembled Au(111)/Down/FePc, Au(111)/Up/FePc, Au(111)/Down/16(Cl)FePc and Au(111)/Up/16(Cl)FePc systems were constructed using the “Up” and “Down” orientations as illustrated in Chart 1. The surface electrochemical characterization was carried by cyclic voltammetry (CV) and square wave voltammetry (SWV), in pH=5 phosphate buffer solution satured whit N2 according to Tebello.57,58 Using CV technique, it was not possible to determine the Fe (II)/(I) and Fe (III)/(II) redox process for FePc on self-assembled system, Figures S5 a and c. However, by changes in the capacitive currents we have monitored the surface modification. Thus, the reduction of the capacitive current was observed after the modification with Down(Up) SAMs. It is a common phenomenon for SAMs and confirms the electrode modification.59 In Au(111)/Down(Up)/FePc there was an increase in the capacitive charging current of the electrode (Figure S5). By SWV technique, Figure 2, it was possible to determine the presence of FePc and 16(Cl)FePc on the gold electrode surface for the different systems. This is a more sensitive technique, that allows to detect redox species, in low concentration on the electrode surface. The SWV responses for the Au(111)/Down(Up)/FePc show two peaks. The first peak near to +0.039 V and +0.065 V, are attributed to the Fe (II)/(I) redox process, and the second peak near to +0.862 V and +0.847 V to the Fe (III)/(II) redox process,38 Figures 2 a and c. The Au(111)/Down(Up)/16(Cl)FePc systems show the peaks at -0.082 and -0.086 V are attributed to Fe (II)/(I) redox process. The peaks +0.714V and +0.714 V are attributed to Fe (III)/(II) redox


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ACS Catalysis

process,60 Figures 2 b and d. To allocate FePc and 16(Cl)FePc redox processes, we have use the Pourbaix diagram for the metal-centered redox processes of different iron phthalocyanines adsorbed on an OPG electrode from reference.38,60

Figure 2. Square wave voltammetric responses for Au(111), Au(111)/Down(Up), Au(111)/FePcs and Au(111)/Down(Up)/FePcs systems: a- Au(111)/Up/FePc. b- Au(111)/Up/16(Cl)FePc. cAu(111)/Down/FePc. d- Au(111)/Down/16(Cl)FePc. Measurements conducted in pH=5 phosphate buffer solution satured whit N2; 25 mV and 15 Hz, 25°C.

The activity of these self-assembled systems for ORR was compared with that of FePc adsorbed directly on the Au(111) surface, see Figure 3a. The catalytic activity for ORR of FePc was the


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highest of all systems studied when the self-assembled system was built using the Up-pyridinium orientation. It is then possible to “tune” or “tailor” the catalytic activity of FePc by changing the configuration of the core positions on the axial ligand. Moreover, this observation corroborates that the axial “pull effect” is an efficient way of increasing the catalytic activity of FePc, and possibly of other similar metal complexes.39 It is important to point out that from previous work,35 a volcano-shaped curve is obtained when comparing the catalytic activity of a great variety of MN4 metal complexes adsorbed on graphite for ORR versus the M-O2 binding energy calculated theoretically. The volcano correlation has two sides, one side for the strongly binding complexes and the other for the weakly binding complexes. On the strongly binding side, the activity increases as the M-O2 binding energy decreases. FeN4 complexes sit on the strongly binding side of the volcano. For the present case, electron-withdrawing groups or ligands (compared to unsubstituted FePc) should increase the catalytic activity since hypothetically they decrease the Fe-O2 binding energy. Surprisingly, when the self-assembled systems were prepared using 16(Cl)FePc, having electron-withdrawing groups on the ligand, the activity for ORR drastically decreased compared with the activity of that of 16(Cl)FePc adsorbed directly on Au(111) surface. It is important to remark here that in the case of the axially linked 16(Cl)FePc both the substituted Pc ligand and the axial ligand exert a pulling effect. Axially linked 16(Cl)FePc exhibits lower activity, and this is evidenced by the shift of the reduction wave to more negative potentials (higher overpotentials), Figure 3b. The reduction currents waves were drastically decreased to one half for both selfassembled systems of Au(111)/16(Cl)FePc system, Figure 3b. The latter indicates that the axial ligand changes the selectivity of 16(Cl)FePc towards ORR, this is in the number of electrons involved in the reduction process. The two-electron reduction of O2 is not desired in energy conversion as in a fuel cell, because this reaction delivers less energy, and the peroxide formed


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ACS Catalysis

can attack the fuel cell components.37 The change in mechanism from four-electron to two-electron reduction probably involves changes in the interaction and orientation of O2 with the Fe center (active site).61 and also a decrease in the M-O2 binding energy.37 This will be discussed further down. There are two models suggested in the literature of the possible orientations of the O2 interacting with Fe center. The end-on (tilted) orientation gives two-electron reduction mechanism for ORR, and side-on orientation (with the O2 molecule parallel to the plane of the complex) gives four-electron.33 However, a decrease in the Fe-O2 binding energy can change the four-electron reduction pathway to a two-electron pathway without necessarily implying a change in the orientation of the O-O molecule concerning to the Fe center. Therefore, the Up and Down pyridiniums molecules in 16(Cl)FePc do not favor the interaction of O2 with the Fe center resulting in a decrease in the Fe-O2 binding energy. According to the scaling relationships,62 this will also decrease the binding energy of superoxide to the Fe center, so superoxide will be released and will not be further reduced to water or OH- in this case. The pyridinium component in self-assembled systems of 16(Cl)FePc causes then an opposite effect for the ORR electrocatalytic activity for compared with systems containing FePc.

The number of electrons for ORR process in FePc systems have been estimated using the Randles-Sevcik equation (i) for irreversible processes controlled by mass transport values are shown in Table 1.

𝑛

3⁄ 2

1⁄ 2

= 𝐼𝑝⁄0.4463 𝐹 𝐴 𝐶𝑂2 [𝐹 𝑣 𝐷𝑂2 /𝑅 𝑇]

(𝑖)

Where Ip is the peak current, n is the total number of electrons, F is the Faraday constant, A is the electrode area, CO2 is the oxygen concentration in the solution (1.38 x 10-6 mol cm-3, taken from


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Page 18 of 46

reference63), DO2 corresponds to oxygen diffusion coefficient in the solution (1.90 x10-5cm2/s, taken from reference63), and v is the scan rate (V/s).

For example, for Au(111) Ip =390 A. The peak currents, Ip, was taken from cyclic voltammetry at 50 mV/s in NaOH 0.1 M saturated with O2 at 25°C, Figure 3. Moreover, it is well known that smooth gold catalyzes the 2-e reduction of O2 only.56 Thus, Assuming n=2 and replacing in equation (i) is obtained the n3/2 = 2.8284 value. From this the factor 0.4463FACO2[FvDO2/RT] 1/2 = Ip/n3/2 can be estimated to be 137.887. With this value, “n” can be estimated for the other systems using maximum currents (Ip) in equation (i). All Ip were taken from the first reduction cycle. It is important to make clear that in this case, the estimated number of electrons is only an approximation. However, it is well known that FePc promotes the four-electron reduction of O2 in alkaline media.35 Another method to calculate “n” is to use all terms in the Randles-Sevcik equation and essentially evaluating the term 0.4463FACO2[FvDO2/RT] 1/2, this by the slope value obtained from a graph of the peak current (Ip) of cyclic voltammograms at different scan rate vs the square root of the scan rate (v)1/2. The Ip are directly proportional to the (v)1/2 when the process is controlled by diffusion.64 With this method there are too many variables involved in the RandlesSevcik equation that can affect the estimation of the value of “n”, e.g., the O2 concentration and diffusion in the solution. In this work we have not done this method (Ip vs (v)1/2), because the 16(Cl)FePc complex does not remain on the surface after the third cycle.


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Figure 3. Cyclic voltammetry for ORR on Au(111) modified with different configurations of pyridiniums with and without FePc (a) and 16(Cl)FePc (b). 0.1 M NaOH O2 saturated; dE/dt= 0.05 Vs-1. Grey curve represents Au(111) in (a) and (b).

To obtain information about the reaction mechanism of the ORR in self-assembled up and down systems, the Tafel plots were obtained from slow potential scan polarization curves (5 mV/s) (Figure S9 in SI) in order to approach steady-state conditions, Figure 4. The lines in the Tafel plots in Figure 4 are essentially parallel with slopes close to -30 mV/decade for ORR for almost all FePc and 16(Cl)FePc systems, Figure 4, which suggests a common electron transfer (ET) mechanism for ORR on these hybrid electrodes containing FePc15,21,65 and16(Cl)FePc, except for the selfassembled system Au(111)/Down/16(Cl)FePc. These values also show that the ET mechanism (not necessarily the selectivity), changes due to the presence of axial ligand Down in16(Cl)FePc. In the other three self-assembled systems ET mechanism is dictated by the nature of the metal center and not by the nature of the ligand or presence of axial ligands. Similar Tafel slope values in the range -30 to -40mV/decade have been observed for ORR on graphite electrodes modified with FePc and


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other Fe macrocycles.15,21 The lower values of the Tafel slopes of -30mV/decade, for the studied systems, on the gold surface from the higher s of -40mV/decade for graphite surface could be attributed to a change in the symmetry energy barrier, namely in the symmetry factor β.66 Due to the ORR is a multi-step reaction, the Tafel slope value of -40mV/decade for ORR, suggests that the rate-determining step of the reaction mechanism involves the transfer of one electron, occurring after a fast electrochemical step, which in this case involves the pH dependent Fe(III)OH/Fe(II) transition21 (ES1 and ES2 in scheme 2). Therefore, the active state is the reduced form Fe(II)Pc of the catalyst interacting with a superoxide intermediate as suggested by DFT calculations.35 However, the Au(111)/Down/16(Cl)FePc self-assembled system shows a change in the Tafel slope (-61mV/dec) and suggests that the rate-determining step, in this case, is a slow chemical step (does not involve electron transfer) occurring after a fast one-electron transfer step.21 A similar mechanim seems to proceed on the bare Au(111) which also shows a -60 mV Tafel slope, this is the behavior for ORR on a gold surface without modification.


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Figure 4. Tafel Plots of a) FePc and b) 16(Cl)FePc systems obtained from linear sweep voltammetry curves (Figure S9 in SI) for oxygen reaction reduction (ORR) at 0.005 V/s, NaOH 0.1 M O2 saturated solution and T=25°C.

The redox potentials obtained for self-assembled Au(111)/Down(Up)//FePc, from square wave voltammetry technique (Figure 2), were no clear to corroborate that Fe(II) is the active form, because, the signal for the Fe(III)/(II) redox couple appears in the potential region where gold oxides are formed. Then, as reference, we can use the redox potentials observed for the complexes adsorbed on graphite (FePc/GPO) obtained from the Pourbaix diagram (redox potential vs. pH) published before,36 Table S2 in SI. Thus, the redox potential reported in the literature for the Fe(III)/(II), seems to confirm that the active state for Fe center in self-assembled systems of FePc, is the Fe(II) state. Conversely to FePc, for 16(Cl)FePc we have not information about the behavior of the Fe(II)/(I) and Fe(III)/(II) redox couples with the pH. Therefore, we could not confirm the redox state of the active site in this complex but presumably is Fe(II) as observed in several publications.16,33,35,63


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Scheme 2. ORR scheme for a Tafel slope of -40 mV/decade in self-assembled Au(111)/Down/(Up)/FePc systems.

Table 1. Parameters obtained for oxygen reaction reduction (ORR) on all systems studied in 0.1 M NaOH at 25°C.

System Au(111)/FePc Au(111)/Down/FePc Au(111)/Up/FePc Au(111)/16(Cl)FePc Au(111)/Down/16(Cl)FePc Au(111)/Up/16(Cl)FePc

Onset-Potential VC V vs (Ag/AgCl 3M)

Onset-Potential VC V vs (RHE)

-0.165 -0.043 -0.038 -0.040 -0.068 -0.071

+ 0.796 +0.918 +0.923 +0.921 + 0.893 +0.890

Number of eper O2 (vs Au) 2.54 3.91 4.25 3.99 2.23 2.70

Tafel Slope

rds

-0.033 -0.028 -0.030 -0.033 -0.061 -0.031

ES2 ES2 ES2 ES2 CS* ES2

CS*; Chemical step

In order to have better understanding of the axial pull effect in self-assembled systems, we carried out XPS measurements with and without O2 coordinated to the iron metal center in Au(111)/Down/16(Cl)FePc - Au(111)/Up/16(Cl)FePc systems (to explore axial pull effect), and Au(111)/Down/FePc - Au(111)/Down/16(Cl)FePc systems (to explore the electron withdrawing effect of Cl groups on the macrocyclic ligand). If the core pyridinium axial pull effect occurs, one would expect a decrease of the electron density at Fe and N pyridine centers and a corresponding increase of electron density at the core pyridinium N atom. It would induce an increase of the binding energy associated to the Fe 2p and N 1s (pyridine) as well a decrease of the binding energy associated to N 1s (pyridinium) (For N 1s region see Table S3, SI). Figure 5 shows the XPS measurements at the Fe 2p3/2 for Au(111)/Down/16(Cl)FePc - Au(111)/Up/16(Cl)FePc (left) and Au(111)/Down/FePc - Au(111)/Down/16(Cl)FePc (right) before and after O2 treatment at room


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temperature. The Fe 2p3/2 shows the presence of two peaks separated by 2.3 eV. The lower binding energy peak is associated with the Fe atoms in the phthalocyanine structure while the higher binding energy peak is associated to the corresponding satellite peak. It is in accordance to the XPS spectra of similar systems existing in the literature.32,67–69 It is possible to see a significant shift of 0.8 eV to lower energies of Au(111)/Up/16(Cl)FePc (before O2 treatment) compared to Au(111)/Down/16(Cl)FePc. The energy shift is an indicator of the existence of the axial pull effect in the systems Up and Down. Similar behavior showed the systems Au(111)/Down/16(Cl)FePc (before O2 treatment) compared to Au(111)/Down/FePc (before O2 treatment), with a significant shift of 1.1 eV to higher energies, showed in Table 2, that confirm the existence of the peripheral electro withdrawing effect due to the presence of substituted iron phthalocyanines with chlorine (Cl). After O2 treatment, there is no significant energy shift for the Fe 2p3/2 core level spectrum indicating that no stable Fe-O2 adducts are formed in the gas phase. These results are expected since these molecules are ORR catalysts, not O2 carriers and they should not bind O2 too strongly.35 Also, the XPS spectra at the N 1s region were analyzed and the energies shifts found are in accordance to that found at Fe 2p. The XPS spectra at the N 1s region shows the presence of four different chemical components associated to N-Fe (at the phthalocyanine compound, named N-Fe (Pc)), C-N=C, N pyridine and N pyridinium (Table S3, SI), shows the binding energies values found

for

each

chemical

component.

When

comparing

Au(111)/Down/FePc

and

Au(111)/Down/16(Cl)FePc, both before the O2 treatment, it is possible to see an energy shift of 0.4 eV to higher binding energies of the N pyridine component for the Au(111)/Down/16(Cl)FePc compared to the Au(111)/Down/FePc. It is consistent with the energy shift observed at the Fe 2p3/2 electronic level. We have added (Figure S6, SI), the XPS measurements of Fe 2p core level in Au(111)/Up/FePc composite. During the experiment was not possible to achieve a good population


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Page 24 of 46

of iron ions over the surface, turning the results obtained from this sample not reliable to be compared with the other SAMs catalysts prepared (shifts over the peak position of Fe 2p region core level spectra). Despite this fact, we carried out XPS measurements with and without O2 coordinated to the iron metal center as shown above.

When comparing Au(111)/Down/16(Cl)FePc and Au(111)/Up/16(Cl)FePc systems, both before the O2 treatment, it is possible to see an energy shift of ~0.4 eV to higher binding energies of the N pyridine, N pyridinium, N-Fe (Pc) and N-C=N components, evidencing once more the pull effect proposed. It is consistent with the energy shift observed at the Fe 2p3/2 electronic level (0.3 eV). After O2 treatment, significant energies shifts were observed for both systems. For system Au(111)/Down/16(Cl)FePc, N pyridine and N pyridinium showed a positive offset of energy. However, for system Au(111)/Up/16(Cl)FePc, both components are shifted to negative energies.

Figure 5 – XPS measurements at the Fe 2p3/2 region before and after O2 treatment at room temperature. Comparison between of Au(111)/Down/16(Cl)FePc and Au(111)/Up/16(Cl)FePc (left). Comparison between Au(111)/Down/16(Cl)FePc and Au(111)/Down/FePc (right). The


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black points represent the experimental data, and the gray line represents the best fitting found. The black solid line at Fe 2p3/2 electronic level represents the Fe-Pc component, respectively. Table 2 – Binding energies values of the different chemical components found in the XPS measurements for the Fe 2p3/2 electronic level before and after treatment with O2. Fe2p3/2 (eV) before Fe2p3/2 (eV) after Self–assembled Systems O2 treatment O2 treatment Au(111)/ Down /FePc 709.2 709.4 Au(111)/Down/16(Cl)FePc 710.3 710.6 Au(111)/Up/16(Cl)FePc 711.1 711.0

Several reactivity descriptors have been proposed in the literature for the activity of MN4 macrocycles for the ORR. The M(III)/(II) formal potential of the catalysts is an excellent very good reactivity predictor, but it requires an accurate determination of this parameter on the surfaceconfined MN4 complexes. These complexes, after being immobilized or adsorbed on graphite and carbon surfaces present evident reversible redox peaks that have been assigned to M(II)/(I) and M(III)/(II) processes supported by spectroscopic evidence. However, in the present case, these processes seem to be masked by redox process associated with Au oxide formation and reduction. Another parameter that seems to track the reactivity trends of these complexes is the donoracceptor intermolecular hardness η = ½( εdonor - εacceptor), where εdonor is the energy of the frontier orbital of the Fe complex and εacceptor is the frontier orbital of O2 which is double degenerated π* antibonding orbital occupied with two unpaired electrons. The activity of FePc and 16(Cl)FePc can then be related directly to the donor-acceptor Fe-O2 intermolecular hardness. When the dioxygen molecule binds to the metal in the FePc, its 2p electrons interact with the partially-filled d-orbitals of the metal in the back bonding process.70,71 The formation of the bond between Fe-O2 requires that the energies of the predominantly d orbitals be the same as or similar to those of the


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charge transfer intermediate species.36 The electron withdrawing groups, although decrease the electron density on the metal center (as chlorine atoms in FePc), favors the donor-acceptor electronic coupling, and increase the catalytic activity of FePc for the ORR.37,67 In similar highly catalytic systems (FePc–Py–CNTs) the presence of pyridine neutral axial ligand in FePc, decreases the electron density on the Fe site, compared with that of FePc adsorbed on the Au surface and higher activity.27 Therefore, these groups decrease the gap between the energy of the frontier orbitals involved.

Figure 6. Computed energy diagram of the HOMO/SOMO (black) and LUMO (grey) levels of the axial coordinated FePc and 16(Cl)FePc systems compared to the doubly degenerated SOMO orbitals of O2. The gap between the frontier orbitals is depicted in the inset.

A robust electronic coupling is expected for a frontier-controlled electron transfer reaction of the more active systems studied here. This case can be observed when the donor and acceptor orbitals are close in energy.67,69 The computed energies of the frontier donor orbitals of the axial coordinated Fe(II) complexes are displayed in Figure 6, and compared with the doubly degenerated SOMO of the O2 molecule (the acceptor orbitals). It is clear that the HOMO orbitals of the Down(Up)/FePc systems (6.46 eV) are slightly higher and are close to the energy of the acceptor


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ACS Catalysis

orbital in O2 (6.49 eV), allowing the electron transfer reaction. However, the axial coordination in the 16(Cl)FePc complex causes a high HOMO stabilization compared to FePc, which results in lower energies for the donor orbitals of the Down(Up)/16(Cl)FePc systems ( 6.80) compared to the acceptor orbitals in O2 (6.49 eV). This mismatching is in agreement with the low electrocatalytic activity of the Down(Up)/16(Cl)FePc systems for the ORR. Additionally, the energy difference (the gap, Fig. 6) between frontier orbitals is lower in the Down/FePc and Up/FePc systems (0.34 and 0.40 eV, respectively) concerning the Down(Up)/16(Cl)FePc systems (0.57 eV). In this regard, the higher energy gaps between donor-acceptor frontier orbitals can be related to the low ORR electrocatalytic activity. Therefore, the electrocatalytic activity of the FeN4 moiety appears mainly determined by the interaction strength with the O2 molecule.

Another essential reactivity descriptor, which is widely accepted in heterogeneous catalysis and electrocatalysis is the binding energy of the intermediate to the active site. In general, the activity varies with the binding energy in a non-linear fashion (activity volcano plots). As the binding energy increases the activity (in this case (log I)E versus the M-O2- binding energy) increases linearly up to a given value of the binding energy which corresponds to a situation where half the active sites are occupied by the bound O2. This situation corresponds to the maximum activity in the volcano plot. For catalysts that exhibit binding energy beyond the maximum, having higher binding energies (strong binding) the activity decreases linearly because gradually more and more active sites are occupied by the adsorbed intermediate, and there are less and less free active sites for promoting the reaction. The computed binding energy for the O2 for the Down/FePc and Up/FePc systems were 0.19 and -0.28 eV, respectively. The Up/FePc shows the highest activity and has an average Fe-


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O2 binding energy according to the volcano plot in Figure 7a. It also shows the highest value of n = 4.0 (see Table 1), so it is selective for the four-electron reduction of O2- to OH-. On the other hand, weakly O2 binding Down/16(Cl)FePc shows low activity and low selectivity for the fourelectron reduction of ORR. According to the estimated values of “n” this catalyst is practically a two-electron catalyst to give mostly peroxide and unable to split the O-O bond. The selectivity of Up/16(Cl)FePc is surprising since it is the catalyst that binds O2 more strongly but reduced O2 to peroxide only. Strongly binding catalysts are expected to break the O-O bond as they will also bind peroxide strongly, preventing its release to the solution. Therefore, differences in the electrocatalytic activity are determined by optimizing the interaction with O2. Due to the mismatching between donor-acceptor orbitals, an increase in the interaction strength with O2 beyond an absolute limit for Down(Up)/16(Cl)FePc systems, could result in lower electrocatalytic activities as discussed above. Indeed, energies were -0.15 and -0.37 eV for binding energy for the O2 onto Fe for the Down/16(Cl)FePc and Up/16(Cl)FePc systems, respectively. Thus, the correlation of electrocatalytic activity for ORR (log I)E versus the M-O2 binding energy on the active site (Fe) in self-assembled systems, gives rise to a “volcano correlation” despite the rather low number of catalytic systems investigated. Volcano correlations in general consist of two linear correlations, one with a negative slope (raising region, weak binding) and another one with positive slope (falling region, strong binding),37 see Figure 7a. The negative sign of the slope in the rising region comes from the fact that the binding energy increases as it becomes more negative. Our results figure suggests that optimal binding energy exists to render the highest catalytic activity for ORR and substantially seems to agree with Sabatier’s principle that states that the highest catalytic activity is obtained when adsorption of the critical intermediate in not too strong, not too weak. However, we are cautious to drawn too many conclusions from Figure 7a as the number of data


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ACS Catalysis

points is somewhat limited and that there only two catalysts on one side of the volcano correlation. However, it is interesting to note that the slopes of both sides of the volcano resemble the slopes of a volcano reported before involving an extensive collection of MN4 complexes adsorbed on graphite. For example, for the stable binding side of the reported volcano, the slope is +0.10 eV/decade whereas for our case the slope is also +0.10 eV/decade. For the weak binding leg of the volcano reported before, the slope was -0.118 eV/decade whereas for this work the slope is 0.103 eV/decade. So it is surprising that with instead few catalysts examined the trends are the same as those reported before and even the slopes of the sides of the volcano are very close to those published.37 This strongly suggest that the M-O2 binding energy is a very good reactivity predictor. It is important to comment up to this point that the formal potential M(III)/(II) in V of the MN4 molecules correlates directly to the M-O2 binding energy.37 Plots of activity as (log I) versus the M(III)/(II) redox potential of the catalysts also give volcano symmetrical plots, with slopes close to +0.12 and -0.12 V/decade. If we want to compare this data with the one in Figure 7a, we have to consider that the energy E in electron-volts (eV) is equal to the voltage V in volts (V), times the electrical charge Q in coulombs (C) divided by elementary charge ~1.6022×10-19 C. If we consider the elemental charge Q for the data in Figure 7a, and equal to ~1.6022×10-19 C then 1 eV proportional to 1 V and we have an equivalence between the slopes in eV/decade and the slopes in V/decade reported before.37 We can explain the slopes of the linear regions of the volcano plots as follows: We propose that the rate determining is an ET step involving the binding of O2 to the Fe(II) active site. This step is preceded by a fast one electron transfer step (ES1) that involves the generation of Fe(II) from Fe(III)OH and depicted in the following scheme: [𝐹𝑒(𝐼𝐼𝐼)𝑂𝐻𝑁4 ]𝑎𝑑 + 𝑒 − ⇄ [𝐹𝑒(𝐼𝐼)𝑁4 ]𝑎𝑑 + 𝑂𝐻 −

(ES1)


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0 Δ𝐺𝑎𝑑𝑂2

[𝐹𝑒(𝐼𝐼)𝑁4 ]𝑎𝑑 + 𝑂2 + 𝑒 − → [𝑂2 𝐹𝑒(𝐼𝐼)𝑁4 ]− 𝑎𝑑

𝑟. 𝑑. 𝑠. (𝐸𝑆2)

Depending on the catalyst equation ES2 will be shifted to the product (strong adsorption) or to the reactants (weak adsorption). The kinetic current density for a given potential and consistent with the Butler-Volmer equation is given by the following equation:

𝑖 = 𝑛𝐹𝑘𝛤𝑀(𝐼𝐼) 𝑝𝑂2 (1 − 𝜃) exp (

−𝛽𝐹𝐸 𝑅𝑇

0 −𝛽 ′ Δ𝐺𝑎𝑑𝑂2

) exp (

𝑅𝑇

)

(𝑖𝑖)

wherein the first exponential factor E is the electrode potential versus some nominal reference electrode, β is the symmetry factor of the ET energy barrier, and the second exponential term comes from the fact that O2 adsorbs in a concerted fashion with the one-electron-transfer step (ES2). M(II) is the surface concentration of the complex in the M(II) active state at the potential = E, (1- θ) is the fraction of Fe(II) active sites that are not occupied by O2 and ∆G°adO2 is the free energy of adsorption of O2 on the Fe(II) active sites. β’ is the Brønsted-Evans-Polanyi (BEP) coefficient that reflects the effect of the binding energy on the activation energy. For a regular energy barrier β’= 0.5; the rest of the terms have their usual meaning. Both the applied potential and the binding energy act as driving forces for the reaction and decreasing the energy barrier for the rate determining step. The coverage of adsorbed O2 can be assumed to follow a Langmuir isotherm: −Δ𝐺0 𝑎𝑑𝑂2 ) 𝑅𝑇 −Δ𝐺0 𝑎𝑑𝑂2 )] [1+𝑝𝑂2 exp( 𝑅𝑇

𝑝𝑂2 exp(

𝜃=

(𝑖𝑖𝑖)

In a volcano plot the kinetic data for different catalysts is compared at constant E, so the Butler-Volmer exponential term for simplicity can be absorbed into a constant k'. For small coverages G° is positive so equation (ii) can be written as:


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ACS Catalysis

𝑘´ = 𝑘𝑒𝑥𝑝 (

−𝛽𝐹𝐸 𝑅𝑇

) ;

𝑖 = 𝑛𝐹𝑘´Γ𝑀(𝐼𝐼) (1 − 𝜃)exp (

0 −𝛽 ´ Δ𝐺𝑎𝑑𝑂2

𝑅𝑇

)

(𝑖𝑣)

We can assume that 𝑝𝑂2 = 1𝑎𝑡𝑚 𝑠𝑜 𝑠𝑜𝑙𝑣𝑖𝑛𝑔 𝑓𝑜𝑟 𝜃

𝑖=

𝑛𝐹𝑘´Γ𝑀(𝐼𝐼) exp(

−𝛽´ Δ𝐺0 𝑎𝑑𝑂2

𝑅𝑇 −𝛥𝐺0 𝑎𝑑𝑂2 )] [1+𝑒𝑥 𝑝( 𝑅𝑇

)

(𝑣)

Equation (v) essentially describes the shape of the volcano plot and that the maximum current density will be observed for G° = 0. For strong adsorption G° is large and has a negative sign so 1

Building Pyridinium Molecular Wires as Axial Ligands for Tuning the

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