Electrochemical and in situ spectroelectrochemical monitoring of the interaction between cobaltphthalocyanines and molecular oxygen in aprotic media

Journal of Electroanalytical Chemistry 655 (2011) 128–139

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Electrochemical and in situ spectroelectrochemical monitoring of the interaction between cobaltphthalocyanines and molecular oxygen in aprotic media Atif Koca ⇑ Marmara University, Department of Chemical Engineering, Faculty of Engineering, Göztepe 34722, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 29 September 2010 Received in revised form 4 February 2011 Accepted 27 February 2011 Available online 2 March 2011 Keywords: Cobaltphthalocyanine Oxygen reduction reaction Electrocatalyst Spectroelectrochemistry

a b s t r a c t In this study, voltammetric and spectroelectrochemical measurements of cobaltphthalocyanines (CoPc) bearing different substituents were performed in aprotic solvents to determine the mechanism of interaction between CoPc and molecular oxygen. Metal-based redox processes of the cobaltphthalocyanine complexes indicate their possible electrocatalytic activities toward many target species. Different substituents of the complexes affect the peak characters and assignments of the processes. Presence of O2 in the electrolyte system influences oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate homogeneous electrocatalytic activity of the complexes for the oxygen reduction reaction. In situ electrocolorimetric method was applied to investigate color of the anionic and cationic forms of the complexes. Electrochemical and in situ spectroelectrochemical analysis indicate interaction of molecular oxygen with monoanionic [CoIPc2]1 and presence of an equilibrium between II II 2  2 2 the reactants [CoIPc2]1 and O2 and the intermediates ½O  and ½O2  . This equi2 —Co Pc 2 —Co Pc librium is disturbed in desired direction by changing the excitation signals of the voltammetric and spectroelectrochemical techniques. The intermediates are very unstable, regenerating the starting form of the 2 catalyst and the products O 2 and O2 . Interaction rate of O2 with the complexes is significantly influenced with the steric and coordination properties of the substituents and solvent of the system. Voltammetric and spectroelectrochemical results indicate that homogeneous electrocatalytic ORR follows an ‘‘inner sphere’’ chemical catalysis process. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Metallophthalocyanines (MPcs) are used in a number of applications due to their high chemical and thermal stability, designing flexibility, varied coordination properties, diverse substitution alternatives and interesting electrochemical properties [1]. It is well documented that phthalocyanines are unique macromolecular complexes that catalyze reactions involving many target species such as CO2 [2–4], CO [5], and H+ [6–10] because of their rich redox behavior [1]. Cobaltphthalocyanines (CoPcs) have been known for a long time as electrocatalysts for the oxygen reduction reactions (ORR). In view of its significance in natural and industrial processes, the reduction of dioxygen has been the object of a very large number of studies. Most of the electrochemical investigations in this connection have been carried out in water. The reduction of dioxygen then leads to hydrogen peroxide or water, which implies that the electron-transfer steps are accompanied by homogeneous or heterogeneous chemical steps involving protonations and oxygen–oxygen bond breaking [11,12]. The electrocatalytic mechanisms of CoPcs for ORR in aqueous solution are well known ⇑ Tel.: +90 216 3480292; fax: +90 216 3450126. E-mail address: [email protected] 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.02.028

[13–15]. Many papers have appeared in the literature dealing with the electrocatalytic activity of CoPcs coated on the working electrode for the reduction of molecular oxygen in generally aqueous solution [12,16]. Most authors agree that the catalytic activity of these heterogeneous catalysts is linked to the CoIII/CoII reversible transition of the metal centers [13–15,17]. On the other hand, there are several reports that have shown that CoPcs promote the electrocatalytic reactions via a redox catalysis mechanism involving the CoII/CoI couple in aqueous solutions [9,18]. Although studies on the heterogeneous catalysts (catalyst immobilized on the electrode) are numerous in the literature and most of them in the aqueous solution, homogenous catalysts (catalyst in solution) for ORR in aprotic media are rare [12,19–21]. ORR mechanisms in the aprotic solvents such as acetonitrile (ACN), dimethylsulfoxide (DMSO) and dichloromethane (DCM) are well known; however, the exact mechanism of electrocatalytic ORR is not known with certainty. There are many studies in the literature deal with the homogeneous catalysis of various target species [22–24]. For example, Saveant and coworkers reported the chemical vs. redox catalysis of electrochemical reactions for the reduction of trans1,2-dibromocyclohexane [24]. According to Saveant, homogeneous catalysis of such slow and irreversible electrochemical processes may be carried out along two conceptually different types of

A. Koca / Journal of Electroanalytical Chemistry 655 (2011) 128–139

mechanisms. In one case, ‘‘redox catalysis’’, the active form of the catalyst, generated at the electrode, exchanges electrons with the reactant in an outer-sphere manner yielding the reaction products and regenerating the starting form of the catalyst. On the other hand, ‘‘chemical catalysis’’ involves the transient formation of an adduct between the active form of the catalyst and the reactant. The chemical bond thus formed has to be cleaved successively or after the exchange of additional electrons for eventually yielding the products and regenerating the starting form of the catalyst. Although the examples of homogeneous catalysis of electrochemical reactions are numerous, it is not always straightforward to distinguish between the two types of catalysis and therefore to find guidelines for designing efficient catalytic systems. While how the catalytic efficiency is related to the nature of the catalyst is well understood in the case of redox catalyst, identification of a chemical catalysis process is more difficult. Thus far, the only approach has been to characterize the intermediate adduct formed by reaction of the active form of the catalyst with the substrate. Although many heterogonous catalytic studies on the ORR and homogeneous catalytic studies on various target species, studies on the homogeneous electrocatalytic activity of MPcs for ORR in aprotic solvents are rare. Mho S. and coworkers reported the

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interactions of reduced cobaltphthalocyanine with oxygen and proposed that the catalytic activities of phthalocyanines are achieved through a conventional regenerative catalytic mechanism [25]. However, it is well illustrated in the literature for homogeneous catalytic process that it is important to identify main catalytic mechanism (outer-sphere redox catalysis or inner-sphere chemical catalysis) and intermediate adducts which reflect the mechanism and illustrate the factors affecting the mechanism in detail. Therefore, in this study we have performed detailed investigation for the interaction of CoPcs having different substituents (Fig. 1) with molecular oxygen with voltammetric and in situ spectroelectrochemical measurements. We have also aimed to illustrate effect of the substituents and solvents to the electrocatalytic activity of the complexes. We have also aimed to determine intermediate adducts with in situ spectroelectrochemical measurements combined with the voltammetric data. The results of this measurements will be a guide for the application of these types of complexes for the possible uses in fuel cell technology as a cathode active material, in the non-aqueous metal-air batteries, as electrocatalyts for the production of superoxide from molecular oxygen in the aprotic solvents, and as electrochemical sensors [26,27].

Fig. 1. Structures of cobalt(II) phthalocyanines bearing tetra(hydroxyethylthio) (CoPc1), 2(3),9(10),16(17),23(24)-tetra{6-hydroxyhexylthiol) (CoPc2), tetra(1,1-(dicarbethoxy)-2-(2-methylbenzyl))-ethyl-tetra chloro (CoPc3), and tetra (pentafluorobenzyloxy) (CoPc4) substituents.

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2. Experimental All reagents were purchased from Aldrich and used without further purification. CoPcs were prepared following literature procedures [28–31]. Electrochemical measurements were done with a Gamry Reference 600 potentiostat/galvanostat. A standard threeelectrode single cell configuration was used for all experiments. This consists of a platinum wire as counter electrode, a saturated calomel reference electrode (SCE) and a Pt disk (geometric area 0.071 cm2) as working electrode. Tetrabuthylammonium perchlorate (TBAP) (0.10 mol dm3) in DMSO or DCM was used as electrolyte for all electrochemical measurements. Prior to use, the working electrode was cleaned by polishing with successively finer grade aqueous alumina slurries (grain size, 5–0.5 lm) on a Buehler polishing cloth. In situ spectroelectrochemical measurements were performed with an OceanOptics QE65000 Diode array spectrophotometer and a three-electrode configuration of thin-layer quartz spectroelectrochemical cell at 25 °C with a Pt gauze working electrode, a Pt wire counter electrode, and a SCE. TBAP in DMSO or DCM (0.20 mol dm3) was used as electrolyte for the spectroelectrochemical measurements. In situ electrocolorimetric measurements under potentiostatic control were obtained at color measurement mode of the spectrophotometer simultaneously with spectroelectrochemical measurements. The standard illuminant A with 2° observer at constant temperature in a light booth designed to exclude external light was used. Prior to each set of measurements, background color coordinates (x, y, and z values) were taken at open circuit, using the electrolyte solution without CoPc under study. 3. Results and discussion 3.1. Voltammetric and in situ spectroelectrochemical measurements Solution redox properties of CoPcs were studied using the voltammetric (CV and SWV) and in situ spectroelectrochemical

measurements in O2-free electrolyte systems and the results were published in our previous papers [28–31]. Table 1 summarizes assignments of the redox couples and the electrochemical parameters, which included the half-wave peak potentials (E1/2), anodic to cathodic peak potential separation (DEp), the ratio of anodic to cathodic peak currents (Ipa/Ipc), and the difference between the first oxidation and reduction processes (DE1/2). Assignments of the redox couples (given on Table 1) were performed with in situ spectroelectrochemical measurements and the results were discussed in detail in our previous papers [28–31]. These data illustrated that voltammetric and spectral changes of target electroactive species, especially metallophthalocyanines bearing redox active metal center, were influenced with introducing of molecular oxygen into the electrochemical and spectroelectrochemical cells due to inefficient purging or maintaining N2 blanket during measurements. Thus in this study, we compare the voltammetric and spectroelectrochemical responses of different CoPcs in the electrolyte system with and without molecular oxygen to investigate the mechanism of the interaction between CoPcs and O2. Comparative CV and SWV responses of the target CoPcs in O2free electrolyte system (case 1) are given in Fig. 2 for comparison with the responses of CoPcs in O2 saturated electrolyte system (case 3) and with the response of CoPcs-free O2 containing electrolyte system (case 2). CoPc1 and CoPc2 dissolve in polar organic solvents. Thus, solution redox properties of these complexes were studied in DMSO. Fig. 2a shows CV and SWV of CoPc1 and CoPc2 within the electrochemical window of TBAP/DMSO electrolyte system. Both complexes undergo two quasi-reversible one-electron oxidation and two reversible one-electron reduction processes at approximately same potentials. Fig. 2b shows CV and SWV of CoPc3 and CoPc4 within the electrochemical window of TBAP/ DCM. Both complexes undergo two quasi-reversible one-electron oxidation and two reversible one-electron reduction processes like those of CoPc1, and CoPc2, but these processes are shifted to positive potential due to the substituent and solvent differences. Redox processes of CoPc3 are recorded at less negative potential than

Table 1 Voltammetric data of CoPcs vs. SCE with the related complexes. Complex

a

CoPc1 (in DMSO)

E1/2 DEp (mV) Ipa/Ipc E1/2 DEp (mV) Ipa/Ipc E1/2 DEp (mV) Ipa/Ipc E1/2 DEp (mV) Ipa/Ipc E1/2 (in DMSO) E1/2 (in DMSO) E1/2 (in DMSO) E1/2 (in DCM) E1/2 (in DCM) E1/2 (in DCM)

CoPc2 (in DMSO) CoPc3 (in DCM) CoPc4 (in DCM) f

CoPc CoPc h CoPc i CoPc j CoPc k CoPc g

d

Peak parameters

Pc Oxd.’s

MIII/MII

MII/MI

Pc red.’s

0.94 – – 0.94 – – 0.67 85 0.73 0.80 80 0.92 0.90

0.38 73 0.97 0.38 60 0.88 1.11e – – 1.26e – – 0.40 0.35 0.13

0.42 83 1.00 0.42 90 0.97 0.23 97 0.95 0.08 90 0.86 0.43 0.43 0.40 0.41 0.40 0.46

1.34 82 0.98 1.34 64 0.92 1.39 91 0.90 1.19 120 0.87 1.39 1.44 1.30 1.20 0.84

0.82 0.42 0.44

c

1.04 450

1.15 600

0.80

0.92 200

1.15 600

0.80

1.20 290

1.31 400

0.90

1.12 270

1.31 400

0.88

II  II 2  2 2 ½O 2 —Co Pc  =½O2 —Co Pc 

1.95e – – 1.95e – –

O2/O 2

b

1.65 1.53

DE1/2 = DE1/2 (first oxidation)  DE1/2 (first reduction). a E1/2 = (Epa + Epc)/2 at 0.100 V s1; DEp = Epa  Epc at 0.100 V s1; Ipa/Ipc for reduction, Ipc/Ipa for oxidation processes at 0.100 V s1 scan rate. b Epc of CoPc–O2 adduct reduction process. c Epc of O2 reduction in CoPc free solution. e The process is recorded with SWV. f Substituted with tetrakis (6-hydroxyhexylsulfanyl) moieties. g Substituted with tetrakis [2,4,6-tris(N,Ndimethylaminomethyl)phenoxy] moieties. h Substituted with tetrakis (7-coumarinthio-4-methyl) moieties. i Substituted with tetrakis [2-(phenylthio)ethoxy] moieties. j Substituted with tetrakis (benzylmercapto) moieties. k Substituted with tetrakis (dodecylmercapto) moieties.

d

DE1/2

Ref.

tw

tw

tw

tw 0.83 0.78 0.53 1.23 0.82 0.90

[32] [33] [34] [35] [36] [36]

A. Koca / Journal of Electroanalytical Chemistry 655 (2011) 128–139

Fig. 2. CVs at 0.100 V s1 scan rate and SWVs (inset) (100 mV pulse size and 25 Hz frequency) of (a) CoPc1 (red) and CoPc2 (blue) in DMSO/TBAP; (b) CoPc3 (blue) and CoPc4 (red) in DCM/TBAP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

those of CoPc4 due to the high electron releasing ability of tetra(1,1-(dicarbethoxy)-2-(2-methylbenzyl))-ethyl -tetra chloro substituents of CoPc3 with respect to tetra (pentafluorobenzyloxy) substituent of CoPc4. These data are in harmony with the MPc complexes in the literature. [37–41]. 3.2. Interaction of CoPcs with O2 in aprotic solvents There are many studies on the redox reaction of O2 in the protic solvents, but studies in the aprotic solvents are deficient and the majorities of these are related to the mechanistic investigation of superoxide and/or peroxide formation from molecular oxygen. In this study for the first time, we performed investigation of CoPcs interaction with O2 to clarify the homogeneous electrocatalytic O2 reduction mechanism in the aprotic media with different electrochemical and in situ spectroelectrochemical techniques. Fig. 3a (inset) gives the CV responses of O2 dissolved in DMSO/ TBAP electrolyte system without CoPcs (case 2). Early investigations of the oxygen reduction reaction (ORR) in the aprotic solvents such as acetonitrile (ACN), dimethylsulfoxide (DMSO) and N,Ndimethylformamide (DMF), showed that molecular O2 reduces via 1e- to superoxide in the non-aqueous environment [25,42, 43]. Similarly clear quasi-reversible oxygen/superoxide (O2/O 2) couple is recorded at 1.00 V in DMSO at 0.10 mV s1 scan rate. O2/O 2 couple shifts to the negative potentials with increasing of O2 concentration as shown in Fig. 3a (inset).

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Fig. 3. (a) CVs (inset: CVs of DMSO/TBAP electrolyte with different O2 concentrations) and (b) SWVs of CoPc1 at 0.100 V s1 scan rate on Pt with increasing amounts of O2 in DMSO/TBAP electrolyte system. (SWV parameters: pulse size = 100 mV; step size: 5 mV; frequency: 25 Hz; Ei = 1.20 V and Ef = 2.20 V for SWV having cathodic currents; Ei = 2.20 V and Ef = 1.20 V for SWV having anodic currents).

The voltammetric responses of dissolved O2 in the DMSO/TBAP electrolyte system containing CoPc1 and CoPc2 complexes (case 3) were recorded to monitor the interaction of CoPcs with O2. Electrochemical responses of O2 recorded in case 3 with increasing O2 concentration are analyzed as: (i) Redox couples of CoPc1, CoPc2, and O2 are considerably influenced with increasing O2 concentration (Figs. 3 and 4). (ii) ORR couple in case 3 shifts ca. 100 mV toward the positive direction and gets more reversible with respect to those recorded in case 2 (DEp values of ORR couple increase from 320 to 582 mV in case 2, while DEp values change from 140 to 234 mV in case 3). (iii) While ORR couple has only one reverse peak in case 2, a split reverse peak is recorded with SWV in case 3. Shifting of the ORR couple to the less negative potentials and increasing reversibility of the ORR process indicate the electrocatalytic activity of CoPcs to the oxygen reduction reaction. Presence of O2 in the electrolyte system also influences the electrochemical properties of CoPcs complexes. Electrochemical responses of CoPcs in case 3 with increasing O2 concentration are recorded as: (i) Couple III gets irreversible (anodic wave of the couple disappears gradually). (ii) Couple IV shifts to the negative potentials with decreasing in current intensity. (iii) Anodic wave of the first oxidation couple shifts to the positive direction with the increasing O2 concentration. These voltammetric data indicate that CoPcs interact with O2 and this interaction affects the voltammetric behaviors of CoPcs as well as O2.

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Fig. 4. (a) CVs and (b) SWVs of CoPc2 at 0.100 V s1 scan rate on Pt with increasing amounts of O2 in DMSO/TBAP electrolyte system. (SWV parameters: pulse size = 100 mV; step size: 5 mV; frequency: 25 Hz; Ei = 1.20 V and Ef = 2.20 V for SWV having cathodic currents; Ei = 2.20 V and Ef = 1.20 V for SWV having anodic currents).

Fig. 5. CVs of CoPc1 at 0.025 V s1 scan rate with different switching potentials on Pt with constant amounts of O2 in DMSO/TBAP electrolyte system. (a). Constant anodic switching potential (Ea,sp = 1.0 V) and various cathodic potentials. (b) Various cathodic potentials without anodic scans.

To clarify the interaction reaction, CVs and SWVs of the complexes were recorded with different switching potentials. While only the anodic potentials were scanned during CV (Ei = 0.0 V; Es = 1.20 V; Ef = 0.00 V) and SWV (Ei = 0.0 V; Ef = 1.20 V) measurements, completely similar oxidation processes were recorded in case 1 and 3. Nevertheless when the anodic potentials were scanned after the cathodic scans (for CV: Ei = 0.0 V; Es1 = 2.20 V; Es2 = 1.20 V; Ef = 0.00 V and for SWV: forward SWV: Ei = 1.20 V; Ef = 2.2 V and reverse SWV: Ei = 2.2 V; Ef = 1.20 V), the oxidation processes changed with increasing O2 concentration. These voltammetric data indicate that [CoIIPc2] and its electrogenerated cationic species do not interact with O2 during the anodic scan. However, interaction of the complexes during the cathodic scans influences both reduction and oxidation processes of the complex. CV and SWV responses of CoPc1 were also recorded at a slow scan rate (0.025 mV s1) with different switching potentials (Fig. 5a). It is clear that the scan rate does not affect general trend of the redox processes. However, varying the switching potentials alters the CV responses of the processes considerably. Fig. 5a represents CV of CoPc1 in case 3 recorded with different cathodic switching potentials at 0.025 mV s1 scan rate. When CV scan is switched before the oxygen reduction reaction, Common redox couples of CoPc1 (I, II, and III) do not change with increasing O2 concentration. However when the switching potential passes the ORR cathodic peak, reverse wave of the couple III decreases in current intensity with shifting to the positive potentials, while the cathodic wave does

not change. At the same time, the couple II shifts from 0.38 V to 0.52 V. These data illustrate that the complex interacts with O2 after the ORR process and the product of this interaction affects the oxidation behavior of the complex. CVs of CoPc1 in case 3 are also recorded with the different cathodic switching potential without the anodic potential scan at 0.025 mV s1 scan rate (Fig. 5b) to determine the effect of the anodic potential scan. When the cathodic potentials were only scanned (anodic potentials were not scanned), CVs of CoPc1 differentiates completely. The reverse wave of the process III decreases in current intensity when the switching potential passes the ORR cathodic peak. At the same time, the cathodic wave of III disappears gradually during the repetitive scans. Disappearance of couple III also affects the ORR couple. Cathodic wave of the ORR couple shifts to the negative potentials and the anodic one shifts to the positive potentials, which increase DEp values of ORR couple. This voltammetric behavior is not observed when the anodic potentials are scanned after cathodic scans (Fig. 5a). These voltammetric data indicate that CoPc1 interacts with reduced O2 and the product of this interaction is not redox active in the cathodic potentials without anodic scans. However when the anodic potentials are scanned after the cathodic scans, the interaction product is oxidized and regenerates the original form of the complex. SWV responses of CoPc1 recorded with different initial potentials (Fig. 6a) support the CV responses of the complex. When SWV scan is performed from 1.10 V to 1.80 V, the redox peaks of the complex and the ORR process are

A. Koca / Journal of Electroanalytical Chemistry 655 (2011) 128–139

Fig. 6. SWVs of CoPc1 on Pt with constant amounts of O2 in DMSO/TBAP electrolyte system; (a) with different initial potentials and (b) with different scan rates (inset: with different equilibrium times at 0.10 V s1 scan rate.

only recorded. However, differentiating the initial potential (from negative to positive potential scan) influences the voltammetric responses of the system considerably. When SWV scan is carried out from 0.70 V to 0.0 V (green SWV), the redox peaks of the complex are only recorded. However when the initial potential of SWV passes to the negative side of the ORR process (1.20 V), a new wave is observed at 0.58 V and this wave increases in current intensity with negative shifting of the initial potential. At the same time, anodic peak current of the process III decreases in current intensity. Initial potential of the SWV measurements also affect the relative peak currents of the split ORR process. While the ORR wave at 0.76 V increases, the ORR wave at 1.06 V decreases in current intensity with the negative shifting of the initial potential. Equilibrium time and scan rate of the SWV measurements also affect the SWV responses of the system (Fig. 6b). At a high equilibrium time and slow scan rate, the SWV response follows the trend of SWV that recorded with the more negative initial potential. These voltammetric data indicate that interaction of CoPc with O2 is affected from O2 concentration, scan rate, switching potential, and equilibrium time of the CV and SWV measurements. If the duration of the interaction is high (negative switching potential, high equilibrium time, and slow scan rate), the indicator of the interaction is more obvious. In situ spectroelectrochemical studies were employed to determine the mechanism of the interaction between CoPcs and O2. Figs. 7 and 8 represents in situ UV–vis spectral changes of CoPc1 in case 1 and 3 respectively in DMSO/TBAP electrolyte system during the

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potential application at the potentials of the redox processes. Under the open circuit potential, spectra of CoPc1 did not change with time both in case 1 and 3. This indicates that [CoIIPc2] do not interact with O2. During the first reduction process, spectral changes assigned to [CoIIPc2] to [CoIPc2]1 reduction reaction are observed in both case 1 (Fig. 7a) and case 3 (Fig. 8a). However final spectrum in case 3 (Fig. 8a) is not completely identical with the spectrum of [CoIPc2]1 recorded in case 1 (Fig. 7a). These spectral changes indicate the interaction of reduced complex [CoIPc2]1 with molecular oxygen, but this interaction does not change the oxidation state of the cobalt center. Different potential applications between 0.60 and 0.90 V were applied to observe the interaction between [CoIPc2]1 and molecular oxygen. During these potential applications, spectra of the complex did not change considerably. To observe the effects of the ORR to the spectra of the complex, spectral changes are recorded under the applied potential at 1.25 V (ORR potential). While there was no spectral changes in case 1, spectra of the complex return back immediately, and approximately the original spectrum of the neutral CoPc1 is obtained in case 3 (Fig. 8b). This spectral changes indicates the interaction of [CoIPc2]1 with the reduced oxygen, (superoxide, O 2) II 2 2 immediately and formation of ½O2 2 —Co Pc  . The Q band in the final spectrum (Fig. 8b) shifts 10 nm to longer wavelength when compared with the original spectrum of CoPc1. This may due to the coordination of the reduced oxygen to the complex as II 2 2 ½O2 2 —Co Pc  . Under the applied potential at 1.50 V, while spectra of the complex slightly change and illustrate the spectrum, which can easily be assigned to CoII/CoI process in case 3 (Fig. 8b inset), any spectral change is recorded in case 1. These data indicate II 2 2 that ½O2 species start to reduce at around 1.50 V, but 2 —Co Pc  the rate of this reaction is too slow. Applying 0.0 V after reduction reactions did not convert the spectrum to the original form in case 3, which indicates the irreversibility of the reduction processes, thus a new solution was used to study in situ spectroelectrochemical behavior of the oxidation reactions. Spectral changes during the oxidation process are similar in case 1 and 3 (Figs. 7c and 8c), although CV and SWV recorded with and without O2 are different. During the first oxidation reaction, the Q band of the complex shifts to higher energy side with and without O2, which is the characteristics of the metal-based oxidation process of CoPc complexes. Spectral changes during the second oxidations are same and characterize the common ring-based oxidation of the complex both in case 1 and 3 (Figs. 7c and 8c as inset). Color change of the solution of the complexes during the redox processes were recorded using in situ colorimetric measurements. All complexes illustrated similar trend of the color changes during the redox processes. Thus the chromaticity diagrams of CoPc1 recorded simultaneously during the spectroelectrochemical measurements are given in Fig. 7d (case 1) and 8d (case 3) as representatives of the in situ recorded chromaticity diagrams of the complexes. As shown in the chromaticity diagrams each redox state of the complex has different color and color changes of the complex with and without O2 are different from each other, which also supports the interaction of CoPcs with molecular oxygen. In summary, according to the results of CV, SWV, and in situ spectroelectrochemical measurements, These are easily concluded that (i) under the applied potential at 0.50 V, [CoIIPc2] species are reduced to [CoIPc2]1 which is reoxidized back to [CoIIPc2] with O2 and form an equilibrium between electrogenerated [CoIPc2]1 and reoxidized [CoIIPc2]. However, spectral changes indicate that rate constant of the electron transfer reaction at the electrode is higher than the reoxidation rate constant of [CoIPc2]1 to [CoIIPc2] with O2, because spectra assigned to [CoIPc2]1 species is dominant at this potential. (ii) Under the applied potential 1.25 V in case 3, decreasing of the band assigned

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Fig. 7. In situ UV–vis spectral changes of CoPc1 in DMSO/TBAP electrolyte without molecular oxygen (purged with N2) (a) Eapp = 0.60 V. (b) Eapp = 1.50 V. (c) Eapp = 0.50 V. (d) Chromaticity diagram (each symbol represents color of the electro-generated species; h: CoIIPc2, s: CoIPc2, 4: CoIPc3, I: CoIIIPc2). (Color coding of spectra: red: spectrum under open circuit; blue: spectrum at the end of the first electron transfer reaction; wine: spectrum at the end of the second electron transfer reaction; green: spectra show the changes between to redox states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to the CoI species at around 470 nm indicate the interconversion of CoI to CoII due to reoxidation of CoI to CoII species with superoxide. II 2 2 Under this potential application, ½O2 form of the com2 —Co Pc  plex is proposed, since the final spectrum illustrates CoII form of the complex and the Q band shift to longer wavelength with respect to [CoIIPc2]. (iii) Changing of the oxidation reactions in the CV and SWVs after reduction processes supports the existence of a different form of complex from [CoIIPc2] at around 0.0 V during the reverse CV and SWV scans. (iv) Disappearance of the reverse wave of [CoIIPc2]/[CoIPc2]1 in the CV and SWV of the complex in Fig. 3 may due to the interconversion of [CoIPc2]1 to II 2 2 ½O2 2 —Co Pc  . (iv) Splitting of the reverse couple of the ORR proI 2 1 cess also support the coordination of O ] and form an 2 to [Co Pc 2 II 2 2 equilibrium between O and ½O —Co Pc  . Split peaks in the 2 2 SWV (Figs. 3 and 6) of the complex may be assigned to the oxidation of superoxide (O and peroxide coordinated as 2) II 2 2 ½O2 2 —Co Pc  . (v) Changing the relative peak currents of these split peaks as a function of initial negative potential, equilibrium time, and scan rate during SWV measurements (Fig. 6) supports this proposed assignment, since above parameters of the SWV can easily shift the proposed equilibrium in the forward or reverse direction.

In situ spectroscopic changes of CoPc2 (Fig. 9) has similar trend; however the final spectrum at the end of the first process is different than that of CoPc1. While CoPc1 give a spectral changes assigned to the [CoIIPc2]/[CoIPc2]1 process in case 1, Spectra of CoPc2 in case 3 do not turn completely to the spectrum of [CoIPc2]1 species during the first reduction process as shown in Fig. 9a (with O2: case 3) and the inset of Fig. 9a (without O2: case 1). These spectroscopic data indicates that [CoIPc2]1 species are reoxidized back to [CoIIPc2] with O2 and the reoxidation rate of [CoIPc2]1 with O2 to [CoIIPc2] is more dominant than the electrochemical reduction reaction rate at the electrode. However, under applied potential at -1.25 V, while any spectral change was recorded in case 1, spectra of the complex return back and approximately original spectrum of the neutral CoPc2 is obtained in case 3. Difference of the final spectrum is only 5 nm shift of the Q band, when compared with the original spectrum. According to CV, SWV and spectral changes, same mechanism of CoPc1 can be assumed for the interaction of O2 with CoPc2. However rate of the interaction reactions should be different. While CoPc2 indicates interaction signals during the first reduction process, this process is dominant during the second reduction reaction with CoPc1. Similar spectral changes of CoPc1 are recorded during the

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Fig. 8. In situ UV–vis spectral changes of CoPc1 in DMSO/TBAP electrolyte saturated with O2 (a) Eapp = 0.60 V. (b) Eapp = 1.25 V (inset: Eapp = 1.50 V). (c) Eapp = 0.50 V. (Color coding of spectra: red: spectrum under open circuit; blue: spectrum at the end of the first electron transfer reaction; wine: spectrum at the end of the second electron transfer reaction; green: spectra show the changes between to redox states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

oxidation processes with CoPc2. The only difference is that the shifting of the Q band is smaller with O2 than without O2. This may due to the coordination of molecular oxygen to the CoIII center, which just affects the degree of the shifting but does not change the assignments of the redox process [44]. According to the voltammetric and spectral responses of the complexes, CoPc2 interacts with O2 more actively than CoPc1. The activity difference of these complexes indicates effect of the substituents to the interaction of O2 with CoPcs. To investigate effect of the solvent to the interactions of CoPcs with O2, voltammetric and spectral responses of CoPc3, CoPc4, and O2 were recorded in DCM/TBAP electrolyte system (Figs. 10– 13). Fig. 10a (inset) gives CV responses of O2 dissolved in DCM (case 2). Irreversible oxygen/superoxide (O2/O 2 ) couple is recorded at 1.10 V in DCM at 0.100 V s1 scan rate. O2/O 2 couple shifts to negative potentials with the increasing O2 concentration. As shown in Fig. 10, O2/O 2 couple is recorded at approximately same potential, but reversibility of the process increases in the solution containing CoPc3 (case 3). Increasing O2 concentration only shifts the couple IV to the negative potential. Existence of O2 does not affect the first reduction couple (III) of the complex. Presence of O2 also influences the oxidation couples of the complex. These changes are well seen

in SWV of the complex (Fig. 10b). CoPc4 gives different CV and SWV responses than CoPc3 in DCM in case 3. As shown in Fig. 11, the reverse wave of the couple III disappears and the couple IV is covered with the ORR peak with increasing O2 concentration. Presence of O2 influences the oxidation couples of the complex differently when compared with CoPc3. Oxidation couples of the CoPc4 shift to the less positive potential with increasing O2 concentration. These voltammetric data indicate that CoPc3 and CoPc4 interact with O2 with different rate. While CoPc3 does not almost interact with O2, interaction of CoPc4 is obvious in CV and SWV responses of the complex. Fig. 12 represents in situ UV–vis spectral changes of CoPc3 in case 1 (inset in Fig. 12) and in case 3 in DCM/TBAP electrolyte system. Initial spectrum of CoPc3 is different from the others due to the aggregation of CoPc3. During the first reduction process, spectral changes of CoPc3 are similar in case 1 and 3 as shown in Fig. 12a. The only difference is that aggregation is not diminished completely during the reduction in case 3. Thus, the band assigned to the aggregated species is still present at 668 nm as shifted at the end of the first reduction reaction. However, spectral changes of the monoanionic [CoIPc2]1 species are different in CoPc3/DCM solution in case 3 and in case 1 during the second reduction process

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Fig. 10. (a) CVs at 0.100 V s1 scan rate (inset: CVs of DCM/TBAP electrolyte with different O2 concentrations) and (b) SWVs of CoPc3 on Pt with increasing amounts of O2 in DCM/TBAP electrolyte system.. (SWV parameters: pulse size = 100 mV; step size: 5 mV; frequency: 25 Hz; Ei = 1.50 V and Ef = 1.80 V for SWV having cathodic currents; Ei = 1.80 V and Ef = 1.50 V for SWV having anodic currents).

Fig. 9. In situ UV–vis spectral changes of CoPc2 in DMSO/TBAP electrolyte saturated with O2 (insets: purged with N2). (a) Eapp = 0.50 V. (b) Eapp = 1.50 V. (c) Eapp = 0.50 V. (d) Eapp = 1.10 V. (Color coding of spectra: red: spectrum under open circuit; blue: spectrum at the end of the first electron transfer reaction; wine: spectrum at the end of the second electron transfer reaction; green: spectra show the changes between to redox states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(Fig. 12b). While spectral changes assigned to [CoIPc2]1/ [CoIPc3]2 process are recorded in case 1 (Fig. 12b inset), spectra of the complex return back and a spectrum similar to the spectrum of the monomeric CoPc3 is obtained in case 3 at the end of the secII 2 2 ond reduction process. Formation of ½O2 from the 2 —Co Pc  I 2 1 monoanionic [Co Pc ] due to the interaction with O 2 may diminish the aggregation, thus the final spectrum recorded in case 3 gives a sharp Q band at the same wavelength with the original complex. Although interaction of the complex with O2 could not be seen obviously with CV and SWV, spectral changes under the applied potential illustrate the interaction clearly. These different changes may be due to the differences of the excitation signals and time scales of these techniques, which can alter the electron

transfer and interaction rates. Spectral changes during the oxidation processes are similar in case 1 and 3 (Fig. 12c), although CV and SWV are also different. Fig. 13 represents in situ UV–vis spectral changes of CoPc4 in case 1 and 3 in DCM/TBAP electrolyte system. During the first reduction process, spectral changes of CoPc4 are same in case 1 and 3. These spectroscopic changes assign the first reduction couple to [CoIIPc2]/[CoIPc2]1 process in case 1 and 3 [36,39–41]. However, during the second reduction process, spectral changes of the reduction of monoanionic species [CoIPc2]1 are different in case 3 (Fig. 13b) and in case 1 (Fig. 13b inset). Under applied potential at 1.50 V, while spectral changes assigned to [CoIPc2]1/ [CoIPc3]2 process are recorded in case 1, spectrum of the complex in case 3, first of all, starts to return back and then the ring reduction process starts at the end of the process. This may be due to the 2 breaking down of the coordination between [CoIIPc2] and O2 2 O2 at the more negative potentials. Spectral changes during the oxidation processes are completely different in case 1 and 3. While the first oxidation process of CoPc4 is ring-based and second one is metal-based in DCM in case 1 (insets of Fig. 13c and d), the order of assignments changes in case 3. Spectral changes in Fig. 13c indicate the metal-based oxidation of CoPc4 under 0.80 V potential application (II couple) and a ring-based oxidation process under the 1.40 V potential application in case 3. These spectral changes may be due to that coordination of O2 with CoPc4 stabilizes CoIII,

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Fig. 11. (a) CVs of CoPc4 at 0.100 V s1 scan rate (inset: CVs of DCM/TBAP electrolyte with different O2 concentrations) and (b) SWVs of CoPc4 on Pt with increasing amounts of O2 in DMSO/TBAP electrolyte system. (SWV parameters: pulse size = 100 mV; step size: 5 mV; frequency: 25 Hz; Ei = 1.50 V and Ef = 1.80 V for SWV having cathodic currents; Ei = 1.80 V and Ef = 1.50 V for SWV having anodic currents).

thus metal-based oxidation is favored at the less positive potentials. It is well documented that while CoPc complexes give a metal-based oxidation process in the electrolyte containing coordinating ligands, it gives a ring-based oxidation process in non-coordinating environments [37]. According to the CV and in situ measurements, a proposed mechanism is given in Scheme 1. According to the proposed mechanism, it is sufficient to recall the following. Initially the active form of the catalyst ½CoI Pc2  is obtained (Scheme 1, Eq. II 2  (1)). Then an intermediate ½O resulting from the 2 —Co Pc  interaction of ½CoI Pc2  with O2 and the electron transfer between the active form of the catalyst and the substrate O2 is produced (Scheme 1, Eq. (2)). Differences in the in situ spectral changes of the complex in case 1 and case 3 indicates formation II 2  of the intermediate ½O and the equilibrium between 2 —Co Pc  the intermediate and the starting form of the catalyst. The interII 2  mediate ½O 2 —Co Pc  is very unstable, regenerating rapidly the starting form of the catalyst ½CoII Pc2  and the product O 2 (Scheme 1, Eq. (3)) or produces the second intermediate II 2 2 ½O2 due to the electron transfer reaction on the 2 —Co Pc  working electrode (Scheme 1, eq.4). This second intermediate reII 3 3 duced to ½O2 species if more negative potentials are 2 —Co Pc  scanned (Scheme 1, Eq. (5)) or regenerating the starting form of the catalyst ½CoII Pc2  and the product O2 (Scheme 1, Eq. (6)). 2 Differences in the in situ spectral changes of the complexes in

Fig. 12. In situ UV–vis spectral changes of CoPc3 in DCM/TBAP electrolyte saturated with O2 (insets: purged with N2). (a) (Eapp = 0.50 V. (b) Eapp = 1.50 V. (c) Eapp = 0.50 V. (Color coding of spectra: red: spectrum under open circuit; blue: spectrum at the end of the first electron transfer reaction; wine: spectrum at the end of the second electron transfer reaction; green: spectra show the changes between to redox states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

case 1 and case 3 indicates formation of the intermediate II 2 2 ½O2 2 —Co Pc  . According to these results, it looks like homogeneous catalytic ORR process occurs via an ‘‘inner sphere’’ chemiII 2  cal catalysis process. Since the intermediates ½O and 2 —Co Pc  II 2 2 ½O2 —Co Pc  are formed between O and active form of the 2 2 catalyst in the solution phase, so rate constant of these interactions should affect the catalytic activity of the complex. It is thus quite possible to predict how the catalytic efficiency, depends on the standard potential of the redox catalyst. Because it is clearly illustrated by Zagal and coworkers [12] that the catalytic activity of the MN4 complexes strongly depends on the nature of the central metal but also on the nature of the ligand. They suggested that formal potential of the complex needs to be in a rather narrow potential window for achieving maximum activity.

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Fig. 13. In situ UV–vis spectral changes of CoPc4 in DCM/TBAP electrolyte saturated with O2 (insets: purged with N2). (a) Eapp = 0.50 V. (b) Eapp = 1.50 V). (c) Eapp = 0.50 V. (d) Eapp = 1.10 V. (Color coding of spectra: red: spectrum under open circuit; blue: spectrum at the end of the first electron transfer reaction; wine: spectrum at the end of the second electron transfer reaction; green: spectra show the changes between to redox states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Scheme 1. Proposed homogeneous electrocatalytic mechanism of oxygen reduction reaction.

They also stated that the catalytic activity of metallophthalocyanines can be ‘‘tuned’’ by manipulating the formal potential, using proper substituents on the macrocyclic ligand. According to Zagal and the data on Table 1, there is no clear correlation between the

redox potentials and the catalytic activity (decreasing the overpotential of ORR) of the complexes. For example, a more negative Co(II)/(I) redox potential of the CoPc1 and CoPc2 does not give higher catalytic activity. Most probably steric effects of the

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substituents and/or the coordination ability of the active OH groups of the substituents to the cobalt center of the complexes affect the catalytic activities of the complexes. The electrocatalytic ORR discussed here is considered as inner-sphere reactions since a bond between oxygen and the active site of the complex (cobalt center) is expected to occur after the electron transfer takes place. It is stated that for inner-sphere reactions catalytic process can be inhibited as the active sites are blocked by the coordinating molecules bound to the active sites in the phthalocyanine or steric effect of the ligand alter the strength of the interaction between oxygen and the active sites in the phthalocyanine which alter the catalytic activity of the complexes [12]. The complexes having OH functional substituents (CoPc1 and CoPc2) decreases the overpotential of the ORR process less than the complexes carrying non-functional substituents (CoPc3 and CoPc4). These results support the steric and coordination effects of the substituents to the catalytic activity. In summary, voltammetric and spectroelectrochemical results of this study indicate that homogeneous electrocatalytic ORR follows an ‘‘inner sphere’’ chemical catalysis process and the proposed mechanism is given in Scheme 1. The results are in harmony with the previous reported homogeneous catalytic studies [22–25]. For example, Mho S. and coworkers reported the homogeneous interactions of unsubstituted cobaltphthalocyanine with oxygen and proposed a conventional regenerative catalytic mechanism. They did not determine the intermediates and just predicted the possible intermediate adducts and proposed mechanism with respect to voltammetric and chronoamperometric responses of the system [25]. In this paper for the first time, we illustrate the proposed intermediates with voltammetric and in situ spectroelectrochemical measurements especially by comparing the spectroelectrochemical changes of the complexes with and without the presence of molecular oxygen in the reaction media.

4. Conclusions Voltammetric and in situ spectroelectrochemical studies show that cobaltphthalocyanine complexes give both metal and ligandbased, diffusion controlled, multi-electron and reversible/quasireversible reduction processes. Substituent variation of the complexes affects the peak potential and assignment of the redox processes. Presence of O2 in the electrolyte system influences both the ORR and the redox couples of the complexes due to the interaction between different redox states of O2 and CoPcs. This interaction depends on the nature of the complex, O2 concentration and excitation signals of the voltammetric and spectroelectrochemical techniques. According to the voltammetric and spectroelectrochemical response of the systems with or without O2, the electrocatalytic activity of the complexes for the ORR depends on the substituents and solvent of the system. While CoPc1 is more active than CoPc2 in DMSO/TBAP electrolyte, CoPc3 is more active than CoPc4 in DCM/TBAP electrolyte for the ORR. Although all complexes interact with O2 with similar mechanism, rate of electron transfer, and interaction reactions and strength of coordination of O2 with the complex change with the substituent environment and electrolyte system of the complexes. Voltammetric and spectroelectrochemical results indicate that homogeneous electrocatalytic ORR follows an ‘‘inner sphere’’ chemical catalysis process.

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