Noncovalent Immobilization of a Pyrene-Modified Cobalt Corrole on

Aug 22, 2016 - However, the design and development of cheap, efficient, and robust catalysts for ORR and OER are still required. High-valent metal cen...
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Noncovalent Immobilization of a Pyrene-Modified Cobalt Corrole on Carbon Supports for Enhanced Electrocatalytic Oxygen Reduction and Oxygen Evolution in Aqueous Solutions Haitao Lei,† Chengyu Liu,† Zhaojun Wang,† Zongyao Zhang,† Meining Zhang,† Xingmao Chang,‡ Wei Zhang,‡ and Rui Cao*,†,‡ †

Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the determinants of the realization of a hydrogen-based society, as sluggish OER and ORR are the bottlenecks for the production and utilization of H2, respectively. A Co complex of 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenylcorrole (1) bearing a pyrene substituent was synthesized. When it was immobilized on multiwalled carbon nanotubes (MWCNTs), the 1/MWCNT composite displayed very high electrocatalytic activity and durability for both OER and ORR in aqueous solutions: it catalyzed a direct four-electron reduction of O2 to H2O in 0.5 M H2SO4 with an onset potential of 0.75 V vs normal hydrogen electrode (NHE), and it catalyzed the oxidation of water to O2 in neutral aqueous solution with an onset potential of 1.15 V (vs NHE, η = 330 mV). Control studies using a Co complex of 5,10,15-tris(pentafluorophenyl)corrole (2) demonstrated that the enhanced catalytic performance of 1 was due to the strong noncovalent π−π interactions between its pyrene moiety and MWCNTs, which were considered to facilitate the fast electron transfer from the electrode to 1 and also to increase the adhesion of 1 on carbon supports. The noncovalent immobilization of molecular complexes on carbon supports through strong π−π interactions appears to be a simple and straightforward strategy to prepare highly efficient electrocatalytic materials. KEYWORDS: oxygen evolution, oxygen reduction, cobalt corrole, electrocatalysis, noncovalent immobilization

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such as Mn,14−19 Fe,20−27 Co,28−38 Ni,39−45 and Cu,46−50 have been shown to be active catalysts for these two processes. However, the design and development of cheap, efficient, and robust catalysts for ORR and OER are still required. High-valent metal centers are generally thought to be involved in the O−O bond formation and breaking processes.4,51−53 As a consequence, effective stabilization of high-valent metal centers is crucial to improve the activity and durability of ORR and OER catalysts. Corrole ligands can afford a stable square-planar coordination environment.54 More importantly, their trianionic nature makes them very effective in stabilizing metal ions of high-valent states.54−56 Associated with this effect, corrole ligands can provide low-valent metal ions large reducing powers.57 All these factors make metal corroles very attractive for ORR and OER catalysis. Recent efforts have resulted in identifying several complexes of earth-abundant transition metals active for these two processes. For example,

INTRODUCTION Globally increasing energy demands and environmental concerns stimulated extensive research to find new energy systems that are sustainable, clean, low-cost, and environmentally benign.1−5 Hydrogen is considered to be an ideal energy carrier, and its generation through light-driven water splitting is a mimic of photosynthesis.6−8 However, efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the determinants of the realization of a hydrogen-based society, as these two processes are the bottlenecks for the production (via water splitting) and utilization (via fuel cell) of H2, respectively.9−11 Therefore, catalysts for OER and ORR are at the heart of renewableenergy technologies, and tremendous research interests have been directed at the exploration of efficient OER and ORR catalysts. Although noble-metal elements and their complexes/ alloys are very active for ORR (i.e., Pt)12 and OER (i.e., RuO2 and IrO2),13 the use of these noble-metal-based catalysts is limited by their low natural abundance and high cost. Alternatively, a variety of molecular complexes and materials made of cheap and earth-abundant transition-metal elements, © 2016 American Chemical Society

Received: June 5, 2016 Revised: August 20, 2016 Published: August 22, 2016 6429

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purchased from commercial suppliers and used as received unless otherwise noted. The dry solvents acetonitrile, tetrahydrofuran, dichloromethane, and dimethylformamide were purified by passage through activated alumina. The ligand 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenylcorrole78 and the Co complex of 5,10,15-tris(pentafluorophenyl)corrole29 (2) were synthesized using modified methods reported previously. Multiwalled carbon nanotubes (>95% purity, 1.23 wt % −COOH content, 20−30 nm o.d., 110 m2 g−1 surface area) and indium tin oxides (ITO) electrodes (thickness ∼2.2 mm, transmittance >82%, sheet resistance Fe > Ni > Mn > Cu.62 For OER, Mn and Co corroles have been shown to be active by Sun and us.29,63 Importantly, Sun and co-workers experimentally verified the O−O bond formation through the nucleophilic attack of a hydroxide ion on a MnVO complex,64 which is relevant to the oxidation of water at the oxygen evolving complex of Photosystem II.65 Dey and co-workers reported Mn corroles as bifunctional electrocatalysts for ORR and OER in water.66 Nocera and co-workers developed hangman metal corroles containing intramolecular acid−base groups to further improve the catalytic activities for both ORR and OER.67,68 Despite these achievements, however, catalytic performance in terms of efficiency and stability has not been satisfactory. Carbon materials have been widely used as supports for various applications due to their large surface areas, high mechanical strengths, structural flexibilities, and electrical conductivities.69−71 In particular, carbon nanotubes (CNTs) have been shown to be able to significantly expedite electrocatalytic processes.32,71−73 In general, there are three strategies for the immobilization of molecular complexes on carbon supports. First, the surface of CNTs is functionalized to attach molecular complexes through covalent bonds.22,74,75 Second, CNTs serve as templates for the formation of covalent organic frameworks, which cover the surface of CNTs.30 Third, molecular complexes are loaded on CNTs by simple adsorption.76,77 The first two strategies are restricted largely due to the difficulty in the functionalization of carbon supports and the critical requirement for molecular complexes to form covalent organic frameworks. For the third method, although it is simple, convenient, and nondestructive, the electrocatalytic activity and durability often depend on the strength of the noncovalent interactions (i.e., π−π interactions) between the catalysts and the supports. In order to fabricate corrole/CNT composites as competent ORR and OER electrocatalysts using a simple and straightforward strategy, we herein report the noncovalent immobilization of a pyrene-modified Co corrole 1 on multiwalled carbon nanotubes (MWCNTs) through strong π−π interactions between pyrene groups and carbon supports. The resulting 1/MWCNT composite displayed enhanced catalytic current and durability during both ORR and OER catalysis in comparison to its pyrene-free analogue. The catalytic ORR performance of 1/MWCNT is even better than that of the commercial Pt/C material in terms of the half-wave and peak potentials. This result demonstrates that the strong noncovalent π−π interactions between the pyrene moiety and MWCNT can lead to fast electron transfer from the electrode to 1 and can increase the adhesion of 1 on carbon supports. This noncovalent immobilization approach is thus valuable to be expanded to other catalyst systems of molecular complexes on carbon supports.

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EXPERIMENTAL SECTION General Methods and Materials. Manipulations of airand moisture-sensitive materials were performed under nitrogen using standard Schlenk line techniques. All reagents were 6430

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∼1.5 mg mL−1 in acetonitrile and ∼120 mg mL−1 in dichloromethane.

Electrochemical Analyzer). Cyclic voltammograms (CV) recorded in acetonitrile (0.1 M Bu4NPF6) used a threecompartment cell possessing a 0.07 cm2 glassy-carbon (GC) electrode as the working electrode, Pt wire as the auxiliary electrode, and Ag/AgNO3 as the reference electrode (BASi, 10 mM AgNO3, 0.1 M Bu4NPF6 in acetonitrile). Ferrocene was added at the end of the measurement as an internal standard, and the potential was converted relative to the normal hydrogen electrode (NHE; all potentials reported in this work are referenced to the NHE) following a literature protocol.79 In aqueous solvents, Ag/AgCl (KCl saturated) was used as the reference electrode. For the experiments conducted with or without O2, the solutions were saturated by bubbling high-purity O2 or N2 for at least 30 min before analysis. For rotating ring−disk electrode measurements, a bipotentiostat (Model CHI 832 Electrochemical Analyzer) and a rotating ring−disk electrode with a rotating GC disk electrode and a platinum ring electrode (ALS RRDE-2) were used. The collection efficiency of the ring−disk electrode was evaluated with the [Fe(CN)6]3−/4− redox couple and was calculated to be 0.47. The gaseous products were analyzed by an SP-6890 Gas Chromatograph. Crystallographic Studies. A complete data set for Co corrole complex 1 was collected. A single crystal of 1 suitable for X-ray analysis was coated with Paratone-N oil, suspended in a small fiber loop, and placed in a cooled gas stream at 150(2) K on a Bruker D8 VENTURE X-ray diffractometer. Diffraction intensities were measured using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection, indexing, data reduction, and final unit cell refinements were carried out using APEX2.80 Absorption corrections were applied using the program SADABS.81 The structure was solved with direct methods using SHELXS82 and refined against F2 on all data by full-matrix least squares with SHELXL-9783 following established refinement strategies. In the asymmetric unit, there are two molecules of 1 and one cocrystallized pyridine solvent molecule. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms binding to carbon were included in the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to. Details of the data quality and a summary of the residual values of the refinements are given in Table S1 in the Supporting Information. Preparation of Catalyst-Loaded Electrodes. Generally, 5 mg of MWCNT was added to 5 mL of DMF, and the mixture was sonicated in an ultrasonic cleaner for 30 min to yield a homogeneous MWCNT suspension (1 mg mL−1). The electrodes were rinsed with ethanol and dried. The MWCNT DMF suspension was dropped on the top of the GC (2 μL), GC disk (4 μL), or ITO (10 μL) electrodes using a pipet, and these electrodes were dried under ambient conditions. The acetonitrile solution of complex 1 or 2 (0.2 mM) was then added dropwise to the MWCNT-coated GC (2 μL), GC disk (4 μL), or ITO (10 μL) electrodes, and these electrodes were dried at room temperature. The electrodes were then dipped into a clean acetonitrile solution for 5 min with gentle agitation to remove weakly adsorbed complexes and were finally dried at room temperature in the dark. At room temperature, the solubility of complex 1 is ∼1 mg mL−1 in acetonitrile and ∼100 mg mL−1 in dichloromethane, and the solubility of complex 2 is

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RESULTS AND DISCUSSION Synthesis and Characterization. The reaction of 5,15bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenylcorrole and Co(OAc)2 in pyridine afforded red crystals of 1 in 78% yield. X-ray diffraction studies revealed that complex 1 crystallized in the monoclinic space group P21/c. In each asymmetric unit, there are two molecules of 1 and one cocrystallized pyridine solvent molecule. As shown in Figure 1, the Co atom is

Figure 1. (a) Molecular structures of Co corroles 1 and 2. (b) Thermal ellipsoid plot (50% probability) of the X-ray structure of 1.

coordinated by the corrole ligand through four N atoms and by two additional axial pyridine molecules, giving a six-coordinated octahedral geometry. The short Co−N(pyrrole) bond distances of 1.880(7) Å (an average value) and long Co−N(pyridine) bond lengths of 1.980(7) and 1.987(7) Å are consistent with a d6 CoIII electronic structure. The identity and purity of the bulk sample of 1 were confirmed by NMR spectroscopy (Figure S1 in the Supporting Information) and mass spectrometry (Figure S2 in the Supporting Information), which also suggested the dissociation of the coordinated pyridine molecules from the Co center upon dissolution in solution. Complex 2, as a pyrenefree analogue of 1, was also synthesized and fully characterized. CV of 1 in acetonitrile at a GC electrode showed two reduction waves (E1/2 = −0.01 V; E1/2 = −1.25 V) and two oxidation waves (E1/2 = 0.68 V; Epa = 1.26 V) (Figure 2a). The peak separation ΔEp for the two redox couples at −1.25 and 0.68 V was measured to be 67 mV, implying two reversible oneelectron-redox events (the ΔEp value of ferrocene is 70 mV). The redox couple at −0.01 V was a quasi-reversible oneelectron-redox event, as its ΔEp value was 105 mV. CV of 2 6431

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Figure 2. (a) CV of complex 1 in acetonitrile. Conditions: 0.1 M Bu4NPF6, GC electrode, 100 mV s−1 scan rate. (b) CVs of GC electrodes coated with complex 1 or 2 under N2 or O2 in 0.5 M H2SO4 solution (50 mV s−1 scan rate, 20 °C). (c) CVs of GC electrodes coated with 1/MWCNT under N2 and O2 in 0.5 M H2SO4 solution (50 mV s−1 scan rate, 20 °C). (d) Linear scan voltammetry of GC electrodes coated with 1/MWCNT or 2/MWCNT under O2 in 0.5 M H2SO4 solution (50 mV s−1 scan rate, 20 °C).

showed similar redox behaviors under the same conditions: two reduction waves with E1/2 = 0.08 and −1.23 V and two oxidation waves with E1/2 = 0.80 and 1.38 V (Figure S5 in the Supporting Information). In comparison with 1, the redox couples of 2 shift to the anodic direction by ∼100 mV, which is consistent with the replacement of a meso-pyrenylphenyl group in 1 by a strongly electron withdrawing meso-pentafluorophenyl group in 2. After 1 was loaded on the MWCNTs, the 1/MWCNT composite was characterized by using electronic absorption spectroscopy, SEM, TEM, EDX, and XPS, which all confirmed the presence of molecular complex 1 on the carbon support. As shown in Figure 3a, three strong absorption bands at λmax 372, 435, and 610 nm were observed in the UV−vis spectrum of 1/ MWCNT, which were not observed in the spectrum of blank MWCNT. A comparison of the electronic absorption spectra of 1 and 1/MWCNT revealed that these three absorption bands observed for 1/MWCNT could be attributed to complex 1, which showed strong absorption bands at λmax 381, 434, and 611 nm in acetonitrile (Figure S3 in the Supporting Information). The resemblance of the electronic absorption spectra of 1 and 1/MWCNT as well as the observation of a strong Soret band at 435 nm and a Q band at 610 nm indicated that complex 1 was present and maintained its intact molecular structure in the composite with MWCNTs. Importantly, the blue shifts observed for the absorption bands at 346 and 381 nm in complex 1 to 334 and 372 nm in 1/MWCNT strongly indicate interactions between molecules of 1 and MWCNTs. No such shifts were observed for complex 2 and 2/MWCNT (Figure S11 in the Supporting Information). In addition, the interactions between molecules of 1 and MWCNTs were further supported by fluorescence quenching experiments. As shown in Figure S12a in the Supporting Information, complex 1 is emissive, and its emission (with an excitation at 277 nm) is almost completely quenched upon the addition of MWCNTs to the solution (Figure S12b). Unlike 1, complex 2 is not emissive in the same region (Figure S12c), indicating that the emission centered at 437 nm of 1 can be attributed to the appended pyrene moiety. These results together suggest the

Figure 3. (a) UV−vis absorption spectra of 1 (red), MWCNT (black), and 1/MWCNT (blue) in acetonitrile. The blue shifts of the absorption bands at 346 and 381 nm in 1 to 334 and 372 nm in 1/ MWCNT are marked. (b) XPS survey scan spectra of 1 (red), MWCNT (black), and 1/MWCNT (blue). (c) SEM image of blank MWCNT. (d) SEM and (e, f) TEM images of the 1/MWCNT composite.

presence of strong interactions between molecules of 1 and MWCNTs in the ground state due to the introduction of the pyrene moiety to the macrocycle. The presence of 1 in the 1/MWCNT composite was further confirmed by EDX and XPS. In the EDX spectrum of 1/ MWCNT, peaks due to Co, N, and F were clearly observed (Figure S6 in the Supporting Information). Figure 3b displays the XPS survey scans of complex 1, MWCNT, and 1/MWCNT (sections of Co 2p, F 1s, and N 1s energy regions are shown in Figure S7 in the Supporting Information). The observation of peaks attributed to Co 2p, F 1s, and N 1s in the spectra of both 1 and 1/MWCNT as well as the absence of these peaks in the spectrum of blank MWCNT confirmed the presence of 1 in the composite. In addition, the close matching of peaks for Co 2p (796.2 and 779.1 eV), F 1s (688.1 eV), and N 1s (398.6 eV) and also the matching of atomic ratio of these elements (the table after Figure S7) in the XPS spectra of 1 and 1/MWCNT further indicated that the molecular structure of complex 1 was maintained intact in the composite. The morphologies of MWCNT and 1/MWCNT composite were characterized by SEM and TEM (Figure 3c−f). SEM and TEM images disclosed the successful loading of 1 on MWCNTs with average diameters of 20−30 nm. The attached species on MWCNTs with dark color indicates the presence of the heavy Co element compared to the light C element in MWCNTs. No bulky agglomeration was observed from the TEM images, suggesting the fair dispersion of 1 on the surface of MWCNTs. 6432

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ACS Catalysis Electrocatalytic Oxygen Reduction. Electrocatalytic ORR experiments were carried out in 0.5 M H2SO4 aqueous solutions at room temperature. When complex 1 or 2 was directly drop-casted onto the surface of a GC electrode, CVs of 1 and 2 under N2 were the same, showing no catalytic features (Figure 2b, black curve). Under O2 conditions, CV of 1 showed a pronounced catalytic current with an onset potential of 0.56 V (Figure 2b, red curve). The half-wave potential and peak potential are 0.40 and 0.27 V, respectively. This result indicates that complex 1 can efficiently catalyze the reduction of O2. Because the onset potential of ORR catalysis with 1 is >0.50 V more positive than the reduction potential of its CoIII/CoII couple, and it is only slightly negative of its CoIV/CoIII reduction potential, it is suggested that the formal CoIII state is the active species for O2 reduction. This result is consistent with those previously reported: the CoIII state of Co corroles is suggested to initiate O2 reduction.60,61,84 It is worth noting that corrole ligands are redox noninnocent, and thus for simplicity, we used the formal oxidation state of the Co center to describe these redox processes. Similarly, CV of 2 showed a large catalytic current with an onset potential at 0.58 V, a half-wave potential at 0.45 V, and a peak potential at 0.35 V (Figure 2b, blue curve). Although complex 1 displayed a larger ORR current density, the ORR onset potential of 2 was more positive, which is consistent with the more positive redox events of 2. When the GC electrode was modified with 1/MWCNT composite, its CV in 0.5 M H2SO4 aqueous solution showed a pronounced catalytic current under O2, which corresponded to the reduction of O2, as no such current was observed under N2 (Figure 2c). This catalytic ORR current has an onset potential of 0.75 V and a peak potential of 0.56 V. Control experiments using a GC electrode modified with blank MWCNTs showed catalytic ORR current with an onset potential of 0.52 V and a peak potential of 0.35 V (Figure S8 in the Supporting Information). Therefore, the catalytic ORR wave with 1/ MWCNT shifts to the anodic direction by about 230 mV in comparison to blank MWCNTs. The catalytic ORR activity of 1/MWCNT is even better than that of the commercial Pt/C material (with onset and peak potentials of 0.73 and 0.52 V, respectively; Figure S9 in the Supporting Information) in terms of the overpotential. Significantly, 1/MWCNT outperformed 2/MWCNT for catalytic O2 reduction under the same conditions. As shown in Figure 2d, the onset potentials are 0.78 and 0.70 V (measured at j = 0.25 mA cm−2) for 1/ MWCNT and 2/MWCNT, respectively; the peak current densities are 1.05 and 0.76 mA cm−2 for 1/MWCNT and 2/ MWCNT, respectively. As we addressed above, the ORR onset potential with complex 2 directly loaded on a GC electrode was more positive than that with complex 1. The enhanced catalytic performance of 1/MWCNT indicated stronger interactions between 1 and the carbon support. More importantly, after the composite was washed with dichloromethane, the washing solution of 1/MWCNT was almost colorless but the washing solution of 2/MWCNT became purple-green, indicating the extraction of 2 into dichloromethane. As we expected, washed 1/MWCNT maintained the initial electrocatalytic ORR performance, while washed 2/MWCNT lost most of its activity for O2 reduction (Figure 4). It is therefore suggested that the strong noncovalent π−π interactions between the pyrene moiety of 1 and MWCNT is crucial to facilitate fast electron transfer from the lectrode to 1 and also to increase the adhesion of 1 on carbon supports,85 which eventually led to the

Figure 4. Linear sweep voltammetry of GC electrodes coated with 1/ MWCNT (a) and 2/MWCNT (b) before and after washing with dichloromethane. Conditions: 0.5 M H2SO4, 50 mV s−1 scan rate, 20 °C.

enhanced catalytic performance of 1 for O2 reduction. It is worth noting that the solubilities in dichloromethane for 1 (∼100 mg mL−1) and 2 (∼120 mg mL−1) are similar. In addition to activity, selectivity is another important factor to assess the performance of an ORR catalyst. O2 can be reduced by four electrons to produce H2O or by two electrons to produce H2O2. To gain more insights into the catalytic ORR with 1/MWCNT composite, the number of electrons (n) transferred per O2 molecule was determined by rotating disk electrode (RDE) and rotating ring−disk electrode (RRDE) measurements. In RRDE, the disk electrode was scanned from 1.0 to 0 V, while the ring potential was held at 1.15 V to completely oxidize H2O2 that was possibly generated during ORR back to O2. As presented in Figure 5a, 1/MWCNT

Figure 5. (a) RRDE measurements for O2 reduction at the GC disk electrode coated with 1 (red), MWCNT (black), and 1/MWCNT (blue) in an O2-saturated 0.5 M H2SO4 solution at 1000 rpm. The ring electrode was polarized at 1.15 V. (b) n value of O2 reduction with 1 (red), MWCNT (black), and 1/MWCNT (blue). (c) RDE measurements for O2 reduction at the 1/MWCNT-modified GC disk electrode in O2-saturated 0.5 M H2SO4 solution at various rotation rates. (d) Koutecky−Levich plots for O2 reduction at the 1/MWCNTmodified GC disk electrode. Conditions: GC disk (area 0.125 cm2), Pt ring (area 0.188 cm2), 10 mV s−1 scan rate, 20 °C.

exhibited a superior electrocatalytic ORR activity associated with a more positive ORR onset potential, a much larger cathodic disk current density, and a significantly smaller anodic ring current density in comparison to those for 1 and blank MWCNT. According to the current densities detected at the disk electrode and the ring electrode, the amount of H2O2 generated during ORR with 1/MWCNT was calculated to be ∼4.4%, which was much lower than those with 1 (∼34.5% 6433

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On the other hand, sufficient electron transfer flux from the electrode to the catalyst is also a significant determinant for the complete reduction of O2. The further reduction of superoxo or peroxo species for O−O bond cleavage and the removal of these intermediates from the catalytic site via hydrolysis are two competing reaction pathways. As demonstrated by Collman and co-workers, the amount of partially reduced oxygen species will increase if the electron transfer rate decreases.86,87 In other words, under insufficient electron transfer flux, partially reduced oxygen species will be released prior to the O−O bond cleavage. The strong interactions between pyrene and the carbon support will lead to increased electron transfer rates for 1/MWCNT, which may also benefit the four-electron reduction of O2. In addition to enhanced activity and selectivity for the reduction of O2 to H2O, 1/MWCNT showed improved durability during catalysis. In 11 h chronoamperometric durability tests, the current with 1/MWCNT composite decreased by only 6% in an air-saturated 0.5 M H2SO4 solution under an applied constant potential of 0.55 V, while under the same experimental conditions, the currents with Pt/C and 2/ MWCNT decreased by 18% and 15%, respectively (Figure 6a,b). The improved durability of 1/MWCNT in comparison

H2O2) and MWCNT (∼32.8% H2O2). On the basis of our experimental results, the n value of 3.8 was calculated for 1/ MWCNT (Figure 5b), which was much higher than those for 1 (2.5) and for MWCNT (2.6) and was also higher than that using 2/MWCNT (3.6) as the electrocatalyst. In RDE experiments, the n value can be calculated using the Koutecky−Levich (K-L) analysis: 1 1 1 1 1 = + =− − 2/3 −1/6 j jk jd nFkCO2 0.2nFDO2 ν CO2ω1/2

where j is the measured current density, jk and jd are the kinetic and diffusion-limited current densities, respectively, k is the rate constant for ORR, F is the Faraday constant (96485 C mol−1), n is the number of electrons transferred per molecule of O2, ω (rpm) is the rotation rate, CO2 is the concentration of O2 in the bulk solution, DO2 is the diffusion coefficient of O2, and ν is the kinematic viscosity of the solution. The constant 0.2 is adopted when the rotation speed is expressed in rpm. The n value thus determined for 1/MWCNT remained ∼3.7 as the rotation speed was increased from 500 to 2500 rpm (Figure 5c). This value is in good agreement with the n value calculated using the data from RRDE experiments. Moreover, the linearity of the Koutecky−Levich plots and the nearparallelism of the fitting lines under applied potentials of 0−0.4 V suggested a first-order reaction kinetics with respect to the concentration of dissolved O2 and similar electron transfer numbers for ORR at different potentials (Figure 5d). In addition, the ability of 1/MWCNT to electrocatalyze the reduction of H2O2 that possibly formed in the two-electron reduction of O2 was also examined. The result showed that the catalytic activity of 1/MWCNT toward H2O2 reduction was negligible, which was consistent with previous studies using Co corrole based ORR catalysts.28,60 In a short summary, 1/ MWCNT composite can efficiently catalyze the reduction of O2 to H2O through a direct four-electron process. For mononuclear metal complexes, in principle, only those composed of early transition metal elements are thought to be able to catalyze the four-electron reduction of O2. On the other hand, mononuclear metal complexes of late-transition-metal elements have been shown to typically catalyze the twoelectron reduction of O2. This difference in ORR selectivity can be explained by the ease in forming a terminal metal oxo species,58 which is the key intermediate produced upon the heterolytic O−O bond cleavage during O2 reduction. For latetransition-metal elements, such as Co, their electrons in the d orbitals will occupy antibonding orbitals of the metal−oxo unit. As a result, terminal oxo complexes of late-transition-metal elements are not likely to be generated due to the electrostatic repulsion of electrons between their d orbitals and the oxo ligand. However, late-transition-metal complexes can still catalyze the four-electron reduction of O2 through a bimetallic mechanism with the formation of dinuclear peroxo complexes. Subsequent homolytic O−O bond cleavage can lead to the complete reduction of O2 to H2O. In the case of Co corroles, previous studies revealed that mononuclear complexes usually catalyzed the two-electron reduction of O2 to H2O2, while dinuclear complexes could catalyze the four-electron reduction of O2 to H2O.61 Therefore, the formation of dimers or oligmers of 1 on the surface of MWCNTs is considered to be responsible for the observed catalytic activity and selectivity of 1/MWCNT toward the four-electron reduction of O2.

Figure 6. Controlled-potential electrolysis of GC electrodes coated with 1/MWCNT and Pt/C (a) or 2/MWCNT (b) at 0.55 V in airsaturated 0.5 M H2SO4 solutions. CVs of 1/MWCNT (c) and Pt/C (d) in O2-saturated 0.5 M H2SO4 solution with (red) or without (black) the addition of 3.0 M methanol.

to 2/MWCNT was suggested to be a result of the smaller amount of partially reduced oxygen species (potentially destructive to Co corrole catalysts) generated during ORR and the strong adhesion of complex 1 on the carbon support. These results identified the significant role of pyrene in the electrocatalytic O2 reduction. In methanol crossover tests, 1/ MWCNT exhibited an unaffected current response after the introduction of 3.0 M methanol (Figure 6c), but the CV of Pt/ C showed an apparent methanol oxidation current in the presence of methanol (Figure 6d). Electrocatalytic Oxygen Evolution. CV studies of 1 in acetonitrile showed two oxidation events at 0.68 and 1.26 V (Figure 2a), indicating that 1 has a sufficient oxidizing power for water oxidation. In order to examine the electrocatalytic OER properties of 1, we immersed the GC electrode coated with 1/MWCNT into a 0.1 M pH 7.0 phosphate buffer. As shown in Figure 7a, CV of 1/MWCNT displayed a significant 6434

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prepared and washed 1/MWCNT and 2/MWCNT as catalysts (Figure 4). It is also worth noting that the catalytic current of as-prepared 1/MWCNT (11.9 mA cm−2 at 1.55 V) is larger than that of as-prepared 2/MWCNT (7.5 mA cm−2 at 1.55 V). All of these results further confirmed that the OER activity and surface stability of 1 on the carbon support were largely increased due to the presence of strong π−π interactions between the pyrene group and the MWCNT surface. On the basis of these results, we can conclude that the molecular design of catalyst 1 has the following three guiding proposals. First, the Co corrole unit is the active site for both OER and ORR. Second, the two strongly electron withdrawing meso-pentafluorophenyl substituents can shift the redox events of Co corrole to the anodic direction, which is beneficial to ORR to reduce the overpotential for O2 activation. On the other hand, meso-pentafluorophenyl substituents can decrease the electron density of the macrocycle and thus increase its stability against oxidative decomposition during OER catalysis. Third, the introduction of a pyrene unit is considered to increase the π−π interactions between the catalyst and the carbon support. Such strong noncovalent interactions should lead to enhanced electrocatalytic activity and stability of the catalyst on carbon supports. Importantly, our results demonstrate that the combination of these structural factors is successful.

Figure 7. (a) CVs of GC electrodes coated with 1 (red), MWCNT (black), and 1/MWCNT (blue) and a blank GC electrode (pink) in neutral aqueous solutions. (b) Controlled-potential electrolysis at 1.40 V using ITO electrodes coated with MWCNT (black) and 1/ MWCNT (blue) in neutral aqueous solutions. CVs of GC electrodes coated with 1/MWCNT (c) and 2/MWCNT (d) in neutral aqueous solutions before (blue) and after (red) washing with dichloromethane. Conditions: 0.1 M pH 7.0 phosphate buffer, GC (area 0.07 cm2) and ITO (area 0.25 cm2) working electrodes, 100 mV s−1 scan rate, 20 °C.

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CONCLUSIONS In summary, the pyrene-modified Co corrole 1 was synthesized, and its 1/MWCNT composite was shown to be an efficient and robust catalyst for both electrocatalytic ORR and OER in aqueous solutions. The 1/MWCNT composite can catalyze the selective four-electron reduction of O2 to H2O at an onset potential of 0.75 V in 0.5 M H2SO4 and can catalyze water oxidation to O2 in neutral aqueous solution with an onset potential of 1.15 V. This Co corrole 1 thus represents one of the few examples of bifunctional molecular catalysts for ORR and OER. A comparison between 1 and its pyrene-free analogue 2 revealed that the pyrene group played a significant role in improving the catalytic activity and stability of 1 on MWCNTs through its strong π−π interactions with the carbon support. It has been established that such strong π−π interactions are responsible for the increased electron transfer efficiency and adhesion of the supported catalyst. This work demonstrates that noncovalent immobilization of molecular catalysts on carbon supports through strong π−π interactions is a simple and straightforward method for the fabrication of hybrid catalyst−carbon materials with efficient electrocatalytic properties.

catalytic current with the onset potential of 1.15 V (measured at j = 1 mA cm−2). This value corresponds to an onset overpotential of 330 mV, which is smaller than those represented by many other recently reported molecular OER catalysts. For example, in the case of Co-based molecular complexes functioning as heterogeneous OER catalysts, hangman Co β-octafluoro corrole displayed an onset overpotential of 630 mV under the same conditions.67 Other molecular Co complexes functioning as homogeneous OER catalysts include Co porphyrins with onset overpotentials of ∼400 mV in pH 7.0 aqueous solutions88 and Co polypyridines with onset overpotentials of ∼510 mV in pH 9.2 aqueous solutions.89 The durability of 1/MWCNT for water oxidation was also examined. CVs of 50 successive cycles were stable with less than a 5% drop in the catalytic current (Figure S10 in the Supporting Information). Controlled-potential electrolysis at 1.40 V was then performed in a 0.1 M pH 7.0 phosphate buffer using a three-electrode cell equipped with an ITO working electrode coated with 1/MWCNT, a Pt-wire counter electrode, and a Ag/AgCl reference electrode. During 2 h electrolysis, the current density with 1/MWCNT was maintained at ∼0.90 mA cm−2 (Figure 7b). The amount of O2 that formed can be quantitatively determined by analyzing the headspace of a gastight electrochemical cell with gas chromatography, which gives a Faradaic efficiency of greater than 95%. These results confirmed that the 1/MWCNT composite exhibited high stability under catalytic OER conditions. In order to prove that the surface stability was enhanced by π−π interactions between the pyrene moiety and the MWCNT surface, we washed the 1/ MWCNT composite with dichloromethane, and the resulting material was subjected to electrocatalytic OER catalysis. As shown in Figure 7c, the catalytic currents of as-prepared and washed 1/MWCNT are almost the same. However, as a control, the catalytic current of washed 2/MWCNT decreased significantly to the background level (Figure 7d). This result is consistent with the ORR studies presented above using as-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01579. Figures S1−S12, Scheme S1, and Table S1 as described in the text (PDF) Crystallographic data for complex 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.C.: [email protected]. Notes

The authors declare no competing financial interest. 6435

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ACKNOWLEDGMENTS We are grateful for support from the “Thousand Talents Program” of China, the National Natural Science Foundation of China (Grant No. 21101170, 21503126, and 21573139), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China.

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