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Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency Jia Meng, Haitao Lei, Xialiang Li, Jing Qi, Wei Zhang, and Rui Cao ACS Catal., Just Accepted Manuscript • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Catalysis

Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency

Jia Meng,‡ Haitao Lei,‡ Xialiang Li, Jing Qi, Wei Zhang, and Rui Cao*

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China

‡

These authors contributed equally to this work.

*Correspondence E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract Cobalt complexes have been extensively explored in catalyzing the oxygen reduction reaction (ORR), which is the cathode reaction in fuel cells. Although they show high activities, mononuclear Co complexes typically mediate the 2e reduction of O2 to H2O2, and two Co sites are generally required to catalyze the 4e reduction of O2 to H2O. Herein we report the significantly improved efficiency of covalently grafted Co corroles on carbon nanotubes (CNTs) for the 4e ORR. Azide-containing Co corroles can be attached to alkyne-modified CNTs via azide-alkyne cycloaddition. This attachment can avoid the formation of dimeric face-to-face Co corroles. The resulted hybrid catalyzes the 4e ORR in 0.5 M H2SO4 aqueous solutions with a half-wave potential at 0.78 V vs reversible hydrogen electrode (RHE). This performance makes this hybrid one of the most efficient Co-based molecular ORR electrocatalysts. Control studies using Co corrole analogues loaded on CNTs via noncovalent interactions give ORR half-wave potentials at 0.68-0.61 V vs RHE. This work is significant to demonstrate that with proper covalent bond interactions to CNTs, mononuclear Co corroles can become intrinsically active for the 4e ORR with significantly improved efficiency.

Keywords: oxygen reduction, cobalt corrole, covalent immobilization, mononuclear, electrocatalysis.


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Introduction Developing efficient and robust electrocatalysts for O2 reduction has attracted considerable interests because this process is sluggish in kinetics but is important as a cathode reaction in fuel cells.1-3 Platinum and its alloys are very active for ORR,4 but their uses are limited due to the very low natural abundance and high cost of this noble metal. As an alternative, extensive efforts have been made in the last decade to make ORR electrocatalysts derived from earth-abundant and cheap transition metal elements.5-17 For mononuclear transition metal catalysts, O2 can be reduced by four electrons to form H2O or by two electrons to form H2O2 (Scheme 1).3 In general, late transition metal complexes typically catalyze ORR via the 2e pathway because it is challenging for the peroxo intermediates M−OOH to undergo heterolytic O−O bond cleavage, leading to the generation of terminal metal-oxo species that are high in energy. However, the 4e ORR can still be accomplished by late transition metal complexes through the formation of dinuclear peroxo intermediates. Cobalt complexes have been widely studied and are shown to be highly active for ORR.18-30 As inspired by cytochrome c oxidases (CcOs) with heme active sites for the 4e ORR,31 a variety of Co porphyrins and derivatives have been investigated as ORR catalysts. Mononuclear and cofacial dinuclear Co porphyrin catalysts are presented by Anson, Fukuzumi, Kadish and others.32-45 Mononuclear Co corroles and Co complexes of porphyrin-corrole dyads and biscorroles are studied by Kadish, Gross, Cao and others.46-55 Although porphyrin and corrole ligands are negatively charged and thus are effective in stabilizing metal ions in their high valent states to


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benefit the O−O bond cleavage, dimeric Co-based forms are still usually required for the 4e ORR.56-59 Xia and co-workers performed theoretical studies and suggested that the highest-occupied 3d orbital of Co porphyrins has a relatively low energy level, leading to the weak activity for the 4e ORR.60 It is necessary to note that mononuclear Co complexes can form face-to-face dimeric Co species in both solution and solid states to catalyze the 4e ORR. This phenomenon has been demonstrated by Girault and co-workers, who studied ORR using Co porphyrins at liquid-liquid interfaces.61-63

Scheme 1. The reduction of O2 catalyzed by mononuclear transition metal complexes, showing the possible reaction pathways for the 4e and 2e ORR.

Synthetic modeling studies of CcOs provide fundamental knowledge for the rational design of efficient ORR catalysts.64,65 Collman and co-workers revealed that rapid electron transfer to heme was essential for CcOs to catalyze the 4e reduction of O2.66 In nature, this rapid electron transfer is realized by involving extra redox centers (i.e., a tyrosine unit and a Cu ion) located in the vicinity of the heme unit. For biomimetic systems, Dey and co-workers reported that Fe porphyrins with ferrocene units attached at the second coordination sphere were able to catalyze the 4e ORR.67,68


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ACS Catalysis

The Dey’s group also showed the switch between 2e vs 4e ORR by controlling the electron transfer rate from the electrode to the Fe porphyrin unit through different linkers.69 On the other hand, Collman and co-workers revealed that hydrogen bonding interactions were able to stabilize the O2-adduct and to assist the O−O bond cleavage, leading to much improved 4e ORR selectivity.70 Similar effects of hydrogen bonding interactions were reported by Mayer, Dey and others with the use of Fe porphyrin model complexes.71-76 Nocera and co-workers demonstrated that intramolecular acid/base groups of hangman Co porphyrins and corroles could lead to increased 4e ORR activity.77,78 These results not only shed light on understanding the reaction mechanism of CcOs for O2 reduction but also provide valuable information for designing new efficient ORR catalysts. Herein we report the high efficiency of covalently grafted mononuclear Co corroles on CNTs for the 4e reduction of O2. Recently, we reported the covalent immobilization of azide-containing Co corroles onto alkyne-modified CNTs through azide-alkyne cycloaddition.79 CNTs are chosen as the support for electrocatalysis due to their good electrical conductivities and large surface areas.80 With the use of this method, we can graft Co corrole 1 onto CNTs (Figure 1). The resulted hybrid CNT-1 is highly efficient in catalyzing the 4e ORR in 0.5 M H2SO4 aqueous solutions with a half-wave potential at 0.78 V vs RHE. This performance is significantly improved in comparison with Co corrole analogues immobilized on CNTs through noncovalent interactions and is also outstanding as compared with previously reported Co-based molecular ORR catalysts. Because such covalently attached Co corroles cannot form


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dimeric face-to-face Co species, this work provides direct experimental evidence that mononuclear Co corroles can be intrinsically active for the 4e ORR. As demonstrated above, extensive efforts have been made to prove and improve the 4e ORR activity of Fe porphyrins. However, the 4e ORR activity of mononuclear Co complexes has been poorly understood, although a large number of Co-based catalysts have been reported to be able to catalyze ORR with high efficiency. This work is therefore significant to show the significantly improved 4e ORR activity of mononuclear Co corroles and will bring new insights into the ORR with complexes of late transition metal elements.

Figure 1. (a) Molecular structures of Co corroles 1, 2, and 3. (b) Illustration showing the preparation of CNT-1, CNT-2, and CNT-3.


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Experimental Section General methods and materials Materials that are sensitive to air and moisture were manipulated under N2 using standard Schlenk line techniques. All reagents were commercially available and were used directly unless otherwise noted. Dry solvents, including dichloromethane, tetrahydrofuran, acetonitrile, and dimethylformamide were used in this work. Co corroles were synthesized according to the method we reported previously.79 All aqueous solutions were prepared freshly using the Milli-Q water. Multi-walled CNTs (>95% purity, < 8 nm outside diameter, 3 nm intside diameter) were commercially available. MALDI-TOF mass spectra were obtained using Brüker MAXIS. UV-vis spectra were measured using a Hitachi U-3310 spectrophotometer. 1H NMR spectra were acquired on a Brüker spectrometer. Transmission electron microscopy (TEM) images were obtained using JEOL JEM-2100 with a 200-kV accelerating voltage. Hitachi SU8020 cold-emission field emission scanning electron microscope (FE-SEM) was used to study the sample morphologies with an accelerating voltage of 1 kV. Energy dispersive X-ray (EDX) spectra were measured from three areas of each sample to ensure the chemical homogeneity. X-ray photoelectron spectroscopy (XPS) data were collected by ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. The C 1s peak at 284.6 eV arising from adventitious hydrocarbon was used for the correction of binding energies. All samples used were carefully prepared via centrifugation and re-dispersion.


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Synthesis of Co corrole 1 To the solution of 5,15-bis-(pentafluorophenyl)-10-(4)-(1-azido)phenylcorrole (19 mg, 0.025 mmol) and anhydrous NaOAc (30 mg, 0.36 mmol) in 10 mL methanol, were added Co(OAc)24H2O (30 mg, 0.12 mmol) and triphenylphosphine (PPh3, 50 mg, 0.19 mmol). The resulted mixture was stirred for 1.5 h at room temperature, and the solvent was evaporated. The residue was then subjected to silica chromatography (petroleum ether : dichloromethane = 5 : 1) to afford purple solids (22 mg, 85%). 1H NMR (400 MHz, CDCl3): δ = 8.69 (d, J = 4.4 Hz, 2H), 8.34 (brs, 2H), 8.21 (d, J = 4.4 Hz, 2H), 8.01 (d, J = 4.4 Hz, 2H), 7.76-7.69 (m, 2H), 7.57-7.49 (m, 2H), 7.03 (t, J = 7.3 Hz, 3H), 6.69 (t, J = 6.6 Hz, 6H), 4.68 (s, 6H) (Figure 2a). HRMS: calcd for C55H28CoF10N5P [1-N2+H+]+, 1038.1249; found, 1038.1245 (Figure 2b).

Preparation of CNTs modified with 4-ethynylaniline Sonication was performed to disperse CNTs to avoid any agglomeration and sedimentation. CNTs (30 mg) were mixed with 4-ethynylaniline (307 mg, 2.6 mmol) and sonicated together in 70 mL of hydrochloric acid solution (0.5 M). Sodium nitrite (1.5 equivalent to 4-ethynylaniline) dissolved in 15 mL of deionized water was then added to generate the diazonium salt from 4-ethynylaniline. The addition of iron powder (500 mg) could reduce the diazonium salt to generate the corresponding aryl radical. Generated aryl radicals can be grafted onto available surfaces, and successive radical substitutions finally yielded dehydrobenzene coatings on CNTs. The crude products were stirred in excessive hydrochloric acid to remove the residue iron, and


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were washed with ethanol, acetone, and water for several times until no Fe content was detected in both the washing solution and the hybrid. The modified CNTs can be obtained after drying at room temperature.

Caution! The addition of iron powder to the hydrochloric acid solution is potentially explosive and should be handled with care and in small amounts.

Preparation of CNT-1 through the azide-alkyne cycloaddition CNTs (10 mg) modified with 4-ethynylaniline were mixed with 5 mg of Co corrole 1 in 5 mL of dimethylformamide, and the mixture was stirred at 85 °C overnight. The suspension was kept in a sonication bath for 1 h, and the crude CNT-1 was collected by centrifugation. The solids were re-suspended in dichloromethane to remove simply adsorbed complex 1 on CNTs, and then collected by centrifugation again, and were dried in the dark at room temperature. The Co content in the hybrid CNT-1 was determined by using inductively coupled plasma mass spectrometry (ICP-MS, Bruker aurora). CNT-1 was immersed in a concentrated high purity HNO3 solution under ultrasonication to dissolve the Co species. The HNO3 solution was filtered and was then diluted into 20 mL for ICP-MS measurements. The Co ICP-MS standards (AccuStandard) with a concentration of 50, 100, 200, 300, 400, and 500 ppb were used for the calibration.

Preparation of catalyst-loaded electrodes


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In order to prepare CNT-1-loaded electrode, 1.0 mg of CNT-1 was added to 1.0 mL of dimethylformamide (DMF), and the resulted mixture was treated with an ultrasonic cleaner to give a homogeneous CNT-1 suspension (1 mg mL−1). The glassy carbon (GC) electrode was rinsed with ethanol and then dried. The CNT-1 DMF suspension was added onto the GC electrode (5 µL) or GC disk electrode (16 µL) using a pipet, and the electrode was dried slowly in the dark. In order to prepare electrodes loaded with non-covalently immobilized Co corroles, 5 mg of CNTs were added to 5 mL of DMF, and the resulted mixture was treated with an ultrasonic cleaner to give a homogeneous CNT suspension (1 mg mL−1), which was then added onto the GC electrode (5 µL) or GC disk electrode (16 µL) using a pipet. The electrode was then dried slowly. The acetonitrile solution of Co corrole 2 or 3 (1 mg mL−1) was dropwisely added to the CNT-coated GC electrode (5 µL) or GC disk electrode (16 µL), and the electrode was dried in the dark at room temperature. The electrode was then gently rinsed with acetonitrile to remove weakly adsorbed corroles, and was dried in the dark at room temperature. The Co contents in these samples of noncovalent immobilization were also determined by ICP-MS.

Electrochemistry Electrochemical measurements were carried out using CH Instruments (model CHI660D Electrochemical Analyzer). Cyclic voltammogram was collected in 0.1 M Bu4NPF6 acetonitrile solution using a three-compartment cell with a 0.07 cm2 GC working electrode, graphite rod counter electrode, and Ag/AgNO3 reference electrode.


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At the end of each measurement, ferrocene was added as an internal standard to calibrate the potential. In aqueous solutions, Ag/AgCl reference electrode was used. Bipotentiostat (model DY2300 Electrochemical Analyzer) was used for rotating ring-disk electrode measurements. The rotating ring-disk electrode has a GC disk electrode (0.125 cm2) and a platinum ring electrode (0.188 cm2, ALS RRDE-2). The collection efficiency of 0.425 was evaluated for this ring-disk electrode using the [Fe(CN)6]3−/4− redox couple.

Results and Discussion Synthesis and characterization of Co corroles Co corrole 1 (Figure 1a) was synthesized by the reaction of Co acetate and 5,15-bis-(pentafluorophenyl)-10-(4)-(1-azido)phenylcorrole. Triphenylphosphine was added as an axial ligand during the reaction. Purple crystals of 1 were obtained with a yield of 85% and were used for subsequent reaction with the alkyne-modified CNTs. Complex 1 was characterized by 1H NMR (Figure 2a) and high-resolution mass spectrometry (HRMS, Figure 2b). In the 1H NMR spectrum, the eight β-hydrogen atoms of the corrole ring and the four hydrogen atoms of the 10-(4)-(1-azido)phenyl substituent are found as sharp peaks in the range of 8.80-7.50 ppm. The rest three hydrogen peaks at 7.03 (3 H), 6.69 (6 H), and 4.68 (6H) ppm are due to the axial PPh3 unit. The large up-field movement of the PPh3 hydrogen peaks is caused by the aromatic effect of the corrole ring, which is strong evidence to support the binding of PPh3 at the Co axial site. The HRMS of 1 shows an ion with the mass-to-charge ratio


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of 1038.1245. This value matches the calculated value of 1038.1249 for the monocation of [1-N2+H+]+ with the expected isotopic distribution. This result suggested that during the HRMS measurement, the axial PPh3 ligand was retained but the azide group decomposed to lose a N2 molecule. Additionally, the UV-vis spectrum of 1 shows characteristic corrole-based Soret bands at 375 and 411 nm and Q bands at 556 and 587 nm (Figure 2c).81 On the basis of these results, we can confirm the identity and purity of Co corrole 1. Co complexes of 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenylcorrole (2) and 5,10,15-tris(pentafluorophenyl)corrole (3) were synthesized according to our previously reported methods.81 The redox properties of Co corroles 1-3 were then studied by using cyclic voltammetry in acetonitrile. As shown in Figure 2d, the cyclic voltammogram (CV) of 1 on a GC electrode displays two quasi-reversible reduction couples at E1/2 = −0.86 and −1.87 V vs ferrocene in acetonitrile, which are ascribed to the formal CoIII/CoII and CoII/CoI couples, respectively. These values are very similar to those of Co corroles 2 and 3. For complex 2, the two reduction couples have E1/2 = −0.76 and −1.88 V vs ferrocene, and for complex 3, they are E1/2 = −0.72 and −1.83 V vs ferrocene. These results demonstrate that all three Co corroles have very similar redox behaviors, which are important for direct comparison of their ORR activities when loaded on CNTs.


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Figure 2. (a) 1H NMR spectrum of Co corrole 1 in CDCl3. (b) HRMS of 1 in methanol. The observed isotopic distribution pattern is identical to that calculated using the formula C55H28CoF10N5P, [1-N2+H+]+. (c) UV-vis absorption spectrum of 1 in DMF at room temperature. (d) CVs of Co corroles 1, 2, and 3 in 0.1 M Bu4NPF6 acetonitrile. Conditions: GC electrode, 100 mV s−1 scan rate, 20 °C.

Covalent immobilization of 1 on CNTs To graft the azide-containing Co corrole 1 onto CNTs through covalent bonds, CNTs were first functionalized with 4-ethynylaniline to introduce surface alkyne groups.79,82 The as-prepared alkyne-functionalized CNTs were denoted as CNT-alkyne hereafter. Notably, iron powder used in this synthesis was carefully removed by stirring the products in hydrochloric acid and then washing with ethanol, acetone, and water. This process was repeated for several times until no Fe content was detected in the washing solution by using ICP-MS. The as-prepared CNT-alkyne


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was further analyzed by XPS to exclude the presence of residual Fe. This washing step is essential because Fe-doped CNTs are known to have some activities for ORR. The alkyne-functionalized CNTs were analyzed by Raman, showing the decrease of the ID/IG ratio (Figure 3a, ID and IG represents the intensity of the D and G bands at 1341 and 1583 cm−1, respectively). Although diazonium treatment will usually cause the increase of the ID/IG ratio, but the surface-modification of CNTs with alkyne groups, which contain abundant graphite-like C atoms, will lead to the decrease of the ID/IG ratio.83-85 Subsequently, Co corrole 1 was grafted onto the alkyne-modified CNTs through the azide-alkyne cycloaddition to make hybrid CNT-1 (Figure 1b). Notably, during this cycloaddition, no metal catalysts were added to avoid the contamination of the CNT hybrid with other metal species. The successful attaching of molecules of 1 onto CNTs was confirmed. The Raman spectrum of CNT-1 showed further decrease of the ID/IG ratio to 0.64 (Figure 3a), which is consistent with the surface-modification with corrole moieties that contain abundant sp2 C atoms. When a GC electrode was loaded with blank CNTs and was immersed in an acetonitrile solution of 1, the CV showed two quasi-reversible reduction couples at E1/2 = −0.89 and −1.91 V vs ferrocene (Figure 3b, blue line). These values match very well with the reduction couples of 1 measured by using a freshly cleaned GC electrode (Figure 3b, black line).


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Figure 3. (a) Raman spectra of CNT-1, CNT-alkyne, and unmodified CNT. (b) CV of GC electrode loaded with CNT-1 in acetonitrile, CVs of GC electrode loaded with or without blank CNTs in the acetonitrile solution of 1. (c) FTIR spectra of CNT-1, 1, and CNT. (d) XPS survey scan spectra of CNT-1 and CNT. (e) SEM image of CNT-1. (f) TEM image of CNT-1.

Importantly, the CV of GC electrode loaded with CNT-1 showed two small but significant reduction processes at similar potentials in acetonitrile (Figure 3b, red line). This result suggests the covalent attachment of 1 on CNTs, although the small CV current implies the very low loading of Co corrole molecules. Notably, if GC electrodes loaded with CNT-2 or CNT-3 were immersed in acetonitrile, no signals


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from Co corroles could be identified in CV measurements. We rationalize that Co corrole molecules in CNT-2 and CNT-3 have noncovalent interactions with CNTs and will re-dissolve into acetonitrile, making its detection by CV very challenging. This difference is noteworthy and strongly supports the covalent immobilization of 1 on CNTs in the hybrid CNT-1. Additional evidence for the covalent attachment of molecules of 1 in CNT-1 was provided by infrared spectroscopy. As shown in Figure 3c, complex 1 presents a characteristic azido group resonance band at 2113 cm−1. This band disappears in the infrared spectrum of CNT-1, indicating the reaction of this azido group with alkyne groups on the surface of CNTs. The resonance bands of 1 are well reserved in the infrared spectrum of CNT-1, confirming the presence of intact molecules of 1 in the hybrid. Notably, in the infrared spectrum of unmodified CNTs, no such resonance bands are found in the same range. In addition, CNT-1 was characterized by XPS (Figure 3d) and EDX (Figure S4). Comparison of the XPS spectra of CNTs before and after cycloaddition clearly showed the appearance of Co, N, and F signals from 1. The corresponding Co 2p narrow scan spectrum was depicted in Figure S5, showing Co 2p3/2 and Co 2p1/2 signals at 778 and 790 eV. This result implies the presence of CoIII ions in the hybrid, which is consistent with the Co oxidation state of Co corrole 1. Moreover, CNT-1 was analyzed by FE-SEM (Figure 3e) and TEM (Figure 3f), excluding the presence of aggregated particles on CNT surfaces. We also loaded Co corroles 2 and 3 onto CNTs through noncovalent interactions to give CNT-2 and CNT-3, respectively (Figure 1b).


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As we mentioned above, the coordination structures of Co corroles 1-3 and their redox behaviors are almost identical. These factors are essential to analyze the effect of covalent and noncovalent immobilization methods on ORR electrocatalysis.

Electrocatalytic oxygen reduction studies The electrocatalytic ORR features of the three hybrids were then examined in 0.5 M H2SO4 aqueous solutions. The CV of a GC electrode loaded with CNT-1 showed no catalytic current under N2, but showed a large catalytic current under O2 with the half-wave potential at 0.78 V vs RHE (Figure 4a, all potentials reported in 0.5 M H2SO4 aqueous solution in this work are referenced to RHE unless otherwise noted). This result indicates electrocatalytic reduction of O2 by CNT-1. The linear sweep voltammetry (LSV) of CNT-1 loaded on a GC electrode was also measured, giving the same result as obtained by CV (Figure S7). This ORR performance of CNT-1 is close to that obtained with the commercial Pt/C material, which shows the half-wave potential at 0.84 V in 0.5 M H2SO4 solution. The LSV of GC electrodes loaded with CNT-2 and CNT-3 showed catalytic ORR currents with the half-wave potential at 0.68 and 0.61 V, respectively. In comparison with CNT-2 and CNT-3, the catalytic ORR wave with CNT-1 shifts to the anodic direction by more than 100 mV under the same conditions. Moreover, the loading of Co corrole molecules on CNTs can be determined by ICP-MS, giving 2.81, 11.93, and 11.17 µg Co per mg of CNT-1, CNT-2, and CNT-3, respectively (Table S1). By normalizing the LSV data per Co content, we can see that the catalytic performance of CNT-1 is


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much improved in comparison with that of CNT-2 and CNT-3 regarding both the overpotentials and the catalytic currents (Figure 4b). Because complexes 1-3 contain the same Co corrole core structures and they have almost identical redox behaviors, this improved ORR performance of CNT-1 suggested the synergistic effect between molecules of 1 and carbon supports for ORR. More importantly, after ultrasonication in dichloromethane, CNT-1 retained the initial ORR activity, but both CNT-2 and CNT-3 almost completely lost their activities due to the re-dissolution of Co corrole molecules into organic solvents (Figure S8). This result provides strong support for the covalent immobilization of Co corrole 1 on CNTs in CNT-1.

Figure 4. (a) CVs of GC electrode loaded with CNT-1 and CNT under O2 in 0.5 M H2SO4 solution. (b) Normalized LSVs of GC electrode loaded with CNT-1, CNT-2, and CNT-3 under O2 in 0.5 M H2SO4 solution. Conditions: 50 mV s−1 scan rate, 20 °C.

The selectivity of O2 reduction with CNT-1 was then evaluated by using rotating ring-disk electrode (RRDE) measurements. As shown in Figure 5a and 5b, CNT-1 displayed a pronounced catalytic ORR activity in RRDE. The GC disk electrode was scanned by LSV from 1.0 to 0.20 V, while the Pt ring electrode was


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applied with a constant potential at 1.20 V to detect H2O2 that might be produced by partially reduced O2 at the disk electrode (Figure S9). The H2O2 yield can be calculated according to current densities at the disk (Id) and the ring (Ir) electrodes using the following equation.2,50,51,86,87

%H 2O2 = 200

Ir /N Id + Ir /N

in which N = 0.425 is€the current collection efficiency of the Pt ring. For CNT-1, this result gives the number of electrons (n) transferred per O2 molecule to be 3.73 (Figure 5f). Moreover, the catalytic feature of CNT-1 for ORR was studied in 0.1-0.5 M H2SO4 solutions. The ionic strength of these H2SO4 solutions is maintained the same with addition of Na2SO4. As shown in Figure S10, the half-wave potential shows a linear relationship with the solution pH, giving a slope of −63 mV per pH. This is consistent with the proton-coupled electron transfer process of ORR. For selectivity, similar n values were obtained in 0.1-0.5 M H2SO4 solutions (Figure S11). In addition, CNT-2 (Figure 5c), CNT-3 (Figure 5d), and CNT-alkyne (Figure 5e) were subjected to RRDE measurements. Similar to LSV studies mentioned above, the catalytic ORR wave with CNT-1 shifts to the positive direction by more than 100 mV in comparison with that with CNT-2 and CNT-3 as observed in RRDE. The n values for CNT-2 and CNT-3 are 3.68 and 3.61, respectively. Importantly, CNT-alkyne displayed much smaller ORR catalytic current under much more negative potentials, confirming that the ORR activity of CNT-1 is due to covalently attached Co corroles instead of the


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alkyne groups on the surface of CNTs. In addition, we examined the catalytic ORR feature of complex 1 loaded directly on the GC electrode. As shown in Figure S12, molecules of 1 on GC electrode showed very poor ORR activity regarding both the overpotentials and catalytic currents. Moreover, the n value of 1 on GC electrode was determined to be 2.93. These results confirmed the significantly improved ORR performance of 1 by attaching to CNTs through covalent bonds. As shown in Table 1, CNT-1 is one of the most efficient Co-based ORR electrocatalysts regarding both the half-wave potentials and the n value. Additionally, the n value was further determined using the Koutecky-Levich (K-L) and the Levich analysis in rotating disk electrode (RDE) experiments. The n value for CNT-1 calculated from the K-L and Levich analysis was very similar to that obtained from RRDE measurements under the applied potential range of 0.20-0.40 V (Figure S13 and S14). The linearity and the near-parallelism as observed in the K-L plots in the potential range of 0.20 to 0.40 V suggested a first-order reaction kinetics on dissolved O2 concentration and similar ORR n values at different potentials (Figure S13b). It is necessary to note that the Levich analysis is in general only applicable under the totally mass-transfer-limited ORR conditions, which could be regarded at the applied potential range of 0.20-0.40 V for CNT-1. The n values for CNT-2 (Figure S15) and CNT-3 (Figure S16) were also examined from RDE measurements, giving similar values to those from RRDE.


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Figure 5. (a) O2 reduction in RDE at the GC disk electrode loaded with CNT-1, CNT-2, and CNT-3 in O2-saturated 0.5 M H2SO4 solutions at 2500 rpm. O2 reduction in RRDE at the GC disk electrode loaded with CNT-1 (b), CNT-2 (c), CNT-3 (d), and CNT-alkyne (e) in O2-saturated 0.5 M H2SO4 solutions at 2500 rpm. The Pt ring electrode was polarized at 1.20 V. (f) The ORR n value with CNT-1, CNT-2, CNT-3 and CNT-alkyne determined from RRDE. Conditions: GC disk electrode (area 0.125 cm2), Pt ring electrode (area 0.188 cm2), 10 mV s−1 scan rate, 20 °C.


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Table 1. Comparison of electrocatalytic ORR performances for Co-based complexes.

Half-wave potentials

n values

(vs RHE)

from RRDE

0.78 V

3.73

0.68 V

3.68

0.61 V

3.61

0.5 M H2SO4

0.58 V

2.14

0.1 M KOH

0.70 V

3.10

Oxa-Co

0.5 M H2SO4

0.63 V

3.96

59

MWCNT/Co(TCPP)pyr4

0.1 M KOH

0.85 V

3.10

45

Vc/Nf/Co2Pc2

0.5 M H2SO4

0.25 V

3.84

28

rGo/(Co-THPP)7

0.1 M KOH

0.74 V

3.85

21

CoTNPc/PGr

0.1 M NaOH

0.80 V

3.90

29

MWCNT-CoP-1

0.5 M H2SO4

0.65 V

3.93

42

0.1 M KOH

0.82 V

3.65

0.1 M HClO4

0.56 V

3.61

GO-CorCo

0.1 M KOH

0.73 V

2.00

46

[Co(tpfcBr8)]/BP2000

0.5 M H2SO4

0.75 V

3.60

49

CoHPX-1

0.5 M H2SO4

0.45 V

3.40

78

CoTPFC/MWCNT

0.5 M H2SO4

0.62 V

3.60

50

Co-1/MWCNT

0.5 M H2SO4

0.62 V

3.80

51

CoCOF-py-0.05rGO

0.1 M KOH

0.76 V

3.80

43

MWCNT-Co-porphyrin

1.0 M H2SO4

0.51 V

3.70

41

Catalysts

Conditions

CNT-1 CNT-2

0.5 M H2SO4

CNT-3

PloyCoTAC

References

This work

47

C-COP-P-Co

30


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The durability of CNT-1 for ORR was examined by chronoamperometric tests. During 10-h measurements under potential 0.60 V, the current with CNT-1 decreased by only 10% in an air-saturated solution (Figure 6a), while the current with Pt/C decreased by 24%. It is worth noting that although Pt/C shows high durability for electrocatalytic ORR in alkaline media, it is much less stable in acidic solutions.30,88,89 Moreover, CNT-1 exhibited a very small current response in the methanol crossover test (Figure 6b), but Pt/C showed an apparent increase in the electrocatalytic current upon the addition of methanol, which corresponds to the methanol oxidation.

Figure 6. (a) Controlled-potential electrolysis of GC electrode loaded with CNT-1 and Pt/C at 0.60 V in air-saturated 0.5 M H2SO4 solutions. (b) Chronoamperometric responses of CNT-1 and Pt/C upon adding 2.0 M methanol. (c) CV of GC electrode loaded with CNT-1 in N2-saturated 0.5 M H2SO4 after adding H2O2. (d) Histogram illustration of currents of GC electrodes loaded with CNT-1 at different electrolysis potentials under O2-saturated or N2-saturated 0.5 M H2SO4 solutions. In N2-saturated 0.5 M H2SO4 solution, H2O2 was added to get the final concentration of 2.0 mM.


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In order to distinguish the two possible pathways of the 4e reduction of O2, namely the direct 4e ORR (i.e., O2 → 2H2O) and the stepwise 2e + 2e ORR (i.e., O2 → H2O2 → 2H2O),90 we examined the electrocatalytic activity of CNT-1 for the reduction of H2O2. As shown in Figure 6c, CNT-1 is virtually inactive in catalyzing the H2O2 reduction. This result excludes the possibility that H2O2 is first generated from O2 reduction and is subsequently reduced to H2O by CNT-1. In other words, CNT-1 is able to catalyze the direct 4e reduction of O2 to H2O. Importantly, in the reverse scan of this test, we can see a pronounced catalytic current, which is due to the oxidation of H2O2 to evolve O2 (Figure 6c). In the subsequent cathodic scan, the generated O2 is reduced by CNT-1, confirming the generation of O2 from the H2O2 oxidation. Moreover, we did controlled potential electrolysis with CNT-1 at 0.40-0.80 V in either an O2-saturated solution or an N2-protected solution with addition of 2.0 mM H2O2 (Figures S17-S21). The concentration of H2O2 used here is more than the 1.30 mM concentration of dissolved O2 in an aqueous solution under 1.0 atm O2 at 23 °C. As shown in Figure 6d, the currents observed in O2-saturated solutions in the potential range of 0.40-0.80 V are much larger than those observed in N2-protected solutions with addition of H2O2. These results provide strong evidence that CNT-1 is ineffective in catalyzing the reduction of H2O2. Consequently, CNT-1 is an efficient electrocatalyst for the direct 4e reduction of O2 to H2O. In addition, chemical O2 reduction by ferrocene (Fc) with CNT-1 in acetonitrile gives n value of 3.2 based on generated Fc+ and H2O2 (Figure S22). This relatively smaller n value in the chemical


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reduction studies is likely due to the less efficient electron transfer between Fc and CNTs in comparison with that between the electrode and CNTs. By further analyzing the results mentioned above, we can also exclude the possibility that small ring currents during the RRDE measurements of CNT-1 are due to the trapping of H2O2 by CNTs or the disproportionation of H2O2 as catalyzed by CNT-1. First, as shown in Figure 5e, the RRDE measurement of CNT-alkyne shows large ring currents. This result confirms the reduction of O2 to form H2O2 at the disk electrode by CNT-alkyne and suggests that CNTs are not likely to trap H2O2, as large ring currents are observed. It is worthy noting that the disk current of CNT-alkyne is much smaller than that of CNT-1 but the ring current of CNT-alkyne is similar with that of CNT-1 under the same conditions. Second, data from Figure 5e and Figure S12 suggest that the disproportionation of H2O2 by CNTs and complex 1 is at least not significant. Even with these small disk currents as observed for CNT-alkyne and complex 1, large currents can be observed at the ring electrode. In addition, as shown in Figure 6c, if the disproportionation of H2O2 by CNT-1 was quick and significant, we should observe large catalytic ORR currents during the first run of CV (O2 is generated in situ from the disproportionation of H2O2). However, no such current is observed in the first CV run in Figure 6c. This result strongly argues against the possibility that the formed H2O2 at the disk electrode undergoes disproportionation as catalyzed by CNT-1, making its detection at the ring electrode unsuccessful. Moreover, as shown in Figure S17-S21, during the electrolysis with H2O2 under N2, we did not observe the increase of currents, indicating that the disproportionation of


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H2O2 by CNT-1 was also not likely during the long time electrolysis studies. Mononuclear late transition metal complexes, such as Co complexes, are in general thought to catalyze the 2e ORR because the heterolytic O−O bond cleavage of their M−OOH units towards the 4e ORR activity leads to the generation of terminal late transition metal oxo species, which are high in energy. However, mononuclear Co porphyrins and corroles have still shown 4e ORR activities.48-51 In these studies, self-assembled dimers or oligomers of Co porphyrins and corroles are suggested to form on the surface of electrode materials, which play crucial roles in improving the 4e ORR selectivity. For example, Girault and co-workers studied and established that the formation of self-assembled face-to-face dimeric Co porphyrins would improve the 4e selectivity of ORR.61-63 Consequently, dimeric face-to-face Co complexes of bisporphyrins, biscorroles, and porphyrin-corrole dyads have been designed and extensively studied as 4e ORR catalysts.33-36,54,56-59 In our studies, by using the ICP-MS result, we can calculate that each Co atom of CNT-1 is supported by about 400 C atoms of the outmost wall of CNTs (please see Table S1 and Figure S24 and their corresponding discussion for details). Because the diazonium of CNT surfaces is a nonselective process, it is unlikely to form dimeric face-to-face Co corrole species in subsequent azide-alkyne cycloaddition, which gives very low loading density of Co corrole molecules on the surface of CNTs. This is further supported by the absence of blue shift for the Soret band of Co corroles in CNT-1 (Figure S23), which is characteristic for the formation of cofacial “face-to-face” structures.91 Although we cannot completely rule out the presence of


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ACS Catalysis

possible trace amount of dimeric face-to-face Co corrole species, the majority of covalently attached Co corrole molecules have to be presented as monomers. Therefore, our results demonstrate that the mononuclear Co corrole molecules grafted on CNTs can indeed catalyze the 4e ORR. We rationalized that the short conjugated linkers enable rapid electron transfer between Co corroles and electrode materials. As the reduction of peroxo species and the dissociation of peroxo from Co sites are two competing reactions, the increased electron transfer rates will eventually improve the 4e ORR activity.

Conclusions In summary, we report the electrocatalytic ORR features of Co corrole 1 covalently immobilized on CNTs. The resulted CNT-1 is highly efficient to catalyze the 4e ORR in 0.5 M H2SO4 aqueous solutions with a half-wave potential at 0.78 V. As compared with noncovalent immobilization of Co corrole analogues on CNTs, this covalent immobilization causes more than 100 mV anodic shift for ORR with significantly improved activity and stability. More importantly, the covalent attachment with short linkers and very low loading density prevents the formation of dimeric face-to-face Co corrole species. This result demonstrates that with rapid electron transfer rates, mononuclear Co corroles can become intrinsically active for the direct 4e ORR. Although it is shown previously that rapid electron transfer abilities can considerably improve the 4e ORR activity of Fe porphyrins, it is very rare to demonstrate similar effect for Co corroles because of the challenging in


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Page 28 of 44

molecular design and synthesis. This work is therefore significant to demonstrate that with sufficient electron transfer abilities, mononuclear Co complexes can display significantly improved activity for the 4e ORR.

Notes: The authors declare no competing financial interest.

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website. Figures S1-S24 and Table S1 as described in the text (PDF).

Acknowledgments: We are grateful for support from the “Thousand Talents Program” of China, the Fok Ying-Tong Education Foundation for Outstanding Young Teachers in University, the National Natural Science Foundation of China (Grants 21101170, 21573139, and 21773146), the Fundamental Research Funds for the Central Universities, and the Research Funds of Shaanxi Normal University.

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Tetrabutano-

and

Tetrabenzotetraarylporphyrin

Complexes:

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