Covalent Cobalt Porphyrin Framework on Multiwalled Carbon

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Covalent Cobalt Porphyrin Framework on Multi-Walled Carbon Nanotubes for Efficient Water Oxidation at Low Overpotential Hongxing Jia, Zijun Sun, Daochuan Jiang, and Pingwu Du Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Chemistry of Materials

Covalent Cobalt Porphyrin Framework on Multi-Walled Carbon Nanotubes for Efficient Water Oxidation at Low Overpotential Hongxing Jia, Zijun Sun, Daochuan Jiang, Pingwu Du* Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China (USTC), 96 Jinzhai Road, Hefei, Anhui Province, 230026, P. R. China *To whom correspondence should be addressed E-mail: [email protected] Tel/Fax: 86-551-63606207 Abstract. A noble-metal-free, efficient, and robust catalyst made of earth-abundant elements for water oxidation is vital to achieve practical water splitting for future clean energy production. Herein, we report the synthesis of multi-layer covalent cobalt porphyrin framework on multi-walled carbon nanotubes ((CoP)n-MWCNTs) to produce a highly active electrocatalyst for water oxidation. A linear sweep voltammetry curve showed that a catalytic current density of 1.0 mA/cm2 can be achieved under a potential of only 1.52 V (vs. RHE, corresponding to an overpotential of only 0.29 V) in alkaline solution at pH 13.6. Such an onset potential is much lower than that of cobalt porphyrin monomer (CoP-TIPS) and pure MWCNTs. In addition, the chronopotentiometry data confirmed its excellent catalytic activity and suggested that (CoP)n-MWCNTs catalyst has good durability for water oxidation catalysis. A Tafel slope of 60.8 mV per decade was obtained by bulk electrolysis measurement and the Faradaic efficiency of oxygen production was > 86%.

1

ACS Paragon Plus Environment


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Introduction Photo- and electro-catalytic hydrogen evolution via water splitting has attracted increasing attention in the past few decades due to the high global demand for clean energy and the increasing awareness of environmental issues.1-2 Although tremendous efforts have been made, it is still a great challenge to construct a robust, low-cost, and highly efficient system for hydrogen production. Generally, two half-reactions take place during water splitting, including oxygen evolution reaction (OER, also called water oxidation) and hydrogen evolution reaction (HER).3 OER involves a four-electron transfer process coupled with four protons, and is believed to be much more difficult and complicated than HER.4-6 Therefore, OER is considered to be the bottleneck in achieving highly efficient water splitting for hydrogen production.7 Although noble metal oxides such as RuO2 and IrO2 have been used for water splitting owing to their great OER activity,8-9 large-scale application of these catalysts may suffer problems due to their scarcity and high cost. Recently, numerous efforts have been made toward the development of highly efficient catalysts for OER based on earth-abundant first-row transition metals such as manganese,6,10-11 cobalt,12-18 nickel,19-21 iron,22-23 and copper.24-26 Among these, investigation of cobalt oxides as efficient OER catalysts has been one of the hotspots. In addition, researchers are also very interested in molecular catalysts because they are more convenient for mechanistic studies by various spectroscopic methods. In the literature, a few studies have recently been reported on the use of cobalt complexes as molecular catalysts for OER. Representative examples are cobalt corrole complexes27-28 and cobalt 2


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[Co(Py5)(OH2)]2+

(Py5=

2,6-(bis(bis-2-pyridyl)methoxy-methane)-pyridine).29

However, all these molecular catalysts have much higher overpotential to achieve OER catalysis than metal oxide-based catalysts. Porphyrins are important, naturally-occurring organic compounds, and widely applied in catalysis,30-31 dye-sensitized solar cells,32 photodynamic therapy,33 etc. Cobalt porphyrin complexes have been recently reported as active OER catalysts for water oxidation,34-36 but their catalytic activities still had high overpotentials. Covalent organic frameworks (COFs), as a class of crystalline micromesoporous materials with an extended network, have drawn extensive research interest in recent years due to their large surface areas, tunable porous structures, desired functionality, and promising capacity for catalysis.37 In addition, COFs can be easily recycled as heterogeneous catalysts.38 Inspired by previous studies, herein we report that cobalt porphyrin organic frameworks

on

multi-walled

carbon

nanotubes

((CoP)n-MWCNTs)

as

a

noble-metal-free heterogeneous molecular catalyst with high activity and great stability for OER in aqueous solution. MWCNTs were used as the template and support due to their excellent electrical conductivity and outstanding stability.39-40 Moreover, compared with previous studies,41-42 the present method to combine MWCNTs with cobalt porphyrin network can preserve good electronic properties of carbon nanotubes as well as high stability of assemblies in the catalyst, as evidenced by a recent study using it for efficiently catalyzing oxygen reduction reaction (ORR).43 To the best of our knowledge, the (CoP)n-MWCNTs derivatives have not 3



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been previously studied for OER.

Experimental section Materials. All chemicals, including triisopropylsilylacetylene (C11H22Si, 97%), n-butyllithium (2.5 M solution in n-hexane), dimethylformamide (DMF, 99.5%), pyrrole (C4H5N, 99%), boron trifluoride diethyl etherate (BF3·C4H10O, 98%), cobalt acetate tetrahydrate (Co(OAc)2·4H2O, 99.5%), tetrabutylammonium fluoride (1M solution in THF), copper(I) chloride (CuCl, 97%), tetramethylethylenediamine (TMEDA, 98%), N-methylpyrrolidone (NMP, 99.0%), multi-walled carbon nanotubes (MWCNTs, OD:10~20nm, 95%), 5 wt% Nafion solution, potassium hydroxide (KOH, 85%), dipotassium hydrogen phosphate trihydrate (K2HPO4·3H2O, 99%), potassium dihydrogen phosphate (KH2PO4, 99.5%), and boric acid (H3BO3, 99.99%) were purchased from Aldrich or Acros and used as received. Some solvents were used after fresh distillation (THF, Et2O, CH2Cl2, CHCl3, and DMF). Others were used without further purification unless otherwise stated. All aqueous electrolyte solutions involved in the paper were freshly prepared with millipore water (~18.0 MΩ·cm resistivity). Purification of MWCNTs. A suspension of MWCNTs (80 mg) in nitric acid (35 vol%, 200 mL) was gently sonicated with a sonic bath (150 W max) for 30 min before it was heated up to 100 °C. After refluxing for 12 h with stirring, the suspension was cooled and centrifuged to yield purified MWCNTs. The nanotubes were washed by NaOH 2 M (20 mL×2), deionized water (20 mL×3), HCl 1 M (20 mL×2), and deionized water (20 mL×5) sequentially until the supernatant liquid was neutral. 4


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Synthesis of 3-(triisopropylsilyl)propiolaldehyde (2).44 n-BuLi (10 mL, 1.6 M in hexanes) was added to a solution of triisopropylsilylacetylene (1) (3 mL, 13.37 mmol) in THF (80 mL) over 3 min with stirring at -78°C under argon stream. Then, DMF (2 mL, 25.84 mmol) was added. After completion, the solution was allowed to warm up to room temperature slowly. It was then poured into a mixture of 75 mL of Et2O and 160 mL of 10% KH2PO4 at 0°C and aged for 30 minutes. Then it was extracted by Et2O (50 mL×2) and the organic layer was dried over Na2SO4 and concentrated with a rotary evaporator. The crude product was isolated by column chromatography as a colorless oil (2.5 g, 89% yield). 1H-NMR (CDCl3, 400 MHz): δ (ppm) 9.21 (s, 1H, -CHO), 1.10~1.12 (m, 21H, TIPS-H). Synthesis

of

5,10,15,20-tetrakis-(triisopropylsilylethynyl)porphyrin

(3).45

Borontrifluoride etherate (0.22 mL) was added to a stirred solution of 2 (2.39 g, 11.36 mmol) and pyrrole (0.76 g, 11.33 mmol) in CH2Cl2 (1L) at -78°C under argon atmosphere. Then it was allowed to warm up to room temperature slowly. Stirring was continued for a further 1 h and DDQ (1.93 g, 8.50 mmol) was added. 10 minutes later, the solvent was removed under vacuum and the residue was purified by column chromatography (petroleum ether) yielding a purple powder (0.99 g, 33.8% yield). UV-vis (NMP) λmax (nm) 456, 612, 714. 1H-NMR (CDCl3, 400 MHz): δ (ppm) 9.56 (s, 8H, pyrrole–H), 1.45~1.47 (m, 84H, TIPS-H), -1.73 (s, 2H, N-H). Synthesis of cobalt (II) 5,10,15,20-tetrakis-(triisopropylsilylethynyl)porphyrin (CoP-TIPS, 4). Co(OAc)2·4H2O (405 mg, 1.63 mmol) was added to a solution of 3 (165 mg, 0.16 mmol) in CHCl3 (20 mL) and AcOH (20 mL). The mixture was stirred 5



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and refluxed for 1 h at 110 °C under argon atmosphere. After the reaction mixture was cooled to room temperature, the solvent was removed under vacuum and the dark solid was purified by column chromatography (petroleum ether) to produce a dark green solid (147 mg, 84.0%). UV-vis (NMP) λmax (nm) 471, 602, 656. MALDI-TOF-MS, m/z = 1087.6214 (M+), Calcd. for C64H92CoN4Si4: 1087.5731 (Figure S1). Synthesis of cobalt (II) 5,10,15,20-tetraethynyl-porphyrin (CoP, 5). 360 µL of tetrabutylammonium fluoride (TBAF, 1 M in THF) was added to a solution of 4 (88 mg, 0.08 mmol) in anhydrous THF (20 mL) under argon atmosphere. Then it was stirred at room temperature for 2 h. After that, 20 mL of water was added and a precipitate formed. The solution was evaporated under reduced pressure to remove most of the THF, and then the precipitate was separated by centrifugation. The crude product was washed with MeOH (20 mL×3), CH2Cl2 (20 mL×3), deionized water (20 mL×3), and acetone (20 mL) sequentially, resulting in a dark red solid (37 mg, 87.1%). UV-vis (NMP) λmax (nm) 459, 588, 634. MALDI-TOF-MS, m/z = 463.0793 (M+), Calcd. for C28H12CoN4: 463.0394 (Figure S2). Synthesis of (CoP)n-MWCNTs. CoP (15 mg, 0.032 mmol) was added into a solution of MWCNTs (10 mg) in NMP (300 mL). Then it was softly sonicated and stirred at room temperature for 2 h. After that, a freshly prepared suspension of 100 µL of TMEDA and 25 mg of copper (I) chloride (0.25 mmol) in 2 mL NMP was added. Then the mixture was stirred and bubbled with O2 for 48 h at room temperature. After completion, the crude product was obtained by centrifugation, then washed with NMP 6


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(20 mL×3), deionized water (20 mL×3), solution of NH3 (5%, 20 mL×3), deionized water (20 mL), NMP (20 mL), deionized water (20 mL), THF (20 mL), and CH2Cl2 (20 mL) sequentially to get the purified product. Preparation of CoP-TIPS mixed with MWCNTs (CoP-TIPS/MWCNTs). CoP-TIPS (15 mg, 0.014 mmol) was added into a solution of MWCNTs (10 mg) in THF (15 mL). Then it was sonicated for 30 min. The THF solvent was then removed by evaporator under vacuum to give the dark powder. Characterization. 1H-NMR spectra were recorded using a Bruker AV400 instrument operating at 400 MHz. High-resolution mass spectra were obtained via a Bruker Daltonics Inc. LTQ Orbitrap XL hybrid Fourier Transform High-resolution Mass Spectrometer. UV-vis spectroscopy curves were analyzed on a UNIC-3802 spectrophotometer, with samples in standard glass cuvettes. SEM images were obtained by a SIRION200 Schottky field emission scanning electron microscope (SFE-SEM) with an acceleration voltage of 5 kV or 10 kV. Before loading into the instrument, samples were coated with Pt to improve their conductivity. The morphologies of samples were further analyzed by TEM measurement via a JEM-2011 electron microscope at an acceleration voltage of 200 kV. Elaborate morphologies of the samples were investigated by high-resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectroscopy (XPS) spectra were acquired with a Thermo ESCALAB 250 X-ray photoelectron spectroscopy instrument using a 150-watt Al Kα excitation source at 1486.6 eV. The spot size of the X-ray was

7



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about 500 µm. This analyzer works at a fixed passing energy of 30 eV. The spectra are referenced to the C 1s peak (285.0 eV). Cyclic voltammetry (CV). All electrochemical experiments were performed in a standard three-electrode electrochemical system at room temperature using an electrochemical work station (CHI760E, purchased from Shanghai Chen Hua Instrument Co., Ltd.). The catalyst inks were prepared by sonicating 2 mg of catalyst powders in a mixture solution of 1800 µL ethanol and 200 µL 5 wt% Nafion solution. Before every test, 5 µL of (CoP)n-MWCNTs ink or 4 µL of CoP-TIPS/MWCNTs ink was carefully dropped onto clean glassy carbon (GC) disk (diameter =3 mm, 0.071 cm2) and dried in air. The GC electrode coated with catalyst was used as the working electrode, with an Ag/AgCl electrode (3 M KCl) as reference electrode and a platinum wire as counter electrode during CV measurements. Potentials in this paper were all versus reversible hydrogen electrode (RHE), according to the formula: E = Eappl + EAg/AgCl + 0.059pH. Eappl corresponds to the applied potential vs. Ag/AgCl and EAg/AgCl was corrected before each measurement. All the CV data were measured with iR compensations with no stirring. Bulk electrolysis and Chronopotentiometry. For bulk electrolysis, a GC electrode coated with (CoP)n-MWCNTs or CoP-TIPS/MWCNTs was used as the working electrode in 0.1 M or 1 M KOH aqueous solution under a constant overpotential without iR compensation. For chronopotentiometry, measurements were performed with a fixed current density of 1 mA/cm2 or 10 mA/cm2 and all other conditions were the same as for bulk electrolysis. 8


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Tafel plot. The current density of (CoP)n-MWCNTs on GC electrode was measured in electrolytes at a variety of anodic potentials ranging from 1.24 V to 2.54 V. Bulk electrolysis lasted for at least 5 min until a steady current was achieved. The intervals of potentials were 20 mV or 25 mV for each test. The solution resistance was measured with iR compensation before bulk electrolysis and the overpotential was corrected. The Faradaic efficiency. The amount of O2 evolution was monitored by a fluorescence-based oxygen sensor (Ocean Optics). The alkaline solution was bubbled with pure nitrogen or argon under stirring for 30 minutes. After calibration, the FOXY probe was converted into the gas-tight electrochemical cell and installed at one neck of the cell. A settling time of 5 minutes was required for the reading to settle before bulk electrolysis was started and readings were recorded.

Results and discussion The synthesis procedure of (CoP)n-MWCNTs is provided in Figure 1. The synthesis of porphyrin ligand can be found in experimental section. Cobalt metal ions were inserted into porphyrin scaffold by refluxing 3 and ten equivalents Co(OAc)2·4H2O

in

mixed

AcOH/CHCl3

(v/v,

1:1)

solution

to

produce

5,10,15,20-tetrakis-(triisopropylsilylethynyl)porphyrin (CoP-TIPS). The removal of the TIPS protecting group was performed by tetrabutylammonium fluoride in THF to yield the monomer CoP as a dark red solid. The mass spectrometry data of CoP-TIPS and CoP are shown in Figures S1-S2. Then the as-prepared CoP was used to 9



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synthesize a covalent cobalt porphyrin framework on MWCNTs ((CoP)n-MWCNTs) via Hay-coupling.46. The as-synthesized (CoP)n-MWCNTs hybrid was further characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Its catalytic property toward OER was analyzed by a series of electrochemical measurements. The morphology of (CoP)n-MWCNTs was characterized by SEM and TEM (Figures 2a-2c). MWCNTs was used for comparison (Figures 2d-2e). The SEM images revealed that the (CoP)n-MWCNTs material contained intertwined and disordered nanotubes with average diameter of 15-20 nm, which were similar to the pristine carbon nanotubes. Interestingly, high-resolution transmission electron microscopy (HRTEM) images of (CoP)n-MWCNTs showed that the nanotubes were coated with a thin-layer nanoshell (Figures 2b-2c). The thickness of thin layer around the nanotubes was ~1.5 nm and no lattice fringes were observed. The results indicate that the coating layer around the nanotube surface was probably amorphous organic polymers made of covalent cobalt porphyrin frameworks. In contrast, no thin layer was seen in the HRTEM images of MWCNTs (Figures 2e-2f). In addition, the SEM and TEM images of the hybrid material made by simple mixing CoP-TIPS with MWCNTs (CoP-TIPS/MWCNTs) were also obtained (Figure S3). The SEM images showed the presence of nanotubes and a large amount of particles or clumps in various shapes and sizes (Figure S3a), which might be formed by the cobalt porphyrin complex upon drying. The HRTEM image revealed that there were some discontinuous thin layer on the nanotubes. The results prove that the attachment 10


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between cobalt porphyrins and carbon nanotubes in (CoP)n-MWCNTs is much stronger than that in the mixed CoP-TIPS/MWCNTs hybrid. Based on previous discussion,43 the non-bonding integration of monomeric porphyrin with the outer walls of MWCNTs was previously realized by π-π stacking interactions,40,47 which contributed to the adsorption of porphyrin monomer on the nanotube surface. Similarly, in our case the π-π stacking interaction might be the main driving force to form the thin layer of covalent cobalt porphyrin framework around the nanotubes. The MWCNTs served as templates for polymerization. The CoP monomers, adsorbed onto the nanotube surface, reacted with each other in situ via Hay-coupling. As a consequence of oxidative coupling, the porphyrins were linked with each other via butadiyne linkers and then a huge and stable shell around the nanotubes was formed. Polymerization also occurred between the free CoP monomers in N-Methylpyrrolidone (NMP). These resultant pieces of polymer were stacked onto the first layer by π-π stacking between porphyrins in different layers, resulting in a multilayer porphyrin shell around the nanotubes. To further analyze the surface composition of (CoP)n-MWCNTs, XPS experiments were performed (Figure S4). The survey scan indicated the presence of Co, O, N, and C elements in the hybrid material (Figure S4a). The high resolution C 1s spectrum was split into four Gaussian curves at 284.7 eV, 285.5 eV, 286.8 eV, and 289.1 eV (Figure S4b). The peaks were assigned to the binding energy of the various carbon atoms (Csp, Csp2, Csp3, C=O, and COOH) from the cobalt porphyrin framework and carbon nanotubes.43,48 In the N 1s spectrum (Figure S4c), two peaks 11



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located at 399.6 eV and 401.1 eV appeared, corresponding to the nitrogen atoms from the porphyrin scaffold and the residual NMP inside carbon nanotubes, respectively.43 As for the Co 2p spectrum (Figure S4d), two peaks were observed at 781.3 eV and 796.3 eV, suggesting the presence of Co (II) in the porphyrins.49 The strong signals located at 532.9 eV represented the element O 1s, which could be attributed to the oxidation on the surface of the nanotubes during the purification of MWCNTs with nitric acid.40,48 In addition, the survey scan of XPS data showed that the elemental molar ratio of Co and C was about 1: 88. Since one CoP molecule contains 28 C atoms, this ratio means that there is an average of one porphyrin for every 60 carbon atoms on nanotubes. Based on a simple geometric model (Figure S5) and a previously reported calculation method,43 a 1 nm2 section (the equivalent area of one CoP molecule) on the MWCNTs with 15 walls and a diameter of ~20 nm contains about 457 carbon atoms. These results suggest that a large number of cobalt porphyrin molecules were attached to the outer wall of the nanotubes and formed a multilayer structure, which is consistent with the SEM and TEM observations. For comparison, the XPS data of CoP-TIPS/MWCNTs were also collected (Figure S6). The spectrum showed the presence of silicon (from TIPS in CoP-TIPS ) at 100.9 eV and 152.1 eV.43 The ratio of Co and C was estimated to be 1 : 104, which corresponds to an average of one CoP-TIPS molecule for every 40 carbon atoms on nanotubes.

The

different

proportions

of

Co:C

in

(CoP)n-MWCNTs

and

CoP-TIPS/MWCNTs suggest that it is not fair to compare the catalytic properties of (CoP)n-MWCNTs

with

CoP-TIPS/MWCNTs

under

the

same

amount

of 12


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electrocatalyst loading (~0.14 mg/cm2) on a glassy carbon (GC) electrode. Therefore, when

we

compare

the

catalytic

properties

of

CoP-TIPS/MWCNTs

with

(CoP)n-MWCNTs, less catalyst material should be used to modify onto the GC electrode. Figure

3

showed

the

linear

sweep

voltammetry

(LSV)

curves

of

(CoP)n-MWCNTs in a standard three-electrode system. In Figure 3a, the (CoP)n-MWCNTs exhibited excellent activity for OER in alkaline solution (pH ~13.6) with a catalytic current density of 1.0 mA/cm2 at ~1.52 V (vs. RHE) during the LSV scan, corresponding to a low overpotential at only 0.29 V. For physisorbed CoP-TIPS/MWCNTs hybrid catalyst, the onset potential for OER appeared at ~1.67 V (vs. RHE), which is ~150 mV higher than that of (CoP)n-MWCNTs catalyst. In sharp contrast, no obvious catalytic responses were observed for the bare GC electrode and pure MWCNTs in the range of 1.30-1.80 V (vs. RHE). These results suggest that (CoP)n-MWCNTs is a highly efficient catalyst for OER and can significantly lower the overpotential for water oxidation catalysis. The pH-dependent results show that the catalytic activity of the (CoP)n-MWCNTs material toward water oxidation reaction increased with the rise of the pH values (Figure 3b), which is consistent with shifts in the thermodynamic potential for water oxidation. For a fixed catalytic current at 0.1 mA/cm2, a plot of the overpotentials versus pH values showed a nearly linear relationship from pH = 13.6 to pH = 7 (insert of Figure 3b). The linear relationship is consistent with previous studies.27,36 The optimal onset potential of (CoP)n-MWCNTs for OER is much lower than the 13



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performance of molecular OER catalysts reported in the literature, including cobalt corrole catalyst,27-28 heterogenous cobalt polyoxometalate catalyst,50 and molecular cobalt porphyrin catalysts.35-36 Moreover, such an onset overpotential is even lower than those of many metal oxide-based heterogeneous OER catalysts such as Mn3O4/CoSe2 catalyst (1.68 V vs. RHE, pH 13),51 N/C-NiOx catalyst (1.70 V vs. RHE, pH 13),52 Co3O4/N-graphene (1.63 V vs. RHE, pH 14),53 NixCo3-xO4 (1.62 V vs. RHE, pH 14),54 Nitrogen doped graphene-NixCo3-xO4 (1.54 V vs. RHE, pH 14),55 and ZnxCo3-xO4 (1.55 V vs. RHE, pH 14).56 Therefore, the present (CoP)n-MWCNTs material is, for OER in alkaline solutions, among the most efficient electrocatalysts made of earth-abundant elements. The chronopotentiometry data of (CoP)n-MWCNTs for OER were examined to check its durability during water oxidation. When the current density was fixed at 1.0 mA/cm2 and 10 mA/cm2 (Figure 4), the performance of (CoP)n-MWCNTs for water oxidation showed very good stability and only a slight decrease of overpotential was observed in the beginning. As for CoP-TIPS, its performance for water oxidation showed much higher overpotential and poorer durability than (CoP)n-MWCNTs. In Figure S7, CoP-TIPS/MWCNTs showed a steady potential under a current density of 1.0 mA/cm2, but when the fixed current density increased to 10 mA/cm2, the stability of CoP-TIPS/MWCNTs was much less than that of (CoP)n-MWCNTs. This was probably because there was much weaker contact between cobalt porphyrin and MWCNTs in the CoP-TIPS/MWCNTs composite and a significant number of oxygen bubbles during catalysis could induce partial desorption of the cobalt porohyrin 14


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molecules from the surface of MWCNTs. In Figure 4, the required overpotential is ~0.30 V for (CoP)n-MWCNTs to achieve a current density of 1.0 mA/cm2. For comparison, CoP-TIPS/MWCNTs required an overpotential of ~0.35 V (Figure S7) to achieve 1.0 mA/cm2. When the current density increased to 10 mA/cm2, the required overpotential for (CoP)n-MWCNTs and CoP-TIPS/MWCNTs increased to ~0.43 V and ~0.54 V, respectively (Figure 4). Therefore, the chronopotentiometry data indicate good stability and high activity of the as-synthesized (CoP)n-MWCNTs for OER in alkaline solutions. To further investigate the stability of (CoP)n-MWCNTs during water oxidation, the morphology and the surface composition of the catalyst was measured by TEM and XPS. After bulk electrolysis over 3 hrs, the catalyst was collected for physical characterization. As shown in TEM image (Figure 5a), no appreciable change in morphology was observed for the (CoP)n-MWCNTs catalyst, suggesting its good morphological stability. Moreover, the XPS data of the (CoP)n-MWCNTs before and after electrolysis showed that the cobalt porphyrin frameworks might not decompose into cobalt oxide materials during catalysis (Figures 5b).35-36 The stability of the cobalt porphyrin scaffold in alkaline solution was also examined. A small amount of water-soluble

cobalt

(II)

5,10,15,20-tetrakis-(4-carboxylphenyl)porphyrin

(CoP-COOH) was dissolved into 1.0 M KOH and the solution was examined by UV-vis spectroscopy, as shown in Figure S8. After 24 hrs, no appreciable change could be observed in the typical absorption peaks of a cobalt porphyrin complex. During long-term bulk electrolysis using (CoP)n-MWCNTs, no color change was 15



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observed in the electrolyte solution. The UV-vis spectrum showed a featureless curve (Figure S9), indicating that there was no leaching of porphyrinic materials from MWCNTs during electrolysis. In addition, CoP-COOH and a mixture of CoP-COOH with MWCNTs (CoP-COOH/MWCNTs) were also dropped using a pipette onto GC to test the chronopotentiometry performance for OER (Figure S10). Since no strong interaction between CoP-COOH and the GC electrode and CoP-COOH is water-soluble in alkaline solution, CoP-COOH was gradually dissolved into the electrolyte solution, as evidenced by the UV-vis spectra (Figure S9). Both electrodes using CoP-COOH also showed much lower catalytic activity for OER than that using (CoP)n-MWCNTs. The results further demonstrated the advantages of the present study by attaching (CoP)n on MWCNTs. A Tafel plot can reflect the electrochemical kinetics between the water oxidation reaction and the overpotential. To gain insight into the OER kinetics, we used (CoP)n-MWCNTs to obtain the Tafel plot by measuring the stable current density (j) as a function of the overpotential in both 0.1 M KOH and 1.0 M KOH (Figure 5c). The catalyst exhibited a Tafel slope of 60.8 mV per decade in 1.0 M KOH and 76.4 mV per decade in 0.1 M KOH. The Tafel slope of 60.8 mV per decade is a very low value for a molecular-based OER catalyst, confirming its great catalytic activity for water oxidation. Previous studies have shown different Tafel slopes of molecular catalysts for OER, such as CoHβFCX-CO2H (120 mV per decade, pH 7),27 Ru(bpa)(pic)2

derivative-MWCNTs

(160

mV

per

decade,

pH

7),47

polyoxometalate-ruthenium @MWCNT catalyst (280 mV per dacade, pH 14).57 At η 16


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= 0.30 V in 1.0 M KOH, the current density was ~1.0 mA/cm2, which is consistent with the results in the linear sweep voltammetry (LSV) curves (Figure 3a). In addition, the Tafel slope in 1.0 M KOH is smaller than that in 0.1 M KOH, a fact indicating that (CoP)n-MWCNTs could catalyze water oxidation more effectively under a higher pH. The evolution of oxygen using (CoP)n-MWCNTs catalyst was confirmed by gas chromatography during bulk electrolysis. The Faradaic efficiency of the catalyst was measured in a gas-tight electrochemical cell under argon with a fluorescence-based oxygen sensor in 1.0 M KOH at 1.73 V (vs. RHE). The experiment was performed for more than half an hour and the efficiency plot is shown in Figure 5d. In the initial reaction, no oxygen production could be detected by the fluorescent sensor, probably because the produced oxygen dissolved into aqueous solution or attached to the GC electrode. After the initial electrolysis, the amount of oxygen rose with the increase of passed charge, which matched well with the theoretical value (assuming that all of the current was from 4e- oxidation of water according to Faraday's law). At the end of the experiment, the Faradaic efficiency was >86 %.

Conclusions In conclusion, we report for the first use of noble-metal-free (CoP)n-MWCNTs for OER catalysis. The noncovalent combination between (CoP)n organic frameworks and MWCNTs was realized by in-situ polymerization using MWCNTs as the template. A catalytic current density of 1.0 mA/cm2 can be achieved under an overpotential of only 0.29 V at pH 13.6. And the lowest Tafel slope was 60.8 mV per decade. All the 17



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results indicate that covalent cobalt porphyrin framework on carbon material is an excellent heterogeneous molecular catalyst made of earth-abundant elements for water oxidation.

Notes The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21271166, 21473170), the Fundamental Research Funds for the Central Universities (WK3430000001, WK2060140015, WK2060190026), the Program for New Century Excellent Talents in University (NCET), and the Thousand Young Talents Program.

Supporting Information Available. HRMS data of CoP-TIPS and CoP; simple geometric model of 1 nm2 section of MWCNTs; TEM and SEM images of CoP-TIPS/MWCNTs; XPS analysis for (CoP)n-MWCNTs and CoP-TIPS/MWCNTs; chronopotentiometry of (CoP)n-MWCNTs, CoP-TIPS/MWCNTs, CoP-COOH, and CoP-COOH/MWCNTs; UV-vis spectra of CoP-COOH in 1 M KOH; UV-vis spectra after

long-term

bulk

electrolysis

for

(CoP)n-MWCNTs,

CoP-COOH,

and

18


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CoP-COOH/MWCNTs. This information is available free of charge via the Internet at http://pubs.acs.org.

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Figures

Figure 1. Molecular structures of CoP-TIPS and CoP and synthesis of (CoP)n-MWCNTs.

24


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Figure 2. SEM image (a) and TEM images (b, c) of (CoP)n-MWCNTs; SEM image (d) and TEM images (e, f) of pure MWCNTs.

25



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Figure 3. (a) LSV curves using different materials in 1.0 M KOH. (b) LSV curves using (CoP)n-MWCNTs as the OER catalyst in aqueous solutions with different pH values of 13.6, 13.0, 11.0, 9.2, and 7.0, respectively. The inset image showed the linear relationship between overpotentials (fixed under 0.1 mA/cm2) and pH values. The slop was calculated to be -55 mV per pH unit. The scan rate was 20 mV/s.

26


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Figure 4. Chronopotentiometry data for CoP-TIPS monomer and (CoP)n-MWCNTs catalysts with a fixed catalytic current density at (a) 1.0 mA/cm2 and (b) 10 mA/cm2 with iR compensation.

27



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Figure 5. (a) TEM image of (CoP)n-MWCNTs after long-term bulk electrolysis. (b) XPS survey spectra of (CoP)n-MWCNTs before and after long-term bulk electrolysis (BE). The inset image is the high resolution XPS spectra of Co 2p before and after long-term BE. (c) Tafel plots of (CoP)n-MWCNTs in 1.0 M KOH and 0.1 M KOH. (d) Theoretical and experimental data for oxygen production by bulk electrolysis at an overpotential of 0.50 V in 1.0 M KOH.

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Table of contents

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Covalent Cobalt Porphyrin Framework on Multiwalled Carbon

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