Gas Uptake and Supercapacitor Performance of a Highly Connected

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Gas Uptake and Supercapacitor Performance of a HighConnected Porous Co-MOF Induced by Ligand Bulk Xiang-Yang Hou, Xiao Wang, Shu-ni Li, Yucheng Jiang, Mancheng Hu, and Quan-Guo Zhai Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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Crystal Growth & Design

Gas Uptake and Supercapacitor Performance of a High-Connected Porous Co-MOF Induced by Ligand Bulk Xiang-Yang Hou,†,‡ Xiao Wang,†,‡ Shu-Ni Li,† Yu-Cheng Jiang,† Man-Cheng Hu,† Quan-Guo Zhai*,† †

Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry &

Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, China ‡

Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical

Reaction Engineering, Yan’an University, Yan’an, Shaanxi 716000, China

ABSRACT: The mimic of (3,9)-connected xmz framework generates a novel high-connected Co-MOF (SNNU-80) constructed by trigonal prismatic [Co3(OH)(COO)6] building blocks and tripodal pyridyl-dicarboxylate ligands (BPYDB = 4,4’-(4,4’-bipyridine-2,6-diyl)dibenzoic acid). Compared to the non-porous Co-pyridine-3,5-dicarboxylate xmz MOF, the extension of triangular linker leads to a different geometrical distribution of ligands and thus a porous structure with the topology of 3,3,9T2 for SNNU-80. Four types of polyhedral cages co-exist together, which produce permanent micro- and meso-porosity. Different to MCF-18 assembled by BPYDB with [Ni3(OH)(COO)6] showing drastic framework breathing, SNNU-80 are not flexible. As a result, SNNU-80 shows remarkable CO2 uptake capacity as well as high CO2 and C2-hydrocarbons over CH4 selectivity. Furthermore, the electrochemical measurements show that SNNU-80 is an idea electrode material for supercapacitor due to its high-connected stable porous framework and the pseudocapacitor contribution introduced by the mixed valent system.

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■ INTRODUCTION In the past twenty years, metal-organic frameworks (MOFs) have been rapidly developed and extensively used for gas storage and separation, and so on.1-6 The high surface area, tunable pore surface, and scalability have made MOFs an attractive porous material.7-12 Up to now, a number of factors have been explored to develop novel high-performance MOFs materials. However, the directional design of high porous MOFs is still very challenging. An effective route to design novel metal-organic frameworks is the utilization of extended geometrically equivalent organic linkers. For a given framework topology, the introduction of longer ligands usually can increase its pore size as well as surface area.13-15 On the base of this strategy, several famous MOF platforms have been successfully developed.16 We are specially interested in the trinuclear cluster [M3(O/OH)(COO)6]-based high-connected MOF platforms,17,18 such as 3,9-connected xmz MOFs19,20 and 9-conencted ncb MOFs21 due to their high stability and potential flexibility. More recently, we have also developed another 3,9-connected MOF platform, namely pacs MOFs22 (pacs = partitioned acs), which can accommodate a large variety of metal centers, and allow for extraordinary tunability in gas sorption properties. Herein, 4,4’-(4,4’-bipyridine-2,6-diyl)-dibenzoic acid (H2BPYDB), an extended version of pyridine-3,5-dicarboxylic

acid,

is

selected

to

mimic

the

(3,9)-connected

Co-pyridine-3,5-dicarboxylate xmz MOF (Scheme 1). However, the torsion of longer organic linker generates a new isomeric 3,3,9-connected framework, [Co3(OH)(BPYDB)3] (SNNU-80). Compared to the non-porous Co-pyridine-3,5-dicarboxylate xmz MOF, SNNU-80 shows remarkable CO2 uptake capacity as well as high CO2 and C2-hydrocarbons 2


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over CH4 selectivity. Moreover, Co3+/Co2+ mixed metal system exist in SNNU-80, which are rarely observed in both xmz and ncb MOF platforms. Taking the advantage of its high-connected stable porous framework and the pseudocapacitor contribution introduced by the mixed valent Co2+/Co3+ system, SNNU-80 shows good potential as electrode material for supercapacitors.

Scheme 1. The mimic of xmz MOF for SNNU-80.

■ EXPERIMENTAL SECTION Materials and Methods. All chemicals were obtained commercially. The ligand 4,4’-(4,4’-bipyridine-2,6-diyl) dibenzoic acid (H2BPYDB) was synthesized according to the literature method.23 Bruker EQUINOX-55 spectrophotometer was used to record the FT-IR spectra in the range 400-4000 cm–1. Perkin-Elmer 2400 elemental analyzer was used to achieve the C, H, and N analysis. Thermogravimetric analyses were carried out on a Q1000DSC+LNCS+FACS Q600SDT thermal analyzer. Raman spectra were done by an Invia Raman Microscope in the 400-3000 cm–1 range. D/Max2550VB+/PC diffractometer (CuK α, λ = 1.5406 Å, 40 kV and 40 mA) was used to finish the powder X-ray diffraction (PXRD) measurements.

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Synthesis. [Co3(OH)(BPYDB)3]n (SNNU-80). A mixture of H2BPYDB (0.008 g, 0.02 mmol), Co(NO3)2·6H2O (0.018 g, 0.06 mmol), N,N’-dimethylacetamide (DMA, 3 mL), and HBF4 (40%, 0.4 mL) was stirred for 30 minutes in air. The clear solution was transferred into a screw-capped vial (20 mL) and placed in an oven at 90 °C for 7 days. Purple block shape crystals were collected and washed with fresh DMA. Yield ～65% (based on H2BPYDB). Anal. Calcd. for C72H42N6O13Co3: C, 62.85; H, 42.34; N, 6.21%. Found: C, 62.16; H, 43.01; N, 6.19%. FT-IR (cm-1): 3422 (broad), 1595(s), 1387(m), 1347(s). X-ray Structural Studies. Single-crystal data of SNNU-80 were collected at 293 K on a Bruker FRAMBO CCD diffractometer using graphite-monochromatized CuKα radiation (1.54178 Å). The structure of SNNU-80 was solved using the SHELXS-97 by the direct method. The SHELXL-97 program package24,25 was further used to refine the structure. The full matrix least squares method was used to refine anisotropically for all non-hydrogen atoms. The SQUEEZE program in the PLATON software package26 was utilized to treat the disordered solvents entrapped in the pores. Gas Adsorption and Selectivity Prediction. Gas sorption isotherms were measured on a Micromeritics ASAP 2020 HD88 surface-area and pore-size analyzer, and all used gases were of 99.99% purity. The gas sorption isotherms for CO2, CH4, C2H2, and C2H4 were measured at 273 and 298 K, respectively. The gas sorption isotherms for N2 and H2 were measured at 77 K with liquid nitrogen. In order to ensure the actual equilibrium was reached, the equilibration interval time was set longer than 45 seconds for all experiments. Ideal adsorbed solution theory (IAST)27 was further used to predict binary mixture adsorption from the experimental pure-gas isotherms. The single-component isotherms for CO2, C2H2, and C2H4 and CH4 of 4


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SNNU-80 are firstly fitted by the Langmuir–Freundlich (LF) equation.28 On the base of the equation parameters, the separations of CO2/CH4, C2H2/CH4 and C2H4/CH4 was further calculated by the IAST model. Electrochemical Measurements. 80% activated SNNU-80 (10 mg), 15% acetylene black together with 5% polyvinylidone fluoride (PVDF) binder were mixed to prepare the electrodes for supercapacitors. The mixture was made as homogeneous slurry with small amount of ethanol. The nickel foam (1 cm2) was selected as a current collector to load the slurry under 10 MPa pressure. Thus, the finial loading density of SNNU-80 was 10 mg cm-2. Before the electrochemical experiments, this self-made electrode was dried at 90 ºC for 12 h. A typical three-electrode experimental setup was utilized to run all electrochemical measurements. SCE electrode, platinum foil and self-made MOF electrode act as the reference, counter and working electrodes, respectively. 1 M KOH solution was used as the electrolyte. CHI660E electrochemical working station system (Shanghai, China) was used to measure the cyclic voltammograms and galvanostatic charge–discharge in a given potential range and the electrochemical impedance spectroscopy in the frequency range of 105 to 10-2 Hz at the room temperature.

■ RESULTS AND DISCUSSION Synthesis and Crystal Structure. It should be pointed out that using different cobalt salts to react with H2BPYDB resulted in different MOFs. Chen and co-workers reported a sulfate-supported (3,12)-connected framework on the base of octacobalt cluster through the reaction of CoSO4 and H2BPYDB under solvothermal conditions.29 When Co(NO3)2 is

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utilized, Zhou and co-works obtained PCN-121 on the base of 1D Co-chains with helical channels.30 Furthermore, the addition of acetic acid to the reaction mixture, PCN-122 with Co-dimers was produced.30

(a)

(b)

Figure 1. (a) The distribution of nine BPYDB ligands around each [Co3(OH)(CO2)6] cluster. (b) The 3-D porous framework of SNNU-80 viewed along the a-axis direction.

In this work, when HBF4 is utilized instead of acetic acid under the same condition, SNNU-80 formed. Single-crystal X-ray diffraction analysis reveals that SNNU-80 crystallizes in the Pnma space group with the asymmetric unit containing crystallographically independent two Co, one µ3-OH and one and half BPYDB ligands for the framework. The central oxygen links three Co cations to form a planar [Co3(OH)] cluster with the Co–O bonds and Co···Co separations of 2.022(5)-2.110(5) and 3.5083(15)-3.5127(13) Ǻ, respectively. Each pair of Co centres are bridged by two carboxylate groups from separate BPYDB ligands above and below the [Co3(OH)] plane, which is encapsulated by six carboxylate groups and three pyridyl groups to form a [Co3(OH)(CO2)6] building block. As shown in Figure 1a, each [Co3(OH)(CO2)6] trimer is connected to nine BPYDB ligands via six carboxylate groups and

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three Co-N bonds, and each BPYDB is connected to three Co-trimer clusters to give a 3-D porous framework (Figure 1b) of SNNU-80. Overall, SNNU-80 exhibits a (3,3,9)-connected 3-D framework with hydroxyl-centered [Co3(OH)(CO2)6] tricapped trigonal prismatic clusters as 9-connected nodes and two unique BPYDB ligands as 3-connected nodes. The whole framework exhibits a 3,3,9T2 topological type with the short Schläfli symbol of {4.62}{42.6}2{48.619.89}. It should be pointed out that xmz Co-MOF has a (3,9)-connected net with the short Schläfli symbol of (42.6)3(46.621.89). 20 Clearly, the torsions induced by the large bulk of BPYDB ligands make SNNU-80 exhibiting a different high-connected topological net compared to Co-pyridine-3,5-dicarboxylate MOF (Figure S1). On the base of BPYDB, Zhang and Co-works have synthesized an xmz-Ni MOF (MCF-18), which shows dramatical framework flexibility.19 It is interesting that on the base of nine-connected tricapped trigonal prismatic building blocks, another type of (3,9)-connected net (pacs MOFs) has also been explored by our group more recently.22 Each 9-connected trimeric building blocks in pacs MOFs connects to six adjacent trimers and three triangular ligands, which leads to a different short Schläfli symbol of (43)(421.615) (Figure S2). Moreover, the local connectivity of these three (3,9)-connected MOFs are reminiscent of the inorganic compound LaCl3 because it also has similar 9- and 3-connected nodes (Figure S2), and shows a short Schläfli symbol of (43)3(412.615.89).20 Overall, such (3,9)-connectivity effectively fixes the framework and leads to well-defined and robust structures of MOFs, which are important for their further applications.

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Figure 2. Four types of polyhedral cages and the corresponding tilings for SNNU-80: a - cage A, [42.64]; b - cage B, [4.62]; c - cage C, [43]; d - cage D, [63].

On the other hand, thanks to the longer BPYDB ligands, SNNU-80 clearly shows a porous structure. As defined by the tiling of 3,3,9T2 topology ([42.64]+3[4.62]+[43]+2[63]), SNNU-80 possesses distorted octahedral, tetragonal-pyramidal, trigonal-bipyramidal and tetrahedral cavities (denoted as cage-A, cage-B, cage-C and cage-D) in a 1:3:1:2 ratio (Figure 2). The dimensions of four types of cages are about 25 Å × 18 Å (cage A), 18 Å × 16 Å (cage B), 14 Å × 8 Å (cage C) and 17 Å × 16 Å (cage D), respectively. As shown in Figure S3, each cage-A connects with two cage-B, two cage-C, and two cage-D. Each cage-B connects with one cage-A, one cage-B, and one cage-D. Each cage-C connects with two cage-A and one cage-B. Each cage-D connects with one cage-A and two cage-B. Overall, four types of cages connected with each other via the face-sharing fashion to furnish a hierarchical channel-cavity biporous system of SNNU-80. Also, the guest accessible volume calculated by PLATON 8


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program is 11085.1 Å3, which occupies about 69.8% of the whole crystal volume. Gas uptakes and Separations of CO2 or C2-Hydrocarbons from CH4. The gas-adsorption studies were carried out with the activated sample to confirm the permanent porosity of SNNU-80. Freshly prepared SNNU-80 sample was confirmed by PXRD patterns (Figure S4) and can be easily activated through being soaked in volatile solvents such as ethanol, acetone or CH2Cl2 to remove the DMA guest molecules. Different to MCF-18 assembled by the same ligand with [Ni3(OH)(COO)6] showing drastic framework breathing upon inclusion of different guests, SNNU-80 is not flexible. This should be caused by their different 3,9-conencte topological nets. The TGA results for fresh and activated samples all are shown in Figure S5. Solvent molecules of the unactivated sample are lost before 200 °C and the thermal decomposition product at 800 °C is Co3O4 in air. But the activated Co-MOF loses solvent molecules below 100 °C and shows the stability of the framework between 100 and 375 °C. PXRD patterns for samples heated at different temperatures have also been given in Figure S5, which clearly prove the framework thermal stability. Such high thermal stability renders SNNU-80 a suitable candidate for gas sorption applications. As depicted in Figure 3a, the N2 adsorption isotherm indicates a Type-IV sorption behavior with a BET surface area of 461 m2 g-1. It is typical for microporous materials with additional mesoporosity as a small degree of hysteresis was observed upon desorption.31 The permanent micro- and meso-porosity of SNNU-80 prompted us to further investigate their adsorptive properties of small gas molecules such as H2, CO2, CH4, C2H2 and C2H4.

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(a)

(b)

(c)

(d)

Figure 3. Adsorption and desorption isotherms of N2 and H2 at 77 K (a), CO2, C2H2, C2H4, and CH4 at 273 and 298 K (b), and the gas selectivity predicted by IAST (c and d) for SNNU-80.

The low-pressure H2 sorption isotherms of SNNU-80 were given in Figure 3a. At 77 K and 1 atm, the H2 uptake is 1.0 wt% (112 cm3 g-1), which is comparable to many famous frameworks such as MOF-5 (1.32 wt%)32 and MOF-177 (1.39 wt% at 1.4 atm)33 under the same conditions. The sorption behaviors of SNNU-80 toward CO2 (Figure 3b) were investigated carefully. At 1 atm, the CO2 uptakes are 45.7 and 24.7 cm3 g-1 at 273 and 298 K, respectively. Furthermore, SNNU-80 can take up CH4 (8.3 cm3 g-1), C2H2 (70.2 cm3 g-1) and C2H4 (46.4 cm3 g-1) at 1 atm and 273 K. At 298 K and 1 atm, these values are CH4 (3.8 cm3

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g-1), C2H2 (37.4 cm3 g-1) and C2H4 (28.8 cm3 g-1), respectively. To further understand the adsorption properties of SNNU-80, the isosteric heat of adsorption (Qst) was studied at 273 and 298 K by the virial model. At zero coverage, the Qst for C2H2, C2H4, CH4, and CO2 are 6.2, 12.6, 17.6 and 20.8 kJ/mol (Figures S6). Due to the absence of open metal sites in SNNU-80 framework, all the Qst values are remarkable lower than most of MOFs with open metal sites. Moreover, the CO2 and CH4 adsorption data are similar to PCN-121,30 another micro- and meso-porous MOF material with [Co3(OH)(COO)6] building blocks and BPYDB ligands. Such adsorption capacities also inspired us to further investigate the separation of CO2, C2H2 and C2H4 from CH4 by SNUU-80. An ideal adsorbed solution theory (IAST) calculation27,28 based on a Langmuir–Freundlich (LF) simulation was employed to establish the feasibility of these gas separations. At 273 K, for the mixtures composed of equimolar CO2 or C2-hydrocarbons to CH4 mixtures, the initial selectivity of SNNU-80 are 12.7, 17.5 and 11.3 for CO2/CH4, C2H2/CH4 and C2H4/CH4, respectively (Figures 3c, 3d and S7). At 1 atm, these values increase to 170.0, 66.8 and 55.9. The CO2/CH4 selectivity is comparable to those of ZJU-60,34 MCF-18,19 JLU-Liu22,35 and 4-In-EBPTC.36 Furthermore, the C2H2/CH4 and C2H4/CH4 selectivities are also higher than many reported MOFs such as CoTZB(INT)37 and ZJU-3138 under the similar conditions. In our opinion, the different polarizability of small gas molecules may help for the CO2 or C2-hydrocarbons selectivity over CH4.39 Supercapacitor Performance. The high-connected stable framework, hierarchical pore structure together with the mixed valent Co2+/Co3+ system of SNNU-80 provoke us to further investigate its potential applications in supercapacitors. To the best of our knowledge, 11



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although metal oxides synthesized from MOF precursors have been extensively reported as electrode materials of supercapacitor,40 rare MOFs have been directly utilized up to now.41-44 Figure 4a shows that the cyclic voltammetric (CV) behaviors of SNNU-80 has obvious redox peaks at various scan rates from 5 to 100 mV s-1 within the range 0.1–0.4 V for the character of faradaic pseudocapacitance due to Co3+ and Co2+ ions’ redox reaction in alkaline electrolyte. Under different scan rates, the CV curves of the SNNU-80 MOF electrode reveal that the electrochemical capacitance for SNNU-80 should result from pseudocapacitance. The average specific capacitances have been calculated 109.3, 97.8, 89.4, 76.2, and 67.8 F g-1 at scan rates of 5, 10, 20, 50, and 100 mV s-1, respectively. Clearly, with increase of the scan rate, the capacitance decreases continuously. This may caused by the circuitous diffusion of electrolyte ions into the pores of SNNU-80 MOF and the electrode resistance.45 That is to say, the Co-MOF electrode exhibits pseudocapacitance induced by electrochemical reactions. This result is quite different from the double-layer capacitor, which basically has an ideal rectangular shaped CV curve. Moreover, the peak current increases continuously with the scan rate ranging from 5 to 100 mV s-1. The constant current charge-discharge tests in the potential range between 0 and 0.4 V were investigated to obtain the specific capacitances of the SNNU-80 electrode (Figure 4b). The specific capacitance (Cm) can be calculated use the following equation: Cm = I∆t/m∆V

(1)

where I is the current of charge-discharge, ∆t is the time of discharge, m is the mass of active materials in the work electrode, and ∆V is 0.4 V. Obvious voltage drop can be observed from the curves and that display approximate 12


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triangular shapes at various currents. As a result, the specific capacitance values of SNNU-80 electrode are measured to be 106, 94 and 88 F g-1 at charge-discharge current densities of 1, 2, and 3 A g-1, respectively. These values are much high than those of Co8-MOF-541 and comparable to the results of other Co-MOFs44 (Co-BDC, Co-NDC, and Co-BPDC), and mesoporous 437-MOF.43

(a)

(b)

(c)

(d)

Figure 4. Supercapacitor performance of SNNU-80 MOF electrode: (a) cyclic voltammograms in 1.0 M KOH electrolyte at different scan rates; (b) galvanostatic charge–discharge curves at different current densities; (c) average specific capacitance versus cycle number at a galvanostatic charge–discharge current density of 5 A g-1; (d) cyclic voltammograms at the scan rate of 10 mV s-1 before and after galvanostatic charging–discharging for 3000 cycles at the current density of 5 A g-1.

Furthermore, the long-term cycling performance of SNNU-80 electrode was measured at 13



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constant current density of 5 A g-1 as shown in Figure 4c. There was only 5.29% capacitance loss observed over 3000 cycles. Also, the PXRD patterns (Figure S4), FT-IR and Raman spectra (Figure S8) of the working electrode before and after electrochemical performance test clearly showed the stability of SNNU-80 MOF electrode. Clearly, the 9-connected net effectively helps to stabilize the framework during the electrochemical process. Moreover, the Nyquist plots of Co-MOF electrode indicate that the equivalent series resistance was 3.1 Ω (Figure S9). This suggests that Co-MOF material can enable high electron mobility through the electrode to facilitate the electron transfer. Moreover, in the low-frequency region, the linear response represents an interfacial diffusive resistance. SNNU-80 electrode exhibiting slight deviation from the imaginary axis indicates that the electrode has a low diffusive resistance. From these observations, this Co-MOF electrode’s high specific capacitance can be attributed to its low serial and diffusive resistance. The change of CV curves for SNNU-80 MOF electrode before and after 3000 cycles shown in Figure 4d. After 3000 cycles, the redox peaks are more obvious due to sufficient faradic redox reaction at higher current densities of the electrode materials, which suggests its good reversibility.

■ CONCLUSIONS In summary, a unique high-connected micro- and meso-porous Co-MOF material on the base of Co3-trigonal prismatic building blocks and tripodal pyridyl-dicarboxylate ligands has been successfully designed by the ligand extension method. The torsions induced by the large bulk of organic linker make the Co-MOF exhibiting a different topological net compared to initial 14


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xmz MOF. Four types of polyhedral cages connected with each other via the face-sharing fashion to furnish a hierarchical channel-cavity biporous system of the Co-MOF. Thanks to high-connected robust porous framework and the pseudocapacitor contribution, the Co-MOF shows remarkable gas adsorption and separation properties as well as supercapacitor performance.

■ ASSOCIATED CONTENT Supporting Information Powder X-ray diffraction patterns, TGA curves, FT-IR and Raman spectra, gas adsorption isotherms, gas selectivity results, additional crystal structure figures, crystallographic tables, and CIF files for SNNU-80. The Supporting Information is available free of charge on the ACS Publications website.

Accession Code CCDC 1532258 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q. G. Zhai) Notes 15



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The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21671126 and 21503183), the Fundamental Research Funds for the Central Universities (GK201701003), and the Natural Science Foundation of Shaanxi Province (2014KJXX-50).

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For Table of Contents Use Only

Gas Uptake and Supercapacitor Performance of a High-Connected Porous Co-MOF Induced by Ligand Bulk Xiang-Yang Hou,†,‡ Xiao Wang,†,‡ Shu-Ni Li,† Yu-Cheng Jiang,† Man-Cheng Hu,† Quan-Guo Zhai*,†

The combination of trigonal prismatic building blocks and tripodal pyridyl-dicarboxylate linkers produces a mixed-valent high-connected hierarchical porous Co-MOF exhibiting remarkable gas uptake and separation properties as well as supercapacitor performance.

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Gas Uptake and Supercapacitor Performance of a Highly Connected

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