From Metal–Organic Framework to Porous Carbon Polyhedron

Article pubs.acs.org/IC

From Metal−Organic Framework to Porous Carbon Polyhedron: Toward Highly Reversible Lithium Storage Hai-Jun Peng,† Gui-Xia Hao,‡ Zhao-Hua Chu,‡ Ying-Lin Cui,† Xiao-Ming Lin,*,†,§ and Yue-Peng Cai*,† †

Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China ‡ College of Chemistry and Environmental Engineering, Hanshan Normal University, Chaozhou, Guangdong 521041, P. R. China § Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China S Supporting Information *

ABSTRACT: By application of a newly designed T-shaped ligand 5(4-pyridin-4-yl-benzoylamino)isophthalic acid (H2PBAI) to assemble with Zn(II) ions under solvothermal conditions, a novel porous polyhedral metal−organic framework (Zn-PBAI) with pcu topology has been obtained. When treated as a precursor by annealing of ZnPBAI at various temperatures, porous carbon polyhedra (PCP) were prepared and tested as an anode material for lithium-ion batteries. The results show that PCP carbonized at 1000 °C (PCP-1000) manifest the highest reversible specific capacity of about 1125 mAh g−1 at a current of 500 mA g−1 after 200 cycles, which is supposed to benefit from the large accessible specific area and high electric conductivity. Moreover, PCP-1000 electrode materials also exhibit superior cyclic stability and good rate capacity. nanostructured carbon,17 mesoporous carbon,18 and Nmodified carbon coated graphene19 have been investigated for new electrode materials. In our previous work, we have prepared a porous carbon microtube material by facile pyrolysis of Cd-MOF {[Cd(HBTPCA)]2H2O}n.20 As a continuous work, herein, we designed and synthesized a novel T-shaped ligand, 5(4-pyridin-4-yl-benzoylamino)isophthalic acid ligand (H2PBAI, Figure S1), with pyridine and isophthalate groups connected through amide group, to construct a porous Zn-based MOF with pcu topology. Direct pyrolysis of this MOF gave rise to porous carbon polyhedra, which could be used as anode materials for LIBs with enhanced reversible capacity and excellent cyclability.

1. INTRODUCTION Rechargeable lithium-ion batteries (LIBs), as one of the most promising power sources, are widely used in portable electronic devices.1 Among the diverse rechargeable batteries, carbon materials are commonly used as anode for their low cost. However, traditional graphite cannot meet the demands of next-generation LIBs due to its low capacity of 372 mAh g−1.2 Over the years, a great deal of research effort has been devoted to explore new high performance carbon anodes. Various carbonaceous materials, such as carbon nanotubes (CNTs),3 carbon nanofibers (CNFs),4 nanospheres,5 nanosheets,6 nanocones,7 hierarchically porous carbon,8 graphenes,9 ordered mesoporous carbon,10 and nitrogen/boron-doped carbon,11,12 have been successfully prepared and applied as the electrode of electrochemical energy devices. However, it still remains difficult to produce carbonaceous materials with controlled pore texture and improved capacity, which are considered to be the important factors in optimizing the performance in LIBs. Therefore, exploring and developing a novel method to prepare carbon materials are urgently required. Interestingly, metal−organic frameworks (MOFs) consisting of organic ligands and metal ions/clusters have emerged as special functional solid-state materials for gas storage and separation,13 catalysis,14 sensing,15 and drug delivery16 due to their large surface area and tunable porosities. Recently, MOFs have been successfully utilized as templates or precursors to prepare porous carbon materials for applications in LIBs and supercapacitors. Several novel carbon materials including © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Functional groups of the materials were assessed using Fourier transform infrared spectroscopy (Thermo Nicolet NEXUS 670 FT-IR, USA). A PerkinElmer 240 elemental analyzer (USA) was used to record the elemental analyses. Crystalline phase was identified by a Bruker D8 Advance X-ray diffraction pattern diffractometer (BRUKER-AXS, Germany). Raman spectra were obtained from a Renishaw inVia confocal Raman microscope (U.K.). Netzsch Thermo Microbalance TG 209 F1 Libra (Germany) was used for thermogravimetric analyses (TGA) in order to know the thermal behavior (mass) of the material. The sorption isotherms were assessed using a Belsorp max gas sorption analyzer (Japan) at 77 K. The surface Received: June 17, 2017 Published: August 3, 2017 10007

DOI: 10.1021/acs.inorgchem.7b01539 Inorg. Chem. 2017, 56, 10007−10012

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Inorganic Chemistry morphology and architecture were recorded by applying scanning electron microscopy (SEM, TESCAN Maia 3, Czech). X-ray photoelectron spectroscopy (XPS) was recorded by an ESCALAB 250Xi XPS spectrometer (USA). Varian Mercury Plus 300 MHz spectrometer was used to record 1H NMR spectra (USA). 2.2. Preparation of the Zn-PBAI MOF. To dimethylformamide (DMF, 6 mL) in a 20 mL scintillation vial were added Zn(NO3)2· 6H2O (14.9 mg, 0.05 mmol) and H2PBAI (18.1 mg, 0.05 mmol). The synthetic route of the H2PBAI ligand is given in Figure S1 and further confirmed by the H1 NMR spectrum (Figure S2). The vial was tightly capped and heated in an 80 °C oven for 72 h. After cooling, colorless block crystals were removed by filtration and dried in air (20 mg, 40% yield). FT-IR (KBr, cm−1): 3447 (vs), 2348 (w), 1633 (m), 1545 (w), 1294 (w), 770 (w), 623 (m), 483 (w). Anal. Calcd for C26H30N4O9Zn (607.89): C 51.33, H 4.92, N 9.21%. Found: C 51.26, H 4.97, N 9.17%. 2.3. Preparation of the Porous Carbon Materials. The porous carbon polyhedral (PCP) materials were fabricated by annealing of the as-synthesized Zn-based MOF particles under flowing nitrogen gas at temperatures of 800 to 1000 °C. The resultant products were placed under nitrogen atmosphere for 1 h. After the carbonization temperature was retained in the tube furnace for 6 h, the materials were cooled to room temperature at a rate of 1 °C min−1. Subsequently, the obtained product was thoroughly washed by hydrofluoric acid to remove all Zn elements. Finally, the sample was washed three times with ethanol and dried at 70 °C to obtain the black powder samples. 2.4. Electrochemical Measurement. Test electrodes consisted of the conductive carbon black (10 wt %), active powder material (80 wt %), and poly(vinylidene fluoride) (10 wt %). The slurry was formed after thoroughly mixing the electrode components, and coated on copper foil. Laboratory-made CR2032-type coin cells were fabricated in an argon-filled glovebox with the separator (Celgard 2400), lithium metal, and electrolyte composed of 1 M LiPF6 in mixed solvent of ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/ DMC/DEC, 1:1:1 in volume). Cyclic voltammetry (CV) curves were carried out at a scanning rate of 0.1 mV s−1 (CHI605C electrochemical workstation, voltage range: 0.01−3.0 V). The electrochemical impedance spectra were tested by the same workstation for various electrodes at 25 °C with the frequency between 100 kHz and 0.01 Hz. 2.5. X-ray Crystallographic Studies. X-ray reflection intensities were collected on an Oxford Gemini S Ultra diffractometer equipped with Cu Ka radiation (λ = 1.54178 Å) at 150 K by using ϕ and ω scan.21 The empirical absorption correction was used to correct the reflections. The structure was solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL program.22 Nonhydrogen atoms were refined with anisotropic displacement parameters at the final cycles. Isotropic displacement parameter was used to place the organic hydrogen atoms in calculated positions. Due to the highly disordered solvent molecules, as a result, we remove the diffraction contribution by PLATON/SQUEEZE.23,24 Crystallographic data for the structure is provided in Table S1, and selected bond lengths and bond angles are listed in Table S2. CCDC 1523821 contains the supplementary crystallographic data.

Figure 1. Crystal structure of Zn-PBAI. (a) Dinuclear Zn paddlewheel motifs and coordination environment of ligand. (b) Octahedral cage. (c) 3D metal−organic polyhedral framework. (d) Primitive cubic network if the octahedra are regarded as octahedral nodes.

locate at each vertex (Figure 1b). These octahedral cages are further connected by sharing the SBUs to give rise to a 3D porous metal−organic polyhedral framework (MOP, Figures 1c and S3), showing the hexagonal channel with open window of 9.5 × 9.5 Å2 along the c axis (the centroid−centroid distances of the paddlewheels, Figure S4). If the SBB of the octahedral cage can serve as an octahedral node, the whole framework would be regarded as a primitive cubic topology (Figure 1d). After removal of the guest molecules, the solvent-accessible void is estimated to 36% as calculated by PLATON. 3.2. Characterization of Zn-PBAI MOFs. Scanning electron microscopy (SEM) was first applied to observe the morphology of the as-made Zn-PBAI MOFs. As seen in Figure S5, the Zn-PBAI MOF particles show uniform shapes and sizes overall: rhombic prisms with ca. 30 μm edges. In the IR spectrum, the carbonyl band at 1716 cm−1 for H2PBAI disappeared, indicating that the carboxylic groups of the ligand were completely deprotonated (Figures S6 and S7). The measured powder X-ray diffraction pattern (PXRD) was well matched with the simulated one (Figure S8), indicating the high phase purity of the samples, in which the preferred orientation was reflected by the intensity differences. To examine the thermal stability and behavior of Zn-BPAI, TGA measurements were conducted under N2 atmosphere (Figure S9). The TG curve shows two major losses between 30 and 800 °C. The weight loss of 64.51% during the first step (30−170 °C) corresponds to the release of two free DMF solvent molecules and two guest H2O moieties (exptl 30.4%, calcd 29.9%). Above 350 °C, the abrupt weight loss occurred, indicating the decomposition of the whole frameworks. 3.3. Characterization of the Porous Carbon Materials. The as-prepared crystalline samples were carbonized under N2 atmosphere at 800, 900, and 1000 °C, respectively. Then resulting products were further washed using hydrofluoric acid for 3 days to remove the residual Zn component. Corresponding porous carbon polyhedra were designated as PCP-800 (yield: 31%), PCP-900 (yield: 28%), and PCP-1000 (yield: 24%), respectively. Optical micrographs of Zn-PBAI and

3. RESULTS AND DISCUSSION 3.1. Crystal Structure Description. Zn-PBAI crystallizes in the space group of trigonal R3̅m and possesses a threedimensional (3D) polyhedral framework with paddlewheel Zn2(COO)4 as the basic secondary building units (SBUs). As shown in Figure 1a, two Zn ions are bonded by four carboxylate functional groups to generate the paddlewheel SBU with a Zn− Zn internuclear separation of 2.985(6) Å, while two N atoms occupy the axial positions. Six Zn-paddlewheel motifs are linked together by six PBAI ligands to give an octahedral supramolecular building block (SBB), in which six ligands cover the alternating faces of the octahedron and the paddlewheel SBUs 10008

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peaks at 1599 cm−1 for G band and 1347 cm−1 for D band (disordered carbon structure) were observed simultaneously. In general, the intensity of D band increases when the number of defects increases.26 The intensity ratios (R = ID/IG) of D band to G band for PCP-800, PCP-900, and PCP-1000 are 0.85, 0.90, and 1.02, respectively. The greater ID/IG value of PCP1000 than others indicates that the higher carbonization temperature generated more defects in the graphitic sheets. To examine both the chemical composition and bonding nature of the elements, X-ray photoelectron spectroscopy (XPS) spectra of the PCPs were analyzed as shown in Figure 3c. The full XPS spectrum confirms the existence of O, N, and C elements in all samples. Obviously, as the temperature rises from 800 to 1000 °C, the N content increases to 4.64, 5.11, and 5.33 atom %, while the C content decreases to 86.38, 85.15, and 83.72 atom %, respectively. The N 1s XPS spectrum in Figure S11 can be deconvoluted into three peaks at 398.3 eV (pyridinic (N-6)), 399.7 eV (pyrrolic (N-5)), and 401.1 eV (graphitic N atoms).27 In the C 1s XPS spectrum (Figure S12), the peaks at 288.6, 286.9, 285.7, and 284.7 eV correspond to the nonoxygenated graphitic carbon, the oxygenated carbons, carbonyl C, and carboxylate C, respectively.28 To better investigate the pore characteristics, nitrogen adsorption−desorption isotherms of all three samples were measured at 77 K. All isotherms follow the typical I curve which is characteristic for microporous solids and exhibits steep uptake and release of nitrogen gas at low pressure (Figure 3d). The calculated porosity parameters and the pore size distribution curves are presented in Table 1 and Figure S13.

carbon material depicted in Figure S10 indicate that the morphologies of PCP-800 and PCP-900 are similar to that of PCP-1000. Therefore, only the morphology of PCP-1000 is selected as a representative example. As shown in Figures 2a

Figure 2. SEM images of PCP-1000 at different magnifications. (a, b) Polyhedral structure. (c, d) Rough surface with irregular gullies and pores.

and 2b, PCP-1000 shows similar shapes and sizes as those of Zn-PBAI precursors. The high magnification SEM image exhibits the rough surface of porous carbon polyhedron with irregular gullies and pores (Figures 2c and 2d). As presented in Figure 3, PXRD patterns suggest that PCP800, PCP-900, and PCP-1000 samples have similar diffraction

Table 1. Pore Characteristics of the Obtained Carbon Particles samples

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

pore diameterc (nm)

PCP-800 PCP-900 PCP-1000 Zn-PBAI

1037 1160 1254 728

0.46 0.48 0.52 0.36

0.41 0.50 0.59 0.37

a The specific surface area (SBET) was calculated by the Brunauer− Emmet−Teller (BET) method. bVtotal represented the total pore volume. cMicropore size is calculated by the Horvath−Kawazoe (HK) method.

Obviously, the parameters, which can affect significantly the electrochemical performances of PCPs, increase as the carbonization temperature rises from 800 to 1000 °C. Compared with the porosity of Zn-PBAI precursor (BET of 728 m2 g−1, pore volume of 0.47 cm3 g−1, Figure S14), the larger BET surface of PCPs not only provides a convenient pathway to electrolyte diffusion and Li ion transfer but also facilitates accommodating the volumetric variations during the lithium storage process, which is highly favorable for the electrochemical performance enhancement as electrode. 3.4. Electrochemical Analysis as an Anode Material. To confirm our hypothesis that larger N contents, more defects, and greater porosity are beneficial for providing more active sites toward Li ions, the electrochemical behaviors of PCP electrodes were discussed in detail. The charge−discharge cycling performances of PCP-800, PCP-900, and PCP-1000 electrodes were evaluated in the range from 0.01 to 3.0 V at 0.5 A g−1 (Figure 4). Obviously, the specific capacity increases when the carbonization temperature rises from 800 to 1000 °C. The electrodes still retained a discharge capacity of 1125 mAh

Figure 3. Prepared PCP-800, PCP-900, and PCP-1000 samples. (a) PXRD. (b) Raman spectra. (c) Full XPS. (d) N2 sorption isotherms at 77 K.

features with two broad peaks at about 23° and 43°, respectively, corresponding to the graphite (002) and (101) peaks. The diffraction peak at 23° is characteristic of graphitization degree in the graphitic carbon materials.25 No other impurity signal was observed in the XRD pattern, which indicates that the Zn species were completely removed by the HF treatments. Raman spectra of all three PCP samples are shown in Figure 3b. Typically shown in amorphous carbon materials, two broad 10009

DOI: 10.1021/acs.inorgchem.7b01539 Inorg. Chem. 2017, 56, 10007−10012

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Figure 4. Cycling performance and Coulombic efficiency of PCP-800, PCP-900, and PCP-1000 at 0.5 A g−1 for 200 cycles. Figure 5. PCP-1000 electrode. (a) Discharge−charge curves from 0.01 to 3.0 V at 0.5 A g−1. (b) Rate capability test. (c) Cyclic voltammetry measurements during the first three cyclings. (d) Nyquist plots. The inset is the proposed equivalent circuit.

g−1 (PCP-1000), 943 mAh g−1 (PCP-900), and 771 mAh g−1 (PCP-800) after 200 cycles, respectively. Stable and reversible capacity can be observed and Coulombic efficiencies of all the electrodes were more than 99% after the initial few cycles. Compared with PCP-800 and PCP-900, the PCP-1000 electrode presents higher lithium storage and better cycling stability, which mainly originated from the following reasons: (i) The electrical conductivities of PCP-800, PCP-900, and PCP-1000 are 10.2, 13.5, and 18.4 S cm−1, respectively. The enhanced electrical conductivity facilitates the fast charge transfer during the charge/discharge process. (ii) Temperature is critical for the structural evolution of resultant PCP samples. Higher carbonization temperature improves the accessible BET surface area and provides more defects, which offered more active sites for Li storage and increased the ability for the accumulation of charges.29 (iii) As demonstrated in previous literature, the amount of N-doping composition can improve the Li storage capacity of the PCP materials to some extent.30 The N content of PCP-1000 (5.33 atom %) is larger than that of PCN-900 (5.11 atom %) and PCP-800 (4.64 atom %). Specifically, the total pyrrolic (N-5) N and pyridinic (N-6) atoms are as much as 67 atom % in the PCP-1000 particles (Table S3), making the structure electron-deficient, which can more strongly bind lithium atoms and accept more charge from Li ions.31 As a representative example, electrochemical performances of PCP-1000 will be described in detail. Figure 5a displays the galvanostatic charge/discharge curves of PCP-1000 within a cutoff window of 0.01−3.0 V at 500 mA g−1. The first discharge capacity is 2156 mAh g−1, while the first charge capacity is 1316 mAh g−1. The large irreversible capacity loss with a low initial Coulombic efficiency of 61.0% can be attributed to the generation of a solid electrolyte interface (SEI) layer and electrolyte decomposition.32 In addition, the rate performances presented in Figure 5b show that the reversible capacities are ∼1125, 1046, 850, and 698 mAh g−1 at 0.5, 1, 2, and 3 A g−1, respectively. More importantly, the discharge capacity can recover back to 1123 mAh g−1 after deep cycling at 3 A g−1. In comparison with other carbon hybrid materials (Table S4), the PCP-1000 electrode presents higher Li storage properties. In comparison with other porous carbon materials derived from MOFs, PCP-1000 electrode shows competitive capacity and superior rate capacity, demonstrating the excellent long-term cycling stability for LIBs. Figure 5c shows the typical cyclic voltammogram (CV) curves of PCP-1000 electrode. In the first

cathodic polarization process, a reduction peak at 0.52 V is found, which might be ascribed to the SEI formation. During the charge process, a wide oxidation peak at 1.36 V is detected. The cathodic peak disappears in the subsequent two cycles, and overlapping curves are observed, demonstrating that the electrochemical reversibility is gradually built. Moreover, electrochemical impedance spectra (EIS) of PCP-1000 were investigated before and after 200 charge/discharge cycles in order to get further insight into Li storage properties (Figure 5d). Similar curves for the two Nyquist plots are composed of a high-frequency semicircle (SEI resistance), a medium-frequency semicircle (charge-transfer resistance), and a lowfrequency straight line (Warburg diffusion resistance).33 The slightly decreasing depressed semicircle indicates that PCP1000 architectures reach a stabilized state and led to facile charge transfer.

4. CONCLUSION In summary, a novel Zn-PBAI metal−organic polyhedron with a primitive cubic network was developed as template for preparing porous carbon polyhedral material. When tested as LIB anode material, PCP-1000 presents improved capacity of 1125 mAh g−1. The enhanced electrochemical performance could result from the unique porous structure, N-doping, and high electrical conductivity. Many defects and large numbers of pores in the obtained material would increase the active sites for Li+ storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01539. Details of experimental results, SEM image, FT-IR spectrum, 1H NMR spectrum, PXRD pattern, TGA curve, and XPS spectrum (PDF) Accession Codes

CCDC 1523821 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 data_ 10010

DOI: 10.1021/acs.inorgchem.7b01539 Inorg. Chem. 2017, 56, 10007−10012

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[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected];. *E-mail: [email protected]. ORCID

Xiao-Ming Lin: 0000-0001-8835-103X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from National Natural Science Foundation of P. R. China (Grants 21401059, 21471061, and 21671071), Innovation team project of Guangdong Ordinary University (No. 2015KCXTD005), Applied Science and Technology Planning Project of Guangdong Province (Project 2017A010104015 and 2015B010135009), Natural Science Foundation of Guangdong Province (2014A030311001), and the Great Scientific Research Project of Guangdong Ordinary University (No. 2016KZDXM023).

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DOI: 10.1021/acs.inorgchem.7b01539 Inorg. Chem. 2017, 56, 10007−10012

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DOI: 10.1021/acs.inorgchem.7b01539 Inorg. Chem. 2017, 56, 10007−10012