Conjugated Microporous Polycarbazole Networks as Precursors for

Article pubs.acs.org/cm

Conjugated Microporous Polycarbazole Networks as Precursors for Nitrogen-Enriched Microporous Carbons for CO2 Storage and Electrochemical Capacitors Haige Wang,†,# Zhonghua Cheng,†,# Yaozu Liao,*,†,‡,§ Jiahuan Li,† Jens Weber,∥ Arne Thomas,*,‡ and Charl F. J. Faul*,§ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ‡ Department of Chemistry, Functional Materials, Technische Universität Berlin, Hardenbergstrasse 40, 10623 Berlin, Germany § School of Chemistry, University of Bristol, Bristol, England BS8 1TS, United Kingdom ∥ Department of Chemistry, Hochschule Zittau/Görlitz (University of Applied Science), Theodor-Körner-Allee 16, D-02763 Zittau, Germany S Supporting Information *

ABSTRACT: The design and synthesis of novel microporous materials have received tremendous attention in both CO2 storage and sequestration (CSS) and electrochemical energy storage (EES). We report molecular design and synthesis of conjugated microporous polycarbazole networks as new precursors for nitrogen-enriched porous carbons. As-prepared porous carbons exhibit a high nitrogen content (6.1 wt %), ultramicropore size (0.7−1 nm), and large surface area (1280 m2 g−1). As a result, these novel nitrogen-enriched carbons show highly efficient and reversible CO2 capture (can store 20.4 wt % at 1 bar and 11.1 wt % at 0.15 bar and at 273 K, while maintaining 100% CO2 uptake capacity after five cycles). Moreover, they can be applied as electrodes and enable high-performance EES devices with a fast charge/discharge rate (8 s), high electrochemical capacity (558 F g−1), and good cycle ability (retain 95% capacity after 1000 cycles).

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INTRODUCTION

polymer networks (MPNs) with permanent porosity, high surface areas, and enhanced thermal and chemical stability.10−14 Carbazole-functionalized monomers can be readily polymerized into such MPNs by chemical or electrochemical oxidative polymerization.15 As example, self-condensation of 2,7-bis(Ncarbazolyl)-9-fluorenone under acidic conditions successfully allowed for the generation of novel conjugated microporous polymers with high specific surface areas (SBET) up to 2250 m2 g−1 and hydrogen storage capacities of up to 1.7 wt % (1.0 bar and 77 K).10 Direct oxidative coupling polymerization of starlike carbazole monomers produced polycarbazole networks with SBET up to 2220 m2 g−1 and CO2 storage capacities of up to 21.2 wt % (1.0 bar and 273 K).11,12 Furthermore, it was shown that the nitrogen-containing conjugated structure can enhance the interaction with specific adsorbate molecules.13 Recently, microporous polymer networks have been also described as suitable precursors for the generation of high-

Recently, the design and synthesis of novel microporous materials have received tremendous attention in both fundamental research and practical applications.1−3 Because of their large surface areas, permanent nanopores, structural modularity, and the possibility to easily tailor their functionality,3 microporous materials have been proven to be useful for diverse applications.4−7 Two applications are of particular interest and have contributed greatly to the development of microporous materials: first, CO2 storage and sequestration (CSS), for which microporous materials promise a cost-effective physical adsorption pathway;3 and second, electrochemical energy storage (EES), as in supercapacitors, for which porous materials enable fast charge/discharge rates, high specific power densities, long cycling life, and a wide working temperature range.8 Polycarbazoles are a class of conjugated polymers, which have been studied for almost three decades9 and have found to be promising materials for various applications such as sensors, photovoltaic cells, field-effect transistors, and electrochromic devices.9 The rigid conjugated backbone of polycarbazoles is also beneficial for the formation of covalently bound, microporous © 2017 American Chemical Society

Received: March 1, 2017 Revised: May 15, 2017 Published: May 15, 2017 4885

DOI: 10.1021/acs.chemmater.7b00857 Chem. Mater. 2017, 29, 4885−4893

Article

Chemistry of Materials Scheme 1. Synthetic Routes To Polycarbazole Networks and Their Derived Carbons

Figure 1. (a−c) photographs, (d−f) SEM images, and (g−i) TEM images of (a, d, g) PTCT, (b, e, h) PTCA, and (c, f, i) PBCP.

surface-area, heteroatom-doped carbons.16−18 In this study, we prepared a series of nitrogen-rich conjugated microporous polycarbazole networks (Scheme 1) by chemical oxidative polymerization, which were subsequently applied as precursors for microporous heteroatom-doped carbons. It is noteworthy that we made use of nitromethane as a co-solvent for our reactions ( see Scheme 1): FeCl3 is fully soluble in this solvent,

which has led to interesting new properties in these well-known materials. The three precursor networks have been reported by Kim and co-workers (PTCT)19 and Han and co-workers (PTCA and PBCP),14 respectively. However, since the properties of the polymer networks should be directly compared with the subsequently produced carbon materials, and our modified method has led to significantly different properties, we will also 4886


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compared to the reported protocols in dichloroethane or chloroform.14,19 Pore size distributions (PSDs) derived by the nonlocal density functional theory (NLDFT) method indicate that all the polymers mainly exhibit microporosity (Figure S4 in the Supporting Information). PTCT, PTCA and PBCP prepared in this study show CO2 uptakes of 99.7, 66.5, and 34.9 cm3 g−1 (19.6, 13.1, and 6.9 wt %, respectively) at 273 K and 1 bar (see Figure S5 in the Supporting Information), respectively, and even a high uptake at elevated temperatures (Table 1). Note that the CO2 uptake observed in PTCT and PTCA are much higher than the values reported for the same networks prepared using the original protocols, while for PBCP lower values are observed (see Table S1 in the Supporting Information).14,19 The thermogravimetric analyses (TGA) of both pure monomers and polymers were conducted under nitrogen. Pyrolysis of PTCT, PTCA, and PBCP at 1000 °C in an inert gas (N2), yielded 75−80 wt % black carbon chars (see Figure S6 in the Supporting Information). In contrast, pyrolysis of the pure monomers produced no or only very limited carbonaceous residues (TCT (0 wt %), TCA (29 wt %), and BCP (0 wt %)), as confirmed by TGA scans. Elemental analyses show high amounts of nitrogen remain within the carbonaceous structures, namely, 6.07, 3.60, and 2.99 wt % for PTCT-C, PTCA-C, and PBCP-C, respectively. Compared with the monomers, the much-improved thermal stability found in the polymers implies a high level of cross-linking within the networks. Also, the high carbon yields indicated a high efficiency of carbon conversion. Note that the TCA indeed also shows some tendency to carbon formation, as evaporation is hindered by its higher molecular weight. As observed for the polymer networks, TEM images show that also the black carbons consist of aggregated nanoparticles with average diameters of ∼100 nm and apparent microporosity (see Figures 3a−c). The aggregated nanoparticles lead to the formation of some large mesopores and macropores from the interparticular voids, generating a hierarchical pore structure in the obtained carbon materials. Notably, HRTEM and XRD analyses indicate that the carbons are amorphous, showing no obvious amount of graphitization, which is unusual for carbonization treatment at 1000 °C (see Figure 3c, as well as Figure S3b in the Supporting Information). This result can be due to the amorphous and contorted structure of the polymer networks precursor, which aggravates graphitization. Note that the first noticeable weight loss of the polymers, as a sign for the start of carbonization, starts just above 600 °C. This finding also shows that Fe(III) residues from the oxidant can be almost completely washed out of the networks (also confirmed by energy-dispersive X-ray spectroscopy), as it can be expected that the amount of graphitization observed would otherwise be much higher. Compared with the polymeric precursors, the PTCT-C and PBCP-C exhibit higher nitrogen uptake in the low-pressure region (p/p0 < 0.01), implying increased microporosity. Obviously, the isotherms for the carbonized samples are much closer to type I isotherms than those observed for the polymer networks. Especially, the continuous nitrogen uptake over the entire pressure range and the large hysteresis are significantly reduced for the carbonized samples, indicating a considerable increase in stiffness of the microporous carbons, compared to the polymers. The BET surface areas of PTCT-C, PBCP-C, and PTCA-C were calculated to be 1280, 910, and 790 m2 g−1, respectively (Figure 2b), i.e., for PTCT-C and PBCP-C, the surface area significantly increases during carbonization, whereas, for PTCA-C, a reduction in surface area is observed. The latter

briefly report on selected aspects of the structure and properties of the polymer networks, compared to previous reported methods.14,19

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RESULTS AND DISCUSSION The synthesis of the networks was achieved using a slightly modified synthesis protocol from the reported protocols, i.e., by oxidation with FeCl3 in nitromethane/chloroform mixtures, instead of acetonitrile, dichloroethane, or chloroform, as usually used in chemical oxidative polymerizations.11−14,20 We observed that the oxidant, iron chloride (FeCl3), is fully soluble in nitromethane, while, e.g., in chloroform, just dispersions can be prepared. This ensures a real homogeneous solution reaction, which is in contrast to the conventional reported reactions in a suspension of FeCl3. Chemical oxidative polymerization of the commercially available monomers, namely 4,6-tri(9H-carbazol9-yl)-1,3,5-triazine (TCT), 4,4′,4″-tri-9-carbazolyltriphenylamine (TCA), and 4,4′-bis(9H-carbazol-9-yl)biphenyl (BCP) (Scheme 1) in a homogeneous CHCl3/CH3NO2 solution, yields light-yellow PTCT, brown PTCA, and white-yellow PBCP solids (see Figure 1), respectively. The chemical identity of the samples was confirmed by Fourier transform infrared (FT-IR) spectroscopy, solid-state 13C crosspolarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectroscopy (see Figures S1 and S2 in the Supporting Information), and elemental analysis, which compared well to the reported results for these networks.14,19 All the polymers are amorphous (Figure S3a in the Supporting Information), consisting of aggregated nanoparticles with diameters of 50−100 nm (Figure 1), as determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Note that the spherical morphologies of PTCT found here, upon changing the solvent and, consequently, the reaction conditions, are in big contrast to the fibrous structures observed in previous investigations (i.e., PCBZ).19 The porous properties of polycarbazole networks were investigated by nitrogen adsorption/desorption measurements at 77.4 K. As shown in Figure 2a, all the polymers exhibited a

Figure 2. N2 adsorption/desorption isotherms of (a) polycarbazole networks and (b) their derived carbons.

sharp nitrogen uptake in the low-pressure region (p/p0 < 0.01), implying significant microporosity. Curiously, the slightly varied synthesis protocol yield a much higher Brunauer−Emmett− Teller (BET) surface area for PTCT (895 m2 g−1 vs 341 m2 g−1), a comparable surface area for PTCA (1020 m2 g−1 vs 1050 m2 g−1), and a smaller one for PBCP (399 m2 g−1 vs 630 m2 g−1), 4887


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Chemistry of Materials Table 1. Pore Characteristics and CO2 Uptake Capacities of Obtained Porous Polymers and Carbons CO2 Uptake [cm3 g−1] sample

surface area [m2 g−1]

PTCT PTCA PBCP PTCT-C PTCA-C PBCP-C

895 1020 399 1280 790 910

a

pore width [nm] 0.75 1.08 1.13 0.71 0.93 0.94

b

c

d

micropore volume [cm3 g−1]

pore volume [cm3 g−1]

273 K, 0.15 bar

293 K, 0.15 bar

273 K, 1 bar

293 K, 1 bar

1.04 1.55 0.36 1.64 1.31 1.98

1.17 1.85 0.62 1.82 1.82 2.24

35.2 23.5 12.3 56.6 44.6 53.5

26.6 17.9 8.1 34.7 27.9 30.8

99.7 66.5 34.9 103.7 81.1 97.6

75.5 50.7 25.2 75.3 59.5 66.3

a

Surface area calculated based on adsorption isotherms. bPore width calculated based on NLDFT method. cPore volume calculated based on MP method. dPore volume calculated based on p/p0 = 0.994.

Figure 3. (a) Photograph, (b) TEM micrograph, and (c) HRTEM micrograph of PTCT-C; (d) N 1s core-level XPS spectra of PTCT and PTCT-C.

might be explained by the predetermined breaking point of the central amine of the triphenylamine tecton, which, in the polymer, is mainly responsible for the high surface area. The PSDs of these carbons determined by the NLDFT method are shown in Figure S4b in the Supporting Information. The PTCTC showed the smallest micropore size (0.71 nm) and the secondhighest micropore volume (1.64 cm3 g−1). Note that decreased micropore size but increased micropore volume was observed for all the carbons, showing a higher amount of smaller pores were created during carbonization (Table 1). As shown in Figure 4, all the porous carbons exhibit an increased CO2 uptake capacity, compared to the polymer networks, up to 103.7 cm3 g−1 (20.4 wt %) at 273 K and 1 bar. Specifically, PBCP-C showed a factor of 2.8 increase in CO2 uptake, compared to the values of the PBCP precursor [(97.6 cm3 g−1 vs 34.9 cm3 g−1) or (19.2 wt % vs 6.9 wt %)], because of the enhanced surface area (910 m2 g−1 vs 399 m2 g−1) and micropore volume (1.98 vs 0.36 cm3 g−1). Notably, all the carbons exhibited even better CO2 uptake at low pressures (p/p0 = 0.025−0.20; see Figure S7 in the Supporting Information), with PTCT-C storing 56.6 cm3 g−1 (11.1 wt %) and 34.7 cm3 g−1 (6.8 wt %) CO2 at 0.15 bar at 273 and 293 K, respectively. The high CO2 uptake capacities of these carbon materials at such low pressure are comparable to most inorganic porous adsorbents,21 such as polymeric carbons nZDC-700 (6.1 wt % at 0.15 bar, 15.4

Figure 4. CO2 adsorption (solid circle) and desorption (open circle) isotherms of polycarbazole-derived carbons at (a) 273 and (b) 293 K. The dashed lines indicate the CO2 adsorption at 0.15 bar.

wt % at 1 bar, 298 K),22 SU-MAC-500 (7.9 wt % at 0.15 bar, 19.8 wt % at 1 bar, 298 K),23 and active carbon CS-6-CD-4 (5.3 wt % at 0.15 bar, 20.2 wt % at 1 bar, 298 K).24 We believe that the structure and composition of the MPN-derived microporous carbons have a great effect on CO2 storage efficiency. Micropores could help to provide a higher degree of accessible CO2-philic sites,25,26 while the nitrogen-rich structures would increase the 4888


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Chemistry of Materials CO2-adsorptive heats4,5,27 and therefore enhance the interaction between the adsorbents and CO2 molecules. The chemical nature of polycarbazole networks upon carbonization was further investigated by X-ray photoelectron spectroscopy (XPS). As an example, the N 1s core-level XPS spectrum of PTCT show two peaks at 398.66 and 400.37 eV (see Figure 3d, as well as Table S2 in the Supporting Information), originating from nitrogen within the triazine and the carbazole moiety, respectively.28−30 The N 1s spectrum of the carbonized material PTCT-C can be deconvoluted into two peaks at 398.5 and 400.7 eV, which, in nitrogen-doped carbon materials, are attributed to pyridine-like and graphitic nitrogen,31 respectively. XPS revealed that the nitrogen content decreased from 13.97% to 6.08% in PTCT-C, still showing a high amount of nitrogen doping in the microporous carbons. The residual nitrogens can serve as binding sites for CO2 uptake through Lewis acid−base interactions, explaining the high CO2 storage efficiencies achieved in both the polycarbazole networks and the carbon materials derived therefrom. To further elucidate the interaction between CO2 and the surface of the microporous materials, the isosteric heats of adsorption (Qst) were calculated by fitting the CO2 adsorption isotherms at 273 and 293 K, using the Clausius−Clapeyron equation.32 The Qst values of the pristine and carbonized polycarbazole networks at low adsorption amounts were calculated to be 31−33 kJ mol−1 (see Figure S8 in the Supporting Information), pointing to a strong dipole−quadrupole interaction between the CO2 molecules and the nitrogen-rich polymers and carbons.33 Notably, the CO2 adsorption isotherms of the PTCT-C are fully reversible (Figure 5) and match the

equipped with a three-electrode configuration (see Figures 6a and 6b, as well as Figures S9a and S9b in the Supporting Information). The CV curves of all the carbons showed roughly rectangular shapes and a few undefined features, because of the combination effects of electric double-layer (EDL) capacitance and pseudocapacitance.46−48 These CV curves also showed some deviations with changes in the scan rates (5−50 mV s−1), which were due to the quick pseudocapacitance or Faradaic reactions, which are frequently found in heteroatom-doped carbon materials such as nitrogen-doped carbon fibers and graphene oxides.49,50 PTCT-C displayed the largest area under the CV curves compared with the other two carbons, suggesting the highest electrochemical capacitance, which is most likely due to the highest surface area (1280 cm2 g−1) and nitrogen content (6.07 wt %) of this material.44,51 In addition, previous studies showed that the efficiency of pore filling, i.e., of double layer formation, is optimal when the pore size is ∼0.7 nm (which exactly fits the pore size observed in PTCT-C) in aqueous media.52−56 Such pores allow the desolvated ions to interact with the surface of the electrodes, thereby increasing the electrochemical capacitance. Note that PTCA-C and PBCP-C have comparable micropore sizes (0.93 vs 0.94 nm) and surface areas (790 vs 910 m2 g−1). However, PTCA-C exhibits higher capacitances than PBCP-C probably due to the relatively higher nitrogen-doping level (3.60 wt % vs 2.99 wt %). Moreover, galvanostatic charge/discharge (GCD) measurements were carried out in 6.0 M KOH using a two-electrode cell configuration to obtain accurate specific capacitances. As shown in the GCD curves (see Figure 6c, as well as Figures S9c and S9d in the Supporting Information), all porous carbons showed very fast charge/discharge rates (8−300 s). However, PTCT-C exhibited a much longer discharge time at 1.0 A g−1, in comparison with the other two carbons, which is in agreement with the CV results. On the basis of the discharge time, the specific capacitances of PTCT-C, PTCA-C, and PBCP-C were calculated to be 558, 385, and 290 F g−1 at 1.0 A g−1, as well as 492, 357, and 264 F g−1 at 2.0 A g−1, respectively. It is believed that the high content of nitrogen and appropriate size of micropores would contribute to the pseudocapacitance and EDL capacitance, respectively, as confirmed by similar nitrogenenriched porous carbons derived from poly(o-phenylenediamine).57 This explains why PTCT-C showed a high capacitance (558 F g−1), which is almost equal to the theoretical capacitance of graphene (550 F g−1).58 When the current density increased from 1.0 A g−1 to 10 A g−1, a sharp decrease in capacitance was observed for PTCT-C, from 558 F g−1 to 140 F g−1. Surprisingly, PTCA-C with a relatively low surface area (790 m2 g−1) and intermediate nitrogen content (3.6 wt %), but bigger micropore size (0.93 nm), retained a high capacitance of 218 F g−1 at 10 A g−1. Taking the internal resistance (IR) drop into consideration, we assumed that the significant decrease in capacitance at high current loads could be ascribed to the large quantity of micropores (mainly at 0.71−0.94 nm). They may afford insufficient time for rapid ion diffusion and electron transfer, thereby increasing the inner resistance.46,59 At the high current, the discharge lagged behind the charge (i.e., low Columbic efficiency), implying that some of the ions that entered into ultramicropores after the charge process were trapped during the discharge process.60,61 Also note that, besides micropores, the hierarchical pore structures observed by TEM (Figures 3b and 3c) are beneficial for supercapacitors, because the interconnected pore channels facilitate ion transport.

Figure 5. Five cycles of CO2 uptake of PTCT-C at 273 K (p/p0 = 0.025−0.995).

desorption isotherms well (Figure 4), thus indicating recyclability and potentially lower regeneration cost of our materials, in comparison with the industrial monoethanolamine (MEA) solution.34 In addition to these attractive properties and performance related to CSS, the potential application of the MPN-derived Ndoped microporous carbons as electrode materials was further explored.35−41 Previous studies reported that nitrogen-enriched porous carbon-based electrodes show enhanced performance and excellent cyclability in electrochemical capacitors.42−46 Motivated by the architecture of the carbon materials with high nitrogen content, abundant micropores, and high surface areas presented here, the EES characteristics for the carbonized samples were evaluated. Initially, cyclic voltammetry (CV) scans were performed in 6.0 M KOH using a conventional cell 4889


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Figure 6. Electrochemical performance of porous carbons obtained in 6.0 M KOH: (a) CV curves of porous carbons at a scan rate of 5 mV s−1, (b) CV curves of PTCT-C obtained at different scan rates, (c) GCD curves of PTCT-C obtained at different current densities, and (d) specific capacitance of porous carbons obtained at different current densities.

Moreover, cycling stability is also an important concern for supercapacitors, especially if pseudocapacitance exists. The GCD cycling of PTCT-C was performed at a current load of 2.0 A g−1 (Figure 8) to investigate the electrochemical stability of the

Electrochemical impedance spectroscopy (EIS) measurements were conducted in 6.0 M KOH using a two-electrode system; the Nyquist plots of all the carbons are shown in Figure 7.

Figure 7. EIS curves of porous carbons obtained with an amplitude of 5 mV (10−2−106 Hz). Inset figure shows a magnified region (1.45−1.75 Ω).

Figure 8. Cycle stability of PTCT-C at a current load of 2.0 A g−1. Insert picture shows the GCD cycling curves (1000 cycles).

nitrogen-enriched carbazole-based carbons. After 1000 cycles, 95% capacitance was retained, indicating the excellent cycle stability of our carbon materials.

At low frequencies, the vertical line in Figure 7 indicates good ion diffusion and almost ideal capacitive behavior.52,54−56,61 At high frequencies, the electrodes behaved similar to pure resistors, which is normally found for nonmetallic carbons.59 For the middle-range frequencies, the plots indicated small deviations, representing low conductivity, possibly caused by microporosity and pseudocapacitance,51,54−56,61 which are consistent with the IR drops observed by GCD measurements. Therefore, we conclude that the microporous carbons could promote electrochemical capacitance greatly but impair the transport of ions, leading to charge-transfer resistance.

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CONCLUSION In summary, nitrogen enriched porous carbons with high surface areas were successfully prepared from polycarbazole networks as the precursors by a direct pyrolysis method. The BET surface area of the best carbon prepared by carbonization of a triazinebased polycarbzole (PTCT) network at 1000 °C reaches 1280 m2 g−1 and still shows a high nitrogen content of 6.07 wt %. Because of the rigid and cross-linked structures of the precursors, 4890



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nitrogen-enriched carbons were obtained with high yields of 75−80 wt%. As a result of their high microporosity, surface area and nitrogen content, the MPN-derived carbons show promising performance as sorbents for CO2 capture (19.6 wt%, 273 K, 1 bar). They furthermore show high capacities of up to 558F g−1 in 6.0M KOH aqueous electrolytes, because of the synergetic effect of double layer capacitance and pseudocapacitance. With a charge/discharge cycle stability retaining 95% capacity after 1000 cycles, this makes the here-presented carbons interesting electrode materials for supercapacitors. Even though the applied synthesis method makes use of simple and commercially available monomers and reactants and yield the final nitrogenenriched carbon materials in high yields, we are aware that the carbon materials presented here are still rather high-priced, when compared to other carbon materials such as activated charcoals, which is a major drawback for industrial applications. However, the possibility to create carbon materials with high and tailorable surface area and nitrogen content make the materials presented here certainly interesting model compounds, showing the effect of these structural properties for both carbon capture and energy storage technologies.

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REFERENCES

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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.chemmater.7b00857. Experimental section, FT-IR, NMR, and XRD spectra, TGA curves, additional gas adsorption and desorption isotherms, pore size distribution, the heat of adsorption, CV curves, and GCD curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Liao). *E-mail: [email protected] (A. Thomas). *E-mail: [email protected] (C. F. J. Faul). ORCID

Yaozu Liao: 0000-0001-9263-6281 Arne Thomas: 0000-0002-2130-4930 Charl F. J. Faul: 0000-0001-6224-3073 Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Alexander von Humboldt Fellowship and Marie Curie Fellowship (FP7-PEOPLE-2012-IIF TANOGAPPs No. 326385), the National Natural Science Foundation of China (No. 51673039), the Shanghai Pujiang Talent Program (No. 16PJ1400300), and the Fundamental Research Funds for the Central Universities (No. 16D110618) for the generous support of this project. We furthermore acknowledge the support from the Sino-German Center for Research Promotion (No. GZ879). Initial gas adsorption data were collected on a Quantachrome Autosorb-1MP system that was purchased under EPSRC CDT Capital Grant No. EP/K035746/1. 4891


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