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Preparation and properties of metal organic framework/ activated carbon composite materials Ohad Fleker, Arie Borenstein, Ronit Lavi, Laurent Benisvy, Sharon Ruthstein, and Doron Aurbach Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00528 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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Preparation and properties of metal organic framework/ activated carbon composite materials Ohad Fleker§, Arie Borenstein§, Ronit Lavi, Laurent Benisvy, Sharon Ruthstein, and Doron Aurbach* The Chemistry Department, Faculty of Exact Sciences, Bar-Ilan University, Ramat-Gan 5290002, Israel * Corresponding authors: [email protected]

KEYWORDS. Electron conductive MOF, MOF@AC, HKUST-1 Abstract Metal organic frameworks (MOFs) have unique properties that make them excellent candidates for many high-tech applications. Nevertheless, their non-conducting character is an obstacle to their practical utilization in electronic and energy systems. Using the familiar HKUST-1 MOF as a model, we present a new method of imparting electrical conductivity to otherwise non-conducting MOFs by preparing MOF nanoparticles within the conducting matrix of meso-porous activated carbon (AC). This composite material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), gas adsorption measurements and electron

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paramagnetic resonance (EPR) spectroscopy. We show that MOF nanoparticles grown within the carbon matrix maintain their crystalline characteristics and their surface area. Surprisingly, as a result of the composition process, EPR measurements revealed a copper signal that was not achieved so far. For the first time, we could analyze the complex EPR response of HKUST-1. We demonstrate the high conductivity of the MOF composite and discuss various factors that are responsible for these results. Finally, we present an optional application for using the conductive MOF composite as a high-performance electrode for pseudo-capacitors. Introduction Metal organic frameworks (MOFs) have been in the forefront of research for the last decade. These crystalline materials display a wide range of useful properties. They have high porosity and surface area, good stability in various environments and are inexpensive to prepare. The MOFs' unique characteristics mark them as candidates for various applications such as hydrogen storage, filtering and purification, catalysis and sensor and optical devices. However, one major drawback is their electric resistance, which makes them incompatible for use in electrical devices. The high durability of the MOFs has prompted researchers to search for solutions to this problem. Indeed, many studies have focused on preparing ion-conductive MOFs. One main method is to induce proton conductivity by either the use of acidic linker molecules1 or by incorporating H-donor guest molecules such as H2SO4.2,3 Both enable proton transfer between acidic functional groups and thus generate charge migration. Despite the promising results, proton-conducting MOF materials still have only relatively low conductivities in the order of 10−5 S/m. Moreover, this method requires special conditions such as an electrolyte environment, elevated temperatures and humidity.


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Although ionic conductivity of MOF material has been achieved to some extent,4 for most applications this kind of conductivity is insufficient. For example, many electrical devices do not contain an electrolyte. Those that do (such as devices containing electrochemical cells) require electric conductivity to complete the electric circle. Overall, MOF's electron conductivity is still inadequate. Attempts to achieve electronically conductive MOF materials have resulted in only limited success. In one attempt,5 iodine (I2) was used to oxidize a MOF compound, thus maintaining its porosity while increasing the conductivity by four orders of magnitude to 10-4 S/m. Sheberla et al.6 reported the highest electron conductivity to date for a Ni3(HITP)2 MOF, displaying pellet and film conductivities of 2x10-2 and 40x10-2 S/m, respectively. This high conductivity was achieved by using a conductive linker molecule that can possibly participate in the electron transfer process. Indeed, computational calculations and optical measurements support a slipped-parallel MOF 2D layer orientation and a high degree of bond conjugation, both of which contribute to its conductive behavior. These results suggest that a parallel-layers morphology and a high degree of conjugation greatly enhance MOF conductivity. Talin et al. demonstrated a very high conductivity of 7 S/m by introducing redox active conjugated conductivity-inducing guests such as the 7,7,8,8-tetracyanoquinododimethane (TCNQ) molecule into films of the isolating MOF HKUST-1.7 It was suggested that such guests form a bridge between dinuclear copper paddlewheel units, enabling electronic coupling. Similar to TCNQ, iodine (I2) was also used as a guest molecule to induce conductivity.8 HKUST1 energy levels and the influence of TCNQ on them were thoroughly studied by Hendon et al.9 Note that guest infiltration methods markedly decrease the available surface area, interfere with the metal site by blocking it and prevent utilization of the porous structure. Consequently, due to


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the high guest molecule load, the entire MOF crystal structure may be changed. MOFs have already been demonstrated to be good candidates for electrochemical systems. For example, Lee et al.10 have used a Co-based MOF-71 as electrode material for supercapacitors, which displayed good specific capacitance. Unfortunately, Lee did not give a detailed electro-conductivity analysis. Wu et al. suggested using combinations of MOFs and a conductive carbonaceous carrier such as O2 electrodes for Li-O2 batteries.11 This mixture was moderately conductive, owing to the carbon carrier, and enhanced the system's discharge capacity. It has been suggested that this enhanced discharge capacity is due to the MOF open metal sites. Similar mixtures have been studied, with similar results.12–14 Nevertheless, all of the above lab-scale solutions, although representing a major advancement towards using MOF materials in electronic devices, are not suitable for production scale-up. Several additional studies report production of MOF with a carbon component combination. In these reports, reagents were not adsorbed into the carbon matrix prior to the synthesis, and thus the MOF did not grow inside the activated carbon (AC) nano-size porous structure. Thus, these compounds do not show significant change in the MOF properties, and specifically, do not show conductivity.15–20 Many reports aim to produce conductive materials by pyrolyzing MOF compounds, thereby losing all of the original MOF properties in the process.21–23 Here we present a new, simple, and cost-effective approach for preparing conductive MOF composite materials. We aim at incorporating non-conductive MOF materials into a highly electrically-conductive matrix instead of focusing on MOF modification, which may have an adverse effect on their properties. Thus, we profit from the conductivity of the matrices while


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maintaining all of the MOF properties. By this approach, we extend the applications of these promising complex materials. The conductive host chosen was AC, which is widely used as electrode material. This conductive porous material displays an extremely high specific surface area (SSA) of up to 2000 m2/gr while maintaining a high conductivity of approximately 100 S/m. AC has unique electrical and thermal stability, high corrosion resistance, low thermal expansion coefficients, low densities and cost efficiency. These properties put activated carbon at the top of the list for use in electronic and electrochemical systems. The porosity and morphology of activated carbon can be easily controlled and shaped by a simple oxidation process. In addition, various studies demonstrate that AC has the ability to adsorb chemical species while maintaining or even increasing its conductivity and charge storage.24,25 Our method can be adapted for all MOFs, without any limitation to specific linkers, metal-ion centers or crystal structures. For a model system, we chose one of the most studied MOF materials, HKUST-1 (Hong Kong University of Science and Technology MOF 1). HKUST-1 is prepared by combining a benzene tricarboxylic acid (H3BTC) linker molecule and Cu2+-ions to form an endless array of metal-organic complexes (Cu3BTC2, see Supplementary Information (SI) Figure S1).26 HKUST-1 has a high surface area, good stability and a straightforward preparation. Since its discovery in 1998, it has been suggested for use as adsorption,26–28 filtering,29,30 electrode7 and sensor31,32 materials. The preparation process that we used in order to incorporate HKUST-1 into the highly electronconductive matrix AC is based on physical interactions. This process requires no additional chemical reagents, special equipment or radiation. The produced composite material samples were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning


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electron microscopy (SEM), nitrogen adsorption, and electron paramagnetic resonance (EPR) spectroscopy. The effect of the composition on both the carbon and the MOF's physical properties will be demonstrated and discussed. Unlike the pristine MOF HKUST-1 EPR spectrum, the composite material exhibits a clear Cu(II) EPR signal at room temperature. The unique EPR signal of the composite material will be discussed in light of the interactions between the guest molecule and its host. Conductivity measurements for this composite material indicate that its electrical conductivity is higher than that of the isolated MOF. Last, we present the specific capacity obtained by utilizing this composite material in an electrode placed in a full electrochemical cell. In order to confirm the validity and applicability of our approach and methodology, we prepared and measured an additional set of composite materials based on another MOF, Mil-100(Fe) and AC (see SI Figures S2 and S5). Combining the crystallinity and advanced properties of MOF with the enhanced conductivity of the activated carbon opens up a new range of possibilities for many electrical applications. Experimental section Synthesis The activated carbon in this research was obtained from Energy2 (United States. Detailed characterization can be found in the Supplementary Information). HKUST-1@AC composite material 1 was prepared by suspending 250 mg of oven-dried AC in a 10 ml di-methyl formamide, DMF solution of H3BTC (103 mg, 4.9x10-4 mol). The suspension was stirred at room temperature for two hr. Then, Cu(OAc)2◦H2O salt (147 mg, 7.3x10-4 mol, 1.5 eq. with respect to H3BTC) was added and the mixture was stirred for two additional hours at room temperature. The mixture was then heated overnight in an oven at 40 °C. The resulting


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suspension was then centrifuged and decanted, and the solid was washed with 15 ml of DMF. These washing cycles of centrifuging and decanting were repeated 3 times. The resulting solid was then suspended in acetone (5 ml) overnight and dried on a rotavapor. The dried sample was then placed in an oven at 40 °C, gradually heated to 150 °C (1 ˚C/min) and kept at that temperature for about 4 hr. Compositions 2 and 3 were prepared according to the same procedure, using 68.4 and 51.5 mg of H3BTC, respectively, and appropriate quantities of Cu(OAc)2◦H2O (1.5 eq) and AC. The resulting HKUST-1@AC composites contain 37% (composite 1), 23% (composite 2), and 16% (composite 3) of HKUST-1 by weight. The prepared compositions were stored in open air. To compare our results to recent work,11,12 we also prepared HKUST-1:AC mixtures 1-3 by mixing appropriate HKUST-1 and AC quantities. The obtained mixtures have the same HKUST-1:AC ratios, as in compositions 1-3. Mil-100(Fe)@AC composite materials (a)-(c) were prepared in the same way as compositions 13, based on the synthesis procedure reported by Zhang et al., by using Fe(acac)3 as the iron precursor (see the detailed synthesis description and characterization in the Supplementary Information).33 SCANNING ELECTRON MICROSCOPY (SEM) images and the corresponding elemental analyses

were obtained by FEI Inspec SEM (FEI Company, USA), equipped with an energy dispersive spectroscopy (EDS) accessory. GAS ADSORPTION PROPERTIES were measured using an Autosorb-1 MP (Quantachrome,

Florida USA) system. The specific surface area was calculated using the Brunauer-EmmettTeller (BET) model. The activation (solvent removal) of compositions 1-3 was done by drying under high vacuum at 150 °C for 12 hr prior to the surface area measurements.


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THERMAL GRAVIMETRIC ANALYSIS (TGA) was carried out using a TGA/DSCI 1 analyzer

(Mettler-Toledo, OH, USA) in the range of 25 – 600 °C under a nitrogen atmosphere. POWDER X-RAY

DIFFRACTION

(PXRD) spectra were recorded using a Bruker AXS D8

Advance XRD. Compositions 1-3 were not activated (not dried under high vacuum) prior to pXRD measurements. CONDUCTIVITY MEASUREMENTS were carried out with a Haioky BT3562 (Japan) battery tester.

Pellet models were prepared by adding a 5% polyvinylidene fluoride (PVDF) binder to powdered samples, using a house-built pellet compartment and a hydraulic press under 5 tons of pressure. ELECTRON PARAMAGNETIC RESONANCE (EPR) spectra were recorded at room temperature and

at 120-140 K at 9.7 and 9.3 GHz, respectively, using an E500 Elexsys Bruker spectrometer equipped with a Bruker ER4131VT variable-temperature unit. The spectra were recorded in 1.5 mm quartz tubes at 6.25 mW microwave power and with a modulation amplitude of 5.0 G. For EPR simulations we used the Easyspin package in a Matlab environment.34 COMPOSITE ELECTRODES FOR ELECTROCHEMICAL CHARACTERIZATION were prepared by

mixing the composite powder (active mass – MOF@AC) with 8% PVDF, dissolved in Nmethyl-2-pyrrolidone (NMP) as a binder, and 5% carbon black. The slurry was pasted on a glass plate and dried in vacuum overnight. The electrodes were peeled from the glass plate as free standing films and were cut into 11 mm-diameter discs. Their active mass weight was 1 mg. ELECTROCHEMICAL

MEASUREMENTS

were

carried

out

with

potentiostat/galvanostat

computerized instruments from Bio-Logic Inc. The Cell used in this study was obtained from Ell-Cell ® type ECC-Combi (France) with gold wire as a quasi-reference electrode.


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RAMAN MEASUREMENTS were carried out using a Jobin Yvon Horiba HR-800 Raman analyzer

with a laser wavelength of 632 nm and an intensity of 10 mV. Results and discussion The isolating nature of MOF materials has limited their integration into various fields that require electrical conductivity.5 To solve the MOF conductivity problem, we suggest using a composition method in which nano-crystalline MOF particles are prepared inside pores of conducting carbon. This method allows us to benefit from the conductivity properties of the carbon material while maintaining the advantages of MOF materials. Since the MOF reagents are physically adsorbed into the carbon matrix without the help of any catalysts, the process is rapid, simple, cost-effective and eco-friendly. To test this composition concept, we chose the wellstudied HKUST-1 (Cu3(BTC)2) as the MOF model compound and meso-porous activated carbon as the highly conductive environment.

Crystallinity and structure HKUST-1 is one of the most-studied MOF materials. Its crystal structure is known to be controlled by the method and conditions of its preparation.35 Different solvent mixtures, preparation temperatures and synthesis methods (such as the solvo-thermal and mechanochemical methods) may lead to SSA values ranging between 600 and 2100 m2/g.36 Since the crystal-habit of HKUST-1 is highly sensitive to preparation and activation conditions, one might expect that incorporating MOF into a carbon matrix will cause changes in its crystal structure. However, the composite HKUST-1@AC material displays the same pXRD pattern as that of the pristine HKUST-1 in its activated form (See SI, Figure S4).26 The pristine AC exhibits a typical amorphous pattern with a broad peak between 2θ angles ranging from 10 to 24° (Figure 1). In our


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experience, pre-activation produces clearer pXRD measurements of HKUST-1 by removing excess water. However, the composite material HKUST-1@AC did not require any preactivation before measuring pXRD, suggesting that the HKUST-1 MOF is located inside the pores and is not bound to excess solvent molecules even when stored in open air. The highly adsorptive nature of AC probably prevents water molecules from reaching the HKUST-1, and so no drying is needed before measuring pXRD. The amount of HKUST-1 that is loaded into the activated carbon has a crucial effect on the composition's conductivity, and must therefore be used moderately. Excess HKUST-1 used in the preparation will precipitate on the outer surface of the carbon particles. Since HKUST-1 is an isolating material, if it covers the carbon particles, each particle will still be conductive, but it will be isolated from neighboring particles, forming a non-conductive material. The desired ratio will yield a composite material in which the AC pores are completely filled with MOF nanoparticles, whereas their outer surface will remain clear of any isolating material. To optimize the composition, a composite material consisting of three different MOF:AC ratios (1:1, 1:2, 1:3, w:w) was prepared and scanned by SEM, as well as bare AC and HKUST-1 particles for reference. The SEM image of AC (Figure 2a) reveals a micron square shape with relatively smooth surface particles. Bulk HKUST-1 MOF particles (Figure 2b) display their typical diamond-like crystals. The SEM images of compositions 1 and 2 are similar to those of a typical AC particle (Figures 2c and 2d). HKUST-1 crystals are not detected at all, indicating that all the HKUST-1 MOF has formed inside the AC particles. This explains why only small, cloud-like remains are observed on the outside of the carbon particle. Moreover, elemental analysis of these particles by energy dispersive analysis by X-rays (EDAX) reveals the homogeneous dispersion of copper inside the carbon pores (Figures 2e and 2f). This type of analysis is suitable for


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activated carbon because its high porosity enables excellent penetration of the X-rays. Figures 2e and 2f display the copper content of the surface as well as the deeper cavities (on a scale of several microns). Elemental mapping identifies the remains on the particles as copper. However, the amount of copper inside the carbon particle is much greater than that present on its surface. We noted small areas of the particles that are covered by a cloudy material. Mapping analysis reveals that these areas have a relatively high copper concentration, suggesting that they contain a small amount of HKUST-1 MOF that crystallized on the outer surface of the carbon particle. In composite 1, consisting of a 1:1 HKUST-1:AC ratio, a large percentage of the carbon particle is covered by this cloudy material (Figures 2c and 2e). In contrast, this area is significantly smaller in composite 2 (Figures 2d and 2f). The high concentration of MOF reagents used in preparing composite 1 results in the saturation of the carbon pores and in excess reagent sediment on the outer surface of the carbon particles. This HKUST-1 MOF layer can severely decrease the conductivity of the composition. Following this trend, the SEM image of 3, consisting of a lower amount of MOF incorporated into the carbon matrix (1:3 ratio), reveals no residual MOF outside of the carbon particle (see SI Figure S6).

TGA measurements Thermogravimetric analysis (TGA) of composition materials can easily determine the precise weight ratio of each component in a sample. Moreover, by comparing the TGA curve of each component separately to its curve in a composition material, one can learn about the level of interactions between the composition ingredients. TGA results of composite 2 and of the bulk HKUST-1 sample, in comparison, are presented in Figure 3 (solid lines). TGA derivative curves (Figure 3, dashed lines) are presented to assist in characterizing the weight loss steps. Dried AC


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is not affected by thermal changes up to 2000 °C (see SI, Figure S7).37 For instance, using composite 2, 66% of the composition is supposed to remain inert despite thermal changes within the measured range of temperatures. The remaining 33% of the composition, related to MOF, is expected to decompose in response to thermal exposure. Owing to AC thermal stability and its thermal conductivity properties, organometallic complexes adsorbed into AC may display increased thermal stability compared with the free, non-adsorbed organometallic complex.24 HKUST-1 decomposes, between 250 and 480 °C, by losing its organic linker molecules and forming copper oxide.38 The TGA of a non-activated HKUST-1 sample displays several water loss steps, a final organic content loss at 350 °C and an overall weight loss of 80% (Figure 3, solid line). Indeed, the TGA of composite 2 reveals a total of 35% weight loss owing to water and organic linker molecules, confirming HKUST-1 content in composite 2 to be approximately 30%. Similar water molecule release steps, as monitored for HKUST-1, suggest that the water content in composite 2 is about 15%. Comparing the derivative curves, composite 2 shows a higher thermal stability than HKUST-1 (Figure 3, dashed lines). Each composite material displays four major weight loss steps. Whereas the initial and final weight loss steps are identical for both samples measured, they differ in the two intermediate steps. For composite 2, these two steps occur at higher temperatures and for a longer duration of time than for HKUST-1 (measured at about 145 and 270 °C) (inset in Figure 3, marked by arrows). This confirms the stabilizing effect of MOF interactions with the AC matrix. Furthermore, although the final disintegration step takes place at 350 °C for both samples, the HKUST-1 sample displays a steeper weight loss than composite 2. The slower, less steep disintegration of composite 2 indicates higher thermal stability, owing to the AC stabilizing effect.


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Surface area and pore distribution Both HKUST-1 MOF and AC are known to have a high SSA. As obtained by N2 adsorption measurement techniques, HKUST-1 MOF has an SSA of 940 m2/g, whereas the AC used in this study displays an SSA of 1430 m2/g. Furthermore, the porosity of the two materials is somewhat different, since the pristine HKUST-1 MOF is micro-porous, with an average pore size of 22.8 Å, whereas the AC is meso-porous (Figure 4). Comparison of pore-size distribution (slit pore, NLDFT equilibrium model and Quantachrome NovaWin software option) of the pristine AC with HKUST-1@AC at various AC to HKUST-1 ratios, shows a decrease in the volume associated with meso-pores, proving the presence of MOF inside the porous AC and not on its outer surface. This decrease in meso-pore volume measured for compositions 1-3 is associated with the percentage of HKUST-1 MOF in the composition. Composite 3 displays a pore size distribution most similar to that of the pristine AC, having the highest concentration of mesopores of all compositions tested (with an average pore size of 40.8 Å). As more MOF was inserted into the AC in composites 2 and 1, the average pore sizes were reduced to the microporous scale (29.4 Å for composite 2 and 20 Å for composite 1). This suggests that the MOF particles block the meso-pores of the AC. Composite 2 has the highest SSA of the three compositions tested, whereas composite 3 exhibits a slightly lower SSA. The lowest SSA was obtained in composite 1 due to excessive MOF infiltration. The gas adsorption measurements may answer whether the HKUST-1 MOF was covering the outer side of the activated carbon particles or penetrated into the pores. The major change caused by introducing MOF to the AC was the decrease of meso-pores concentration. This can only be


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explained by a partial blockage of the AC’s meso-pores by the MOF nano-particles. This supports our understanding that the MOF is included within the porous carbon matrices. This conclusion is also supported by SEM images. While the MOF synthesized without carbon shows a clear crystalline morphology, any indication for a separate MOF phase is not seen in the SEM images of the composite materials 1-3.

Electron paramagnetic resonance EPR spectroscopy is a technique used for studying materials with unpaired electrons.

The

paramagnetic species Cu(II) has an electron spin of S=1/2 and a nuclear spin of I=3/2 and it has a characteristic EPR signal.39–41 However, the antiferromagnetic nature of HKUST-1 caused by its dimeric Cu(II) species, as well as fast relaxation processes due to high local Cu(II) concentrations, makes obtaining a clear EPR signal for HKUST-1 difficult, even at low temperatures.42 Various attempts have been made to measure the EPR spectrum of HKUST-1 MOF, for example, by using the field-swept electron spin echo (FS–ESE) technique and magnetic dilution to replace some copper ions with zinc ions.43 However, all measurements displayed an unresolved Cu(II) spectrum, in which the g-parameters and the hyperfine parameters were difficult to estimate. A resolved Cu(II) spectrum was obtained only after dilution with Zn(II), at 7K. Figure 5 shows the EPR spectra recorded at room temperature (RT, 298K) and at 150K for HKUST-1 and composite 2. The low-temperature CW-EPR measurements for HKUST-1 yielded data consistent with published results44 (see SI Table S2), displaying the five characteristic Cu(II) peaks. HKUST-1 samples are highly sensitive to the preparation method used, which can affect physical properties such as the crystal pattern and surface area, and may also affect additional electronic properties. Pöppl et al. suggested that the


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EPR spectrum of HKUST-1 is characterized by a mixture of copper centers where S=1/2 and S=1.42 The centers in which S=1 probably contribute to line broadening, prohibiting the determination of a straightforward EPR signal. EPR measurements of composite 2 revealed several interesting results. First, the signals obtained were completely different from those of the bulk HKUST-1 sample. Owing to the spin dilution effect of the AC matrix, the spectrum is much more resolved. The spectrum of composite 2 taken at room temperature is even more resolved than that of HKUST-1 taken at 150K. Second, shifts in giso value as well as in the hyperfine coupling (A values) were revealed (see SI Table S2). For comparison, the EPR signals of the pristine AC and of the Cu precursor adsorbed into the AC can be found in the Supplementary Information, Figure S9. The EPR signal of the HKUST-1@AC composition is significantly different from that of mixture of AC and HKUST-1 (Figure S10 in the SI). Unlike the EPR signal of the HKUST-1@AC composition, mixture of AC and HKUST-1 preserve the dual signals of AC and HKUST-1, emphasizing the differences between an homogenous mixture of MOF and AC (where MOF particles are located outside of AC particles), to the composition, where MOF particles are prepared inside the AC porous matrix. We would like to propose a complete description that can explain all of the above observations. The HKUST-1@AC composite material yielded a resolved EPR spectrum with a clear electronnuclear spin coupling pattern (hyperfine interaction) even at RT (Figure 5c). The AC we used as a host matrix for MOF growth has a meso-porous nature (mainly containing pores of 6-7 nm diameter, see Figure 4), with pores located relatively far from each other. Since HKUST-1 MOF particles were grown only in AC nano-pores, these particles grew separately from each other, consequently diluting the Cu(II) particles in the composite. Thus, the spin-spin coupling interaction between nearby Cu(II) centers was significantly suppressed. This description explains


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the clear peaks of Cu(II) observed in the composition even at RT. In the bulk HKUST-1 sample, these couplings are responsible for signal broadening. The separation between the copper centers caused by the carbon produced the clear HKUST-1 spectrum. Preliminary results show this influence also in our Mil-100(Fe)@AC samples, a-c (data not displayed). This explanation is also attributed to the decrease in the g-value of the composite material compared with that of HKUST-1. The giso value of the composition is shifted to 2.16-2.17, while for the bulk HKUST-1 the value is 2.19. By decreasing the number of Cu(II) neighbors around each Cu(II) center, we decreased the local magnetic field around it, leading to a lower g-value. However, we would like to suggest an in-depth explanation that relates to the direction of change in the g-value. MOF nano-particles with a high surface/volume ratio, growing densely, are likely to be affected by their surrounding environment, displaying strong electric interactions between the host and the guest materials. This influence is even more prominent if the environment of the MOF is electron-conductive. This 0.02 shift towards the value of the free electron (g-value of 2.0023) indicates that the Cu(II) electron in the composition has gained a higher level of freedom. Remarkably, the notable decrease in the giso value can explain the change in the conductivity of the composition without any change in the structure of the conducting component, namely, the activated carbon. Simulating the values for the EPR response of composite 2 at 150K leads to fascinating discoveries (see the detailed values in the Supplementary Information). Two copper species were found to contribute to the overall signal (Figure 5d). One species (species A), as expected, has values similar to those of bulk HKUST-1, measured at 150K. This demonstrates that within the diluted environment of composite 2, some copper centers are still close enough to one another to induce the bulk HKUST-1 signal. The other species (species B), surprisingly, has properties


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almost identical to those obtained for composite 2 at RT. Since no MOF is present outside the carbon matrix (confirmed by SEM analysis, Figure 2d), it is reasonable to conclude that each MOF nano-particle has a core shell-like structure, reflected in EPR by two appearances. One appearance, which was shown to be significantly influenced by the activated carbon, and had a totally different electronic arrangement than the bulk HKUST-1 signal, is likely to be at the outer shell, forming the surface of the particle and strongly interacting with the surrounding AC. The other appearance, retaining the same response as the bulk HKUST-1 and apparently not affected by the activated carbon, is likely to be the inner part of the MOF particle. If we take this explanation a step further, it is possible to calculate the ratio between the two layers of MOF. The bulk-like part (the particle's inner volume, not in contact with the AC pore walls) was found to contribute 29% of the total signal, whereas the AC-affected part (the particle's surface) contributes 71%. A cell unit of HKUST-1 has a diameter of 0.9 nm. Considering this cell-unit size and the surface:volume ratio, the spherical MOF particle should have a diameter of approximately 6 nm, which is exactly the diameter of the average AC pore. Note that although the MOF material by itself remains non-conductive, the EPR spectra analysis point out that it is strongly interacting with its encompassing conductive carbon. We can therefor conclude that the MOF inside the composite material will be polarized when an electric voltage will be applied. This paves the way for utilization of non-conducting MOF in electric devices.

Electrical conductivity We set out to develop materials with high conductivity while preserving the unique properties of MOF materials. For practical needs, we prepared and measured pellets of the powdered materials by adding 5% PVDF binder. This binder is well-known and commonly used in energy


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storage systems. It is important to remember that powder-based pellet conductivity is relatively low compared to that of a single grain. PVDF is known to have thermal and physical stability as well as inert behavior for redox reactions between (-3) and 4 V. Although the binder addition hinders the conductivity of the pellets, the composite material still exhibits good conductivity (as presented in Figure 6). We measured compositions having three different MOF:AC ratios (37%, 23%, and 16% by weight). As expected, increasing the ratio of AC in the composite material also increases electrical conductivity. Composite 3 has a good conductivity of 17.2 S/m. This high conductivity, which is in the same order of magnitude as the pristine AC, could be explained by noting that electron transfer in AC is based on the flow of electrons (electron tunneling) through the carbon matrix.45 Filling the matrix voids with an insulator should not significantly interfere with electrical conductivity, as long as an insulator layer does not form on the outer-side of the AC particles. This, in turn, further supports the possibility that HKUST-1 MOF has formed inside the pores of the carbon material and that it does not cover the particle surface, as observed in the SEM images (Figures 2c and 2d). However, the decrease in conductivity by a factor of four is not negligible. A reasonable explanation for this decreased conductivity must address the electrostatic interactions of MOF on the surface of the carbon pores. These kinds of interactions were also detected in other aromatic materials adsorbed on activated carbon.24 The aromatic BTC ligand of HKUST-1 is strongly adsorbed into the carbon, probably by π-π stacking interactions with the graphite-like structure of the AC. It is suggested that this arrangement affects free electron migration in the carbon matrix, localizing it near the copper metal ion. The electrons flowing on the valence band of the carbon interact with the d-orbital electrons of the copper metal. As a result of these interactions, the overall conductivity of the composition decreases. Increasing the


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weight percent of HKUST-1 in the composition to 23% (composition 2) still results in a relatively good conductivity of 7.6 S/m. At this ratio, although most of the HKUST-1 is present inside the pores of the carbon, small amounts of MOF partly cover the outer layer of the carbon particle, directly influencing the conductivity of the composite pellet. An attempt to increase HKUST-1 content to 37% resulted in a diminished conductivity of 0.2 S/m, owing to an excess of insolating MOF covering the AC particles (Figure 2e) and interfering with the charge transfer between them. We also prepared pellets by mixing HKUST-1 powder that was not synthesized in carbon and pristine AC, using the same ratios as compositions 1-3. The results presented in Figure 6 show that the conductivity was hardly affected by the amount of AC in these pellets. This is because the insulating HKUST-1 particles separate the conducting AC particles from each other and mechanical mixing of these components does not produce electronically conducting mixtures. A similar trend is shown for the Mil-100(Fe)-based composition. While Mil-100(Fe) is also an isolating material, the Mil-100(Fe)@AC compositions display conductivities of 0.03, 8.3 and 16.6 S/m for compositions a-c, respectively (Figure S11 in the SI). The conductivity of both HKUST-1@AC and Mil-100(Fe)@AC compositions can be explained by the incorporation of the MOF nano-particles in the AC pores. These particles do not interfere with the AC electron conductivity. After increasing the MOF concentration in the composition, some MOF particles are synthesized outside of the AC particles, separating one from another and lowering the AC electron conductivity. We emphasize that the MOF (by itself) within the composite material remains non-conductive. The electrical conductivity composite measured, is gained only by the conductive carbon matrix. The high conductivity of the composite matrices indicates that the electrically isolating MOF is not covering the outer-part of the AC particles, but rather is


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included therein. This result supports further the conclusion raised by SEM images and gas adsorption measurements. The preparation of these composite materials aims at retaining all the properties of the MOF (including the electronic properties), while providing the MOF appropriate electrically conducting matrices. Electrochemical measurements Adjusting the conductivity of the material enables direct electrochemical characterization of the bulk MOF composite material. Cyclic voltammetry was conducted to determine the redox potentials and electrochemical behavior of HKUST-1@AC as the active mass in composite electrodes (Figure 7). We demonstrate a typical steady-state voltammetric behavior of electrodes comprising HKUST-1@AC composites. The cyclic voltammetric curves thus obtained exhibit the overall shape of typical pseudo-capacitors with flat current responses for the electric doublelayer capacitance (EDLC) of the porous carbon and peak areas related to the Faradaic reaction of the MOF, which is electrochemically active. This is in contrast to the linear CV curve of the pristine AC, which involves only electric double layer interactions and lacks any faradaic reaction (Figure S12 in the SI). Within the potentials measured, clear redox peaks are observed at -0.25 V for the reduction reaction [Cu3(BTC)2 Cu3(BTC)2-1] and at 0.34 V for the reverse oxidation reaction [Cu3(BTC)2-1 Cu3(BTC)2], both versus gold pseudo-reference electrodes. The oxidation peak has a shoulder at lower potentials (-0.1V). Thus, we can assume that the electrochemical reaction is somewhat complex. This behavior agrees with our previous results for composite materials in which part of the electro-active material that is attached to the carbon has electronic environments that differ from those of the bulk electro-active material. After ten cycles the second peak disappears, as commonly happens after the system reaches stabilization. The wide bandgap (0.59V) between the two redox reactions indicates a low electrochemical


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reversibility of the electro-active material. The specific gravimetric capacitance of the composite material tested electrochemically is 124 F/g. Full supercapacitor cells were built and they demonstrated similar capacitance (Figure S13 in the SI). This pseudo-capacitance is higher by 30% than that of the activated carbon (which is around 100 F/g, purely EDLC), demonstrating one potential use of these composite materials. Initial testing over hundreds of cycles showed perfect stability of the symmetric super-capacitor cells, with electrodes comprising the HKUST1@AC composite as their active mass. It should be noted, however, that Figure 7 reflects only preliminary electrochemical investigations, aimed at feasibility studies. Further work (in progress) demonstrates prolonged cycling and high-rate capabilities, which are mandatory requirements for practical super-capacitors. We conducted the same electrochemical measurements with electrodes comprising Mil-100(Fe)@AC. These results validate the possibility of alternatively using the Mil-100(Fe) MOF in electrochemical systems. The observed oxidation potential was 0.15 V versus Au and -0.2 V for reduction. The specific gravimetric capacitance of the composite material tested electrochemically is 110 F/g (Figure S14 in the SI).

Conclusions Harnessing the unique and exclusive properties of MOF for electric applications faces great difficulties because of the non-conducting nature of most MOF material. Here we established a new route for fabricating composite material with meso-porous activated carbon as the hosting structure for MOF nano-particles. This process is simple, safe, cost effective, up-scalable and eco-friendly. HKUST-1, a well-studied insulating MOF, was prepared inside a conductive AC matrix. The presence of HKUST-1 inside the AC meso-pores was proven by SEM, gasadsorption, EPR, and conductivity measurements. The composition exhibits high conductivity


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values obtained thank to the AC host matrix, while preserving the chemical and physical properties of HKUST-1 MOF. In this paper, the key properties of the composition were intensively studied, including the surface area, porosity, crystalline structure, thermal stability, electron paramagnetic resonance, conductivity and electrochemical characterization. The results demonstrate the mutual effects within the composition. The interaction between the two components of the composition was revealed and discussed. Preparation of MOF within the carbon matrix allowed us to present for the first time an EPR clear signature of MOF even at room temperature. Without changing the intrinsic resistivity of the MOF, the goal of high electrical conductivity was extrinsically achieved, and it was found to be controlled by the MOF:AC composition ratios of the material. This composition method may enable the introduction of MOF material into electronic devices, without the need to use specially designed conductive MOF material. The ability to adjust and simplify chemical and physical properties of MOF while circumventing its poor conductivity may increase its potential for commercial applications.

FIGURES


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Figure 1. Powder XRD spectrum of HKUST-1. Composite HKUST-1@AC material (pink), HKUST-1 pattern simulated from its crystal structure (blue),26 and the pristine AC (green). The HKUST-1 pXRD planes are marked in brackets.46

a

b


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c

d

e

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Figure 2. SEM images of pristine AC (a), bulk HKUST-1 (b) and compositions 1 (c) and 2 (d). The corresponding EDAX copper mapping of the same area for compositions 1 (e) and 2 (f).


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Figure 3. The TGA (solid line) of HKUST-1 (blue) and composite 2 (pink) and their corresponding first derivatives (dashed lines). Similar weight loss steps are marked with arrows (inset).


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Figure 4. Pore size distributions of pristine AC, pristine HKUST-1 and compositions 1-3. Isotherms displayed in SI, Figure S8.


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Figure 5. EPR analysis of experimental (blue) and simulated spectra (red) for HKUST-1 (a,b) and composite 2 (c,d), measured at room temperature (left, a,c) and at 150K (right, b,d).


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Figure 6. Electrical conductivity of pellet samples of AC, HKUST-1, compositions 1-3, mixtures 1-3 and comparisons with select studies. (Inset: magnification of the relevant area.)


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Figure 7. A typical CV-curve of HKUST-1@AC in a three-electrode cell, a reference gold electrode, in PC/1.4 M (TEA)BF4 electrolyte solution, displaying the first cycle (purple), 10th cycle (red), and 100th cycle (green). Scan rate = 10mV/sec. The electrode weight was 1 mg.

ASSOCIATED CONTENT Supplementary Information. Additional synthesis details, Raman, elemental analysis, pXRD, SEM, TGA, BET isotherms, EPR and electrochemical measurements. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected] Author Contributions The manuscript was written by all the authors. All the authors have given approval to the final version of the manuscript. §These authors contributed equally. Funding Sources This research was funded by BIU funds.

Acknowledgments The authors thank Prof. Omar Yaghi and Dr. Kyungmin Choi for a valuable disscussion regarding HKUST-1 synthesis methods.

ABBREVIATIONS MOF metal organic framework; AC activated carbon; HKUST-1 Hong Kong university of Science and Technology; MOF metal organic framework; NLDFT

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Table of Contents, Graphics and Synopsis

A new method of imparting electrical conductivity to otherwise non-conducting MOFs is presented, by preparing MOF nanoparticles within the conducting matrix of meso-porous activated carbon.


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