Article pubs.acs.org/IC
Ultrahigh Ionic Conduction in Water-Stable Close-Packed MetalCarbonate Frameworks Biplab Manna,†,‡ Aamod V. Desai,†,‡ Rajith Illathvalappil,∥ Kriti Gupta,† Arunabha Sen,† Sreekumar Kurungot,*,∥ and Sujit K. Ghosh*,†,§ †
Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune 411 008, India Physical & Materials Chemistry Division, National Chemical Laboratory (NCL), Dr. Homi Bhabha Road, Pune 411 008, India § Centre for Research in Energy & Sustainable Materials, IISER, Dr. Homi Bhabha Road, Pune 411 008, India ∥
S Supporting Information *
ABSTRACT: Utilization of the robust metal-carbonate backbone in a series of water-stable, anionic frameworks has been harnessed for the function of highly efficient solid-state ionconduction. The compact organization of hydrophilic guest ions facilitates water-assisted ion-conduction in all the compounds. The dense packing of the compounds imparts high ion-conducting ability and minimizes the possibility of fuel crossover, making this approach promising for design and development of compounds as potential components of energy devices. This work presents the first report of evaluating ion-conduction in a purely metal-carbonate framework, which exhibits high ion-conductivity on the order of 10−2 S cm−1 along with very low activation energy, which is comparable to highly conducting well-known crystalline coordination polymers or commercialized organic polymers like Nafion.
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INTRODUCTION Solid electrolytes with high ion-conductivity have commanded significant research attention in recent years owing to applications for storage and generation in a range of electronic devices.1 Fundamental advancement in terms of controlling the intermolecular interactions is of much importance for actuating the development of materials as components of future energy devices.2 In this regard, crystalline solids, metal−organic frameworks (MOFs), and coordination polymers (CPs), in particular, have emerged as suitable candidates for investigation as solid-state ion-conductors.3 The benefit of tunable architecture and feasible access to precise structure−property correlation affords key advantages to this class of materials. Although MOFs have shown affirmative promise as conductors for a range of ions, much work remains to be done with regard to issues of hydrolytic stability and concerns of gas penetration for real-time applications.2b,4 Thus, to overcome such issues without affecting the performance, newer approaches are being sought after. Typically, the material that is able to overcome these bottlenecks should (i) have a dense anionic framework to minimize gas leakage through the compound, (ii) possess residual ion-carriers for efficient transport under a conductive environment, and (iii) be water-stable and robust to changes in the microenvironment to seek real-time applicability. Integration of all these facets is not trivial, and hence, only a few reports of ionic conduction in coordination polymers satisfying all the above features are known.5 This necessitates the © 2017 American Chemical Society
development of different kinds of robust crystalline materials which can conform to the prerequisites. We applied the Cambridge Structural Database (CSD) screening approach to choose a compound that could meet the criteria. Metal-carbonate networks are potentially useful in this regard, as they afford reasonably stronger coordination bonds which are resistant to disintegration in the presence of water, and variable denticity results in higher density of anions around metal nodes.4b,6 In addition, carbonates generally are costeffective and can adopt binding modes that are analogous to the way carboxylate ligands function in MOFs as building blocks of infinite polymers. These features in turn provide facile access to fabricate anionic frameworks having uncoordinated cations as ion-carriers. It is worth noting that although several coordination networks built from other metal−anion backbones have been investigated as solid-state ion-conductors such as metal-oxalates, metal-sulfates, metal-phosphates, etc.,7,2c,3f,5a metal-carbonate frameworks have not been evaluated for this function yet. With this background, we narrowed our interest to a dense Cu(II)-sodalite framework,8 viz., {[Cu6(CO3)12·(CH6N3)8]· 2Gd·2K·OH·H2O}n (hereafter referred to as MCF-Gd; MCF stands for metal-carbonate framework, Gd represents the uncoordinated lanthanide cation, Gd3+) (Figure 1). Abrahams Received: May 12, 2017 Published: July 31, 2017 9710
DOI: 10.1021/acs.inorgchem.7b01217 Inorg. Chem. 2017, 56, 9710−9715
Article
Inorganic Chemistry
noticed that the hydrated lanthanide cation in the framework is in close proximity with other extraframework ions (Figures S4, S5, and S7). Thus, we hypothesized that the presence of such residual ions held by intermolecular interactions in the confined space can significantly influence ion-transport in the presence of moisture. To examine this further, we synthesized three new isostructural compounds based on the same metal-carbonate framework by varying the free lanthanide cation (Dy3+, Tb3+, Er3+; referred to as MCF-Dy, MCF-Tb, MCF-Er, respectively, hereafter). Herein, a novel function of the robust metalcarbonate networks as solid-state ion-conductors has been demonstrated in a series of water-stable, close-packed anionic frameworks, which bear the compact presence of hydrophilic uncoordinated ions for facilitating water-assisted ultrahigh ionconduction.
Figure 1. Representation of synthetic scheme employed for formation of dense framework of MCF-Gd (crystal structure adapted from CCDC194987; WABWEC).8 Color code: carbon, gray; oxygen, orange; copper, green; nitrogen, blue; potassium, yellow; gadolinium, dark purple.
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EXPERIMENTAL SECTION
Materials. All the reagents and solvents were commercially available and used without further purification, unless otherwise specified. Synthesis of MCF-Gd. The compound was synthesized via a previously reported protocol.8 In a typical synthesis, a solution of CH6N3Cl (1.25 mmol) in water (0.5 mL) was added to an aq solution (1 mL water) containing K2CO3 (2.4 mmol), KHCO3 (1.25 mmol), Cu(NO3)2 (0.17 mmol), and Gd(NO3)3 (0.092 mmol). The mixture was kept at room temperature for 24 h without stirring, and blue crystalline product was then filtered. Elemental analysis (%) calcd for {[Cu6(CO3)12(CH6N3)8]·2Gd·2K·4OH·xH2O}n: C 10.63, N 14.88. Found: C 9.98, N 13.89. CCDC194987 (Refcode: WABWEC) contains crystallographic data for this compound.8 Synthesis of MCF-Tb. In a typical synthesis a solution of CH6N3Cl (1.25 mmol) in water (0.5 mL) was added to an aq solution (1 mL water) containing K2CO3 (2.4 mmol), KHCO3 (1.25 mmol), Cu(NO3)2 (0.17 mmol), and Tb(NO3)3 (0.092 mmol). The mixture was kept at room temperature for 24 h without stirring, and blue crystalline product was then filtered. Elemental analysis (%) found for MCF-Tb: C 11.15, N 15.21. Elemental analysis gave the formula of the compound as {[Cu6(CO3)12(CH6N3)8]·2Tb·2K·4OH·xH2O}n. Synthesis of MCF-Dy. In a typical synthesis a solution of CH6N3Cl (1.25 mmol) in water (0.5 mL) was added to an aq solution (1 mL water) containing K2CO3 (2.4 mmol), KHCO3 (1.25 mmol), Cu(NO3)2 (0.17 mmol), and Dy(NO3)3 (0.092 mmol). The mixture was kept at room temperature for 24 h without stirring, and blue crystalline product was then filtered. Elemental analysis (%) found for MCF-Dy: C 9.87, N 13.58. Elemental analysis gave the formula of the compound as {[Cu6(CO3)12(CH6N3)8]·2Dy·2K·4OH·xH2O}n. Synthesis of MCF-Er. In a typical synthesis a solution of CH6N3Cl (1.25 mmol) in water (0.5 mL) was added to an aq solution (1 mL water) containing K2CO3 (2.4 mmol), KHCO3 (1.25 mmol), Cu(NO3)2 (0.17 mmol), and Er(NO3)3 (0.092 mmol). The mixture was kept at room temperature for 24 h without stirring, and blue crystalline product was then filtered. Elemental analysis (%) found for MCF-Er: C 10.76, N 14.88. Elemental analysis gave the formula of the compound as {[Cu6(CO3)12(CH6N3)8]·2Er·2K·4OH·xH2O}n. FE-SEM Sample Preparation. Less than 1 mg of each sample was dispersed in dry CH2Cl2 via 10 min sonication and then was loaded on a silicon wafer which was then used for FE-SEM imaging after drying overnight. Activation of Compounds. Each powdered sample was heated at its respective first weight loss temperature for 24 h under vacuum to obtain guest-free phase. PXRD measurements were done for each activated sample. Test for Moisture Stability. Each activated powdered sample was kept under humidified conditions for 6 h, and then, subsequently, PXRD measurements were performed for each humidified sample. Postimpedance Stability Check. Each pelletized sample after impedance analysis was tested with PXRD measurements.
et al. had reported the crystal structure of MCF-Gd (CCDC194987; WABWEC), which is among the well-known benchmark compounds.8 The compound is known to be waterstable and rigid owing to the presence of strong metalcarbonate bonds building up the backbone. The lattice has the desired presence of uncoordinated ions in close proximity, which can facilitate conductivity by functioning as ion-conduction carriers (Figure 2). The overall 3D packing is
Figure 2. Structural diagram depicting the stacked arrangement of ionconduction carriers. Cu(II) and carbonate (CO32−) ions have been omitted for clarity (crystal structure adapted from CCDC194987; WABWEC).8 Color code: carbon, gray; hydrogen, light yellow; oxygen, orange; nitrogen, blue; potassium, yellow; gadolinium, dark purple.
built on a network of carbonate (CO32−) anions and possesses an H-bonded array of guanidinium (CH6N3+) cations (Figures S2 and S3). Additionally, the compound has uncoordinated and hydrated gadolinium (Gd3+) cations arranged in a stacked manner. Hydroxide (OH−) and potassium (K+) ions are present in the confined spaces, which balance the overall charge of the compound. The dense packing of the compound ensures that all the ionic species are in close vicinity and are held by supramolecular interactions (Figures S4 and S5). In addition, the lack of free space (Figure S6) drastically subdues the issues of gas leakage from the perspective of a real-time application. Gas crossover in fuel cells can lead to undesired reactions and loss of efficiency in operating ability; hence, the membrane material should prevent gas leakage.5b,9 In particular, we 9711
DOI: 10.1021/acs.inorgchem.7b01217 Inorg. Chem. 2017, 56, 9710−9715
Article
Inorganic Chemistry Test for Water Stability. Each powder sample was fully immersed in water for 24 h, and then, the corresponding sample was analyzed with PXRD measurements after it dried at room temperature. Physical Measurements. Powder X-ray diffraction (PXRD) patterns were measured on Bruker D8 Advanced X-ray diffractometer at room temperature using Cu Kα radiation (λ= 1.5406 Å) with a scan speed of 0.5° min−1 and a step size of 0.01° in 2θ. Thermogravimetric analysis was recorded on a PerkinElmer STA 6000, TGA analyzer under N2 atmosphere with heating rate of 10 °C/min. The IR spectra were recorded on a Thermoscientific−Nicolet-6700 FT-IR spectrometer. FT-IR spectra were recorded on NICOLET 6700 FT-IR spectrophotometer using KBr pellets. Impedance Analysis. Bio-Logic VMP-3 instruments were used for the impedance analysis of the samples. A two-electrode assembly having stainless steel disc electrodes was used for the measurements. Samples in the form of solid pellets were kept in between the steel disc electrodes by applying a spring load of 0.5 N/m2. For control of the temperature and humidity, the whole cell assembly was kept in an Espec environmental test chamber. A frequency range 106−0.1 Hz was applied against the open circuit potential with sinus amplitude of 10 mV. EC-Lab Software V10.19 was used to fit all the EIS data.
copy confirmed the formation of the framework with peaks corresponding to carbonate (848 cm−1) and guanidinium cations (1701 cm−1) (Figure S22). FESEM (field emission scanning electron microscopy) images exhibited the formation of crystalline morphologies (Figure S23), and the EDX elemental mapping confirmed the homogeneous distribution of constituent elements throughout the crystallites (Figure S24). The thermogravimetric analysis (TGA) profile indicated ∼11% loss of guest molecules up to ∼148 °C (Figure S17), after which no further weight loss was observed until 195 °C. Variable temperature powder X-ray diffraction (VT-PXRD) patterns substantiated the thermal stability of the compound up to 175 °C (Figure S9). Upon activation, the guest-free phase was obtained which was devoid of the presence of lattice guest molecules, and the bulk phase purity was retained (Figures S8 and S17). Further, the moisture and hydrolytic stability of the compound was evaluated, and in both the cases the structural integrity of the compound was retained (Figure S8, S21). Likewise, compounds MCF-Tb, MCF-Dy, and MCF-Er were synthesized at room temperature by varying the lanthanide salts. PXRD patterns (Figures S10, S12, and S14) substantiated the formation of isostructural phases, and EDX elemental mapping (Figures S26, S28, and S30) validated the homogeneous inclusion of respective elements. The TGA profiles of these phases showed initial loss of guest molecules up to 140 °C (for MCF-Tb), 124 °C (for MCF-Dy), and 180 °C (for MCF-Er) (Figures S18−S20). The thermal and hydrolytic stability of the 3 compounds was found to be in accordance with the observation for MCF-Gd (Figures S10− S16). FESEM images demonstrated the retention of similar crystalline morphologies (Figures S25, S27, and S29) in the newly formed phases as well. The compounds were found to be nonporous to H2 (77 K) and O2 (195 K) even at low temperature, validating their potential as solid-state ionconductors which do not have the issues of fuel crossover (Figures S31 and S32). We also checked the water adsorption (298 K) profiles for all 4 compounds and found a similar uptake profile and amount (Figure S33). The structural features along with the stability of the compounds propelled us to evaluate their ion-conduction behavior. The ion-conductivity property of the compounds was tested using an alternating current (ac) impedance analyzer of the pelletized form of the polycrystalline phase by both a temperature and a humidity swing (Figure S1). Initially, the conductivity of MCF-Gd was measured at ambient temperature and with varying humidity. The conductivity was found to increase incrementally in the humidity at 25 °C (Figure 5a). The conductivity of MCF-Gd at 70% RH was found to be 1.2 × 10−5 S cm−1, which reached 0.99 × 10−2 S cm−1 at 95% RH (Figure 4a and Figure S34). The increasing conduction values with increasing RH values agree well with the observation of low pressure water uptake by all four compounds (Figure S33), suggesting the effect of hydration in the observed conduction. The same experiment was performed for MCF-Tb, MCF-Dy, and MCF-Er which gave values of conductivity at 1.50 × 10−2, 0.89 × 10−2, and 2.50 × 10−2 S cm−1, respectively, under 95% RH (Figure 4b− d). Like the previous case, the conductivities for MCF-Tb, MCF-Dy, and MCF-Er rose with increasing humidity (Figure 5a and Figures S36, S38, and S40). Notably, these values are among the best known for crystalline coordination networks and the commercialized Nafion measured under similar conditions (Table 1).10 The evaluation of stability of the
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RESULTS AND DISCUSSION MCF-Gd was synthesized at room temperature via the reported protocol.8 All the other new compounds were prepared in a similar manner by using nitrate (NO3−) salts of respective lanthanide cations. Structural analyses reveal that the compound has a sodalite structure with cage-like subunits built via copper−carbonate coordination bonds. The guanidinium cations are closely packed (C···C distance 3.28 Å) and are held by cooperative H-bonding interactions with the oxygen atoms of carbonates. Each cage further accommodates K+ and hydrated Gd3+ cations, with the former directed inside the cage owing to the lack of a cavity between O4 donating groups of the carbonates. This leads to the presence of all the residual ionic components of the compound in close proximity. The assynthesized phase was carefully characterized before proceeding with impedance analysis. Powder X-ray diffraction (PXRD) patterns and elemental analyses validated the bulk phase purity of MCF-Gd (Figure 3 and Figure S8). Fourier-transform infrared (FT-IR) spectros-
Figure 3. PXRD patterns of the various synthesized phases MCF-Gd simulated (green; crystal structure data adapted from CCDC194987; WABWEC),8 MCF-Gd as-made (red), MCF-Tb as-made (blue), MCF-Dy as-made (orange), MCF-Er as-made (violet). 9712
DOI: 10.1021/acs.inorgchem.7b01217 Inorg. Chem. 2017, 56, 9710−9715
Article
Inorganic Chemistry
cycles, and the structural integrity was retained (Figure S42b). As anticipated from the structure, the dense packing of the compounds keeps the relevant ion-carriers in close proximity, resulting in cooperative ion-transport and yielding high conduction values. More importantly, the presence of uncoordinated lanthanide cations makes the confined space highly hydrophilic. This is reflected in the water adsorption profiles, and subsequently leads to conjunctive water-assisted ion-conduction behavior. The similar performance of the 4 compounds substantiates the role of having the free polar cation in an ionic framework. To gain mechanistic insight into the ion-transport pathway, we performed temperature dependent impedance analysis at 95% RH (Figures S35, S37, S39, and S41). The Arrhenius plots yielded the activation energies to be 0.189, 0.116, 0.125, and 0.167 eV for MCF-Gd, MCF-Er, MCF-Tb, and MCF-Dy, respectively (Figure 5b). It is well-known in the literature that low activation energies (
