Single Crystal Proton Conduction Study of a Metal Organic Framework

Dong Won Kang , Jeong Hwa Song , Kyung Jin Lee , Han Geul Lee , Jeong Eun Kim , Hwa Young Lee , Jin Young Kim , Chang Seop Hong. Journal of Materials ...
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Single Crystal Proton Conduction Study of a Metal Organic Framework of Modest Water Stability Biplab Joarder,† Jian-Bin Lin,† Zaida Romero, and George K. H. Shimizu* Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada S Supporting Information *

bonding for the formation of proton conduction paths. In this context, introduction of free ammonium cations and sulfonic acid groups would form Brønsted acid−base pairs could allow the proton transfer process along hydrogen bonding chains under ambient conditions. Herein, we report a sulfonated MOF, {[In(5-Hsip)2(Me2NH2)]·DMF·(H2O)1.4}n (5-H3sip = 5-sulfoisophthalic acid), PCMOF-17, with hydrogen-bonded dimethylammonium cation (Me2NH2+)/water proton conduction paths that shows high proton conduction value over 10−3 S cm−1 at 25 °C and 40% RH. Notably, PCMOF-17 is not stable to higher humidity and so dissolution and conduction through grain boundary pathways rather than the bulk material is a possibility which raises the value the crystallographic insights.7 Triaxial proton conductivity data was collected on single crystals which corroborated the data on pressed pellets. The data affirm that the modest stability MOF was demonstrating bulk conductivity at 40% RH even when it degrades significantly at 60% RH and that the conduction pathways observed in the crystal structure offer meaningful insights. PCMOF-17 crystallized in the tetragonal noncentrosymmetric space group P4̅21m (Table S2) and adopted an anionic layered structure with interlayer Me2NH2+ cations and water molecules. In PCMOF-17, each eight-coordinate In3+ ion is coordinated with four ligands through O8 donor set and each ligand bonds to two In3+ sites forming an anionic [In(−COO−)4] subunit (Figure S1). Each [In(−COO−)4] connects with another four adjacent subunits, building the 2D anionic structure in the {001} plane (Figure 1b and S2). The 2D sheets are eclipsed and equivalent (Figure S3). The 2D anionic framework is neutralized by in situ generated interlayer Me2NH2+ cations (Figure 1a) because nitric acid and DMF were used in the reaction mixture.8 Energy-dispersive X-ray (EDX) spectroscopy confirms the absence of any sodium ions in the compound (Figure S8). The site occupancy of each Me2NH2+ cation and water O1W is 0.25. As shown in Figure 1c, 3-fold disordered sulfonic acid groups, Me2NH2+ cations and lattice water molecules form a complicated hydrogen bonding network along the {001} crystallographic plane. The bridging Me2NH2+ cations donate not only hydrogen bond branches to 3-fold disordered sulfonic acid O3A, O3B, O4A, and O5A (D···A = 2.34(5) to 3.07(6) Å) but also a bifurcated hydrogen bond to another two symmetrically equivalent water molecules O1W (D···A = 2.38(5)−3.12(6) Å). The bridging water O1W is further hydrogen bonding to another sulfonic

ABSTRACT: A sulfonated indium (In) metal organic framework (MOF) is reported with an anionic layered structure incorporating hydrogen-bonded dimethylammonium cations and water molecules. The MOF becomes amorphous in >60% relative humidity; however, impedance analysis of pelletized powders revealed a proton conduction value of over 10−3 S cm−1 at 25 °C and 40% RH, a very high proton conduction value for low humidity and moderate temperature. Given the modest humidity stability of the MOF, triaxial impedance analyses on a single crystal was performed and confirmed bulk proton conductivity over 10−3 S cm−1 along two axes corroborating the data from the pellet.

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etal−organic frameworks (MOFs) have attracted considerable attention as proton conducting materials with potential application for hydrogen fuel cells.1 Structurally, the interest stems from the ability to tune MOF structures in a modular manner in regards to pore dimensions, chemical groups lining pores, and protic species inside the pores. Another value of MOFs to proton conducting systems is the insights to design imparted by crystallinity and potential for direct visualization of proton conduction pathways. Most proton conducting MOFs only show high conductivity (>10−3 S cm−1) at high relative humidity (RH > 90%).1a−k,2 These conditions are seemingly challenging for application in fuel cells due to the substantial costs associated with maintaining the appropriate level of humidity, as well as the possibility of flooding the cathode leading to loss in fuel cell performance.3 Furthermore, anhydrous proton conducting MOFs require high temperatures (>100 °C), increasing energy consumption.4 Therefore, the development of novel electrolyte MOF materials suitable for high proton conduction at ambient condition is strongly desired. Reports of MOF materials with high proton conduction at ambient conditions are rare.5 Incorporation of functional linkers with hydrophilic groups (e.g., hydroxyl groups or sulfonic, phosphonic, carboxylic acids) and proton carrier molecules or ions (e.g., water, Nheterocycles, NH4+, H3O+, etc.) can create proton conducting pathways in MOFs.1e,5d,6 Only incorporation of such functional linkers would promote hydrogen bonding with guest water molecules to enhance proton conduction, but this type of material suffers from a large tendency to lose water at low RH, leading to a decrease in conductivity. Therefore, a strategic approach could be the decoration of MOFs with Brønsted acid−base pairs, which can form intermolecular hydrogen © 2017 American Chemical Society

Received: April 5, 2017 Published: May 16, 2017 7176

DOI: 10.1021/jacs.7b03397 J. Am. Chem. Soc. 2017, 139, 7176−7179

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Figure 2. Nyquist plots from AC impedance data of PCMOF-17 in its powder form at 25 °C and 40% RH (inset: white powder sample).

Figure 1. Single crystal structure of PCMOF-17 showing: (a) a view parallel to the layers with interlayer Me2NH2+ cations and water molecules; (b) anionic layers view along {001} crystallographic axis; (c) view of the interlayer hydrogen bonding between 3-fold disordered sulfonic acids, Me2NH2+ cations and lattice water molecules along {001} crystallographic plane.

The Arrhenius plot for the bulk phase of PCMOF-17 gave a good linear fit, and the activation energy (Ea) of the proton conduction was estimated from impedance spectra recorded at 40% RH between 25 and 50 °C as 0.31 eV (Figure S15). This number is within the range typically attributed to a Grotthuss transfer mechanism (0.1−0.4 eV).12 The activation energy value is quite reasonable for proximal and continuous yet heterogeneous functional groups with varying pKa values in the hydrogen bonding network.1d,g The reproducible conduction data and the PXRD pattern at 40% RH (Figure 3) suggested integrity of PCMOF-17 but, given the PXRD data acquired at higher humidities, this assumption was further explored.

acid O3A and O4A (2.63(4)−3.25(6) Å). The other lattice water molecule O2W forms a bifurcated hydrogen bond to four adjacent symmetrically equivalent sulfonic acid O3B (D···A = 3.16(4) Å). However, because the occupancy of O2W is estimated as 0.4 and O3B is 0.25, O2W is not acting as a bridging element for hydrogen bonding (Figure S5). In addition, the strong peaks at 3465, 2784, and 1000−1130 cm−1 observed in the FTIR spectra also supports the hydrogen bonding between sulfonic acid groups and Me2NH2+ cations (Figure S6).9 On the other hand, the FTIR spectrum of PCMOF-17 clearly demonstrates the characteristic peaks of both −SO3H and −SO3− groups (Figure S6), attributed to the dynamic nature of protons. The presence of a hydrogen bonding 2D network encourages us to explore the proton conducting properties of PCMOF-17. As an extended hydrogen bonding network is key for an efficient proton conducting MOF, alternating current (AC) impedance spectroscopy of a pelletized sample with controlled humidity and temperature was used to analyze the proton conduction behavior of PCMOF-17. Prior to the impedance measurement, phase purity of the bulk sample was confirmed by powder X-ray diffraction analysis (PXRD, Figure S10). The proton conductivity (σ) of PCMOF-17 in its powder form at 25 °C and 40% RH is found to be 1.17 × 10−3 S cm−1 from the Nyquist plot (Figure 2). It is worth mentioning that this is the highest reported value of proton conductivity at ambient conditions in the domain of any porous crystalline materials (Table S1).10 Notably, it is also similar to Nafion 117, (conductivity is 1.4 × 10−3 S cm−1 at 31 °C and RH = 40%).11 The Nyquist plots under different humidity conditions (from 30% to 50% RH) at 25 °C show an increasing trend in conductivity, because the adsorbed water molecules help the diffusion of protons (Figure S12). The conductivities at different temperatures at 40% RH were also measured after equilibration for 24 h (Figure S13) giving consistent results after consecutive heating and cooling cycles (Figure S14).

Figure 3. PXRD of PCMOF-17 at different relative humidities showing amorphization above 60% relative humidity.

Figure 3 shows that treating PCMOF-17 with even moderately higher levels of relative humidity (60% RH) led to decomposition of the framework as confirmed by PXRD. Although this limits the practical application of PCMOF-17 itself, it did offer the opportunity to address a broader issue surrounding proton conducting coordination materials. Any proton conducting powder conducts through a combination of bulk and grain boundary pathways. In a material with modest 7177

DOI: 10.1021/jacs.7b03397 J. Am. Chem. Soc. 2017, 139, 7176−7179

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be >200 μm on edge (Figure S7). From our experience, this is larger than typical MOF proton conducting samples. This observation correlates with the easy growth of single crystals and the agreement in powder and single crystal proton conduction data. For a structure activity correlation, as expected, the σ value was maximized when the Pt wires were attached to the {-110} and {110} faces. Conduction between these faces is enabled by the continuous 2D hydrogen bonding network shown in Figure 1c. These two values are above the proton conductivity of the powder sample. On the other hand, the σ value along the c axis (perpendicular to the {001} plane) was found to be only 8.66 × 10−5 S cm−1, more than 1 order of magnitude smaller than in the other two directions. Owing to the absence of a grain boundary, impedance measurements of the single crystals along all three {-110}, {110}, and {001} faces gave the single relaxation response for impedance and modulus, and the overlap of the Z″ and M″ peaks in the spectra indicates longrange proton migrations (Figures 4d,e).4d,13 The maintained single crystallinity at room temperature and 40% RH was verified through SC-XRD measurements with a pre/posthumidity treated single crystal (Figure S19). The following assertions emerge: (i) Even though PCMOF-17 is a material of modest water stability, the observed proton conduction at 25 °C and 40% RH in pelletized samples is not resulting from dissolution at the grain boundary as confirmed by the single crystal study. (ii) Given the layered structure, even in the single crystal, one could suggest that increased hydration brings about local dissolution and augments proton transfer. Although this may occur to some extent, if this was a dominant effect, the proton conduction along c axis would be comparable to the other two axes. (iii) The main conduction pathways in PCMOF-17 are through the continuous hydrogen bonded pathways in the ab crystallographic plane. (iv) Although much of MOF chemistry focuses on designing pores in solids, the pore structure in PCMOF-17 does not present an efficient proton transfer pathway and acts to restrict only proton conduction. In summary, we have successfully synthesized a 2D sulfonated metal organic framework that exhibits high proton conductivity at room temperature and low relative humidity (40% RH). The MOF material has modest water stability, which raised questions about the origin of the observed proton conduction. Although single crystal proton conductivity measurements of organic or inorganic−organic hybrid materials are still rare,4d,14,10a,14,15 this method was employed at different relative humidities to confirm that the MOF was indeed a bulk proton conductor. Furthermore, the triaxial conductivity measurements confirmed that dominant proton transfer pathways in the crystal are tied to the continuous hydrogen bonding pathway and independent of the MOF pore structure.

stability to water, the grain boundary contribution can be augmented or even dominated by dissolution of the solid at the interface and facile movement of ions through a more fluid interface. In such a case, structure activity relationships such as those often extracted from crystalline MOF materials may be of questionable value. As a modest stability MOF, but also one that could grow as sizable single crystals, the proton conduction of PCMOF-17 was explored in the single crystal form at 40% RH. AC impedance analysis was performed on large single crystals of PCMOF-17 along specific crystallographic axes at 25 °C and 40% RH to confirm bulk conductivity and to correlate structure and function. Single crystal X-ray diffraction (SCXRD) was used to index the crystal faces and identified them as {-110}, {110}, and {001} (Figures S16−18). Pt wires were carefully attached to three different crystals using conductive silver paste along opposite faces (Figure 4a−c) and AC

Figure 4. Nyquist plots from AC impedance data of PCMOF-17 in its single crystal form (a) {-110}, (b) {110}, and (c) {001} faces at 25 °C and 40% RH (inset: Pt wires are glued with the single crystals along (a) {-110}, (b) {110}, and (c) {001} faces, respectively). Impedance (filled) and modulus (unfilled) spectra along (d) {-110}, (e) {110}, and (f) {001} faces at 25 °C and 40% RH.

conductivity was measured. The σ values, found from the Nyquist plot, are 1.25 × 10−3, 1.20 × 10−3, and 8.66 × 10−5 S cm−1 when the Pt wires were attached to the {-110}, {110}, and {001} crystallographic faces, respectively (Figure 4a−c). The {-110} and {110} faces are crystallographically equivalent in the tetragonal cell and ideally identical conductivity values would be obtained. The values compare very well to the 1.17 × 10−3 S cm−1 obtained for the powder sample under the same conditions in that higher conductivity would be expected for the monolithic system along preferred axes compared to the randomly oriented powder with more grain boundaries. SEM images of PCMOF-17 show typical grain sizes in the powder to

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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/jacs.7b03397. Data for C13H3In0.50N3O11S (CIF) Sample preparations, crystal structures, SEM image, EDX data, PXRD, TGA, IR, RH dependent conduction for 7178

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crystal and powder, temperature dependent conduction values, and crystallographic data (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

George K. H. Shimizu: 0000-0003-3697-9890 Author Contributions †

These authors contributed equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.J. is thankful to AITF postdoctoral research fellowship. We are grateful to Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support. We are also thankful to IISER Pune for SEM image.

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DOI: 10.1021/jacs.7b03397 J. Am. Chem. Soc. 2017, 139, 7176−7179