Achieving Superprotonic Conduction in Metal-Organic Frameworks

terials, PCMOF2½(Pz) and PCMOF2½(Tz) (Pz= 1H-pyrazole, Tz= 1H-1,2,4-triazole), have their proton conduction raised almost two orders of magnitude ...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 1077−1082

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Achieving Superprotonic Conduction in Metal−Organic Frameworks through Iterative Design Advances SiRim Kim, Biplab Joarder, Jeff A. Hurd, Jinfeng Zhang, Karl W. Dawson, Benjamin S. Gelfand, Norman E. Wong, and George K. H. Shimizu* Department of Chemistry, University of Calgary, Calgary, AB T2N 1N4, Canada S Supporting Information *

ABSTRACT: Two complementary design strategies, isomorphous ligand replacement and heterocycle doping, have been applied to iteratively enhance the proton conductivity of a metal−organic framework, β-PCMOF2. The resulting materials, PCMOF21/2(Pz) and PCMOF21/2(Tz) (Pz = 1H-pyrazole, Tz = 1H-1,2,4-triazole), have their proton conduction raised almost 2 orders of magnitude compared to β-PCMOF2. The bulk conductivities of these materials are over 10−1 S cm−1 at 85 °C and 90% relative humidity (RH), while maintaining the parent MOF structure. A solid state synthetic route for doping 1-D channels is also presented.

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INTRODUCTION Metal−organic frameworks (MOFs) have received growing interest as proton conducting materials in recent years.1−14 This is mainly because of the modular nature of MOF design and synthesis, as well as their crystallinity, allowing structure− activity relationships to be developed.15−21 Many protonconducting MOFs have focused on incorporating proton transfer agents within the pores, to improve both conductivity and the upper operating temperature, on functionalizing coordinatively unsaturated metal sites, or on tuning the acidity of the pores by incorporating specific functional groups.22−36 The following Arrhenius equation (1), which is derived from the Nernst−Einstein relation, provides a fundamental guideline in designing high proton conducting MOFs.37−45 σ=

( ) exp⎛ −E ⎞ → σ = σ

ne 2Do exp kT

ΔSm k

number of charge carriers, (2) by decreasing the activation energy of proton transfer, and (3) by increasing the motional entropy. Increasing the number of charge carriers can be achieved by introducing acidic moieties such as H2SO4, RSO3H, H3PO4, water/hydronium, RCO2H, or protonated N-heterocycles into the framework or as guests in the proton conduction channels.46−56 Only about 50% of the charge carriers should be protonated as proton hopping requires nonprotonated sites to act as proton acceptors. Finally, a proton carrier will need to reorient itself following a proton transfer, and the ease with which this occurs is determined by how the carriers interact with the pore and each other; this is intimately related to the structure of the MOF. Therefore, a desirable proton conducting metal−organic framework would have a 3-dimensional pore structure with a large number of available protons and proton carriers that have high rotational freedom. Proton carriers would interact weakly with the framework, and they should be at a distance to allow for facile proton hopping but not be so close as to hinder reorientation following proton transfer. A proton conducting MOF named β-PCMOF2 (Figure 1, left) was reported in our previous study, and the conductivity value was found to be 1.3 × 10−3 S cm−1 at 90% relative humidity and 85 °C.57 β-PCMOF2 consists of a trisodium 2,4,6-trihydroxy-1,3,5-trisulfonate benzene (Na3L1, Scheme 1)

⎜

a⎟

⎝ kT ⎠

⎛ −E ⎞ exp⎜ a ⎟ ⎝ kT ⎠ T o

(1)

where σ = ionic conductivity, n = number of charge carriers (temperature independent mobile site occupancy), e = charge of the mobile ion, Do = a constant related to the mechanism of ionic conductivity, k = Boltzmann constant, T = temperature, ΔSm = motional entropy, and Ea = motional enthalpy (activation energy for ion transport). According to eq 1, better and high conducting MOFs can be achieved via three different strategies: (1) by increasing the © 2017 American Chemical Society

Received: October 24, 2017 Published: December 22, 2017 1077

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compared under common conditions to PCMOF2(Tz) and PCMOF21/2. The ligand H3L2 had previously been reported in a structure named PCMOF3,59 and so the mixed ligand systems were named PCMOF21/2. The systematic variation of components provides insights to the extent different factors can affect total proton conduction and activation energy. To obtain good heterocycle loadings in these systems, solid state syntheses are employed, a method that is potentially applicable to other MOF syntheses where competition from solvent is problematic. Ultimately, the application of the two design strategies results in the synergistic enhancement of the βPCMOF2’s proton conductivity to yield MOFs that conduct protons over 10−1 S cm−1.

Figure 1. (left) The space-filling cross section structure of β-PCMOF2 with its 1-dimensional proton conduction channel illustrated with an arrow and the schematic illustration of isomorphous replacement. (right) A 2-dimensional crystal layout of β-PCMOF2 where the pores are impregnated with heterocycles (teal pentagons).

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EXPERIMENTAL PROCEDURES

General Synthesis of Mixed Ligand-Heterocycle Loaded MOFs. As-synthesized PCMOF2(heterocycle) and Na3H3L2 were individually ground into fine powders using a mortar and pestle and mixed in a 2:1 ratio with respect to Na3L1 and Na3H3L2. The ground powders were placed in a vial and mechanically shaken. The resulting mixture was placed into a silicon tube, sealed, and pressed under a hydrostatic pressure of 10000 pounds/inch2 for 2 min. The resulting pellet was removed from the silicone tube and placed into a 23 mL Teflon autoclave, along with a vial of water (1.7 mL). The Teflon autoclave was then sealed in a stainless steel jacket and heated to 80 °C for 48 h then cooled back to room temperature over 12 h. The pellet was then placed in a desiccator for a minimum of 12 h to remove excess moisture and ground into a fine powder using a mortar and pestle. The resultant powder was analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), elemental analysis, and 1H and 31P NMR to verify the completion of the isomorphous ligand replacement as well as the chemical composition of the mixed ligand-heterocycle loaded PCMOFs. Detailed compositional analysis on all PCMOFs is presented in the Supporting Information. We have observed that higher phosphonate ligand loadings than 33% give new phases in the PXRD. AC Impedance Measurements. Powdered samples of PCMOF21/2(Pz), PCMOF21/2(Tz), PCMOF2(Pz), PCMOF2(Tz), (25 to 30 mg each) were placed in a glass cell and compressed between two solid titanium electrodes (0.3175 cm diameter). The sample length was measured by the difference between the empty cell and the filled cell, which was typically to 1 to 2 mm in length. The sample cells were placed inside a humidity and temperature controlled chamber (ESPEC BTL-433) and connected to a Princeton Applied Research VersaSTAT 3 impedance analyzer using a 2 probe setup. AC impedance data was collected by cycling between 106 and 1 Hz with 10−200 mV of applied potential using VersaStudio software. Samples were equilibrated for between 20 and 24 h after each step in temperature (20 to 85 °C) and for 48 to 120 h for each step in relative humidity (30% to 90% RH). These lengthy times are not arbitrary as short equilibration times, especially for RH, yield non-Arrhenius behavior and much lower conduction values. Exposure of the samples to humid environment was performed for conditions at and below 90% RH using the ESPEC BTL-433 humidity control oven. Proton conductivity of all samples was measured under 90% RH condition at various temperatures. The temperature was varied from 20 to 85 °C for minimum of two heating and cooling cycles with sufficient time for sample equilibration between each step. Following the heating and cooling cycles, the temperature was held at 25 °C and the humidity was decreased from 90% to the minimum of 30% to measure the humidity dependent proton conductivity of the samples.

Scheme 1. Chemical Structure of L1, H3L2, 1H-Pyrazole, and 1H-1,2,4-Triazole

complex arranged in a 1D columnar structure where the pores are lined with sulfonate oxygen atoms.57 The pores are 5.6 Å in diameter and are hydrophilic in nature. Further, the proton conductivity value of β-PCMOF2 was improved 1.5 orders of magnitude by isomorphous ligand replacement with a more protic ligand of comparable size and shape (1,3,5-benzenetriphosphonic acid, Na3H3L2, Scheme 1), and the resultant MOF, named β-PCMOF21/2, showed 2.1 × 10−2 S cm−1 at 90% relative humidity and 85 °C.57 This enhancement in conductivity is due to increase the number of freely available acidic protons from the backbone.57,58 In another study, we successfully enhanced the anhydrous proton conductivity of βPCMOF2 by loading the pores with less volatile, amphiprotic guest molecules (Figure 1, right). The report describes a βPCMOF2(1H-1,2,4-triazole) system where β-PCMOF2 was loaded with 0.3 equiv of triazole in the pores and showed a five orders of magnitude higher anhydrous conductivity relative to unloaded β-PCMOF2 to reach 5 × 10−4 S cm−1 at 150 °C.49 Thus, combination of nonvolatile heterocyclic compound loading and isomorphous ligand replacement in β-PCMOF2 would be an excellent strategy to further enhance both the water assisted and anhydrous proton conductivity for MOF materials. Herein, we report a study assessing the effects of heterocycle loading, isomorphous protic ligand replacement, and merging these approaches on proton conduction in a single family of isostructural MOFs. Three new compounds, PCMOF21/2(Pz), PCMOF21/2(Tz) and PCMOF2(Pz), are reported and

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RESULTS AND DISCUSSION Three new crystalline compounds, PCMOF2 1 / 2 (Pz), PCMOF21/2(Tz), and PCMOF2(Pz), were synthesized and characterized through PXRD, elemental analysis, thermogravimetry, and NMR analyses (See Supporting Information). In 1078

DOI: 10.1021/jacs.7b11364 J. Am. Chem. Soc. 2018, 140, 1077−1082

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Journal of the American Chemical Society synthesizing PCMOF21/2(Pz) and PCMOF21/2(Tz), a three component mixed ligand MOF, it was critical to employ appropriate synthetic parameters to enable mutual compatibility between the two design strategies. At first the syntheses of PCMOF2(heterocycle) (heterocycle = pyrazole and triazole) were carried out. This synthesis process of PCMOF2(heterocycle) was tailored to kinetically capture pyrazole within the β-PCMOF2 framework via a solution precipitation. In the second design strategy, PCMOF2(heterocycle) was then mixed with Na3H3L2 and placed under solid state reaction conditions to yield corresponding PCMOF21/2(heterocycle) (Supporting Information). Pelletization accelerates the process because it increases the contact between the multiphases. Additionally, the high temperature and humidity condition accelerates the mobility of the molecules and drives the reaction because it increases the positional and vibrational entropy of the solid constituents. This synthetic approach is conceptually similar to that of Accelerated Aging, which describes the natural mineral weathering process.60 Figure 2 shows the powder XRD analysis

powder XRD of PCMOF21/2(Pz) and PCMOF21/2(Tz) resembled that of β-PCMOF2 with a slight 2θ shift for the peaks that correspond to the (2,1̅,0) plane and the (0,0,1) plane with a decrease of ∼0.1 and 0.1 Å and 0.03 and 0.04 Å in the dspacing, respectively (Figure S11 and Table S1). The presence of heterocycles were demonstrated by 1H NMR studies, where the digested samples of PCMOF2(heterocycle) showed peaks corresponding to the heterocycle molecules in the framework (Supporting Information). The molecular formulas of all the compounds were calculated merging elemental analysis and thermogravimetric analysis (Supporting Information). The maximum ratio of H3L2 that can be incorporated with retention of structure is 33%. While we do not have direct structural support, a possible arrangement based on this ratio would be a phosphonate ligand between two sulfonate ligands. To characterize the proton conductivity, AC impedance measurements were performed on samples of PCMOF2, PCMOF2(heterocycle), and PCMOF21/2(heterocycle) at 90% RH in air. Table 1 summarizes the conductivity values Table 1. List of PCMOF Samples Featured in This Work and Their Conductivity Profiles conductivity, S cm−1

name 1

a

PCMOF2 /2(Pz) PCMOF21/2(Pz)a PCMOF21/2(Pz)a PCMOF21/2(Tz)a PCMOF21/2(Tz)a PCMOF2(Pz)b PCMOF2(Pz)b PCMOF2(Pz)b PCMOF2(Tz)b PCMOF2(Tz)b PCMOF2(Tz)c PCMOF21/2d PCMOF21/2d β-PCMOF2b,d β-PCMOF2d β-PCMOF2c PCMOF2(Im)a,j PCMOF21/2(Im)a,j

Figure 2. Powder XRD patterns of (A) PCMOF21/2(Pz), postimpedance; (B) PCMOF21/2(Tz), postimpedance; (C) PCMOF21/2, postimpedance; (D) PCMOF2(Pz), postimpedance; (E) PCMOF2(Tz), postimpedance; (F) β-PCMOF2, postimpedance; (G) PCMOF21/2(Pz), preimpedance; (H) PCMOF21/2(Tz), preimpedance; (I) mechanical mixture of PCMOF2(Pz) and Na3H3L2 (resembles J + M); (J) PCMOF2(Pz); (K) mechanical mixture of PCMOF2(Tz) and Na3H3L2 (resembles L + M); (L) PCMOF2(Tz); and (M) Na3H3L2.

1.1 1.23 7.2 1.17 1.08 4.6 3.28 1.3 1.9 3.18 5 2.1 2.4 1.3 1.8 1

× × × × × × × × × × × × × × × ×

−1

10 10−3 10−7 10−1 10−2 10−2 10−4 10−3 10−2 10−5 10−4 10−2 10−5 10−3 10−6 10−9

T, °C

RH, %

Ea,e eV

85 25 150 85 25 85 25 150 85 25 150 85 20 85 20 100

90 40 0 90 40 90 40 0 90 40 0 90 50 90 50 0

0.16 k 0.20,f 0.98g 0.22 k 0.10 k 0.42h 0.10 k 0.34i 0.21 k 0.28 k k

a New compounds and new conductivity data. bCompounds previously synthesized by Hurd et al.; conductivity was measured under new conditions.49 cCompound and conductivity previously reported by Shimizu et al.49 dCompounds and conductivity previously reported by Shimizu et al.57 eActivation energy measured between the temperature of 85 and 25 °C. fActivation energy measured between the temperature of 150 and 110 °C. gActivation energy measured between the temperature of 110 and 90 °C. hActivation energy measured between the temperature of 150 and 60 °C. iActivation energy measured between the temperature of 150 and 90 °C. jThese PCMOFs exhibited proton conductivity below 10−6 S cm−1 at 85 °C and 90% RH. kNot applicable.

of the various PCMOFs featured in this work with the focus on PCMOF2 1/2 (Pz) and PCMOF21 /2(Tz). We performed powder XRD analysis throughout the synthesis to confirm the preservation of the original β-PCMOF2 structure (Figure 2F). In synthesizing PCMOF21/2(Pz) and PCMOF21/2(Tz), finely ground powders of PCMOF2(Pz)/PCMOF2(Tz) and Na3H3L2 were mechanically mixed. The PXRD pattern of the mechanical mixture exhibited a simple superpositioning of the patterns of its two components (Figure 2, spectra I (J and M) and K (L and M)). The mechanical mixture was then pressed into a pellet and placed under high temperature and humidity conditions to achieve isomorphous ligand replacement, a thermodynamically driven solid state reaction.57 The resultant

and the corresponding activation energies of the various PCMOFs under their respective conditions. The Nyquist plots obtained from the second heating cycle, to ensure humidity equilibration, are shown in Figure 3, and it shows that PCMOF21/2(heterocycle) exhibited very low resistances, resulting in only the tail end of a semicircle being observed at high frequencies due to instrument limitations. At lower frequencies, the capacitive tail is also observed, as expected for 1079

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lowered from 90% to 40%, where as the conductivity of PCMOF2(Pz) and PCMOF(Tz) experienced a much more abrupt decrease (Table 1 and Figure S8). The activation energy for proton transfer in the PCMOF2 series under humid condition is between 0.10 and 0.28 eV,58 which is consistent with a Grotthuss transfer mechanism (Figure 3). β-PCMOF2 had the greatest activation energy, 0.28 eV, among all the PCMOFs investigated in this study. PCMOF21/2 showed activation energy of 0.21 eV. PCMOF2(Tz) and PCMOF2(Pz) showed very low activation energies (0.10 eV). PCMOF21/2(Pz) and PCMOF21/2(Tz) showed activation energies of 0.16 and 0.22 eV, respectively (Table 1 and Figures S6 and S7). The activation energy (Ea) of proton transfer is dependent on the facility of H+ transfer and the reorientation of proton carriers post-transfer.58 Subsequently, proton mobility is a function of activation energy, and proton conductivity is a function of mobility. It may seem that a PCMOF with the lowest activation energy would yield the highest proton conductivity, but based on the results of this work (Figure 4

Figure 3. Nyquist plot for (a) PCMOF2 1 / 2 (Pz) and (b) PCMOF21/2(Tz) at 90% RH.

blocking effects of the mobile charge at the electrode interface. The conductivity was calculated from the real-axis intercept of the Nyquist plot (Figure 3). As a control, PCMOF2(Pz) and PCMOF2(Tz) were measured and their proton conductivities reproduced previously measured values on samples prepared by a different coworker: PCMOF2(Pz) (4.6 × 10−2 S cm−1) and PCMOF2(Tz) (1.9 × 10−2 S cm−1) at 85 °C and 90% RH (Table 1). The mixed ligand loaded samples were then measured and gave data almost an order of magnitude higher under identical conditions: 1.1 × 10−1 S cm−1 and 1.17 × 10−1 S cm−1 for PCMOF21/2(Pz) and PCMOF21/2(Tz), respectively, at 85 °C and 90% RH. Since both PCMOF21/2(Pz) and PCMOF21/2(Tz) showed proton conducting properties over 10−1 S cm−1 (at the time of initial measurement, the highest in any proton conducting MOF), new samples were remade (by a second researcher) and conductivity was remeasured for four heating and cooling cycles to confirm stability and reproducibility. Each cycle was measured at 90% RH and from 20 to 85 °C with at least an 8 h equilibration time between temperatures. The integrity of the sample was maintained, as suggested by the reproducible conduction data, and corroborated by the pre- and postimpedance PXRD patterns (Figure 2). Notably, all PCMOFs investigated showed the same PXRD pattern as β-PCMOF2 both before and after impedance measurements, confirming the preservation of the structure. The conductivity was found to be highly dependent on the humidity, which suggests that water plays a critical role in proton conduction. Interestingly, both PCMOF21/2(Pz) and PCMOF21/2(Tz) showed a steady decrease in conductivity when the relative humidity was

Figure 4. Proton conductivity data (90% RH) for various samples. Lines are present only as visual guides.

and Table 1), lowest activation energy does not necessarily equal highest conductivity. In order to address this, multiple factors that affect conductivity (amphiprotic proton donor/ acceptor sites, pKa values, host-guest interactions, and the size of the heterocycles) need to be addressed since they all affect the resultant conductivity and the activation energy. Empirical comparison of conductivity can be made using the relative pKa values (Table 2) of the groups lining the pores and the conjugate acid form of the heterocycles. Table 2. List of pKa Values Relevant to This Work

1080

name

pKa

benzenesulfonic acid phenylphosphonic acid 1H-pyrazol-2-ium 1H-1,2,4-triazol-2-ium 1H-imidazol-3-ium hydroxonium

−0.60 1.88 2.50 2.39 6.90 −1.74 DOI: 10.1021/jacs.7b11364 J. Am. Chem. Soc. 2018, 140, 1077−1082

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reported now with accounts of values exceeding 10−1 S cm−1.45 These advances are enabled by design advances in parallel with increasingly more robust MOF structures being reported. In this case, the MOF materials are not highly robust and would not survive a long-term fuel cell test; the value of this work is the design principles put forth. The fundamental chemistry governing proton transfer is identical regardless of the specific material or even class of solid. There must be a continuous proton transfer pathway with minimal potential wells to trap the protons. MOFs, with their crystalline structures, offer the ability to correlate crystallographic structure and function. This coupled with the well-established routes for systematic structural variation, as we have shown here, make iterative cycles of design and assessment a rational rather than stochastic process.

PCMOF2(Pz) and PCMOF2(Tz) have an activation energy for proton transfer of 0.10 eV, a very low value indicative of highly facile proton transport pathway. PCMOF21/2(Pz) and PCMOF21/2(Tz), on the other hand, have activation energies of 0.16 and 0.22 eV, respectively, double that of PCMOF2(Pz) and PCMOF2(Tz). PCMOF2 itself has a higher activation energy than PCMOF21/2.57 This result is in accord with the pKa values in Table 2 as the sulfonate group has lower affinity for protons. However, despite the higher activation energy, in all three cases, the PCMOF21/2(heterocycle) complexes show higher conductivity than the pure sulfonate counterparts (PCMOF2(heterocycle)). Revisiting eq 1, the conductivity in the PCMOF21/2(heterocycle) series is greater because the higher activation energy is offset by the increase in the number of freely available acidic protons resulting in enhanced conductivity. Concurrently, the greater difference in pKa will also lead to greater host−guest interaction between Na3H3L2 and the heterocycles, inhibiting reorientation and increasing the activation energy. pKa is an empirical solution measurement and may be a good predictor for the resultant conductivity, but in the PCMOF framework, other factors such as their size, hydrogen bonding proficiency, and host-guest interactions (which in turn affect the motional entropy and the rotational freedom of the heterocycles) also impact the resultant mobility and conductivity. Being similar in terms of pKa and molecular size, both triazole and pyrazole showed similar conductivity. Perhaps this is because the pKa of pyrazolium and triazolium cation is much closer to the pKa of hydronium cation (−1.74), enabling a more facile reversible proton transfer between the two conjugate acids. The pKa values indicate that the triazole− triazolium or pyrazole−pyrazolium pair would form a more balanced mix of protonated and unprotonated molecules; it is just as important to provide sufficient unprotonated sites that can act as facile proton acceptors in addition to maximizing the number of proton donors. As a contrast, we also investigated imidazole (Im, pKa1 = 7.18, pKa2 = 14.52) as a dopant. We synthesized PCMOF2(Im) and PCMOF21/2(Im) and measured their respective conductivities. Neither sample showed significant proton conductivity at 85 °C and 90% RH. We hypothesized the minimal conductivity to be caused by the much higher pKa of imidazolium meaning much more localized protons as well as the resulting stronger host−guest interaction between the imidazole molecule and the MOF framework limiting reorientation. Comparisons of the proton conductivity in these systems was also carried out under anhydrous conditions. PCMOF21/2(Pz) was dehydrated, and the conductivity was found to be 7.2 × 10−7 S cm−1 at 150 °C in air, a much lower value than the anhydrous conductivity of PCMOF2(Pz) and PCMOF2(Tz) yet a higher value than that of pure β-PCMOF2 (10−8 S cm−1) (Figure S9). It is speculated that the loaded heterocycle in PCMOF21/2(Pz) still imparts some degree of anhydrous enhancement but nowhere as drastic as the 5 orders of magnitude increase as PCMOF2(Pz) versus β-PCMOF2. We hypothesized that the conductivity enhancement by Na3H3L2 requires sufficient amount of pore water molecules to activate/ dissociate its free acidic protons. This is supported by the rapid decrease in PCMOF21/2(Pz)’s and PCMOF21/2(Tz)’s conductivity when the humidity was decreased from 90% to 40% RH. The field of proton conducting MOF materials has seen steady progress in the levels of proton conduction being

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CONCLUSION We have successfully applied two design strategies, isomorphous ligand replacement and heterocycle doping, to enhance the proton conductivity of a proton conducting metal−organic framework, β-PCMOF2. The rational and sequential variation of structures gives proton conductivity data that correlates with expected performance based on molecular properties. We have also shown a solid state route to controllably loading 1-D pores in the MOFs with proton carriers. Of six PCMOFs investigated in this study, two materials, PCMOF2 1 / 2 (Pz) and PCMOF21/2(Tz), show exceptionally high proton conduction values (>1 × 10−1 S cm−1) at 85 °C and 90% RH, while maintaining the parent MOF structure. The principles outlined are applicable to other proton conducting systems beyond MOF materials.

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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.7b11364. Details of synthesis, powder XRD, and impedance analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Norman E. Wong: 0000-0003-3850-8638 George K. H. Shimizu: 0000-0003-3697-9890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Science and Engineering Research Council (NSERC) of Canada for support of this research and for graduate scholarships to S.K. and B.S.G. We thank Alberta Innovates Technology Futures for a doctoral scholarship to K.W.D. and a postdoctoral fellowship to B.J.

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REFERENCES

(1) Yamada, T.; Otsubo, K.; Makiura, R.; Kitagawa, H. Chem. Soc. Rev. 2013, 42, 6655. (2) Ramaswamy, P.; Wong, N. E.; Shimizu, G. K. H. Chem. Soc. Rev. 2014, 43, 5913. (3) Meng, X.; Wang, H.-N.; Song, S.-Y.; Zhang, H.-J. Chem. Soc. Rev. 2017, 46, 464. 1081

DOI: 10.1021/jacs.7b11364 J. Am. Chem. Soc. 2018, 140, 1077−1082

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DOI: 10.1021/jacs.7b11364 J. Am. Chem. Soc. 2018, 140, 1077−1082