High Proton Conductivity Achieved by Encapsulation of Imidazole

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High proton conductivity achieved by encapsulation of imidazole molecules into proton conducting MOF-808 Hong-Bin Luo, Qiu Ren, Peng Wang, Jin Zhang, Lifeng Wang, and Xiaoming Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01075 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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ACS Applied Materials & Interfaces

High proton conductivity achieved by encapsulation of imidazole molecules into proton conducting MOF-808 Hong-Bin Luo,a,b Qiu Ren,a Peng Wang,b Jin Zhang,a Lifeng Wang,c Xiao-Ming Ren*a,d a

State Key Laboratory of Materials-Oriented Chemical Engineering and College of

Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, P. R. China b

Department of Chemistry and Biochemistry, University of Maryland, College Park,

MD 20742, USA c

Institute for Frontier Materials (IFM), Deakin University, 75 Pigdons Road, Waurn

Ponds, Victoria 3216, Australia d

State Key Laboratory of Coordination Chemistry, Nanjing University 210093, P. R.

China

Fax: 86-25-58139481 Tel: 86-25-58139476 E-mail: [email protected]

1

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Abstract Metal-organic frameworks, as newly emerging materials, show compelling intrinsic structural features, e.g., the highly crystalline nature, designable and tunable porosity as well as tailorable functionality, rendering them suitable for proton conducting materials. The proton conduction of a MOF is significantly improvable using the post-synthesis or encapsulation strategy. In this work, the MOF-based proton conducting material Im@MOF-808 has been prepared by incorporating the imidazole molecules into the pores of proton conducting MOF-808. Compared with MOF-808, Im@MOF-808 not only possesses higher proton conductivity of 3.45×10-2 S cm-1 at 338 K and 99% RH, superior to that of any imidazole-encapsulated proton conducting materials reported to date, but also good durable and stable proton conduction. Moreover, the thermal stability of H-bond networks is much improved owing to the water molecules partially replaced by higher boiling point imidazole molecules. Additionally, it is further discussed for the possible mechanism of imidazole encapsulation into the pores of MOF-808 to enhance proton conduction.

Keywords: Metal-organic frameworks; imidazole encapsulation; superior proton conductor; isotropic effect

2


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Introduction During the past decade, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have been demonstrated to be good candidates as proton conducting materials,1-5 because MOFs show compelling intrinsic structural features, including the highly crystalline nature, designable and tunable porosity as well as tailorable functionality. The high crystallinity of MOFs offers crystallographically defined proton conduction pathway, as a powerful platform to well understand the proton conduction mechanism, therefore, providing theoretical guidance for design of novel crystalline superior proton conducting materials.6-9 Remarkably, the proton conducting nature of a MOF is significantly improvable using the post-synthesis strategy. By modifying the surface of pores in MOF framework with functional groups (-SO3H or -COOH etc.),10-14 the proton conducting nature of a MOF may be effectively enhanced owing to the increase of protonic carrier concentration and the optimization of proton transfer pathway. By contrast with the post-synthesis strategy involving the complicated preparation process, to encapsulate the special guests into the pores of a MOF is a facile strategy for upgrading the performance of MOFs-based proton conductors.15-18 For example, Lan et al. recently achieved a superprotonic conductor H2SO4@MIL-101-SO3H ( = 1.82 S cm-1 at 345 K and 90%RH) by encapsulation of nonvolatile H2SO4 into pores of MIL-101-SO3H.10 Kitagawa and co-workers have firstly shown that the facile encapsulation of imidazole molecules into porous MOFs would prepare composite superprotonic conducting materials,19 up to now, substantial research efforts have been devoted to incorporate the N-heterocyclic molecules (e.g., imidazole, triazole, histamine) into the pores of various porous materials including MOFs,20-26 covalent organic frameworks (COFs),27-29 mesoporous organosilicons,30 aiming to develop synergistic high performance proton conducting materials. However, it is still an enormous challenge to develop N-heterocyclic-encapsulated proton conducting materials with high proton conductivity and long-term durable proton conduction, 3



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particularly for MOFs-based proton conducting materials. For instance, the N-heterocyclic molecules probably coordinate to the metal ions in the framework of MOF via N atoms to lead to the framework of MOF collapsing. As a result, the more investigations are required for exploration of the MOFs-based proton conducting materials with encapsulation of N-heterocyclic molecules. In previous study, it was found MOF-808 possesses not only excellent proton conductivity over 10-3 S cm-1 at ambient temperature and high relative humidity (RH), but also the admirable chemical stability to water, acids and other ligands.31-34 and these outstanding natures are attributed to the unique framework structure of MOF-808.35 Three OH groups per cluster of Zr6O5(OH)3 in MOF-808 could ionize to produce protons, which respond to the high proton conduction at ambient temperature. However, the proton conductivity drops rapidly at the temperature above 315 K, owing to the release of water molecules from the pores leading to the H-bond network damage and the proton transfer pathways broken down.31 Here, we demonstrate that substituting water and DMF molecules in pores with higher boiling point imidazole would significantly strengthen structure stability of H-bond networks in framework of MOF-808, and then result in advanced performance proton conduction. By encapsulating imidazole molecules into pores of MOF-808, we have successfully obtained the new composite proton conducting material Im@MOF-808, which features with the highest proton conductivity of 3.45×10-2 S cm-1 (338 K and 99% RH) for all the imidazole-encapsulated proton conducting materials reported hitherto, together with good durability and stability in the proton conduction process. Results and discussion Preparation and characterization The polycrystalline sample of MOF-808 was prepared following the synthetic 4


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procedure previously reported and characterized by PXRD technique, which showed the sample has high phase purity (Figure 1a). The imidazole-encapsulated sample Im@MOF-808 was prepared by the process that the activated MOF-808 adsorbed imidazole vapor at 120 C in a vacuum oven, and the amount of imidazole molecules encapsulated into the pores of MOF-808 were evaluated according to the elemental analysis (Table S1). The time dependent amount of adsorbed imidazole were investigated for this preparing process, as shown in Figure S4, the maximum encapsulation amount of imidazole was achieved as the adsorbing time was 72 hours, which is designated as Im@MOF-808 in this work. The PXRD profile of Im@MOF-808 in Figure 1a is in good agreement with the PXRD pattern of the as-synthesized MOF-808, implying the framework of MOF-808 is maintained without any damaged after the imidazole molecules were encapsulated into the pores. The FT-IR spectra of MOF-808, Im@MOF-808 and imidazole are displayed in Figure S8, compared with IR spectrum of MOF-808, several new bands located at 1260 cm-1 and 1056 cm-1 appear in IR spectrum of Im@MOF-808, and these bands stem from the C-N stretching vibrations in imidazole.36 Besides, the N-H stretching vibration band in IR spectrum of imidazole crystal locates at 3125 cm-1, while, shifts to 3145 cm-1 for Im@MOF-808, indicating that the imidazole molecules in Im@MOF-808 have different environment from the imidazole crystal. Im@MOF-808 shows distinct behavior of losing weight from MOF-808, as shown in Figure 1b, MOF-808 displays one step of gradual weight loss process until decomposition, and the ~20% losing weight is related to the release of lattice solvents, including water and DMF. However, Im@MOF-808 exhibits two steps of weight loss processes, the 5.8% weight loss occurred in the range of 300-375 K is ascribed to the liberation of adsorbed water, obviously, the water residual in the pores of Im@MOF-808 is much less than that in the pores of MOF-808 due to the imidazole molecules accommodated. The further 35.1% weight loss underwent in the range of 402-573 K corresponds to the liberation of accommodated imidazole molecules in the 5



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MOF-808 framework. On the basis of the losing weight percentage of ~35.1%, it is estimated

that

there

are

~12

imidazole

molecules

per

unit

of

[Zr6O5(OH)3(C9H3O6)2(HCOO)5] encapsulated in the framework of MOF-808, and this result is in accordance with the elemental analysis. The porosity of MOF-808 and Im@MOF-808 have been characterized by N2 adsorption-desorption technique at 77 K, and the N2 adsorption-desorption isotherms together with pore size distribution profiles are shown in Figures 1c and 1d, both of them display type-I sorption isotherm, a typical characteristic of microporous materials. MOF-808 shows 1287.82 m2/g of the Brunanner-Emmett-Teller (BET) surface area, and 0.53 cm3/g of the pore volume, by comparison with MOF-808, Im@MOF-808 has lower BET surface area (495.37 m2/g) and smaller pore volume (0.23 cm3/g), and these results further demonstrate that the imidazole molecules were certainly encapsulated into the pores, not adsorbed on the crystal surface of MOF-808. All of the supporting evidences, obtained from elemental microanalysis, IR spectra comparison, TG and N2 adsorption-desorption isotherm analyses, suggest that the imidazole molecules have been successfully encapsulated into the framework of MOF-808. Furthermore, on the basis of TG (Figure 1b) and variable-temperature PXRD (Figure S2) measurements, it is also revealed that the frameworks in both MOF-808 and Im@MOF-808 are thermally stable up to ~573 K, and their framework also preserves the structural integrity up to ~573 K. These findings revealed that the encapsulation of imidazole molecules into pores does not degenerate the thermal and structural stabilities of MOF-808 framework.

(a)

(b)

6


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(d)

(c)

Figure 1: (a) Experimental PXRD profiles of MOF-808 and Im@MOF-808 as well as simulated PXRD pattern of MOF-808. (b) TG plots of MOF-808 and Im@MOF-808. (c) N2 adsorption-desorption isotherms and (d) pore size distribution profiles for MOF-808 and Im@MOF-808. Proton conduction of Im@MOF-808 Alternating current (ac) impedance analyses are carried out on the compressed pellet samples to evaluate the proton conductivity of Im@MOF-808. In these measurements, the ac frequency ranges from 102 to 106 Hz and the temperature spans from 288 to 338 K at 99% relative humidity (RH). As shown in Figure 2a and Figure S9a, all of the Nyquist plots display an imperfect arc in the high frequency region together with a tail in the low frequency region, in this case, the resistance of a proton-conducting material is estimated from the Z’-axis intercept value.24 The temperature-dependent proton conductivity of Im@MOF-808 is plotted in Figure S9b, showing that the proton conduction increases with rising temperature. The proton conductivity is 8.89×10-3 S cm-1 at 288 K, reaches to 2.04×10-2 S cm-1 at 313 K, and is further up to the maximum value of 3.45×10-2 S cm-1 at 338 K. Noticeably, at room temperature (298 K) and 99%RH, Im@MOF-808 exhibits a promising proton conductivity (1.22×10-2 S cm-1) that is over 10-2 S cm-1, and this value is not only superior

to

many

well-studied

proton

conducting

materials,

e.g.,

(NH4)2(adp)[Zn2(ox)3]·3H2O with  = 8×10-3 S cm-1 at 298 K and 98%RH,37 HOF-GS-10 having  = 7.5×10-3 S cm-1 at 303 K and 95%RH,38 Ca-PiPhtA-NH3 7



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showing  = 6.6×10-3 S cm-1 at 297 K and 98%RH,39 but also competivable with the most efficient proton conducting MOFs, such as {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n with  = 4.2×10-2 S cm-1 at 298 K and 98%RH,40 Fe-CAT-5 with  = 5.0×10-2 S cm-1 at 298 K and 98%RH,41 UiO-66(SO3H)2 with  = 1.4×10-2 S cm-1 at 298 K and 90%RH.12 Remarkably, the proton conductivity of Im@MOF-808 achieved up to 3.45×10-2 S cm-1 at 338 K and 99% RH, and this value is much higher than that of the well-known imidazole-encapsulated proton conducting MOFs, i.e., Im-Fe-MOF (1.21×10−2 S cm−1 at 333 K and 98% RH),21 Im@(NENU-3) (1.82×10−2 S cm−1 at 343 K and 90% RH)20 and HIm11VNU-17 (5.9×10−3 S cm−1 at 343 K and 85% RH).23 To the best of our knowledge, this value is the highest proton conductivity for imidazole-encapsulated proton conducting materials (Table 1). As a comparison, the proton conductivity of activated MOF-808 has been determined (Figure S10), revealed that its proton conductivity is less than two orders magnitude with respect to that of imidazole-encapsulated sample, Im@MOF-808. Moreover, the proton conductivity in Im@MOF-808 is also significantly larger than that in MOF-808 at the same conditions.31

(b)

(a)

8


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(d) (c)

Figure 2: (a) Nyquist plots of Im@MOF-808 at different temperatures and 99%RH. (b) Comparison of maximum proton conductivity for Im@MOF-808 with representative imidazole-encapsulated proton conductors. (c) Time-dependent Nyquist plots of Im@MOF-808 at 338 K under 99% RH. (d) Humidity-cycling study of Im@MOF-808 between 43% RH and 99% RH at 300 K. Table 1 Proton conductivity value of Im@MOF-808 in comparison with other high-performing imidazole-loaded proton conducting materials. Materials

Conductivity

Conditions

References

(S cm-1) Im@MOF-808

3.45×10−2

338 K 99% RH

This work

Im@(NENU-3)

1.82×10−2

343 K 90% RH

20

Im-Fe-MOF

1.21×10−2

333 K 98% RH

21

Im@Fe-MOF

4.23×10−3

HIm11⊂VNU-17

5.9×10−3

343 K 85% RH

23

[Him]2Tb2(ox)4(H2O)2·2H2O

5.0×10−3

298 K 98%RH

26

[Him]2Eu2(ox)4(H2O)2·2H2O

2.8×10−3

Im@TPB-DMTP-COF

4.37×10−3

403 K anhydrous

27

Im@s-PMO

7.11×10−3

453 K anhydrous

30 9



1.44×10−3

Im@UiO-67

393 K anhydrous

Page 10 of 26

22

The proton transport activation energy has been estimated using the Arrhenius equation, ln(T )  ln A 

Ea k BT

(1)

in Eq.(1), the symbol σ represents the conductivity, A is the pre-exponential factor, kB is the Boltzmann constant and Ea stands for the proton transport activation energy. The Arrhenius plot of Im@MOF-808 displayed in Figure 2b shows Ea = 0.25 eV (< 0.4 eV), suggesting that the proton transport follows predominantly the typical Grotthuss mechanism,42 that is, the protons hop between the neighboring sites in the hydrogen-bonding networks, which are consist of hydroxyl groups in the Zr6O5(OH)3 clusters of framework, water molecules and imidazole molecules in the pores. The proton transport hopping mechanism in Im@MOF-808 was further supported by the deuterium-hydrogen isotopic effects. Theoretically, the proton conductivity (σ) is inversely proportional to the square root of protonic carrier mass in the proton conductor. In the case of the proton transport following the hopping mechanism, the proton conductor will show significant isotopic effects, and the proton conductivity obeys the equality of σH : σD = 1 : 1/√2, where σH and σD represent the proton conductivities of the samples with the hydrogen and the deuterium, respectively. The ac impedance measurements have been performed for Im@MOF-808 in both environments of H2O and D2O vapor, respectively, and the samples would adsorb H2O or D2O into the framework and reach the equilibrium between the H2O/D2O in the pores and the vapor in the environment during the testing process. As shown in Figure 3a, the proton conductivity in the H2O-adsorbed sample is ca. 1.37 times (smaller but closed to the value of √2) as larger as that in D2O-adsorbed sample, this result is in agreement well with the observations of other reported proton conducting 10


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materials,25,

43-45

in which the proton transport follows the hopping mechanism. In

addition, the activation energy for proton transfer is 0.26 eV in the D2O-adsorbed sample, being comparable to the activation energy of 0.25 eV observed in the H2O-adsorbed sample (Figure 3b). These results further validate that the proton transport in Im@MOF-808 follows the hopping mechanism.

(a)

(b)

Figure 3: (a) Temperature-dependent proton conductivity of Im@MOF-808 performed in H2O and D2O vapor, respectively, and (b) the corresponding Arrhenius plots of proton conductivity. Figure 4 shows the temperature dependent proton conductivity at 99% RH for MOF-808 and Im@MOF-808. The distinction of proton conducting nature between MOF-808 and Im@MOF-808 concerns two aspects, (1) the proton conductivity in MOF-808 increases with rising temperature below 315 K, while drops at the temperature above 315 K owing to the liberation of water molecules in the pores. The proton conductivity in Im@MOF-808 rises monotonically with elevating temperature in the range of 288-338 K, demonstrating that the thermal stability of H-bond networks is indeed enhanced by replacing water in the pores of MOF-808 with higher boiling point imidazole. (2) Im@MOF-808 shows higher proton conductivity than MOF-808 in the whole temperature range under 99% RH.

11



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Figure 4: Comparison of temperature-dependent proton conductivities of MOF-80831 and Im@MOF-808 at 99% RH. In a solid state ion conducting material with negligible electronic conductivity, the electrical conductivity of the material is the sum of all ionic contributions and expressed using equation below,

   niZii

(2)

In Eq.(2), ni is the concentration of the charge carriers with charge Zi and mobility μi. On basis of Eq.(2), it is understandable that the encapsulation of strong acid species into the pores of MOFs could improve the proton conductivity of the MOFs, this is mainly because of the increasement of proton concentration (charge carrier concentration), and the optimization of proton transfer pathways owing to the formation of more dense H-bond networks. Interestingly, the imidazole is a weak base that has the ability to acquire a proton when the pH value in the environment drops less than 6.46 The previous study has indicated that the close-packed imidazole crystal shows quite low intrinsic proton conductivity and high proton transport activation energy as well.47 The pKa value of imidazole in water is 14,48-50 however, the metal ion would induce a significant drop of the pKa value of imidazole, allowing its deprotonation at physiological pH,51 and even so, the imidazole shows rather weak acidity. These facts demonstrate that the encapsulation of imidazole molecules into the pores of MOF-808 is impossible to increase the concentration of protonic carriers. 12


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Accordingly, it is certain that other mechanism would respond to the enhancement of proton conductivity in Im@MOF-808. It is worth noting that the imidazolium ion possess a structural diffusion owing to the existence of lower energy barrier of molecule rotation and the several equivalent resonance structures (see Scheme 1), such type of structural diffusion rather resembles that of water molecule and leads to the proton transport between imidazole and imidazolium easily undergoing,47,52 viz. the intermolecular proton transfer between the imidazolium and the imidazole is achievable along the hydrogen-bond chain via the imidazolium/imidazole molecule reorientation process. Therefore, like water molecules, the imidazole molecules are excellent media for proton hopping. By comparison of MOF-808 and Im@MOF-808, it is found that the proton transport activation energy is higher in MOF-808 (~0.37 eV)31 than that in Im@MOF-808 (~0.25 eV), since proton mobility μi is related to the proton transfer activation energy Ea by the equation of μi = μ0*exp(-Ea/kBT), this meanings that the protonic carriers show higher mobility in Im@MOF-808 than that in MOF-808, giving that Im@MOF-808 has higher proton conductivity than MOF-808 at the same condition. (a)

(b)

(c)

Scheme 1: Illustration of (a) the resonance structure for imidazolium and (b) rotation of imidazolium and imidazole as well as (c) proton hopping along H-bond chain between imidazolium and imidazole. 13



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Proton conducting durability and stability The relationship between the proton conductivity and relative humidity has been inspected for Im@MOF-808, and the proton conductivity has been determined under the selected relative humidity at 298 K. As shown in Figure S11, the proton conductivity is elevated with the increase of relative humidity, with the low proton conductivity 1.2×10−5 S cm−1 at 43% RH, 5.22×10−5 S cm−1 at 75% RH, then rapidly increased to 2.56×10−3 S cm−1 at 89% RH, and attained the maximum value of 1.25×10−2 S cm−1 at 99% RH. This phenomenon has also been observed in many MOF-based proton conducting materials,53,54 indicating that the proton conductivity is tightly associated with the amount of water molecules adsorbed in the pores, because the water molecules adsorbed in the pores could provide facile access to well-established hydrogen-bonding networks in the framework, which would serve as the high-efficiency proton-hopping pathways, then result in a significant enhancement of proton conductivity. To prove this point, the water-adsorption/desorption experiments have been carried out and the plots of water-adsorption/desorption isotherms are depicted in Figure S12, showing that the amount of water adsorbed in the pores is increased as the relative pressure of water vapor increased, this observation demonstrates that Im@MOF-808 can favorably adsorb water molecules into the framework under a certain relative humidity environment. Noticeably, IM@MOF-808 shows similar plot of N2 adsorption/desorption isotherms to that of MOF-808 owing to only the existence of weak interaction between N2 and MOF framework as well as imidazole molecules, but different plot of water adsorption/desorption isotherms from that of MOF-808 because of formation of H-bonds between water molecules and MOF framework as well as imidazole molecules. The proton conductivity stability is a primary requirement for realistic application of a proton conducting material. The time-dependent proton conduction has been investigated for Im@MOF-808 at 338 K and 99%, as displayed in Figure 2c, the 14


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impedance plots coincide nearly with each other, implying that the efficient proton conduction can maintain up to more than 2 hours. Likewise, the humidity-cycling proton conductivity has been investigated between the 43% RH and the 99% RH at 300 K (Figure 2d), and the result manifests that Im@MOF-808 is able to withstand the huge changes in humidity. The PXRD pattern has been recorded for the sample used for impedance measurement, as shown in Figure S5, with respect to the PXRD pattern of the sample prior to impedance measurement, the crystallinity of the sample used for impedance measurement shows only a little bit diminish, and the results suggest that Im@MOF-808 used for impedance measurement still maintains the structural integrity. All the above impressive performances unambiguously demonstrate that Im@MOF-808 possesses good proton conducting durability and structural stability, which afford a wide potential application in the field of proton conduction. Conclusion In summary, we have successfully achieved a high performance MOF-based proton conducting material via the facile encapsulation of the imidazole guests within the pores of robust MOF-808, and demonstrated that Im@MOF-808 possesses high proton conductivity with the value of 3.45×10-2 S cm-1 (338 K and 99% RH). To the best of our knowledge, the proton conductivity in Im@MOF-808 is the highest one among the imidazole-encapsulated proton conducting materials. Simultaneously, the obtained material displays excellent durability and stability for proton conduction. Our study also demonstrate that the encapsulation of imidazole into the pores of MOF-808 not only improve the proton conductivity owing to increase of proton mobility, but also enhance the thermal stability of H-bond networks in the framework of MOF-808 due to the water molecules in pores partially replaced by higher boiling point imidazole molecules. This study will expand the opportunity to realize superior protonic conducting materials, as well as inject fresh impetus to the practical application of MOF-based proton conducting materials. 15



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Experimental Chemicals and materials All the employed chemical reagents and solvents including zirconium oxychloride octahydrate (ZrOCl28H2O), 1, 3, 5-benzenetricarboxylicacid (H3BTC), formic acid, deuterium oxide (D2O), N, N-dimethylformamide (DMF) and Imidazole were bought from commercially available sources, and used as received without further purification. MOF-808 was prepared according the previously reported synthetic procedure31, 33 and characterized by powder X-ray diffraction technique. Elemental microanalysis calculated for [Zr6O5(OH)3(C9H3O6)2(HCOO)5](DMF)(H2O)2: Found: C, 21.42; H, 2.13; N, 1.08%. Calcd.: C, 21.88; H, 1.76; N, 0.98%. Preparation of Im@MOF-808 Im@MOF-808. Initially, the activated MOF-808 was obtained following the reported solvent exchange method, namely, the as-synthesized MOF-808 was successively treated with DMF, water and anhydrous acetone. The acetone-exchanged sample was firstly evacuated at room temperature for 24 hours and then at 150 C for 24 hours to yield activated MOF-808. Afterwards, the activated MOF-808 was heated together with imidazole in a vacuum drying oven at 120 C for 72 hours under vacuum (ref. Figure S3), and the vaporized imidazole molecules diffuse into the pores of the activated MOF-808 to yield Im@MOF-808. The amount of encapsulated imidazole in the pores was evaluated according to the thermogravimetric analysis and elemental

microanalysis.

Elemental

microanalysis

calculated

for

[Zr6O5(OH)3(C9H3O6)2(HCOO)5](C3N2H4)12(H2O)3: Found: C, 31.90; H, 3.38; N, 15.11%. Calcd.: C, 32.37; H, 3.13; N, 15.36%. Chemical and Physical characterizations Elemental analyses of C, H and N were carried out on an Elementar Vario EL III 16


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elemental analyzer. Powder X-ray diffraction (PXRD) patterns at room temperature were recorded in the range of 2θ = 5-50° with a step of 0.01° on a Rigaku MiniFlex600 diffractometer with Cu K radiation (λ = 1.54059 Å), operated at 40 kV and 15 mA. Fourier transform infrared (FTIR) spectra were collected on a Nicolet iS5 spectrometer with KBr pellets in the spectral regime of 400-4000 cm−1. The variable-temperature PXRD data were collected on SHIMADZU XRD-6100 diffractometer with Cu K radiation (λ = 1.5418 Å), operated at 40 kV and 40 mA, and the 2 angle ranges from 5 to 50° with a step of 0.01° and the temperature changes from 303 to 673 K. Thermogravimetric analyses (TGA) were performed on a TA2000/2960 thermogravimetric analyzer under N2 atmosphere and in the temperature range of 298-1073 K. Water-adsorption/desorption isotherms were achieved using the Belsorp-Max instrument at ambient temperature. Nitrogen adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 system at 77 K. The measurements of alternating current (ac) impedance spectroscopy were performed on a Gamry Reference 600+ electrochemical workstation with a conventional three-electrode method, and the frequencies ranged between 106 Hz to 102 Hz with signal amplitude of 5 mV. The impedance spectra were recorded under different relative humidity and temperature, wherein the relative humidity was controlled using the standard saturated aqueous solutions of certain salts. The proton conductivity () was calculated using the formula  = L/RS, in which L is the thickness of sample and S denotes the cross-sectional area, and R stands for the resistance. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: PXRD profiles of the samples of MOF-808 soaked in water, hydrochloric acid 17



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aqueous solution (6.0 mol L-1) for 7 days; Variable-temperature PXRD profiles at selected temperatures for MOF-808 and Im@MOF-808; Schematic for encapsulating imidazole molecules into MOF-808 via vaporization method; Results of elemental analyses for C, H and N for each sample; The amount of imidazole molecules in the Im@MOF-808 against vaporization time; Experimental PXRD profiles of MOF-808 and Im@MOF-808; XPS spectra of Zr3d, C1s, O1s, N1s in MOF-808 and Im@MOF-808; FT-IR spectra of MOF-808, Im@MOF-808 and bulk imidazole; Nyquist plots of Im@MOF-808 at selected temperatures under 99% RH; Temperature-dependent proton conductivity of Im@MOF-808 under 99% RH; Nyquist plots of activated MOF-808 at selected temperatures under 99% RH; Temperature-dependent proton conductivity of activated MOF-808 under 99% RH; Nyquist plots of Im@MOF-808 at 298 K with different relative humidity; Humidity-dependent proton conductivity at 298 K; Water-adsorption/desorption isotherm of MOF-808 and Im@MOF-808 at 298 K; Temperature-dependent Nyquist plots of D2O-adsorbed Im@MOF-808 measured in D2O vapor. Acknowledgment This research is financially supported by Priority Academic Program Development of Jiangsu Higher Education Institutions, National Nature Science Foundation of China (grant no. 21671100). Hong-Bin Luo is greatly indebted to the Postgraduate Research & Practice Innovation Program of Jiangsu Province of China (grant no. KYCX18_1064) and the Joint Ph. D Program of China Scholarships Council (Grant CSC No. 201708320366) for financial support.

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TOC

Im@MOF-808, a proton conducting MOF, achieved by encapsulating imidazole molecules into pores of MOF-808, possesses the highest proton conductivity in all imidazole-encapsulated proton conducting materials reported to date, together with excellent durable and stable proton conduction.

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High Proton Conductivity Achieved by Encapsulation of Imidazole

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