Impressive Proton Conductivities of Two Highly Stable Metal–Organic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Impressive Proton Conductivities of Two Highly Stable Metal−Organic Frameworks Constructed by Substituted Imidazoledicarboxylates Xiaoxin Xie, Zhehua Zhang, Jian Zhang, Lifen Hou, Zifeng Li, and Gang Li*

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College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China S Supporting Information *

ABSTRACT: There is great interest in the promising applications of proton-conductive metal−organic frameworks (MOFs) in the field of electrochemistry. Thus, seeking more types of MOFs with high proton conductivity is of great importance. Herein, we designed and prepared two substituted imidazoledicarboxylate-based MOFs, {[Cd(p-TIPhH2IDC)2]·H2O}n [1; p-TIPhH 3 IDC = 2-p-(1H-1,2,4-triazolyl)phenyl-1H-4,5-imidazoledicarboxylic acid] and [Sr(DMPhH2 IDC) 2 ] n [2; DMPhH3IDC = 2-(3,4-dimethylphenyl)-1H-imidazole-4,5-dicarboxylic acid], and fully explored their water-assisted proton conduction. The best conductivity for 1 of 1.24 × 10−4 S·cm−1 is higher than that of most previous conductive Cd-MOFs under similar conditions. 2 has the highest conductivity (0.92 × 10−3 S·cm−1) among the reported conductive Sr-MOFs. Via structural analysis, Ea values, water vapor adsorptions, and powder X-ray diffraction and scanning electron microscopy tests, reasonable proton pathways and conduction mechanisms were highlighted. It should be emphasized that the N-heterocyclic units (imidazole and triazole) and carboxyl and hydrogen-bonding networks in the frameworks all play crucial roles in the transmission of proton conductivity. Our research offers more choice for the preparation of desired proton-conductive materials.

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sulfonic acid, phosphoric acid, NH4+, and H+) into MOFs is the simplest and most effective way. For instance, Zhou′s group has introduced imidazole molecules into Fe-MOF to obtain a novel compound, Im−Fe-MOF, with an excellent conductivity of 1.21 × 10−2 S·cm−1.21 Kitagawa’s group has successfully synthesized a ZnII-MOF containing NH4+ ions with a conductivity of 0.8 × 10−2 S·cm−1.22 Liu’s group has prepared a MOF containing mixed-metal ions, whose conductivity is 1.11 × 10−3 S·cm−1.23 Several reviews have concluded the latest progress of proton-conducting MOFs.16−20 However, the synthesis of high proton-conductive MOFs with high water and chemical stabilities is still a challenging topic. Lots of previously described MOFs are sensitive to water especially the acidic or basic solutions because of the lability of ligand−metal bonds.24 Low water stability is a fatal disadvantage for solid proton conductors. Also, MOFs with high chemical stability will benefit their real-world various applications.

INTRODUCTION Proton conduction is an important phenomenon not only in ecosystems but also in solid materials and is the core of fuel cells.1−3 In addition to the traditional commercial perfluorosulfonic acid films, Nafion and Nafion-like films,4 used as proton conductors, there has been strong interest in seeking promising crystalline solids, covalent organic frameworks,5 hydrogen-bonded organic frameworks,6 and metal−organic frameworks (MOFs)/coordination polymers (CPs)7−20 as novel proton conductors. In this context, MOFs/CPs as rapidly developing crystalline materials have received wide attention as a good class of solid electrolyte materials because of their structural variability and controllability and rich hydrogen-bonding networks and water clusters conducive to fast and efficient proton exchange. Therefore, efforts have been devoted to obtaining high proton conductivity or modulating the proton conductivity of MOFs.7−20 Up to now, a large number of studies have shown that the introduction of proton carriers (imidazole, carboxylic acid, © XXXX American Chemical Society

Received: January 29, 2019

A

DOI: 10.1021/acs.inorgchem.9b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

0.05 mmol), Cd(NO3)2·4H2O (15.4 mg, 0.05 mmol), and 7.0 mL of deionized water was transferred into a 25 mL Teflon-lined autoclave and heated at 150 °C for 4 days. After natural cooling to 25 °C, paleyellow bulk crystals of 1 were produced, washed with water, and airdried. Yield: 65.7% based on Cd. Elem anal. Calcd for C26H18N10O9Cd: C, 42.92; H, 2.48; N, 19.25. Found: C, 42.55; H, 2.24; N, 18.97. IR (cm−1, KBr): 3438 (s), 3154 (m), 2926 (m), 1710 (s), 1533 (s), 1512 (s), 1381 (m), 1284 (s), 1227 (w), 1142 (m), 1052 (w), 976 (s), 900 (w), 858 (m), 828 (w), 780 (w), 744 (m), 714 (w), 670 (m), 662 (w), 565 (m). Compound 2 was synthesized by a solvothermal method: A mixture of DMPhH3IDC (13.0 mg, 0.05 mmol), Sr(NO3)2 (22.1 mg, 0.1 mmol), and CH3CN/water (1/6; 7.0 mL) was transferred into a 25 mL Teflon-lined autoclave and heated at 150 °C for 4 days. After natural cooling to 25 °C, colorless transparent bulk crystals of 2 were obtained, washed with water, and air-dried. Yield: 54.5% based on Sr. Elem anal. Calcd for C26H18N4O8Sr: C, 51.47; H, 3.63; N, 9.23. Found: C, 51.12; H, 4.01; N, 9.03. IR (cm−1, KBr): 3438 (m), 3096 (m), 2942 (w), 1734 (w), 1692 (s), 1613 (w), 1542 (s), 1399 (m), 1378 (m), 1260 (m), 1228 (w), 1141 (w), 1105 (w), 1056 (m), 1022 (w), 976 (w), 950 (w), 916 (w), 879 (s), 734 (s), 707 (w), 669 (w), 549 (m), 513 (m), 494 (w), 443 (w). Crystal Structure Determinations. The crystallographic data of the two compounds are shown in Table 1. Crystal samples of appropriate

On the other hand, to synthesize high proton-conductive MOF-based materials, it is necessary to understand the proton transport sites and mechanisms. Usually, the proton transport sites are divided into three types: (1) acid proton-transport sites, such as −COOH, −OHSO2, PO3H2−, etc. (in general, the number of H+ of acidic groups dissociating protons determines the conductivity of the MOF protons);25 (2) basic proton-transport sites, such as NH2−, −NH2−, chitosan, etc.;26 (3) acid−base to proton-transfer sites.27 These protons can be combined with adjacent water molecules or backbone molecules to transport protons along the pore or hydrogen-bonding grid. The general proton-conduction mechanism is mainly the Grotthuss and Vehicle mechanisms.28,29 However, a deeper and clear understanding of the proton-conduction mechanism is still a challenging topic. On the basis of the above considerations, we have recently developed a strong interest in the study of the proton conductivity of MOFs constructed with imidazole dicarboxyate derivatives as bridging ligands. The reason lies in the two following aspects. First, the imidazoledicarboxylic acid ligand itself contains the imidazole and carboxylic acid groups, which are both good proton carriers and building blocks of hydrogen bonds. Second, previous studies have found that such ligands have strong coordination abilities and varied coordination fashions30−35 and can prepare various MOFs with good stability, which is very conducive to the investigation of proton conduction. Up to now, although the proton conductivities of some MOFs built by imidazoledicarboxylate ligands have been preliminarily studied by our group,36−44 the related research is very limited, and much work is still needed in order to reveal the structure−activity relationship in depth. Therefore, herein, our group selected two substituted imidazoledicarboxylate ligands, 2-p-(1H-1,2,4-triazolyl)phenyl-1H-4,5-imidazoledicarboxylic acid (p-TIPhH3IDC; Scheme S1a) and 2-(3,4-dimethylphenyl)-1H-imidazole-4,5dicarboxylic acid (DMPhH3IDC; Scheme S1b) to synthesize two novel MOFs, {[Cd(p-TIPhH2IDC)2]·H2O}n (1) and [Sr(DMPhH2IDC)2]n (2), with high water and chemical stabilities. Interestingly, the conductivity values of 1 and 2 can reach up to 1.24 × 10−4 and 0.92 × 10−3 S·cm−1, respectively, at 98% relative humidity (RH) and 100 °C. Note that the optimized conductivity of 1 is higher than the value of most previous conductive Cd-MOFs and of 2 is the highest value among the previous conductive Sr-MOFs. Their protonconductive mechanisms will be highlighted herein.

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Table 1. Crystallographic Data and Structure Refinement Information for 1 and 2 Dr,max and Dr,min/e·Å−3 empirical formula fw temperature/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z Dc/g·cm−3 Μ/mm−1 F(000) 2θ range for data collection (deg) index ranges

EXPERIMENTAL SECTION

Reagents and Apparatus. All reagents adopted are of analytical purity herein. The ligands p-TIPhH3IDC and DMPhH3IDC were prepared by the previous literature.45,46 Elemental analysis was tested on a FLASH 1112 elemental analyzer. The IR spectra were determined by a Nicolet NEXUS 470 Fourier transform infrared (FTIR) spectrometer (400−4000 cm−1; KBr pellet). Thermal gravimetric analysis (TGA) was conducted on a Netzsch STA 409PC synchronous thermal analyzer (heating rate of 10 °C·min−1; in air). Powder X-ray diffraction (PXRD) was performed on a Panalytical X’pert PRO X-ray diffractometer (λ = 1.5418 Å). SEM images were obtained on a Zeiss SIGMA 500 SEM apparatus. A water vapor adsorption test was conducted with a 3H-2000PW multistation gravimetric adsorption instrument [BeiShiDe Instrument Technology (Beijing) Co. Ltd.]. N2 adsorption/desorption isotherms were obtained by a ASAP 2420 adsorptometer at 77 K. Preparation of MOFs 1 and 2. Compound 1 was synthesized by a hydrothermal method: A reactant mixture of p-TIPhH3IDC (15.0 mg,

reflns collected indep reflns data/restraints/ param GOF on F2 final R indexes [I⩾ 2σ(I)] final R indexes (all data)

1

2

0.32 and −0.57

0.521 and −0.387

C26H18CdN10O9 726.9 293(2) monoclinic P21/n 10.0030(3) 17.0024(6) 16.0933(5) 90 92.0870(10) 90 2735.25(15) 4 1.765 0.874 1456 5.44−55.2

C26H18SrN4O8 602.06 273.15 tetragonal I41/a 16.7950(5) 16.7950(5) 17.5814(5) 90 90 90 4959.2(3) 8 1.613 2.234 2432 5.766−55.064

−13 ≤ h ≤ 13, −22 ≤ k ≤ 22, −18 ≤ l ≤ 20 57942 6316 (Rint = 0.0384) 6316/0/433

−21 ≤ h ≤ 21, −21 ≤ k ≤ 21, −22 ≤ l ≤ 22 22067 2853 (Rint = 0.0262) 2853/12/179

1.052 1.173 R1 = 0.0206, wR2 = 0.0544 R1 = 0.0318, wR2 = 0.1021 R1 = 0.0259, wR2 = 0.0565 R1 = 0.0368, wR2 = 0.1048

size were selected from the mother liquor and collected on a Bruker Smart 1000 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The crystal structure was resolved by direct methods, extended by the FTIR technique, and refined by anisotropy. Finally, a full-matrix least-squares method was adopted to correct the variable parameters and observable diffraction data. All of the data were corrected by the Lorentz polarization factor. Direct B

DOI: 10.1021/acs.inorgchem.9b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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method. The H atoms adopted isotropic thermal parameters, and all atoms except H used the anisotropic thermal parameter method. The calculation used was the SHELXT47 program. Selected bond distances and angles and hydrogen-bonding parameters are deposited in Tables S1 and S2, respectively. Proton Conductivity Measurement. Approximately 30 mg of the sample of compound 1 or 2 was weighed and pressed into a disk having a diameter of 0.5 mm under a pressure of 2 MPa. The resistance was measured on a PARSTAT 2273 Electrochemical Workstation over 1 Hz to 1 MHz with an external voltage of 100 mV. The wafer was attached to a couple of silver electrodes, put in a quasi-fourprobe electrochemical cell, and fully hydrated at least 24 h at different RHs. The determinations were made under 30−100 °C and 75−98% RH. The impedance plots were fitted by ZSimpWin software. The conductivity was calculated according to the formula σ = L/RS. The total resistance (R) of the pellet could be obtained by arc extrapolation to the Z′ axis, S is the surface area of the wafer, and L is the thickness of the wafer. The activation energy (Ea) was acquired by the equation Tσ = σ0 exp(−Ea/kT). Analysis on the Impedance Plots. The ZSimpWin program was used to obtain the equivalent circuit diagrams R(QR(CR)(RW))(CR) for 1 and R(C(R(Q(R(C(RW)))))) for 2 under 30 or 100 °C and 98% RH. Detailed fits are shown in Figures S1 and S2.

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RESULTS AND DISCUSSION Crystal Structures of 1 and 2. MOF 1 belongs to the monoclinic system and P2 1 /n space group. The sixcoordinated CdII ion is coordinated by two O and four N atoms, in which the O8, N3, O3, and N1 atoms are derived from two organic ligands and N7 and N12 are from the two

Figure 1. (a) 1D chain of 1. (b) 3D supramolecular structure of 1. methods were used to obtain all of non-H atom coordinates, and the H-atom coordinates were obtained by the difference FTIR synthesis

Figure 2. (a) 3D structure of 2. (b) Topological consideration of the 3D network of 2.

Figure 3. PXRD patterns of 1 (a) and 2 (b): simulated from the single-crystal data, as-prepared, and after water treatment of solids. C

DOI: 10.1021/acs.inorgchem.9b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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O and N atoms involve coordination to the SrII atom. The Sr− O lengths are between 2.4845(16) and 2.7624(16) Å, and the Sr−N length is 2.7456(16) Å. The bond angles surrounding the SrII atom vary from 63.24(2) to 155.36(5)°. Each HDMPhIDC2− ligand employs a μ3-k-N,O-k-O′-k-O′′ connection fashion (Figure S4b) linking neighboring SrII ions to construct a complicated 3D structure (Figure 2a). To understand the framework better, the structure of 2 was simplified as a topology symbol of 6^{6} (Figure 2b). It is worth noting that there are hydrogen bonds in 2, and the presence of 1D channels in the 3D framework facilitates the formation of protontransfer channels. TGA of 1 and 2. TGA experiments of MOFs 1 and 2 were performed to determine their thermal stability (Figure S5). MOF 1 completely removed one crystallization water in the range of 30−250 °C (obsd 2.2%; calcd 2.48%). The second weight loss of 31.12% (calcd 32.73%) from 316.2 to 399 °C could be inferred to lose the triazole and carboxyl groups. From 399 to 518 °C, another carboxylate group was lost. Then the continuous weight loss up to 633 °C can be presumed to lose the remaining part of the organic ligand (obsd 34.98%; cald 38.78%). The remaining residue is CdO (obsd 17.66%; calcd 19.20%). Compound 2 has a very high thermal stability and can maintain structural stability before 332 °C. The sharp weight loss in the range of 332−464 °C corresponds to the

Figure 4. SEM images of MOFs 1 and 2: (a) as-synthesized crystals of 1; (b) water-treated crystals of 1; (c) as-synthesized crystals of 2; (d) water-treated crystals of 2.

triazole moieties of another two organic ligands (Figure S3a). Each organic ligand p-TIPhH2IDC− coordinates with a metal atom using the coordination mode μ2-k-N,O-k-N′ (Figure S3b). The Cd−N distances are from 2.3123(13) to 2.3855(15) Å, and the Cd−O distances are 2.3141(11) and 2.3519(12) Å, which are consistent with the values of related cadmium(II) imidazoledicarboxylate-based MOFs.30,31,45 The bond angles surrounding the CdII ion are in the range of 72.71(4)− 179.42(5)°, suggesting a distorted octahedron. The p-TIPhH2IDC− anion links the adjacent CdII ions by N,O-chelated or N-bridged means, forming a double-stranded chain (Figure 1a). The intrachain Cd···Cd distance is 10.8031(2) Å. Consequently, these chains interact with an intermolecular force to obtain a 3D supramolecular structure (Figure 1b). It should be noticed that there are a large number of uncoordinated N and O atoms and free water molecules in 1, which can form complicated hydrogen-bonding networks with exotic water molecules. That is an important factor for the formation of proton-conductive materials. MOF 2 is assigned to the tetragonal I41/a space group and presents a complicated 3D structure. The symmetric unit contains an eight-coordinated SrII ion and six H2DMPhIDC− anions (Figure S4a). Among them, the N2, O4, N2#1, and O4#1 atoms are from two organic ligands, the O#2, O#3, O#4, and O#5 atoms are from four distinct organic ligands, and SrII is located in the distorted center of the octahedron. Different from the previous SrII-MOFs with the same ligand,48 both the

Figure 6. SEM images of 1 and 2: (a) crystals of 1 immersed in a solution of pH = 1 for 24 h; (b) crystals of 1 immersed in a solution of pH = 11 for 24 h; (c) crystals of 2 immersed in a solution of pH = 1 for 24 h; (d) crystals of 2 soaked in a solution of pH = 11 for 24 h.

Figure 5. PXRD patterns of 1 (a) and 2 (b): simulated and after soaking of the solid in various pH solutions. D

DOI: 10.1021/acs.inorgchem.9b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Impedance spectra of 1 from 30 to 100 °C under 98% RH.

Figure 8. Impedance spectra of 2 from 30 to 100 °C under 98% RH.

indicate very similar surfaces and morphologies, indicating their stability toward acidic or basic solutions. N2 and Water Vapor Adsorption of 1 and 2. The porosities of the two MOFs were studied by N2 adsorption/ desorption under −196 °C. Both adsorption isotherms were expressed as type III isotherms (Figure S7). The saturated absorption rates of MOFs 1 and 2 are 14.30 and 18.75 cm3·g−1, respectively. The Brunauer−Emmett−Teller surface areas are 5.42 m2·g−1 for 1 and 5.62 m2·g−1 for 2. The average pore diameters of adsorption are 15.57 and 17.44 nm for 1 and 2, respectively. Figure S8 shows the water adsorption/desorption isotherms under 25 °C. With P0/P equal to 0.05, the adsorption amounts of water molecules reached 53.38 and 68.21 mg·g−1 of MOFs 1 and 2, respectively. As the relative pressure was increased, the water adsorption amounts also gradually increased until P/P0 was 0.95. The maximum water vapor adsorption amounts are 103 and 120 mg·g−1 for 1 and 2, respectively. These absorbed water molecules can exist in the pores or channels of the frameworks of 1 and 2. Proton Conduction of 1 and 2. Proton conduction was studied by the alternating-current (ac) impedance method. The Nyquist plots for 1 demonstrate similar features, with one semicircle at high frequency and a characteristic spur at low frequency showing the blocking of H+ ions at Ag electrodes (Figures 7 and S9−S11).49 For 2, in the ac impedance plots (Figures 8 and S12−S14), there are two obvious depressed semicircles at high or low frequencies corresponding to the bulk phase and grain boundary, respectively.50,51 The tail that is not apparent at low frequency may be caused by the mobile

decomposition of 3,4-dimethylbenzene, imidazole, and one carboxylate groups on the organic ligand (obsd 65.46%; calcd 63.96%). Then another weight loss process can be found from 464 to 981 °C, being the decomposition of the rest of the organic ligand. The final SrO residue (obsd 17.41%; calcd 17.20%) remains. The excellent structural stability of these two complexes demonstrated before 250 or 316 °C provides a good basis for further electrochemical testing. Water and Chemical Stabilities of MOFs 1 and 2. The water and chemical stabilities of the MOFs play key roles in real-world applications. We know that the PXRD test is a very powerful tool to demonstrate the framework stability of the MOFs. The PXRD patterns of the crystalline samples for 1 or 2, which were immersed in water for 7 days or heated under reflux for 1 day, were recorded and compared with the simulated ones from the single-crystal data. As shown in Figure 3, the PXRD peaks of water-treated solids overlap their simulated ones very well, suggesting strong structural stabilities. In addition, as displayed in Figure 4, the crystal morphology of 1 or 2 remains basically unchanged after water treatment. As illustrated in Figure S6, the PXRD patterns of 1 and 2 with evacuation for 24 h matched their simulated ones well, indicating high stability under vacuum. To study the chemical stabilities of the two MOFs, we immersed the solids in various pH aqueous solutions (pH = 1−11) for 24 h. The overlapping PXRD peaks before and after immersion showed that the two MOFs could maintain their structural rigidity (Figure 5). Also, the morphologies of the two compounds were characterized by SEM. As shown in Figure 6, both MOFs E

DOI: 10.1021/acs.inorgchem.9b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ions being blocked by the electrode−electrolyte interfaces and/or electrode polarization.52 At 30 °C and 75−93% RHs for 1 and at 30 °C and 75−85% RHs for 2, the Nyquist plots show irregular and disordered electrochemical data. This may be in the low-humidity and low-temperature conditions, the adsorption water units are limited, and the water units could not obtain the necessary activation energy to form H+ or H3O+ ions. First, we explored the conductivity of 1 or 2 under constant RH and varying temperatures. For 1, the proton conductivities vary from 5.72 × 10−7 S·cm−1 (40 °C) to 5.57 × 10−5 S·cm−1 (100 °C), and the conductivities of 2 have similar trends of 1.07 × 10−7 S·cm−1 at 40 °C and 1.71 × 10−5 S·cm−1 at 100 °C. From Nyquist plots, it can be seen that, as the temperature rises, the semicircular arc becomes smaller (Figures S9−S14, 7, and 8). As denoted in Tables 2 and 3, the conductivities of both MOFs increase with increasing temperature at settled RH, which is consistent with the phenomena described in the literature.37−40,53 For example, the σ values of 1 are from 5.99 × 10−7 S·cm−1 (40 °C) to 6.98 × 10−5 S·cm−1 (100 °C) (Figure S10) under 85% RH [at 93% RH the σ values are from 1.16 × 10−6 S·cm−1 (40 °C) to 7.96 × 10−5 S·cm−1 (100 °C) (Figure S11); at 98% RH the σ values are from 1.69 × 10−5 S·cm−1 (30 °C) to 1.24 × 10−4 S·cm−1 (100 °C) (Figure 7)]. For 2, the conductivities at 85% RH vary from 1.71 × 10−7 S·cm−1 (40 °C) to 6.20 × 10−5 S·cm−1 (100 °C) (Figure S13) [at 93% RH they are from 4.03 × 10−7 S·cm−1 (30 °C) to 5.43 × 10−4 S·cm−1 (100 °C) (Figure S14); at 98% RH, they are from 1.31 × 10−5 S·cm−1 (30 °C) to 0.92 × 10−3 S·cm−1 (100 °C) (Figure 8)]. At settled temperature, the two MOFs exhibit humiditydependent conducting behaviors (Tables 2 and 3). For example, the conductivities of 1 increase from 5.57 × 10−5 S·cm−1 (75% RH) to 1.24 × 10−4 S·cm−1 (98% RH) under 100 °C. In general, the conductivities of MOFs 1 and 2 can be heightened with an increase of the temperature and humidity.

Table 4. Comparison of the Proton Conductivities of MOFs 1 and 2 with Those of Some Related MOFs under Similar Tested Conditions MOFa

−1

Table 2. Proton Conductivities (S·cm ) of 1 and 2 at Different RHs and Temperatures

30 40 50 60 70 80 90 100

75 5.72 1.70 6.01 1.22 2.25 3.67 5.57

× × × × × × ×

85 −7

10 10−6 10−6 10−5 10−5 10−5 10−5

5.99 1.87 6.15 1.25 2.57 4.63 6.98

× × × × × × ×

93 −7

10 10−6 10−6 10−5 10−5 10−5 10−5

1.16 2.26 6.34 1.30 3.15 4.06 7.96

× × × × × × ×

98 −6

10 10−6 10−6 10−5 10−5 10−5 10−5

1.69 2.41 3.05 4.65 5.96 8.23 1.08 1.24

× × × × × × × ×

10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4

RH (%) 30 40 50 60 70 80 90 100

75 1.07 1.36 3.46 3.83 7.06 1.05 1.71

× × × × × × ×

85 10−7 10−6 10−6 10−6 10−6 10−5 10−5

1.71 1.45 5.38 1.50 3.22 4.94 6.20

× × × × × × ×

93 10−7 10−6 10−6 10−5 10−5 10−5 10−5

4.03 1.42 6.00 1.23 3.25 8.51 3.44 5.43

× × × × × × × ×

98 10−7 10−6 10−5 10−5 10−5 10−5 10−4 10−4

1.31 1.71 2.86 6.05 1.31 3.53 5.32 0.92

× × × × × × × ×

3.61 × 10−3

{Na[Cd(MIDC)]}n

1.04 × 10−3

{Cd2(D-pmpcH)(H2O)2Cl2}n

1.38 × 10−4

[Cd(HDMPhIDC)(H2O)]n

1.30 × 10−4

[Cd(p-TIPhH2IDC)2(H2O)]n (1) [Cd2(btc)2(H2O)2]n·nH2bmib· 6nH2O {[Cd2L3(DMF)(NO3)]·2DMF· 3H2O}n [Cd4(cpip)2(Hcpip)2]n· nH2bmib·nH2O {[Cd2(LA)3(DMF)(NO3)] (DMF)3(H2O)8}n [Cd0.5(tpy)]·0.75H2O

1.24 × 10−4 5.4 × 10−5 3.25 × 10−5 2.2 × 10

−5

1.3 × 10−5 6.03 × 10−6

[Cd5(TCA)2(H2O)2]·8DMA· 16H2O [Cd(L-tart)(bpy)(H2O)]n· 9nH2O [CdL0.5(DMF)2]n

1.45 × 10−6

[Sr(H2DMPhIDC)2]n (2)

0.92 × 10−3

{[Sr2(BPTC)(H2O)6]·H2O}n

2.7 × 10−4

Sr-SBBA

4.40 × 10−5

[Sr(2,5-pzdc)(H2O)4]·H2O

0.91 × 10−5

[Sr(H2PhIDC)2(H2O)4]·2H2O

1.91 × 10−6

1.3 × 10−6 2.49 × 10−7

28 °C and 98% RH 100 °C and 98% RH 50 °C and 97% RH 100 °C and 98% RH 100 °C and 98% RH 60 °C and 85% RH 25 °C and 98% RH 60 °C and 85% RH 25 °C and 98% RH 95 °C and 97% RH 80 °C and 85% RH 85 °C and 95% RH 95 °C and 95% RH 100 °C and 98% RH 90 °C and 98% RH 25 °C and 98% RH 20 °C and 98% RH 90 °C and 98% RH

ref 61 44 62 39 this work 63 64 63 64 65 66 67 68 this work 53 54 55 40

The reason may be that, with an increase of the humidity, the number of water molecules adsorbed by the frameworks also increases, leading to an increase in the proton carriers. In addition, these extra absorbed water molecules could constitute more useful hydrogen-bonding networks with O and N atoms of the organic ligands, which can benefit the transport of protons in the MOFs.53−60 As illustrated in Table 4, the optimized conductivity of MOF 1 (1.24 × 10−4 S·cm−1 under 98% RH and 100 °C) is only lower than the value of one N-heterocyclic carboxylatebased MOF, [Cd-5TIA]61 (5-TIA = 5-triazoleisophthalic acid), but close to that of three MOFs39,44,62 and much higher than that of most reported CdII-MOFs.63−69 For 2, the optimized conductivity of 0.92 × 10−3 S·cm−1 at 98% RH and 100 °C is the largest value among the described Sr-MOFs.40,53−55

Table 3. Proton Conductivities (S·cm−1) of 2 at Different RHs and Temperatures temp (°C)

Cd-5TIA

tested conditions

a 5-TIA = 5-triazoleisophthalic acid; H3MIDC = 2-methyl-1Himidazole-4,5-dicarboxylic acid; D-H3pmpc = D-1-(phosphonomethyl)piperidine-3-carboxylic acid; bmib = 1,4-bis(2-methylimidazol-1′-yl)butane; L = 1,3-bis(4-carboxyphenyl)imidazolium chloride; H2LA = 1,3-bis(4-carboxyphenyl)imidazolium; tpy = 2-1H-1,2,4-triazol-5ylpyrazine; TCA = 4,4′,4″-tricarboxytriphenylamine; tart = tartaric acid; bpy = 4,4-bipyridine; DMF = dimethylformamide; BPTC = 2,2′,6,6′-tetracarboxybiphenyl; SBBA = 4,4′-sulfobis(benzoic acid); 2,5-H2pzdc = pyrazine-2,5-dicarboxylic acid; H3PhIDC = 2-phenyl4,5-imidazoledicarboxylic acid.

RH (%) temp (°C)

σ (S·cm−1)

10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−3 F

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Figure 9. Arrhenius plots of 1 (a) and 2 (b).

Figure 10. PXRD patterns of 1 and 2: simulated ones and samples after impedance measurements under 75, 85, 93, and 98% RHs, respectively.

substituents carried by the organic ligands in the frameworks. However, because of the complexity of the proton conduction mechanism, a more accurate explanation is required for further study. As can be seen from Figure 10, the well-matched PXRD patterns of MOFs 1 and 2 can be observed before and after electrochemical experiments, indicating the durability of the material used, toward various RHs, which is very important for practical applications. It can be seen from the SEM images (Figures S15 and S16) that the morphology of MOFs 1 and 2 did not change before and after the proton conductivity test. In addition, in order to ensure the practical application of both MOFs, we recorded the relationship between the proton conductivity and time at 98% RH and 100 °C. As indicated in Figure S17, we can see that the proton conductivities remain almost constant for both MOFs even after 8 h.

The activation energies (Ea) of MOFs 1 and 2 were calculated. As displayed in Figure 9, the Arrhenius plots indicated a perfect linear fit. Ea values of compound 1 at 98% and 75% RH are 0.32 and 1.11 eV, respectively. Obviously, two different conducting mechanisms lie in the compound.21 At 98% RH, Ea is 0.32 eV lower than 0.4 eV, indicating a Grotthuss mechanism.28,29 According to structural analysis, N atoms and carboxylic groups, which are not involved in coordination in MOF 1, can build up hydrogen-bonding networks with adsorbed water units. In the network, protons are passed through the hydrogen-bonding network to adjacent lattice water molecules. With less than 75% RH, Ea is 1.11 eV because water molecules as proton carriers require an endothermic process to dissociate during proton transfer, thereby forming various hydrated ions required for transport. It can be considered that the transfer of protons in compound 1 under low-humidity conditions is a vehicle mechanism.28,29 The Ea values for MOF 2 at 98% and 75% RH are 0.67 and 0.52 eV, respectively, suggesting a vehicle mechanism.28,29 As exotic water molecules entered the 1D channels, these absorbed water molecules can gather around the imidazoledicarboxylic acid ligands. Consequently, the −COOH groups can interact with the exotic water molecules to form H3O+ ions, which can migrate along the 1D hydrophilic channels, and the protons are transported. Note that, at the same temperature and RH (e.g., 98% RH), the Ea value of MOF 2 is higher than that of MOF 1. However, at 75% RH, the result is adverse. There are two possible reasons for this: one is the structural difference (1 is a 1D structure, while 2 is a 3D structure), and the other is the difference of the phenyl

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CONCLUSION Two substituted imidazoledicarboxylate-based proton-conductive MOFs were successfully synthesized, and their proton conduction was fully explored under water vapor conditions. The results demonstrate that both compounds feature temperature- and humidity-dependent conducting behavior. Their best proton conductivities under 100 °C and 98% RH can reach 1.24 × 10−4 S·cm−1 for 1 and 0.92 × 10−3 S·cm−1 for 2, which are higher than those of most reported CdII- or SrIIMOFs. At the same time, we investigated their water and chemical stabilities by PXRD and SEM determination. The time-dependent measurements reveal that the MOFs also have excellent electrochemical stability. The conductive mechanism G

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has been discussed according to the structural analyses, Ea calculations, and water and N2 adsorptions. Our research offers more choice for the design and preparation of new crystalline proton-conductive 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/acs.inorgchem.9b00274. Details of the crystal data, impedance analysis, PXRD patterns, and gas adsorption/desorption (PDF) Accession Codes

CCDC 1520155 and 1817135 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gang Li: 0000-0001-9049-4208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 21571156 and J1210060). REFERENCES

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J

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