Effective Approach to Promoting the Proton Conductivity of Metal

Jul 19, 2017 - We explored the proton conductivities of two 3D CoII metal–organic frameworks (MOFs), {[Co3(m-ClPhIDC)2(H2O)6]·2H2O}n [1; m-ClPhH3ID...
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Effective Approach to Promoting the Proton Conductivity of Metal− Organic Frameworks by Exposure to Aqua−Ammonia Vapor Xi Liang, Bin Li, Minghao Wang, Jing Wang, Ruilan Liu, and Gang Li* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China S Supporting Information *

ABSTRACT: We explored the proton conductivities of two 3D CoII metal−organic frameworks (MOFs), {[Co3(mClPhIDC)2(H2O)6]·2H2O}n [1; m-ClPhH3IDC = 2-(m-chlorophenyl)imidazole-4,5-dicarboxylic acid] and {[Co3(pClPhHIDC)3(H2O)3]·6H2O}n (2; p-ClPhH3IDC = 2-(p-chlorophenyl)imidazole-4,5-dicarboxylic acid), under water and aqua−ammonia vapors, respectively. The experimental results revealed that the proton conductivities of 1 and 2 at aqua− ammonia vapor were 2.89 × 10−2 and 4.25 × 10−2 S/cm, respectively, and approximately 2 orders of magnitude greater than those at water vapor. On the basis of the activation energy, water and ammonia vapor absorption, and powder X-ray diffraction patterns, their proton-conduction mechanisms have been discussed. We believe that this is a novel approach to drastically improving the proton conductivity of MOFs. KEYWORDS: substituted imidazole dicarboxylate, CoII MOFs, proton conduction, promotion, aqua−ammonia vapor

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imidazole-4,5-dicarboxylic acid (H3IDC) and its 2 position on the imidazole ring-substituted derivatives have indicated outstanding coordination features and various coordination modes.12−14 By such ligands as mentioned above, many MOFs with pleasing structures have been built up, but studies on the proton conduction are very scarse. Now considering the promising structural features of imidazole dicarboxylate based MOFs (they contain both imidazole and carboxylate groups, which may be helpful for the proton-conducting pathway), we hope to first explore the proton conductivity of relevant MOFs under different humidities. On the other hand, previous research demonstrated that an ammonium ion (NH4+) was directly introduced into the pores as a counterion and effectively improved the proton conductivity of MOFs.15−18 Inspired by these results, we want to know what will happen if crystalline MOFs are exposed to aqua−ammonia vapor during testing. Our experimental results indicate that there is remarkable promotion of the proton conductivity of imidazole

he solid materials with high proton conduction have attracted much attention because of their potential applications to fuel cells. Recently, the investigations about proton conduction on metal−organic frameworks (MOFs) illustrate that MOFs are an emerging family of protonconductive materials.1−4 The reasons why people choose MOFs as proton conductors mainly lie in two aspects: one is that the single crystals of MOFs can be easily obtained and structurally characterized by X-ray crystallography,1,2 which may provide a better understanding of the proton-conduction mechanism; the other is that the structures, especially the pores of MOFs, can be tuned in a flexible manner (choosing different metal ions or altering and modifying the multifunctional organic ligands and so on), which can improve the proton conductivity of the MOFs.1,3−7 That is to say, as a new class of proton-conduction materials, MOFs are very promising. Thus, the exploration of MOFs with high proton conductivity has become an important goal. Although people have researched the proton conductivity of some MOFs constructed by ligands with COOH−, SO3H−, or PO3H2− groups5−7 or introduced by protonic guest molecules like imidazole, triazole, histamine, and so on,8−11 there is no example concerning imidazole dicarboxylate based MOFs as proton conductors. From previous reports, we found that 1H© 2017 American Chemical Society

Received: May 31, 2017 Accepted: July 18, 2017 Published: July 19, 2017 25082

DOI: 10.1021/acsami.7b07635 ACS Appl. Mater. Interfaces 2017, 9, 25082−25086

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ACS Applied Materials & Interfaces

maintain transport of the protons below 100 °C. At relatively high temperature, water retention of the two complexes benefits the proton conduction. To determine the stability of 1 and 2 toward water, the complexes were soaked in deionized water for 1 week or refluxed for 12 h under boiling water, filtered, dried at room temperature, and subsequently subjected to powder X-ray diffraction (PXRD) characterization. It can be seen in Figure S1a,b that the PXRD peaks of the samples before and after water treatment are consistent with the PXRD peaks simulated by the single-crystal data. At the same time, the crystalline samples of 1 or 2 were placed in contact with vapors from an NH3·H2O solution of 7.4 M in a closed container over 1 week. Then, the samples were air-dried and determined by PXRD measurements. As indicated in Figure S2a,b, the experimental patterns matched well the simulated ones based on the singlecrystal structures of 1 and 2. The results confirmed the better stability of the complexes toward water or aqua−ammonia vapor, which supplies a good foundation as proton transfer. The porosity of MOFs 1 and 2 was determined by N2 absorption and desorption experiments at 77 K. In accordance with the IUPAC classification (Figure S3), both absorption isotherms reveal type II isotherms. The Brunauer−Emmett− Teller surface areas are 13.79 and 4.97 m2/g for 1 and 2, respectively. The saturated uptakes are 20.64 and 9.18 cm3/g for 1 and 2, respectively. Assessments of the Barrett−Joyner− Halenda (BJH) method simulation from the N2 absorption curves show that the average pore sizes are 7.31 and 8.48 nm for 1 and 2, respectively. The proton conduction of compounds 1 and 2 was measured as a function of the relative humidity (RH) and temperature by alternating-current (ac) impedance spectroscopy. For 1, at 30 °C and 68−93% RHs, because of the irregular impedance pattern and disorderly data points, it was not possible to find the corresponding impedance value at different humidities (Figure S4 and Table S2). The conductivity of 1 increases with increasing RH, and the plots tend to be smooth at 98% RH with a conductivity value of 2.73 × 10−6 S/cm. This may be in the low-humidity condition and adsorption in the 1D channel limited the proton carrier, while at the low-temperature condition, the water molecules did not reach ionization into the H+ or H3O+ required activation energy, resulting in protons that cannot pass and with low-conductivity values. To check the effect of the RH on the conductivity at a fixed temperature, we continue to measure the impedance of 1 by heating to 100 °C. The conductivity value was found to be 8.69 × 10−5 S/cm at 68% RH, and it increased to 7.62 × 10−4 S/cm at 98% RH (Figure 3a and Table S3). The increased conductivity with an increase in the humidity is due to, under high humidity, more water molecules entering the 1D channel, where additional absorption of water molecules and carboxyl oxygen atoms is built into the hydrogen-bonding network and the extra adsorbed water molecules act as proton carriers for proton transport. To study the effect of temperature on the proton conductivity, we measured the impedance under the same humidity conditions by changing the temperature. Figure 3b shows the Nyquist plots from 30 to 100 °C at 98% RH (Table S4). In 1, the conductivity attains a value of 2.61 × 10−6 S/cm at 30 °C, and it gradually increases with increasing temperature and obtains a maximum value of 7.62 × 10−4 S/cm at 100 °C. From above observations, we anticipate that the increased acidity (pKa) of water molecules with an increase in the temperature enhances the proton conductivity under high RH.

dicarboxylate based 3D MOFs from water to ammonia−water vapors. Herein, we selected two 3D CoII MOFs, {[Co3(mClPhIDC)2(H2O)6]·2H2O}n (1)13 and {[Co3(p-ClPhHIDC)3(H2O)3]·6H2O}n (2),14 based on substituted imidazole dicarboxylate ligands, 2-(m-chlorophenyl)imidazole-4,5-dicarboxylic acid (m-ClPhH 3 IDC) and 2-(p-chlorophenyl)imidazole-4,5-dicarboxylic acid (p-ClPhH3IDC) with infinite 1D channels to investigate their proton conductivity. The synthesis and characterization of 113 and 214 have been previously reported by our team. To express both MOFs as candidates for proton conductors, we purposely give their 3D crystal structures in Figures 1 and 2.

Figure 1. (a) View of 1D channels with water molecules (red dots present water oxygen atoms, and green presents hydrogen bonds). (b) 1D water chain built by hydrogen bonds and carboxyl oxygen atoms.

Figure 2. (a) Guest water molecules flooded in the 1D channels (red dots present water oxygen atoms). (b) 1D water chains supported by hydrogen bonds.

As shown in Figure 1a, 1 contains infinite cylindrical 1D channels, in which the six coordination and two free water molecules of one asymmetric unit all involve many hydrogen bonds. Figure 1b indicates a 1D water chain constructed by hydrogen bonds (Table S1) and carboxyl oxygen atoms that will be more favorable in proton transfer. As indicated in Figure 2a, the MOF 2 is still a 3D structure with 1D open channels like 1. Nevertheless, their structures have some differences: the type of channel and number of coordination and crystallization waters. Different from 1, the asymmetric unit of 2 includes three coordination and six free water molecules. Similarly, these water molecules in the channel form rich hydrogen bonds (O−H···O, O−H···N, and O−H··· Cl; Figure 2b and Table S1) and the consequent water chains. This allows us to study its proton transfer with greater space. As stated in the previous references,13,14 although 1 and 2 first lose lattice water molecules from 72.7 and 22 °C, respectively, the coordinated water molecules still present in the structures below 226.1 °C for 1 and 213 °C for 2. This means that some of the water molecules can continue to 25083

DOI: 10.1021/acsami.7b07635 ACS Appl. Mater. Interfaces 2017, 9, 25082−25086

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(Figure S7). Apparently, the possible pathway of proton migration in 2 belongs to the Grotthus mechanism,19−21 which can occur in the process of proton transfer among uncoordinated imidazole nitrogen atoms and hydrogen-bondrecombined water molecules. The PXRD patterns of 1 and 2 after water conductivity measurements reveal that there is no change in the skeleton, showing the stability of the frameworks (Figures S8 and S9). The water conductivities of both complexes increase with increasing temperature and humidity. The different proton-transfer mechanisms of 1 and 2 are due to their different structures built by similar organic ligands with chlorine atoms on different positions of the phenyl rings. Although the two MOFs show maximum values for proton conductivity of 7.62 × 10−4 S/cm at 100 °C under 98% RH and 2.47 × 10−4 S/cm at 90 °C under 93% RH, the values are smaller than those of some reported MOFs, such as UiO66(Zr)(COOH)2 (2.3 × 10−3 S/cm at 90 °C and 95% RH),23 UiO-66(SO3H)2 (8.4 × 10−2 S/cm at 80 °C and 90% RH),6 {H[Cu(Hbpdc(H2O)2)2[PMo12O40]]·nH2O (Hbpdc = 2,2′bipyridyl-3,3′-dicarboxylic acid) (1.25 × 10−3 S/cm at 100 °C and 98% RH), {H[Cu(Hbpdc(H2O)2)2[PW12O40]]·nH2O (1.56 × 10−3 S/cm at 100 °C and 98% RH),24 and imidazole[FeIII2FeII(μ3-O)(CH3COO)6(H2O)3] (1.21 × 10−2 S/cm at 60 °C and 98% RH).8 Therefore, we hope to explore the proton conduction of the two MOFs following exposure to different concentrations of aqua−ammonia vapor, which was obtained from various concentrations of an NH3·H2O solution ranging from 2.11 to 7.40 M. As indicated in Figures S10 and S11 and Tables S7 and S8, at 30 °C the proton conductivities of 1 and 2 increase from 7.44 × 10−7 to 4.39 × 10−5 S/cm and from 5.71 × 10−7 to 4.23 × 10−5 S/cm, respectively, with increasing concentrations of aqua−ammonia vapor. To check the effect of NH3·H2O concentrations on the conductivity at 100 °C, we measured the impedance of 1 and 2 by heating an NH3·H2O solution to 100 °C. For 1, the conductivity value is 0.92 × 10−3 S/cm at 2.11 M and increases to 2.89 × 10−2 S/cm at 7.40 M (Figure 5a and Table S9). For 2, the conductivity also steadily increases from 3.02 × 10−3 S/cm (at an NH3·H2O concentration of 2.11 M) to 4.25 × 10−2 S/cm (at an NH3·H2O concentration of 7.40 M) (Figure 5b and Table S10). Consequently, to check the temperature effect on the conductivity, we continuously explored the impedance by altering the temperature from 30 to 100 °C for 1 at an NH3· H2O concentration of 4.93 M and for 2 at an NH3·H2O concentration of 2.96 M. As depicted in Figures S12 and S13 and Tables S11 and S12, the proton conductivities of both compounds gradually increase with increasing temperature from 6.51 × 10−6 S/cm for 1 and 9.77 × 10−7 S/cm for 2 at 30 °C to the maximum values of 1.24 × 10−2 S/cm for 1 and 0.75 × 10−2 S/cm for 2 at 100 °C. Obviously, both the large concentration of aqua−ammonia vapor and high temperature benefit proton conduction of compounds 1 and 2. It is to be pointed out that the maximum proton conductivity values of 1 (2.89 × 10−2 S/cm) and 2 (4.25 × 10−2 S/cm) are close to the highest values of other MOFs ever reported8,9,25−28 and comparable to those of organic polymers, for example, Nafion, which has been used practically in fuel cells.29 The activation energies for 1 at an NH3·H2O concentration of 4.93 M were calculated to be 0.25 eV from 30 to 60 °C and 0.73 eV from 70 to 100 °C (Figure 6a). This means that, at temperatures below 60 °C, the proton conduction is the Grotthus mechanism, which shows that the main H+ cations, dissociated from NH4+ or H+(H2O)n, were transferred to

Figure 3. Impedance spectra of 1: (a) at 100 °C under different RHs; (b) from 30 to 100 °C at 98% RH.

We calculated the activation energies (Ea) for 1 under 68% and 98% RHs by the Arrhenius equation, which are 0.90 and 1.58 eV, respectively (Figure S5), and indicate that proton conduction follows the typical Vehicle mechanism.19−21 Such high activation energy values may be due to water molecules needing a thermal activation process to dissociate into various hydrated protons, H+(H2O)n.22 Afterward, the hydrated protons could move along the 1D channels or water chains of 1 to produce conduction. Compound 2 shows a proton conductivity of 1.08 × 10−7 S/ cm at 30 °C and 68% RH, and the conductivity value increases with increasing RH and reaches a value of 1.1 × 10−4 S/cm at 30 °C and 93% RH (Figure S6 and Table S5). The dependence of the conductivity on the temperature in the range 30−100 °C at 93% RH was tested, which increased with temperature, reached a maximum value of 2.47 × 10−4 S/cm at 90 °C, and was slightly larger than the conductivity of 1.47 × 10−4 S/cm at 100 °C (Figure 4 and Table S6). The slight conductivity decrease above 90 °C may be associated with the water vapor desorption rate being greater than the rate of adsorption even at 93% RH. The activation energy under 93% RH is 0.20 eV

Figure 4. Nyquist plots of 2 at 30−100 °C under 93% RH. 25084

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2 occurs before a relative pressure (P/P0) of 0.1, where the adsorbed water amount reached ca. 166 and 120 mg/g, respectively. With P/P0 increasing to 0.95, the adsorbed water molecules are gradually stored in the 1D channel of polymers, with maximum absorption values of ca. 240 mg/g for 1 and ca. 180 mg/g for 2. Apparently, there is a slight difference of the water vapor absorption between complexes 1 and 2. However, this leads to optimized proton conductivities for 1 and 2 of 7.46 × 10−4 and 2.47 × 10−4 S/cm, respectively. The ammonia vapor absorption and desorption isotherms for 1 and 2 indicated that, at a P/P0 of 0.05, the ammonia vapor absorption values are ca. 137.3 and 104 mg/g for 1 and 2, respectively. With P/P0 increasing to 0.95, the absorption isotherms reach maximum ammonia vapor uptake of ca. 182.2 mg/g for 1 and ca. 175.3 mg/g for 2 (Figure S17). The results revealed that both complexes 1 and 2 can adsorb lots of ammonia molecules, which led to tremendous changes of the proton conductivities following exposure to aqua−ammonia vapor. In general, it is to be emphasized herein that the optimized conductivity values of 2.89 × 10−2 S/cm for 1 and 4.25 × 10−2 S/cm for 2 under aqua−ammonia vapor are about 2 orders of magnitude larger than the values of 1 (7.62 × 10−4 S/cm) and 2 (2.47 × 10−4 S/cm) under water vapor. We speculate that the proton-conducting media, such as water and ammonia, and protonation of the media from the aqua−ammonia vapor exhibit a synergistic effect on the proton conduction process. Especially, ammonium cations play a key role in high proton conduction through hydrogen-bonding networks in our MOFs. In all events, our experimental results provide an innovative approach to improving the proton conductivity of MOFs.

Figure 5. Impedance spectra for 1 (a) and 2 (b) at 100 °C under aqua−ammonia vapors from different concentrations of NH3·H2O solutions.

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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/acsami.7b07635. Details of impedance analysis, PXRD patterns, and gas sorption (PDF)

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Figure 6. Arrhenius plots of the proton conductivities for (a) 1 under an NH3·H2O concentration of 4.9 M and (b) 2 under an NH3·H2O concentration of 2.96 M in the range of 30−100 °C.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. crystallized water molecules and carried by hydrogen-bonding networks. At temperatures above 60 °C, the higher activation energy of 0.73 eV indicates that the proton conduction is the Vehicle mechanism,19−21 which originates from the moving of exotic and crystallized water molecules inside the channels, accompanied by NH4+ or H+(H2O)n. For 2, Ea was calculated to be 1.16 eV (Figure 6b), which is obviously a Vehicle mechanism.19−21 The exotic and crystallized water molecules and ammonium ions can transfer from the hydrogen-bonding network inside the channels of 2. The structural integrities of 1 and 2 were identified by the PXRD patterns before and after aqua−ammonia vapor proton conductivity measurements (Figures S14 and S15), which revealed that both compounds contain structural stabilities under aqua−ammonia vapor. To gain insight into the proton conduction mechanism, the water and ammonia vapor absorption and desorption isotherms for compounds 1 and 2 were measured at 25 °C using dehydrated samples in the low-pressure region. As shown in Figure S16, the first-step rapid adsorption of dehydrated 1 and

ORCID

Gang Li: 0000-0001-9049-4208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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

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