Enhancing Proton Conductivity of a 3D Metal–Organic Framework by

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Cite This: Inorg. Chem. 2018, 57, 11560−11568

Enhancing Proton Conductivity of a 3D Metal−Organic Framework by Attaching Guest NH3 Molecules Ruilan Liu, Lili Zhao, Shihang Yu, Xi Liang, Zifeng Li, and Gang Li* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001 Henan, P. R. China

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S Supporting Information *

ABSTRACT: By reaction of a newly designed organic ligand, [3-(naphthalene-1-carbonyl)-thioureido] acetic acid (C10H7C(O)NHC(S)NHCH2COOH; H3L), with Cu(OAc)2, a metal−organic framework [(CuI4CuII4L4)·3H2O]n (1) containing unique mixed-valence [CuI4Cu4IIL4] subunits has been successfully synthesized and structurally characterized. MOF 1 displays a three-dimensional open framework bearing one-dimensional channels. Consequently, a new derivative MOF [CuI4CuII4L4]n-NH3 (2) is procured upon exposure of 1 to NH3 vapors from 28 wt % aqueous NH3 solution, which bears 2 NH3 and 4 H2O molecules in accordance with the elemental and thermal analyses. Both 1 and 2 exhibit relatively high water stability, whose proton conduction properties under water vapor have been researched. Notably, 2 shows an ultrahigh proton conductivity of 1.13 × 10−2 S cm−1, which is 2 orders of magnitude larger than that of MOF 1 (4.90 × 10−4 S cm−1) under 100 °C and 98% RH. On the basis of the structural data, Ea values, H2O and ammonia vapor absorptions, and PXRD measurements, the proton transfer mechanisms were suggested. This is an efficient and convenient way to obtain suitable and highly protonconducting materials by attaching NH3 molecules.

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INTRODUCTION Metal−organic frameworks (MOFs) usually show conventional applications, including gas adsorption, heterogeneous catalysts, luminescence, drug delivery, etc.1−8 Apart from above descriptions, MOFs now emerge with a potential application as proton conductors in the field of energy materials.9−18 This is mainly due to the highly ordered MOF materials frequently containing different shapes of pores and a rich hydrogen bonding network, which is very conducive to the distribution of water molecules or other volatile molecules in the pore. Furthermore, the crystallinity and tunability of MOFs can provide fine information on structure to study the proton conduction mechanism deeply. Therefore, the research into MOF materials in the field of proton conductivity has been rapidly expanding in recent years. Until now, several MOFs with ultrahigh proton conductivities have been described to reach up to a magnitude of 10−1−10−2 S cm−1.19−22 Through careful analysis of the above-mentioned literature, we found that the researchers mainly used two strategies to design and prepare MOFs with excellent proton-conductive properties. The first strategy is to consciously introduce acidic organic © 2018 American Chemical Society

linkers (such as carboxyl groups, sulfonic groups, phosphate groups, etc.) into the frameworks of the MOFs by careful molecular design.9,10,19 Another strategy is to load additional protonic molecules or carriers (for instance, imidazole, histamine) into the pores of the MOFs through a postsynthesis method.21,22 However, the report on MOFs with prominent proton conductivity is overwhelmingly limited. By ingenious molecular design or otherwise by adjustment of external influence factors to acquire excellent proton conductivity, MOFs still require extensive research. On the other hand, several conductive MOFs have recently been discovered to show outstanding ammonia electrochemical recognition properties.23−25 This further stimulates us to achieve more highly proton-conductive MOFs. Recently, our laboratory has shown great interest in improving the proton conductivities of MOFs by altering the composition of ambient gases. By placing four cobalt and one strontium MOFs in different concentrations of NH3·H2O Received: June 9, 2018 Published: August 28, 2018 11560

DOI: 10.1021/acs.inorgchem.8b01606 Inorg. Chem. 2018, 57, 11560−11568

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Inorganic Chemistry

solution (20 mL) containing 1-naphthoyl chloride (3.92 g, 0.02 mol) at 25 °C. After stirring for 1 h, glycine (1.50 g, 0.02 mol) was added to above mixture, and afterward, the reaction mixture was heated under reflux for 6 h. After the temperature was lowered to 25 °C, the yellow resulting solution was poured into 550 mL of ice water. The light yellow precipitates were collected and recrystallized from EtOH/H2O (2:1) solution. Yield: 66%. Mp: 188−190 °C. IR (cm−1, KBr): 3414(m), 3300(w), 13048(w), 1720(s), 1683(s), 1594(w), 1522(s), 1518(s), 1433(m), 1410(m), 1278(m), 1248(s), 1199(s), 1277(w), 1248(s), 1199(s), 1176(m), 783(s), 689(w), 627(w), 577(w), 505(w). 1H NMR (300 MHz, DMSO-d6): δ 13.02 (s, 1H, COOH), 11.89 (s, 1H, NH), 11.12 (t, J = 5.1 Hz, 1H, NH), 8.12− 8.01 (m, 3H, C10H7), 7.76−7.55 (m, 4H, C10H7), and 4.39 (d, J = 5.1 Hz, 2H, CH2). Preparation of [(CuI4CuII4L4)·3H2O]n (1). A solution of H3L (28.8 mg, 0.1 mmol) in CH3OH/DMF (1:1, 12 mL) was dropwise added to a CH3OH/H2O (3:1, 9 mL) solution of Cu(OAc)2·H2O (39.8 mg, 0.2 mmol). The blackish green solution is then placed at 25 °C. Good quality dark-green crystals of 1 were collected after 7 days (76% yield based on Cu). Calcd for C14H9N2O3SCu2(H2O)3: C, 36.05; H, 3.25; N, 6.02; S, 6.87%. Found: C, 36.33; H, 3.49; N, 6.34; S, 7.16%. IR (cm−1, KBr): 3441(m), 2924(w), 1671(w), 1593(s), 1533(m), 1458(s), 1368(s), 1291(w), 1263(w), 1229(s), 1179(s), 1090(m), 1061(m), 936(w), 782(s), 724(m), 659(w). Preparation of [CuI4CuII4L4]n-NH3 (2). A 100 mg portion of 1 was put into the vapors from NH3·H2O solution (28 wt %) in a closed container for 2.5 h. Subsequently, the deep dark-green product 2 was moved from the container and air-dried. Consequently, the samples were used for PXRD, elemental and thermal analyses, and electrochemical determinations. Calcd for C 14 H 9 N 2 O 3 SCu 2 (H2O)4(NH3)2: C, 32.43; H, 4.47; N, 10.82; S, 6.18%. Found: C, 32.28; H, 4.19; N, 10.98; S, 6.26%. Crystal Structure Determinations. Crystal data for 1 were collected on a Bruker Smart APEXII CCD X-ray diffractometer with the graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) at 293(2) K by ω−2θ scan technique and corrected for Lorenzpolarization effects. An appropriately sized single crystal was arranged on a glass fiber. Modification for secondary extinction was adopted. We use the direct method to solve the structure expanded by Fourier technique. All calculations were accomplished by the SHELXL software.32 Note that the unit cell contains a big area of disordered solvent units, which were not modeled as discrete atomic locations. PLATON/SQUEEZE was adopted to calculate the diffraction contribution of the solvent units to get a set of solvent-free diffraction intensity. The final formula was determined by combing element and TG analyses and the electron count of the SQUEEZE results,33−36 about 3 H2O molecules per asymmetric unit for MOF 1. CCDC: 1817802. Water Treatment and Activation. The crystalline powders of the two MOFs were soaked in H2O for 30 days and heated to reflux in water for 1 day, and then separated and air-dried to get water-treated solids. Activated solids for the MOFs were achieved as follows: The crystalline solids were put into the EtOH for 1 day to exchange the H2O units inside the frameworks, and then were vacuum-dried under 75 °C for 16 h. Last, these activated solids were used for N2, H2O, and NH3 vapor adsorption determinations. Proton Conductivity Measurement. Alternating current (ac) impedance spectra were recorded on a Princeton Applied Research PARSTAT 2273 impedance analyzer covering a frequency from 1 Hz to 1 MHz by a quasi-four-probe method and ac voltage of 100 mV with Pt electrodes. The pressed pellets for ac testing were prepared as in the previous method.27−29 The water-assisted conductivity was determined at various RHs. To fix the water content and obtain a stable conductivity value, pellets were equilibrated for 18 h under various RHs. Impedance data were collected by the Power-Suite program. The general impedance (R) (Ω) is obtained from the arc extrapolation to the low frequency Z′ side axis on the Nyquist plot. Proton conductivity was deduced from the following equation:

vapors, it indicates that their proton conductivities are significantly enhanced, and the ammonia molecule performs a key function in the proton transfer course.26−29 Another example also illustrates the crucial influence of NH3 on the proton conduction of the MOFs. Cabeza and coauthors discovered that the derivative MOF Ca-DPPA-NH3 [5(dihydroxyphosphoryl)isophthalic acid = DPPA] prepared by exposing Ca-DPPA-I to NH3 gas has higher conductivity than that of PiPhtA-I.29 From the above descriptions, it can be found that both the introduction of ammonia into the external environment and the introduction of ammonia molecules inside the MOFs can greatly strengthen the proton conduction of the MOFs. Nevertheless, consequences and further studies on this aspect are extremely infrequent. More data are required to support the aforementioned judgments. On the other hand, we have previously reported two acylthiourea carboxylate-based MOFs bearing mixed-valence CuIICuI clusters,30,31 which indicates antiferromagnetic coupling between the neighboring CuII ions. However, there is no report of study on proton transfer of such MOFs. Now, the organic ligands containing carbonyl, thiocarbonyl, carboxylato, and two −NH− groups inside the two MOFs have captured our attention, which would be beneficial to the formation of intramolecular or intermolecular hydrogen bonds. It is now recognized that these hydrogen bonds always exert a vital part in proton transfer process. In order to extend our previous work and deepen our understanding of proton conductivity for MOFs, we designed and prepared one new acylthiourea carboxylate ligand, [3(naphthalene-1-carbonyl)-thioureido] acetic acid (C10H7C(O)NHC(S)NHCH2COOH; H3L), which was employed to react with Cu(OAc)2 under mild conditions. Fortunately, a new MOF, [(CuI4CuII4L4)·3H2O]n (1), containing unique mixed-valence [CuI4CuII4L4] subunits has been synthesized. MOF 1 exhibits excellent water and thermal stability and displays a 3D framework with 1D open channels. After MOF 1 was placed in a closed container and put in contact with concentrated aqueous ammonia vapors for 2.5 h, derivative MOF [CuI4CuII4L4]n-NH3 (2) was newly obtained and contains 2 ammonia and 4 water molecules as determined by elemental and thermal analyses. The proton conduction of MOFs 1 and 2 at water vapor has been fully explored. The influences of the guest species (ammonia molecules) on the proton conduction properties would be highlighted.

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EXPERIMENTAL SECTION

Reagents and Apparatus. All reagents are analytically pure, without further purification, unless otherwise specified. Elemental analysis was conducted on a FLASH SMART analyzer. The infrared spectrum was determined on a BRUKER TENSOR II FTIR spectrophotometer (KBr pellets). Thermogravimetric analysis (TGA) was recorded on a Netzsch STA 449F3 differential thermal analyzer. The powder X-ray diffraction (PXRD) pattern was recorded on a Rigaku D/MAX-3 with Cu Kα (λ = 1.5418 Å) irradiation. N2 (at −196 °C), H2O, and NH3 (at 25 °C) adsorption−desorption isotherms were determined on a ASAP 2420 adsorptometer, and a 3H-2000PW Multistation Weight method analyzer (BeiShiDe Instrument Technology (Beijing) Co. Ltd.), respectively. The 1H NMR spectrum was determined with a Bruker DPX 300 spectrometer. The X-ray photoelectron spectroscopy (XPS) was measured on a PHI 5000 Versaprobe II scanning XPS microprobe instrument. A solidstate UV−vis spectrum was obtained on a Varian Cary 5000 UV− vis−NIR spectrophotometer. Preparation of H3L. A CH3CN solution (40 mL) containing anhydrous KSCN (2.00 g, 0.02 mol) was added to the CH3CN 11561

DOI: 10.1021/acs.inorgchem.8b01606 Inorg. Chem. 2018, 57, 11560−11568

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Figure 1. (a) Three-dimensional structure of 1 with 1D channel. (b) Drawing of the subunit of 1. (c) The topological consideration of the 3D network of 1.

942.06 and 961.09 eV suggest that 1 has CuII ions.30,31,37 As mentioned previously, the CuII ions can be reduced by thiourea ligands to CuI ions.38,39 The four CuI ions inside the tetranuclear cluster bond to each other with CuI−S bonds, and the two CuI−S lengths are 2.2734(8) and 2.2928(8) Å, respectively, which are close to those in the previous CuI cluster.30 The CuI−N length is 2.035(2) Å. The neighboring CuI···CuI distances vary from 2.630(1) to 2.736 (2) Å, which indicates the existence of a strong Cu···Cu interaction. This case is similar to the previously reports.30,31 Notably, the four CuI ions are located at four vertices of the tetrahedron. As shown in Figure S6, the coordination locations of the central Cu2 atom were occupied by S1, S1#3, and N2#2 from three L3− ligands. Unlike Cu2, each Cu1 ion is in a 5coordinated environment, in which the O1, O2, N1#1, O1#1, and O3#1 atoms are from two individual L3− species. The bond angles surrounding the Cu1 ion vary from 85.78(10)° to 178.36(11)°. The Addison parameter (τ = 0.033) demonstrates that the environment around the CuII cation is in nearly a square-pyramidal geometry.40 The bond length of CuII−N is 1.919(2) Å. The CuII−O bond lengths are between 1.897(2) and 1.964(5) Å (Table S2), which are similar to the reported values.30,31,41 Each organic ligand uses the same coordinating fashion, μ5kN; kS; kS; kO,N′,O′; kO′,O″ (Figure S7), to link one CuI and two CuII atoms. Furthermore, the [CuI4S4] subunits and CuII

σ = L /(dA) where σ = proton conductivity (S cm−1), L = thickness of the pellet (cm), and A = flat surface area of the pellet = πr2 (cm2) (d = radius of the pellet (cm)). The Ea value was derived from the Arrhenius equation:

Tσ = σ0 exp( − Ea /kT ) Analysis on the Impedance Plots. At 30 or 100 °C and 98% RH, the impedance spectrum shows a single semicircle at high frequency being ascribed to bulk and grain boundary resistance, and a spur at low frequency corresponding to the blocking at the electrode− electrolyte interface. 28 The equivalent circuit (RbCPEb)(RgbCPEgb)CPEe was tried as a fit to the Nyquist plots of MOFs 1 and 2 by the ZSimpWin program. The representative fitting results are demonstrated in Figures S1−S4.

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RESULTS AND DISCUSSION Structural Descriptions of MOFs 1 and 2. MOF 1 crystallizes in the tetragonal system I4̅ space group (Table S1) and exhibits a 3D framework (Figure 1a) bearing a distinct mixed-valence [CuI4CuII4L4] subunit (Figure 1b). As displayed in Figure 1, the four copper cations in the [Cu4S4] moiety can be assigned as CuI, and the four copper cations surrounding this cluster are CuII. This could be welldemonstrated by XPS measurements. As illustrated in Figure S5a, two distinct peaks of 2p3/2 at 931.5 eV and 2p1/2 at 951.2 eV indicate the existence of CuI in 1. The small peaks at 11562

DOI: 10.1021/acs.inorgchem.8b01606 Inorg. Chem. 2018, 57, 11560−11568

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Figure 2. PXRD patterns of (a) 1 and (b) 2 for the simulated, as-synthesized, and water-treated solids.

range 713.7−850 °C. The black residue is the mixture of CuO and 0.5Cu2S (obsd 34.38%, calcd 34.31%). For 2, it first loses weight between 25 and 235 °C, attributed to the loss of 4 H2O and 2 NH 3 molecules (obsd 20.56%, calcd 20.46%). Subsequently, two steps of continuing weight losses in the ranges 230−485.8 °C and 485.8−721.2 °C correspond to the decomposition of the organic ligands (obsd 48.17%, calcd 48.65%). The black residue of CuO and 0.5Cu2S (obsd 31.27%, calcd 30.89%) remained. Additional evidence for the above judgment is that the elemental analyses of 2 are consistent with the molecular formula [CuI4CuII4L4]n attaching 2 NH3 and 4 H2O. The PXRD analysis of 2 illustrates the profound structural changes of 2 compared to 1. As depicted in Figure S10, the PXRD of simulated MOF 1 and as-synthesized MOF 1 cannot overlap with as-synthesized MOF 2. To obtain useful structural information from the PXRD data of 2, the PXRD pattern was collected at a scan rate of 2°/min in order to get high quality data. Then, the Rietveld refinement was performed by applying JADE software. The best fitting cell parameters of 2 reflect the following: monoclinic; P21/c; a = 8.163(0.002) Å; b = 13.549 (0.000) Å; c = 8.111(0.002) Å; and β = 93.127(0.003)°. In a comparison of the above parameters with those of MOF 1 (Table S1), it can be found that, from the crystal system and space group to the cell parameters, the structural information has changed greatly, which shows the framework of 2 has undergone a tremendous change. Nevertheless, the similar XPS spectrum (Figure S5b) of MOF 2 with MOF 1 also indicates the presence of CuI and CuII ions. As shown Figure S11, the solid products of MOFs 1 and 2 have little color difference; only the color of 2 is slightly deepened. We further determined the solid-state UV−vis spectra of MOFs 1 and 2, and a traditional Cu-NH3 complex, [Cu(NH3)4]SO4 (Figure S12). In a comparison of the solidstate UV−vis spectra of the three compounds, it was found that, in MOFs 1 and 2, the wide absorption peaks around 360 nm corresponding to the electronic transition of conjugate aromatic groups changed little, but the wide peak at 632 nm in 1 moved to 606 nm in 2. The maximum absorption peak in [Cu(NH3)4]SO4 appeared around 577 nm, which is obviously caused by the Cu-NH3 interaction. The absorption peak (606 nm) of 2 is between 1 (632 nm) and [Cu(NH3)4]SO4 (577 nm), which indicates that NH3 molecules may coordinate to copper atoms in MOF 2. N2 gas absorption/desorption measurements were used to prove the porosity of MOFs 1 and 2 (Figure S13). According to the IUPAC classification, they indicate type II absorption

ions are bridged by L3− ligands resulting in a complicated 3D structure bearing open 1D channels (Figure 1a). In a comparison of the 3D framework of 1 with the 2D structure of {[Cu I 6 Cu II 6 (C 6 H 5 C(O)NC(S)NCH 2 COO) 6 (H 2 O) 3 (MeOH)6]·5H2O·3MeOH}n,30 only the different aromatic groups (naphthalene or benzene rings) on the organic ligand were found to result in the difference between the subunits {[CuI4CuII4L4] or [CuI6CuII6L16]} and the final solid-state structures (3D or 2D). This once again illustrates the key role of multifunctional organic ligands in the synthesis of novel MOFs.42 To comprehend the 3D structure of 1 better, the building blocks of 1 are simplified. Each [CuI4S4] cluster can be one node, and the four CuII ions can be another node. Therefore, the complicated structure of 1 was simplified as a dia net with point symbol {6^6} (Figure 1c). After MOF 1 was put into the vapors from a 28 wt % NH3· H2O solution, a new derivative [CuI4CuII4L4]n-NH3 (2) was produced having 2 ammonia and 3 water molecules. However, we cannot get a good crystal of MOF 2 for X-ray diffraction collection. Then, we will amply discuss its structure in accordance with IR spectra, elemental and thermal analyses, XPS and UV−vis spectra, and Rietveld refinement based on PXRD data. The IR spectrum of MOF 2 was mostly compatible with that of 1. This hints that the organic ligand in the two MOFs has a similar coordinating fashion (Figure S8a,b). The carboxylate ν(CO) vibration of H3L at 1700 cm−1 vanished upon coordination. The characteristic bands ν(CS) and ν(CO) (carbonyl group) at 1180 and 1634 cm−1, respectively, can be observed. The wide strong ν(OH) band around 3400 cm−1 for MOF 1 shows the existence of lattice water molecules. Notably, the infrared spectrum of 2 illustrates a shift of the low wavenumber in the water stretching vibration region. This indicates that strong H-bonds may exist between intercalated H2O and NH3 units. More obviously, the NH stretching vibration around 2200 cm−1 newly emerged in 2, which is indicative of the existence of NH3 units in 2 (Figure S8c).43 The TG analyses (Figure S9) of MOFs 1 and 2 indicate that they have similar thermal decomposition trends. The large difference is the number of solvent molecules lost in the initial weight loss process. For 1, an initial weight loss being 11.70% from 25 to 215 °C can be attributed to losses of three solvent H2O units (calcd 11.63%). Afterward, two steps of continuing weight losses can be observed in temperature ranges 215−465 °C and 465−713.7 °C, which can be attributed to decomposition of the organic ligands (obsd 53.92%, calcd 54.06%). Eventually, a plateau region can be observed in the 11563

DOI: 10.1021/acs.inorgchem.8b01606 Inorg. Chem. 2018, 57, 11560−11568

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

Table 1. Conductivities (S cm−1) of 1 at Various RHs and 30−100 °C 68% RHa 30 °Cb 40 °Cb 50 °Cb 60 °Cb 70 °Cb 80 °Cb 90 °Cb 100 °Cb

6.86 0.92 2.85 9.99 1.64 2.40 3.01 1.09

× × × × × × × ×

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

75% RHa 7.64 2.03 4.00 1.01 2.38 3.29 3.80 1.57

× × × × × × × ×

85% RHa

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

2.02 2.63 3.29 3.78 5.95 8.79 1.09 2.43

× × × × × × × ×

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

93% RHa 2.84 3.40 4.53 5.58 8.49 1.08 1.74 4.29

× × × × × × × ×

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

98% RHa 4.62 6.18 1.16 1.79 2.04 2.18 4.60 4.90

× × × × × × × ×

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

a

Relative humidity. bTemperature.

Figure 4. Impedance spectra of MOF 2: (a) at 30 °C and various RHs and (b) from 30 to 100 °C at 98% RH.

isotherms. A large BET surface area for 1 (113.55 m2/g) and small BET surface area for 2 (3.41 m2/g) can be observed. Average BJH pore sizes are 7.16 nm for 1 and 8.43 nm for 2. As stated above, although 1 and 2 first started to lose solvent molecules from room temperature, this weight loss process lasted over 200 °C. This means that part waters for 1 and part NH3 and H2O molecules for 2 can hold the proton transfer below 100 °C. Water Stability. As shown in Figure 2, the determined PXRD data of as-synthesized and H2O-treated solids overlap the simulated ones from the single crystal data well, which indicates the outstanding water stabilities of the two MOFs. This would be beneficial to future proton conductivity tests under water vapor. However, the reflection of as-synthesized material of 1 seen from the PXRD patterns (Figure 2a) has a little offset to the right (5−10°). To confirm the integrity of the structure, we determined the IR spectra of water-treated samples for 1 (Figure S8d,e). As displayed in Figure S8, the IR spectra of as-synthesized and water-treated solids are almost

same, illustrating the structural integrity and stability of MOF 1. Proton-Conductive Properties. Alternating current (ac) impedance determination was performed on the pressed pellets at various RHs and temperatures.44,45 Samples had fixed water content, and stable conductivities were equilibrated at controlled RH for 16 h. For accuracy of the data, the cell was equilibrated for 20 min after each step from 30 to 100 °C. Nyquist plots of MOF 1 were measured from 30 to 100 °C at 98%, 93%, 85%, 75%, and 68% RHs, respectively (Figure 3a and Figures S14−S17). To be noticed, the Nyquist plots reveal one semicircle bearing a spur, suggesting that the conduction types are H+ ions. As denoted in Table 1, at fixed RH, conductivities of 1 increase along with the temperature increase. For instance, at 98% RH, the calculated conductivities of 1 vary from 4.62 × 10−5 S cm−1 at 30 °C to 4.90 × 10−4 S cm−1 at 100 °C (Figure 3a). Under fixed RH, the conductivity of 1 at high temperature is 1−3 orders of magnitude higher than that at low temperature. 11564

DOI: 10.1021/acs.inorgchem.8b01606 Inorg. Chem. 2018, 57, 11560−11568

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Inorganic Chemistry Table 2. Conductivities (S cm−1) of 2 at Various RHs and 30−100 °C 68% RHa 30 °C 40 °Cb 50 °Cb 60 °Cb 70 °Cb 80 °Cb 90 °Cb 100 °Cb b

3.40 5.48 7.00 8.5 1.13 1.31 1.70 2.13

× × × × × × × ×

−4

10 10−4 10−4 10−4 10−3 10−3 10−3 10−3

75% RHa 4.64 8.50 9.80 1.28 1.82 2.55 3.40 4.25

× × × × × × × ×

85% RHa

−4

10 10−4 10−4 10−3 10−3 10−3 10−3 10−3

0.82 1.02 1.34 1.76 2.55 3.40 4.64 6.00

× × × × × × × ×

−3

10 10−3 10−3 10−3 10−3 10−3 10−3 10−3

93% RHa 0.88 1.15 1.64 2.55 4.25 5.67 7.39 0.93

× × × × × × × ×

−3

10 10−3 10−3 10−3 10−3 10−3 10−3 10−2

98% RHa 0.96 1.34 1.92 2.86 4.36 7.85 9.45 1.13

× × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−2

a

Relative humidity. bTemperature

As indicated in Figure 3b and Figure S18, at fixed temperature, the ambient RH-dependence of proton conductivity can be observed. The calculated conductivity of 1 increases from 1.09 × 10−4 (6.86 × 10−7) to 4.90 × 10−4 (4.62 × 10−5) S cm−1 as RH increases from 68% to 98% at 100 °C (or at 30 °C). Apparently, the conductivity of 1 increases with increasing RH at a fixed temperature. The conductivities of 2 at water vapor were determined the same way as those of 1. Identical humidity-dependent and temperature-dependent proton conduction behaviors of MOF 2 can also be found. As shown in Figure 4a and Table 2, the conductivities augment from 3.40 × 10−4 S cm−1 (68% RH) to 0.96 × 10−3 S cm−1 (98% RH) at 30 °C, and vary from 2.13 × 10−3 S cm−1 (68% RH) to 1.13 × 10−2 S cm−1 (98% RH) (Figure S19) at 100 °C. Figure 4b and Figures S20−S23 give the ac spectra of 2 at 98%, 93%, 85%, and 75% RHs from 30 to 100 °C, respectively. Under fixed RH, the calculated conductivities for 2 also increase with the increase of the temperature. As indicated in Table 2, at 30 and 100 °C, the conductivities augment from 3.40 × 10−4 to 2.13 × 10−3 S cm−1, respectively, at 68% RH (4.64 × 10−4−4.25 × 10−3 S cm−1 at 75% RH; 0.82 × 10−3−6.00 × 10−3 S cm−1 at 85% RH; 0.88 × 10−3−0.93 × 10−2 S cm−1 at 93% RH; 0.96 × 10−3−1.13 × 10−2 S cm−1 at 98% RH). To investigate the proton-conductive mechanism of 1 and 2, we were prompted to undertake a study of their activation energy (Ea). Under 98% RH, the calculated Ea values are 0.37 and 0.39 eV for 1 and 2, respectively (Figure 5), which is a representative hydrated proton conductor and shows a Grotthuss mechanism for them.46,47 The possible pathway of proton transport in MOF 1 is that the adsorbed H2O units or

crystallization H2O units can conveniently form rich Hbonding nets inside the framework for proton H+(H2O)n hopping. In 2, the attached ammonia molecules can be involved in the proton hopping.46,47 In the two MOFs, the protons transfer from H+(H2O)n donors to the crystallization and adsorbed H2O, and NH3 receptors were incorporated by H-bonding nets. Additionally, the PXRD data of samples used for electrochemical experiments of 1 and 2 were measured. As exhibited in Figure S24, the PXRD patterns of the samples and the simulated ones are overlapped, which reveals that the MOFs maintain their structural stability after electrochemical investigation showing the potential value of application. In a comparison of the proton conductivity of 1 and 2 under the same conditions, it can be found that the conductivity of 2 is about 2 orders of magnitude larger than that of 1 (Tables 1 and 2). The optimization conductivities of 2 and 1 are 1.13× 10−2 and 4.90 × 10−4 S cm−1, respectively, under 98% RH and 100 °C, which can be interpreted from the structural differences. As discussed in the structural part, the additional NH3 molecules in 2 can interact with the crystallization H2O units to from more hydrogen bonds inside the channels, which benefit the proton transfer process. To get an insight into the interaction of MOF 1 to NH3 molecules, the NH3 vapor absorption and desorption isotherms for 1 are measured at 25 °C. The results (Figure S25a) illustrate that the NH3 gas absorption value is ca. 41 mg g−1 at P/P0 = 0.05. As P/P0 equals to 0.95, the maximum NH3 gas absorption is ca. 101 mg g−1. Similar phenomena of MOFs capturing ammonia vapor have been recently reported.48 The NH3 uptake of 1 indicates that it can interact with NH3 molecules. To investigate the relationship between RH and adsorbed H2O units, the H2O vapor adsorption/desorption isotherms of the two MOFs were determined (Figure S25b,c). For 1, the water vapor absorption value is ca. 121 mg g−1 as P/P0 reaches 0.05. As P/P0 reaches 0.95, the water adsorption value arrives at about 160 mg g−1 for the dehydrated samples (Figure S25b). For 2, the H2O vapor absorption value is ca. 126 mg g−1 at P/ P0 of 0.05. As the P/P0 is 0.95, the maximum H2O vapor uptake is ca. 180.5 mg g−1 for the dehydrated samples (Figure S25c). Obviously, the water absorption capacity of MOF 2 is stronger than that of MOF 1. This illustrates why proton conductivities of 2 are larger than those of 1 under the same conditions. It is apparent that the highly hydrophilic ammonia molecules inside MOF 2 are beneficial for the proton conduction process. Interestingly, the maximum conductivity of 2 (1.13 × 10−2 S cm−1) under 98% RH and 100 °C can be comparable to that of Nafion,49,50 and those of several recently reported MOFs under similar conditions. As denoted in Table S3, the

Figure 5. Arrhenius plots of the proton conductivities of 1 and 2 at 98% RH. 11565

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transform the MOFs to obtain promising proton-conductive materials with high conductivity.

optimized conductivity of MOF 2 is one of the best conductivities of the recently prepared MOFs.51−59 Apparently, MOF 2 presents a promising proton-conductive candidate for electrochemical applications in the future. As described above, MOF 1 has undergone large structural changes under ammonia vapor. But in our recent report on the proton transfer of four CoII and one Sr(II) MOFs under ammonia−water vapor, their structures have not changed after under the ammonia−vapor electrochemical testing.25−29 We argue that this may be due to the different structural features of them. The complicated organic ligand in MOF 1 contains carbonyl, thiocarbonyl, carboxylato and two −NH− groups, which all can be beneficial to the interaction with ammonia molecules other than the crystallization water molecules. In addition, the mixed-valence CuI and CuII ions in 1 may be coordinated by NH3 molecules, especially the coordination unsaturated CuII ions. Therefore, the structural change of 1 arose, and a new derivative 2 is obtained. Taking into account the need for future practical applications, the time-dependent proton conductivity for MOF 2 was measured under high humidity (98% RH) and temperature (100 °C). As denoted in Figure 6, after at least 8 h

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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.8b01606. Details of crystal data, impedance analysis, PXRD patterns, and gas adsorption/desorption (PDF) Accession Codes

CCDC 1817802 contains 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 (21571156 and J1210060). We are very grateful for Dr. Yubing Si, Dr. Liming Fan, and Mr. Yongjian Li giving us helpful suggestions related to the aspects on PXRD Rietveld refinement and topology analysis.

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REFERENCES

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Figure 6. Time-dependent conductivity of MOF 2 under 100 °C and 98% RH.

of continuous determination, the conductivity of 2 remains basically stable, with merely negligible changes occurring. Furthermore, as shown in Figure S26, the PXRD data after the time-dependent conductivity for 2 also show its high structural stability.

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CONCLUSION In general, we succeeded in preparing a MOF containing mixed-valence subunits, [(CuI4CuII4L4)·3H2O]n (1), and a new derivative attaching NH3 molecules, [CuI4CuII4L4]n-NH3 (2). Their proton conduction under various RHs and temperatures has been explored and compared. MOF 2 features an ultrahigh proton-conductive value of 1.13 × 10−2 S cm−1 at 98% RH and 100 °C which is greatly larger than that of MOF 1. Their proton-conducting mechanisms have been discussed according to the structural comparison, E a values, H 2 O vapor absorptions, and PXRD measurements. Both Ea values are smaller than 0.4 eV, and there are typical Grothuss mechanisms for them. Moreover, the high structural stabilities of the two MOFs toward water and electrochemical determination give a solid foundation for future practical applications. This work offers a convenient and easy way to 11566

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