Enhanced Intrinsic Proton Conductivity of Metal–Organic Frameworks

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Enhanced Intrinsic Proton Conductivity of Metal−Organic Frameworks by Tuning the Degree of Interpenetration Published as part of a Crystal Growth and Design virtual special issue on Crystalline Functional Materials in Honor of Professor Xin-Tao Wu

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Lizhen Liu, Zizhu Yao, Yingxiang Ye, Quanjie Lin, Shimin Chen, Zhangjing Zhang,* and Shengchang Xiang* Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, PR China S Supporting Information *

ABSTRACT: Control of interpenetration degree of the metal−organic frameworks (MOFs) can be a wise tactic to enhance their proton-conductivities. Two anionic MOFs, [(CH3)2NH2][In(L)]·2.5DMF·2H2O (FJU-16) and [(CH3)2NH2][In(L)]· 4.5DMF·16H2O (FJU-17) (H4L= 4,4′,4″,4‴-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid), with different degrees of interpenetration have been synthesized. The resulting 2-fold interpenetrated FJU-17 shows a higher intrinsic proton-conductivity (1.08 × 10−2 S/cm, 100 °C) and wider performing temperature range (−40 to 100 °C) under the conditions of without additional humidity than the 4-fold interpenetrated FJU-16, owing to the more available void space and abundant guest molecules in FJU-17.

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To introduce proton media (e.g., [(CH3)2NH2]+, imidazole, NH4+, and H+) into the cavities of MOFs is the simplest and effective method.9 Several highly proton-conductive MOFs have been achieved with the introduction of different organic hydroxyl molecules,12 and imidazole molecules13,14 into the cavities or channels as proton media. It is worth noting that the above strategies can improve their conductivity by introducing proton carriers, but little attention has been paid in accessible void space of intrinsic proton carriers.15 The large void space of MOFs can enhance the mobility of the ions and guest molecules; however, an extremely large void space is not conducive to the stability of MOFs. To achieve two contradictory functions, we need to design a stable pore structure with inherent proton-conductivity material. Controlling the degree of interpenetration is a wise strategy in MOFs for tuning pore size.16,17 On the one hand, interpenetration usually significantly enhances the stability of MOFs.18,19 On the other hand, interpenetration can strengthen the interactions

nvironmentally friendly renewable energy achieves the goals of green energy development.1 Fuel-cells (FCs) have been recognized as a promising energy candidate owing to its high power density, wide fuel adaptability, and environmentally friendly characteristics for a wide scope of applications, for instance, electric vehicles, portable electronic devices, and smart grids.2 The proton conductor is an essential component of FCs to ensure safe and efficient operation.3 The commercial Nafion based proton-conducting materials can reach 10−1−10−2 S/cm conductivities under high relative humidity (RH = 98%) and moderate temperatures (60−80 °C). But the significant drops in its conductivity above 80 °C, the freezing damage caused by freeze/thaw cycles, and the high-cost principally limit its largescale applications.4,5 Hence, the development of cheaper and better-performing proton-conductive electrolytes that work in a wide-temperature range, even at subzero conditions, would be expected for applications to FCs. Metal−organic frameworks (MOFs) have proved to be potential proton-conductor candidates because their intrinsic structural merits endue them with structural controllability and functional designability.6−11 Efforts have been devoted to high proton conductivity or modulate proton conductivity of MOFs. © 2018 American Chemical Society

Received: April 12, 2018 Revised: May 27, 2018 Published: June 8, 2018 3724

DOI: 10.1021/acs.cgd.8b00545 Cryst. Growth Des. 2018, 18, 3724−3728

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100 °C) than FJU-16, and the proton conductivity is the highest among intrinsic proton-conductive MOFs materials under the conditions without additional humidity at 100 °C. The crystal data show that FJU-16 and FJU-17 have similar secondary building units (SBUs): an organic secondary building unit consists of a 4-connected ligand which shows square planar node (Figure 1a); an inorganic SBU is formed by an [In(COO)4]− node which could be considered as a 4-linked tetrahedral vertex (Figure 1b). The tetrahedral SBUs are further connected by four different square planar linkers into a single net of the framework as shown in Figure 1c. FJU-16 has a 4fold interpenetrated structure, whereas FJU-17 has a 2-fold interpenetrated structure. They are connected by weak π···π interactions, as evidenced by the center-to-center range between the pyridyl rings of H4L and the phenyl rings of adjacent H4L of 4.324 and 3.703 Å, respectively (Figure 1d,g). On the basis of the different degrees of interpenetration, FJU16 and FJU-17 provide two types of square channels with dimensions 8.352 Å × 11.091 Å and 6.794 Å × 7.831 Å for FJU-16 along the a and b axes; 8.062 Å × 14.759 Å and 20.022 Å × 25.928 Å for FJU-17 along the a and c axes (Figures S6− S7). Detailed structural investigation reveals that both are anionic frameworks, and the overall framework charge is balanced by (Me2NH2)+ cations. These (Me2NH2)+ cations and guest molecules are disordered in pores. After elimination of the guest molecules, the porosity of FJU-16 and FJU-17 calculated by PLATON22 is 36.4% and 73.8% of the unit cell, respectively. From the topology point of view, both structures could be divided into sqc515 topology with a Schläfli symbol of (62· 84)(62·8)2 (Figure 1f,i). Prior to the tests, the powder X-ray diffraction (PXRD) of two compounds has been studied. The PXRD diagrams of synthesized compounds are matched well with the simulated PXRD of single crystal data, demonstrated structural integrity, and high purity (Figures S1−S2). The thermal gravimetric (TG) data are used to confirm thermal stability of FJU-16 and FJU-17. As illustrated in Figure S5, the thermal stability of FJU-16 and FJU-17 is ∼350 °C. For FJU16, the first weightlessness is 24.28% at 30−230 °C which was attributed to the removal of [(CH3)2NH2]+, H2O, and DMF (cal. 24.33%). Hereafter, the thermal gravimetric curve maintained unaltered to ∼350 °C, and subsequently the framework begin to collapse. For FJU-17, due to a large number of guest molecules in the cavity, a sharp drop of weight 44.57% at 30−179 °C is attributed to the loss of DMF, (Me2NH2)+, and H2O (cal. 44.61%), and there is a plateau observed from 179 to 350 °C before the framework decomposition. On the basis of TG and elemental analysis, the molar H2O content of two compounds could be calculated as 2 water molecules and 16 water molecules per structural formula unit in FJU-16 and FJU-17, respectively see (Experimental Section in Supporting Information and Figure S5). Considering that the abundant [(CH3)2NH2]+ and H2O molecules reside in the channels, we seek to explore the protonconductivity properties of FJU-16 and FJU-17.23−25 The impedance measurements of FJU-16 and FJU-17 were performed on pellets under the conditions without additional humidity, and the obtained impedance curves are shown in Figure 2a,b. The compounds show perfect semicircles under subzero temperature (−40 °C), and the diameter of the arc decreases with the rise of temperature in the Nyquist plots, indicating the increment of the conductivity. FJU-16 and FJU-

between pore surface and guest molecules, and endow functional improvement of MOFs.17,20 In general, structural interpenetration is applied to improve the adsorption and separation of gas,17,21 while, to the-best of our knowledge, this research has not yet been published on enhancing proton conductivity by tuning the degree of interpenetration. In this work, by using different solvents, two different interpenetration framework In(III) based MOFs have been synthesized (Scheme 1). As the single crystal X-ray diffraction Scheme 1. Schematic of Syntheses and Optical Images of FJU-16 and FJU-17a

a

They show distinct intrinsic proton conductivities.

(SCXRD) analyses display, one has a 4-fold interpenetrated framework [(CH3)2NH2][In(L)]·2.5DMF·2H2O (FJU-16), while the other possesses a 2-fold interpenetrated framework [(CH3)2NH2][In(L)]·4.5DMF·16H2O (FJU-17) (Figure 1 and Table S1). They have the same single net structures, but FJU17 has a larger solvent accessible void than the FJU-16. As is expected, FJU-17 exhibits much higher intrinsic proton conductivity and wider operating temperature ranges (−40−

Figure 1. Single-crystal structure for FJU-17 and FJU-16: (a) H4L organic connector as a square-planar linker; (b) [In(CO2)4]− inorganic linker as a tetrahedral vertex; (c) the single net for FJU17 and FJU-16; π−π interactions among adjacent molecules of FJU16 (d) and FJU-17 (g); the interpenetrating modes of FJU-16 (e) and FJU-17 (h) along the c axis; the simplified topology network for FJU16 (f) and FJU-17 (i). Color scheme: In, olive; O, red; C, gray; N, blue. For clarity, the guest molecules and H atoms are omitted. 3725

DOI: 10.1021/acs.cgd.8b00545 Cryst. Growth Des. 2018, 18, 3724−3728

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and is proportional to the accessible void space for the guests (36.4% for FJU-16 and 73.8% for FJU-17). The larger void space can accommodate more guest molecules, leading to significant improvement of proton conductivity. To further prove guest molecules play a vital role in the proton conductor, we performed a proton-conduction study on the FJU-16 and FJU-17 sample preheated at 250 °C for 12 h. The preheated FJU-16 and FJU-17 do not display any proton-conductivity, presumably owing to the loss of guest molecules in the framework (Figure S14). Herein, we speculated the enhanced conductivity is attributed to the larger void space of framework that imparts higher capability of mobile protons hopping and forms a more extensive hydrogen-bond network for facile proton transfer23,32,33 (Figure S15). Moreover, we measured temperature dependent proton-conductivity during heating− cooling cycles for FJU-16 and FJU-17 (Figure 2d). The proton conductivity rises at the heating regime and decreases during the cooling process. At the same temperature, the proton conductivity of two compounds almost coincides during the heating−cooling process, indicating the two compounds have good repeatability and stability in a wide temperature range. PXRD was performed to further confirm structural integrity after proton-conduction tests (Figures S1−S2). To explore the proton-conduction mechanism, the protonconducting activation energy Ea of FJU-16 and FJU-17 was estimated from the curve of ln(σT)−1000 T−1 (Figure 2c), following fitting of the data in the −40 to 100 °C range using the Arrhenius equation. The activation energies of FJU-17 (0.29 eV) show lower activation energy values than FJU-16 (0.38 eV), which is similar to those of Nafion (0.22 eV),34 UiO66(SO3H)2 (0.32 eV),35 and many other reported conductive materials (Table S2). These values are within the 0.1−0.4 eV range typically attributed to a predominant Grotthuss mechanism.36−38 Obviously, the low activation energy and high proton-conductivity of FJU-17 in the wide temperature range (−40−100 °C) indicate that FJU-17 belongs to the kind of fast ion proton-conducting materials without additional humidity.39,40 In summary, we have successfully synthesized two different interpenetrated 3D MOFs FJU-16 and FJU-17, using H4L as the organic ligand and In3+ as metal ion. They display a distinct distinction in proton-conductivity due to the framework interpenetrated variation. Two-fold interpenetrated FJU-17 shows a higher intrinsic proton conductivity and wider operation temperature range (−40 to 100 °C) than 4-fold interpenetrated FJU-16 due to the large available space and abundant guest molecules confining the framework, which contribute to the migration of protons. Our work demonstrates that proton-conduction properties of MOFs can be reasonably tuned by appropriately governing the framework interpenetration and further provides guidance for the rational design of novel fuel-cells with wide operating temperature range protonconduction materials.

Figure 2. Impedance plots of FJU-16 (a) and FJU-17 (b) under the conditions of without additional humidity and different temperatures. (c) Arrhenius plots of FJU-16 and FJU-17. (d) The heating and cooling cycles log plots of FJU-16 and FJU-17 at different temperatures and without additional humidity. (e) Comparison of intrinsic proton-conductivity of FJU-16 and FJU-17 with other proton-conducting materials at 100 °C and comparison of protonconducting MOFs containing [(CH3)2NH2]+ (f). The green (cyan) bars indicate proton-conducting materials tested under humidity (without additional humidity) conditions.

17 show proton-conductivities value of 2.90 × 10−6 S/cm and 9.13 × 10−5 S/cm at subzero temperature (−40 °C), respectively. They display good proton conductivities and match some already published proton-conducting MOFs materials at a subzero temperature to date26 (Table S2). Such high conductivity at subzero temperatures endows them with the potential to work as PEMs of FCs for automotive application in cold climates. From the Arrhenius plots (Figure 2c), their intrinsic proton conductivity increases linearly as the temperature increases. The intrinsic conductivity (σ) of FJU-17 is 3.17 × 10−3 S/cm at 30 °C, close to 9 times higher than 3.64 × 10−4 S/cm for FJU16 and higher than most reported proton-conducting materials (Table S2). As the temperature continues to rise, FJU-16 shows highest intrinsic conductivity, and its conductivity at 80 °C reaches 1.25 × 10−3 S cm−1, while FJU-17 shows the highest intrinsic conductivity and its conductivity at 100 °C reaches 1.08 × 10−2 S/cm, which is the highest among already published intrinsic proton-conductor MOFs at same conditions and comparable to that of dimethylammonium-containing MOFs (e.g., MROF-1, 2 7 {[((CH 3 ) 2 NH 2 ) 3 (SO 4 )] 2 [Zn2(ox)3]}n,28 Fe-CAT-529), but the σ value is lower than that of reported humidity-dependent proton-conductive MOFs BUT-8(Cr)A30 and H2SO4@MIL-10131(Figure 2e,f). The working temperature range of FJU-17 is −40−100 °C, wider than most proton-conducting materials tested under humidity conditions, similar to other proton-conducting materials without additional humidity conditions (Table S2). Taking into account these results, the proton conductivity of FJU-17 is higher than that of FJU-16 over the whole temperature range

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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.cgd.8b00545. Experimental section, IR, TG, PXRD, DSC and Nyquist plots (PDF) 3726

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Accession Codes

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CCDC 1831798−1831799 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 Authors

*(Z.Z.) Tel: +8613609524277. E-mail: [email protected]. *(S.X.) E-mail: [email protected]. ORCID

Yingxiang Ye: 0000-0003-3962-8463 Shengchang Xiang: 0000-0001-6016-2587 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (21673039, 21273033, and 21573042), Fujian Science and Technology Department (2014J06003, 2016J01046, and 2014H6007). S.X. gratefully acknowledges the support of the RecruitmentProgram of Global Young Experts.

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