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Enhanced Intrinsic Proton Conductivity of MOF by Tuning the Degree of Interpenetration Lizhen Liu, Zizhu Yao, Yingxiang Ye, Quanjie Lin, Shimin Chen, Zhangjing Zhang, and Shengchang Xiang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00545 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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Crystal Growth & Design
Enhanced Intrinsic Proton Conductivity of MOF by Tuning the Degree of Interpenetration Lizhen Liu, Zizhu Yao, Yingxiang Ye, Quanjie Lin, Shimin Chen, Zhangjing Zhang,* 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 Supporting Information ABSTRACT: Control of interpenetration degree of the MOFs can be a wise tactics to enhance their proton-conductivities. Two anionic metal-organic frameworks (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‴-(1,4Phenylenbis(pyridine-4,2-6-triyl))-tetrabenzoic acid) with different degree of interpenetration have been synthesized. The resulting two-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 oC) under the conditions of without additional humidity than four-fold interpenetrated FJU16, owing to the more available void space and abundant guest molecules in FJU-17.
Environmentally friendly renewable energy is 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 environment-friendly for wide scope of applications, for instance electric vehicles, portable electronic devices, and smart grids.2 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 highly relative-humidity (RH=98%) and moderate temperatures (60-80 °C). But the significant drops of its conductivity above 80 °C, the freezing damage caused by freeze/thaw cycles and the high-cost principally limit its large-scale applications.4,5 Hence, the development of cheaper and better-performing proton conductive electrolytes work in a wide-temperature range, even at sub-zero conditions would be expected for applications to fuel cells. Metal organic frameworks (MOFs) have proved to be potential proton conductor candidates because their intrinsic structural merits endue them structural controllability and functional designability.6-11 Efforts have been devoted to high proton conductivity or modulate proton conductivity of MOFs. To introduce proton media (e.g. [(CH3)2NH2]+, imidazole, NH4+, and H+) into the cavities of MOFs is the simplest and effective methods.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 between pore surface and guest molecules, and endow functional improvement of MOFs.17, 20 In general, structural interpenetration are applied to improve the adsorption and separation of gas, 17, 21 while, to thebest of our knowledge, it has not yet been published on enhancing proton conductivity by tuning the degree of interpenetration.
Scheme 1. Schematic of syntheses and optical images of FJU16 and FJU-17. They show distinct intrinsic proton conductivities. 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 (SCXRD) analyses display, one has a four-fold interpenetrated framework [(CH3)2NH2][In(L)]·2.5DMF·2H2O (FJU-16), while the other possesses a two-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 FJU-17 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 - 100 oC) than FJU16, and the proton conductivity is the highest among intrinsic proton conductive MOFs materials under the conditions of without additional humidity at 100 oC. The crystal data shows that FJU-16 and FJU-17 have similar secondary building units (SBU): an organic secondary building units is consisted of 4-connected ligand which shows square planar node (Figure 1a); a inorganic secondary building units is formed by [In(COO)4]− node which could be considered as a 4linked tetrahedral vertex (Figure 1b). The tetrahedral SBUs are further connected by four different square planar linkers into a
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single net of the framework as shown in Figure 1c. FJU-16 has a four-fold interpenetrated structure whereas FJU-17 has a two-fold interpenetrated structure. They are connected by weak π···π interactions, as evidenced by center-to-center range between pyridyl rings of H4L and the phenyl rings of adjacent H4L of 4.324 Å and 3.703 Å, respectively (Figure 1d and 1g). Based on the different degree of interpenetration, FJU-16 and FJU-17 provide two types of square channels with dimensions 8.352×11.091 Å and 6.794×7.831 Å for FJU-16 along the a and b axes; 8.062×14.759 Å and 20.022×25.928 Å for FJU-17 along the a and c axes (Figures S6-S7).
Figure 1. Single-crystal structure for FJU-17 and FJU-16: (a) H4L organic connecter 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 FJU-16(d) and FJU-17(g); the interpenetrating modes of FJU16(e) and FJU-17(h) along the c axis; the simplified topology network for FJU-16(f) and FJU-17(l). Color scheme: In, olive; O, red; C, gray; N, blue. For clarity, the guest molecules and H atoms are omitted. Detailed structural investigation reveals that both are anionic framework and 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 (Figures 1f and 1i). 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 is used to confirm thermos stability of FJU-16 and FJU-17. As illustrated in the Figure S5, the thermal stability of FJU-16 and FJU-17 are ~350 °C. For FJU-16, the first weightlessness is 24.28% at 30230 °C that was attributed to the removal of [(CH3)2NH2]+, H2O and DMF (cal. 24.33%). Hereafter, the thermal gravimetric curve maintained unaltered to ~350 °C, 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 30179 °C attribute to the loss of DMF, (Me2NH2)+ and H2O (cal.
44.61%), there is a plateau observed from 179 °C to 350 °C before the framework decomposition. Based on 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 ESI and Figure S5).
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 oC and comparison of proton-conducting MOFs containing [(CH3)2NH2]+ (f). The green (cyan) bars indicate proton conducting materials tested under humidity (without additional humidity) conditions. Considering that the abundant [(CH3)2NH2]+ and H2O molecules are resided 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 condition of without additional humidity, and the obtained Impedance curve are showed in Figures 2a-2b. The compounds show the perfect semicircles under subzero temperature (-40 oC) 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-17 show proton-conductivities value of 2.90×10-6 S/cm and 9.13×10-5 S/cm at subzero temperature (-40 oC), respectively. They display good proton conductivities and match to 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 fuel cells 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 oC, close to 9 time higher than 3.64×10-4 S/cm for FJU-16 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 oC reaches 1.25×10-3 S
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Crystal Growth & Design cm−1, while, FJU-17 shows highest intrinsic conductivity and its conductivity at 100 oC reaches 1.08×10-2 S/cm, which is thehighest among already published intrinsic proton conductor MOFs at same conditions and comparable to that dimethyl ammonium27 containing MOFs (e.g., MROF-1 , {[((CH3)2NH2)3(SO4)]2[Zn2(ox)3]}n 28 , Fe-CAT-5 29 ), but the σ value is lower than that of reported humidity-dependent proton conductive MOFs BUT-8(Cr)A 30 and H2SO4@MIL-101 31 (Figures 2e-2f). The working temperature range of FJU-17 is -40-100 °C, wider than most proton conducting materials tested under humidity conditions, similar to other without additional humidity conditions proton-conducting materials (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 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 proved guest molecules play a vital role in the proton conductor, we have performed proton conduction study on the FJU-16 and FJU-17 sample preheated at 250 oC for 12 h. The preheated FJU-16 and FJU-17 does not display any proton-conductivity, speculation owing to the loss of guest molecules in the framework (Figure S14). Herein, we speculated the enhanced conductivity is attributed that larger void space of framework 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 heating regime and decreases during the cooling process. At the same temperature, the proton conductivity of two compounds almost coincides during heating-cooling process; indicated two compounds have good repeatability and stability in a wide temperature range. PXRD was performed to further confirm structural-integrity after proton conduction tested (Figures S1-S2). To explore the proton-conduction mechanism, the protonconducting activation-energy Ea of FJU-16 and FJU-17 were 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, UiO-66(SO3H)2 (0.32 eV)35 and many other reported conductive material (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 oC) indicates that FJU-17 belongs to the kind of without additional humidity fast ion proton conducting materials.39,40 In summed, we have successfully synthesized two different interpenetrated 3D metal-organic frameworks FJU-16 and FJU-17, using H4L as 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 four-fold interpenetrated FJU-16 due to the large available space and abundant guest molecules confining in the framework, which contribute to the migration of proton. Our work demonstrated that proton conduction properties of MOFs can be reasonably tuning by appropriate govern of framework-interpenetration; further provides guidance for rational design of novel fuel-cells with wide operating temperature range proton conduction materials.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.0000000 Experimental section, IR, TG, PXRD, DSC and Nyquist plots (PDF). SCXRD data in CIF format (CIF)
Accession Codes CCDC 1831798-1831799 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.
AUTHOR INFORMATION Corresponding Author *Tel: +8613609524277.
[email protected] *E-mail:
[email protected];
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT 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 Recruitment-Program of Global Young Experts.
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For Table of Contents Use Only Enhanced Intrinsic Proton Conductivity of MOF by Tuning the degree of Interpenetration Lizhen Liu, Zizhu Yao, Yingxiang Ye, Quanjie Lin, Shimin Chen, Zhangjing Zhang,* Shengchang Xiang*
Two different anionic framework MOFs are the first examples to demonstrate that tuning the degree of interpenetration can be an efficient strategy to improve the proton conductivity and to widen operating temperature range.
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