40-Fold Enhanced Intrinsic Proton Conductivity in Coordination

Article pubs.acs.org/IC

40-Fold Enhanced Intrinsic Proton Conductivity in Coordination Polymers with the Same Proton-Conducting Pathway by Tuning Metal Cation Nodes Xuelian Su,† Zizhu Yao,† Yingxiang Ye,† Heng Zeng,† Gang Xu,‡ Ling Wu,† Xiuling Ma,† Qian-Huo Chen,† Lihua Wang,† Zhangjing Zhang,*,†,‡ and Shengchang Xiang*,†,‡ †

Fujian Provincial Key Laboratory of Polymer Materials, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China

‡

S Supporting Information *

ABSTRACT: Three isostructural imidazole-cation-templated metal phosphates (FJU-25) are the first examples to demonstrate that the tuning of metal cation nodes can be an efficient strategy to significantly improve the proton conductivity without changing the structure of the proton-conducting pathway.

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proposed. In type I, functional groups (e.g., sulfonic,9 phosphonic,4b,10 carboxylic,11 and hydrophilic functionalities12) are incorporated because the proton conductivity depends on the pKa value of the functional groups.13 In type II, guest molecules (e.g., water,14 triazole,15 imidazole,16 histamine,17 ammonium cations,18 hydronium ions,19 and sulfonic and phosphonic acid4b,20) are involved in the empty pores, so proton-conducting pathways can be obtained by a network through hydrogen bonding between the backbone and guest molecules. In type III, the mobility or concentration of proton carriers can be strongly enhanced by phase transformation,21 heating,22 ligand substitution,23 ligand substitution defects,24 and structure diversity.25 It is worth noting that the strategies above imparting or improving the proton conductivity accompany changes of the structure of the protonconducting pathway, probably a principal factor for changes of the proton conductivity. It is desirable that the strategy to design better proton-conducting materials does not change the structure of the proton-conducting pathway for an in-depth understanding of the relationship between the structure and proton conductivity of the crystalline materials. Nevertheless, to

INTRODUCTION Control of the proton conductivity is important because of the potential application in fuel cells. State-of-the-art protonexchange membrane (PEM) fuel cells with Nafion as electrolytes can reach a conductivity on the order of 10−1− 10−2 S cm−1 under a limited condition with moderate temperatures (60−80 °C) and high relative humidity (98% RH).1 Platinum catalysts are necessary for PEMs under this operating temperature, which will lead to high cost and potential for CO poisoning.2 In recent years, crystalline materials, such as metal−organic frameworks (MOFs)/ coordination polymers (CPs)3 and covalent organic frameworks,4 extensively applied in gas sorption,5,6 catalysis,7 and magnetism,8 have been used to construct proton-conducting materials because of their rich structural tunability. These materials can provide abundant structural information via diffraction experiments to reveal the mechanism of the proton conduction pathway and the sequential structure−property relationship and can lead to better electrolyte design enabled via further adjustment and functioalization. As a result, proton conductivity control using the crystalline materials has attracted increased attention and become an interesting topic.4b,9−28 Up to now, three types (types I−III) of strategies to control the conductivity of the crystalline materials have been © XXXX American Chemical Society

Received: November 19, 2015

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DOI: 10.1021/acs.inorgchem.5b02686 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Asymmetric units of compound FJU-25-Fe. Asymmetric code: a, 0.5 − x, 0.5 − y, 0.5 − z; b, −x, −0.5 + y, 0.5 − z; c, 0.5 + x, 1 − y, z. (b) View of FJU-25-Fe along the a axis. FeO6 and PO4 are represented as green octahedral and yellow tetrahedral, respectively. Hydrogen bonds are shown as dotted lines.

Figure 2. (a) Arrhenius plots of the anhydrous conductivity of compounds from −10 to +90 °C under anhydrous conditions as determined by an ac impedance analyzer. (b) Typicl Nyquist plots of compounds at 90 °C. Scatters are experimental data, and lines are simulated values from equivalent circuits.

date, such a case has been uniquely observed in a layered MOF via interlayer cation substitution.26 Herein, we synthesized three isostructural imidazole cation (ImH2)-templated layered metal phosphates, [ImH2][X(HPO4)2(H2O)2] (FJU-25-X, X = Al, Ga, and Fe), based on two aspects: (1) the isostructural CPs have the same structure of the proton-conducting pathway; (2) imidazole is a good carrier for the proton conductor,27 even over a wide temperature range.28 Interestingly, their intrinsic proton conductivities increase with increasing size of the metal cations: Al3+ (53.5 pm) < Ga3+ (62 pm) < Fe3+ (64.5 pm).29 The intrinsic value for FJU-25-Fe can reach 5.21 × 10−4 S cm−1 at 90 °C, about 40-fold that for the aluminum analogue and comparable with the best anhydrous proton conductors Im@ UiO-67 (5.25 × 10−4S cm−1)16 and His@[Al(OH)(1,4-ndc)]n (7.02 × 10−4 S cm−1).17 Our work provides the first example demonstrating that the tuning of metal cation nodes may provide an efficient route to remarkably enhance the proton conductivity for the MOFs/CPs, even in the case that the structure of the proton-conducting pathway does not change.

The crystallographical data, bond lengths, and bond-valence sums32 for the compounds are listed in Tables S1 and S2. All metal cations look to be trivalent. FJU-25-Fe is taken as an example to describe the isostructural structures. FJU-25-Fe has corrugated metal−phosphate anionic layers constructed from alternating FeO6 octahedra and PO4 tetrahedra, which are separated by imidazole cations. There are one crystallographically independent Fe3+ ion and one P site. The coordinated atmosphere of Fe cations is completed by two water molecules and four O atoms from four phosphate groups (Figure 1a), with the Fe−O bond lengths ranging from 1.942 to 2.0578 Å. All P atoms are connected to four O atoms, two of which (O1 and O3) are coordinated to Fe atoms, with bond lengths typical of those found previously in other iron(III) phosphates materials.33 One of two remaining P−O bond lengths is considerably longer than the other (P1−O4 1.579 Å), consistent with the presence of distinct P−OH groups. This assignment is supported by the location of H4 in Fourier maps and by bond-valence calculations. Another P−O length (P1− O2 1.521 Å) is relatively short, suggesting some multiple-bond character. The imidazole-templated cations occupy the sites between the layers (Figure 1b) and balance the overall negative electrostatic charge of the FePO layers. The two N atoms of each template molecule are symmetry-related, and they are hydrogen-bonded to phosphoryl O atoms of both adjacent metal−phosphate layers (N1···O2 2.809 Å). As the size of the metal nodes in the three isostructural CPs increase, the unit cell

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RESULTS AND DISCUSSION The three CPs obtained through hydrothermal reaction are isostructural and crystallize in monoclinic space group C2/c. FJU-25-Al and FJU-25-Ga are the same as those obtained via ambient-temperature synthesis in silica gels.30 FJU-25-Fe is a new compound, while its isomer previously obtained from synthesis in silica gel crystallizes in the space group Pnam.31 B

DOI: 10.1021/acs.inorgchem.5b02686 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(30 °C), much higher than the values for solid bulk imidazole (10−8 S cm−1)27 as well as for other imidazole-trapped materials (3.3 × 10−8 and 1.69 × 10−7 S cm−1 for [Zn(HPO4)(H2PO4)2](ImH2)2 and [ImH2][Cu(H2PO4)2Cl]·H2O, respectively).21a,22 The value of FJU-25-Fe is higher than the highest material Im@Td-PPI (7.64 × 10−5 S cm−1)28 at 30 °C reported to date. Their proton conductivity increases linearly as the temperature increases. Until a temperature of 90 °C, their conductivity values reach a maximum of 1.30 × 10−5 (X = Al), 1.80 × 10−4 (X = Ga), and 5.21 × 10−4 (X = Fe) S cm−1, respectively. In particular, the value of FJU-25-Fe at 90 °C is comparable with the highest values (5.25 × 10−4 S cm−1 for Im@UiO-6716 and 7.02 × 10−4 S cm−1 for His@[Al(OH)(1,4ndc)]n17) among the anhydrous proton-conducting materials. The operating temperature for stable proton conduction on FJU-25-Fe ranges from −10 to +90 °C, wider than that for most of the representative proton conductors measured under moist conditions, although its maximum intrinsic proton conductivity is still lower than those for some conductors measured under moist conditions (Figure S9 and Table S4), similar to the cases in other anhydrous proton conductors. On the basis of the Arrhenius equation (Figure 2), the activation energies of FJU-25 are 0.20 (X = Al), 0.25 (X = Ga), and 0.22 eV (X = Fe), respectively, clearly smaller than those of the reported anhydrous imidazole-loaded MOFs.16,21a,22,27 The low activation energy reflects the Grotthuss mechanism34 of proton conduction for FJU-25. Notably, the observation of the high proton conductivity and low energy value of FJU-25-Fe from −10 to +90 °C suggests that it belongs to anhydrous fast-ion conductors.12

parameters (a, b, c, β, and volume) for FJU-25 increase, which may further affect their physical properties. Multiple hydrogen bonds between protonated imidazole (ImH2+) ions and orthophosphates (Figure S5) suggest the possibility of proton conductivity behavior. Therefore, the temperature dependence of proton conductivities was measured under anhydrous conditions at temperatures ranging from −10 to +90 °C by using alternating-current (ac) impedance spectroscopy. A noticeable phenomenon has been observed: the intrinsic proton conductivity of FJU-25-Fe is the highest, FJU-25-Ga is the second highest, and FJU-25-Al is the latest over the temperature (Figure 2a). The proton conductivities increase with increasing size of the metal cation nodes. The intrinsic value for FJU-25-Fe is about 40-fold that for the aluminum analogue over the tested temperature range. Because the CPs FJU-25-X possess completely coincident architectures except the metal cations, the density of imidazole in a unit cell depends on the sizes of the metal cations, which are 0.431 (X = Al), 0.421 (X = Ga), and 0.413 (X = Fe) g/cm3, respectively. Similar to the cases for imidazole-loaded porous organic polymers,28 the improved proton conductivity in FJU-25-Fe is attributed to the lower density and higher mobility of imidazole guests, which resulted from the increasing size of the metal cation nodes. A comparison of the conductivity of FJU-25 with those of some representative crystalline materials under anhydrous conditions is shown in Figures 3 and S9 and Table S4.

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CONCLUSIONS In conclusion, we have synthesized three isostructural CPs (FJU-25-X, where X = Al, Ga, and Fe) and first demonstrated that tuning of the metal cation nodes may provide an efficient way to remarkably enhance the intrinsic proton conductivity for crystalline materials by maintaining the structure of the protonconducting pathway. The value for FJU-25 with Fe3+ nodes can reach 5.21 × 10−4 S cm−1 at 90 °C, 40-fold that for the Al3+ analogue and comparable with the best anhydrous protonconducting materials. Because of the widespread existence of the isostructual MOFs, we believe that our findings will encourage further work in proton conductivity control using MOFs to enhance their proton-conducting properties through metal-node substitution.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 3. Arrhenius plots for FJU-25 in comparison with other representative proton-conducting materials under anhydrous conditions.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02686. Details of experimental methods and procedures on the proton conductivity, Fourier transform infrared spectra, thermogravimetric analysis curves, powder X-ray diffraction patterns, differential scanning calorimetry profiles, Nyquist plots, and selected bond lengths and bond-valence sums (PDF) X-ray crystallographic data in CIF format (CIF)

Anhydrous proton conductivities for most of the crystalline materials reported are remarkably reduced below room temperature, similar to the hydrated PEMs suffering from significant damage by freezing. Even under subzero temperature (−10 °C), obvious proton conductivity for FJU-25 has been observed with respective values of 1.88 × 10−6 (X = Al), 1.16 × 10−5 (X = Ga), and 5.15 × 10−5 (X = Fe) S cm−1, with few shown in crystalline materials, and the conductivity of FJU-25Fe is the highest among all reported anhydrous proton conductors at −10 °C. Upon heating, their conductivity values gradually increase to 5.34 × 10−6 (X = Al), 3.83 × 10−5 (X = Ga), and 1.79 × 10−4 (X = Fe) S cm−1 at ambient temperature

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +8615005999348. E-mail: [email protected]. C

DOI: 10.1021/acs.inorgchem.5b02686 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry *Tel: +8615005999348. E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21207018, 21273033, 21203024, and 21573042) and Fujian Science and Technology Department (Grants 2014J06003 and 2014H6007). S.X. gratefully acknowledges support of the Recruitment Program of Global Young Experts, Program for New Century Excellent Talents in University (NCET-10-0108), and the Award “MinJiang Scholar Program” in Fujian Province.

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DOI: 10.1021/acs.inorgchem.5b02686 Inorg. Chem. XXXX, XXX, XXX−XXX