Proton Conduction in Organically Templated 3D Open-Framework

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Proton Conduction in Organically Templated 3D Open-Framework Vanadium−Nickel Pyrophosphate Le Zhang,† Xinxin Liu,‡ Xuejiao Sun,† Juan Jian,† Guanghua Li,† and Hongming Yuan*,† †

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State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China ‡ Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, PR China S Supporting Information *

ABSTRACT: A new proton-conducting material (C6H14N2)[NiV2O6H8(P2O7)2]·2H2O (1) was hydrothermally synthesized by using 1,4-diazabicyclo[2,2,2]octane (DABCO) as the template. Its inorganic framework, determined by single crystal X-ray diffraction, is constructed by the connection of V/NiO6 octahedral to P2O7 pyrophosphate units through sharing oxygen atoms, giving rise to three-dimensional (3D) intersecting 6-, 8-, and 12-ring channels along the [100], [010], and [001] directions, respectively, in which there are ordered protonated DABCO cations balancing negative charge of the framework and disordered water molecules. Complex impedance measurements on polycrystalline samples gave proton conductivities of 4.9 × 10−3 and 2.0 × 10−2 S cm−1 at 25 and 60 °C under high humidity conditions, respectively. The activation energy is 0.38 eV.

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INTRODUCTION Proton-conducting materials have significant technical applications in both fuel cells and sensors.1 Over the past decades, many high proton conductivity conductors have been developed. Most of them possess 3D framework structures with various channels, such as micro- or mesoporous organic solids,2,3 polymers, and inorganic materials. Among them, the well-known Nafion membrane has high proton conductivity of 7.8 × 10−2 S cm−1 under 100% relative humidity (RH) at room temperature.4 Some of mesoporous silica materials exhibit proton conductivities of 2 × 10−1 and 2 × 10−2 S cm−1 at 140 and 50 °C under high RH conditions, respectively.5,6 In addition, metal−organic frameworks (MOFs) with welldefined channels are also excellent proton-conducting materials.7−9 Some of them achieve very high proton conductivities from 10−3 to 10−1 S cm−1 at 80−150 °C and high humidity due to the forming water hydrogen bond networks in MOF structures.10−13 At present, focus on metal pyrophosphates is growing rapidly due to their outstanding properties in terms of luminescence,14−16 nonlinear optical behavior,17−19 and ionic conduction.20−24 However, there are only several isostructural TP2O7 (T = Sn, Ti, Si, Ge, Ce, and Zr) that have exhibited proton conductivities on the order of 10−3−10−2 S cm−1 in the temperature range from 100 to 400 °C under anhydrous conditions.25−28 The frameworks of these materials are built up of metal−oxygen polyhedral and pyrophosphate units through sharing their corners and edges to form closely packed structures. As is well-known, organic amines are able to be used © XXXX American Chemical Society

as templates to synthesis microporous inorganic materials. An amine-templated vanadyl pyrophosphate with 1D chain structure has been reported.29 Nevertheless, so far there has been no example of 3D porous inorganic framework containing organic amine molecules as guest species with which possesses proton conduction property. In this work, we report the first organo-templated vanadium−nickel pyrophosphate, (C6H14N2)[NiV2O6H8(P2O7)2]·2H2O (1). Its 3D framework structure is constructed by corner-sharing V/NiO6 octahedral and P2O7 pyrophosphate units. Moreover, compound 1 exhibits high proton conductivity at temperatures of 25−60 °C in the presence of water.

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EXPERIMENTAL SECTION

Synthesis of Compound 1. The reactants and reagents used in our experiments were purchased from commercial sources. The mixture of V2O5 (1.0 mmol), H3BO3 (5.0 mmol), Ni(CH3COO)2 (1.0 mmol), H3PO4 (7.5 mmol), DABCO (3.5 mmol) and H2O (10 mL) was first stirred for 2h at room temperature, and then directly transferred into a 25 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 5 days. Blue plates were collected by filtration, washed with deionized water and ethanol and dried at room temperature for 12 h. The yield is around 60% (based on V2O5). Elemental analysis (wt %) for compound 1: Calcd: C, 10.10; H, 3.51; N, 3.93. Found: C, 9.98; H, 3.40; N, 3.75. Received: December 18, 2018

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DOI: 10.1021/acs.inorgchem.8b03526 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Characterization. Powder X-ray diffraction (PXRD) experiments were measured on a Rigaku D/max-2550 diffractometer using Cu Kα radiation (λ = 1.5418 Å) at room temperature. Inductively coupled plasma (ICP) analysis was performed on a PerkinElmer Optima 3300Dv spectrometer. Thermogravimetric analyzer (TGA) was tested on a TGA Q500 thermoanalyzer in temperature range of 25−800 °C in air. C, H, and N elemental analyses were carried out on a Vario MICRO elementar. Fourier transform infrared (FT-IR) spectrum was collected within the 4000−400 cm−1 region with a Bruker-IFS 66 V/S spectrometer using KBr pellets. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a VG Scienta R3000 spectrometer with Al Kα (1486.6 eV) as the X-ray source. Water absorption isotherm was measured using Quantachrome instrument. X-ray Crystallography. The structure of compound 1 was obtained on a Bruker Apex II CCD single crystal diffractometer with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 25 °C. The structure was determined by direct methods and refined using the SHELXL-2014 software.30 All non-hydrogen atoms in the lattice were determined using anisotropical technology. According to the crystallographic data and element, TGA, and ICP analysis results, the formula of compound 1 was derived. The CCDC number of compound 1 is 1853304. The related crystallographic data are summarized in Table 1, and bond lengths and angles are summarized in Tables S1.

simulated pattern on the basis of the structure, indicating the product is in pure phase (Figure S1). From crystallographic data, there is only one vanadium, nickel, and phosphorus atom site in the asymmetric unit, as shown in Figure 1, where both

Figure 1. Asymmetric unit of compound 1.

V4+ and Ni2+ ions are octahedrally coordinated with six oxygen atoms and a P5+ ion occupies the tetrahedral site. In VO6 octahedron, there are two unusual V−O distances, 1.617(4) and 2.298(4) Å, involving terminal VO bond and V−OH2, and the remaining four V−O distances are in the range of 1.996(2)−2.014(2) Å. These V−O distances have also been observed in vanadium pyrophosphates reported previously.31−33 The Ni−O bond lengths vary within 2.032(2)− 2.077(3) Å with average value of 2.047 Å. These results indicate that both of VO6 and NiO6 octahedra are distorted. In PO4 tetrahedron, the P−O bond length ranges from 1.499(2) to 1.6254(17) Å, and the value of O−P−O angle ranges within the range of 105.39(14)−113.17(13)°, indicating that the P atom lies at the center of a regular tetrahedron surrounded by four oxygen atoms as found in other pyrophosphates.18,34,35 Every last phosphorus atom shares three oxygen atoms with adjacent V and Ni atoms. To determine the valence state of V and Ni ions, XPS measurement of compound 1 was carried out. The spectrum gives the V 2P3/2 peak at 516.8 eV and Ni 2 P3/2 at 855.7 eV (Figure S2), which is consistent with the values reported for V4+ and Ni2+.36−39 The FT-IR spectrum exhibits absorption bands at 595, 712, 891, and 1060 cm−1 corresponding to the symmetric, asymmetric bending and stretching vibrations of P2O7 unit (Figure S3). The bands at 1460, 1620, 3100, 3333, 3480, and 3573 cm−1 can be ascribed to the bending and stretching vibrations of OH or water and NH.36,37 TGA of compound 1 was tested to confirm the existence of DABCO and free water molecules (Figure S4). The first weight loss of 4.6% (calcd 5.2%) attributes to the release of two water molecules from 25 to 300 °C. The second weight loss of 15.1% (calcd 16.5%) is corresponding to the removal of [N2C6H14]2+ in the temperature range of 300−540 °C. The structure of 1 consists of a three-dimensionally connected framework of strictly alternating V/NiO6 octahedra and P2O7 units. The VO6 octahedra and P2O7 groups linked together by sharing four vertexes to form 1D chain along the aaxis. These chains are cross-linked by sharing vertexes between NiO6 and P2O7 units into an infinite 3D framework, in which there are intersecting channels of 6-, 8-, and 12-rings running parallel to the [100], [010], and [001] directions, respectively (Figure 2). The larger chaplet-like 12-ring channel along the [001] direction is about 7.6 × 3.7 Å (O−O distances, Figure 2c). There are disordered water and protonated DABCO

Table 1. Crystal Data and Structure Refinement for 1 1 empirical formula formula weight temperature (K) space group a (Å) b (Å) c (Å) a (deg) β (deg) γ (deg) volume (Å3) Z calculated density (Mg·m−3) absorption coefficient (mm−1) F(000) theta range for data collection (deg) limiting indices reflections collected/unique data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e Å−3)

C6H26N2NiV2P4O22 762.51 293(2) Pbam 9.895(2) 13.453(3) 7.7295(15) 90.00 90.00 90.00 1028.9(4) 2 2.281 2.210 692 3.028−27.394 −12 ≤ h ≤ 12 −16≤ k ≤ 17 −10 ≤ l ≤ 9 8372 1251/1/110 1.168 R1 = 0.0389, wR2 = 0.1166 R1 = 0.0451, wR2 = 0.1212 2.119/−0.546

Proton Conductivity Measurements. Pellets of compound 1 with dimensions of 1.0 cm diameter and 0.92 mm thickness were prepared by pressing ground samples at room temperature in air. Each side of the pellet was attached to silver wires with conducting sliver paste to ensure good electrical contacts and then dried overnight. The impedance measurements were carried out by using Solartron SI 1260 impedance analyzer with a sweep frequency range from 1 to 105 Hz.

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RESULTS AND DISCUSSION Crystal Structure. The analysis of structure showed compound 1 belongs to orthorhombic space group Pbam. The PXRD pattern of as-synthesized sample agreed well with B

DOI: 10.1021/acs.inorgchem.8b03526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Compound 1 ac impedance plots at 25 °C and (a) 34% and 50% RH and (b) 86% RH; (c,d) ac impedance plots at different temperatures and 100% RH. Figure 2. 3D channels viewed along the (a) [100], (b) [010], and (c) [001] directions, respectively.

in proton-conducting MOFs materials.10−13 When RH was increased to 100%, its proton conductivity unexpectedly increased to 4.9 × 10−3 S cm−1, which is comparable to those of excellent proton conductors under the same RH,10−13 attributed to the formation of continuous hydrogen-bonding networks. The temperature dependence of proton conductivity in the temperature range of 25−60 °C at 100% RH are shown in Figure 4c,d. With temperature increasing, the conductivity increased to 2.0 × 10−2 S cm−1 at 60 °C, which outdoes the values measured below 400 °C for MP2O725−28 and is comparable to that of Nafion at 25 °C.2 Besides, the PXRD pattern of the pellet after carried out ac impedance measurements does not exhibit any change, confirming its intact structure. (Figure S6) The conductivities measured at each temperature and 100% RH were plotted log(σT) vs 1000/T, as shown in Figure 5. The linear plots indicate proton conduction in 1 follows the Arrhenius equation. On the basis of the slope of the line, the activation energy is calculated to be 0.38 eV, which is close to that of Nafion, indicating that this

molecules in the channels, which act as not only the agents to balance the negative charges of [NiV2O6H8(P2O7)2]2− but also the template through N−H···O hydrogen bonds (2.940(5)− 2.981(6) Å) (Tables S2) between the protonated DABCO and the oxygen atoms of the framework, as shown in Figure 3.

Figure 3. Protonated DABCO molecules inside channels and the Hbond network viewed along the [001] direction.

Proton Conductivity. The PXRD pattern obtainedafter thepellet of compound 1 was immersed in water did not show a detectable impure phase, implying that it is water-stable at room temperature (Figure S1). The room temperature proton conductivityof compound 1 was measured by alternating current (ac) impedance spectroscopy at 34, 50, 86, and 100% RH, respectively. With increasing RH, the conductivities obtained from the semicircles in the Nyquist plots (Figure 4a−c) increased from 3.4 × 10−6 S cm−1 at 34% RH to 6.7 × 10−5 S cm−1 at 86% RH. Moreover, water absorption increased with increasing humidity (Figure S5). The increase in RH obviously improves the proton conductivities, suggesting that the presence of hydrogen-bonding networks inside 3D channels of compound 1 is very important for proton conduction similar to the water-mediated situation occurring

Figure 5. Arrhenius conductivity plots of compound 1 at 100% RH. C

DOI: 10.1021/acs.inorgchem.8b03526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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proton conduction process follows the Grotthuss mechanism.40 Our results show that compound 1 has remarkable ability to conduct proton.

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CONCLUSION In summary, we have successfully synthesized vanadium− nickel pyrophosphate with high proton conductivity of 10−2 S cm−1 at high RH, which is the first example of metal pyrophosphate with 3D intersecting 6-, 8-, and 12-ring channels obtained by using organic amine as the template. Due to its high thermal stability, such proton-conducting compound is a potential candidate as solid electrolyte material for hydrogen fuel cells and sensors in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03526. PXRD, XPS, IR, TGA, bond lengths and angles of the compound 1 (PDF) Accession Codes

CCDC 1853304 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

Guanghua Li: 0000-0003-3029-8920 Hongming Yuan: 0000-0001-7677-5935 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.inorgchem.8b03526 Inorg. Chem. XXXX, XXX, XXX−XXX