Communication pubs.acs.org/IC
Significantly Dense Two-Dimensional Hydrogen-Bond Network in a Layered Zirconium Phosphate Leading to High Proton Conductivities in Both Water-Assisted Low-Temperature and Anhydrous Intermediate-Temperature Regions Daxiang Gui,†,‡,§ Tao Zheng,†,‡,§ Jian Xie,†,‡ Yawen Cai,†,‡ Yaxing Wang,†,‡ Lanhua Chen,†,‡ Juan Diwu,†,‡ Zhifang Chai,†,‡ and Shuao Wang*,†,‡ †
School for Radiological and Interdisciplinary Sciences (RAD-X) and ‡Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Jiangsu 215123, China S Supporting Information *
pathways. Proton-conducting CPs/MOFs can be generally divided into two categories based on two different operating temperature regions. The majority of materials are operated in a water-mediated environment below 373 K,6 which is very similar to other well-developed water-dependent materials such as the Nafion membrane.7 The proton conductivity for these materials can reach as high as 10−1 S cm−1, and the long-term stability is no more a technical issue.6a,8 The other category contains only a few examples, which can be operated in dry/wet gas (H2, N2, and air) in the intermediate-temperature range of 373−573 K and have better practical significance.9 However, the proton-transfer behavior for these materials mostly originates from dehydration or phase transition, and this often results in temporary proton conduction that cannot be used for practical applications.10 Therefore, the long-term stability of proton-conducting materials used at the intermediate-temperature range remains a significant challenge. One way to resolve this issue is to build a robust hydrogen-bond network that can be maintained over a wide range of temperature, so that the proton can efficiently transport within the network in both low-temperature water-assisted and intermediate-temperature anhydrous regions. Herein, we introduced a new layered zirconium phosphate, (NH4)2[ZrF2(HPO4)2] (denoted as ZrP-1), synthesized by the ionothermal reaction of ZrCl4 with H3PO3 in a solvent of 1-ethyl3-methylimidazolium hexafluorophosphate ([Emim]PF 6 ), where NH4+ was generated by the thermal decomposition of urea as the starting material (Supporting Information). This compound contains a significantly dense two-dimensional (2D) hydrogen-bond network that is thermally stable up to 573 K and can be indeed used as a two-in-one material with excellent proton-conducting properties in both temperature regions. Single-crystal X-ray diffraction (XRD) analysis reveals that ZrP-1 crystallizes in the space group P21/c. The asymmetric unit of ZrP-1 consists of a zirconium cation coordinated by four O atoms from phosphate groups and two F atoms, forming an octahedral coordination geometry (Figure S1). ZrO4F2 octahedra and PO4 tetrahedra are further connected in a corner-sharing manner to achieve zirconium phosphate anionic layers. Chargebalancing NH4+ cations are accommodated between the adjacent
ABSTRACT: A highly stable layered zirconium phosphate, (NH4)2[ZrF2(HPO4)2] (ZrP-1), was synthesized by an ionothermal method and contains an extremely dense two-dimensional hydrogen-bond network that is thermally stable up to 573 K, leading to combined ultrahigh waterassisted proton conductivities of 1.45 × 10−2 S cm−1 at 363 K/95% relative humidity and sustainable anhydrous proton conductivity of 1.1 × 10−5 S cm−1 at 503 K.
Z
irconium phosphates have been widely developed during the past 50 years, in many potential application fields, such as intercalation, ion exchange, catalysis, biotechnology, and solidstate proton conductivity.1 Many new zirconium phosphate structures have been synthesized via a hydrothermal or solvothermal method with organic amines as the structuredirecting agents.1a,2 Among these, α-Zr(HPO4)2·H2O (α-ZrP) and γ-ZrPO4(H2PO4)·2H2O (γ-ZrP) and their derivatives are the most famous examples. Nevertheless, for these traditional synthetic approaches it is difficult to form large single crystals of zirconium phosphates because of the high hydrolysis tendency of the Zr4+ ion or rapid precipitation of amorphous products, leading to precise structural determination being quite challenging. In contrast, ionothermal synthesis using an ionic liquid as both the structure-directing agent and the solvent is an effective way of inhibiting the hydrolysis of zirconium and leads to the formation of several novel zirconium phosphates such as layered ZrPOF-Epy as well as three-dimensional open frameworks ZrPOF-EA and ZrPO4-DES3.3 On the other hand, the proton-exchange membrane fuel cell is a promising energy technology because of its environmental friendliness and high-power-density features.4 Developing electrolyte materials possessing high proton conductivity and stability over a wide range of temperatures (298−573 K) is critical and one of the major hurdles for the next generation of the fuel-cell systems. During the past few years, crystalline materials, such as coordination polymers (CPs) or metal− organic frameworks (MOFs), have been extensively investigated for proton-conducting materials because of their various architectures and designable functionalities.5 Meanwhile, atomic-resolution structural information is also a powerful tool for understanding the mechanisms of proton transportation and © XXXX American Chemical Society
Received: September 23, 2016
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DOI: 10.1021/acs.inorgchem.6b02308 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
range where the hydrogen bond can form. An extremely dense 2D hydrogen-bond network (Figure 1b) is therefore obtained, potentially giving rise to a confined space where the protons are able to effectively transport.14 More importantly, the crystallinity of ZrP-1 can be completely maintained after treatment in a series of solutions including boiling water, CH3OH, and CH3CH2OH (Figures 2 and S4).
layers, where the interlayer spacing is 7.034 Å, as shown in Figure 1. The structure of ZrP-1 possesses a topology similar to that of
Figure 2. (a) TGA curve of ZrP-1 measured from 30 to 900 °C. (b) Variable-temperature and long-term-stability powder XRD data of ZrP1.
Thermogravimetric analysis (TGA) shows that the 2D hydrogen-bond network in ZrP-1 is stable up to 573 K without any noticeable weight loss in the nitrogen atmosphere (Figure 2a), indicating that the whole structure is highly robust over a wide temperature range. This is also confirmed by the variabletemperature powder XRD data (Figure 2b) and FTIR analysis (Figure S3). In addition, the long-term stability of ZrP-1 in dry air at 503 K was further checked to be excellent (the initial results shown in Figure 2b indicate that ZrP-1 can maintain the crystallinity for at least 6 days without any sign of crystal degradation). All of these significant features have endowed this material with a variety of prerequisites for practical applications as a wide-temperature-range solid electrolyte material under fuelcell operation conditions including both hydrogen- and alcoholbased fuel-cell systems. The proton conductivity was evaluated by alternating-current impedance spectroscopy using a pellet sample of ZrP-1. The conductivities of ZrP-1 at various combinations of humidity and temperature were measured (Figures S5 and S6). At 313 K and 50% relative humidity (RH), the conductivity of ZrP-1 is relatively low at 1.37 × 10−7 S cm−1, which is similar to other materials intercalated by the NH4+ cation.15 The conductivity increases from 1.37 × 10−7 to 1.84 × 10−4 S cm−1 as the temperature increases from 313 to 363 K under 50% RH. As the humidity further increases, ZrP-1 exhibits an ultrahigh proton conductivity of 1.45 × 10−2 S cm−1 at the conditions of 363 K and 95% RH, suggesting that the transportation of a proton in ZrP-1 is highly associated with water. This is similar to the majority of traditional low-temperature proton-conducting materials, where water plays an important role in improving the efficiency of proton transportation. A series of samples with three different particle sizes (i.e., different surface area) were selected using stainless steel cell cribbles for proton conductivity measurements, and the values of proton conductivity for these samples do not vary significantly (Figure S7). Therefore, we deduced that most of the water molecules are intercalated between the layers in ZrP-1, further aiding with proton conduction. The Grotthuss hopping mechanism is further elucidated by the calculated activation energy of 0.19 eV derived from the temperaturedependent behavior.16 This is consistent with the structural feature of ZrP-1 that a highly dense hydrogen-bond network is present, which is predominately responsible for the efficient
Figure 1. (a) View of the crystal structure of ZrP-1 along the b axis. ZrO4F2 and PO4 are shown as blue octahedra and yellow tetrahedra, respectively. Other atom color codes: O, red; N, marine; F, green. (b) View of the dense hydrogen-bond network in ZrP-1. The hydrogen bonds are shown as rendered multibond cylinder lines when the X−X bond distances are shorter than 3.4 Å (X = O, N, or F).
[ImH2][X(HPO4)2(H2O)2] (FJU-25-X, where X = Al, Ga, and Fe).11 The bond distance of Zr1−F1 is 1.980 Å, while the Zr1− O1 and Zr1−O3 bond distances are 2.069 and 2.068 Å, respectively. Within each PO4 tetrahedron, O2 and O3 atoms are bound to Zr atoms, with the P1−O1 and P1−O3 bond distances being 1.509 and 1.509 Å, respectively. The bond distance of P1− O4 is 1.550 Å, where the corresponding phosphate O atom is protonated, as suggested by bond-valence-sum calculations. P1− O2 is a double bond because the PO bond distance is 1.4781 Å.12 Typical symmetric and antisymmetric stretch vibration peaks of PO4 groups are observed in the range of 1186−1039 cm−1 in the Fourier transform infrared (FTIR) spectrum. Meanwhile, the broad peak located between 3313−3052 cm−1 (Figure S3 and Table S6) is attributed to the vibrations of the interlayer NH4+ cation.13 As shown in Figure 1b, the protonated O atoms within the phosphate group face toward the interlayer space and strongly interact with the NH4+ cation given the N1···O4 distance is 3.048 Å, suggesting the presence of strong hydrogen bonds. Additionally, the O4···O4 distance of the neighboring protonated O atoms in phosphate is 3.361 Å, while three sets of N1···O2 distances between the adjacent O atoms within the PO unit and NH4+ are 2.973, 3.170, and 2.984 Å, respectively, while the N1···F1 distance is 2.961 Å. All these values fall well within the B
DOI: 10.1021/acs.inorgchem.6b02308 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
vehicle-transfer mechanism is predominant.5b,17 Most likely, this behavior originates from the efficient thermal activation of the NH4+ cation as the proton carrier, which is also effectively stabilized by the 2D hydrogen-bond network at relatively high temperatures. Notably, this phenomenon substantially deviates from the dehydration/phase transition mechanisms that account for most other intermediate-temperature proton-conducting materials.18 Much more powerful evidence to demonstrate this hypothesis comes from the heating and cooling cycles showing almost overlapped dependent conductivity values, as shown in Figure 3c. This well reveals the advantage derived from the thermal stability of the dense 2D hydrogen-bond network and potential application of ZrP-1 as a practical proton conductor that can be utilized in the intermediate-temperature region, showing a clear advantage over previously reported zirconium phosphate proton-conducting materials.19 In summary, a new layered zirconium phosphate, ZrP-1, has been successfully synthesized using a facile ionothermal synthesis method. This compound possesses a two-in-one protonconducting behavior in two different temperature regions. One is in an ultrahigh value of 1.45 × 10−2 S cm−1 at 363 K and 95% RH, while the other reaches 1.1 × 10−5 S cm−1 at 503 K with a sustainable proton mechanism distinct from the typical dehydration/phase transition cases. These properties originate from the thermally robust and hydrolytically stable 2D hydrogenbond network in the structure, making ZrP-1 a truly comprehensive and promising solid electrolyte material in the fuel-cell system.
transportation of protons. It should be noted that, although this is not a substantial breakthrough in the value of low-temperature proton conductivity, stable proton-conducting materials with ultrahigh conductivity higher than 10−2 S cm−1 are still quite scarce to date.5b Given that the 2D hydrogen-bond network in ZrP-1 is thermally stable up to 573 K, its proton-conducting behavior in the intermediate-temperature range (373−503 K) with zero humidity was therefore investigated. Impressively, as shown in Figure 3b, the conductivity of ZrP-1 is very promising and increases from 1.18 × 10−7 to 1.1 × 10−5 S cm−1 as the temperature increases from 383 to 503 K. The calculated activation energy in this region is 0.46 eV, indicating that the
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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.6b02308. Experimental methods, crystal data, IR spectroscopy data, TGA, scanning electron microscopy−energy-dispersive spectroscopy analysis, and proton conductivity measurement (PDF) X-ray crystallographic file in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shuao Wang: 0000-0002-1526-1102 Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Foundation of China (Grants 21422704 and 91326112), Science Foundation of Jiangsu Province (Grants BK20140303 and BK20140007), Young Thousand Talented Program, and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Figure 3. (a) Impedance spectrum of ZrP-1 at 363 K under 95% RH conditions. (b) Impedance spectrum of ZrP-1 at 503 K under anhydrous condition. (c) Heating (black line) and cooling (red line) cycles of ZrP-1 showing almost overlapped dependent conductivity values.
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