A Two-Dimensional Inorganic–Organic Hybrid Solid of Manganese(II

Article pubs.acs.org/IC

A Two-Dimensional Inorganic−Organic Hybrid Solid of Manganese(II) Hydrogenophosphate Showing High Proton Conductivity at Room Temperature Hai-Rong Zhao,† Chen Xue,† Cui-Ping Li,† Kai-Ming Zhang,*,† Hong-Bin Luo,† Shao-Xian Liu,† and Xiao-Ming Ren*,†,‡,§ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering and College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China § State Key Laboratory & Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China ‡

S Supporting Information *

ABSTRACT: The inorganic−organic hybrid metal hydrogenophosphate with a formula of (C2H10N2)[Mn2(HPO4)3](H2O) (1) shows layered crystal structure. The inorganic anion layer is built from Mn3O13 cluster units, and the interlayer spaces are filled by the charge-compensated ethylenediammonium dications together with the lattice water molecules. The thermogravimetry, variable-temperature powder X-ray diffraction, and the proton conductance under anhydrous and moisture environments were investigated for 1, disclosing that 1 shows high thermal stability and high proton transport nature, and the proton conductivity reaches to 1.64 × 10−3 S·cm−1 under 99% RH even at 293 K. The high proton conductivity is related to the formation of denser H-bond networks in the lattice.

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INTRODUCTION Proton-conducting materials have displayed a range of practical applications in a variety of electrical and electrochemical devices, such as the proton exchange membrane fuel cell,1−6 electrochemical sensor,7 electrochromic display device,8 and others.9 In the area of proton conductors, the organic polymers and perovskite-type oxides have been extensively studied so far. One of the most important organic polymer-based proton conductors is Nafion,10 which possesses high proton conductivity (1 × 10−1 to 1 × 10−2 S·cm−1 under humidity at room temperature), while it is costly. Some doped perovskite-type oxide ceramics possess oxide vacancies in the lattice; the water vapors or hydrogen molecules in ambient gas can be absorbed into the oxide vacancies to give rise to protons. Such types of oxide ceramics generally show high proton conductivity owing to the protons hopping between the oxide vacancies in the lattice; however, the efficient proton transport undergoes only at elevated temperature (>600 °C).11 Accordingly, it has attracted much research attention to develop new protonconducting material with low cost and high conductivity. In the past decade, the metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) have emerged as a new type of proton-conducting material.12−17 With respect to the conventional proton conductors of organic polymers and perovskitetype oxide ceramic, the MOFs/PCPs-based proton conductors show significant advantages, for example, the pore size in the framework of MOFs/PCPs is designable and controllable, and the large pores favor increase in the movement of proton carriers that improves proton conductance characteristics as © 2016 American Chemical Society

well. Nevertheless, in most cases, the poor stability to water is an obvious drawback for MOFs/PCPs, and this is because of the weak coordination bonds between the nodes (metal ions) and the linkers (organic ligands). It is predictable to overcome this issue of stability to water, if the porous framework or sheet in the MOFs/PCPs is built from the robust inorganic units. And therefore, the porous or layered inorganic−organic hybrids are one of the best candidates in new types of protonconducting materials. Metal phosphate-based inorganic−organic hybrids have been widely explored in various fields due to their excellent chemical and thermal stabilities, designable structures, and tunable functionalities.18−21 To the best of our knowledge, the studies have been scarcely reported on the proton conductance for metal phosphate-based inorganic−organic hybrids.22,23 Herein we present the investigation of proton conductance for a layered metal−hydrogenophosphate (C 2 H 10 N 2 )[Mn 2 (HPO4)3](H2O) (1, which crystal structure is shown in Figures S1 and S2). Hybrid 1 hydrothermally synthesized24 exhibits high thermal stability and high stability to water. Most importantly, this hybrid shows high proton conductivity, 1.64 × 10−3 S·cm−1, under 99%RH even at room temperature.

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EXPERIMENTAL SECTION

Chemicals and Reagents. All reagents and chemicals were purchased from commercial sources and used without further purification. Received: June 16, 2016 Published: August 10, 2016 8971

DOI: 10.1021/acs.inorgchem.6b01438 Inorg. Chem. 2016, 55, 8971−8975

Article

Inorganic Chemistry Sample Preparation. The sample was hydrothermally synthesized according to the previously reported procedure.24 MnCl2·4H2O (0.1099g) and H3PO4 (0.42 mL) were dissolved in 5 mL of H2O, and ethylenediamine (Aldrich) was added dropwise to the mixture to control pH ≈ 6.4 of the solution, which was transferred to and sealed in a 25 mL Parr Teflon-lined stainless steel autoclave and heated at 165 °C for 15 h. The pink plate crystals of (C2H10N2)[Mn2(HPO4)3](H2O) (1) was obtained. Elemental microanalysis calculated for C2H15N2Mn2P3O13 (1): C, 5.03; H, 3.16; N, 5.86%. Found: C, 5.04; H, 3.06; N, 5.80%. Material Characterizations. Elemental analyses (C, H, and N) were performed with an Elementar Vario EL III analyzer. Powder Xray diffraction (PXRD) data were collected on a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the temperature range of 303−493 K (30−220 °C). Thermogravimetric analysis (TGA) experiments were performed using an STA449 F3 thermogravimetric analyzer in the temperature range of 298−1073 K (25−800 °C) at a warming rate of 10 °C·min−1 under a nitrogen atmosphere and the polycrystalline samples were placed in an Al2O3 crucible. Conductivity Measurements. The powdered sample was pressed into a pellet with a diameter of 13 mm and a thickness of ca. 0.62 mm. Impendence data were collected at the anhydrous condition using a Concept 80 system (Novocontrol, Germany), and the frequency ranged from 100 to 10 MHz in the temperature range of 273−518 K (0−245 °C). The direct-current (DC) offset was zero. The measurements of proton conduction at various relative humidities were performed using a conventional three-electrode method with a CHI 660D electrochemical workstation, and the reference electrode was shortened with auxiliary electrode; the cable connecting the copper plate electrodes with the electrochemical workstation was ca. 1.2 m. The copper plate electrode was 13 mm in diameter. The frequency of the applied alternating current (ac) field ranged from 100 Hz to 2 MHz with 5 mV of signal amplitude. The DC offset is zero. The powdered pellet of sample was sandwiched between two copper plate electrodes, which is suspended in a wide-mouth bottle with a rubber stopper and water solution of salt on the bottom; such a bottle was placed in an oven. The humidity was tuned by the concentration of salt and monitored using a humidity probe.

with this weight-losing process. The subsequent mass loss (ca. 5%) that occurred between 550 and 569 K is probably attributed to the dehydroxylation of hydrogen phosphates (HPO42−). The charge-compensating cations occupied the interlayer spaces, and the anionic sheets decompose when the temperature is further elevated. The TG and DTA analyses demonstrated that the anhydrous phase of 1 is thermally stable below ca. 550 K. The temperature dependences of PXRD profiles are displayed in Figure 2 for 1. The PXRD patterns below 413 K are almost the same as that at 303 K, indicating that the crystal structure within the sheet and the interlayer spaces are not altered in the temperature range of 303−413 K, albeit the lattice water molecules start to be released in this temperature range. As the lattice water molecules are further lost at elevated temperatures, the PXRD profiles show slight change; for example, the peak corresponding to (2 0 0) plane diffraction is split into two peaks at 443 K. One of them shifts toward the higher 2θ angle, indicating that the partial interlayer spaces are reduced owing to losing lattice water. When the temperature is up to 473 K, the (2 0 0) diffraction peak in 1 completely disappears, demonstrating that the lattice water molecules were wholly removed, and this observation is in good agreement with the TG analysis. Meanwhile, the diffraction with 2θ = 10.3°, which corresponds to the (1 1 0) diffraction in 1, is split into two peaks; however, the whole diffraction pattern still shows the high similarity with the simulated one at room temperature for 1, disclosing that the lattice water releasing only slightly affects the crystal structure within the inorganic [Mn2(HPO4)3]∞ sheet, which structure is shown in Figures S1 and S2. Proton Conductivity. The proton-conducting behavior of 1 was studied by ac impedance measurements under an anhydrous N2 atmosphere between 328 and 518 K, and the Nyquist plots were shown in Figure 3 and Figure S3. The arc in the high-frequency region together with a spike in the lowfrequency range were observed; they are due to the bulk resistance and electrode contribution, respectively. As shown in Figure 3b, two arcs in the high-frequency region with a spike in the low-frequency range appear when the temperature was increased to 433 K. The new arc may be caused by the grain boundary, and the analogous behavior has been also observed in other proton-conducting materials.25 Proton conductivity was calculated by fitting the arc at the selected temperatures using the ZView program, and the temperature dependences of conductivity (σ) are plotted in Figure 3c,d in both the forms of σ versus T and ln(σT) versus 1000/T. The proton conductivity σ = 1.97 × 10−10 S·cm−1 at 328 K; the value quickly increases with the temperature rising and reaches to 2.97 × 10−6 S·cm−1 at 398 K owing to the activation energy being reduced for the proton transport at elevated temperature. It is noted that the proton conductivity is over 1 × 10−5 S·cm−1 when the temperature is higher than 450 K. In the temperature region of 328−518 K, as displayed in Figure 3c, two maxima occur in the σ versus T plot of 1, which are, respectively, located at 403 and 488 K, and rather close to the peak temperatures of two corresponding losing weight process at 404 and 482 K in the DTA plot. As a result, two maxima in the σ versus T plot are related to the process of losing lattice water. The ln(σT) is plotted against 1000/T in the temperature range of 328−518 K and shown in Figure 3d. Obviously, the ln(σT) versus 1000/T plot shows nonlinear relationship across the entire temperature range. To roughly estimate the proton

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RESULTS AND DISCUSSION Thermogravimetry and Powder X-ray Diffraction. TG and differential thermal analysis (DTA) plots of 1 are displayed in Figure 1. The lattice water molecules start to release at ca.

Figure 1. TG and DTA plots of 1 in the temperature range of 298− 1073 K.

353 K, and the loss of lattice water continues to 473 K; the percentage of weight loss is estimated as 4.04% in the temperature range of 353−473 K, which is close to the calculated value (3.77%) that corresponds to losing one water molecule per formula unit of (C2H10N2)[Mn2(HPO4)3(H2O)]. In the DTA plot of 1, a peak temperature at 404 K is associated 8972

DOI: 10.1021/acs.inorgchem.6b01438 Inorg. Chem. 2016, 55, 8971−8975

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Figure 2. Variable-temperature PXRD profiles of 1 with 2θ ranges of (a) 5−50° and (b) 7−18°.

Figure 3. Temperature-dependent (a, b) Nyquist plots and (c, d) the corresponding conductivity in the form of (a) σ vs T and ln(σT) vs 1000/T under N2 atmosphere for 1.

transport activation energy Ea, we fitted the temperaturedependent conductivities in two different temperature regions (328−383 K and 428−470 K) using the Arrhenius eq 1,26 where the corresponding ln(σT) against 1000/T plot shows approximately linear relationship. ln(σT ) = ln A −

Ea kBT

of 278−293 K for 1. The ac impedance measurements were performed at 33%, 43%, 60%, 75%, 85%, and 99%RH, and the corresponding Nyquist plots are shown in Figure 4a and Figure S4. The RH dependences of proton conductivity are displayed in Figure 4b, demonstrating that the proton conductance is quite sensitive to the RH. In addition, with regard to the anhydrous environment, the proton conductivity shows tremendous enhancement in the moisture atmosphere. For example, the proton conductivity is ca. 1.0 × 10−7 S·cm−1 under 60%RH and reaches to 1.64 × 10−3 S·cm−1 under 99%RH at 293 K. Such high conductivity at 99%RH in 1 is comparable to that recently reported for the several high proton-conducting MOFs/PCPs; for example, PCMOF-5 with σ = 1.3 × 10−3 S· cm−1 at 294.5K under 98%RH,27 (NH4)2[MnCr2(ox)6]3·4H2O with σ = 1.0 × 10−3 S·cm−1 at 295 K under 96%RH,28 MgH6ODTMP·6H2O (σ = 1.6 × 10−3 S·cm−1 at 292 K under 100%RH),12b and CaPiPhtA-NH3 (σ = 6.6 × 10−3 S·cm−1 at 297 K under 98%RH).12a Plots of ln(σT) versus 1000/T at selected RH are displayed in Figure 5a, and the RH dependences of conductivity at the

(1)

In eq 1, the symbol σ represents the proton conductivity, Ea is the proton transport activation energy, kB is the Boltzmann constant, and A is the pre-exponential factor. The best fits gave the activation energy Ea = 1.17 eV in the range of 328−383 K versus Ea = 0.67 eV in the range of 428−470 K, implying that the proton-conduction process mainly follows the vehicle mechanism in the anhydrous environment, and this is due to the H-bond network for proton hopping being interrupted when the lattice water molecules were removed. The relative humidity (RH) dependences of proton conductance were also investigated in the temperature range 8973

DOI: 10.1021/acs.inorgchem.6b01438 Inorg. Chem. 2016, 55, 8971−8975

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Figure 4. Nyquist plots of 1 at (a) 60%, (b) 75%, (c) 85%, and (d) 99%RH and the selected temperatures.

Figure 5. (a) Plots of ln(σT) vs 1000/T at selected RH and (b) plots of σ vs RH at the selected temperatures for 1.

hydrogenophosphate and discovered that this hybrid shows high proton conductivity under higher RH even at ambient temperatures. The high proton conductivity is attributed to the formation of denser hydrogen-bonding networks in the lattice of 1, which not only provide efficient proton-transfer pathways for protons hopping but also block the migrating of the ethylenediammonium cations to reduce the activation energy for proton transport. Such a type of high conducting material at ambient temperature maybe has promising application in the proton conductor-based devices.

selected temperatures are shown in Figure 5b. By means of linearly fitting the plots of ln(σT) versus 1000/T, the activation energy of proton transport is estimated to be 0.22 eV at 99% RH, 0.37 eV at 85%RH, 0.87 eV for 60%RH, and 1.13 eV for 75%RH for 1, indicating that the proton transfer in 1 follows an efficient Grotthuss mechanism at higher RH, while the vehicle mechanism is under lower RH. It is understandable that the RH strongly affects the proton conductivity and the activation energy of proton transport in 1, since there exists an adsorption−desorption equilibrium of water in the certain moisture condition for the crystal of 1, and more water molecules are absorbed into the interlayer spaces of 1 at higher RH. However, more water molecules within the interlayer spaces of 1 not only results in the formation of denser hydrogen-bonding networks, which provide efficient protonhopping pathways, but also blocks the migrating of the cations, so that the Grotthuss mechanism is governed at higher RH, while the vehicle mechanism is predominant at lower RH.

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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.6b01438. Crystal structure description together with figures of Mn3O13 cluster unit, the connectivity of three HPO42− anions, inorganic sheet structure, the layered packing along a-axis and H-bond interactions in 1; Nyquist diagrams of 1 at different temperatures (below 100 °C) under N2 atmosphere as well as at 33% and 43%RH in

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CONCLUSION In summary, we presented the study on proton-conducting behavior for a layered inorganic−organic hybrid metal 8974

DOI: 10.1021/acs.inorgchem.6b01438 Inorg. Chem. 2016, 55, 8971−8975

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278−293 K; table that summarizes the proton conductors published with high proton conductivity (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (KMZ) *E-mail: [email protected]. (XMR) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the National Nature Science Foundation of China (Grant No. 21271103) for financial support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01438 Inorg. Chem. 2016, 55, 8971−8975