An Alkali Metal Ion-Exchanged Metal-Phosphate (C2H10N2)xNa1–x

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

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An Alkali Metal Ion-Exchanged Metal-Phosphate (C2H10N2)xNa1−x[Mn2(PO4)2] with High Proton Conductivity of 10−2 S·cm−1 Kai-ming Zhang,†,§ Feng-yun He,‡ Hai-bao Duan,‡ and Hai-rong Zhao*,‡ †

Department of Material Engineering, Nanjing Institute of Technology, Nanjing 211167, P. R. China School of Environmental Science, Nanjing Xiaozhuang University, Nanjing 210009, P. R. China § Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, 1 Hongjing Road, Nanjing 211167, P. R. China

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‡

S Supporting Information *

ABSTRACT: A two-dimensional layered inorganic−organic hybrid metal hydrogenophosphate (1) was treated with 0.1 M NaOH−ethanol solution, which resulted in a Na+-ion substitution product that exhibits excellent thermal and aqueous stability with 1, as well as much higher proton conductivity (σ = 10−2 S·cm−1) even at low temperature (283 K). This is because Na+ ions in aqueous solution make a more dense and extensive H-bonding network of water molecules, which enables protons to more easily transfer along the network.

1. INTRODUCTION Porous proton-conducting materials, such as metal−organic frameworks (MOFs),1−11 porous coordination polymers (PCPs),12,13 and covalent organic frameworks (COFs),14,15 have attracted the interest of many researchers in the last several years because they can be used as key components in electrochemical devices.16−21 Among the candidates, MOFs have been extensively studied. On the basis of the extensive work of many researchers, some MOFs are reported to exhibit very high proton conductivities of up to 10−2 S·cm−1,22−25 which approaches the proton conductivity of commercial Nafion-based proton-conducting materials (σ = 10−1−10−2 S· cm−1). Moreover, MOFs can maintain their high proton conductivity over a wider range of temperatures than Nafion, the conductivity of which drops well above 80 °C owing to the loss of its inner water.26,27 Accordingly, great efforts have been made to identify desirable MOFs and their composite membranes28−31 to replace the Nafion-based proton-conducting materials used in fuel cells. However, most MOFs exhibit poor aqueous or chemical stability, and these shortcomings limit their application in electrochemical devices. Open-framework metal phosphate, another porous solid material, has a designable structure similar to that of MOFs, but better thermal and aqueous stability due to its robust inorganic units. In recent decades, it has shown considerable potential for application in separation processes, gas adsorption, and heterogeneous catalysis.32−36 More recently, open-framework metal phosphates have been recognized as © XXXX American Chemical Society

promising new candidate porous materials for proton conduction. Several open-framework phosphates, including JU103,37 JU102,38 and AlPO-CJ70,39 have been reported to achieve proton conduction. Our group also reported two types of inorganic−organic metal−hydrogenophosphate materials, (C 2 N 2 H 10 ) 0.5 CoPO 4 40 and (C 2 H 10 N 2 )[Mn 2 (HPO 4 ) 3 ](H2O),41 both of which exhibit good proton conductivity, 2.05 × 10−3 S·cm−1 at 329 K and 98% RH and 1.64 × 10−3 S· cm−1 at room temperature and 99% RH, respectively. However, the proton conduction is still lower than those of Nafion and some MOFs. For instance, in {[(Me2NH2)3(SO4)]2[Zn2(ox)3]}, the proton conductivity reached 4.2 × 10−2 S·cm−1 at 298 K and 98% RH,24 and that of Fe-CAT-5 reached 5.0 × 10−2 S·cm−1 at 298 K and 98% RH.42 Thus, improving their proton conductivities will promote their use in electrochemical devices, although this poses technical challenges. Enabling the effect of water is key to achieving high proton conductivity for water-mediated proton-conducting materials because protons can be transferred via the rearrangement of the H-bonding network (i.e., Grotthus mechanism) or as H3O+ with H2O molecules as vehicles (i.e., vehicle mechanism). In addition, high proton conductivity is often observed at high relative humidity. Therefore, it is vital to identify a material or effective method to enhance the formation of hydrogen bonds Received: November 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) The Mn4O20 cluster unit, (b) the connectivity of two PO43− anions, (c) the structure of an inorganic sheet, which is parallel to the ab plane, and (d) the layered packing along the c-axis for 1 (the coordination polyhedra of Mn(1) and Mn(2) are represented by dark green and light green, respectively).

2. EXPERIMENTAL SECTION

between water molecules and protons/proton carriers to provide effective proton-transport pathways, and ultimately heighten proton conductivity. It is widely known that Na+ ions, which are a kind of alkali metal ion with high ion potentials (Z/r, where Z is charge and r is ionic radius), can effectively enhance the formation of hydrogen bonds.43 However, there are only a few studies addressing the effect of Na+ ions on the proton conductivity of materials that contain both water and Na+ ions.44 Herein, we present a two-dimensional layered metal phosphate (C2H10N2)[Mn2(PO4)2]·2H2O (1),45 in which the inorganic layerprotonated ethylenediammonium that fills in the interlayer spacesshows high thermal and aqueous stability. Immersing 1 in an ethanol solution of NaOH results in an altered layer compound (C2H10N2)xNa1−x[Mn2(PO4)2]· 2H2O, in which some of the protonated ethylenediammoniums are replaced by Na+ ions via ion exchange. ICP and EDS data indicate that about 0.35% and 0.5% of Na+ ions are inserted into 1 when 1 is immersed in a NaOH−ethanol solution twice (2) and four times (3), respectively (immersing 1 in 0.1 M NaOH−ethanol solution for 3 days each time). It is exciting that both 2 and 3 exhibited high proton conductivity (10−2 S· cm−1), even at 283 K and 99% RH, which bear comparison with that of Nafion, which has the best performance under similar conditions.

2.1. Chemicals and Reagents. All reagents and chemicals were purchased from commercial sources and used without further purification. 2.2. Synthesis of (C2H10N2)[Mn2(PO4)2]·2H2O (1). Compound 1 was hydrothermally synthesized as in the reported method.45 Ethylenediamine (Aldrich) was dropped into a solution containing 0.1099 g of MnCl2·4H2O, 0.42 mL of H3PO4, and 5 mL of H2O to maintain a solution pH ≈ 8.4. Then, the solution was poured into a 25 mL stainless-steel reactor lined with Teflon, the lid was tightened, and it was put into an oven (165 °C) for 15 h. Synthesis of (C2H10N2)1−xNax[Mn2(PO4)2]·2H2O. We immersed the crystals of compound 1 (0.2000 g) in ethanol solution of 0.1 M NaOH for 3 days and then filtered and washed the reaction product with absolute ethyl alcohol three times. Then, the reaction product was vacuum-dried at 50 °C. The experimental operations were repeated twice (obtained compound 2) and four times (obtained compound 3), respectively. 2.3. Material Characterizations. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 diffractometer for Cu Kα radiation with λ = 1.5418 Å in the temperature range of 303− 493 K. Thermogravimetric analysis (TGA) experiments were carried out in an STA449 F3 thermogravimetric analyzer in 298−1073 K at a warming rate of 10 °C·min−1 under a nitrogen atmosphere. A BelsorpMax instrument was used to measure the water-adsorption/ desorption isotherms of 1, 2, and 3. 2.4. Conductivity Measurements. Proton conductivity measurements at various relative humidities were performed using a conventional three-electrode method on a CHI 660D electrochemical workstation, in which the reference electrode was shortened with an B

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. H-bond interactions in the inorganic [Mn2(PO4)2(H2O)]∞ sheet and between the guests and the inorganic [Mn2(PO4)2(H2O)]∞ sheet in the crystal of 1.

Figure 3. (a) PXRD patterns of 1 for the simulated, synthesized, and post-NaOH-treated samples in 5−50°. (b) PXRD patterns of 1 for the simulated and post-NaOH-treated samples in 8.5−11°. auxiliary electrode, and powdered polycrystalline pellets with a diameter of 13 mm and thicknesses of roughly 0.7, 0.49, and 0.45 mm for 1−3, respectively. 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 was zero. Two copper plate electrodes were used to clamp the pellet sample, which was then suspended in saltwater solution in a glass bottle with a rubber plug. This bottle was put into an environmental test box. The RH was adjusted by the content of salt in the salt solution and measured using a moisture meter.

P(1), and the last is contributed by the coordination water molecule. In contrast, for Mn(2), three O atoms (O(1), O(2), and O(3)) are supplied by PO43− ions with P(1), two others (O(5) and O(6)) come from PO43− ions with P(2), and the coordination water molecule provides the last O atom coordinated with Mn(2). The connectivity of two PO43− anions is shown in Figure 1b, where two different PO43− ions link four Mn2+ ions (two Mn(1) and two Mn(2)). Two crystallographically different MnO6 (Mn(1), Mn(2)) corner-sharing octahedrals develop into an Mn4O20 cluster unit (refer to Figure 1a). This cluster forms a two-dimensional planar structure along the ab plane via corner-sharing. As shown in Figure 1c, two crystallographically inequivalent Mn2+ ion intervals form a grid of inorganic layers [Mn2(PO4)2(H2O)]∞. The charges of the anionic sheets are compensated by ethylenediammonium dications, which fill the interlayer spaces of the inorganic layer (Figure 1d). There is a significantly shorter O···O distance in the inorganic [Mn2(PO4)2(H2O)]∞ sheet between O(2w) and O(7) with d = 2.548 A, between O(3) and O(1w) with d = 2.499 Å, and between the inorganic [Mn2(PO4)2(H2O)]∞ sheet and the ethylenediammonium with dN(2)···O(4) = 2.716 Å, dN(1)···O(4) = 2.731 Å, dN(2)···O(8) = 2.921 Å, dN(1)···O(8) = 2.711 Å (refer to Figure 2).

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. The crystal structure of 1 has been previously reported;45 so here, we offer a simple description for the convenience of discussing its proton-conducting behavior. Compound 1 crystallizes in the triclinic space group P1; its asymmetric unit contains two crystallographically independent Mn2+ ions (labeled as Mn(1) and Mn(2)), two different PO43− ions (labeled as P(1) and P(2)), and two coordination water molecules (labeled O(1W) and O(2W)) together with one protonated dication of ethylenediammonium. As shown in Figure 1a, two Mn2+ ions show an octahedral coordination sphere of six O atoms. For Mn(1), half of the O atoms (O(5), O(6), and O(7)) come from PO43− ions with P(2), two others (O(1) and O(2)) come from PO43− ions with C

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) TGA and (b) DTA plots for 1−3 between 298 and 1073 K.

Figure 5. (a) Humidity-dependent proton conductivities of 1−3 at 303 K. (b) Corresponding conductivities in the form of (a) σ vs T at 99% RH for 1−3.

coordination water in 1 at elevated temperatures. The peak corresponding to the (001) plane diffraction is split into two peaks at 433 K and some of the peaks have migrated at high angles, which indicates that loss of the coordination water reduces the interlayer spacing between the two layers of 1. The diffraction peak of the (001) plane disappears completely as the temperature rises to 478 K, demonstrating that the coordination water molecules in 1 were wholly removed, which is consistent with the TGA. The whole diffraction pattern continues to show high similarity with the simulation at room temperature for 1 at 533 K. This means that the loss of coordination water has no great influence on the crystal structure within the inorganic sheet. To investigate the effect of Na+ ions on improving the proton conductivity of 1, we replaced the ethylenediammonium in the interlayer spaces of 1 with Na+ ions using an ionexchange method. ICP-OES analysis indicated that, after two and four repetitions, 0.37 and 0.54 Na+ ions entered the inorganic layer of 1 after immersion in NaOH ethanol, which means 0.37 and 0.54 Na+ ions per formula, respectively (refer to Table S1). Furthermore, this result is illustrated by the EDS elemental mapping (refer to Table S2). Panels (a) and (b) in Figure 3 show the PXRD patterns of 2 and 3, which accord with the simulated profiles of 1 and indicate that the target

Figure 3a shows the PXRD profiles of 1 and its sample after exposure in air for 1 year, together with the simulated PXRD pattern of 1. All of the profiles show high similarity, which indicates that 1 has ultrastrong storage stability. Figure S1 shows the TGA and differential thermal analysis (DTA) curves of 1, in which a two-step decomposition process appears in the ranges of 395−468 K and 500−680 K, respectively. The weight loss percentage is estimated to be 9.74% in the first step, which is close to the calculated value (9.1%) corresponding to the loss of coordination water in 1. This weight-loss process is illustrated in the DTA plot of 1, in which there is a corresponding peak temperature at 443 K. Another decomposition process starting at 500 K corresponds to the decomposition of charge-compensating cations occupying the layered space. The mass loss (∼12.2%) is also close to the calculated value (12.8%). The TGA and DTA results demonstrate that 1 is thermally stable below approximately 680 K. Figure S2 shows the temperature dependences of the PXRD profiles of 1, in which the PXRD patterns are unchanged below 418 K. This indicates that the structure of the inorganic layer and interlayer of 1 is stable, although loss of the coordination water in 1 had begun between 303 and 418 K. The PXRD profiles show a slight change due to the further loss of the D

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Arrhenius eq 1, where the plot of ln(σT) against 1000/T shows an approximately linear relationship (Figure 7)

samples remain integrated after soaking in the NaOH ethanol. Figure 3b shows that the (0 0 1) diffraction peaks of 2 and 3 increase to a high angle compared with 1, indicating that the interlayer spacing had reduced after the insertion of Na+ ions. The TGA and DTA plots of 2 and 3 further demonstrate that the structure of 1 remains stable after undergoing ion exchange with Na+. As shown in Figure 4a, both 2 and 3 undergo a twostep thermal decomposition process, which is similar to that of 1. A slight difference is that the decomposition temperature of the exchange product is higher than that of 1. For example, the corresponding peak in the DTA increased from 445 to 472 K in the first decomposition process and the weightlessness of the ethylenediamine increased from 609 to 631 K in 3 (refer to Figure 4b). This can be attributed to the weak interaction of the inserted Na+ ions and ethylenediammonium, as well as the ion−dipole interactions between the Na+ ion and water molecules. We measured the ac impedances of the powdered sample of 1 at 303 K in the region of 60−99% RH. In the resultant Nyquist plots shown in Figure S3, we can observe a regular semicircle in the high-frequency region at low humidity, which is due to the bulk resistances. At high humidity, however, 1 gradually exhibits an irregular arc in the high-frequency region that corresponds to the bulk and grain boundary resistances. It also shows a spike in the low-frequency range caused by the electrode contribution, in which the values at the Z′-axis intercept are regarded as the resistance. We calculated the proton conductivity by fitting the Nyquist plots with the ZView program (the proton conductivity data shown in Table S3), which resulted in a proton conductivity of 3.25 × 10−8 S· cm−1 at 60% RH, 2.33 × 10−6 S·cm−1 at 85% RH, and a maximum value of 2.22 × 10−5 S·cm−1 at 99% RH (Figure 5a). These results indicate that the proton conductivity of 1 was improved by an increase in relative humidity, which is consistent with many reported water-mediated protonconducting materials.46,47 Figure 6 shows the temperature-dependent Nyquist plots of 1 measured in the range of 283 to 303 K at 99% RH. The

ln(σT ) = ln A −

Ea kBT

(1)

Figure 7. Corresponding conductivities in the form of ln(σT) vs 1000/T at 99% RH for 1−3.

in which σ is the proton conductivity, Ea represents the activation energy, kB is the Boltzmann constant, and A represents the pre-exponential factor. As determined from the least-squares fits of the slopes of the Arrhenius plots, the activation energies were 0.61 eV. As such, we conclude that the observed proton conductivity follows the vehicle mechanism, which means that the proton hopping of the adsorption water molecules in the layer space is inferior to the proton transfers. The Nyquist plots of 2 and 3 (shown in Figures S4−S6) are for the same temperature and humidity regions as 1, and the resultant proton conductivity also increases with increases in temperature and humidity, before reaching a maximum value of 1.3 × 10−2 S·cm−1 for 2 and 2.1 × 10−2 S·cm−1 for 3 at 303 K and 99% RH (Figure 5). Compared to 1, the proton conductivities of 2 and 3 have significant differences. First, the proton conductivities of 2 and 3 increase faster than that of 1 with increasing RH, with a 6 orders of magnitude increase for 2 from 60% RH to 99% RH, and just a 3 orders of magnitude increase for 1 in the same humidity range. Second, the proton conductivities of 2 and 3 are much higher than that of 1 over the whole temperature and humidity testing region, and the gap between 2, 3 and 1 is especially large at high humidity. For example, the proton conductivity is 6.91 × 10−8 S·cm−1 for 2 and 1.60 × 10−7 S·cm−1 for 3 at 60% RH and 303 K, which are 1.5 and 3.6 times that of 1. At 85% RH, the proton conductivity increased to 8.94 × 10−6 S·cm−1 for 2 and 2.48 × 10−5 S·cm−1 for 3 at 85% RH, which are 3.8 and 10.6 times that of 1 for the same conditions. The proton conductivity further increased to 1.3 × 10−2 S·cm−1 for 2 and 2.1 × 10−2 S· cm−1 for 3 at 99% RH, which are 585.6 and 945.9 times that of 1 (refer to Figure 5). This shows that the huge differences in the proton conductivities of 1 and 2, 3 are closely related to the RH. Finally, it is significant that the proton conductivities of 2 and 3 reached 10−2 S·cm−1 at 283 K, which bears comparison with that of Nafion. Thus, they have obvious advantages compared with many water-facilitated protonconducting materials that exhibit similar proton conductivities at higher temperature, such as UiO-66(SO3H) (σ = 8.4 × 10−2

Figure 6. Nyquist plots of 1 at 99% RH and selected temperatures.

results demonstrate that the proton conductivity of 1 increases with the rise in temperature. For example, the proton conductivity increased from 4.52 × 10−6 S·cm−1 at 283 K to 1.15 × 10−5 S·cm−1 at 295 K and reached a maximum value of 2.22 × 10−5 S·cm−1 at 303 K (Figure 5b). To further understand the proton-transport mechanism, we calculated the activation energies for the proton conduction of 1 using the E

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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S·cm−1 at 353 K, 90% RH),9 In-Cr-MOPs (σ = 5.8 × 10−2 S· cm−1 at 295 K, 98% RH),48 PCMOF-10 (σ = 3.55 × 10−2 S· cm−1 at 343 K, 95%RH),22 and Fe(THO)·Fe(SO4)(Me2NH2)3 (σ = 5 × 10−2S·cm−1 at 298 K, 98% RH).42 To estimate the activation energies of 2 and 3, we used the Arrhenius equation to fit their Nyquist plots. The activation energies of 2 and 3 were 0.085 and 0.14 eV (refer to Figure 7), respectively, which demonstrates that the proton transport process follows the Grotthuss mechanism. In addition, this indicates that there is a difference in the proton conduction mechanism between 1 (vehicle mechanism) and 2, 3 (Grotthuss mechanism). To gain insight into the influences on these mechanisms, we measured the water sorption isotherms of 1, 2, and 3 at 303 K. The results show that the amount of water sorption sharply increases after ion exchange and 3 exhibits the largest amount of water sorption, which is consistent with the proton conductivity results shown in Figure S7. Considering that the structure remains unchanged with the ion-exchange treatment (shown in Figure 3a), the ion−dipole interaction between the Na+ ion and water molecules is probably the main factor leading to the different water sorption properties before and after ion exchange. This observation agrees with the effect of Na+ ions in aqueous solution, which can form a dense and extensive hydrogen-bonding network of water molecules. As such, the proton transfers along the hydrogen-bonding network occur more easily, which increases the proton conductivity, but decreases the activation energy. Besides that, the transport of Na+ ions also probably promotes the increased conductivities of 2 and 3 so that the rise in conductivity maybe attributable to both protons and Na+ ions.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 25 13914700426. ORCID

Hai-rong Zhao: 0000-0001-8977-1319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grant No. 51604155), the Foundation of the Jiangsu Education Committee (17KJB150028), the Introducing Talents Fund of Nanjing Institute of Technology (YKJ201707), and the School Project of Nanjing Xiaozhuang University (2016NXY42) for their financial support. This work was supported by the outstanding scientific and technological innovation team of the colleges and universities of Jiangsu province.

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REFERENCES

(1) Wei, Y. S.; Hu, X. P.; Han, Z.; Dong, X. Y.; Zang, S. Q.; Mak, T. C. W. Unique Proton Dynamics in an Efficient MOF-Based Proton Conductor. J. Am. Chem. Soc. 2017, 139, 3505−3512. (2) Colodrero, R. M. P.; Angeli, G. K.; Bazaga-Garcia, M.; OliveraPastor, P.; Villemin, D.; Losilla, E. R.; Martos, E. Q.; Hix, G. B.; Aranda, M. A. G.; Demadis, K. D.; Cabeza, A. Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids. Inorg. Chem. 2013, 52, 8770−8783. (3) Colodrero, R. M. P.; Papathanasiou, K. E.; Stavgianoudaki, N.; Olivera-Pastor, P.; Losilla, E. R.; Aranda, M. A. G.; León-Reina, L.; Sanz, J.; Sobrados, I.; Choquesillo-Lazarte, D.; García-Ruiz, J. M.; Atienzar, P.; Rey, F.; Demadis, K. D.; Cabeza, A. Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphousto-Crystalline Transformations. Chem. Mater. 2012, 24, 3780−3792. (4) Inukai, M.; Horike, S.; Itakura, T.; Shinozaki, R.; Ogiwara, N.; Umeyama, D.; Nagarkar, S.; Nishiyama, Y.; Malon, M.; Hayashi, A.; Ohhara, T.; Kiyanagi, R.; Kitagawa, S. Encapsulating Mobile Proton Carriers into Structural Defects in Coordination Polymer Crystals: High Anhydrous Proton Conduction and Fuel Cell Application. J. Am. Chem. Soc. 2016, 138, 8505−8511. (5) Hurd, J. A.; Vaidhyanathan, R.; Thangadurai, V.; Ratcliffe, C. I.; Moudrakovski, I. L.; Shimizu, G. K. H. Anhydrous Proton Conduction at 150 °C in a Crystalline Metal-Organic Framework. Nat. Chem. 2009, 1, 705−710. (6) Bureekaew, S.; Horike, S.; Higuchi, M.; Mizuno, M.; Kawamura, T.; Tanaka, D.; Yanai, N.; Kitagawa, S. One-Dimensional Imidazole Aggregate in Aluminium Porous Coordination Polymers with High Proton Conductivity. Nat. Mater. 2009, 8, 831−836. (7) (a) Horike, S.; Umeyama, D.; Inukai, M.; Itakura, T.; Kitagawa, S. Coordination-Network-Based Ionic Plastic Crystal for Anhydrous Proton Conductivity. J. Am. Chem. Soc. 2012, 134, 7612−7615. (b) Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent Proton Conduction in a 2D Coordination Framework. J. Am. Chem. Soc. 2012, 134, 12780−12785. (8) (a) Horike, S.; Chen, W. Q.; Itakura, T.; Inukai, M.; Umeyama, D.; Asakurae, H.; Kitagawa, S. Order-to-disorder structural transformation of a coordination polymer and its influence on proton conduction. Chem. Commun. 2014, 50, 10241−10243. (b) Inukai, M.; Horike, S.; Chen, W. Q.; Umeyama, D.; Itakura, T.; Kitagawa, S. Template-directed proton conduction pathways in a coordination framework. J. Mater. Chem. A 2014, 2, 10404−10409. (9) Phang, W. J.; Jo, H.; Lee, W. R.; Song, J. H.; Yoo, K.; Kim, B.; Hong, C. S. Superprotonic Conductivity of a UiO-66 Framework

4. CONCLUSION In summary, in this paper, we presented a two-dimensional layered inorganic−organic hybrid metal hydrogenophosphate 1 and its Na+ ions substitution products 2 and 3 and found that all compounds possessed excellent thermal and chemical stability. Moreover, 2 and 3 exhibited high proton conductivities even at low temperature. The high proton conductivities are attributed to the formation of a more dense and extensive hydrogen-bonding network of water molecules caused by the effect of Na+ ions in aqueous solution, wherein protons transfer more easily along the hydrogenbonding network. This type of high conductive material, together with its excellent thermal and chemical stability, has promise for application in electrochemical devices. The mechanism of the alkali metal ion-exchange product requires further study.

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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.8b03278. TGA and DTA plots together with the variabletemperature powder X-ray diffraction profiles of 1, Nyquist diagrams of 1, 2, and 3 at 303 K and the selected relative humidity. Nyquist plots of 2 and 3 at 99% RH and the selected temperatures. Wateradsorption/desorption isotherms of 1, 2, and 3 at 298 K. Tables that summarize the Na, Mn, and P contents for 1 exchange using NaOH and the proton conductivity data of 1, 2, and 3 (PDF) F

DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03278 Inorg. Chem. XXXX, XXX, XXX−XXX