Visualization of the Diffusion Pathway of Protons in (NH4)2Si0.5Ti0

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

Visualization of the Diffusion Pathway of Protons in (NH4)2Si0.5Ti0.5P4O13 as an Electrolyte for Intermediate-Temperature Fuel Cells Chunwen Sun,*,†,◊ Lanli Chen,∇ Siqi Shi,*,∇ Berthold Reeb,‡ Carlos Alberto López,⊥ José Antonio Alonso,*,§ and Ulrich Stimming‡,¶,□ †

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China ∇ School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China ‡ Bavarian Center for Applied Energy Research (ZAE Bayern), Division Energy Storage, Walther-Meißner-Straße 6, Garching D-85748, Germany ⊥ Instituto de Investigaciones en Tecnología Química (INTEQUI), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Química, Bioquímica y Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, San Luis 5700, Argentina § Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (CSIC), Cantoblanco, Madrid 28049, Spain ¶ School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU Newcastle, United Kingdom □ TUM CREATE, 1 CREATE Way, #10-02 CREATE Tower, Singapore 138602, Singapore ◊ School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: We demonstrate that (NH4)2Si0.5Ti0.5P4O13 is an excellent proton conductor. The crystallographic information concerning the hydrogen positions is unraveled from neutron-powder-diffraction (NPD) data for the first time. This study shows that all the hydrogen atoms are connected though H bonds, establishing a two-dimensional path between the [(Si0.5Ti0.5)P4O132−]n layers for proton diffusion across the crystal structure by breaking and reconstructing intermediate H−OP bonds. This transient species probably reduces the potential energy of the H jump from an ammonium unit to the next neighboring NH4+ unit. Both theoretical and experimental results support an interstitial-proton-conduction mechanism. The proton conductivities of (NH4)2Si0.5Ti0.5P4O13 reach 0.0061 and 0.024 S cm−1 in humid air at 125 and 250 °C, respectively. This finding demonstrates that (NH4)2Si0.5Ti0.5P4O13 is a promising electrolyte material operating at 150− 250 °C. This work opens up a new avenue for designing and fabricating high-performance inorganic electrolytes.

1. INTRODUCTION

PEMFCs or direct-alcohol fuel cells operating at higher temperatures show improved electrode-reaction kinetics and enhanced impurities tolerance to the fuels. Thus, solid-state proton conductors with high conductivities operating at 150− 400 °C are highly desired for developing intermediatetemperature fuel cells (ITFCs).4−7 Ammonium polyphosphate based electrolytes are possible candidates.4,5,8 However, purephase ammonium polyphosphate is not stable at temperatures higher than 200 °C. It is noteworthy that using a composite with highly dispersed oxides is an effective strategy to improve stability and ionic conductivity.9,10 Although Wang et al.11

The use of fuel cells as clean-power sources have attracted wide attention because of their high energy-conversion efficiency as well as their low emissions.1−3 Notably, fuel-cell-powered vehicles were introduced into the market by several vehicle companies in 2014. However, the present polymer-electrolytemembrane fuel cells (PEMFCs) with Nafion as the protonconducting electrolyte are usually operated below 100 °C. CO poisoning is a concern if the cell works with carbon-based fuels at these temperatures. Thus, increasing the loadings of noblemetal catalysts and alloying Pt with other oxophilic elements are usually used to enhance the catalytic activity and mitigate CO poisoning. © XXXX American Chemical Society

Received: September 29, 2017

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DOI: 10.1021/acs.inorgchem.7b02517 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

example, the phosphorus in the tetrapolyphosphate groups have sp3 hybridization, indicating a chemical constraint to tetrahedral geometry. Hence, the angles O−P−O can be constrained to 109.5°. Additionally, the P−O distances can also be constrained to 1.55 Å, because this value is maintained (within a tolerance range) in the crystal structures of several polyphosphate materials. Considering that the Si and Ti atoms are in octahedral coordination, the three angles formed by the opposite oxygen and Si (or Ti) atoms were constrained to 180°. Furthermore, the shapes of the ammonia groups also were constrained. For this purpose, the H−N−H angles were constrained at 109.5°, and the N−H distances at 1.00 Å. It is noted that each constraint with its standard deviation is considered to be minimized in the function in the Rietveld method. Therefore, some chemical restrictions are established while maintaining the molecular flexibly. After fitting the unit-cell and instrumental parameters, the atomic coordinates were refined incrementally using an overalldisplacement parameter in the following order: the silicon and titanium, phosphorus, oxygen, nitrogen, and finally hydrogen atoms. Then, this overall-displacement parameter was replaced by the isotropic-displacement parameters for each atom type, except the Si and Ti site because of the opposite signs in Si and Ti’s scattering lengths; for this site, the value was fixed to 1.2 Å2. Finally, the relative occupation of Si and Ti was also refined, reaching the convergence. The good agreement between the observed and calculated NPD patterns after the refinement is shown in Figure 1. The structural parameters of

reported the preparation, structural and transport properties of the (NH4)2Si1−xTixP4O13 (0 ≤ x ≤ 1), the proton-transportation mechanism in these compounds is not yet clear. In this work, we have prepared (NH4)2Si0.5Ti0.5P4O13 by a solid-state reaction. The crystal structure of (NH4)2Si0.5Ti0.5P4O13 and its structural evolution with increasing temperature were studied by neutron-powderdiffraction (NPD) experiments and X-ray diffraction (XRD). The proposed diffusion pathway was confirmed by densityfunctional-theory (DFT) calculations. The relationship between the structure and conductivity property of (NH4)2Si0.5Ti0.5P4O13 as a proton conductor for ITFCs is unraveled, and the microscopic origin of the observed good performance has been revealed, which is instructive for designing and fabricating high-performance inorganic electrolytes.

2. RESULTS AND DISCUSSION Both XRPD and NPD patterns show that this material is isostructural with a (NH4)2SiP4O13 phase, as reported in 1976 by Durif et al.12 from X-ray single-crystal diffraction. The backbone of this phase (excluding the H atoms) is defined in the P1̅ space group with the unit-cell parameters a = 15.14(1) Å, b = 7.684(5) Å, c = 4.861(5) Å, α = 97.86(1)°, β = 96.74(1)°, and γ = 83.89(1)°. The structure was described by considering that the SiP4O132− anions form a bidimensional network along the bc planes, composed of tetrapolyphosphate groups (P4O136−) connected to silicon atoms in octahedral coordinations. The ammonia groups (NH4+) are located between the SiP4O132− units. The crystallographic positions of these groups were reported,12 but the hydrogen positions were unknown because they were not observed from X-ray diffraction. Subsequently, a Ti-substituted phase was described by Wang et al.;11 however, they only provided the unit-cell parameters, determined from the XRPD data, and there was no crystallographic information concerning the atomic positions. In a first approximation, the XRPD pattern of (NH4)2Si0.5Ti0.5P4O13 was refined using the Le Bail method, obtaining the following unit-cell parameters in the P1̅ space group: a = 15.253(1) Å, b = 7.7340(6) Å, c = 4.9333(5) Å, α = 97.846(5)°, β = 96.1941(5)°, and γ = 83.9336(5)°. Figure S1 (Supporting Information) shows the excellent Rietveld fit of the XRPD pattern. On the other hand, the refinement of the atomic positions was not possible from XRPD data, because of the low symmetry and the high number of nonequivalent atoms. NPD data collected at room temperature (RT) were essential to resolve this novel structure and in particular to locate the hydrogen positions. This information is of paramount importance for gaining insight into the protonconduction mechanism in this material. However, there are two issues to consider in the structural resolution. First, the fact that the hydrogen has a large inelastic component originates an important, incoherent background. In principle, this should not affect the quality of the Rietveld refinement if correct statistics are reached for the crystallographic peaks. The second restriction is related to the inherent limitations of the powder method; this aspect becomes more complicated if we consider the triclinic symmetry with the 28 different atomic positions. These facts do not enable full structural resolution with free atomic coordinates for all the atoms. Fortunately, several well-known chemical geometries for different coordination environments can be used to partially constrain the atomic parameters during the refinement. For

Figure 1. Neutron-diffraction patterns of (NH4)2Si0.5Ti0.5P4O13 after refinement at room temperature.

this phase at RT are listed in Table S1 (Supporting Information). The crystal structure at 100 and 200 °C can be refined with the same triclinic model from the NPD data. The Rietveld profiles at 100 and 200 °C after the refinements are shown in Figures S2 and S3 (Supporting Information), and the structural parameters are included in Tables S2 and S3. Also, a comparison between the main distances for the three temperatures is listed in Table S4. Figure 2 shows a schematic view of the crystal structure after the NPD refinement. The tetraphosphate units (P4O136−) are aligned along the [011] direction. Four of these tetraphosphate units are bonded to Si or Ti atoms with an octahedral geometry. As a result, a [(Si0.5Ti0.5)P4O132−]n layer extending parallel to the (100) plane is built up (see Figure S4, Supporting Information). These layers are interleaved with B

DOI: 10.1021/acs.inorgchem.7b02517 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Schematic view of the crystal structure of (NH4)2Si0.5Ti0.5P4O13 obtained from the NPD refinement at RT.

ammonium ions (NH4+), bonded via interactions of H bonds (N−H···O−P). The most important parameters showing the H-bond interactions (the distance, angle, and type of H bridge) are listed in Tables S5, S6, and S7 for RT, 100, and 200 °C, respectively. Three types of hydrogen bridges are found: normal, bifurcated, and trifurcated H bonds. According to the classification of Jeffrey for H bonds13 and the parameters listed in Tables S5, S6, and S7, the observed interactions for this compound are in the moderate and weak ranges. Very importantly, the interatomic distances show that the Hbond network is reorganized as the temperature increases, as illustrated in Figure S5. This behavior is at first manifested in the thermal expansion of the a axis, which is greater than those of the other axes. The most indicative parameters of the ionic mobility are the displacement factors of the H atoms, which show a noticeable increase: 3.6, 4.9, and 10.4 Å2 at 25, 100, and 200 °C, respectively. Analyzing the H-bond interactions between the [(Si0.5Ti0.5)P4O132−]n layer and the ammonium groups, it is possible to note that all the hydrogen atoms can be connected though H bonds, establishing channel-like paths between [(Si0.5Ti0.5)P4O132−]n. Hence, it is possible to assume that the proton mobility occurs through the formation of intermediate H−OP bonds. This H-bond network probably stabilizes interstitial hydrogens, which can promote conductivity. Difference Fourier maps were calculated from the final refinements at 200 °C. Figure 3a shows that the map overlaid with the crystal structure reveals subtle negative regions, which can be assigned to interstitial H atoms. Thus, an interstitial-H-conducting mechanism may be involved. To gain further insights into the proton-diffusion mechanism, the energy barriers of the protons in (NH4)2Si0.5Ti0.5P4O13 are obtained via DFT calculations and illustrated in Figure 3b,c. For convenience, the modeling of Hi+ (interstitial H) and VH− (H vacancy) diffusion in (NH4)2Si0.5Ti0.5P4O13 at 200 °C is taken as the representative one in the following discussion. As shown in Figure 3b, the interstitial H+ ions at the initial and final states coordinate with four O2− ions, and the total energies of the corresponding configurations are ∼1 eV lower than those of others. This open space at the 2i site at the center of four O2− atoms is noted as site Hi+ (IS) or Hi+ (FS). For the purpose of comparison, we also have constructed a negatively charged H vacancy (VH−) by removing any one lattice H+ and have examined its diffusion. It is clearly seen from Figure 3b that the energy barrier of Hi+ (0.65 eV) is much lower than that of VH− (3.17 eV), which indicates that (NH4)2Si0.5Ti0.5P4O13 is an interstitial-H-type conductor. Furthermore, it is found that more energy is needed to break the N−H bond for H-vacancy diffusion in

Figure 3. (a) Negative regions observed from a difference Fourier map at 200 °C. (b) Path and (c) energy barriers for Hi+ (interstitial H) and VH− (H vacancy) diffusion in (NH4)2Si0.5Ti0.5P4O13. The projection view suggests the jumping path of the protons in (NH4)2Si0.5Ti0.5P4O13 at 200 °C. IS, FS, and TS represent the initial, final, and transition states, respectively.

(NH4)2Si0.5Ti0.5P4O13. However, it is much easier for a proton to hop from an interstitial site near a given NH4 unit to another interstitial site near a neighboring NH4 group. During this process, it needs to break the O−H bond (1.06 Å) and form another O−H bond (1.013 Å). Note that these two values of O−H bond lengths are almost the same under the DFT framework. The DFT results are consistent with the proton interstitials revealed by difference Fourier map with subtle negative regions, as shown in Figure 3a. Therefore, both theoretical and experimental results support the interstitialproton-conduction mechanism. The feasibility of (NH4)2Si0.5Ti0.5P4O13 as an intermediatetemperature electrolyte material was investigated by impedance-spectra measurements. The temperature dependence of the proton conductivity of (NH4)2Si0.5Ti0.5P4O13 is shown in Figure 4. This material exhibits high proton conductivities of 0.0061 and 0.024 S cm−1 in humid air at 125 and 250 °C, respectively. The conductivities of (NH4)2Si0.5Ti0.5P4O13 in humid H2 are slightly higher than those in wet air.12 The

Figure 4. Temperature dependence of the proton conductivity of (NH4)2Si0.5Ti0.5P4O13 in comparison with those of ceramic14 and polymeric proton conductors.15 C

DOI: 10.1021/acs.inorgchem.7b02517 Inorg. Chem. XXXX, XXX, XXX−XXX

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temperatures after 1 h equilibration times. The data were analyzed using the ZView software (version 2.1). DFT Calculations. The calculations were performed by the Vienna ab initio simulation package (VASP), on the basis of the firstprinciples density functional theory and generalized gradient approximation (GGA).20,21 The lattice parameters were fully relaxed in all the calculations with a force-convergence criterion of less than 0.02 eV/Å and a total energy convergence within 10−4 eV per unit cell. The cutoff energy for the plane-wave basis was set at 540 eV. All calculations were performed in a 1 × 1 × 2 supercell with a 1 × 2 × 3 Monkhorst−Pack k-point grid. The ion-migrated barriers were obtained using the climbing-image-nudged-elastic-band (CINEB) method.22

conductivity of (NH4)2Si0.5Ti0.5P4O13 is much higher than that of propanesulfonated polybenzimidazole (PBI−PS) (∼0.00158 S cm−1 at 140 °C)3 and comparable to that of the solid acid proton conductor CsHSO4 (∼0.04 S cm−1 at 180 °C).14 It is believed that conductivity is dependent on the testing conditions.11 The diffusion mechanism is still hydrogen hopping via external hydrogen bonds. Figure 4 also shows the typical ceramic proton conductors, such as BaZr0.8Y0.2O3‑δ (BZY) and BaCe0.7Zr0.1Y0.2O3‑δ (BCZY),15 and polymeric conductors, such as NAFION,16 which are electrolyte materials operating at high and low temperatures, respectively. Thus, (NH4)2Si0.5Ti0.5P4O13 could well satisfy the technological requirements in the intermediate-temperature ranges. These results demonstrate that (NH4)2Si0.5Ti0.5P4O13 is a promising electrolyte material operating in 150−250 °C.

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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.7b02517.

3. CONCLUSIONS In summary, the crystallographic information concerning the hydrogen atomic positions in (NH4)2Si0.5Ti0.5P4O13 is unraveled in an NPD study. A two-dimensional path between [(Si0.5Ti0.5)P4O132−]n layers for proton diffusion across the crystal structure is elucidated; a temperature increase leads to many connections between the protons of neighboring NH4+ units. DFT calculations unveil that this compound is an interstitial-H-type conductor. (NH4)2Si0.5Ti0.5P4O13 shows high proton conductivities of 0.0061 and 0.024 S cm−1 in humid air at 125 and 250 °C, respectively. These results demonstrate that (NH4)2Si0.5Ti0.5P4O13 is a promising electrolyte material for intermediate-temperature fuel cells (150−250 °C) and electrolyzers, filling well the window left between ceramic and polymeric proton conductors.

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XRD, NPD patterns of (NH4)2Si0.5Ti0.5P4O13, schematic view of a [(Si0.5Ti0.5)P4O132−]n layer extending parallel to the (100) plane, thermal evolution of the H-bond connectivity, crystallographic data for (NH4)2Si0.5Ti0.5P4O13 from NPD data at various temperatures, main interatomic distances and angles for the main H-bond interactions at different temperatures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-82854648, Fax: +86-10-82854648, E-mail: [email protected] (C.S.). *Tel.: +86-21-66133141, Fax: +86-21-66133141, E-mail: [email protected] (S.S.). *Tel.: +34-91-3349071, Fax: +34-91-3720623, E-mail: ja. [email protected] (J.A.A.).

4. EXPERIMENTAL SECTION Preparation of (NH4)2Si0.5Ti0.5P4O13. Commercial APP (AP423, Clariant GmbH), SiO2 (quartz, Alfa Aesar), and TiO2 (anatase, Alfa Aesar) were used to synthesize (NH4)2Si0.5Ti0.5P4O13 by a solid-state reaction process in an ammonia atmosphere as reported previously.11 Stoichiometric amounts of the APP, SiO2, and TiO2 powders were thoroughly mixed with a mortar and pestle and were then calcined in an ammonia atmosphere at 400 °C for 24 h. Then, the ammonia atmosphere was switched to argon, and the temperature was reduced to room temperature. After this procedure, the product was washed with distilled water, dried at a reduced temperature (RT), and crushed at 10 °C. Then, the obtained product was sieved to collect an electrolyte powder with diameters less than 100 μm. Characterization of Materials. The XRD patterns were measured on a Bruker-AXS D8 diffractometer with Cu Kα radiation in the 2θ range of 5 to 90° with increments of 0.006°. To study the crystallographic structure, NPD patterns were measured by the HRPT diffractometer of the SINQ spallation source (Paul Scherrer Institute, Villigen, Switzerland) with a wavelength of 1.494 Å at 25, 100, and 200 °C. Both XRPD and NPD diffraction patterns were analyzed with the Rietveld method using the FullProf program.17,18 The coherent scattering lengths for Si, Ti, P, O, N, and H were 4.1491, −3.438, 5.13, 5.803, 9.36, and −3.739 fm, respectively. Further details on the crystal structure can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247−808−666, e-mail: crysdata@fiz-karlsruhe.de); the depository numbers 432599, 432632, and 432633 correspond to the phases of formula H8N2O13P4Si0.5Ti0.5 at RT, 100, and 200 °C, respectively. For impedance-spectra measurements, the method is that reported in the literature.4,19 The measurement was performed in humid air. Wet air with a water-vapor partial pressure of 3.4 kPa was obtained by bubbling gas through water at 25 °C. Before recording impedance spectra, the pellet was equilibrated under humid gas for 24 h in the test system. The impedance spectra were recorded at constant

ORCID

Chunwen Sun: 0000-0002-3610-9396 Carlos Alberto López: 0000-0002-5964-7103 José Antonio Alonso: 0000-0001-5329-1225 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51372271, 51672029, U1630134, 51622207, and 51372228). This work was also supported by National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702). J.A.A. acknowledges the Spanish Ministry of Economy and Competitivity, Spain, for granting the project MAT2013-41099-R. C.A.L. acknowledges Agencia Nacional de ́ Promoción Cientifica y Tecnológica (ANPCyT), Consejo ́ Nacional de Investigaciones Cientificas y Técnicas (CONICET), and Universidad Nacional de San Luis (UNSL), Argentina, for financial support (projects PICT2014-3576, PIP 112-201501-00820, and PROICO 2-2016). The authors acknowledge the Paul Scherrer Institute (PSI), Switzerland, for making all facilities available for the neutron-diffraction experiments. D

DOI: 10.1021/acs.inorgchem.7b02517 Inorg. Chem. XXXX, XXX, XXX−XXX

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REFERENCES

(1) Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345−352. (2) Sun, C. W.; Stimming, U. Recent Anode Advances in Solid Oxide Fuel Cells. J. Power Sources 2007, 171, 247−260. (3) Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Approaches and Recent Development of Polymer Electrolyte Membranes for Fuel Cells Operating above 100°C. Chem. Mater. 2003, 15, 4896−4915. (4) Sun, C. W.; Stimming, U. Synthesis and Characterization of NH4PO3 Based Composite with Superior Proton Conductivity for Intermediate Temperature Fuel Cells. Electrochim. Acta 2008, 53, 6417−6422. (5) Paschos, O.; Kunze, J.; Stimming, U.; Maglia, F. A Review on Phosphate Based, Solid State, Ptotonic Conductors for Intermediate Temperature Fuel Cells. J. Phys.: Condens. Matter 2011, 23, 234110. (6) Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Solid Acids as Fuel Cell Electrolytes. Nature 2001, 410, 910−913. (7) Nagao, M.; Takeuchi, A.; Heo, P.; Hibino, T.; Sano, M.; Tomita, A. A Proton-Conducting In3+-Doped SnP2O7 Electrolyte for Intermediate-Temperature Fuel Cells. Electrochem. Solid-State Lett. 2006, 9, A105−A109. (8) Cappadonia, M.; Niemzig, O.; Stimming, U. Preliminary Study on the Ionic Conductivity of A Polyphosphate Composite. Solid State Ionics 1999, 125, 333−337. (9) Liang, C. C. Conduction Characteristics of Lithium Iodide Aluminum Oxide Solid Electrolytes. J. Electrochem. Soc. 1973, 120, 1289−1292. (10) Maier, J. Ionic-Conduction in-Space Charge Regions. Prog. Solid State Chem. 1995, 23, 171−263. (11) Wang, H.; Tealdi, C.; Stimming, U.; Huang, K.; Chen, L. Preparation and Conductivity Measurements of Ammonium Polyphosphate-Based Proton Conductors. Electrochim. Acta 2009, 54, 5257−5261. (12) Durif, A.; Averbuch-Pouchot, M. T.; Guitel, J. C. Structure Cristalline de (NH4)2SiP4O13: Un Nouvel Eexemple de Silicium Hexacoordiné. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, B32, 2957. (13) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1997. (14) Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Solid Acids as Fuel Cell Electrolytes. Nature 2001, 410, 910−913. (15) Volkov, A.; Gorbova, E.; Vylkov, A.; Medvedev, D.; Demin, A.; Tsiakaras, P. Design and Applications of Potentiometric Sensors Based on Proton-Conducting Ceramic Materials. A Brief Review. Sens. Actuators, B 2017, 244, 1004−1015. (16) Alberti, G.; Casciola, M.; Capitani, D.; Donnadio, A.; Narducci, R.; Pica, M.; Sganappa, M. Novel Nafion-Zirconium Phosphate Nanocomposite Membranes with Enhanced Stability of Proton Conductivity at Medium Temperature and High Relative Humidity. Electrochim. Acta 2007, 52, 8125−8132. (17) Rietveld, H. M. A. Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (18) Rodríguez-Carvajal, J. Recent Advances in Magnetic-Structure Determination by Neutron Powder Diffraction. Phys. B 1993, 192, 55−69. (19) Sun, C. W.; López, C. A.; Alonso, J. A. Elucidating the Diffusion Pathway of Protons in Ammonium Polyphosphate: a potential Electrolyte for Intermediate Temperature Fuel Cells. J. Mater. Chem. A 2017, 5, 7839−7844. (20) Kresse, G.; Furthmüller, J. Efficiency of ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (22) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904.

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DOI: 10.1021/acs.inorgchem.7b02517 Inorg. Chem. XXXX, XXX, XXX−XXX