Unique Proton Transportation Pathway in a Robust Inorganic

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Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity Daxiang Gui, Xing Dai, Zetian Tao, Tao Zheng, Xiangxiang Wang, Mark A. Silver, Jie Shu, Lanhua Chen, Yanlong Wang, Tiantian Zhang, Jian Xie, Lin Zou, Yuanhua Xia, Jujia Zhang, jin zhang, Ling Zhao, Juan Diwu, Ruhong Zhou, Zhifang Chai, and Shuao Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02598 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity Daxiang Gui,1# Xing Dai,1# Zetian Tao,2# Tao Zheng,1 Xiangxiang Wang,1 Mark A. Silver,1 Jie Shu,3 Lanhua Chen,1 Yanlong Wang,1 Tiantian Zhang,3 Jian Xie,1 Lin Zou,4 Yuanhua Xia,4 Jujia Zhang,5 Jin Zhang,5 Ling Zhao,6* Juan Diwu,1 Ruhong Zhou,7* Zhifang Chai,1 and Shuao Wang1* 1

State Key Laboratory of Radiation Medicine and Protection, School for Radiological and interdisciplinary Sciences (RADX) and Collaborative Innovation Centre of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China 2 Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224001, Jiangsu China 3 Analysis and Testing Center, Soochow University, 199 Renai Road, Suzhou 215123, China 4 Key Laboratory of Neutron Physics and Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics(CAEP), Mianyang 621999, China 5 Beijing Key Lab of Bio-inspired Energy Materials and Devices & School of Space and Environment, Beihang University, Beijing 100191, China 6 Department of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China 7 Computational Biology Center, IBM Thomas J Watson Research Center, Yorktown Heights, NY 10598, USA Supporting Information ABSTRACT: Although comprehensive progress has been made in the area of coordination polymer (CP)/metal-organic framework (MOF)-based proton conducting materials over the past decade, searching for a CP/MOF with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell (PEMFC) assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton conducting CP, (NH4)3[Zr(H2/3PO4)3] (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered chargebalancing NH4+ cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid-base pairs (N−H•••O−P), leading to a stable anhydrous proton conductivity of 1.45 × 10−3 S•cm−1 at 180 °C, one of the highest values among reported intermediatetemperature proton conducting materials. First principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH4+ and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H2/O2 fuel cell, which showed a record-high electrical power density of 12 mW•cm−2 at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell (DFMC) for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power lost.

INTRODUCTION Proton-exchange membrane fuel cells (PEMFCs) are promising candidates for the partial substitution of fossil fuel energy owing to their high power density, green features, and mild operating conditions1,2. Intermediate-temperature (100-300 °C) PEMFCs exhibit several clear advantages over those assembled with commercialized Nafion, which can be operated only at low temperature (below 100 °C) and high hydration levels3,4. These advantages include faster electrode kinetics, no requirement of humidified inlet streams or large radiators to dissipate waste heat, and minimized CO poisoning of the platinum catalysts when equipped with methanol fuel, i.e., in a direct methanol fuel cell (DMFC)5. The development of electrolyte materials exhibiting durable, high proton conductivity

that can be operated over long time periods at intermediate temperature under anhydrous conditions is one of the primary technological barriers that must be overcome for the next generation of PEMFCs3,6. There are two general types of intermediate-temperature proton conducting materials: “superprotonic” solid acids, such as CsHSO4 and CsH2PO47,8, and organic polymers impregnated with relatively less-volatile inorganic acids, mostly represented by polybenzimidazole (PBI)H3PO49,10. Both types exhibit decent anhydrous proton conductivity at elevated temperature. However, both face long-term stability issues, especially under operating conditions. The former is limited by chemical reduction and dehydration,

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whereas the latter is subject to acid molecule leaching upon contact with water. Over the past decade, coordination polymers (CPs) and metal-organic frameworks (MOFs) have been widely investigated as new types of crystalline proton conducting materials, where their primary advantage is that the desired proton conducting behavior can be controllably achieved, for example, by crystal engineering, ligand functionalization, and guest doping11-19. Meanwhile, CP/MOF materials provide a powerful platform for understanding proton-transport pathways at the molecular level20-24. Proton conductivity at the level of 10-1 S•cm−1, close to that of Nafion, has recently been achieved under low-temperature and high-humidity conditions for CP/MOF-based proton conductors25-30. However, CPs/MOFs exhibiting high anhydrous proton conductivity at temperatures above 120 °C are much less reported, with the majority realized by the post-synthetic encapsulation of proton carriers (e.g., imidazole) into the pores of the CPs/MOFs25,31-36. Although the relatively weak interactions between these proton carriers and the host material lead to their high mobility, which is advantageous for elevating the proton conductivity, they are also responsible for the potential loss of proton carriers under working conditions, giving rise to similar long-term stability issues as (PBI)-H3PO4, representing a critical disadvantage that impedes practical applications. A challenge lies in this system of attaining both high proton conductivity and stability in a single material. Therefore, CPs/MOFs that show intrinsic anhydrous proton conductivity that does not originate from guest encapsulation/dehydration/superprotonic transition are highly desirable. In addition, even though a handful of examples of CPs/MOFs exhibiting high anhydrous proton conductivity that can be maintained at working conditions have been reported, studies showing CPs/MOFs that can be directly used as the electrolyte in a practical fuel cell assembly are almost nil, and understanding of the durability of cell performance remains elusive37-41. This mostly originates from the incompatibility between the CPs/MOFs and electrodes, questioning the real application of CPs/MOFs as practical electrolytes in PEMFC and DMFC systems. In this work, we present a highly crystalline zirconium phosphate (ZrP) synthesized via the ionothermal method42,43. One of the most important structural features of this compound is the presence of charge-balancing NH4+ cations, which can act as proton carriers and are located at a very suitable position, connecting all the partially protonated phosphate oxo atoms from the neighboring zirconium phosphate chains through strong hydrogen bonds. This affords an inherent onedimensional proton-transport pathway that does not rely on external water molecules, as demonstrated by a series of characterization techniques, including X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy, and further visualized by combined firstprinciples and molecular dynamics simulations. Additionally, these NH4+ cations are fully ordered in the crystal lattice of ZrP and therefore possess intrinsic thermal and water stability, contrasting with cases involving the post-synthetic encapsulation of mobile proton-carrier species, leading to an anhydrous proton conductivity of 1.45 × 10−3 S•cm−1 at 180 °C. This represents one of the highest values of intrinsic anhydrous proton conductivity in the intermediate-temperature region. More importantly, ZrP can be directly used as the electrolyte in intermediate-temperature PEMFC and DMFC assemblies for durable power generation. To the best of our

knowledge, this is the first report on the application of CPs/MOFs as solid electrolytes for a DMFC device, and the first device holds the record electrical power density among reported PEMFCs assembled by crystalline proton conducting electrolytes, therefore representing one-step forward towards practical applications of CP/MOF-based proton conducting materials. EXPERIMENTAL SECTION Chemicals and reagents. All chemicals were obtained from commercial sources. Orthophosphoric acid (H3PO4, 85 wt% in water) was purchased from J&K Chemical, urea and ZrCl4 were purchased from Adamas Chemical Reagent, and the ionic liquid 1-butyl-2,3-dimethylimidazolium chloride ([BMMim]Cl) was purchased from Lanzhou Greenchem ILS, LICP, CAS, China. All other reagents and solvents were used as received from commercial suppliers without further purification. Preparation of (NH4)3Zr(H2/3PO4)3 (ZrP): A mixture of ZrCl4 (0.15 g), H3PO4 (0.2 g), (NH2)2CO (0.15 g), and 1-butyl2,3-dimethylimidazolium chloride ([BMMim]Cl) (0.3 g, 1.325 mmol) was added to a 10 mL stainless-steel PTFE autoclave liner, heated at 180 °C for 12 h in a furnace, and then cooled to room temperature. Colorless block crystals of ZrP were obtained as a pure phase with a yield of ca. 80% (based on zirconium). Elemental analyses (%) for ZrP: Calcd.: C 0.000, N 9.716, H 2.776. Found: C 0.000, N 9.308, H 2.444. Single-crystal X-ray Diffraction and Powder X-ray Diffraction Analysis. A Crystals of the compound were mounted on a Bruker D8-Venture diffractometer with a Turbo X-ray source (Mo Kα radiation, λ = 0.71073 Å, 50 kV/50 mA power), adopting the direct-drive rotating anode technique and a CMOS detector at 123 K. The data frames were collected using the APEX3 program and processed using the SAINT routine in APEX3. The structures were solved by direct methods and refined by the full-matrix least-squares on F2 method using the SHELXTL-2014 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed at calculated positions44. Details of the crystal data and structural refinement can be found in Supplementary Table S1. Powder patterns were collected from 5° to 80° with a step of 0.02° over 0.5 s using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) and a Lynxeye one-dimensional detector. The powder X-ray diffraction pattern of the bulk production of each reaction was compared with simulated patterns generated based on single-crystal structures. Alternating Current Impedance Measurements. Alternating current impedance measurements were carried out on a Solartron SI 1260 Impedance/Gain-Phase Analyzer with applied ac voltage amplitude of 500 mV over a frequency range of 5 MHz to 1 Hz. A bulk crystalline powder was compressed into a pellet with 1000 kg of pressure, the diameter of the pellet was 3 mm, and the thickness ranged from 1 to 2 mm. For measurement below 100 °C, the pelletized sample attached with two gold electrodes was placed in a temperature- and humidity-controlled chamber. To determine the conductivity of the sample above 100 °C, the pelletized sample was placed into a tube furnace under air, and silver electrodes were used to avoid the complexation reaction between the pellet and gold at elevated temperature. All temperature points were calibrated



by an external thermocouple. The conductivity was calculated by the following equation: σ = L/RS where S and L are the cross-sectional area and thickness of the pellet, respectively, and R, which was obtained from the impedance plots, is the resistance45. Contributions to resistance from bulk and grain boundary were difficult to be distinguished in the impedance plots. The activation energy (Ea) for the ZrP conductivity was calculated from the following Arrhenius equation: Eୟ σT = σ଴ exp ൬− ൰ k୆T where σ is the ionic conductivity, kB is the Boltzmann constant, and σ0 is the pre-exponential factor, T is the temperature. Fabrication of the Fuel Cell Assembly. The ZrP membrane was prepared by a hot-press process. The as-prepared ZrP powder was physically mixed with 10 wt% PVDF binder, which was used to increase the density and strength of the ZrP disk. 0.12 g of this mixture was uniformly distributed in a stainless steel mold and subsequently was hot-pressed under 300 MPa at 180 °C for 0.5 h using an electric hot compacting press (Kejing HP100). After cooling down to room temperature, the mold was depressurized to form a ZrP membrane with a diameter of ~19 mm and a thickness of ~0.3 mm. A single cell consisting of a Pt/C anode, ZrP membrane, and Pt/C cathode was assembled for electrochemical assessment. The Pt/C catalyst with a Pt loading of 1.0 mg/cm2 was used for both the anode and cathode. The catalyst ink was prepared by blending Pt/C powders with a PTFE solution (3.1 wt%, DMF) under ultrasonication. The catalyst ink was then brushed onto a microporous gas diffusion layer (GDL) to form the electrodes. The ZrP membrane was sandwiched between two electrodes to obtain a single cell with an effective area of 1 cm2. The performances of the single fuel cells were tested at 180 °C by a fuel cell testing station (Greenlight G20, Canada). Humidified (4.54% H2O) H2 at a flow rate of 70 SCCM and O2 at a flow rate of 150 SCCM were input at the anode and cathode, respectively. The electrochemical performances were measured by an electrochemical workstation (Gamry Ref 3000). Impedance curves were recorded under open-circuit conditions over a frequency range of 0.1 MHz to 1 Hz. Molecular Dynamics Simulations. DFT calculations were performed using the Gaussian 09 program46. The geometry optimizations and frequency calculations of the structures in Figure 2 and Figure 4 were carried out at the B3LYP/6-31G* level47-49. The structures in Figure 2 and Figure 4 were extracted from the crystal structure. During the geometry optimizations, the atomic positions of the corresponding P, O1 and O2 atoms were frozen to maintain the stable local framework of ZrP, while the corresponding O3, O4, N and H atoms were allowed to freely relax. Molecular dynamics simulations based on DFTB theory were carried out by using the DFTB+ code50. The initial structure (Figure 5, 0 fs) was constructed by adding OH- near PP-2 and H+ to PP-1 on the base structure in Figure 4a to simulate the species produced at the PEMFC cathode and anode. The self-consistent charge-density functional tight bonding (SCCDFTB) Hamiltonian51 was employed. The mio-1-1 parameters set, which was developed for organic molecules, was used for the P, O, N and H atoms. The molecular dynamics simulations were performed by employing the Nosé-Hoover chain thermostat with a target temperature 300 K and were carried out for 15000 fs with a time step of 0.25 fs. During the simulations,

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the P and O atoms were frozen, while the N and H atoms were allowed to relax. Variable-temperature Solid-state NMR Measurement. Solid-state NMR (SNMR) experiments were performed on a Bruker Avance III 400 HD instrument equipped with a double-resonance magic-angle-spinning (MAS) probe supporting rotors with a 3.2 mm outer diameter. The Larmor frequency of 1 H is 400.25 MHz. The 1D 1H spectrum and 1D DQ-filtered spectrum were measured at a MAS frequency of 18 KHz with 16 accumulations. The DQ signals were excited and reversed by using the back-to-back sequence (BaBa).52 Variabletemperature (VT) 1H spectra were recorded at 10 kHz MAS frequency. In addition, 2D 1H-1H exchange spectrum were recorded with a mixing time of 5 ms. All spectra were referenced with respect to tetramethyl silane (TMS), using adamantane (1H, δ = 1.85 ppm) as a secondary reference53,54. Neutron Powder Diffraction Measurements. Neutron powder diffraction (NPD) experiments were carried out at room temperature using a high-resolution neutron powder diffractometer (HRND) (λ = 1.884 Å) at China Mianyang Research Reactor (CMRR). In order to improve the signal-tonoise ratio, ZrP was prior refluxed in deutium oxide(D2O) for two days to exchange proton into deutium partially or totally. Powder patterns were collected from 10° to 150° with a step of 0.1°. The obtained NPD patterns were solved by Rietveld profile refinement method using FULLPROF software. Using the single-crystal X-ray Diffraction results as the starting model, the crystal structure parameters and all atoms parameters were refined. Almost all the diffraction peaks of the NPD patterns can be indexed。 Other Physical Measurements. Elemental analyses (C, H, and N) were carried with a Vario EL CHNOS elemental analyzer. Scanning electron microscopy images and energydispersive spectroscopy data were recorded on an FEI Quanta 200FEG Scanning Electron Microscope with electron beam energy of 30 keV. The samples were mounted directly on carbon conductive tape with an Au coating. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449F3 instrument over the range of 30−900 °C under nitrogen flow at a heating rate of 10 °C/min for dried ZrP samples. A Quantachrome Autosorb Gas Sorption analyzer (IQ2) was used to perform the water adsorption measurements. The detection pressures ranged from 0 to 760 Torr. Before the adsorption measurement, the treated ZrP sample was activated using the “outgas” function of the surface area analyzer for 10 h at 100 °C. Water vapor adsorption measurements for ZrP were taken at 298 K in a water bath, similar to gas adsorption. The IR spectrum of the powder of the pure phase was recorded by a Thermo Scientific Nicolet iS50 FT-IR over the range of 400-4000 cm-1. RESULTS AND DISCUSSION Syntheses, Structure and Stability Characterizations. Colorless crystals of ZrP were derived from the ionothermal reaction of ZrCl4, H3PO4, and (NH2)2CO in the ionic liquid 1butyl-2,3-dimethylimidazolium chloride. Single-crystal X-ray diffraction analysis performed at 123 K and 298 K both revealed that ZrP crystallizes in the trigonal space group R-3. The structure of ZrP is composed of a series of stacked onedimensional anionic zirconium phosphate chains running along the c axis (Figure 1a), with charge-balancing NH4+ cations filling the intervals between chains (Figure 1d). The NH4+ cations originate from the in situ thermal decomposition of


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(NH2)2CO during the reaction, which can be further confirmed by FT-IR spectroscopy (Figure S4-5) and elemental analysis. Each Zr4+ atom is coordinated by six O atoms from six independent but symmetrically related PO43- groups, forming an octahedral coordination geometry (Figure 1b). Figure 1c shows the detailed coordination component within the chain. For each PO43- group, O1 and O2 atoms are bound to the Zr4+ metal center at bond distances P1-O1 and P1-O2 of 1.529(5) Å and 1.518(5) Å, respectively (All bond distance information were extracted from 123 K structure). The Zr1-O2 and Zr2-O1 bond distances are 2.057(5) Å and 2.065(4) Å, respectively. Clearly, these oxygen atoms are not protonated. For the remaining terminal O3 and O4 atoms, the P1-O3 and P1-O4 bond distances are 1.552(6) Å and 1.477(5) Å, indicating that P1-O4 is a double bond while O3 should be protonated, based on the bond valence sum (BVS) calculation (see SI for detailed discussion).

short O3•••O3 distance implies that the adjacent O3 atoms must share one proton, as shown in Figure 1e. In order to verify this speculation, we performed density functional theory (DFT) calculations based on a designed (H2PO4)•••H+•••(H2PO4)- system to simulate the proton sharing mode (Fig. 1f). The two neighboring phosphate groups (abbreviate as phosphate pair, PP, in the following text) were extracted from the crystal structure with the dangling O1 and O2 saturated by hydrogen. An additional proton was placed between the two O3 atoms. Through geometry optimizations, we obtained two stable structures (Figure 2a left and right) and one transition state (Figure 2a middle), which can describe an overall proton exchange process between the two O3 atoms. The calculated energy requirement for the proton exchange is only 0.44 kcal/mol, suggesting that the shared proton is highly active and mostly delocalized between two O3 atoms even at low temperature, consistent with the neutron powder diffraction data. It is noteworthy that the calculated O3•••O3 distance in the transition state is about 2.464 Å, in agreement with the experimentally determined crystal structure data (2.465 Å). As a comparison, we also check the structure of the (H2PO4)•••H+H+•••(H2PO4)- system with fully protonated O3. The DFT calculated O3•••O3 distance in this case is about 2.933 Å (Figure 2b), obviously longer than the experimental data. Furthermore, half-protonated O3 has a high electronegativity and strongly interacts with the NH4+ cation with a N1•••O3 distance of 2.872 Å, giving rise to an infinite one-dimensional spiral hydrogen-bonded chain with a series of proton carriers (acids) and proton vacancies (base) (Figure 1f)55. In principle, protons should be highly active within the chain, with driving forces much like those in an acid-base neutralization reaction, which led us to investigate the proton conducting behavior.

Figure 1. Crystal structure of ZrP. (a) View of the crystal structure of ZrP along the c axis. ZrO6 and PO4 are shown as yellow octahedra and blue tetrahedra, respectively. Other atom color codes: O, red; N, marine; H, white and green. (b) View of the infinite negatively charged [Zr(H2/3PO4)3]n3n- chains of ZrP. (c) Detailed local geometric parameters of the chains. (d) View of the charge-balancing NH4+ cations filled between the adjacent chains. (e) Bond distances of P1-O3 and the adjacent protonated O3•••O3 atoms sharing a positive H+. The zirconium chain is depicted as a yellow column. (f) Strongly hydrogen-bonded chains constructed by acid-base pairs running along the c axis in ZrP.

Figure 2. Performed density functional theory (DFT) calculations based on a designed (a) (H2PO4)-•••H+•••(H2PO4)- system to simulate the proton sharing model and (b) (H2PO4)-•••H+ H+•••(H2PO4)- system.

Before these measurements, the thermal and chemical stabilities of ZrP were comprehensively investigated. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed that ZrP is thermally stable up to 200 °C (Figure S6), which was also confirmed by the variabletemperature powder X-ray diffraction measurements (Figure 2f). Above 200 °C, ZrP gradually converts to ZrP2O7. Furthermore, ZrP can maintain its crystallinity after being soaked in aqueous solutions with pH values ranging from 2 to 12, menthol, or ethanol for longer than one month. ZrP can also be boiled in water for at least two weeks without structural changes (Figure S7-8). These features suggest that ZrP could be a durable solid electrolyte in intermediate-temperature (100-200 °C) PEMFC and DMFC systems.

Considering the charge-balancing principle and that the smallest repeating component of ZrP is (NH4+)3[Zr4+(PO43-)3], two protons should be distributed in this unit. Therefore, the PO43- group can be only partially protonated, giving rise to a series of proton vacancies, which are critical for intrinsic proton transport, as demonstrated below. This begs the question of whether these vacancies are ordered. Therefore, we collected neutron powder diffraction data to locate the position of protons in ZrP. Intriguingly, all protons in the NH4+ cations can be clearly located, while all charge-balancing protons are fully disordered and therefore cannot be located in the structure (Figure S2). To further probe the state of these missing protons, we found that the O3•••O3 distances from two neighboring independent chains are very close to 2.465 Å. Such a



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[Zn(H2PO4)2(TzH)2]41, [Zn(HPO4)(H2PO4)2]•2H2Im59, [Zn3(H2PO4)6(H2O)3]•Hbim40, and (Me2NH2)[Eu(L)]37, and is comparable to that of [ImH2][Cu(H2PO4)2Cl]•H2O60, [Eu2(CO3)(ox)2(H2O)2]•4H2O38 and [Zn(H2PO4)2(C2N3H3)2]n32. We propose that this behavior originates from the efficient thermal activation of NH4+ cations as proton carriers and "proton conduction switches" described by the theory below, which are also effectively stabilized by the dense hydrogenbonding network and electrostatic interactions at relatively high temperatures. Notably, this phenomenon is an intrinsic property originating from the crystal structure of ZrP, given that all proton carriers are fully ordered in the crystal lattice, substantially deviating from the guest doping/dehydration/phase transition mechanisms that account for most other intermediate-temperature proton conducting materials. Assumingly, this could lead to elevated durability of proton conduction even under working conditions. Confirming this point, a durability test showed that ZrP maintains its proton conductivity when heated at 180 °C for at least 60 h (Figure 3e). To gain initial insight into the proton conducting mechanism, we calculated the activation energy for ZrP in both the heating and cooling cycles. According to the least-squares fit of the Arrhenius plot, the proton transfer Ea of ZrP was calculated to be 0.26 eV (Figure 3d), again suggesting the Grotthuss hopping mechanism (< 0.4 eV) in this temperature region58. This further supports the idea that the strongly hydrogenbonded chains constructed by acid-base pairs in ZrP can yield facile and efficient proton transfer with the help of electrostatic interactions.

Proton Conductivity Measurements. The proton conductivity under humidified conditions was first evaluated by alternating current impedance spectroscopy using a pellet sample of ZrP.56,57 This is similar to the majority of traditional lowtemperature proton conducting materials, where water plays an important role in improving the efficiency of proton transport. Note that although ZrP has a nonporous structure, it exhibits decent water uptake capacity (Figure S10), possibly due to the abundant hydrophilic active sites on the sample surface. As the humidity is further increased, ZrP exhibits an ultrahigh proton conductivity of 1.21 × 10−2 S•cm−1 under the conditions of 90 °C and 95% relative humidity (RH), suggesting that proton transport in ZrP is highly associated with water (Figure 3a). Meanwhile, ZrP exhibits a high proton conductivity of 0.81 × 10−2 S•cm−1 under the conditions of 25 °C and 100% RH. The Grotthuss hopping mechanism was further elucidated by the calculated activation energy (Ea) of 0.30 eV, derived from the temperature-dependent behavior (Figure 3b)58. Although this is not a substantial breakthrough in the area of lowtemperature hydrous proton conductors, stable materials with ultrahigh conductivities above 10−2 S•cm−1 are still quite rare, as summarized in Supplementary Table S5.

Figure 3. Proton conductivities of ZrP under anhydrous and different humid conditions. (a) Impedance spectrum of ZrP at 180 °C under anhydrous conditions. (b) Proton transfer activation energy (Ea) of ZrP under anhydrous conditions at temperatures above 100 °C. (c) Impedance spectrum of ZrP at 90 °C and 95% RH. (d) Proton transfer activation energy (Ea) of ZrP in an aqueous environment. (e) Proton conductivity measurements and durability test under anhydrous conditions at 180 °C. (f) Variabletemperature powder X-ray diffraction data of ZrP.

Figure 4. DFT studies of the proton-transport pathways and mechanisms. (a) The optimized local structure extracted from ZrP. This local structure contains two phosphate pairs (PO4•••H+•••PO4; PP-1 and PP-2), one NH4+, and two adjacent phosphate groups. (b) The addition of an extra proton to PP-2. (c) Transition state structure for proton transfer between PP-1 and PP-2. (d) The added proton was transported to PP-2.

Much more significantly, proton conductivities from 90 to 180 °C under strictly anhydrous conditions were also evaluated for ZrP. The samples were equilibrated for 2 to 4 h at each temperature. The real (Z’) and imaginary (Z’’) parts of the impedance spectrum are shown in Figure S18. At 90 °C, the proton conductivity of the ZrP pellet sample was 1.69×10−5 S•cm−1, which further increased with temperature, reaching a maximum of 1.45 × 10−3 S•cm−1 at 180 °C (Figure 3c). This value is notably higher than that of the majority of CPs/MOFs exhibiting anhydrous proton conductivity, such as

Theoretical Study of the Proton Conduction Mechanism. To further elucidate the mechanism of proton conduction in ZrP on an atomic scale, we theoretically investigated a local structure extracted from the crystal structure of ZrP based on DFT calculations. This local structure contains two phosphate


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pairs (PO4•••H+•••PO4; PP-1 and PP-2), one NH4+ cation, and an additional two adjacent phosphate groups, as shown in Figure 4a. These six phosphate groups together constitute an enclosed cavity, which can prevent the escape or diffusion of the inner NH4+ cation, contributing to its ordered state in the structure. Each O3 atom of the phosphate pair is half-protonated and therefore has a formal negative charge of -0.5e, forming strong hydrogen bonds with the positively charged NH4+ cation. The DFT geometry optimization showed that when all O3 atoms from both PP-1 and PP-2 are half-protonated, the NH4+ cation is located almost in the middle of the two phosphate pairs (Figure 4a; theoretical: O3(PP-1)•••N = 2.80 Å, O3(PP2)•••N = 2.90 Å; experimental: O3(PP-1)•••N = 2.872 Å, O3(PP-2)•••N = 2.983 Å), indicating equivalent hydrogenbonding interactions. When an additional proton is added to PP-1 to mimic proton injection from the anode, the two O3 atoms in PP-1 are fully protonated and lose their ability to attract NH4+, giving rise to an obvious shift of NH4+ towards PP-2 (Figure 4b; O3(PP-2)•••N = 2.61 Å, O3(PP-1)•••N = 3.15 Å). In this case, the O3 atom in PP-2 forms a much stronger hydrogen bond with NH4+, generating the necessary conditions for the proton capture from NH4+. Figure 4c shows a critical transition state for proton transfer between the two phosphate pairs, where the NH4+ cation donates one proton to PP-2, affording a neutral, planar NH3 molecule. The N atom of NH3 still forms hydrogen bonds with the O3 atoms from both PP-1 and PP-2 (O3(PP-1)•••N = 2.79 Å, O3(PP-2)•••N = 2.80 Å). The characteristic frequency (imaginary frequency) at this state is the out-of-plane vibration of the N atom (towards H on O3) accompanied by the stretching vibrations of the two O3-H bonds of the two phosphate pairs (towards N atoms), indicating that the planar NH3 molecule is metastable and available for another proton. Figure 4d shows another stable state corresponding to Figure 4b, in which the central NH3 captures a proton from PP-1 to reform NH4+. Meanwhile, PP-1 returns to its original half-protonated state. This brings NH4+ closer to PP-1 (O3(PP-1)•••N=2.60 Å) and away from the fully protonated PP-2 (O3(PP-2)•••N = 3.06 Å). Through the above discussion, the unique half-protonated phosphate pair containing proton vacancies along with NH4+ in ZrP act as a springboard and medium for proton transfer, much like an equilibrated neutralization reaction between the weak base HPO42- and weak acid NH4+. Because the local structure of PP•••NH4+•••PP is continuously distributed in ZrP as an infinite chain along the c axis, the macroscopic proton-transport process should consist of many segments of proton hopping between phosphate pairs and NH4+, as described above. To visually confirm the DFT-predicted proton transfer path, we performed molecular dynamics simulations based on the density functional tight bonding (DFTB) method. To the structure in Figure 4a, we added an OH- near PP-2 and H+ to PP-1 to simulate the species produced at the PEMFC cathode and anode. A video directly revealing this process is also provided as a supplementary file. This designed structure was used as the initial state for the dynamics simulation and is labeled 0 fs (Figure 5). Due to the complete protonation of PP1, the central NH4+ is quickly attracted by the half-protonated PP-2, forming a strong hydrogen bond at 10 fs. At 40 fs, NH4+ donates a proton to PP-2, becoming NH3 with a trigonal prismatic structure. At 52 fs, the OH- near PP-2 begins to capture the proton of the fully protonated PP-2, and this transfer process quickly ends at 60 fs, resulting in the formation of H2O. At 80 fs, the tetrahedral NH3 molecule transforms into a meta-

stable planar structure, reproducing the DFT-predicted transition state (Figure 4c). It rapidly re-converts to the trigonal prismatic structure and forms a hydrogen bond with the fully protonated PP-1 at 95 fs. At 145 fs (not shown in Figure 5), the half-protonated PP-2 restores its proton-sharing state. At 1312 fs, the proton of PP-1 begins to migrate to the NH3 and completely transfers to NH3 at 1330 fs, and PP-1 returns to the half-protonated state. The molecular dynamics simulation lasted for a total of 15000 fs (15 ps), and the two halfprotonated PP and NH4+ molecules remained stable until the end.

Figure 5. Snapshots of the DFTB molecular dynamics simulation. The simulated system was constructed by adding a proton to PP-1 and an OH- near PP-2 to the structure given in Figure 4a. The two adjacent phosphate groups (see Figure 4a) are omitted for clarity.

From the DFTB molecular dynamics simulation, we proved that the possible proton conduction path predicted by the DFT calculation is feasible. Notably, our theoretical studies demonstrate that the proton conduction mechanism in ZrP is a truly transitive and intrinsic conduction mechanism (proton hopping between phosphate pairs with NH4+ as the medium)61. This type of conduction overcomes the unavoidable steric hindrance and other unfavorable factors in "proton carrier" conduction mode. In addition, proton conduction in ZrP completely utilizes the composition of the material itself and does not depend on additional reinforcing media, such as water. Solid-state Nuclear Magnetic Resonance. To experimentally determine the proton mobility and its correlation with the conductivity of the phosphate pairs and NH4+ in ZrP, we performed solid-state 1H nuclear magnetic resonance (SSNMR) spectroscopy.62 As shown in Figure 6a, two broad resonance peaks appear in the room-temperature SSNMR 1D 1H spectrum (above) under one-pulse excitation SSNMR test conditions, indicating that protons experience two types of chemical environments in diamagnetic ZrP. Based on the crystal structure of ZrP, the two peaks at 7.34 and 13.57 ppm can be assigned to the protons of phosphate pairs and NH4+, respectively. In addition, the 1D 1H double-quantum (DQ)-filtered spectrum was also recorded. This type of measurement can clearly discriminate active protons in a flexible environment from



those in an inactive rigid environment. Different from the typical 1H spectrum, the 1D 1H DQ-filtered method excites only protons with low local mobility. Interestingly, as depicted in Figure 6a (below), no signal was observed using the DQfiltered method, which implies the high local mobility of all protons in the system. From the molecular dynamics simulation, proton transport in ZrP was accomplished by successive proton hopping between phosphate pairs and NH4+ cations, which was also confirmed by the 2D 1H-1H exchange spectrum. As shown in Figure 6b, cross-correlation signals were observed between 7.34 ppm and 13.57 ppm in the 2D 1H-1H exchange spectrum, indicating that proton chemical exchange occurs between the N-H and P-OH units. We also measured a 2D 1H-31P exchange spec-

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ton conductivity of ZrP, the next logical step was to directly test ZrP as a solid-state electrolyte within a real H2/O2 fuel cell assembly. In current study, an ultrathin and dense ZrP electrolyte membrane with the thickness of only ~0.3 mm was prepared by a facile hot press technique and fully characterized (Figures S5, S9, and S24). A sandwich cell, consisting of PtRu/C anode, ZrP membrane, and Pt/C cathode (Figure 7a), was assembled for electrochemical test. The electromotive force curves were measured from 100 to 180 °C. As shown in Figure 7b, the maximum open-circuit voltage of the H2/O2 fuel cell is 0.72 V, comparable to reported PEMFCs assembled from other CP/MOF-based electrolytes and lower than the theoretical maximum of 1.16 V. This likely originates from nonnegligible fuel crossflow, which can be further improved by membrane fabrication and crystal engineering of the pore size. Impressively, the maximum power density is 12 mW•cm−2, which is more than four times higher than that of the best reported CP-based PEMFC32,63. This is mainly attributed to the ultrathin electrolyte layer and the enhanced electrode activity at raised operation temperature as well as the possibly improved electrode/electrolyte contact. Related results from PEMFCs assembled from other CP/MOF-based electrolytes are summarized in Table S4 for comparison. A steady current density without obvious degradation is achieved over 15 h at 180 °C (Figure 7c), indicating the practical feasibility of the ZrP electrolyte.

trum at room temperature, showing intense cross peaks labeled between 31P 11.42 ppm and 1H 13.57 ppm, 7.34 ppm (Figure S21).

Furthermore, the variable-temperature SSNMR 1H spectra were recorded from 25 to 125 °C to elucidate the temperaturedependent proton conduction behavior. The stacked spectra are plotted in Figure 6c. As depicted in Figure 6d, the full height at half width (FHHW) of the 7.34 ppm signal slightly increases initially and then decreases upon further heating. Simultaneously, the peak at 13.57 ppm gradually merges with the peak at 7.34 ppm. The 1H chemical shift slightly increases from 7.34 ppm to 7.95 ppm as the temperature increases, suggesting protons at different chemical shifts are thermally activated, which then tend to produce isotropic protons with a higher exchange rate when the temperature is elevated. As a result, the thermal activation effect on confined H+ atoms in proton hopping sites along the c axis is responsible for exchange and diffusion, where short N-O distances enable “soft” H+ atoms to jump between adjacent N-H and P-OH units, giving rise to so-called “oscillatory proton transfer”.

Figure 7. H2/O2 fuel cell containing ZrP. (a) Schematic representation of the PEMFC system and PEMFC pellet in which ZrP is used as a solid electrolyte. (b) Performance of the H2/O2 fuel cell utilizing ZrP as the electrolyte at 180 °C. The orange hollow spheres and orange solid spheres represent current−voltage and current-power measurements, respectively. (c) Constant voltage test of a single cell containing ZrP electrolyte at 0.4 V for 15 h.

Additionally, we provided the first demonstration of power generation through a DMFC with CPs/MOFs as the solid-state electrolyte. The maximum open-circuit voltage of the DMFC using ZrP as the electrolyte is 0.44 V, and the maximum power density is 0.13 mW•cm−2, as shown in Figure S23. The open-circuit voltage and the obtained power density of the DMFC are lower than those of the H2/O2 fuel cell. Several reasons may be responsible for this, such as the low catalytic efficiency of methanol, unoptimized fuel concentration, and more severe crossflow issue.

Figure 6. Results of the variable-temperature solid-State NMR measurement of ZrP. (a) Line shape analysis of the 1D 1H spectrum (above). Two resonances are deduced with chemical shifts at 7.34 and 13.57 ppm. The recorded DQ-filtered 1H spectrum (below) reveals that protons in the system are mobile. (b) 2D 1H-1H exchange spectrum. The intense cross peaks labeled in red at 13.57 ppm and 7.34 ppm indicate the existence of proton exchange. (c) Variable-temperature 1H spectra of ZrP. (d) Plots of chemical shifts and the FHHW of the 7.34 ppm signal (in ambient) as a function of the experimental temperature.

CONCLUSIONS

H2/O2 and Direct Methanol Coordination Polymer Fuel Cell. After observing the stable, inherent, high anhydrous pro-


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One of the most common and widely utilized strategies that leads to intermediate-temperature anhydrous proton conducting CPs is the post-synthetic encapsulation of non-volatile proton-carrier species into the pores. Those species are often neutral in charge and therefore experience very weak interactions (mostly hydrogen bonding and van der Waals interactions). Although this is beneficial to proton transport owing to the high mobility of the carriers themselves, it also greatly sacrifices the long-term stability by enhancing the chance of potential loss of these carriers during practical applications. In traditional systems, high proton conductivity and stability are difficult to achieve simultaneously. In the ZrP structure, ordered NH4+ cations form in situ and act as proton carriers. The strong electrostatic interaction between the cations and anionic CPs endows the proton carriers with improved stability, which one would assume to be detrimental to proton conduction. However, these NH4+ cations are located at amazingly suitable positions to undergo very efficient proton exchange with phosphate pairs containing proton vacancies, affording a unique one-dimensional proton-transport pathway, which was directly revealed by DFT and molecular dynamics simulations. This proton transfer network has a notably high thermal stability up to 200 °C and long-term stability under operating conditions, which is scarcely reported. This unique proton conducting CP was further utilized as a solid-state electrolyte in a PEMFC, which showed the record power density for PEMFCs based on proton conducting CPs, and a DMFC for the first time. The intrinsic stability of the proton carriers gives rise to the stable performance of these cells over a long time period. These efforts suggest that practical applications of proton conducting CPs are viable by further optimization of the electrode contact with the CP electrolyte, the thickness of the electrolyte, and the mechanical properties of the CP electrolyte with minimized fuel crossflow.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details and data (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] (Shuao Wang), [email protected] (Ruhong Zhou), and [email protected] (Ling Zhao).

Author Contribution #

These three authors contributed equally.

Notes The authors and Soochow University have filed a patent on the presented results.

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21790370, 21790374, 21761132019, and 51402266), "Young Thousand Talented Program" in China, the General Financial Grant from the China Postdoctoral Science Foundation (2016M591901) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Table of Contents (TOC)


Unique Proton Transportation Pathway in a Robust Inorganic

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