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An open-framework chalcogenide showing both intrinsic anhydrous and water-assisted high proton conductivity Hong-Bin Luo, Mei Wang, Jin Zhang, Zheng-Fang Tian, Yang Zou, and Xiaoming Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17189 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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ACS Applied Materials & Interfaces

An open-framework chalcogenide showing both intrinsic anhydrous and water-assisted high proton conductivity Hong-Bin Luo,a,b Mei Wang,a,b Jin Zhang,a,b Zheng-Fang Tian,*c Yang Zou,a,b Xiao-Ming Ren*a,b,d

a

State Key Laboratory of Materials-Oriented Chemical Engineering and College of

Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, P. R. China b

College of Materials Science and Engineering, Nanjing Tech University, Nanjing

210009, P. R. China c

Hubei Key Laboratory for Processing and Application of Catalytic Materials,

Huanggang Normal University, Huanggang 438000, P. R. China d

State Key Laboratory of Coordination Chemistry, Nanjing University 210093, P. R.

China

Fax: 86-25-58139481 Tel: 86-25-58139476 E-mail: [email protected]

1

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Abstract Proton conducting materials have attracted increasing interest because of the promising technological applications as key components in various electrochemical devices. It is of great significance for technique application to seek for superior proton conducting materials, operating under both anhydrous and humidified conditions in a wide temperature range. Herein we demonstrate the proton conductance of an open-framework chalcogenide, (CH3NH3)2Ag4Sn3S8 (1), and the postsynthesis product 2 achieved by doping hydrochloric acid into 1. Hybrid 2 displays both intrinsic anhydrous and water-assisted high proton conductance, with σ = 1.87×10-4 S⋅cm-1 at 463 K under N2 atmosphere and 1.14×10-3 S⋅cm-1 at 340 K 99%RH, and these conductivities are comparable to that in the efficient MOF-based proton conducting materials. Moreover, hybrid 2 shows excellent thermal stability and long-term stability of proton conduction.

Keywords: Open-framework chalcogenide; postsynthesis; proton conductance; thermal stability; long-term stability

2


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Introduction Motivated by wide application in a range of electrochemical devices, as the key components,1-3 the development of proton conducting materials have attracted increasing interest over the past few years. Currently, the commercial Nafion-based proton conducting materials possess excellent proton conductivity (10−1−10−2 S·cm−1) at moderate temperature (60−80 °C) and high relative humidity (98%RH), but also have some limitations, such as high cost, hazardous manufacturing processes and low thermal stability,4,5 and these disadvantages spurred researchers to search alternative proton conducting materials. In response to obtaining the proton conducting polymer membranes with high thermal stability and good water retention, several efficient strategies have been developed, which include use of chemical cross-linking agents, incorporation of hydrophilic inorganic fillers into the polymer proton conducting membranes, or replacement of water with non-aqueous and low-volatility proton carriers, e.g. protic ionic liquids.6,7 Most recently, great efforts have been offered to exploring desirable crystalline porous proton conducting materials, for example, covalent organic frameworks (COFs), and metal–organic frameworks (MOFs) or porous coordination polymers (PCPs).8-11 To date, these types of proton conducting materials could be divided into two subcategories based on the operating temperature regions. The water-mediated proton conducting materials often show extremely high proton conductivity at low temperature (< 100 °C) under high relative humidity, however, the proton conducting performance of a material relies strongly on the presence of hydrogen-bonded water molecules since the hydrogen-bonding interaction between protons/proton-carriers and water molecules in a material provide effective proton transport pathways;12-20 the anhydrous proton conducting materials operate at the temperature above 100 °C under anhydrous conditions, and this can be realized when the nonvolatile proton carriers (such as H3PO4, H2SO4, imidazole, histamine and triazole etc.) are incorporated into the pores or defect sites of crystalline porous materials.21-27 Therefore, it is of great significance and challenge for technique application to seek for superior proton conducting materials operating under both anhydrous and humidified conditions in a 3



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wide temperature range.28,29 Open-framework chalcogenides, as one member of crystalline porous family, integrating zeolite-like architecture, high anionic framework polarizability and high concentrations of charge carriers in pores as well, have been extensively investigated for promising application in the area of fast-ion conductors. In 2003, Zheng and co-workers presented a series of three-dimensional sulphides and selenides, which contain highly mobile alkali metal ions as charge-balancing cations of framework, among of which ICF-5 (CuInS-Na), ICF-21 (InSe-Na) and ICF-22 (InS-Li) exhibit high ionic conductivity at ambient temperature and from moderate to high humidity.30 Kaib et al. reported several lithium chalcogenidotetrelates that exhibit promising Li+ ion conductivity.31,32 Kim and co-workers demonstrated the layered chalcogenide with planar pathways for ion diffusion resulting in fast Na+ ion conduction.33 While, the study of ion conduction in chalcogenides is limited to alkaline ions, and only few examples have been reported for proton conduction so far.34,35 In comparison with the burgeoning MOFs/PCPs that have been widely investigated in the proton conducting application, the open-framework chalcogenides possess better chemical and thermal stability since they have solid inorganic frameworks,36,37 thanks to these distinct features, the encapsulation of functional guest molecules into pores would impart high proton conducting performance to open-framework chalcogenides, and the study of proton conducting open-framework chalcogenides gives a fresh stimulus to explore new types of solid proton conducting materials. In this contribution, we report an open-framework hybrid chalcogenide, (CH3NH3)2Ag4Sn3S8 (1),38 features with superior thermal stability and surprising acidic stability. The crystals of 1 imbued with hydrochloric acid gave an altered hybrid crystal, HCl@1 (2), and such postsynthesis hybrid possesses intrinsic anhydrous proton conductance with σ = 1.87×10-4 S⋅cm-1 at 463 K (190 °C) and water-assisted high proton conductance with σ = 1.14×10-3 S⋅cm-1 at 340 K (67 °C) 99% RH. Moreover, the resultant 2 shows excellent thermal stability and long-term stability for practical application. 4


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Experimental Chemicals and materials The chemicals and reagents with analysis pure grade, including silver chloride, sulfur sublimed, tin and methylamine (27−32% alcohol solution) were obtained from commercial sources and used as received. Syntheses of compound 1 and 2 (CH3NH3)2Ag4Sn3S8 (1). The hybrid crystals were prepared by means of the slight modification of the literature method,38 using AgCl (95.3 mg), Sn (78.7 mg), S (85.3 mg) and methylamine (27−32% alcohol solution, 2.7 mL) as solvent, the above chemicals and solvent were mixed at ambient temperature, followed by transferred into a 20 mL Teflon reactor, heated at 160 °C for 8 days and then cooled down to ambient temperature. The obtained hybrid crystals were washed with ethyl alcohol and deionized water, and subsequently dried at 80 °C in an airflow drying oven for 12 hours. Elemental microanalysis calculated for (CH3NH3)2Ag4Sn3S8: C, 2.17; H, 1.09; N, 2.53%. Found: C, 1.93; H, 0.87; N, 2.21%. (CH3NH3)2Ag4Sn3S8·0.44HCl·H2O (2). This sample was prepared by soaking crystals of 1 in 4.0 mol⋅L-1 hydrochloric acid (HCl) solution for 5 days, and then the postsynthesis crystalline sample are separated by suction, washed with deionized water, dried at 80 °C in an airflow drying oven for 12 hours, sequentially. The amount of HCl doped in the crystals of 1 was evaluated in accordance with the content of Cl element using Energy Dispersive Spectrometer and the amount of H2O in 2 was calculated according to thermogravimetric analysis (TG). The chemical formula of 2 is roughly expressed as (CH3NH3)2Ag4Sn3S8·0.44HCl·H2O. Elemental microanalysis calculated for (CH3NH3)2Ag4Sn3S8·0.44HCl·H2O (2): C, 2.10; H, 1.27; N, 2.45%. Found: C, 1.87; H, 0.81; N, 2.47%. Chemical and Physical characterizations Elemental analyses (C, H and N) were achieved using an Elementar Vario EL III 5



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elemental analyzer. The water-adsorption/desorption isotherm measurement was carried out utilizing the Belsorp–Max instrument. The energy-dispersive spectroscopy (EDS) characterization was conducted using a Hitachi S-3400N Scanning Electron Microscope. Powder X-ray diffraction (PXRD) data at room temperature were collected on a Rigaku MiniFlex600 diffractometer, operated at 40 kV and 15 mA, with Cu Kα radiation (λ = 1.54059 Å), and the measurement was performed in the range of 2θ = 5-50° with 0.01°/step. The variable-temperature PXRD measurements were performed using SHIMADZU XRD-6100 diffractometer, operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 1.5418 Å). The 2θ = 5-50° with 0.01°/step and the temperatures ranges from 303 to 493 K (from 30 to 220 °C). During measurements, the changing rate of temperature is 10 K⋅min−1, and the sample was kept at the set temperature for 15 minutes to ensure that the sample and probe have the same temperature. TG measurement was performed on a TA2000/2960 thermogravimetric analyzer in the temperature range of 293-1073 K (20-800 °C) under nitrogen atmosphere. The relative humidity (RH) dependence of proton conductivity was derived from the alternating current (ac) impedances, measured using a CHI 660D electrochemical workstation with a conventional three-electrode method. The frequencies spanned from 10 Hz to 106 Hz with signal amplitude of 5 mV. The anhydrous proton conductivity was evaluated by ac impedance measurements under N2 atmosphere using a Concept 80 system (Novocontrol, Germany), the scanning frequencies ranged from 1 Hz to 10 MHz. The conductivity (σ) was calculated using the equation of σ = L/RS, where L is the sample thickness and S is the cross-sectional area, and R represents the resistance. Results and discussion Preparation and characterization of 2 Polycrystalline sample of 1 were immersed in a concentration of 4 mol⋅L-1 HCl solutions for five days, and then the postsynthesis polycrystalline sample separated by suction, and thoroughly washed with deionized water, dried at 80 °C under vacuum for 12 hours, which are the target samples and labeled as the hybrid 2. 6


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Experimental PXRD profiles of 1 and 2 together with simulated PXRD pattern of 1 are illustrated in Fig. 1a, all of them show rather high similarity, indicating that the crystal structure of 1 still maintains integrated after soaked in a concentration of 4 mol⋅L-1 HCl solutions for 5 days. TG curves of 1 and 2 are depicted in Fig. 1b, although 1 and 2 show similar two-step decomposition processes, the difference of TG curve shape is obvious. Two platforms appear in the temperature range of 303-486 K and 540-836 K in the TG plot of 1, respectively. The abruptly losing weight occurring at ca. 486 K in 1 is related to the release of CH3NH2 and H2S. The next decomposition process starting at 836 K corresponds to the collapse of inorganic framework. However, the gradual weight loss process undergoes at the temperature below the decomposition process of CH3N3+ and release of H2S in 2, which is related to the adsorbed water as well as a part of HCl liberating; and with respect to the platform at higher temperature region in 1, the weight loss of ca. 1.5% process is probably associated with the release of the residual HCl. The guest-free framework in 2 is thermally stable up to 836 K, which is almost the same as that of 1, and this observation is further confirmed by the variable-temperature PXRD analysis for 1 and 2 (ref. Fig S2).

(a)

(b)

Fig. 1 (a) Experimental PXRD profiles of 1 and 2 as well as simulated PXRD pattern of 1 (b) TG plots of 1 and 2. As shown in Fig. 2, the EDS elemental mapping indicated a homogenous distribution of Ag, Sn, S and Cl as well, confirming that the HCl molecules were indeed encapsulated into the pores or defects of the hybrid crystals. EDS was also employed to approximately estimate the content of HCl in the crystalline sample of 2. 7



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The measurements were performed for four times, as shown in Fig S3, the relative contents of S, Ag and Sn in the inorganic framework show somewhat difference from each measurement, as a result, the averaged HCl content of four times measurements is calculated to be ca. 0.44 HCl molecules per formula unit of (CH3NH3)2Ag4Sn3S8. Combined analysis of TG and EDS, the chemical formula of 2 is roughly expressed as (CH3NH3)2Ag4Sn3S8·0.44HCl·H2O. (a)

(c)

(b)

(d)

Fig. 2 EDS elemental mapping images of (a) S Kα1 where the inset: photo of the crystal (b) Ag Lα1 (c) Sn Lα1 and (d) Cl Kα1 in 2, respectively. X-ray photoelectron spectroscopy of 1 and 2 is shown in Fig 3 and Fig S6, indicating that 1 and 2 have comparable core levels of C1s, N1s, S2p, Ag3d and Sn3d. In contrast with almost the same C1s peaks (282.30 eV; ref. to Fig 3a) in 1 and 2, the N1s peak located at 399.10 eV in 1 shifts slightly to the higher energy region (399.31 eV; ref. Fig 3b) in 2,39 and a similar finding has been observed in other HCl doped open-framework chalcogenide.35 The Ag3d core level spectrum of 1 exhibits two peaks of Ag3d5/2 and Ag3d3/2 with binding energy at 365.42 and 371.41 eV together a peak splitting of 6.0 eV, indicating that Ag is in the +1 oxidation state. With respect to 1, the binding energies of Ag3d5/2 and Ag3d3/2 increase to 365.60 and 371.61 eV in 2, respectively. Peaks corresponding to the core levels of Sn3d5/2 and Sn3d3/2 are visible 8


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at binding energies of 483.96 and 492.32 eV in 2, showing a small shift towards higher energy region regarding the corresponding binding energy of 483.80 and 492.23 eV in 1. The S2p3/2 and S2p1/2 peaks locate at 158.86 and 159.37 eV in 1, respectively, and the corresponding binding energies at 159.10 and 160.12 eV in 2, all of which fall within the 160-164 eV range anticipated for S atoms in the form of sulfides. With regard to the XPS spectrum of 1, the changes of binding energy of N1s, S2p3/2 and S2p1/2, Ag3d5/2 and Ag3d3/2 as well as Sn3d5/2 and Sn3d3/2 in the XPS spectrum of 2 are probably related to the interactions between the protons in HCl molecules and the S atoms on the surface of framework as well as between the chlorides in HCl molecules and the silver ions on the surface of framework. In addition, the characteristic peaks belonging to Cl2p3/2 and Cl2p1/2 located at 196.47 and 197.84 eV are also found in 2 (Fig 3f). Regarding 1, all changes in TG and XPS of 2 demonstrate HCl molecules are successfully imbued into the lattice of open-framework of 1.

(a)

(b)

(d)

(c)

9



(e)

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(f)

Fig. 3 XPS spectra of (a) C1s (b) N1s (c) S2p3/2 and S2p1/2 (d) Ag3d5/2 and Ag3d3/2 (e) Sn3d5/2 and Sn3d3/2 in 1 and 2 (f) Cl2p3/2 and Cl2p1/2 in 2. Proton conductance under certain humidity The proton conductivity of 1 and 2 was determined by the alternating current (ac) impedance on the compressed pellet samples at different temperatures and 99%RH. The typical Nyquist plots at the selected temperature and 99%RH are shown in Fig S7a-e, all of which exhibit an imperfect arc in the higher frequency region corresponding to the bulk and grain boundary resistance, and a spur in the lower frequency region related to the mobile ions that are blocked by the electrode-electrolyte interface. In this case, the value at the Z’-axis intercept is approximately regarded as the resistance at the selected temperature.40 It is obvious that the resistance decreases with the increase of temperature, indicating that the proton

conductivity

is

improved

owing

to

the

thermal

activation.

The

temperature-dependent proton conductivity of 1 is also shown in Fig S7f, demonstrating that the proton conductivity increase gradually with elevating temperature, for example, the σ value increases from 6.61×10-5 S cm-1 at 289 K to 1.03×10-4 S cm-1 at 305 K, 1.54×10-4 S cm-1 at 317 K and further reached a maximum of 2.56×10-4 S cm-1 at 329 K. As for 2, it also displays a liner increase in conductivity with increasing temperature. Noteworthy, the value of proton conductivity of 2 is as several times as higher than that of 1, e.g., the proton conductivity of 2 at 298 K and 99%RH, with the value of 4.53×10-4 S cm-1, which is also higher than the proton conductivity in the well-studied porous proton conducting materials, such as K2(H2adp)[Zn2(ox)3]·3H2O (with σ = 1.2×10-4 S⋅cm-1 at 298 K and 98%RH),41 10


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[{In2(OH)2(SO4)4}·{(LH)4}·nH2O]n (with σ = 4.4×10-5 S⋅cm-1 at 303 K 98%RH),42 [NH2(CH3)2][In(FDA)2] (with σ = 1.0×10-4 S⋅cm-1 at 304 K and 99.5%RH),43 and can be comparable to some efficient MOF-based proton conducting materials, such as MFM-500(Ni) (with σ = 4.5×10-4 S⋅cm-1 at 298 K and 98%RH),44 GdHPA-Ⅱ (with σ = 3.2×10-4 S⋅cm-1 at 294 K and 98%RH),45 CoLa-II-SC (with σ = 3.05×10-4 S⋅cm-1 at 298 K and 95%RH).46 Notably, the proton conductivity at 334 K and 99%RH reaches to 8.97×10-4 S⋅cm-1 that is nearly more than twice the proton conductivity at room temperature, and the highest proton conductivity (with σ = 1.14×10-3 S⋅cm-1) over 10-3 S⋅cm-1 was attained at 340 K and 99%RH, this prominent proton conductivity can compete with that of water-facilitated proton conducting materials, such as NENU-530

(with

σ

=

1.5×10-3

S⋅cm-1

at

348

K

and

98%RH),16

[(H2dmp)(SO4)2][Zn3(ox)2(H2O)7] (with σ = 2.6×10-3 S⋅cm-1 at 333 K and 98%RH),47 [La(H4NMP)(H2O)2]Cl·2H2O (with σ = 2.0×10-3 S⋅cm-1 at 353 K and 95% RH).48 By comparing with 1, the enhancement of conductivity in 2 is attributed to that the absorbed HCl not only gives higher concentration of mobile protons but also constructs more extensive hydrogen-bond networks for facile proton conduction. To get deeper insight into the proton-transport mechanism, the activation energies of proton conduction were estimated from the Arrhenius equation, expressed below, ln(σT ) = ln A −

Ea k BT

(1)

where the symbols σ, Ea, kB and A represent the proton conductivity, the proton-transport activation energy, the Boltzmann constant and pre-exponential factor, respectively. Fig. 4 shows the Arrhenius plots of the proton conductivity, and the activation energy was estimated to be 0.299 eV and 0.199 eV for 1 and 2 via linear fit of the plots, respectively. The activation energies of 1 and 2 lie within the range of Ea < 0.4 eV, demonstrating that the proton conduction process mainly follow the typical Grotthuss mechanism,49 namely, the protons hopping within a hydrogen-bond network. The activation energy is lower in 2 than in 1, and this observation is easily understandable that the absorbed HCl in the pores or defects of framework would bring more dense H-bond networks in the crystal of 2 to promote proton hopping 11



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process.

Fig. 4 Plots of ln(σT) against 1000/T for 1 and 2 under 99%RH. In a water-mediated proton conducting material, the framework should readily adsorb water molecules into the lattice, which conduce to generate well-established hydrogen-bond networks and provide efficient proton transport pathways. The water-adsorption/desorption isotherm measurement was performed for 1 and 2 at 298 K (Fig. S8), the amount of adsorbed water increases with the increase of water vapor pressure, suggesting that water molecules are successfully adsorbed into 1 and 2 at a certain relative humidity. By comparison of 1 and 2, the amount of adsorbed water in 2 is significantly larger than that in 1 at the same condition. On the other hand, the EDS images show the surface of the open-framework crystal becomes roughness, demonstrating that the lattice defects increase after the open-framework doped by HCl. Accordingly, the fact that the amount of adsorbed water in 2 is significantly enhanced regarding 1 perhaps arises from the formation of more defects in the lattice of 2 owing to adsorption of HCl. To clarify the correlation between proton conductance and relative humidity, the humidity-dependent proton conductivity was further measured for 1 and 2 at 301 K. As shown in Fig. S9 and S10, the proton conductivity raised with the relative humidity ranging from 43% to 99%RH, the proton conductivity increases from 1.13×10-8 S⋅cm-1 (2.69×10-6 S⋅cm-1) at 43%RH to 2.3×10-6 S⋅cm-1 (1.13×10-4 S⋅cm-1) at 88%RH, then reached the maximum values of 9.07×10-5 S⋅cm-1 (4.67×10-4 S⋅cm-1) at 99%RH for 1 (2), respectively. This finding is consistent with the results of many reported water-mediated proton conducting materials,50,51 and also indicates that water 12


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molecules play a vital role in building the effective proton transport pathways. The proton conducting stability in a material is an essential term for practical application. The time-dependent proton conductivity of 2 at 301 K and 99%RH were further investigated, as shown in Fig. S11, hybrid 2 maintains its proton conductivity for 1-5 days, without any sizable loss in performance, which demonstrate that this material possess excellent long-term stability of performance. Furthermore, the humidity-cycling proton conductivity has also been measured between 56%RH and 99%RH at 301 K (Fig. 5 and Fig. S12), and the result reveals that the material is resistant to a wide range of humidity changes, suggesting that it has good repeatability and humidity-cycling stability. All of the impressive features of 2 suggest a promising potential for practical application in proton conduction.

Fig. 5 Humidity-cycling study of 2 between 56%RH and 99% RH at 301 K. Proton conductance under anhydrous conditions Given the excellent thermal stability of 1 and 2, and they are likely to be good candidates in anhydrous proton conducting materials, which can be operated in high temperature region (> 373 K). Therefore, the proton conduction behaviors have been investigated under anhydrous conditions. As shown in Fig. S13, open-framework hybrid 1 displays increased proton conductivity with rising temperature, with the low σ value of 1.33×10-9 S⋅cm-1 at 253 K, 6.34×10-7 S⋅cm-1 at 373 K, 3.55×10-6 S⋅cm-1 at 423 K, and the maximum proton conductivity, of 2.06×10-5 S⋅cm-1, was attained at 463 K. Regarding 1, hybrid 2 possesses higher proton conductance over the whole temperature range, and performs promising proton conductivity in a wide temperature range (253-463 K) under anhydrous conditions. As displayed in Fig. 6a and Fig. S14, 13



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the proton conductivity of 2 was also increased with the increasing temperature, and enhanced by more than four orders of magnitude as temperature increases from 253 K to 463 K, with the σ value increased from 1.70×10-8 S⋅cm-1 at 253 K to 2.25×10-7 S⋅cm-1 at 298 K, 2.80×10-6 S⋅cm-1 at 373 K, and then reached to the highest proton conductivity of 1.87×10-4 S⋅cm-1 at 463 K, and this valve is comparable to a series of well-known

anhydrous

proton

[Zn(HPO4)(H2PO4)2](ImH2)2 [Zn-(H2PO4)2(TzH)2]n

conducting

(with

σ

σ

=

(with

{[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n

(with

2.6×10-4

=

1.2×10-4 σ

materials,

=

S⋅cm-1 S⋅cm-1

1.0×10-4

such at

at

S⋅cm-1

403 423

at

423

as K),52 K),53 K),28

β-PCMOF2(Tz)0.45 (with σ = 5.0×10-4 S⋅cm-1 at 423 K).25 It is worth mentioning that there appears a turning point on σ-T plot of 2 at ca. 370 K, which is probably related to the adsorbed water as well as a part of HCl liberating, and this supposition is supported by TG analysis (ref. Fig. 1b). The curves of ln(σT) against 1000/T are plotted in Fig. 6b for 1 and 2 under anhydrous condition, the activation energy was estimated to be 0.479 eV for 1, suggesting the proton transport is mainly dominated by vehicle mechanism. However, the activation energy of 2 was estimated to be 0.325 eV in the lower temperature region below 363 K and 0.646 eV in the higher temperature region above 363 K, corresponding to Grotthuss and vehicle meachanism,54 respectively.

(a)

(b)

Fig. 6 (a) temperature-dependent proton conductivity of 2 and (b) plots of ln(σT) against 1000/T of 1 and 2 under anhydrous condition. Possible proton transport and water-assisted proton conductance mechanism The crystal structure of 1 was previously reported in the literature.38 In the crystal 14


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structure of 1 with orthorhombic space group Pnma, as displayed in Fig.7a, each [Ag4SnIVSnIIS8]n6n− ribbon links four adjacent [Ag4SnIVSnIIS8]n6n− ribbons via tetrahedral Sn4+ ions to form three-dimensional [Ag4SnIV2SnIIS8]n2n− anionic open-framework with one-dimensional irregular channels, and the CH3NH3+ ions are residual in the channels to balance the charge of inorganic framework.38 There only exist weakly charge-assisted hydrogen-bonding interactions between N atoms in the CH3NH3+ cations and S atoms on the surface of framework (ref. Fig.7b), where the interatomic distances of H…S and N…S are slightly less than or comparable to the sums of van der Waals radii of corresponding atoms (H, N and S atoms: rH = 1.2 Å, rN = 1.55 Å and rS = 1.8 Å).55 The hydrogen-bond parameters are summarized in Table 1, and two different H-bond chains, formed via the N atoms in the CH3NH3+ cations and S atoms on the surface of framework along b-axis direction, are shown in Fig.7c and 7d. Obviously, two types of H-bond chains do not provide efficient proton hopping pathway, and it is unavailable that the proton transport realizes via proton hopping along the hydrogen-bond chains. Thus, the proton transport process should follow by vehicle mechanism in 1 under anhydrous condition, and this is in agreement with the activation energy analysis under anhydrous condition in 1. In the humidity environment, the water molecules are adsorbed into the channels or defects, which not only increases the packing density of guests in the channels, but also results in the formation of efficient hydrogen-bond networks between N atoms in the CH3NH3+ cations and water molecules/S atoms on the surface of framework. Although the increase of guest packing density in the channels obstruct the migration of CH3NH3+ cations, the efficient hydrogen-bond networks favor the proton transport via hopping manner. Therefore, the Grotthuss mechanism dominates the proton transport process in 1 and 2 in the high relative humidity environment.

15



(a)

Page 16 of 26

(b)

(c)

(d)

Fig. 7 (a) Inorganic open-framework showing one-dimensional channels along b-axis direction (b) hydrogen-bonds between S and N atoms (c, d) the H-bond chains, viewed along [1 0 -1] direction, formed via the N atoms in the CH3NH3+ cations and S atoms on the surface of framework in the crystal of 1 (the symmetric codes: #1 = 0.5-x, 1-y, -0.5+z; #2 = x, 0.5-y, z). Table 1 Parameters of weak hydrogen bonds formed between N atoms in the CH3NH3+ cations and S atoms on the surface of framework in 1 D-H…A

D-A (Å)

H…A (Å)

D…A (Å)

∠D-H…A (°)

N2-H2A…S3 N2-H2A#2…S3 N2-H2A#2…S6#1 N1-H2…S6#1 Symmetric codes

0.83

2.95

3.359(8)

112.7

0.83

2.95

3.359(8)

112.7

0.83

2.99

3.383(6)

111.1

0.80

2.71

3.281(6)

130.7

#1 = 0.5-x, 1-y, -0.5+z; #2 = x, 0.5-y, z

Conclusion In summary, we have successfully obtained a new type of proton conducting material based on open-framework chalcogenide via a simple but effective 16


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydrochloric acid impregnation, and our study demonstrated that this hydrochloric acid doped material exhibits both intrinsic anhydrous and water-assisted high proton conductance, with σ = 1.87×10-4 S⋅cm-1 at 463 K under anhydrous conditions and 1.14×10-3 S⋅cm-1 at 340 K 99%RH. The high proton conductance could compete with those in many reported MOFs/PCPs and COFs-based proton conducting materials. Furthermore, this material possesses the unique excellent thermal stability and long-term stability, which suggest a great potential for practical application. And this study will broaden the scope of proton conducting materials for practical application in electrochemical devices. Supporting Information Available: PXRD profiles of 1, 2 at room temperature, Variable-temperature PXRD patterns at selected temperatures for 1 and 2, EDS analysis for 2, EDS analysis for 1 immersed in concentrated H3PO4 and H2SO4 for five days, XPS spectra of 1 and 2, Nyquist plots of 1 and 2 at selected temperatures under 99%RH and temperature-dependent proton conductivity of 1 and 2 under 99% RH, water-adsorption/desorption isotherm of 1 and 2 at 298 K, Nyquist plots of 1 at 301 K with different relative humidity and humidity-dependent proton conductivity at 301 K, Nyquist plots of 2 at 301 K with different relative humidity and humidity-dependent proton conductivity at 301 K, Time-dependent Nyquist plots and proton conductivity of 2 measured at 301 K under 99% RH, Nyquist plots of 2 under 56% RH and 99% RH at 301 K for humidity-cycling study, Nyquist plots of 1 at selected temperature under anhydrous condition and temperature-dependent proton conductivity of 1 under anhydrous condition, Nyquist plots of 2 at selected temperatures (263-463 K) under anhydrous condition and temperature-dependent proton conductivity of 2 under anhydrous condition, the environments around the Ag, Sn and S atoms in the crystal structure of 1. Acknowledgment The authors thank the Priority Academic Program Development of Jiangsu Higher Education Institutions, the National Nature Science Foundation of China (grant no. 21671100), and the Postgraduate Research & Practice Innovation Program of Jiangsu 17



Page 18 of 26

Province (grant no. KYCX17_0930) for financial support.

18


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Multifunctional

Luminescent

Carboxyphosphonate

and

Open-Framework

Proton-Conducting Hybrids

Exhibiting

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25



Page 26 of 26

TOC

Open-framework chalcogenide, (CH3NH3)2Ag4Sn3S8 (1), was doped by HCl to give hybrid 2, which shows both intrinsic anhydrous and water-assisted high proton conductivity.

26


Open-Framework Chalcogenide Showing Both ... - ACS Publications

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