Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior

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Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior and Related Spectroscopic Studies Ramanpreet Kaur, Diptikanta Swain, Dipak Dutta, Kumar Brajesh, Priyank Singh, Aninda Jiban Bhattacharyya, Rajeev Ranjan, Chandrabhas Narayana, Jürg Hulliger, and Tayur N. Guru Row J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03215 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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The Journal of Physical Chemistry

Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior and Related Spectroscopic Studies Ramanpreet Kaura, Diptikanta Swaina, Dipak Duttaa, Kumar Brajeshb, Priyank Singhc, Aninda J. Bhattacharyyaa, Rajeev Ranjanb, Chandrabhas Narayanac, Jürg Hulligerd, Tayur N. Guru Row*a a

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India.

b

Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

c

Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

d

Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, 3012 Berne, Switzerland

ACS Paragon Plus Environment

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ABSTRACT

One of the key challenges of fuel cell technology is to find solid electrolytes which are cheap and environmental friendly with high proton (H+) conductivities. In this context, designing new materials based on organic co-crystals/salts appear very promising to expand the scope of H+ ion conductors. In our approach, we have synthesized a quaternary organic salt consisting of gallic acid, isoniazid, sulfate (SO 4 2-) ion and water by slow evaporation method which exhibits high proton conductivity of 0.2 mScm-1 at 293K to serve as a model system. The proton conductivity value observed in our system is comparable and in some cases better than recently published coordination polymers, metal organic frameworks and covalent organic frameworks. The system crystallizes as monoclinic with space group P2 1 /c, (Z’=3; Z=12) which depicts a layered structure with extensive O-H⋅⋅⋅O and N-H⋅⋅⋅O hydrogen bonding networks. Further it exhibits interesting order-disorder phase transitions at both high and low temperatures. Calculation of activation energy (∼0.39 eV) from conductivity plots for the system reveals the mechanism of proton conduction to be Grotthuss type. Thus, our novel design strategy of preparing organic salt for proton conduction applications opens up a pathway to generate easy synthesis, cheap and environmental friendly materials.

1. Introduction Proton-conducting solids are extremely important materials for their application in fuel cell technologies.1 The classical example is the Nafion membranes which recently being used as proton conductors in fuel cells.2-4 The expensive nature of these polymer membranes and dehydration at high temperature demands alternate cheap by proton conducting materials. For this propose, a number of materials such as inorganic oxides, co-ordination polymers, covalent


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organic frameworks (COFs) and metal organic frameworks (MOFs) have been explored as possible chioces.5-7 In addition, these systems have been studied extensively for their efficient values of proton conductivities at moderate temperatures8 but their utility is somewhat limited due to complex synthesis procedures. The facile synthesis, light and comparatively cheap nature of organic systems could be attractive factors for designing materials for proton conducting applications. In this direction, co-crystals/salts are versatile crystalline solids that are currently being investigated for various applications such as nonlinear optics, electronics and pharmaceutics etc.9 The ability to design and tune the structural architectures of these materials to yield desired physical and chemical properties using principles of crystal engineering10 make them attractive as possible functional materials. Although it is difficult to reach the proton conductivity values comparable to Nafion, structure-property co-relation could eventually help to generate the more efficient materials to replace these costly membranes. To test this hypothesis, we set out to synthesize a quaternary molecular salt by incorporation of sulfuric acid in a ternary salt reported by us in our earlier studies.11 It is noteworthy that the disorder nature of sulfuric acid and resultant enhancement in the proton conductivity values has been well understood in inorganic materials.12-22 The same idea has been utilized to prepare organic molecular crystals to analyze the proton conducting properties of this salt. A simple method of solvent evaporation for crystallization of gallic acid (3,4,5-trihydroxybenzoic acid) and isoniazid with aqueous solution of sulfuric acid is used. The crystallization yielded a hydrated sulfuric salt of gallic acid and isoniazid (GISH, hereafter). Conductivity measurements on GISH show high proton conductivity values in the order of 10-4 Scm-1 at 293K in presence of 98% relative humidity which is better and comparable to several MOFs and co-ordination polymers reported in the literature.23-27 GISH also displays very interesting phase transitions at both low and high temperatures with change in


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Z values (number of formula unit per unit cell). The unique feature of this study is the reversibility in phase transition upon dehydration in GISH, a feature which will render the compound for repeated usage, unlike the earlier reported ternary compound.11 Further, careful structural analysis of GISH and spectroscopic techniques uncovered the mechanism of proton conduction. Thus, we illustrate a novel design strategy for efficient proton transfer in organic salts by incorporating sulfate ions and structure-property co-relation which will through light to design more organic salts based proton conductors.

2. Experimental section 2.1. Synthesis. Commercially available compounds (Sigma-Aldrich) were used without further purification. Solvents were of analytical or chromatographic grade and purchased from local suppliers. 3,4,5-trihydroxybenzoic acid (hereafter GA) and isoniazid (hereafter INZ) in 1:1 molar ratios combined on the 100 mg scale were subjected to neat grinding for 15 min manually using a mortar-pestle. Ground mixtures were kept for crystallization at ambient conditions using 1 molar (1M) aqueous solution of H 2 SO 4 as solvent of crystallization. The crystallization resulted yellow single crystals of plate morphologies after 5 days. 2.2. Thermal analysis. Differential scanning calorimetry (DSC) [Mettler Toledo DSC 822e module and thermo gravimetric analysis (TGA) [Mettler Toledo TGA/SDTA 851e module] were carried out over a temperature range of 303-623K with a heating rate of 5Kmin-1. Samples were placed in crimped but vented aluminum pans for DSC and open alumina pans for TGA and were purged by a stream of dry nitrogen flowing at 50 mLmin-1. 2.3. Single crystal X-ray diffraction. X-ray diffraction on suitable single crystals were collected on an Oxford Xcalibur (Mova) diffractometer equipped with an EOS CCD detector and a microfocus sealed tube using MoK α radiation (λ = 0.71073 Å). Data collection and reduction


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were performed using CrysAlisPro (version 1.171.36.32).28 OLEX2 (version 1.2)29 was used to solve and refine the crystal structures. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on O and N were located from difference electron density maps whereas all CH hydrogen atoms were fixed geometrically using HFIX command. The WinGX package30 was used for refinement and production of crystallographic tables. 2.4. Powder X-ray Diffraction (PXRD). PXRD were recorded on PANalytical X'Pert diffractometer using CuK α X-radiation (λ = 1.54056 Å) at 40 kV and 30 mA. X'Pert High Score Plus (version 1.0d)31 was used to collect and plot the diffraction patterns over 2θ range of 5-40° using a step size of 0.02° and time per step of 1 sec. Profile fitting refinements were carried out to confirm the phase purity before performing impedance measurements. Profile refinements (LeBail fit) were carried out using the crystallographic package JANA2006. Profile parameters such as GU, GV, GW, LX and LY are refined using Pseudo-Voigt function, in such a way that the profile fits best with the experimentally observed PXRD pattern. The values of cell parameters, (a, b, c) and symmetry unrestricted angles (e.g. β angle in case of monoclinic system) of the known phase are given as input which are further refined. This technique has been employed to examine the powder phases, as it is a superior way to check the phase purity than a simple peak matching. Any impurity, if present, will appear as a difference peak (Y obs .-Y calc. ) indicative of the presence of a different phase. The following representations are used in the powder X-ray diffractogram: crosses: observed pattern, red line: fitted profile, black line: difference between observed and calculated profile (Y obs. -Y calc. ), tick marks: reflection position. Mercury program was used to prepare packing diagrams. 2.5. Conductivity measurement. To measure the conductivity properties, sample was prepared by mechanochemistry32-33 in which GA and INZ (1:1 molar stoichiometry) were ground


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using 1M aqueous solution of H 2 SO 4 in a mortar and pestle for 30 minutes. The ground mixtures were analyzed by PXRD followed by profile fitting refinements to ensure the purity of the samples. The powder samples were then compacted (12.5 kN/mm) to form pellets (10 mm in diameter and 1.3 mm in thickness). These were used instead of a single crystal sample (ideally suggested to evaluate the anisotropy) since growing large sized crystals were unsuccessful. Both surfaces of the pellet were coated with gold paste electrodes. The pellet was then inserted between two home-built stainless-steel electrodes and exposed to relative humidity and different temperatures. The real and imaginary parts of the electrical impedance were measured using Alpha-A high performance frequency analyzer (Novo control) combined with Julabo cryostat. The measurements were performed in the temperature range 298-318K by varying frequency from 1Hz to 10MHz at the oscillation voltage of 0.05V. The impedance measurements allow the estimation of proton conductivity from the intercept of the low frequency spike on the real axis. The equivalent circuit are used for the analysis of the impedance plots recorded at ambient condition is shown along with the corresponding Nyquist plot. The fitting of the data by using equivalent circuit allows the evaluation of resistance of the sample. The fitting was performed by using the ZViewTM [Scribner Associates Inc.] software and conductivity was found out by using σ =

1 L equation, where L and A are the × R A

thickness (cm) and cross-sectional area (cm2) of the pellet respectively, and R is the bulk resistivity of the sample (Ω). 2.6. Dielectric spectroscopy. Dielectric measurements were carried out using pelletized sample by Novocontrol (Alpha AN) impedance analyzer in the frequency range [100 Hz to 10 MHz (signal amplitude = 0.05 V] from 300-420 K. Prior to measurement, gold was sputtered on both sides of the pellet for better ohmic contact. Thin silver wires were soldered on both sides of


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the pellet. Diameter and thickness of the pellet were approximately 10 mm and 1-1.5 mm, respectively 2.7. Raman spectroscopy. The temperature evolution of the Raman spectra of GINZH was monitored in the 180° backscattering geometry, using a 532 nm excitation from a diode pumped frequency doubled Nd:YAG solid state laser (model GDLM-5015 L, PhotopSuwtech Inc., China) and a custom-built Raman spectrometer equipped with a SPEX TRIAX 550 monochromator and a liquid nitrogen cooled CCD (Spectrum One with CCD 3000 controller, ISA JobinYovn). Laser power at the sample was ∼ 8 mW, and a typical spectral acquisition time was 3 min. The spectral resolution chosen was 2 cm-1. The temperature was controlled with an

accuracy of (0.1 K) by using a temperature controller (Linkam TMS 94) equipped with a cooling stage unit (Linkam THMS 600).

3. Results and discussion Thermal analysis of the freshly prepared crystals of GISH using thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) show several interesting features (Figure 1). The analysis is carried out in the temperature range of 303-623K with a heating rate of 5K min-1 in N 2 atmosphere. The DSC plot shows two consecutive endothermic peaks at 383K and 390K whereas the third and fourth endothermic peaks appear at 477K and 562K (Figure 1a). The first two endothermic peaks may correspond to loss of hydration from crystal structure and other two peaks correspond to melting and decomposition of the sample. Further, the experimental weight loss of around 4.7% loss of water is estimated by TGA analysis till 390K. This corresponds approximately to the calculated weight loss of 4.3% loss of three water molecules from crystal structure. The discrepancy of 0.4% between experimental and calculated weight loss can be


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attributed to the surface water adsorbed on the sample. The occurrence of a stable phase after dehydration of the sample indicates the presence of a high temperature phase of the sample.

Figure 1. (a) Thermal analysis of GISH confirms the water loss around 390K. (b) Reversibility associated with water loss and gain also supports the reversible nature of phase transition.


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The final weight loss in the temperature range 474-573K corresponds to the melting of the anhydrous compound. The in situ PXRD measurements (as shown in Figure S1, supporting information) of the compound at high temperature also indicated that the solid retains crystallinity even after 390K hence resulting a stable anhydrous phase. This prompted us to examine the possibility of reversible water adsorption-desorption behavior in the compound. A modified TGA setup containing a port for introducing the water vapor was used to investigate reversibility of water adsorption in crystal. The sample (5mg) was taken in a TGA crucible and heated beyond 390K in an atmosphere of flowing dry nitrogen (50mLmin-1). The samples were subsequently cooled to room temperature, and nitrogen gas bubbling through water was allowed to pass into the system for possible rehydration to an extent of 60 minutes. During this process, we observed that the compound exhibited only partial reabsorption of water molecules. For first cycle, on heating the weight loss accounted is 4.4% and re-gain is 3.27%. In the second cycle weight loss and gain corresponds to 3.24% and 3.04% respectively. Similarly, 3.33% and 2.56% are the weight gain and loss of water molecules in third cycle (Figure 1b). It may be noted that we could not reach the expected weight gain during the rehydration process, which may suggest that the water uptake is somewhat sluggish. To understand the crystal structure features of these phases, Single crystal X-ray diffraction (hereafter, SCXRD) measurements were performed. 3.1. Room temperature Phase (RTP). The asymmetric unit of GISH consists of three molecules each of GA, protonated INZ, SO 4 2- ions along with lattice water and crystallizes in monoclinic system with space group P2 1 /n (Z´=3). It is to be noted that during crystallization process, sulfuric acid transfers two of its protons, one to the pyridine nitrogen and other to the hydrazide group of INZ forming SO 4 2- ion in the crystal structure. The conformations of


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hydroxyls in GA have a cis-cis-trans geometry. It is observed that two of three SO 4 2- moieties and one of three water molecules in the asymmetric unit display disorder. Extensive hydrogen bonding dominated by O-H∙∙∙O and N+-H···O- contacts generate sheet like structures (Figure 2a). A closer examination of the sheets reveals that two of three asymmetric unit contents form a Layer (Layer A; Figures 2b and 2c) built from octameric units shown as highlighted while the third asymmetric unit contents form similar octameric interactions leading to the formation of Layer B with its symmetry related counterpart. These layers are arranged as …A-A-B-A-A-B-AA… fashion to generate a three-dimensional motif utilizing well defined π⋅⋅⋅π interactions (Figures 2a and 2c). 3.2. High temperature Phase (HTP). As suggested by thermal and PXRD experiments (Figure 1 and S1 respectively), a single crystal was heated in situ on the single crystal diffractometer and the cell parameters abruptly changed at 385K which may indicate the loss of water molecules from the crystal structure. The single crystal data were collected at 385K and examination of HTP phase shows the crystal structure of anhydrous phase with 1:1:1 gallic acid: isoniazid and SO 4 2- in the asymmetric unit. All the molecules of asymmetric unit are heavily disordered as shown in figure 3 and hydrogen atoms could not be assigned to the crystal structure because of the same reason. The disordered SO 4 2- moiety acts as a rotor and interacts with both gallic acid and isoniazid molecules. The disordered moieties of gallic acid and isoniazid further forms a sheet structure as shown in Figure 3. There are π-π stacks between the acid and base molecules which results further stabilization of the crystal structure.


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Figure 2. (a) The packing of GISH at RT showing layer arrangements by π-π stacks which stabilizes the crystal structure, (b) the sheet like structure where disordered octameric unit is shown in pink circle, (c) cartoon depiction of layering structure where maroon layer represents A and yellow represents B layer which results in a packing of …A-A-B-A-A-B-…


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Figure 3. High temperature phase of GISH showing disorder related to all molecules in crystal structure. 3.2. Low temperature Phase (LTP). Although, we could not capture any low temperature phase transition in thermal analysis of GISH, we decided to perform low temperature SCXRD measurements to resolve the room temperature disorder of sulfate ions. To our surprise, GISH showed abrupt change in cell parameters at 145K indicating a new phase. The room temperature phase undergoes an interesting reversible phase transition around 145K retaining the same space group P2 1 /n with Z´=4 resulting the changes of c-axis of the crystal from 23.357(2) Å to 30.653(1) Å (as shown in Table S1, supporting information). The reversible nature has been established by in situ measurements using the same crystal realizing a single crystal-single crystal (SCSC) phase transition. The crystal structure at 100K retains the hydrogen bonding pattern similar to RT phase containing octameric packing motifs between two independent pairs of GA: protonated INZ: SO 4 2-: H 2 O (Figure 4). However, the layers now are packed as …-A-AB-B-A-A-B-B-… with the layers constituted by molecules in the asymmetric unit [two asymmetric units in layer A and the other two units in layer B respectively; Figure 4(c)]. Figure 4c also describes the packing motifs generated by well defined π-π interactions.


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Figure 4. LT phase of GISH shows the sheet like structure (a) and ordered octameric unit is shown in pink circle. (b) Cartoon depiction of layering structure in LT phase where maroon layer represents A and yellow represents B layer and results in a packing of …A-A-B-B-A-A-… (c) The arrangement of these layers by π-π stacks stabilizes the crystal structure.


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Figure 5. Temperature dependence of the unit-cell parameters and volume/Z in temperature range of 100-300K, where a_R, b_R, c_R and β_R, Vol/Z_R corresponds to reversible cell parameters while heating in situ from 100-300 K.


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3.4. Thermal evolution of lattice parameters. Variable temperature in situ single X-ray crystal diffraction measurements were carried out to investigate the structural evolution, the nature of phase transitions and to establish the exact temperature of the phase transitions. The changes in the cell dimensions as a function of temperature in the range of 100-300K are shown in Figure 5. Anomalous behavior of cell parameters is displayed around the transition temperature (T c = 145K). The b cell parameter remains relatively constant, whereas a and c cell parameters depict abrupt change at the transition temperature. The change at T c is more pronounced (Figure 5a) for the ‘c’ axis [23.357(2) Å at 300K to 30.736(2)Å at 145K] while ‘a’ increases marginally [12.966(2)Å at 300K and 13.324(4)Å at 145K]. The value of angle β (Figure 5b) also changes from 102.654(8) ° at 300K to 109.434(5) ° at 145K. The volume variation with temperature plotted as Vol/Z also shows an abrupt change (Figure 5c). It is noteworthy that the transition is reversible on heating the crystal from 100K to RT, where T c on heating cycle is observed to be 170K indicating a hysteresis of about 35K, exhibiting a characteristic of first order transition. Similarly, all cell parameters display an abrupt change at 385K on heating from the room temperature (as shown in Figure S2, supporting information). The a and b cell parameter remains relatively constant with increasing temperature, while the change at T c is more pronounced for c in which c decreases from 23.357(2) Å at 300K to 7.516(2) Å at 385K. The β angle also undergoes a significant change from 102.654(8) ° at 300K to 106.33(17) ° at 385 K. The volume variation plotted as Vol/Z from 300K to 400K and a molar volume discontinuity at 385K confirms the first order nature of the phase transition as shown in Figure S2, supporting information. The expansion/contraction coefficients along the three crystallographic axes with temperature can be characterized by distortion of a crystal structure.34 The linear thermal


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expansion/contraction coefficient along a particular crystallographic axis ‘a’ is defined as 𝛼𝛼𝑎𝑎 =

1

𝑎𝑎293𝐾𝐾

µ𝑎𝑎 ; Where µa =

δa δT

and a = µ0 T + a0 . µa is the slope of the ‘a’ vs T plot which

could be calculated from the measured changes in lattice parameters with temperature and a 0 is

the unit-cell parameter value at 0 K. a 0 of a given crystallographic phase could be calculated from ‘a’ vs T plot by extrapolating the cell parameter to 0 K.34 The principal linear thermal expansion/contraction coefficients along different crystallographic axes calculated for the 100145 K (Low temperature phase; LTP) and 150-380 K (Intermediate temperature phase; ITP) phases are summarized in Table 1. These values reveal that the linear thermal expansion/contraction, along a axis (αa ) increases from 10.09 to 11.93×10-5 K-1 when the crystal undergoes the phase transition on heating, whereas αb decreases from 1.1 to -1.54 ×10-5K-1. Similarly, (α c ) decreases from 9.5 to -2.7 ×10-5K-1 in both LTP and ITP phases. Table1. Principal linear thermal expansion/contraction coefficient Unit cell parameter

Coefficient

100-145K

145-380K

a

a 0 (Å)

13.0998

12.5496

µ a ×10-4 (ÅK-1)

14.8

14.4

α a ×10-5 (K-1)

10.9

11.93

b 0 (Å)

17.2393

17.4458

µ b ×10-4 (ÅK-1)

1.9

-2.68

α b ×10-5 (K-1)

1.1

-1.54

c 0 (Å)

30.3109

23.499

µ c ×10-4 (ÅK-1)

29.7

-6.22

α c ×10-5 (K-1)

9.5

-2.7

b

c


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Figure 6. The three unit cells showing the phase transition at low and high temperatures following the nature of order-disorder phase transition. 3.5. Mechanism of Phase transition. It is essential to understand the structural phase transitions to correlate structure-property relationship. As discussed above, room temperature (RT) phase with Z´=3 on cooling to 145K switches to the low temperature (LT) phase with Z´=4 in the asymmetric unit. It is of interest to note that two SO 4 2- moieties and one water molecule are disordered at room temperature. On cooling the crystal, the disorder associated with the SO 4 2- moieties and water molecule at RT is resolved with Z´=4 at LT represented by a SCSC transition. Similarly, the high temperature phase transition shows a change to Z´=1 at 385K with the loss of three water molecules from the crystal lattice. It is of interest to note that the crystal remains intact even after the loss of water molecules. The gallic acid, isoniazid and SO 4 2-


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moieties depict the SO 4 2- as a rotor with the ring atoms of both acid and base molecules displaying disorder. It is remarkable that on cooling the crystal from HT phase to RT in situ on the diffractometer, uptake of water molecules from the surroundings (humidity driven) is observed. It is noteworthy that depending on the humidity of the surroundings, it takes more than 10 hours for the sample to restore the RT phase (Figure S2, supporting information). On the other hand, in a separate experiment with a new crystal, the sample when subjected to rapid cooling (liquid nitrogen flow) converts form HT phase to RT phase instantly. Figure 6 indicates the relationship between the three crystal forms proceeding from the high temperature anhydrous disordered structure to fully ordered hydrated structure at 100K. 3.6. Proton Conductivity measurements and mechanism. The detailed understanding of the three phases and presence of moieties like SO 4 2-, H 2 O and NH 3 + in the crystal structure prompted us to investigate the possibility of proton migration in GISH. These measurements were performed on powder pressed pellets. Powder X-ray diffraction followed by profile fitting refinements were carried out to confirm the phase purity of RT phase before performing impedance measurements (Figure S3, Supporting Information). We observed a proton conductivity value of 1.36×10-8 S cm-1 at ambient conditions and at 298K (Figure 7). It is to be noted that the reversibility associated with the sample in terms of dehydration-rehydration may affect the proton conductivity values. To address this phenomenon, the pellet was heated (externally in the oven) to 403K for 24 hours to form the anhydrous phase. The heated sample was used for impedance analysis and it is observed that proton conductivity value is reduced by 2 orders of magnitude. (3.02×10-10 S cm-1; Figure S4; supporting information). The pellet was kept at 98% relative humidity in a home built humidity chamber and in situ conductivity


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measurements were performed after an hour resulting in a value of 1.42×10-8 Scm-1, indicating that the phase has reverted to the RT phase (Figure S5; supporting information).

Figure 7. Nyquist plot for proton conduction in GISH at RT and under ambient humidity. Further, an interesting feature is observed when the sample was placed in 98% RH over a long period of time in the in situ chamber. The sample exhibits a remarkably high proton conductivity value of ∼10-4 Scm-1 after 24 hours at 298K (Figure S6; supporting information). The value of the conductivity remains almost the same order when heated to 308K at 98% RH (Figure 8a).

The impedance plot of the sample shows two semi-circles (Figure S7-S9, supporting information) except for 98% RH conditions suggesting that the contributions to the net proton conductivity are from both grain and grain boundaries.35 The calculated activation energies [E a (g) and E a (gb)] given in Table 2 for various RH values support the above observation. The range of E a (0.1-0.5 eV) corresponds to a Grotthuss type mechanism for proton conduction.36 Table 2 summaries the results of proton conductivities and activation energies corresponding to different


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relative humidity observed in this system where E a (g) and E a (gb) are the activation energies corresponding to grain and grain boundaries.

Figure 8. Temperature dependent (a) Nyquist plots of GISH at 98% RH (b) plot showing activation energy.


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3.7. Proposed mechanism of proton conduction. A molecular level mechanism for proton conduction has been proposed based on the crystal structure (Figure S10). A similar mechanism was suggested for the ternary system (Gallic acid-Isoniazid-water) and the role of participation of the water molecules is explained in our earlier reported work.11 The present system also shows the possibility of Grotthuss type mechanism of proton conduction supported by the values of the activation energy (E a ) values. NH 3 + ion of the hydrazide group initiates the proton migration on application of electric field. The proton hopping pathway further can be guided by para-hydroxyl group of gallic acid molecule which promotes the proton hopping to SO 4 2- moiety. The disorder associated with SO 4 2- and water molecules is masked for clarity of the pathway in Figure S10. With increasing relative humidity from 75% to 98%, the contribution of the grain boundary vanishes and the conductivity of the system increases drastically by 4 orders. It may be argued that the water uptake of the anhydrous phase with higher RH assists in removing the grain boundaries present in the sample. Further investigations on the effect of relative humidity show some interesting trends. Table 2 (Figures S7-9 in supporting information) lists the experiments conducted in the in situ chamber at different RH values. However experiments done with varying humidity conditions shown in supporting information (Figures S7-9, Table S2) do not clearly bring out the mechanism involved with role of water.


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Figure 9. Temperature dependent dielectric constant (a) in heating cycle and (b) cooling cycle at variable frequencies.


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3.8. Dielectric spectroscopy. The structural changes and order-disorder phase transition induce changes in physical properties like the dielectric constant. Temperature dependent dielectric constant for GISH was measured on powder pressed pellets in heating-cooling cycle modes in the frequency range of 100Hz to 100 KHz. Figure 9(a) shows that the dielectric constant (έ) gradually increases with temperature and around 380K, depicts the expected dielectric anomaly consistent with the phase transition and the peak height decreases with increasing frequency. The behavior of the dielectric constant in the cooling-cycle mode is shown in Figure 9(b). Unlike the heating cycle there is no sharp peak observed in the cooling-cycle mode, however, a step like change at around 353K is observed characteristic of organic cocrystals reported in literature.37-44 The dielectric loss as observed (Figure S11; supporting information) also supports the observed dehydration phenomena with temperature. 3.9. Raman spectroscopy. Temperature dependent Raman spectroscopy studies have been carried out to obtain insights into the dynamics of structural transition. Figure 10 shows Raman active modes in GISH and assignments to each mode is made based on available literature.45 Significant changes can be observed in the internal modes with symmetric and asymmetric S-O stretching (940-1040 cm-1) frequencies. At room temperature, the structure contains three crystallographically independent SO 4 2- units whereas high temperature phase contains only a disordered SO 4 2- ion. This phenomenon is well reflected in the symmetric S-O stretching region which is depicted in temperature evolution of the Raman spectra in the temperature range 300420K (Figure 10). Major changes observed in the spectral features around the phase transition temperature (T c = 388K) is in agreement with the structural phase transition observed by thermal and SCXRD analysis. The disappearance of the Raman modes in the region of 960-980 cm-1 and


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1040-1080 cm-1 indicate lesser number of independent molecules in the asymmetric unit consistent with the observed unit cell in the single crystal data at high temperature.

Figure 10. Temperature dependent Raman spectroscopy showing changes in the region of 9401040 cm-1 corresponding to symmetric and asymmetric S-O stretching. 4. Conclusions This study establishes the possibility of designing co-crystal/salt based systems for applications as potential proton conductors. We have demonstrated a simple quaternary salt consisting of gallic acid, isoniazid, sulfate (SO 4 2-) ion and water molecule in the asymmetric unit would serve as a model system to generate multifunctional materials. While the hydrated salt displays 10-4 Scm-1 value in 98% RH, the anhydrous salt shows reduction in proton conductivity


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values. The detailed crystallographic study is substantiated with both Raman and dielectric studies. Exhaustive study on SCSC transitions across the dehydration-rehydration pathway suggests the systems designed are sufficiently robust and reliable for repeated operations suggesting their utility in devices. Even though the values of conductivity obtained is not significantly different from those reported for MOFs and COFs, the study points out to the use of simple organic salts based proton conductors in the near future. ASSOCIATED CONTENT Supporting Information. Variable temperature in situ powder X-ray diffraction, Temperature dependence of the unit cell parameters and volume in range of 300-400K, Nyquist plots showing experimental proton conductivity and activation energy for the proton conduction at 92% relative humidity, 87% relative humidity, 75% relative humidity for GISH, possible proposed pathway of proton migration, temperature dependent dielectric loss at variable frequencies, crystallographic details of three phases of GISH. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT RPK acknowledges Indian Institute of Science for SRF. DS acknowledges Department of Science and Technology (DST), India for post-doctoral fellowship. We acknowledge Mr. I. S. Jarali for DSC, TGA measurements, and Mr. Raj for helping in preparation of conductivity


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samples. DD thanks "DST Nano-Mission" for postdoctoral fellowship. TNG thanks Department of Science and Technology of India for J.C. Bose fellowship. We thank IISc for all facilities. ABBREVIATIONS GA gallic acid; INZ isoniazid; RTP room temperature phase; LTP low temperature phase; ITP Intermediate temperature phase.

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TOC Graphics.


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Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior

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