Giant {Mo₁₃₂} polyoxometalate isolated with diverse organic cations: a systematic proton conductivity study

Abstract

The development of efficient and stable proton conductors is a pivotal area of research due to their transformative potential in alternative energy technologies. Recently, there has been a surge of interest in synthesizing proton conductors based on polyoxometalate (POM) materials, attributed to their highly negatively charged and oxygen-rich surfaces. In this study, we report on a highly water-soluble giant POM, (NH₄)₄₂[Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄ (designated as {Mo₁₃₂}), which was rendered insoluble in water by exchanging its ammonium cations with larger organic cations, specifically histidinium, pyridinium, bipyridinium, and methyl viologen, resulting in His-Mo₁₃₂, Py-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂, respectively. These ion-exchanged compounds were thoroughly characterized through comprehensive spectral analyses, elemental analyses and microscopic studies. The substitution with organic cations containing nitrogen centres not only rendered {Mo₁₃₂} insoluble, but also increased the number of proton hopping sites, thereby enhancing proton transport. Consequently, His-Mo₁₃₂, Py-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂ demonstrated impressive proton conductivity. Among these, Py-Mo₁₃₂ stood out with a proton conductivity of 1.07 × 10⁻² S cm⁻¹ under 98% relative humidity at 80 °C. All four compounds exhibited proton conduction predominantly via the Grotthuss mechanism. Furthermore, stability assessments of these Mo₁₃₂-based proton conductors were conducted under operational conditions to evaluate their performance in practical applications.

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Introduction

The current global energy demands necessitate a thorough understanding and management of energy resources. With depleting fossil fuels and increasing pollution from a growing population, research on sustainable energy sources is crucial. Some of the cleaner energy devices are fuel cells, which use a redox reaction to convert chemical energy to electrical energy.^1–4 Among different types of fuel cells, proton exchange membrane fuel cells or polymer electrolyte membrane fuel cells (PEMFCs) use a solid electrolyte as a proton conducting membrane.^5–8 Nafion, a synthetic polymer with terminal sulfonic acid groups, is widely used in PEMFCs due to its high proton conductivity under humid conditions.^9,10 However, Nafion's performance degrades at temperatures above 100 °C due to dehydration, breaking proton transport channels and reducing conductivity.^11,12 Additionally, Nafion's high cost drives the need for new proton-conducting materials that are more affordable, effective over a broader temperature range, and highly efficient for use in fuel cells.¹³

Polyoxometalates (POMs),^14–20 metal–organic frameworks (MOFs),^21,22 covalent organic frameworks (COFs),^23–25 graphene oxide,^26,27 and perovskites^28,29 are notable materials that have recently proven effective in developing efficient proton conductors. Each class has unique properties that make them suitable candidates for proton conduction. POMs, composed of metal–oxygen clusters from early transition metals, stand out due to their oxygen-enriched surfaces, Lewis and Brønsted acidity, electron-accepting ability, and customizable synthesis.^14–20 Along with these, their high negative charge also contributes to their efficiency as proton conductors.^30–41 However, POMs’ high solubility in water and low structural stability in solution limit their use in proton exchange membranes (PEMs).^31,32 To address this, strategies such as incorporating POMs with materials like TiO₂, SiO₂, and ZrO₂, and ion exchange with organic cations have been developed.^42–45

Nakamura et al. first conducted a detailed study of the proton conductivity of POMs, specifically dodecamolybdophosphoric acid (H₃PMo₁₂O₄₀·29 H₂O) and dodecatungstophosphoric acid (H₃PW₁₂O₄₀·29·H₂O).³⁰ In recent years, numerous reports on POM-based proton conductors have emerged. Wang et al. reported assembling discrete POM units into a 3-D inorganic framework to enhance proton conductivity and stability.³² Our group has reported two polyoxovanadate-based materials with acidic protons as cations, exhibiting proton conductivity on the order of 10⁻³ S cm⁻¹ under mild conditions (50 °C and 70% RH).³³ Lin et al. described a POM with a high nuclearity hollow dodecahedron cage, showing proton conductivity between 10⁻² and 10⁻¹ S cm⁻¹ over a wide temperature range.³⁴ Recently, Supriya and co-workers reported a sparingly water soluble giant POM, {Mo₇₂Fe₃₀} [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂ {H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O, exhibiting proton conductivity on the order of 10⁻³ S cm⁻¹ under moderate humidity (R.H. 50%) and temperature (<50 °C) conditions.³⁵

The family of giant Keplerate-type polyoxometalates emerged in 1998. {Mo₁₃₂}, bearing the formula (NH₄)₄₂[Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄, is a well-known member of this family.⁴⁶ It has a spherical shape with icosahedral symmetry and a diameter of ∼2.9 nm. This giant {Mo₁₃₂} polyanion consists of 12 {Mo₁₁} fragments with central Mo^VI existing as an Mo^VIO₇ pentagonal bipyramid. This unit is then surrounded by 5 Mo^VIO₆ octahedral groups. Each of these 12 {(Mo)Mo₅} units is connected to each other by 30 Mo^v₂ linkers, stabilized by bidentate acetate ligands, to form a giant sphere. Each {Mo₁₃₂} polyanion carries 42 negative charges, which is balanced by 42 ammonium cations. The ammonium cations in the structure and the numerous oxygen atoms on the POM surface, which can engage in extensive hydrogen bonding, make {Mo₁₃₂} an excellent candidate for study as a proton conductor. However, its high water solubility is a significant challenge. To address this, one can utilize the large hollow cage structure of the {Mo₁₃₂} anion, which can host smaller cations. Using this strategy, Neumann and his team encapsulated first-row transition metals within {Mo₁₃₂} and replaced the NH₄⁺ cations with quaternary ammonium cations, making the system water-insoluble.⁴⁷

Another effective method is the ion exchange of NH₄⁺ cations in {Mo₁₃₂} with organic cations, as demonstrated by Lan and co-workers.⁴⁸ They synthesized a series of insoluble {Mo₁₃₂} systems using imidazolium cation derivatives and a tetrabutylammonium cation, producing HImMo₁₃₂, HMeImMo₁₃₂, ILMo₁₃₂ and TBAMo₁₃₂, which primarily operated via the vehicular mechanism of proton conduction. This work encouraged us to explore the role of diverse organic cations that can be exchanged with the NH₄⁺ cations of parent NH₄⁺-{Mo₁₃₂} to form {Mo₁₃₂}-based proton conductors. The diversity of organic cations, in terms of the pK_a value and the number and type of nitrogen atoms, has provided useful insights into the proton conducting behaviour of these systems.

Keeping this view in mind, in this study, we report the synthesis, characterization, and proton conducting properties of four new water-insoluble {Mo₁₃₂}-based compounds: His-Mo₁₃₂, Py-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, derived from histidine (His), pyridine (Py), bipyridine (Bpy), and its dicationic methylated analogue, methyl viologen (MV²⁺), respectively. A schematic illustration of the preparation of these products is shown in Scheme 1. Here, it is to be noted that pyridine has one N-center, bipyridine and methyl viologen have two, and histidine has three N-centers along with a carboxylic acid group. The choice of these organic molecules offers an interesting series of compounds, providing insights into the role of nitrogen heteroatoms and cations in modifying proton conductivity.

Scheme 1 Schematic representation of the formation of Py-Mo₁₃₂, Bpy-Mo₁₃₂, MV-Mo₁₃₂ and His-Mo₁₃₂.

Experimental section

The synthetic procedure for the ion-exchanged products, Py-Mo₁₃₂, Bpy-Mo₁₃₂, MV-Mo₁₃₂ and His-Mo₁₃₂, was adapted from the work of Lan and co-workers.⁴⁸ Even though it is known that the strong hydration shell of smaller ammonium cations present in the solution prevents other organic cations from directly interacting with the {Mo₁₃₂} polyanion having a low charge density,⁴⁹ the excess concentration of organic cations ultimately results in the exchange of some of the ammonium ions, forming new water insoluble {Mo₁₃₂} complexes. The detailed procedures are provided below. The synthesis procedures of pristine {Mo₁₃₂} and methyl viologen are provided in the ESI (Section S1†).

Synthesis of (NH₄)₁₃(PyH)₂₉[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·77H₂O: (Py-Mo₁₃₂)

0.3 mL of pyridine (3 mmol) was mixed with 3 mL of water, and the pH was adjusted to 4.0 using glacial acetic acid. To this solution, 0.8 g of {Mo₁₃₂} (0.03 mmol) was added, which immediately resulted in the precipitation of the ion exchanged product. A dark brown colored product was filtered after stirring for 5 minutes, washed with water, and dried in air. Yield: 0.67 g (83% based on Mo).

IR (cm⁻¹): 1607(m: δ_CH3), 1536(m: ν_COO⁻), 1487(m: δ_NH4⁺), 1403(br, m), 1329(w), 1249(w), 945(ν(Mo=O)), 771(w), 660(w).

Elemental CHN (carbon, hydrogen and nitrogen) analysis (EA) of C₂₂₅H₆₆₄N₅₂O₅₉₁Mo₁₃₂ (MW – 26 219.73): % calcd (found): C 9.61(9.8), H 2.41(2.6), N 2.30(1.63); inductively coupled plasma (ICP) analysis % calcd (found): Mo 48.30(52). The number of pyridinium ions was calculated to be 29 by ICP-OES analysis and CHN analysis.

Synthesis of (NH₄)₂₀(HisH)₂₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·110H₂O·10CH₃COONH₄: His-Mo₁₃₂

0.232 g of histidine (1.5 mmol) was dissolved in 3 mL of water and the pH of the solution was adjusted to 4.0 using glacial acetic acid. 0.8 g of {Mo₁₃₂} (0.03 mmol) was then added to the solution and stirred for 5 minutes. A dark brown colored product was filtered, washed with water, and dried in air. Yield: 0.69 g (88% based on Mo).

IR (cm⁻¹): 1614(w: δ_CH3), 1533(w: ν_COO⁻), 1403(m: ν_NH4⁺), 1326(w), 970(ν(Mo=O)), 787(vs), 697(vs).

Elemental CHN (carbon, hydrogen and nitrogen) analysis (EA) of C₂₁₂H₈₀₂N₉₆O₆₇₈Mo₁₃₂ (MW – 28 210.92): % calcd (found): C 9.03(10.08), H 2.87(2.69), N 4.77(4.42); ICP analysis % calcd (found): 44.89(47.5). The number of histidinium ions was calculated to be 22 from ICP-OES analysis and CHN analysis.

Synthesis of (NH₄)₁₈(BpyH)₂₄[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·128H₂O·10CH₃COONH₄: Bpy-Mo₁₃₂

0.234 g of bipyridine (1.5 mmol) was dissolved in 3 mL of water and the pH of the solution was adjusted to 4.0 using glacial acetic acid. To this solution, 0.8 g of {Mo₁₃₂} (0.03 mmol) was added and stirred. A dark brown precipitate formed immediately, which was then filtered, washed with water and dried in air. Yield: 0.71 g (88% based on Mo).

IR (cm⁻¹): 1614(m: δ_CH3), 1539(m: ν_COO⁻), 1484(w), 1410(m: ν_NH4⁺), 1205(w), 948(w): (ν(Mo=O)), 846(w), 784(vs), 697(vs).

Elemental CHN (carbon, hydrogen and nitrogen) analysis (EA) of C₃₂₀N₇₆H₈₄₈O₆₅₂Mo₁₃₂ (MW – 28 858.32): % calcd (found): C 13.32(13.79), H 2.96(2.63), N 3.69(3.02); ICP analysis % calcd (found) 43.8(47). The number of bipyridinium ions was calculated to be 24 from ICP-OES analysis and CHN analysis.

Synthesis of (NH₄)₁₄(MV)₁₄[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·94H₂O: MV-Mo₁₃₂

0.330 g of methyl viologen (0.75 mmol) was dissolved in 3 mL of water. The pH of this solution was already at 3.0, so it did not require acidification. To this solution, 0.8 g of {Mo₁₃₂} (0.03 mmol) was added and stirred for 5 minutes. A dark brown product was then filtered, washed with water and dried in air. Yield: 0.64 g (85% based on Mo).

IR (cm⁻¹): 1645(m: δ_CH3), 1546(w: ν_COO⁻), 1428(m: ν_NH4⁺), 1329(w), 1267(w), 1186(s), 948(w): (ν(Mo=O)), 901(w), 787(m), 703(m)

Elemental CHN (carbon, hydrogen and nitrogen) analysis (EA) of C₂₄₈H₇₂₄N₅₂O₆₀₈Mo₁₃₂ (MW – 26 828.44): % calcd (found); C 10.44(11.01), H 2.59(2.86), N 2.24(1.7); ICP-OES analysis % calcd (found): 52.4(47.2). The number of methyl viologen ions was calculated to be 14 from ICP-OES analysis and CHN analysis.

Physical characterization studies

The products were characterized using different analytical techniques such as IR spectroscopy, powder X-ray diffraction (PXRD), Raman spectroscopy, thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), UV-visible spectroscopy, elemental analyses [inductively coupled plasma optical emission spectrometry (ICP-OES) and carbon, hydrogen and nitrogen (CHN) elemental analysis], dynamic vapor sorption (DVS) and X-ray photoelectron spectroscopy (XPS). PXRD was performed using a PANalytical X'Pert³ powder instrument, and data were analyzed using Bruker DIFFRAC plus EVA software. Raman spectra were obtained with the help of a HORIBA LabRAM HR800 instrument using a 633 nm argon ion laser. IR spectroscopy was performed using an iD7 ATR Thermo Fisher Scientific Nicolet iS5 instrument. TGA was performed using a PerkinElmer Simultaneous Thermal Analyzer STA-6000. The samples were heated from 30 °C to 800 °C at 10 °C per minute. FESEM imaging was done using a Carl Zeiss model Ultra 55 microscope. Before imaging, all the samples were gold-coated. Diffuse reflectance UV-visible spectroscopy was carried out using a Shimadzu 2600 spectrophotometer. CHN analysis was conducted using a Thermo Scientific FLASH 2000 HT analyzer and ICP-OES analysis was performed using a PerkinElmer Optima 5300 DV ICP-OES. DVS analysis was conducted using a TA Q5000 SA instrument under a N₂ atmosphere at a temperature of 25 °C. Methyl viologen synthesized was analyzed by IR spectroscopy and liquid chromatography-mass spectrometry (LC-MS). LC-MS analysis was performed using a Shimadzu LCMS-8040 instrument.

Electrochemical impedance spectroscopy (EIS)

The proton conductivity of all the samples was studied using electrochemical impedance spectroscopy. Prior to the measurement, the samples were powdered well, and 400 mg of each powdered sample was made into a pellet by placing it between two carbon papers in a pellet maker and applying 3 tons of pressure for 2 minutes. The pellet's thickness was measured using a screw gauge, and the contact surface area was also calculated.

The pellet was then placed between two stainless-steel electrodes, ensuring contact with the carbon papers. For temperature-dependent studies, the setup was placed in a 5 L sealed flask with circulating hot air and distilled water at the bottom. The temperature inside the flask was monitored with a digital thermometer. Impedance spectra were recorded at ≥95% relative humidity and temperatures ranging from 35 °C to 80 °C. Humidity-dependent impedance measurements were conducted in a programmable humidity chamber, keeping the temperature constant while varying the humidity.

Results and discussion

The synthesized products were characterized using various spectroscopic techniques and the data obtained were compared with those of parent {Mo₁₃₂}, as and when required. To estimate the number of remaining NH₄⁺ cations and exchanged organic cations for each of the products, CHN and ICP analyses were performed. All the products were then studied for proton conductivity. The mechanism of proton transfer and the stability of the materials under the operational conditions were evaluated.

The IR spectra of all the products, Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂, and parent {Mo₁₃₂}, are shown in Fig. 1a. For all the samples, two clusters of peaks can be observed: one in the 1650–1350 cm⁻¹ range and the other in the 970–600 cm⁻¹ range. The characteristic peaks of {Mo₁₃₂}, originating from the asymmetric stretching of Mo–O–Mo bridges and the asymmetric stretching of terminal Mo=O bonds, can be observed at ∼784 cm⁻¹ and ∼950 cm⁻¹, respectively, for all the samples.^46,47 The stretching of –COO⁻ and NH₄⁺ groups is represented by the relatively weak bands at 1542 cm⁻¹ and 1401 cm⁻¹, respectively, in all the spectra. This shows that the structure of the {Mo₁₃₂} anion remains intact in all the cation exchanged products. Now, since all the organic cations chosen are aromatic in nature with C=C bonds, its presence can be confirmed by the peak at 1625 cm⁻¹. The broad band in the range of 3100–3600 cm⁻¹ (see ESI, Section S2.6†) represents the O–H stretch along with the aromatic C–H bonds. In addition to it, the presence of the characteristic peaks at ∼1480 cm⁻¹ and ∼1250 cm⁻¹, originating from the N–H bending modes of aromatic secondary amine and C–N bonds, further confirms the presence of organic cations in the products.

Fig. 1 (a) Infrared spectra and (b) Raman spectra of Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, MV-Mo₁₃₂ and pristine {Mo₁₃₂}.

The Raman spectra of the four products, Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, MV-Mo₁₃₂, and {Mo₁₃₂}, are shown in Fig. 1b. For {Mo₁₃₂}, the two most prominent peaks appear at ∼820 cm⁻¹ and ∼993 cm⁻¹. The peak at ∼820 cm⁻¹ originates from the symmetric breathing of oxygen atoms connecting the {(Mo)Mo₅} units with Mo^V₂ units in the {Mo₁₃₂} ball.^50,51 A stretching of the terminal oxygen atoms (Mo=O) in the {Mo₁₃₂} ball generates the peak at ∼993 cm⁻¹ in the spectrum. The peak at ∼650 cm⁻¹ with very low intensity is due to the scissoring motion of bridging oxygen atoms. Another peak of medium intensity is observed at ∼345 cm⁻¹, originating from the bending motion of pentagon non-terminal oxygen atoms. In the Raman spectra of Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, the positions and intensities of the peaks corresponding to the {Mo₁₃₂} ball are similar to those in pristine {Mo₁₃₂}, which indicates the preservation of the {Mo₁₃₂} ball (icosahedral cluster) in all four products.

The powder X-ray diffraction (PXRD) pattern of the synthesized {Mo₁₃₂} matches well with the simulated pattern (Fig. S3, ESI†). The PXRD patterns of the four ion-exchanged products, Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, provided in Fig. 2, show that the products are crystalline in nature. A careful analysis of these reveals that the patterns for ion-exchanged products are similar to that of pristine {Mo₁₃₂} but not identical to it in the sense that there are differences in the relative intensity of the peaks. The exchange of the NH₄⁺ ions in pristine {Mo₁₃₂} with the larger organic cations, probably and quite expectantly, caused this discrepancy compared to pristine {Mo₁₃₂}. The overall similarity of the PXRD patterns of the synthesized ion-exchanged compounds with that of parent {Mo₁₃₂} indicates that the title compounds crystallize in the same crystal system as that of pristine {Mo₁₃₂}. Indeed, we were successful in obtaining the unit cell parameters of the single crystals for two such compounds, Py-Mo₁₃₂ and His-Mo₁₃₂, both belonging to the cubic crystal system, which is the crystal system of the parent {Mo₁₃₂} molecule, the details of which are provided in the ESI (Section S2.3†). This again suggests that {Mo₁₃₂} retains its structure, i.e., the cubic crystal system, in the ion-exchanged products in the present study.

Fig. 2 Powder X-ray diffraction (PXRD) patterns for (a) Py-Mo₁₃₂, (b) His-Mo₁₃₂, (c) MV-Mo₁₃₂ and (d) Bpy-Mo₁₃₂ in comparison with that of pristine {Mo₁₃₂}.

To further verify whether the ion-exchanged products are indeed new crystalline ion-exchanged materials and not a physical mixture of the starting materials, we did a comparative analysis of these products with their respective starting organic molecules and pristine {Mo₁₃₂} using PXRD and FT-IR spectroscopy. The obtained results have been provided in the ESI (Sections S2.4 and S2.6†). From Fig. S4 (ESI†), it can be observed that the PXRD patterns of His-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂ do not contain any characteristic peak from their respective parent organic molecules, indicating that the synthesized products are not physical mixtures.

However, here it is interesting to find that the IR spectra of all the ion-exchanged products (Fig. S7, ESI†) show the characteristic features of both {Mo₁₃₂} and the parent organic molecules, along with the generation of several new peaks. From the results obtained for these experiments, we can safely conclude that the products Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂ are not physical mixtures of their starting materials, but indeed new crystalline ion-exchanged materials.

The UV-visible absorption spectra of parent {Mo₁₃₂} and the ion-exchanged products were recorded, and no notable changes were observed in the higher wavelength region of 450 nm–750 nm, which corresponds to the Mo^V d–d transition and Mo^V–Mo^VI inter-valence charge transfer transition (Section S2.7, ESI†).⁵²

To determine the oxidation state of Mo in the {Mo₁₃₂} cluster of the ion-exchanged products, X-ray photoelectron spectroscopy (XPS) was performed (Fig. 3 and Section S2.8, ESI†). The XPS plots for all the ion-exchanged products display the characteristic Mo 3d doublet consisting of 3d_5/2 and 3d_3/2 peaks. The spectra were deconvoluted using Lorentzian–Gaussian functions, enabling the identification of two sets of doublets corresponding to Mo^V and Mo^VI centers. Moreover, a quantitative analysis was conducted for determining the ratio between the peak areas corresponding to Mo^V and Mo^VI, and for all the products, the ratio was found to be approximately 1.2 : 1.⁵³ This proves that there are 72 Mo^VI and 60 Mo^V centers in the {Mo₁₃₂} cluster for all four ion-exchanged products, further substantiating the structural integrity of {Mo₁₃₂}.

Fig. 3 XPS plot of Py-Mo₁₃₂.

The thermal stability of all four products and {Mo₁₃₂} was analyzed using thermogravimetry. Weight loss corresponding to water, both lattice and coordinated, can be observed in the range of 80–180 °C, for all the compounds (Fig. S12, ESI†). Beyond 220 °C and up until 380 °C, weight loss corresponding to the loss of organic cations and the degradation of the {Mo₁₃₂} ball can be observed for pristine {Mo₁₃₂} and Py-Mo₁₃₂. However, in the case of MV-Mo₁₃₂ and Bpy-Mo₁₃₂, the degradation of the systems completes at ∼450 °C, while for His-Mo₁₃₂, it completes at a much higher temperature of 550 °C.

The morphology of all the samples, Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, was studied by field emission scanning electron microscopy (FESEM), as shown in Fig. 4. The FESEM image of the synthesized {Mo₁₃₂} is provided in Fig. S5 (ESI†). As can be seen from the images, the products are micro-crystalline in nature with crystallite sizes ranging from 0.2 μm for Py-Mo₁₃₂ to 4 μm for Bpy-Mo₁₃₂.

Fig. 4 FESEM images of Py-Mo₁₃₂ (a and b), MV-Mo₁₃₂ (c), His-Mo₁₃₂ (d and e), and Bpy-Mo₁₃₂ (f).

Now, to determine the number of NH₄⁺ ions that are partially exchanged with their respective organic cations to form Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, ICP and CHN analyses were performed. According to the estimation, for every {Mo₁₃₂} anion in His-Mo₁₃₂, there are approximately 22 molecules of histidine (detailed data have been provided in the ESI, Sections S2.10 and S2.11†). Similarly, for Py-Mo₁₃₂, MV-Mo₁₃₂ and Bpy-Mo₁₃₂, there are approximately 29 molecules of pyridine, 14 molecules of methyl viologen, and 24 molecules of bipyridine, respectively, per formula unit. Here, it is to be noted that even though pyridine, bipyridine and histidine have one, two and three N-heteroatoms in the structure, respectively, their numbers being exchanged with the ammonium cations in {Mo₁₃₂} are quite similar. While in the case of MV-Mo₁₃₂, the number of ions exchanged (14) is considerably lower than the rest of the ion exchanged products. Now, to understand this observation, we need to consider the pK_a values of the N-centers for all the organic molecules employed. The pK_a value of pyridine is 5.2, which means that at pH = 4.0, where the ion exchange reaction was performed, more than half of the pyridine concentration had been protonated. For bipyridine, the pK_a values are 2.5 and 4.9, which means that the concentration of mono-protonated species was much larger than that of di-protonated species at the pH of synthesis (that is, pH 4.0). In histidine, the carboxyl group has a pK_a value of 1.82, the amine group has a pK_a value of 9.3, and N-atom on the imidazole side chain has a pK_a value of 6. Thus, at pH 4, almost all the carboxyl groups and amine groups would have remained as –COO⁻ and –NH₃⁺, respectively, while most of the imidazole side chain would have been protonated. Due to such responses to pH, the numbers of exchanged cations for pyridinium, bipyridinium, and histidinium are closer to each other. However, for methyl viologen, which inherently exists in its dicationic form, almost half as many NH₄⁺ cations were exchanged as in Py-Mo₁₃₂.

Proton conductivity studies

The samples Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂ were studied for their proton conducting properties, using electrochemical impedance spectroscopy. The experiments were conducted using a two-electrode setup, under varying temperature and humidity conditions. The conductivity values were calculated by fitting the obtained Nyquist plots to the most suitable equivalent electrical circuit. A detailed description of the experiment methodology, along with the calculations, Nyquist plots, and the equivalent electrical circuit, are provided in the ESI (Section S2.12†). In brief, each sample was thoroughly ground into fine powder and then sandwiched between two carbon wafers to compress all of them into a pellet, under high pressure.⁵⁴

The study of humidity-dependent proton conductivity of the samples was carried out in a humidity chamber, at a constant temperature of 45 °C, with relative humidity (RH) varying from 98% to 70%. The temperature-dependent proton conductivity studies were carried out at 98% RH by varying the temperature from 35 °C to 80 °C. Fig. 5a shows the Nyquist plot for Py-Mo₁₃₂ at various temperatures and 98% relative humidity (RH). Nyquist plots for the other three compounds, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, are provided in the ESI (Section S2.12†). The Nyquist plots for all the samples consist of an incomplete semicircle in the high frequency region, followed by a straight line in the lower frequency region, as shown in Fig. 5 and in Section S2.12 (ESI†).

Fig. 5 Nyquist plots for Py-Mo₁₃₂: (a) at 98% relative humidity (RH) and at different temperatures; (b) at 45 °C and at different RH values. The solid lines represent the fitted data, and the points denote the experimental data.

The proton conductivity value for each sample was calculated from the resistance value obtained in the high frequency region of the respective Nyquist plots, which corresponds to the intrinsic resistance of that sample.^54,55Py-Mo₁₃₂ shows a conductivity of 3 × 10⁻³ S cm⁻¹ at 35 °C, which increases to 1.07 × 10⁻² S cm⁻¹ at 80 °C and 98% RH. Similarly, the proton conductivity of the other samples also increases with an increase in temperature (complete data have been tabulated in the ESI; Tables S3, S5, S7 and S9†). His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂ show their highest conductivities of 6.7 × 10⁻³ S cm⁻¹ (at 60 °C, 98% RH), 2 × 10⁻³ S cm⁻¹ (at 80 °C, 98% RH), and 3.9 × 10⁻³ S cm⁻¹ (at 80° C, 98% RH), respectively. Among these four compounds, Py-Mo₁₃₂ shows the highest proton conductivity (1.07 × 10⁻² S cm⁻¹), followed by His-Mo₁₃₂ (6.6 × 10⁻³ S cm⁻¹), MV-Mo₁₃₂ (3.9 × 10⁻³ S cm⁻¹), and then Bpy-Mo₁₃₂ (2 × 10⁻³ S cm⁻¹). The above-mentioned order is quite unexpected, since histidine having three N-heteroatoms and one carboxyl group was expected to facilitate building extensive H-bonded networks, thereby improving the proton conduction pathway. However, His-Mo₁₃₂ is found to be structurally unstable above 60 °C. This might be because of the presence of the negatively charged carboxylate group on the alkyl side chain of the histidine ion. This negatively charged segment of histidine possibly faces electrostatic repulsion from the giant {Mo₁₃₂} cluster anion, ultimately lowering the structural stability and also the proton conductivity of the His-Mo₁₃₂ product. On the other hand, between Bpy-Mo₁₃₂ and MV-Mo₁₃₂, both of which have a similar structure, MV-Mo₁₃₂ shows ∼2 times higher conductivity than Bpy-Mo₁₃₂, throughout the temperature range of study. This is probably because in Bpy-Mo₁₃₂, bipyridine is mostly singly protonated and there are 24 of them, whereas in MV-Mo₁₃₂, there are 14 dicationic methyl viologens, which means that the organic content is higher in Bpy-Mo₁₃₂ than in MV-Mo₁₃₂. Additionally, the non-protonated N-center in bipyridine probably does not interact with the {Mo₁₃₂} ball and therefore does not contribute to proton conduction. Another possible explanation for the higher conductivity of Py-Mo₁₃₂ compared to the others could be the size of the organic cations. The {Mo₁₃₂} framework, with its cubic crystal system (vide supra), contains channels for proton conduction and a void space of approximately 5 Å. The pyridine molecule, being smaller (about 4.5 Å), likely fits within this void, allowing for closer interaction with {Mo₁₃₂} even from within the internal space (Fig. S27, ESI†). This advantage is not available to the other cations, which are larger than the void space in Mo₁₃₂, limiting their proximity and interaction with the {Mo₁₃₂} structure. Thus, Py-Mo₁₃₂ containing the smallest of all – pyridinium cation – has the optimum interaction with the {Mo₁₃₂} ball, which results in Py-Mo₁₃₂, having the most efficient proton conductivity amongst all.

We have made this assumption because all the synthesized compounds crystallize in the cubic crystal system similar to parent {Mo₁₃₂} (referring to the similar PXRD patterns and unit cell parameters of Py-Mo₁₃₂ and His-Mo₁₃₂). In order to understand the mechanism of proton conduction, Arrhenius plots were constructed for all the samples from the conductivity values at different temperatures. Fig. 6a shows Arrhenius plots for all four compounds. The activation energy for each of them was determined from the slopes of the Arrhenius plots as 0.24 eV, 0.18 eV, 0.27 eV, and 0.29 eV, for Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂, respectively. These values indicate that the proton conduction in all these compounds takes place predominantly through the Grotthuss mechanism. Here, it is noteworthy that these activation energies are much lower compared to previously reported {Mo₁₃₂}-based proton conductors (0.44–0.68 eV), which functioned predominantly through the vehicular mechanism.⁴⁸ A comparison among previously reported giant-POM based proton conductors and the materials reported in this present work has been tabulated in Table S12 (ESI†).

Fig. 6 (a) Arrhenius plots for activation energy and (b) humidity-dependent proton conductivity values for Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂.

The dependence of proton conduction on relative humidity was also studied for all four compounds. Fig. 5b shows the Nyquist plots at 45 °C and at different relative humidity values for Py-Mo₁₃₂. The Nyquist plots and proton conductivity data for the other three compounds are provided in the ESI (Section S2.12†). Here, it is found that the values of proton conductivity increase sequentially with increasing humidity for each one of them (Fig. 6b; see also the ESI†). This indicates a water-assisted proton conduction process for all the samples in the present work. Among the four samples, His-Mo₁₃₂ has the highest rate of increase in proton conduction with respect to RH (relative humidity), while Bpy-Mo₁₃₂ shows the lowest rate of increase in proton conduction with increasing RH.

To gain a better overview of the role of humidity for the different samples, the dynamic vapour sorption (DVS) technique was used (Section S2.14, Fig. S28, ESI†). At 90% relative humidity (RH), His-Mo₁₃₂ shows higher water uptake than Py-Mo₁₃₂. This is consistent with the expected behavior, as the additional heteroatoms and the carboxyl group in histidine contribute to its increased hydrophilicity. In hydrophilic materials, the affinity for water molecules supports the formation of a robust hydrogen-bonding network that facilitates proton transfer through the Grotthuss mechanism. In His-Mo₁₃₂, while elevated temperatures could enhance proton conductivity, this compound becomes unstable above 60 °C. This instability may stem from weaker interactions between histidine and the {Mo₁₃₂} cluster as mentioned earlier, which limits the compound's contribution to proton conduction at higher temperatures. Even though His-Mo₁₃₂ exhibits a higher water uptake capacity, Py-Mo₁₃₂ demonstrates more effective proton conductivity.

For a proton conductor functioning through the Grotthuss mechanism, the most crucial factors, which determine the efficiency of conduction, are the concentration and lability of protons. Therefore, to gain further insight into the observed trend of proton conductivity in the series of compounds synthesized, their acidity was determined. Here, the four compounds Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂ were suspended in water by ultrasonication for 30 minutes and stirred overnight; thereafter, their acidity was measured. The pH observed for Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂, and MV-Mo₁₃₂ is 3.5, 4.8, 5.0, and 4.5, respectively. According to this, proton conductivity of the compounds can be expected to be in the order of Py-Mo₁₃₂ > MV-Mo₁₃₂ > His-Mo₁₃₂ > Bpy-Mo₁₃₂, which matches more or less well with the observed trend (Py-Mo₁₃₂ > His-Mo₁₃₂ > MV-Mo₁₃₂ > Bpy-Mo₁₃₂), except for the fact that His-Mo₁₃₂ shows a higher proton conductivity value than that of MV-Mo₁₃₂, which is the opposite trend predicted by the pH values of the respective compounds’ aqueous suspensions. We can explain this anomaly by considering the lowest activation energy (0.18 eV) of His-Mo₁₃₂ towards its proton conduction compared to the highest activation energy (0.29 eV) of MV-Mo₁₃₂ towards its proton conduction. Upon comparing the trend of activation energy among the products, a direct correlation can be observed with the water sorption results obtained from dynamic vapor sorption (DVS) analysis (Fig. S28, ESI†). The amount of water adsorbed per formula unit of the ion-exchanged product is His-Mo₁₃₂ (137) > Py-Mo₁₃₂ (105) > Bpy-Mo₁₃₂ (92) > MV-Mo₁₃₂ (78). This implies that higher uptake of water molecules lowers the activation energy barrier for proton transport in these samples.

Furthermore, to check the stability of these {Mo₁₃₂}-based proton conductors under the operational conditions, impedance measurements were conducted for all four products, during the subsequent heating–cooling cycling. All four compounds, Py-Mo₁₃₂, His-Mo₁₃₂, MV-Mo₁₃₂, and Bpy-Mo₁₃₂, were stable up to the 3rd cycle (Fig. 7). The Nyquist plots for each cycle are provided in the ESI (Section S2.12, ESI†). Furthermore, the pellets of all the samples after impedance measurements were ground, and the material stability was analyzed by PXRD (Fig. S29, ESI†) and IR spectroscopy (Fig. S30, ESI†). The characterization results obtained after the impedance measurements were in good agreement with the data that was acquired prior to the measurement, which indicates the retention of structural integrity for Py-Mo₁₃₂, His-Mo₁₃₂, and MV-Mo₁₃₂. Additionally, the electrical conductivity of all the products was also analyzed using direct current, since it is important that for a material to be used in proton exchange membranes, it should be electronically non-conducting. The relevant experimental details, the results obtained and the calculations are provided in the ESI (Section S2.16†). For all the products, the conductivity was found to be very low (10⁻⁷–10⁻⁸ S cm⁻¹), which indicates that the conductivity observed for the materials in higher humidity (10⁻³ S cm⁻¹) primarily originates from the conduction of protons assisted by water molecules.

Fig. 7 Proton conductivity during the heating–cooling cycles of (a) Py-Mo₁₃₂, (b) His-Mo₁₃₂, (c) MV-Mo₁₃₂ and (d) Bpy-Mo₁₃₂.

Conclusion

In summary, we have successfully synthesized water-insoluble proton conductors based on the giant POM {Mo₁₃₂} by exchanging its ammonium cations with larger organic cations—pyridinium, histidinium, bipyridinium, and methyl viologen—resulting in Py-Mo₁₃₂, His-Mo₁₃₂, Bpy-Mo₁₃₂ and MV-Mo₁₃₂, respectively. The organic molecules used have varying numbers of N-heteroatoms, and their differing pK_a values affect the extent of protonation. Among these compounds, Py-Mo₁₃₂ exhibits the highest proton conductivity, reaching 1.07 × 10⁻² S cm⁻¹ at 80 °C and 98% relative humidity.

Our findings indicate that by increasing the number of heteroatoms in the cations, the water uptake capacity and activation energy of the products could be improved. However, for obtaining stable proton conductivity, an optimal interaction between the cations and the giant {Mo₁₃₂} anion might play a crucial role. These proton conductors primarily operate via the Grotthuss mechanism and demonstrate stability up to 80 °C. This study underscores the potential of the giant POM {Mo₁₃₂} as a promising candidate for developing proton conductors suitable for fuel cell applications.

Data availability

The data supporting this article, including experimental procedures, instrumentation details, characterization studies (including FT-IR, UV-Vis and XPS spectroscopy, TGA and elemental analysis), detailed proton conductivity studies (including Nyquist plots, equivalent circuit and proton conductivity calculations, and post-impedance material analysis), electrical conductivity study, and comparison of the proton conductivity of previously reported POMs, have been included in the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We thank Chaitanya D and Prof. Ashwini Nangia (School of Chemistry, University of Hyderabad) for helping us with the DVS measurements. We thank Prof. Rajeev Gupta (University of Delhi) and Dr Ujjwal Pal (Principal Scientist, IICT, Hyderabad) for helping us with the CHN analyses. We also thank Ingilala Venkataramanaiah from the School of Physics for helping us with the Raman analysis and the School of Physics for use of the PXRD facilities. We are thankful to IoE-UoH (Project No. RC1-20-007) and SERB, DST, Government of India (Project No. CRG/2022/007283) for the financial assistance. P. M. U. thanks DST-INSPIRE, New Delhi, India, and O. B. thanks UGC, New Delhi, India, for their fellowships and R. N. thanks UGC, Government of India, for the Dr D. S. Kothari Post-doctoral fellowship (F.4-2/2006 (BSR)/CH/18-19/0032). We thank Calicut University, IISER Bhopal, and SAIF-IIT Madras, for providing the instrumental facility for the CHN and ICP-OES elemental analyses. S. K. D. thanks the Science and Engineering Research Board (SERB), DST, Government of India, for the J. C. Bose fellowship (JCB/2023/000007).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt02834a

‡ These authors contributed equally to this work.

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