Sustainable Design for High‐Performance Hybrid Proton Exchange Membranes With a Proton‐Conducting Metal Organic Framework

ABSTRACT

In recent years, composite materials based on MOFs have emerged as a promising second‐generation research direction due to their potential applications in developing high‐performance, durable proton conductors, and proton exchange membranes (PEMs) for fuel cells (FCs). In this context, a novel mixed‐ligand MOF, {[Zn₂(4‐pa)(5‐sip)(H₂O)₃]·(H₂O)₃}_n (1), has been synthesized using Zn(II) as the metal center, 4‐picolylamine (4‐pa) and sodium salt of 5‐sulfoisophthalate (5‐sip) as proton‐conducting ligands. Proton conductivity measurements revealed that compound 1 exhibits a conductivity of 3.44 × 10⁻³ S.cm⁻¹ at 80°C and 100% relative humidity. To further enhance its practical applicability, 1 was incorporated into a cost‐effective and appropriate matrix composed of a saturated polyvinylidene fluoride (PVDF) and Polyvinylpyrrolidone (PVP) mixture, forming a proton exchange mixed matrix membrane (PEMMM) referred to as 1@MMM‐XX. Notably, the 1@MMM‐XX containing 40 wt.% of 1 (as 1@MMM‐40) demonstrated a significantly improved proton conductivity 2.81 × 10⁻² S.cm⁻¹ under the same testing conditions. This represents nearly an 8‐fold and a 16‐fold enhancement over pure compound 1 and the blank PVDF/PVP (1.71 × 10⁻³ S.cm⁻¹) thin film, respectively, at 80°C, which is exceptional. The other prepared PEMMMs, for example, 1@MMM‐10, 1@MMM‐20, and 1@MMM‐30, display the convincing conductivity values 2.04 × 10⁻², 2.27 × 10⁻², and 2.60 × 10⁻² S.cm⁻¹, respectively.

A newly developed proton‐conducting MOF, doped into PVDF/PVP mixed matrix membranes (MMM) shows excellent membrane performance, which is 8‐fold higher over pristine MOF and 16‐fold over the blank PVDF/PVP membrane.

1. Introduction

Nonconventional energy has become an important aspect in today's overgrowing demand of energy in the progressive, technology‐driven, and artificial intelligence‐conquered society [1]. The sources of traditional energy are neither sufficient to serve to cater such high demands, nor it realistically suit well in some operational constraints [2]. The fuel cell is one of such tricky and sustainable solutions in this regard, which can deliver a perfect blend of cost and convenience in the sector of non‐conventional energy. Therefore, researchers across disciplines are continuously exploring the development of suitably designed fuel cells, as these devices are regarded as clean, environmentally friendly, and sustainable energy sources [3, 4]. In this juncture, proton (H⁺) conduction has emerged as a crucial phenomenon as it is an integral part of the fuel cell membrane, consequently, contemporary studies are increasingly focused on designing effective materials that exhibit efficient proton conduction. The proton (H⁺) exhibits exceptionally high ionic mobility (36.2 × 10⁻⁴ cm²s⁻¹V⁻¹), which arises from their transport through the hydrogen‐bond networks with partial covalent character, along with reduced electrostatic interactions compared to other ionic species such as Li⁺ (4.01 × 10⁻⁴ cm²s⁻¹V⁻¹) and Na⁺(5.19 × 10⁻⁴ cm²s⁻¹V⁻¹) [5, 6, 7].

In this context, the crystalline metal‐organic frameworks (MOFs) are found promising as a reliable class of proton conductors [8, 9, 10]. The well‐defined crystalline structures of MOFs play a crucial role in elucidating proton transport mechanisms and clarifying structure–property relationships, thereby accelerating enormous progress in the field [11, 12, 13]. Proton transporters in MOF‐based proton conductors are generally classified into two categories: extrinsic and intrinsic [14]. When protic organic molecules (e.g., imidazole, histamine, triazole, etc.) or inorganic mineral acids (e.g., H₂SO₄, H₃PO₄) are introduced into the pores of MOF as guest molecules via simple impregnation, extrinsic proton conductivity is observed [15, 16]. In the intrinsic one, it originates from the functional groups (─SO₃H, ─PO₃H₂, ─COOH, ─OH, ─NH₂) of the framework backbone of MOF, or from charge‐balancing counterions (H₃O⁺, NH₄ ⁺, H₂PO₄ ⁻) located within the channels of the MOF [17, 18, 19]. Some acidic functional groups (e.g., ─SO₃H, ─PO₃H₂), either coordinated or uncoordinated/free, show very high conductivity due to their greater affinity toward protons in transport [20, 21]. It is their acidity, which facilitates the formation of H‐bonding networks, enabling efficient proton donation and transport, making the materials highly conductive [22, 23].

To enhance the ability and durability of the proton‐conducting membrane, the incorporation of proton‐conducting MOFs into the proton exchange membrane (PEM) is found to be effective for advancing their commercial application in fuel cells [24]. Such hybrid systems are expected to play an important role in lowering environmental impact and reducing operational costs, owing to their high stability, flexibility, efficiency, and simplicity [25, 26, 27]. A wide range of both synthetic and naturally occurring PEMs has been investigated, including Nafion, sulfonated poly (ether ether ketone) [SPEEK], sulfonated poly (aryl ether ketones) [SPAEK], polyimide, chitosan, polyvinylidene fluoride [PVDF], polyvinylpyrrolidone [PVP], etc. The Nafion, a well‐known commercial electrolyte membrane based on a polytetrafluoroethylene (PTFE) backbone, achieves high proton conductivity (σ) values of 10⁻¹ to 10⁻² S.cm^{− 1} at 80°C and 98% relative humidity (RH). However, its application is limited by a narrow operating temperature window and high cost [28, 29]. Alternative membranes such as SPEEKs and polyimides (PIs) show Nafion‐like proton conductivity, but show reduced conductivity when water is lost at high temperatures and due to swelling, when they are highly sulfonated [30]. On the other hand, biodegradable Chitosan‐based membranes suffer from poor mechanical and chemical stability and inherent brittleness [31, 32]. Based on the aforementioned limitations, this study employs PVP and PVDF as key components for designing a new proton exchange membrane (PEM) suitable for fuel cell applications over a wide temperature range. PVP exhibits strong potential for PEMs; however, its standalone use is restricted due to its high‐water solubility. In contrast, PVDF, a low‐cost fluoropolymer, provides excellent chemical resistance, thermal stability, and mechanical strength. Consequently, blending PVP with PVDF results in efficient and structurally stable membranes. Moreover, the incorporation of MOFs into PVDF/PVP membranes further enhances their thermal stability and proton conduction performance under both hydrous and anhydrous high‐temperature conditions, which is critical for practical fuel cell applications [33, 34, 35, 36, 37, 38].

Here, a Zn(II)‐based 1D‐MOF, {[Zn₂(4‐pa)(5‐sip)(H₂O)₃].(H₂O)₃}_n (abbreviated as 1), has been synthesized using 4‐picolylamine (4‐pa), and the sodium salt of 5‐sulfoisophthalate (5‐sip) that contains uncoordinated sulfonate groups for promoting the proton conduction. To enhance proton conductivity, varying amounts of 1 were incorporated into a homogeneous PVDF and PVP solution to fabricate the thin‐film mixed matrix membranes (1@MMM‐XX), where XX is the weight percentage of the corresponding MOF in the said membrane. The synthesis and fabrication process are summarized in Scheme 1. Remarkably, the membrane with 40 wt.% 1 loading (1@MMM‐40) achieved a conductivity of 2.81 × 10⁻² S.cm⁻¹, the highest reported for a PVDF/PVP blend, which is more than double that of similar systems obtained with 40 wt.% MOF doping in a homogeneous PVDF/PVP membrane mixture [39]. The compound and its corresponding matrices were extensively characterized by different physicochemical methods, and the results are consistent with the findings.

SCHEME 1.

Synthesis and fabrication strategy of compound 1 and 1@MMM‐XX (vide infra).

2. Experimental Section

2.1. Materials

Zinc (II) nitrate hexahydrate, 4‐picolylamine, and 5‐Sulfoisophthalic acid sodium salt were purchased from the Merck Sigma–Aldrich. All other reagents, such as polyvinylidene fluoride (with average Mw ∼530 000), polyvinylpyrrolidone (average Mw ∼40 000), and solvents methanol (MeOH), N,N‐dimethylformamide (DMF), were purchased from commercial sources and were used without further purification. The compound Na₃‐5‐SIP was synthesized by gradually adding solid NaHCO₃ to an aqueous solution of 5‐sulfoisophthalic acid, and evaporating the solvent.

2.2. Synthesis of {[Zn₂(4‐pa)(5‐sip)(H₂O)₃].(H₂O)₃}_n (1)

At first, 4‐picolylamine (4‐pa) (1 mmol, 0.108 g) was dissolved in 20 mL of methanol and mixed with an aqueous solution (20 mL) of trisodium salt of 5‐Sulfoisophthalate, i.e., [Na₃(5‐sip)] (1 mmol, 0.312 g). The resulting mixture was stirred for 5 min to mix well. The Zn(NO₃)₂·6H₂O (1 mmol, 0.297 g) was dissolved in 20 mL of water in a separate beaker. After that, 3 mL of metal solution and 6 mL of this mixed ligand solution were mixed in a glass tube with 5 mL of buffer (1:1 MeOH and H₂O) between the ligand and the metal solution. All the additions are made slowly and conscientiously at room temperature [40, 41]. After a fortnight, clear, off‐white, pillar‐shaped single crystals were obtained at the wall of the tube with ∼78% yield. The crystals were collected and washed with a methanol–water (1:1) mixture and dried before use.

2.3. Fabrication of Hybrid Membranes (PVDF/PVP)

The hybrid membrane was fabricated using the solution casting technique. A polymer solution was prepared by dissolving 75 mg of PVDF and 225 mg of PVP (PVDF: PVP = 1:3) in 3 mL of DMF. The mixture was stirred at 60°C for 6 h to obtain a homogeneous gel‐like substance. This solution was then cast into a glass Petridis (5 cm inner diameter) and dried in an oven overnight at 80°C. To get 300 mg flexible thin film, the resulting solidified membrane was removed, thoroughly washed with deionized (DI) water, and subsequently dried again at 80°C for 8 h. This thin film (PVDF/PVP) has been primarily characterized by NMR spectroscopy (Figure S1) and other physicochemical methods.

2.4. Fabrication of 1@MMM‐XX

Using the same solution casting method, 300 mg of hybrid mixed matrix membranes (MMMs) were prepared using 10% compound 1 and 90% PVDF/PVP (PVDF: PVP = 1:3 by weight). Initially, a weighed amount of 202.5 mg of PVP and 67.5 mg of PVDF were dissolved in 3 mL of DMF in a beaker. Thereafter, 30 mg of 1 (in powdered form) was incorporated into the polymer solution, followed by ultrasonication for 1 h and mechanical stirring for an additional 6 h to yield a uniform casting solution (1@MMM‐10). The resulting solution was then transferred into a glass Petri dish (5 cm inner diameter) and kept at 80°C overnight for solvent evaporation. The obtained thin film of 1@MMM‐10 was carefully detached, rinsed with deionized water, and subsequently dried in an oven at 80°C for 8 h. Following the same procedure discussed above, 60 mg PVDF, 180 mg PVP (1:3 ratio), 60 mg 1 (20% by mass of the total solid content), in 3 mL DMF, are taken to get the thin film of 1@MMM‐20, whereas 52.5 mg PVDF, 157.5 mg PVP, and 90 mg of 1 (30%) results the formation of 1@MMM‐30. Similarly, 1@MMM‐40 was prepared with 120 mg of 1 (corresponding to 40% of the 300 mg solid content) to incorporate with PVDF and PVP under similar conditions, only with a longer incubation time to allow proper aggregation, a common practice for preparing such films with a high concentration of MOF. The fabrication of 1@MMM‐XX, beyond 40%, was also tried, but the film was not found suitable due to the cracks in it, originating from the brittleness.

3. Results and Discussion

The single crystal X‐ray data collection and the refinement details are presented in the Supporting Information (SI). The crystallographic data, along with structure refinement parameters for 1, are provided in Table S1.

3.1. Structural Description of {[Zn₂(4‐pa)(5‐sip)(H₂O)₃].(H₂O)₃}_n (1)

Compound 1 crystallized in the triclinic P‐1 space group, and the single crystal structural determination shows that it has a 1D ladder‐like structure. The asymmetric unit of 1 consists of two crystallographically independent Zn(II) centers (Zn1 and Zn2), bridged together by 4‐picolylamine (4‐pa), and 5‐sulfoisophthalate (5‐sip) (Figure 1a). Here, Zn1 and Zn2 are 4 and 6‐coordinated with distorted tetrahedral and octahedral geometry, respectively. The Zn1 has the ZnO₃N environment that is furnished by O7, O4 ^a from two individually chelated dicarboxylate groups, O3W from a water molecule, and one nitrogen atom (N1 ^a ) from the pyridinyl ring of the 4‐pa linker. Here, Zn1─O bond distances vary from 1.931(2) to 2.037(3) Å and Zn1─N is of 2.021(2) Å (Table S2). Similarly, the Zn2 with ZnO₄N₂ coordinates to four oxygen atoms (O1W, O2W, O1W ^a , and O2W ^a ) of four different water molecules and two nitrogen atoms (N2 and N2 ^c ) from two different 4‐pa linkers (Figure 1a).

FIGURE 1.

(a) Coordination environment of compound 1; green (Zn) black (C), blue (N), red (O), and yellow (S); (b) 1D chain extended along with b direction; (c) Water molecules in coordination as well as lattice sphere (d) Simplified topological representation of the 1.

The Zn2─O bond lengths are in the range of 2.1332(19) Å–2.146(3) Å and both of Zn2─N and Zn2─N ^c is 2.149(2) Å (Table S2). Here, the bridging 4‐pa ligand acts to connect two Zn1 centers via one Zn2, forming a trinuclear fragment (Figure 1b); that further connected by 5‐sip dicarboxylate via Zn1 centers, forming a 1D ladder‐like polymeric chain along the b axis. In the crystal packing, these 1D ladders are occupied by lattice guest water molecules, and those with coordinated waters facilitate efficient hydrogen bonding (Figure 1c). Further, the overall framework has been additionally stabilized by both the H‐bonding and π···π interactions, resulting in a supramolecular 3D network (Tables S3 and S4, and Figures S2 and S3, respectively). The structural analysis by TOPOS suggests that this framework forms a 3‐c uninodal net with the Schläfli symbol of {4².6} (Figure 1d) [42].

3.2. Characterization of 1 and 1@MMM‐XX

3.2.1. Powder X‐Ray Diffraction (PXRD)

The structural integrity of all the synthesized bulk compounds, as well as the films, has been confirmed through powder X‐ray diffraction (PXRD) analysis. The as‐synthesized compound 1 exhibits well‐defined PXRD peaks that closely match its corresponding simulated pattern, derived from single‐crystal X‐ray diffraction (SCXRD) structures. This strong correlation between experimental and simulated data indicates the phase purity of the bulk materials across all samples (Figure 2a). PXRD analysis of 1 in various solvents also confirmed that compound 1 retained exceptional stability in methanol, ethanol, acetone, DMF, water, and even in boiling water (Figure 2a).

FIGURE 2.

(a) Powder X‐ray diffraction pattern for the compound 1 in different solvents, and (b) PXRD Pattern of 1@MMM‐XX various membranes.

Furthermore, the PXRD analysis is also performed for the hybrid PVDF/PVP membrane and those with variable loading of 1 as fillers. In all such PXRD, there is a broad diffraction peak observed at 10°–15° corresponds to PVP, while the peak at ∼20° is assigned to PVDF [43]. Upon gradual increase of loading of 1 in the composite membrane (1@MMM), the diffraction pattern progressively approaches that of pristine 1, associated with sharper and more intense peaks, leading to the formation of a more crystalline matrix. This observation confirms the successful incorporation of the MOF 1 into the membrane matrix (Figure 2b).

3.2.2. Fourier Transform‐Infrared (FT‐IR) Spectroscopy

To investigate the effect of varying the quantity of loading of 1 in different types of mixed matrix membranes (MMMs), Fourier‐transform infrared (FT‐IR) spectra were also recorded. The stretching frequency at ∼3400 cm⁻¹ corresponds to the presence of water molecules in 1, which is absent in the PVDF/PVP hybrid membrane. Similarly, the band at ∼2940 cm⁻¹ is attributed to the asymmetric C─H vibration of PVP in the PVDF/PVP hybrid membrane [44]. Both of these characteristic bands appear in the spectra of the 1@MMM‐XX films, confirming the incorporation of variable amounts of 1 into the membranes. All other FT‐IR stretching frequencies remain almost consistent across all samples (Figure S4).

3.2.3. Thermogravimetric Analysis (TGA)

The thermal stability of all the compounds has been analyzed through TGA conducted under an N₂ atmosphere with a flow rate of 5°C/min within the temperature range of 30°C–610°C. Compound 1 and all 1‐doped membranes were stable up to ∼300°C, which is considerably higher than the blank membrane of PVDF/PVP, which is stable up to ∼270°C. The observed weight loss of the compound of 1 was 17% in the temperature range 70°C to 110°C, indicating the evaporation of water molecules. In the case of PVDF/PVP membrane, 9% weight loss was observed due to the hydrophilic nature of PVP in the temperature range between 70 and 110°C. Further, all the 1‐doped proton exchange hybrid membranes, 1@MMM‐XXs, show a weight loss of about 15%–20% in the same temperature region of 70°C to 110°C (Figure S5).

3.2.4. Morphological and Topological Analysis

The FE‐SEM analysis has been performed to understand the morphological features of 1 as well as different types of 1‐doped 1@MMM‐XX membranes, and also blank PVDF/PVP membrane. The FE‐SEM image of the 1 is depicted in Figure 3a. In 1, EDAX and elemental analysis confirmed the presence of carbon, nitrogen, oxygen, sulfur, and zinc (Figure 3b,c) atoms and the absence of sodium ions, although the sodium 5‐sulfoisophthalate was used in the complexation reaction. Additionally, the optical image of the 1 is provided in Figure 3d. The FE‐SEM analysis of the blank PVDF/PVP membrane confirms the homogeneous mixing of PVDF and PVP, where nitrogen and oxygen are part of PVP and fluorine is part of PVDF (Figure S6). The homogeneous distribution of the 1 in the 1@MMM‐XX composite membrane was confirmed by the FE‐SEM investigation of the 1@MMM‐40 membrane. As shown in Figure 4a, the membrane surface is smooth and homogeneous, which is suitable for fuel cell applications. The EDAX and elemental mapping show the uniform distribution of the zinc metal from the 1, confirming the successful incorporation of microcrystals 1 throughout the whole surface of the 1@MMM membranes (Figure 4b,c). Also, the optical image of the self‐standing 1@MMM‐40 membrane is illustrated in Figure 4d.

FIGURE 3.

(a) FE‐SEM image, (b) EDAX spectra, (c) Elemental mapping, and (d) Optical image of the compound 1.

FIGURE 4.

(a) FE‐SEM image, (b) EDAX spectrum, (c) Elemental mapping, and (d) Optical image of the 1@MMM‐40 membrane.

The surface topography of the composite membranes was examined by using atomic force microscopy (AFM) in dynamic mode, cantilever type Dyn190Al. The pristine PVDF/PVP membrane shows a relatively smooth and uniform surface, whereas the 1@MMM‐40 membrane exhibits uniformly dispersed 1 filler accompanied by white blotches in 2D or hill‐like features in 3D images, indicating almost homogeneous distribution 1 (Figure S7). The incorporation of 1 notably increases the surface roughness, which contributes to the enhancement of the mechanical properties. The pristine PVDF/PVP membrane displays arithmetic mean roughness R_a and root‐mean‐square (rms) roughness R_q values of 0.98 and 1.36 nm, respectively, while the 1@MMM‐40 membrane shows increased values of 1.72 and 2.26 nm, confirming the rougher surface after 1 incorporation.

3.2.5. Nitrogen and Water Sorption

The nitrogen and water sorption studies have been carried out to understand the potential pore and surface area at 77 and 298K, respectively, using a Quantachrome Autosorb‐iQ adsorption instrument. The samples have been prepared by activating at 393K under a dynamic vacuum (up to 1 × 10⁻¹ Pa) for about 4 h in the presence of helium gas (99.999% purity) to remove lattice solvent molecules. For PVDF/PVP and 1, the maximum nitrogen uptake is 34 and 43 cc/g, with calculated surface areas of 42.714 and 53.083 m²/g, respectively. In the case of 1@MMM‐10, 1@MMM‐20, 1@MMM‐30, and 1@MMM‐40, the maximum uptake found was 47, 49, 55, and 59 cc/g, with corresponding surface areas of 61.452, 67.563, 72.629, and 80.323 m²/g, respectively (Figure S8). The above results indicate the presence of micro‐pores in all cases, where the surface area steadily increases with the percentage of MOF doping in the matrix.

The water vapor sorption isotherms of compound 1 and related fabricated materials were performed after their activation as mentioned above, to understand their response toward relative humidity (% RH). Among all, the blank PVDF/PVP membrane exhibited the highest water uptake value of 67 cc/g, mainly due to the presence of a maximum amount of hydrophilic PVP units in the blank membrane. In contrast, the maximum water uptake values of the 1@MMM‐XX membranes are gradually decreasing from 52, 45, 43, 42 cc/g with increasing the amount of 1‐loading from 10 to 40 wt.% in the PVDF/PVP matrix and 36 cc/g for 1 (Figure S9). This reduction is attributed to the introduction of a larger extent of 1 in the PVDF/PVP matrix, which restricts the facile movement of the polymer chains and increases the rigidity in the composite membrane [45]. However, the maximum value is comparable in all the cases, and this clearly indicates the good affinity of all the matrices toward water, which is the baseline criterion of proton conduction. Moreover, the water vapor adsorption for all the systems exhibits three‐step uptake profiles, initiated at low humidity with a small uptake, followed by a two‐step pore‐filling uptake, indicating their hydrophilic character, which is also noteworthy for the proton conduction process.

3.2.6. Proton Conductivity (σ), Activation Energy (E_a), and Durability

The proton conductivity (σ) measurement of the pristine 1D‐compound 1 was done in a pellet. The pellet has been prepared by pressing the finely ground powder of the crystals of 1 using a steel mold. The proton conductivity measurement of this pellet was carried out between 30°C and 80°C at 100% RH (hydrated conditions). The characteristic Nyquist plot of 1 is depicted in Figure 5a, which shows that the bulk resistance (impedance) value of 1 gradually decreases with the rise in temperature from 30°C to 80°C. This is mainly owing to the upsurge of the H⁺ ion mobility with increased temperature [46, 47]. The σ values also gradually increased from 1.97 × 10⁻³ to 3.44 × 10⁻³ S.cm⁻¹ in the temperature sweep range of 30°C to 80°C (Table S5). The proton conduction‐related Arrhenius‐type plot of ln(σ) vs 1000/T of 1 (Figure 5b), the calculated E_a value of 1 is found to be 0.103 eV, which indicates that proton conduction occurs via the Grotthuss mechanism (E _a <0.4 eV) [48, 49]. Here, it is interesting to note that the 1 shows relatively higher proton conductivity and lower activation energy than some previously reported similar MOFs under comparable testing conditions (Table S6). Therefore, with this superior proton conductivity value of 1, it has been utilized for the fabrication of PEMMMs to achieve better and viable results.

FIGURE 5.

(a) The Nyquist plot of the 1 under hydrated conditions at different temperatures. (b) Arrhenius‐type plot of ln(σ) vs 1000/T of 1.

The conductivity value of the fabricated composite membranes (1@MMM‐XXs) was evaluated by the proton conductivity measurement in the same temperature sweep between 30°C and 80°C under hydrated conditions. The Nyquist plots of the composite 1@MMM‐XX membranes are provided in Figure 6a,b, Figure S10a,b. The σ values of the 1@MMM‐10, 1@MMM‐20, 1@MMM‐30, and 1@MMM‐40 membranes at 80°C are 2.04 × 10⁻², 2.27 × 10⁻², 2.60 × 10⁻², and 2.81 × 10⁻² S.cm⁻¹ respectively (Table 1). Among all the testing membranes, the 1@MMM‐40 demonstrated the highest σ value owing to the highest 1 loading percentage (40 wt.%). All the 1@MMM‐XX membranes exhibited a high σ value compared to that of the reported composite membranes, whereas 1@MMM‐40 shows the highest with such a minimum (40%) MOF‐based doping (Table S7). Here, the 1@MMM‐XX composite membranes demonstrated almost 6 and 23 times higher σ values than the MMM@Cu‐BTC‐Lys‐40 and MMM@Cu‐BTC‐Lys‐20 membranes under comparable testing conditions (Table S7). The 1@MMM‐40 membrane exhibited 8.2 times higher σ value than that of the pristine 1 (Figure 5a), and shows a 16.4‐fold increase in σ value compared to the blank PVDF/PVP membrane (Figure S11).

FIGURE 6.

The Nyquist plots of (a) 1@MMM‐30 and (b) 1@MMM‐40 membranes under hydrated state in different temperatures. (c) Arrhenius‐type plot of ln(σ) vs 1000/T of 1@MMM‐XX membranes, and (d) time‐dependent durability plots of 1@MMM‐40 membrane at 80°C in hydrated conditions.

TABLE 1.

The proton conductivity (σ) and activation energy (E_a) values of the 1@MMM‐XX composite membranes under hydrated conditions.

Material	Proton conductivity (σ, S.cm−1)			Activation energy (Ea, eV)
	30°C	60°C	80°C
1	1.97 × 10−3	2.74 × 10−3	3.44 × 10−3	0.103
1@MMM‐10	1.07 × 10−2	1.49 × 10−2	2.04 × 10−2	0.117
1@MMM‐20	1.19 × 10−2	1.69 × 10−2	2.27 × 10−2	0.116
1@MMM‐30	1.70 × 10−2	2.19 × 10−2	2.60 × 10−2	0.078
1@MMM‐40	1.94 × 10−2	2.33 × 10−2	2.81 × 10−2	0.066

The observed proton conduction in 1 can be correlated with the dynamic nature of its framework. Measurements of proton conductivity under varying relative humidity (RH) and temperature conditions reveal that the conductivity reaches its maximum at 100% RH and 80°C. Additionally, the RH‐dependent proton conductivity of compound 1 was investigated over the range of 60%–98% RH at 80°C to elucidate the influence of humidity on its conductive behavior. The corresponding Nyquist plot is shown in Figure 7a. The conductivity of 1 increases progressively from 1.91 × 10⁻³ to 3.11 × 10⁻³ S.cm^{− 1} with increasing RH (Table S8). This trend is consistent with the temperature‐dependent measurements, indicating that proton transport is facilitated at higher humidity. The enhancement in conductivity at elevated RH can be attributed to the formation of extended hydrogen‐bonding networks involving absorbed water molecules, ionic functional groups, coordinated water, and lattice water within the framework of compound 1. Similarly, the RH‐dependent proton conductivity of the 1@MMM‐40 membrane was evaluated over the range of 60%–98% RH at 80°C, and the corresponding Nyquist plots are shown in Figure 7b. The proton conductivity increased from 1.54 × 10⁻² to 2.29 × 10⁻² S.cm⁻¹ with increasing RH from 60% to 98% (Table S9).

FIGURE 7.

The Nyquist plots of (a) compound 1 and (b) 1@MMM‐40 membrane under variable RH conditions at 80°C.

This performance is attributed to the two primary structural features; the presence of free sulfonate (─SO₃ ⁻) groups and lattice water molecules. These components act as proton donors/acceptors, forming an extensive hydrogen‐bonding network. Notably, the variations in O─H···O distances are found critical for promoting extensive proton delocalization throughout the framework. The hydrogen‐bond distances between hydrogen atoms of lattice or coordinated water molecules (proton donors) and oxygen atoms of carboxylate or sulfonate groups (proton acceptors) fall within the range of 1.86–2.79 Å, indicating their strong nature. Collectively, the synergistic interplay between lattice water molecules and free sulfonate groups forms an extensive hydrogen‐bonding network that establishes efficient proton‐conductive channels within the framework of 1 (Figure S12).

Henceforth, the incorporation of 1 into the pristine PVDF/PVP polymer significantly enhances the σ owing to the presence of free sulfonate (─SO₃ ⁻) groups, lattice water molecules, and metal‐coordinated water molecules within the 1@MMM‐XXs framework. The PVP, with its N‐heterocycle groups acting as proton acceptors, shows strong potential in PEMs due to its hydrophilic nature, as established both theoretically and experimentally [50, 51]. On the other hand, PVDF, a widely used low‐cost fluoropolymer, has good thermal stability, mechanical strength [52], and therefore proper blending of PVP with PVDF always acts as a popularly utilized stable membrane where strong hydrogen‐bonding interactions between the O or N of PVP and the ─CH₂ groups of PVDF are responsible for their stability [53]. These entities collectively generate additional proton migration pathways throughout the composite membrane. Moreover, strong hydrogen‐bonding interactions between the sulfonate and water molecules of 1 with the carbonyl oxygen and pyrrolidinone nitrogen atoms of PVP further facilitate efficient proton transfer. This synergistic interplay between the filler (1) framework and the polymer matrix establishes a continuous, dynamic hydrogen‐bonding network that enables facile, efficient proton conduction throughout the 1@MMM‐XXs composite membrane (Figure S13). The E_a values, calculated from the Arrhenius‐type plot of ln(σ) vs 1000/T of the composite membranes, are 0.117, 0.116, 0.078, and 0.066 eV for 1@MMM‐10, 1@MMM‐20, 1@MMM‐30, and 1@MMM‐40, respectively (Figure 6c and Table 1).

The E_a values of 1@MMM‐XX membranes gradually decrease with the increase in the loading percentage of 1, which indicates that the proton conduction process becomes more facile at a higher 1‐loaded membranes. As shown in Table S7, the E_a values of the 1@MMM‐XX composite membranes are quite lower than those of similar MOF‐loaded PVDF/PVP‐based composite membranes. Additionally, the E_a values (<0.40 eV) of the 1@MMM‐XX membranes further confirm that proton conduction in the composite membranes predominantly follows the Grotthuss mechanism, as observed in the pristine 1. Here, it is interesting to note that the E_a values of 1@MMM‐10 and 1@MMM‐20 are slightly higher than pristine 1, even though their proton conductivities are still higher. This indicates the activation energy (E_a) is not always inversely proportional to proton conductivity, considering the complicated nature of the proton conduction, often governed by multiple factors, including the availability of proton donors/acceptors, the presence of an adequate aqueous medium, and the overall structural characteristics of the material.

In addition to the proton conductivity value, the durability of the PEMMMs is one of the prime parameters for their long‐term applicability and real‐time efficiency. The durability of the 1@MMM‐XX membranes was checked via the conductivity measurements of the 1@MMM‐40 membrane at 80°C and RH 100%; for four consecutive days, and the time‐dependent Nyquist plots of the 1@MMM‐40 membrane are depicted in Figure S14. The σ value of the 1@MMM‐40 membrane marginally decreased (5.7%) from 2.81 × 10⁻² S.cm⁻¹ on day one to 2.65 × 10⁻² S.cm⁻¹ after four consecutive days (Figure 6d; Figure S14). This confirms the long‐term durability of the 1@MMM‐XX membranes under potential fuel cell operating conditions. Furthermore, the PXRD study of the 1 and 1@MMM‐XX membranes (Figure S15) reveals that the structural integrity and stability of the 1@MMM‐XX membranes remain unaltered after the impedance measurements under high‐temperature and high‐humidity conditions.

4. Conclusion

In this study, we have synthesized a novel proton‐conducting MOF (1) containing a free sulfonic group and a dicarboxylic acid. The water molecules are found to be responsible for creating high proton conductivity for the 1D MOF, through the formation of an extended intermolecular hydrogen‐bonded network with other oxygen of water, carboxylic or sulfonic acid groups of 5‐sulfoisophthalate. This excellent proposition of the said MOF prompted us to use this in the fabrication of mixed matrix membranes of PVDF/PVP to enhance their proton conductivity, durability, and flexibility for the design of a more practical, utility‐oriented fuel cell membrane. In this way, a ratio variation in the loading has been explored, starting from 10 weight percentage of 1, leading with PVDF/PVP. The maximum possible loading of 1, which gives a stable membrane morphology, is found 40%, and in that condition, the mixed matrix membrane 1@MMM‐40 shows a remarkably high proton conductivity value, which is unique in this class of materials. In summary, this exploration is a step forward not only for the understanding of the unique properties of mixed matrix membranes, but also a promising approach toward the design of an economically and technologically viable proton‐conducting membrane for application in commercial fuel cells.

Conflicts of Interest

None of the authors have a conflict of interest to disclose

Supporting information

Supporting File: smll73938‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.