Development of novel proton exchange membranes based on cross-linked polyvinyl alcohol (PVA)/5-sulfosalicylic acid … – Springer

Fuel cells have recently been regarded as a possible future clean energy source due to their ability to be used in various applications ranging from mobile devices to large power stations, depending on the type of fuel cell. As a result, fuel cells may become an appropriate solution to today’s energy source issues [1]. Through the help of fuel cells, chemical energy from hydrogen or other fuels is converted into electrical energy. In the case of fuel cells using hydrogen as fuel, hydrogen and oxygen from the air react across an electrochemical cell to produce electricity. It is a clean source of energy that produces electricity as its primary product, with heat and water as by-products [2]. Many fuel cell types operate at different temperatures, are powered by different fuels, need different types of catalysts, and hence experience quite distinct electrochemical processes with various electrolytes [3]. Solid oxide fuel cell (SOFC), proton exchange membrane fuel cell (PEMFC), molten carbonate fuel cell (MCFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), reversible fuel cell (RFC), and direct methanol fuel cell (DMFC) are the most common types of fuel cells [4].

Recently, direct methanol fuel cells (DMFC) represent alternatives to traditional electricity sources and are regarded as the most technologically advanced electricity sources among the other fuel cell (FCs) types because of its unique features, such as high fuel energy density, easy liquid fuel storage, low operating temperature, high energy conversion efficiency, simplified system construction, and low pollutant emission [5]. However, one significant limitation of DMFC is the loss of methanol via the polyelectrolytic membrane and the formation of highly reactive polymeric electrolytes with optimal water management. Furthermore, fouling is one of the difficulties that might impact polymeric membranes by reducing their effectiveness by blocking proton-binding sites, particularly fouling caused by fuel impurities or other biological products and chemical interactions. The fouling polymeric membrane must be recovered or changed to boost operational efficiency. In addition, the cost of the components, especially electrodes and electrolytes, is one of the main obstacles to the advancement of fuel cells [6].

The essential component of DMFC is the proton-exchange membrane (PEM), which works as both a separator to prevent the supplied fuel from contacting the electrodes and an electrolyte for proton (H⁺) conduction from the anode part to the cathode in DMFCs [7].

A suitable PEM should have excellent conductive properties, high water absorption capacity, low permeability to the fuel, good mechanical and oxidative stability, electrical insulators, and the ability to be produced into a membrane electrode assembly (MEA). With all these characteristics, it is essential to the commercial element to keep the cost as low as possible. The main challenge is developing a low-cost alternative membrane with good performance [8].

Nowadays, Nafion® membranes have been the most extensively utilized electrolyte in polyelectrolyte membrane fuel cells (PEMFC) for many factors, including their superior chemical stability, proton conductivity, and mechanical strength. However, these membranes are limited by their high fuel permeability, issues with water management, and instability at high temperatures that lead to membrane deterioration, as well as reduced DMFC performance, in addition to their high cost [9].

To overcome these challenges, alternative polyelectrolytic membranes can be developed by functionalizing and modifying different polymers, which will improve their desired characteristics and increase their proton conductivity while also being inexpensive materials that will lower production costs. Several natural and synthetic polymers have been chemically modified or grafted to functionalize them with acidic groups, mainly sulphonic acid groups (SO₃H) to yield corresponding sulfonated polymers. Polymer blending or the production of polymer composites with inorganic nanoparticles has also been studied [10].

Among the synthetic polymers that have attracted significant attention in the development of polyelectrolyte membranes is polyvinyl alcohol (PVA), which is regarded as a cost-effective, environmentally friendly synthetic polymer with excellent film-forming properties, chemical resistance, and high hydrophilicity [11]. However, the fundamental drawbacks of PVA, such as high-water permeability, minimal proton conductivity, and extensive swelling, require modifications to its structure. Cross-linking is an effective method to modify the polymer, enhancing its surface, oxidative chemical, thermal, and mechanical properties. The cross-linking of PVA involves the creation of three-dimensional networks, improving dimensional stability and reducing permeability to substances like methanol while enhancing proton conductivity.

Because of its high density of hydrophilic functional (–OH) groups, PVA polymer chain may be chemically modified by substitution, grafting, and cross-linking [12]. As a result, mixing PVA with a chemical cross-linker has been described as a possible strategy for producing a suitable polyelectrolyte membrane [13]. Therefore, several functionalization techniques and modifications have been put forth as workable strategies for developing polyelectrolyte membranes with enhanced mechanical stability, proton conductivity, and the barrier effect—which is essential for preventing fuel from switching over. These techniques include mixing PVA with anionic polymers and/or creating composite materials. PVA is frequently mixed with ionomers such as perfluorosulfonic acid polymer (Nafion) [14], polystyrene sulfonic acid (PSS) [15], poly(2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) [16], and chitosan [17] to create films with strong proton conductivity for fuel cell applications.

Sulfosalicylic acid (SSCA) has attracted the interest of researchers due to its unique chemical structure, high water solubility, and proton conductivity, which allows proton transfer across membranes by forming negative-charged sulfonic groups (-SO₃H) ions [18]. Furthermore, SSCA is less costly as compared to Nafion® 117. Additionally, SSCA cannot form a self-supporting PEM because of its solubility and must be joined with another supporting matrix.

As a result, this research aimed to develop a substitute membrane based on PVA and SSCA. Because of their non-toxicity, low cost, biodegradability, and effective matrix formation, PVA and SSCA were chosen for this study [19]. In addition, to increase the mechanical properties, thermal stability, and water resistance of fabricated membranes, chemical cross-linkers such as Glutaraldehyde (GA), Fumaric Acid (FA), and Malic Acid (MA) are homogeneously disseminated in the originating polymer of the composite membranes [20]. A chemical cross-linking interaction takes place between the aldehyde (–CHO) group of (GA) or the carboxyl group (-COOH) of (FA or MA) and the hydroxyl group (-OH) of PVA polymer in order to create the acetyl ring and ester linkage respectively [21]. Functionalized and cross-linked PVA/SSCA-based polyelectrolytic membranes were created by combining and blending SSCA with PVA and using glutaraldehyde (GA), fumaric acid (FA), and malic acid (MA) as chemical cross-linkers. Noteworthy is the fact that the membranes developed in this study showed improved proton transportation properties, high ion exchange capacity, better mechanical and thermally stable properties, and a lower methanol permeability, improving electrochemical performance.

2.1 Materials and chemicals for membrane preparation

PVA (96.5% hydrolyzed, average M. wt. = 85,000–124,000, fine powder) was acquired from Across Organics, Belgium. 5-Sulfosalicylic Acid dihydrate (SSCA, ≥ 99%), Fumaric Acid (FA), and Malic Acid (MA) were received from Merck, Germany. Glutaraldehyde Solution (about 50%) (assay 47–53 w/w, El Nasr Pharmaceutical Chemicals Company, Gesr El Suez, Cairo, Egypt). Sodium Hydroxide (pellets, purity 98%) was obtained from Central Drug House Ltd (New Delhi, India). Sulphuric acid, Hydrochloric Acid, and Methanol (MeOH) were purchased from United Co., Egypt. All chemicals were used as received, with no further purification.

2.2 Synthesis of cross-linked PVA/SSCA-based PEMs

Cross-linked (PVA/SSCA) composite polyelectrolyte membranes were synthesized by solution-casting method as illustrated in Fig. 1. Typically, a homogeneous (8 wt%) PVA polymer solution was obtained by dissolving a pre-weighed amount of PVA in de-ionized water for 5 h with magnetic stirring at 90 °C. For functionalization, different SSCA concentrations were introduced separately into homogeneous PVA solutions, and to ensure that the solution was entirely dissolved, the blend membrane-forming solutions were stirred regularly for 24 h at room temperature. The cross-linking technique was applied to increase the performance of the resultant polymer mix membranes for fuel cell applications. Chemical cross-linkers such as Glutaraldehyde (GA), Fumaric acid (FA), and Malic acid (MA) were individually introduced into PVA/SSCA solutions and stirred at room temperature overnight to generate a chemically restricted network via PVA chain cross-linking [22]. Finally, the homogenous, transparent, and viscous membrane-forming solutions were put in plastic Petri plates after sucking out the air and allowing the water to evaporate at room temperature [23]. The dried membranes were kept in plastic bags till use. Schem 1 depicts the preparation reactions of PVA/SSCA-Glu, PVA/SSCA-FA, and PVA/SSCA-MA composite membranes.

2.3 Membrane characterization

2.3.1 Spectral analysis

By employing a (Shimadzu FTIR-8400 S, Japan) instrument with a resolution of 4 cm⁻¹ and a wavenumber bandwidth of 400–4000 cm⁻¹, the chemical bonding, and functional segments of PVA/SSCA and modified PVA/SSCA-based polyelectrolyte membranes were investigated. A laser Raman scattering spectrometer (SENTERRA-Bruker, Germany) equipped with a Leica microscope additionally was employed to research the chemical bonding and conceivable interactions inside the artificially created polymeric membranes [24].

2.3.2 Morphological features and microstructure

The surface morphology of the polymeric membrane was examined using a scanning electron microscope [SEM, JEOL-JSM-6360LA, Tokyo, Japan] with an acceleration voltage of 10 kV. A layer of gold 10–20 nm thick was sputtered onto membrane samples using a JFC-1100E sputter [JOEL Ltd., Tokyo, Japan], which was double-sided and taped to stainless steel studs [25]. The composition of the elements of the constructed polyelectrolyte films was also studied with an energy-dispersive X-ray spectroscopy (EDX) partnered with a scanning electron microscope.

2.3.3 Thermogravimetric analysis (TGA)

The thermogravimetric properties of the constructed polymeric membranes were examined using a Shimadzu TGA-50 analyzer (Shimadzu, Japan). The aluminum pans containing the 5 mg-sized film samples are heated at 10 °C min⁻¹ from ambient temperature to 800 °C [26].

2.3.4 Mechanical property

A universal tensile machine (UTM, model AG-I, Shimadzu, Kyoto, Japan) was used to assess the tensile strength (TS) and elongation at break (EB) of the membranes. The initial grip separation and crosshead speed settings were 10 cm and 1 mm per minute. Before tensile testing, film specimens were subjected to conditioning for two days in an environmental room at 22 °C and 70% RH using ASTM [D 882-91] Conventional Technique. Each made-cast membrane’s TS and EB values were determined as duplicate experimental units for each type of manufactured polymeric membrane. The relative importance rating of five experiments was utilized to calculate the TS and EB replicate value for each specimen [27].

2.3.5 Contact angle measurement

The hydrophilic capability of polymeric blend-based membranes was examined using a VCA 2500 XE contact angle meter, a CCD camera, and statistical computer programs (AST Products, Billerica, MA) [28].

2.3.6 Ion exchange capacity (IEC)

The number of active sites or groups with functional qualities involved in transferring ions in polymeric electrolytes membranes is referred to as the ion exchange capacity (IEC). The number of sulfonic acid groups bearing onto the membranes was used to compute the IEC. Weighed, and dried specimens of polyelectrolytic membrane (1 × 2 cm²) were immersed into a 2M sodium chloride (NaCl) solution for a full day to be evaluated. The protons released from these samples were quantified using an acid base titration method with 0.01N sodium hydroxide (NaOH) using phenolphthalein as indicator. The IEC of polyelectrolyte membranes was calculated using the Eq. (1) beneath [29].

while V is the volume of titrated NaOH, and C is the NaOH solution concentration. W_d represents the dry membrane’s weight specimen after 12 h of drying at 50 °C.

2.3.7 Water and methanol uptake (WU, MU) and swelling ratio (SR)

The water uptake capacity and swelling ratio of the prepared polyelectrolytic membranes are determined by comparison of weights (W) and dimension (D) in the direction x of the membrane specimens before (W_dry and D_dry) and after (W_wet and D_wet) soaking in water [30]. The membrane-derived specimen was first dried in a vacuum oven at 50 °C for 12 h and then the weight and dimension are measured. The dried polymer membrane is immersed immediately in the distilled water at room temperature for a full day followed by soaking up off the surface moisture with filter paper before weighing the specimen. Water uptake and the swelling ratio of the samples are then calculated respectively, with Eqs. (2, 3) [31].

where (W_wet and D_wet) are the weight and dimension of the wet membrane, while (W_dry and D_dry) are the weight and dimension of the dry membrane, respectively. Methanol uptake was tested using the same procedure of water uptake as previously, using methanol in place of water [32].

2.3.8 Electrochemical measurement

The proton conductivity of the manufactured polyelectrolytic membranes has been assessed using electrochemical impedance spectroscopy (EIS) on a palletized sample using a Gamry Instruments Reference 3000 galvanostatic between 100 Hz and 1 MHz. The following Eq. (4) calculates the protonic conductivity of membranes using impedance data.

while σ is the proton conductivity, measured in S cm⁻¹, L is the length of the distance among electrodes (cm), R is the membrane’s electrical resistance, while W and T are the width (cm) and thickness (cm) of the membranes, respectively [33].

2.3.9 Oxidative stability analysis

The synthesized cross-linked PVA/SSCS-based polyelectrolytic membranes were investigated for oxidation stability through Fenton test. Oxidative stability reflected the lifetime of the membrane to some extent. The dried membranes specimens (1 × 4 cm²) were soaked in in Fenton’s reagent (2 ppm FeSO₄ in 3% H₂O₂) for 1 h. 80 °C [34]. The following Eq. (5) can be used to determine the oxidative stability based on the weight difference between the specimens before and after immersion.

where W_o and W₁ represent the specimen weight in grams before and after oxidation, respectively.

2.3.10 Methanol permeability measurement

Methanol permeability was assessed using a typical glass diffusing cell as shown in Fig. 2, which comprises of two glass cavities (cavity A with a volume of 125 ml for the feed and a cavity B with a volume of 120 ml for the permeate) separated by a specimen the same diameter as the investigated membrane. Cell (A) was filled with 15% methanol, whereas cell (B) was filled with deionized water. To measure the permeability of methanol, 1 ml of liquid samples were obtained from the permeator while both solutions were stirred.

A portable density metre-DMA 35 (Anton Paar) was used to quantify the methanol content in the water compartment [35]. Methanol permeation was calculated using the following Eq. (6) as the slope of the exponential relationship between methanol quantity present in the water cavity and time (t) [26].

where K is the permeation curve slope, V_B is the initial volume of deionized water, L is the membrane thickness, A is the membrane working area (area of permeation), and C_A is the initial methanol concentration in the compartment (A).

2.3.11 Membrane efficiency

To assess the efficiency of polyelectrolyte membranes for employing in DMFCs, the following Eq. (7) was used, which expresses ionic conductivity and methanol permeability.

where E denotes the effectiveness of the membrane as a function of the connection between ionic conductivity (σ) and methanol permeability (P) [36].

3.1 Investigation of the chemical composition of membranes

3.1.1 FT-IR spectra

FT-IR spectra of pristine PVA, PVA/SSCA, and improved cross-linked PVA/SSCA based-polyelectrolyte membranes are presented in Fig. 3. The spectrum of non-crosslinked PVA membrane is shown in Fig. 3A. In this spectrum, a broad peak located around 3264–3500 cm⁻¹ was observed, which corresponds to the stretching vibration of the hydroxyl group (OH) from the intermolecular and intramolecular hydrogen bonds, as well as a characteristic peak around 2935 cm⁻¹ corresponds to the symmetric and asymmetric (C–H) stretching frequency of the alkyl group in the polymer backbone [37].

Meanwhile, the spectrum of PVA/SSCA and modified cross-linked PVA/SSCA polyelectrolyte membranes reveals another absorption characteristic peak in the range of 1500–1700 cm⁻¹ which is an indication of the existence of the C=O group (belonging to the aldehyde group in GA or carboxyl groups in FA and MA) and the O–C–O bond resulting from the interaction of any of GA, FA, MA with PVA (as shown in Scheme 1) which is an indication that the cross-linking process has been achieved [38].

Furthermore, the presence of ester bonds in all cross-linked membranes is due to the presence of SSCA (-ـCOOH), which is bound to (OH) of PVA. This cross-linking was also accompanied by the appearance of the signal centered at 1210 cm⁻¹ confirming the presence of an ether linkage in the polymer chain [39]. New absorption peaks at 1022 and 1152 cm⁻¹ are discovered by comparing the spectra of pristine PVA membranes with those for PVA/SSCA and modified PVA/SSCA polyelectrolyte membranes. These peaks are created by the SSCA symmetric and asymmetric O–S=O stretching vibrations of –SO₃H groups [40].

The S–O bond is related to the band of about 1190 cm⁻¹. The existence of –SO₃⁻¹ in the polymeric matrix was verified by stretching vibrating peaks of absorption at 620 cm⁻¹, 628 cm⁻¹, and 627 cm⁻¹. The high signal seen at 1598 cm⁻¹ was caused by the aromatic ring skeleton [41]. When PVA is present, however, the absorption peaks at 3264 cm⁻¹ induced by stretching of (O–H) appear in all spectra. The strength of this peak is very low in modified PVA/SSCA-based polyelectrolyte membranes, suggesting that chemical cross-linking has taken place and was successfully completed [42].

3.2 Raman spectroscopy

The Raman spectroscopy technique was used to understand the membrane’s microstructure. The spectrum of modified SSCA/PVA-based polyelectrolyte membranes is shown in Fig. 4. The C–S covalent bond is connected to the absorbed peak at 605 cm⁻¹, which is shown in all Raman curves except the pure polymer curve in (Fig. 4I-A). The strength of this band increases when the SSCA molar ratio increases, causing an increase in the –SO₃H groups.

The C–C of the polymeric chain was responsible for the other strong vibrational peak that appeared about 906 cm⁻¹ [43]. The vibration of C–H bonds produced signals at 1415 cm⁻¹ in the spectrum. The asymmetrical vibration of C–O–C bonds at 1095 cm⁻¹ has yielded additional signals that enhance cross-linking investments [44]. Furthermore, the C=O group in PVA produces a strong valence spectrum at 1642 cm⁻¹ and this band’s intensity grows as PVA is functionalized with SSCA and chemically cross-linked with GA, FA, and MA. Because the preparation and functionalization processes did not significantly change the structure of PVA, SSCA, or the chemical cross-linkers, it is interesting to note that all the spectra in Fig. 4. (I, II, and III) share the same pattern and are relatively comparable [45].

3.3 Topography and cross-sectional features of the membranes

The surface and cross-section morphological characteristics of modified cross-linked PVA/SSCA-based polyelectrolyte membranes have been evaluated using a scanning electron microscope (SEM), as shown in Fig. 5. Because of the insertion of the sulfonic group, which ties and aligns polymer chains, as well as the incorporation of chemical cross-linkers GA (Fig. 5B), FA (Fig. 5C), and MA (Fig. 5D), The membranes’ surface morphology is glossy and smooth. They have high mix compatibility with no phase separation or breaking.

The PVA/SSCA-based membrane’s cross-sectional SEM micrographs revealed a compact and dense structure. Chemical cross-linkers, however, increased the compact and rigid structure without causing the formation of loose structures or wrinkled fractures in blend membrane matrices. These outcomes are due to the extended PVA layers embedded in the pure PVA membrane matrix during the cross-linking process. The observed changes in the topographical surface and cross-section are indications of the sulfonation sort through, which has a significant impact on the membrane’s structure and characteristics. All Enhanced PVA/SSCA-based membranes, however, had denser microstructures than unaffected PVA/SSCA-based membranes. These discoveries result from ester linkage bonding between polymer chains with OH groups and conductive material (SSCA) with COOH groups [46].

The membranes exhibited a uniform, compact structure, a rough surface, no phase separation, and a smooth, crack-free surface, as seen by these micrographs. These features may be attributed to PVA and SSCA’s high compatibility and interfacial solid interactions.

It is worth mentioning that surface holes were not visible on the surface of the micrographs and did not extend over the membrane, which might result in smooth proton transmission across the membrane and prevent methanol crossover [47]. After the addition of chemical cross-linkers that enhance the formation of covalent bonds, which hold portions of several polymer chains together through the crosslinking process, the matrix molecular arrangement was altered, which played a role in some of the alterations in the membrane surface and the collection of mixes [48].

3.4 Energy-dispersive X-ray (EDX) spectroscopy

Energy-dispersive X-ray (EDX) microanalysis is a technique for revealing the elemental composition of materials. EDX can map several chemical components of interest. An EDX spectrometer examination of PVA/SSCA-based polyelectrolytic membranes confirmed the presence of elemental C, O, and S. Because of the PVA and SSCA frameworks, as well as the incorporation of chemical cross-linkers, these elements are present in the samples under investigation. Carbon, oxygen, and Sulphur are the primary components of the non-crosslinked PVA/SSCA membrane, as shown in Fig. 6.

All modified cross-linked PVA/SSCA using GA, FA, and MA as chemical cross-linkers exhibited present in the S element. In addition, a decrease in the carbon element band and an increase in the oxygen content was observed, indicating that the functionalization and cross-linking process of membranes occurred successfully. This might be explained by the development of ester and ether linkage between PVA’s hydroxyl groups and chemical cross-linkers GA, FA, and MA by elimination of water, which reduces the polar group’s number as demonstrated by FT-IR and Raman analysis and prevents water from absorbing onto the film surface [49]. The behavior is in harmony with the exchange in the chemical cross-linker type.

These data revealed that the preparation technique did not affect SSCA and that the PVA/SSCA matrix and chemical crosslinkers worked effectively together, as mentioned in the preceding section (SEM) [50]. Finally, all modified cross-linked PVA/SSCA membranes contained elemental S in near proportions and without degradation. This might be read as a successful insertion of SSCA into the holding polymer matrix and an increase in the number of polar groups linked to the membrane, improving ionic conductivity [51].

3.5 Elemental analysis

The chemical constituents of PVA/SSCA and improved cross-linked PVA/SSCA-based polyelectrolytic membranes have been assessed using a carbon, hydrogen, nitrogen, and sulfur (CHNS) analyzer. The findings are shown in Table 1.

As the molar proportion of SSCA in the membrane increases, so does the sulfur intensity, indicating that the amount of –SO₃H groups linked to the SSCA backbone has grown.

On the other hand, as the molar ratio of SSCA rises, there is a slight loss in carbon and hydrogen content. The observed behavior supported the functionalization and modification of PVA/SSCA through increased sulfonation, which occurs without any unexpected degradation or fragmentation, as shown by the C/H ratio [52].

The quantity of carbon is raised for each group (that includes the same ratio of SSCA) by introducing the chemical cross-linker into the PVA/SSCA mix, and the hydrogen percent changes slightly. This is explained by the interaction of the (FA and MA) –COOH group or the –CHO group of GA with the –OH group of PVA to create an ester or ether bond by removing H₂O molecules, resulting in a drop in hydrogen ratios. The insertion of chemical cross-linkers doesn’t significantly affect the amount of Sulphur element (related to –SO₃H groups) for each group, indicating good stability of –SO₃H groups. It has been revealed that raising the amount of carbon and Sulphur content while decreasing the SSCA molar ratio increases the number of –SO₃H groups linked to the membrane and improves ionic conductivity. As a result, the modest shift in C/H ratios for each group suggests membrane composition stability after adding a chemical cross-linker. It provides a favorable indicator that the membrane structure has not altered appreciably. Furthermore, no known loss of backbone carbon has occurred [53]. In contrast, the C/S ratio decreased when SSCA concentration increased, suggesting a stronger polar group (–SO₃H) that enables membranes to have intensified ion exchange capacity and exponentially surface area.

3.6 Thermal stability

Thermal stability greatly influences the polyelectrolyte membrane’s working life in fuel cell applications. TGA thermograms of the obtained polyelectrolytic membrane were analyzed using (a Shimadzu TGA-50 analyzer, in Japan) in N₂ ambient conditions, at a heating rate of 10 °C/min. Figure 7 shows the TGA results for pristine PVA and chemically cross-linked polyelectrolytic membranes using different chemical cross-linkers. Pure PVA thermograms demonstrated three primary heat breakdown stages. The first region between 70 and 200 °C can be attributed to the loss of adsorbed water molecules. In contrast, a peak in the second region between 230 and 340 °C indicates that PVA chains with splitting –OH groups have been thermally degraded. The third region, located between 340 and 450 °C, is attributed to the polymer’s decomposition and carbonization. The TGA thermogram of the synthesized uncross-linked PVA/SSCA and modified cross-linked PVA/SSCA-based polyelectrolytic membrane shows three weight loss zones. For uncross-linked PVA/SSCA membrane the first area has been seen between 50 and 192 °C when both the physically bound water and the solvent become initially volatile. From 200 to 490°C, the SO₃ groups break, while the main polymer chains undergo cleavage between 490 and 800°C. The first region observed in cross-linked PVA/SSCA-GA membranes between 50 and 250 °C is related to water loss. The SO₃ groups and ester bonds, and acetal linkage break between 250 and 470 °C, whereas the major polymer chains cleave at 500–650 °C. For cross-linked PVA/SSCA-MA membrane the first area has been seen between 50 and 200 °C and is attributed to water loss. From 200 to 500 °C, the SO₃ groups and ester bonds break, while the main polymer chains undergo cleavage between 500 and 800 °C. The TGA curves of cross-linked PVA/SSCA-FA were also exhibited in three regions. The first area has been seen between 50 and 250 °C when both the physically bound water and the solvent become initially volatile. After the first session, chemically bonded water evaporated, absorbed, and retained in the hydrophilic PVA matrix [54]. Terminal polymer groups, such as sulfonic and other suspended groups, are broken down in the second weight loss area (290–470 °C). As temperatures rise, the main polymer chains become more prone to collapse. At high temperatures, C–C bonds are typically broken about 330 °C, and C–H, O–H, and C=O bonds are engaged [10]. The breakdown of cross-linking tunnels caused the third zone of weight loss, which ranged from 490 to 750 °C. The TGA curves showed that the temperatures in the weight loss areas increased with the addition of chemical cross-linkers, indicating that chemical cross-linkers improve the stability of PVA/SSCA-based membranes at high temperatures. Thus, these improved cross-linked PVA/SSCA-based membranes exhibit greater residual weights and thermal stability than non-cross-linked membranes [23]. The seamless transition between successive sections demonstrates that when exposed to diverse temperature ranges, the ion-conducting groups may stay attached to the primary polymer matrix longer. In general, adding chemical cross-linkers influenced thermal stability due to superior blend compatibility and the joining of polymer chains via chemical linkages, which gives a material a more rigid structure and perhaps a better-defined form [55].

3.7 Mechanical property

It is essential to evaluate the mechanical performance of prepared membranes to determine whether they can withstand the conditions of operation of a DMFC or any other type of Fuel Cell (FC) while resisting rapid penetration. Membrane mechanical strength is indicated by its tensile stress and stroke strain. Figure 8 displays the mechanical characteristics of the prepared cross-linked PVA/SSCA-based polyelectrolytic membranes, together with the data from Table 2. These results demonstrated that the non-crosslinked (PVA/1%SSCA) membrane’s tensile stress (N/mm²) and stroke strain (%) were valued at 20.27 N/mm² and 84.62%, respectively. These findings are consistent with those in Fig. 5, which showed that the PVA/SSCA membrane had lower cross-linking density because the polymeric matrices had not been further modified or cross-linked, and the van der Walls forces were most likely at work [56]. Increasing the molar ratio of SSCA in PVA/SSCA-based membranes increased tensile stress to a point before decreasing again. This is because the molecular arrangement, which is further modified by including GA, FA, and MA as chemical crosslinkers, alters the mechanical characteristics of the membranes formed [57]. Tensile stress and stroke strain values are 40.93 N/mm² and 280.38% in the presence of 2% SSCA, respectively. This phenomenon has been linked to incorporating extra ionic –SO₃H groups, which boost intramolecular forces rather than covalent interaction between the polymeric components. When the SSCA molar ratio in the membrane is raised to 3 wt%, the tensile stress reduces marginally to 23.88 N/mm². This is demonstrated by the deteriorating effects of adding SSCA, particularly at higher molar ratios [58]. The final stroke strain acts in the opposite direction, increasing from 84.62 to 281.40%, for (PVA/1%SSCA and PVA/3% SSCA) respectively, resulting in higher mobility of the polymeric segment. The incorporation of chemical cross-linkers enhances crystallinity, resulting in greater tensile stress values, with the maximum value of 54.96 N/mm² for PVA/2%SSCA-GA. Furthermore, because GA and FA or MA have a high degree of aldehyde or carboxylation, respectively, and SSCA has more –SO₃H groups, the high adhesion between polymeric matrix and chemical cross-linkers and more polymeric segmental restricting may be the explanation. Therefore, the modified cross-linked polymeric membranes exhibit higher strength than the unmodified membrane [59]. The mechanical strength results of modified PVA/SSCA-based polyelectrolytic membranes were promising for DMFC and operating conditions.

3.8 Water uptake and swelling ratio

Water uptake, and swelling ratio are the significant properties that play a crucial role in order to determine the proton conductance of membranes, with PEM wetting impacting proton conductivity and, consequently, fuel cell efficiency.

Consequently, water uptake, and swelling ratio for the preprepared polyelectrolytic membranes were evaluated, and the results are shown in Figs. 9 and 10, respectively.

These results suggested that combining SSCA and PVA increases the water uptake of the resulting blend-based membranes. Furthermore, the percentage of water intake was gradually enhanced by increasing the fraction of SSCA in the blends. Water absorption was lower in PVA/SSCA-based polyelectrolyte membranes made at a ratio of (1 wt% SSCA) than in those prepared at (3 wt.% SSCA). The likely cause is as follows: as the amount of conducting material (SSCA) increases, so does the availability of hydrophilic moieties, which improves the water sorption capacity of polyelectrolyte membranes in general. This finding shows that SSCA had a more significant effect than pure PVA polymer matrix in enhancing the water-holding capacity of the produced membranes [26]. On the other hand, the various cross-linking agents cause variances in the amount of water uptake utilizing PVA/SSCA membranes for each group. Surprisingly, non-cross-linked PVA/SSCA had a higher swelling ratio than PVA/SSCA-GA, PVA/SSCA-FA, and PVA/SSCA-MA [60]. This behavior may be explained by the degree of cross-linking, which rose most noticeably in the PVA/SSCA-GA and PVA/SSCA-FA membranes and resulted in a stiffer and compacted polymer structure with decreased water absorption. This result is consistent with what Parhi stated [61]. Additionally, Chang et al. [62] discovered that strongly cross-linked hydrogels could not sustain considerable water uptake inside their structure, which led to the discovery that less cross-linked hydrogels could. IEC and contact angle measurements indicated that the amount of hydrophilic (sulphonic) groups in polymer matrices was significantly connected to the water retention of the resulting membranes. The swelling ratio property of synthesized polyelectrolyte membranes is due to the equilibrium of osmotic and other dispersive forces. The osmotic force draws water into the polymer network, while the dispersion force exerted by the polymer chain opposes this [63]. The modified cross-linked PVA/SSCA blend membranes showed lower swelling values than the pristine PVA/SSCA membranes. The swelling variations of these membranes are directly proportional to water uptake, and higher swelling of polyelectrolytic membranes has been attributed to the more hydrophilic membrane surface, which is caused by the high-density hydrophilic groups in SSCA and PVA [64]. The incorporation of cross-linking agents to the PVA/SSCA polymer matrix reduced the accessibility of hydroxyl groups due to decreased segment mobility [65]. So, all modified cross-linked polyelectrolytic membranes have an optimal swelling ratio because much bigger swelling may cause membranes to dissolve in water, which is an undesired feature for a membrane used in PEMFC applications. The cathode side of the fuel cell produces water, and the membrane is always in touch with it. As a result, the fuel cell membrane must be stable in water. Dissolution in water causes the membrane to lose its function. Membranes should not be soluble in water, and if they are, their solubility should be as minimal as feasible.

3.9 Methanol uptake

To prevent fuel crossover in DMFC, when methanol is employed as a hydrogen source, the methanol absorption by the polyelectrolytic membrane must be reduced. The affinity and the gaps in the membrane’s structure that are available for liquid absorption play a significant role in a polymer membrane’s capacity to absorb methanol.

As a result, the ability of prepared PVA/SSCA and modified cross-linked PVA/SSCA electrolytic membranes to uptake methanol was examined. Figure 11 demonstrates the results. These data revealed that as the SSCA molar ratio grew, the percentage of methanol absorption reduced; the methanol uptake values reached 22.8% for (3 wt%) SSCA-based membranes and 26.65% for (1 wt.%) SSCA-based membranes, respectively. This was attributable to the insertion of low sulfonic –SO₃H groups, which lowered membrane affinity to methanol by increasing the distance between sulfonic acid groups as methanol saturation rose [66].

Similarly, adding a chemical cross-linker to PVA/SSCA blend-based membranes reduces methanol absorption, reaching 6.11% and 8.51 for PVA/3% SSCA-GA and PVA/3% SSCA-FA, respectively. These findings showed that the modified PVA/SSCA membranes had a more significant effect on lowering methanol holding capacity than the other non-crosslinked PVA/SSCA membranes, minimizing methanol crossing [67]. In contrast, they developed membranes with better water-holding capacity and absorbed less methanol. The hydrophilic/hydrophobic properties of the polymer matrix support this observation.

3.10 Contact angle measurements

The hydrophilicity of a polyelectrolyte membrane reveals its ability to absorb or swell with water, as well as its relationship to water durability, IEC, and proton conductivity. This attribute was evaluated using the water contact angle (WCA). Figure 12 depicts the water contact angle of PVA/SSCA and chemically cross-linked PVA/SSCA polyelectrolyte membranes.

The contact angles of unmodified PVA/SSCA membranes were 32.27°, 24.94°, and 18.13° for PVA-1%SSCA, PVA-2%SSCA, and PVA-3%SSCA membranes, respectively. This is due to the increased number of hydrophilic sulphonic groups substituted onto polymer chains as the molar concentration of SSCA utilized in the functionalization process is increased. Each group’s modified cross-linked PVA/SSCA membranes have a higher contact angle than non-crosslinked PVA/SSCA membranes.

For instance, the WCA values increased from 32.79° (PVA-1%SSCA) to 95.29° (PVA-1%SSCA-GA) in the case of membrane-based GA as a chemical cross-linker. It signified that the hydrophobicity surface of the PVA/SSCA membrane was enhanced. This might be explained by the development of hydrophobic ester and ether linkage between PVA’s hydroxyl groups and used chemical cross-linkers GA, FA, and MA, which reduces polar group’s number as demonstrated by FT-IR and Raman analysis and prevents water from absorbing onto the film surface [22]. Contrary to the other cross-linked membranes, the WCA value of the MA-crosslinked membrane dropped from 55.56° (PVA-1%SSCA-MA) to 34.83° when (PVA-3%SSCA-MA). This discrepancy might be attributed to free (–COOH and –OH) groups from the acid cross-linker addition, which is apparent in the SEM topography pictures and causes the films’ surface to crystallize, resulting in considerably reduced contact angles [68].

3.11 Ion exchange capacity (IEC)

The existence of exchangeable protons or sulfonic groups in a polymer matrix, which oversees retaining and transporting protons crucial to proton conduction from anode to cathode, makes IEC an indirect and valid estimate for proton conductivity. Figure 13 depicts IEC measurements of manufactured polymer electrolyte membranes.

The IEC value of un-crosslinked PVA/SSCA-based polyelectrolyte membranes grew considerably as the SSCA molar ratio in PVA/SSCA membranes increased, rising from 1.29 meq g⁻¹ for (1% SSCA) to 2.33 meq g⁻¹ for (3% SSCA). This behavior may be demonstrated by increasing the –SO₃H ionic groups on PVA/SSCA-based membranes, resulting in many reactive and exchangeable sites in the polymer matrix [69].

Furthermore, the protonic conductivity values and IEC of enhanced cross-linked PVA/SSCA-based membranes improved significantly, particularly when FA and MA were used as chemical cross-linkers, due to an increase in the number of charged negative ions (–COO H⁺ groups present in FA or MA) and redundancy of the cross-linking process in addition to (–SO₃ H⁺ groups present in SSCA) which were responsible for holding and transporting protons (H⁺) through the polymeric membrane [32]. The IEC values of membranes based on GA as a chemical cross-linker were lower when compared to other enhanced cross-linked membranes because cross-linking in this case restricts the movement of protons due to reduced inter and intra distance between polymer chains [70]. Still, they remained higher than Nafion 117 (0.91 meq g⁻¹) due to the presence of sulfonic groups associated with SSCA. Because cross-linking not only improves the water stability of the crosslinked membranes, but also successfully introduces negative surface charge, the developed membranes with PVA/3% SSCA-FA and PVA/3% SSCA-MA possessed the most substantial IEC data (3.31 meq g⁻¹ and 2.78 meq g⁻¹, accordingly) [71]. The differences in IEC findings were attributed to the type of chemical cross-linker used and how acid modifier agent and type altered the electrochemical properties of membranes. This is explained by the fact that as the acid content grew, so did the ionic conductivity and IEC of completely hydrated PVA/SSCA-modified membranes.

3.12 Proton conductivity (PC) measurement

Proton conductivity assessments of PVA/SSCA, PVA/SSCA-GA, PVA/SAA-FA, and PVA/SSCA-MA electrolytic membranes as a function of SSCA loading are shown in Fig. 14. Because of the presence of ionic groups in a lower ratio connected to PVA polymeric chains that may contain and transport ions, the clean membrane has a lower ionic conductivity (1.6 × 10^–4 Scm⁻¹). The water/methanol ratio and IEC values significantly influence membrane conductivity. Higher water intake promotes proton transport, whereas higher IEC promotes quicker proton conduction by shortening the distance between anionic groups [72].

The incorporation of SSCA boosted the proton conductivity of the membranes, which promotes proton transportation through the membrane due to the presence of a negative charge that may attract protons. The ionic conductivity rose when the quantity of SSCA in the PVA/SSCA polyelectrolytic membrane grew from 1 to 3%, reaching 0.041 and 0.065 Scm⁻¹ at PVA/1% SSCA and PVA/3% SSCA, respectively. This behavior pattern was like the IEC behavior that increased with SSCA content in PVA/SSCA-based electrolytic membranes [73].

The proton conductivity of synthetic membranes based on GA as a chemical cross-linker was lower when compared to non-modified PVA/SSCA and other enhanced cross-linked membranes because cross-linking in this case restricts the movement of protons due to reduced inter and intra polymer chain distance [74].

In contrast, because of the high surface charge density caused by the incorporation of FA and MA chemical cross-linkers to PVA/SSCA-based membranes, proton conductivity increases to 0.072 and 0.078 Scm⁻¹, respectively, for PVA/3%SSCA-FA and PVA/3%SSCA-MA, owing to the existence of such chemical cross-linkers that provide an ionic channel pass way and a wide surface area, especially at functionalized PVA/SSCA-FA [71].

Figure 15 shows the Nyquist and Bode plots of synthetic polyelectrolytic membranes, which are between the real part of the impedance (Z′) and the imaginary part of the impedance (Z″). Nyquist plot is very important to investigate the polarization resistance to determine the proton conductivity of synthetic membranes [75].

The specimens show the detrimental effect of frequency on the pattern of action of the impedance. The linear behavior of the Cole–Cole plot is known as indicating a capacitance and resistance linked in series; in this case, all synthetic samples displayed a circuit consisting of resistance and capacitor connected in parallel [76].

All samples have become more ideal and take semicircle-shape behavior at higher frequencies, which means samples can be excellent candidates for use in DMFC. The addition of chemical crosslinkers such as FA and MA into the membrane matrix boosted conductivity compared to unmodified PVA/SSCA, but the insertion of GA decreased conductivity. The existence of one sort of relaxation process was shown by the appearance of one arc in the impedance plane plot [77].

3.13 Oxidative stability analysis

The polymer electrolyte membrane durability is highly dependent on the resistance to oxidation. The stability of cross-linked PVA/SSCS-based membranes in Fenton’s reagent to oxidation was investigated and the obtained results were illustrated in Fig. 16. In general, all polyelectrolytic membranes exhibit oxidative stability when conductive material SSCA is incorporated along with the cross-linking agents GA, FA, and MA into PVA polymer matrix [78].

The modified cross-linked PVA/SSCA-based membranes exhibited significantly better oxidative stabilities than the uncross-linked membrane. Additionally, the existence of oxygen-containing groups like –SO₃H, –OH, and –COOH inside the polymeric matrix is thought to be the cause of this behavior. These oxygen groups are improving cross-linking density through the creation of hydrogen bonds with the polymer, which may hinder the attack of free radicals, increasing the membrane’s resistance to oxidation. Furthermore, the surface area is increased by the presence of cross-linking agents, improving the oxidative stability [79, 80].

3.13.1 Methanol permeability

Methanol crossing over PEM when DMFCs are functioning consumes fuel and destroys the catalyst, reducing fuel cell performance. In a diffusion cell with the membrane clamped between two units of aqueous methanol solution and deionized water, the methanol permeability of PVA/SSCA and modified cross-linked PVA/SSCA-blend polyelectrolyte membranes were tested. The amount of methanol penetrated vs time was charted, and the findings are shown in Fig. 17. Table 3 shows the results of determining the methanol permeability coefficient using the slope of the obtained line.

These results show that non-modified PVA/SSCA membranes had methanol permeability coefficients ranging from 4.10 × 10^–7 to 5.10 × 10^–7 cm²/s for PVA/1% SSCA and PVA/3%SSCA, respectively. In contrast, modified chemically cross-linked PVA/SSCA-based polyelectrolyte membranes had lower methanol permeability than non-modified PVA/SSCA membranes. As a result, PVA/1%SSCA-GA, PVA/2%SSCA-FA, and PVA/3%SSCA-MA membranes had methanol permeability values of 2.52 × 10^–6 cm²/s, 2.66 × 10^–7 cm²/s, and 3.19 × 10^–7 cm²/s, respectively, lower than non-crosslinked membranes.

Furthermore, the modified cross-linked membranes have methanol permeability values much lower than the Nafion® 117 comparable value (3.39 × 10^–6 cm²/s) [33]. Unlike the methanol absorption findings, the methanol permeability coefficient increased considerably with increasing SSCA concentration, reaching up to 5.40 × 10^–7 cm²/s for produced membranes employing PVA/3%SSCA.

On the one hand, the amount and state of water (freezing and unfreezing) in the prepared membrane matrix can be used to explain these results, and on the other, the predominant hydrophobic or hydrophilic interactions among methanol molecules in the aqueous solution that control their association dynamics and, ultimately, their permeation. Because of the increased temperature during DMFC operation, these effects may become exceedingly entangled and complicated. These findings demonstrated that a trade-off between several functional features was required to boost proton conduction while decreasing methanol crossover [81].

3.14 Membrane efficiency

An effective PEM for DMFC operation should have both a low permeability to methanol and a high proton conductivity. This implies that the proportional ionic conductivity to methanol permeability is more essential than methanol permeability. To measure the performance of polyelectrolyte membranes in DMFCs, the efficiency factor (φ), which is a ratio of ionic conductivity (σ) to methanol permeability (p), is utilized. This factor was determined and shown in Table 3 for PVA/SSCA and enhanced cross-linked PVA/SSCA blend-based membranes. These findings show that all modified cross-linked PVA/SSCA-based polyelectrolyte membranes outperformed non-modified PVA/SSCA membranes in terms of efficiency [82].

Furthermore, as the amount of SSCA in the polymer blend grew, so did the φ value of these membranes. The highest (3.4 × 10⁵) value was obtained for polyelectrolyte membranes produced with PVA/2%SSCA-FA.

In the same context, all blended membranes constructed with uncross-linked PVA/SSCA had a value greater than Nafion 117 (2.68 × 10⁵). However, the PVA/2%SSCA-FA synthesized polymer electrolyte membrane was determined to be the most efficient membrane, and any more SSCA did not significantly improve the membrane efficiency [83].