Development of polymer electrolyte membrane based on poly(Vinyl Chloride)/graphene oxide modified with zirconium phosphate for fuel cell applications

The function of a membrane in the fuel cell is critical to its success. The major component of a direct methanol fuel cell (DMFC) is the proton exchange membrane (PEM) which must have proton conductivity, thermal stability, mechanical qualities, and low methanol permeability. In this study case, the film-forming and structural properties of Polyvinyl chloride (PVC) impelled us to employ them for developing polyelectrolyte membranes (PEMs). To functionalize the resultant PEMs, Graphene oxide (GO) and zirconium phosphate (ZrP) were incorporated into polyvinyl chloride in different proportions. The structural and physical properties of PVC/GO-ZrP membranes were investigated by using a variety of techniques instance, Fourier transform infrared spectroscopy (FTIR), Scanning electron microscope (SEM), Transmission electron microscope (TEM), Thermogravimetric analyzer (TGA), universal testing machine, and water contact angle meter. Furthermore, water uptake, Methanol uptake, and ion exchange capacity (IEC) were measured. The results demonstrated that the membranes developed have enough characteristics to be valid in DMFCs.


Introduction
Population growth has soared worldwide energy consumption by leaps and bounds in recent years. As a result, conventional energy supplies, particularly nonrenewable ones, are in short supply. As a result, there is a global push to use sustainable and eco-friendly energy sources [1]. The membrane-based fuel cell is a revolutionary sustainable energy source with high efficiency, minimal resource use, a compact structure, low noise, and no pollutant emissions.
DMFCs are now of the utmost interest of these fuel cells, mainly for applications of portable batteries, because of their low-temperature and high-power density device and eco-friendly, remarkable efficiency, and cheap fuel use in transportation [2].
Furthermore, PEM is a critical component of DMFC systems since it works as a barrier to fuel crosslinks across electrolytes and for moving protons from anode to cathode.
Ionomeric polymers (ionomers) and PEMs have been employed in batteries, sensors, and low-temperature cells, among other electrochemical devices. Alternative energy is one conceivable application for (PEMFCs).
Lastly, there has been much interest in developing (PEMFC), mainly for vehicle power applications. In a PEMFC, a proton electrolyte membrane is placed between two porous electrodes with a catalyst [3]. The PEMFC is critical to the advancement of fuel cell technology. It has good proton conductivity as well as long-term mechanical stability. Nafion ® N-117, a per-fluorinated ionomer-based polymer with high chemical, thermal, and physical stability as well as high proton conductivity, is one commercially available PEM that has proven to be useful in PEMFC applications [4]. Aside from these benefits, the Nafion ® membrane has two drawbacks: They are quite expensive due to the risky and difficult manufacturing process, and they demonstrated greater methanol permeability in (DMFCs) [5]. High methanol crossover reduces both fuel usage efficiency and cathode performance. These drawbacks have sparked interest in alternative hydrocarbon polymer-based membranes [6].
The polymer electrolyte membranes were made of (PVC) polymer. In contrast, composite membranes were fabricated from inorganic materials, including silica, titania, zirconia, phosphotungstic acid (PWA), graphene oxide (GO), silicotungstic acid (SWA), montmorillonite (MMT), kaolinite, and hydroxyapatite (HAP) [7]. (PVC) was chosen due to its excellent chemical and physical features. In terms of properties of physical and chemicals, ion-exchange organic-inorganic membranes with an organic substrate and a dispersed inorganic phase containing functional (ionic) groups outperform both organic and total membranes inorganic, and they are likely to be employed in DMFC [8]. The phase inorganic is chosen according to its (IEC) and conductivity [9]. Fast proton conductors are a type of inorganic ion exchanger that has recently gained popularity.
These compounds are acid salts of tetravalent metals with structure [M (IV) HPO 4 [10,11]. Zirconium phosphate (ZrP) is a fast proton conductor due to its IEC is high and thermal durability up to 450 °C. However, its proton conductivity is relatively low [12]. Polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyethersulfone (PES), and polypropylene (PP) as polymeric materials are impregnated with a colloidal suspension of inorganic material to produce inorganic-organic composite membranes [13]. According to Previous research, particle size is small (varying from nano to micrometers), and the transfer of ZrP dispersion colloidal particles into a matrix allows inorganic particle dispersion [14] [15]. Solid inorganic proton conductors that are hydrophilic and proton conductors include Heteropoly acids (HPAs), metal hydrogen sulfates, and zirconium phosphates. Low prices, great mechanical in dry and wet states, and good thermal stability are all advantages of composite membranes [16,17]. We improved the chemical and physical properties of the membranes by using graphene oxide and zirconium Phosphate (ZrP) (by converting Zirconium oxychloride on the membrane surface), as well as the method of introducing proton conductors into the polymer, and the membrane was prepared by solvent evaporation.
The thermal and chemical stabilities of prepared (PVC-GO/ZrP) membranes were excellent. It demonstrated outstanding thermal stability up to a temperature of 250 °C, which is sufficient for fuel cell operations because the fuel cell worked between 80 and 140 °C. All the foregoing results show that the PVC-GO/ZrP membrane is suitable for electrochemical applications [18].

Fabrication of (GO) from natural purified Graphite
The GO flakes were created using purified natural Graphite's modified Hummers' technique [19]. The first 3.0 g of flakes of Graphite was mixed with sodium nitrate (1.5 g) and sulfuric acid concentrated (69 ml). The mixture froze in an ice bath to 0 °C then, potassium permanganate (9.0 g) was slowly added. After that, the temperature was raised to 35 °C and 2 h of stirring. Afterward, de-ionized (DI) water (138 ml) was gently added, and then, retain the reaction conditions by heating for fifteen minutes at 98 °C. The solution was chilled for 10 min in a water bath. Then, treatment with 420 mL of (H 2 O) and 3 mL of 30% hydrogen peroxide (H 2 O 2 ) occurred in the solution. After cooling the solution, the supernatant acidic was centrifuged off, and washed the leftover mixture with ethanol and hydrochloric acid until a neutral pH was reached after washing with D.I water. The leftover material was diluted softly and sonicated [20]. (Scheme 1) demonstrates the preparation of graphene oxide from natural Graphite.

Preparation of composite membrane from (PVC/GO)
PVC/GO composite membrane was prepared using the colloidal mixing procedure. Figure 1 depicts a protocol synthesis in which PVC was dissolved with a definite amount at 70 °C in THF, and GO powder Separately was dispersed and sonicated in THF for 1 h. Then, sonicated the mixture for another hour. The resulting dispersion homogeneous solution was cast in a petri dish glass and dried at room temperature, yielding flawless PVC/GO composite films [22]. (Scheme 2) demonstrates the bonding interaction between PVC and GO in schematic form, illustrating that chemical bonding occurs as a result of an ester linkage formation. Previous studies have demonstrated that halogen bonding can be obtained via the GO (which has an oxygen-containing group) and PVC [23,24] and can operate as an active site to anchor functional groups, thereby rendering compatibility between GO and PVC [25].

Preparation of ZrP ion-exchanger
By stirring 20 ml of H 3 PO 4 solution (0.5 M) with 10 (wt.%) of Zirconium oxychloride for 5 min, ZrP powder was synthesized. To keep the pH between 2 and 3, HCl was added to the solution with a few drops. Then, stirred the solution at room temperature for 2 h, and the solution became milky. After that, the precipitate was filtered and washed many times with (DI) water to produce the ZrP powder by removing excess acid before being roasted at 120 °C for 12 h.

Preparation of (PVC/GO-ZrP) membrane
The PVC/GO-ZrP membrane was prepared in two steps; first, the prepared PVC/GO membrane was immersed in various concentrations of ZrOCl 2 aqueous solution (5, 7.5, 10, 12.5, 15 wt. %) separately for 6 h at 80 °C. Second, after removing excessive solution from the surface, the ZrOCl 2 permeated PVC/GO membrane was soaked in H 3 PO 4 (0.5 M) for 48 h. followed by heating at 120 °C for 12 h. for ZrP creation on the PVC/GO membranes surface. [26].

Ion Exchange Capacity (IEC)
The IEC shows the proportion of hydrophilic ionizable groups in the membrane matrix, which are responsible for the membranes' ionic conductivity. The membranes' (IEC) were evaluated using acid-base titration. Membrane specimens with a definite weight were immersed for 24 h in NaCl (2 M).
Titration with phenolphthalein and sodium hydroxide (0.01 N) was used to determine the protons emitted from these samples. The following equation was used to calculate IEC: ( ∕ ) = × Where N, W, and V denote the concentration of the NaOH solution, the sample weight, and the titer of the NaOH, respectively [27].

Water uptake and methanol uptake
The sample was put for 24 h in de-ionized water at room temperature before being withdrawn from the water. The filter paper was used to remove the water from the surface by wiping the sample with it; then, the samples were weighed immediately. The following equation determined the water uptake: where W dry and W wet were the weights of the dry and wet membranes, respectively, a standard approach for testing methanol uptake involved substituting water with methanol. [28].

Tensile strength measurement
A universal testing machine (model AG-I, Shimadzu, Kyoto, Japan) was used to measure the tensile strength (TS) and elongation at break (E) of the polymer-based membranes at room temperature. The measurements were conducted with the cross-heads moving at a fixed rate (3 mm/min.) [29].

Water contact angle measurement
VCA contact angle device 2500 XE with analytic software (AST Products, Billerica, MA) and a CCD camera investigated the hydrophilicity of polymer blend-based membranes [30].

FT-IR spectral investigation
FT-IR Spectral analysis was used to investigate the structure of the polymeric substances and to confirm the success of the synthesis process in the range of 400-4000 cm −1 , Scheme 2 Bonding interaction between GO and PVC [22] with a scanning rate of the 30 s and resolution of 4 cm −1 using (Shimadzu FT-IR-8400 S, Japan) [31].

X-ray Diffractometry (XRD)
Shimadzu (XRD-6000) device with a back monochromatic and a Cu Anticathode. The scan radical was performed in reflection scanning mode with 2θ moved from 0 to 120° [32].

Scanning Electron Microscopy (SEM)
The microstructure and morphological characteristics of polymer-based membranes were examined using JEOL JSM-6360LA, SEM, Japan. Samples of the resulting membranes were tested at a 10 K from 500 to 5,000 magnification power [33].

Transmission Electron Microscopy (TEM)
(TEM) Graphite, graphene oxide, and ZrP were performed using a JEM-2000EX (Japan) at a 200 kV acceleration voltage. Before measurement, the suspended particles in ethanol were transferred to a copper grid (400 meshes) coated with a carbon film [34].

Thermogravimetric analysis (TGA)
Thermal degradation characteristics of polymer blend-based membranes were determined by the TGA device (Shimadzu TGA-50 at a rate of 10 °C/min and 800 °C ambient temperature [35].

Ion Exchange Capacity (IEC)
Ion exchange capacity indicates the number of exchangeable protons in the polymer matrix, which determine the proton conductibility. Therefore, the aforementioned prepared membranes were measured at room temperature. Pristine PVC membrane has no ion exchange capacity because it has no ionized functional groups in its backbones.
The IEC values of prepared PVC/GO composite membranes were measured and recorded in Table 1. It was observed that the IEC values were increased with increasing the proportion of GO, which possesses carboxyl groups that are responsible for IEC.
The IEC values of PVC/GO-ZrP membranes were determined and presented in Table 2. As expected, the IEC values were increased patently from 0.27 to 0.75 meq/g due to the modification (PVC/GO) membrane by immersing Table 1 Mean ± SD of IEC (meq/g) for the three concentrations of PVC membranes at different concentrations of GO -S = Statistically significant at P ≤ 0.05 -NS = Nonsignificant P < 0.05 -ʎ and € are the Overall p-value for Intra and inter-group comparisons, respectively. (ANOVA Test) -Small and Capital letters for Intra and inter-group comparison, respectively. And the means with different superscripts are statistically significantly different (S.D.) at P ≤ 0.05 (Tukey Post Hoc Test) GO  It was observed that the highest IEC value of 0.75 meq/g was recorded for PVC/GO-ZrP membrane at (15% ZrOCl 2 ) due to the increase in the degree of phosphorylation of PVC/ GO and/or increase in the proportion of ZrP in the blends. The electrochemical approach for determining the membrane's surface charge and the total number of exchangeable ion sites in the membrane is measured by IEC [36,37] (Figs. 2, 3, 4, 5 and 6).

Water uptake and methanol uptake
Polyelectrolyte membrane (PEM) proton conductivity is greatly influenced by its water amount, and hydration is a critical step toward improving the fuel cell's efficacy by enhancing its conductivity [38]. To eliminate fuel cross-over in DMFCs that use methanol as a source of hydrogen, the polyelectrolyte membrane's methanol absorption must be lowered [28].
Therefore, water and methanol uptake of prepared PVC/GO composite membranes were determined and presented in Table 3. It was observed that the percentage of water uptake of PVC/GO membranes was enhanced by increasing the molar concentration of GO., which possess hydroxyl groups, which are hydrophilic groups responsible for hydrophilicity and increasing water uptake. It can be shown that the water uptake values of PVC (2%) membranes at various GO concentrations exhibit the highest values when compared to other PVC membrane concentrations. Likewise, the values of water uptakes of PVC/ GO-ZrP membranes were increased by increasing the ZrP concentrations, as shown in Table 4. It is observed that the water uptake percentage increases in the presence of ZrP because this leads to the presence of phosphoric acid groups on the surface of the membrane in addition to the presence of hydroxyl groups and carboxylic acid groups from GO, which improves the hydrophilicity of membranes and thus increase water absorption [39]. The highest water uptake was 25.91% for PVC (3%) /GO (1.5%)-ZrP at (15% ZrOCl 2 ) membrane and this is a lower value in comparison with the water uptake for Nafion-117 (29.6%) [39].
In general, water uptake of the resultant membranes is significantly linked to the presence of hydrophilic (carboxylic and phosphoric) groups in the polymer matrix, as evidenced by IEC and contact angle measurements. On the other hand, pre-prepared membranes with a larger water-holding capacity had a lower methanol uptake. These observations can relate to polymer matrices and solvents' hydrophilic/hydrophobic behavior. [40] ( Table 5).

Mechanical properties
Mechanical characteristics are regarded as one of the most important elements in determining the potential efficiency of DMFCs. Rectangular strips' elongation and tensile strength at break were estimated in the dry state. Tensile strength denotes the highest stress created in a membrane during a tensile test, whereas elongation denotes the membrane's ability to stretch.
Tensile elongation indicates the ability of a material to deform before breaking; it is a more important design factor in choosing a proper material than many of the average prop-erties…" [41]. Later Menges and Boden [42] also reported on the significance of the relationship between elongation and brittleness. The brittleness of materials, whether naturally or aging, significantly affects performance and manifests itself in various properties. We can see from this that the elongation at break is important to brittleness. A material with high elongation at break will not be brittle; thus, an inverse relationship is proposed [43]. The mechanical characteristics of Pristine PVC and PVC/GO composite membranes were investigated, and the results are displayed in Table 6. The influence of tensile strength and membrane elongation was detected as good results as the elongation of the membrane increased due to the incorporation of GO into the PVC membrane, signifying that the PVC/GO composite membrane became more elastic. The mechanical characteristics of pure PVC membranes have been enhanced in general.
The tensile properties of prepared PVC/GO-ZrP composite membranes were measured and presented in Table 7. These results indicated that increasing the ZrP content improves the mechanical properties of the PVC/ GO films; These results are attributed to the addition of ZrP to PVC/GO films, which may enhance the mechanical properties to a certain extent; However, continuous addition of ZrP to PVC/GO films resulted in mechanical decline of the films.  It was observed that the PVC/GO-ZrP membrane at (12.5% ZrOCl 2 ) has a tensile strength of 20.91 ± 0.6 (MPa), which is higher than that of the Nafion-117 membrane, which has a tensile strength of 18.125 (MPa) [44].
Finally, we have found that adding GOMand ZrP particles to a polymer matrix may change the brittleness of the pure polymer. The change, whether an increase or decrease, depends on multiple factors (particle dispersion, filler matrix adhesion, the amount of filler added, etc.), and typically (but not always), the tensile elongation is reduced for polymer-GO/ ZrP composites [45].

Water contact angle
The surface wettability of a polyelectrolyte membrane indicates its tendency to swell or absorb water, which is related to IEC, water stability, and proton conductivity of the prepared membranes. As a result, the water contact angle was utilized to analyze this property. A lower contact angle, in general, indicates a more hydrophilic membrane surface [46].
Contact angle measurements of pristine PVC and PVC/ GO composite membranes are shown in Table 8. As the GO concentration in the polymer matrix elevated, the contact angle decreased, suggesting that the hydrophilicity of PVC/ GO hybrid membranes enhanced dramatically. Furthermore, water dropped on modified membranes permeated the membranes faster than water dropped on pristine PVC membranes. This is because of including GO with a significant number of hydrophilic functional groups (i.e., -O.H. and -COOH groups) [47].
Contact angle measurements of prepared PVC/GO-ZrP composite membranes are shown in Table 9. It can be seen that the hydrophilicity of the prepared membranes increases with increasing the quantity of zirconium phosphate. This is because zirconium phosphate contains phosphoric acid groups that increase the attraction of hybrid films to water molecules [48].

FT-IR Spectroscopy analysis
FT-IR spectra of Graphite and GO were presented in Fig. 7. It was observed that no significant peak was found in Graphite. Hence, the existence of carbonyl and carboxyl groups in the spectrum of GO suggested that Graphite was successfully oxidized into GO by the modified Hummers process [49,50]. A prominent band at 3419.90 cm −1 was mainly related to (O-H) vibration, which considerably improves the wettability of GO [46]. There was a peak at 1730 cm −1 , which is typical of carbonyl and/or carboxyl group vibration [51]. The 1629 cm −1 peak is connected with stretching vibration (C = C), which indicates the graphene structure. Moreover, the 1392 cm −1 peak is due to the bending vibration of (C-O-C), whereas the stretching vibration of (O = C-O) produced from the O-H group appeared at the 1220 cm −1 peak, reflecting the production of GO [52].  Fig. 8. The spectrum of ZrOCl 2 .8H 2 O powder shows that the band at 1623 cm −1 is attributed to the coordinated water "scissor" bending mode [53].
FT-IR spectral assignment of ZrP powder reveals the existence of distinctive peaks with ZrP (confirming ZrP production), leading to hydrogen bond formation at the P = O group. The 1639.55 cm −1 peaks were caused by the H-O-H. The two peaks remaining, at 511 cm −1 and 599 cm −1 , were caused by the Zr-O and (PO 4 ) groups, respectively. The peak at 1066.88 cm −1 was due to (P = O), and the 2378.31 cm −1 peak was caused by (POH). The peak at 3433.41 cm −1 was caused by -O.H. stretching and suggested the existence of water bound in ZrP [15,54].
FT-IR spectral assignments of pristine PVC, PVC/GO, and PVC/GO-ZrP composite membranes are shown in Fig. 9. The spectrum of pristine PVC membrane exhibit a characteristic band at 2939.61 cm −1 due to the C-H stretching vibrations. The bands at 960 cm −1 and 1259.56 cm −1 are attributed to Trans CH wagging vibrations and C.H. rocking, respectively. The 1384.94 cm −1 and 1066 cm −1 bands are caused by stretching CH 2 and C-C wagging, respectively. The C-Cl band stretching was found at 642 cm −1 [55].
FT-IR spectral assignment of PVC/GO composite membranes shows an increase in the peak intensity at 2923.82 cm −1 , which is attributed to the C-H stretching 17.14 ± 0.6 0.53 ± 0.03 Table 8 The water contact angle of prepared pristine PVC and PVC/ GO composite membranes

X-ray Diffraction (XRD) patterns
Natural Graphite and GO were analyzed by XRD patterns, as shown in Fig. 10. depicts the patterns of XRD used to characterize the crystalline nature and phase purity of the Graphite and a synthesized GO In the case of pristine Graphite, strong diffraction at (2θ = 26.4°) was observed, which is related to the (002) plane hexagonal lattice [57]. The equivalent interlayer distance was 0.33 nm. The oxidation of Graphite into GO increased interplanar space. This is connected to the creation of interpenetrating groups containing oxygen. A broad peak for GO, at 2θ = 10.8° due to the basal plane (001), shows intercalated different oxygen-containing functional groups and H 2 O molecules [58]. The equivalent gap between layers was 0.88 nm.
The increasing gap between layers from 0.33 nm to 0.88 nm gives rise to shifting the 2θ peak shift from 26° to 10.8°, indicating that Graphite has been fully oxidized [59]. The gap between layers elevates due to the intercalation of different oxygen-containing functional groups and water molecules [60].
XRD patterns of pristine PVC, PVC/GO, and PVC/ GO-ZrP composite membranes are shown in Fig. 11. It was observed that the amorphous phase of pristine PVC by detecting a distinct peak at about (2θ = 23°). The resulting PVC/GO, and PVC/GO-ZrP composite membranes have the same diffraction peaks as virgin PVC, according to XRD patterns. However, crystalline regions of PVC were suppressed by increasing the portion of GO in the blend membranes. These findings can be explained by the probable interactions between the membrane-forming components, which could prevent the polymer chains from stacking [61]. of prepared PVC/GO composite membranes with varying GO content (0.5, 1, and 1.5%) were shown in Fig. 12, compared to pristine PVC, which illustrates the influence of GO incorporation on membrane surface morphology. In general, the integration of GO substantially influences the characteristics of PVC, even at a modest percentage of added GO flakes [62]. It was observed that the surface of incorporating GO with PVC becomes rougher than a pure PVC membrane surface. On the surface of modified PVC membranes, a pore-like structure may be noticed in SEM pictures, which was strengthened by the inclusion of GO, which can decrease the stability of thermodynamics of PVC polymer solution and, therefore, considerably aid membrane development by enabling phase inversion [63]. The potential motive is that the PVC solution with large viscosity, loaded with GO (1.5%), acts as a physical impediment to solvent and non-solvent mass transfer, slowing and inhibiting the growth of macro-voids in the PVC/GO films. This is reliable to previous research on various nanoparticles, such as ZnO, Ag, TiO 2, and SiO 2 , which have were published in the literature [64,65]. The researchers discovered that increasing the GO loading induces a surface modification morphology of the prepared films, exposing an interconnected network of macro-pores. In addition, SEM micrographs of PVC/GO composites membrane indicate that the bulk of the GO is dispersed uniformly throughout the polymer matrix, with no agglomeration. Furthermore, because polymers form in the pores and galleries of GO, distinguishing the various phases of GO and PVC in PVC/GO composites is challenging [66,67].

Morphological and microstructure features
On the other side, morphological features of the PVC/ GO-ZrP composite membrane are shown in Fig. 13. It was observed that ZrP molecules were filling the pores of the PVC/GO composite membrane. Furthermore, these micrographs revealed that these membranes have a more uniform and compact structure, as well as a rougher surface than their corresponding based on ZrP content.
As a result, SEM analysis showed that GO and ZrP created a homogeneous composite with PVC filling the gaps between the GO sheets, resulting in a microporous morphology [22].

Transmission Electron Microscope (TEM)
TEM micrographs of surface examinations of natural graphite and synthetic graphene oxide are shown in Figs. 14 and 15, respectively to indicate the morphological properties. As the material was manufactured in powder form, a good view of the images revealed that the samples have multiple layers.
After oxidation of purified natural flake graphite (NFG) using the Hummers method, the synthesized GO sample has a laminar and thin structure with a size of around 1 mm 2 . Meanwhile, a wrinkled structure in GO was found due to edge-to-edge contacts. This can potentially increase the surface area of GO [68]. Furthermore, the unique structure of GO may facilitate its interaction with PVC polymers, increasing the mechanical properties of hybrid membranes [69].
The morphology of graphene oxide showed thick flat wafer layers, rough surface, uneven shape, and irregular particle size. Since the oxygen atoms were not removed and there was a large degree of exfoliation during the oxidation process, the structure was turbulent and not wrinkled. Moreover, fracture of the structure occurred during the oxidation process with an unexpected temperature or oxidizing amount, resulting in uneven shape and disordered particle size [70].
The wrinkled shell structure is morphologically similar to the thin film of graphene sheets. Rapid removal of intercalated oxygen and other functional groups between the layers led to exfoliation of the reducing medium and sonication, resulting in multi-layered thin layers and wrinkled plates [71]. As a result of the exfoliation process, the solid graphene structure became disordered, resulting in the plates crunching. As a result of correlating the results of graphene

Thermogravimetric analysis (TGA)
The outcome of the thermogravimetric analysis, as is widely known, is depicted as curves illustrating the thermal degradation of organic molecules against increasing temperature.
TGA analysis of pristine PVC and PVC/GO composite membranes is shown in Fig. 16. As can be seen, all membranes showed the same two-step decomposition pattern as pristine PVC films, with first-stage dissociation occurring between 200 and 300 °C, possibly due to loss of HCl (dehydrochlorination) [74]. In this temperature range, PVC dechlorination and loss of plasticizers and other additives may occur. This can be described as follows: With increasing temperature, Cl radicals from (-C-Cl) bonds accept hydrogen radicals from surrounding C-H groups to create a covalent bond (H-Cl), which leads to the liberation of HCl  [75,76].
The second stage reaches its apex at 495.2 °C due to events involving the breakage of (C-H) bonds and the fragmentation of the backbone.
On the other hand, first-stage dissociation of PVC/GO composite films was detected in the range of 200-300 °C, which might be attributed to the removal of oxygen-containing functional groups from GO. A second weight loss was seen in the temperature range of 300-500 °C, which could be attributed to the degradation of the alternative single and double carbon bonds, leading to the development of a carbonaceous residue [77]. This implies that PVC and GO have a significant interaction, leading to the composites' improved thermal stability.
TGA analysis of PVC/GO-ZrP composite membranes is shown in Fig. 17. As can be seen, the weight loss in the PVC/GO-ZrP membrane was caused by the loss of structurally bound water linked with ZrP particles. TGA results reveal the moisture retention capability of PVC/GO-ZrP membranes at greater temperatures than Nafion-117. ZrP particles demonstrated exceptional thermal stability and water-retaining capacity even at high temperatures. The degradation of the polymeric chain of the PVC support caused significant weight loss in the PVC/GO-ZrP membrane at 420 °C [78]. Finally, it was discovered that integrating GO and ZrP into the PVC matrix increases the thermal stability of the composites. It is also clear that modified membranes have sufficient thermal characteristics for use in DMFCs because thermal breakdown is found between 200-300 °C.  A slight change at 100 °C is found for all membranes, which corresponds to the weight loss of absorbed water. It also observed that phosphate group thermal degradation induces the transition at about 250 °C, while main chain scission causes the change at 380 °C (Tables 10 and 11).

Conclusion
This study focuses on preparing PVC/GO and PVC/GO-ZrP composite membranes using variable ratios of PVC, GO, and ZrP to optimize the performance and membrane properties in DMFCs operation. The structural and functional properties of prepared PVC and its blend-based membranes were characterized. The results show that polyelectrolyte composite membranes based on PVC modified with GO were prepared using the phase inversion method to improve their mechanical properties and hydrophilicity by incorporating GO modified by the Hummer's method and displayed many hydrophilic groups on the surface. The influence of GO incorporation on hybrid membranes' shape, thermal stability, mechanical stability, and hydrophilicity was primarily studied. The fabricated PVC/GO, and PVC/GO-ZrP composite membranes exhibited high thermal stability, so they withstood the high temperature (200-300 °C) for thermal decomposition onset. The IEC value increased with increasing the proportion of GO and ZrP in the polymer blend. Ion exchange capacity reached its maximum (0.75 meq/g) in the case of PVC/GO-ZrP composite membranes using (3%) PVC, (1.5%) GO, and the highest level of (ZrOCl 2 ).
Incorporation of the ZrP ion exchanger into the pores of the PVC/GO membrane increases the ion exchange capacity (IEC) of the membranes compared to the PVC/GO membranes, for example, the IEC values increased from 0.27 to 0.75 meq /g, when the PVC/GO composite films were   immersed at different concentrations of ZrOCl 2 solution (5 to 15 wt%), due to the increased content of ZrP on the membrane surface. The compositional and structural properties of prepared PVC/GO and PVC/GO-ZrP composite membranes were studied using FTIR, XRD, and SEM techniques. FT-IR spectra indicated the complete oxidation and conversion of Graphite into GO by indicating the characteristic peaks of carboxylic and carbonyl groups and the conversion of ZrOCl 2 .8H 2 O into ZrP by indicating the characteristic peaks of (P = O) group. XRD pattern indicated the conversion of natural Graphite to Go and indicated the presence of crystalline phase in the composite membranes. In summary, it was reasonable to conclude that the performance of PVC/GO composite membranes could be optimized by incorporating the ZrP appropriately to tune proton conduction and methanol barrier characteristics. In summary, it was plausible to deduce that the performance of PVC/GO composite membranes could be improved by effectively incorporating ZrP to improve proton conductivity and methanol barrier properties. As a result, these novel proton-conducting membranes are potential low-cost candidates for DMFC applications.

Conflicts of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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