Granular polyether ether ketone (PEEK) grade 90P from Victrex. Chitosan (CS) by R&M Chemicals. Sulphuric acid (95–98 wt.%) by Chemiz and sodium hydroxide by Merck KGaA. Sodium chloride (99.0%) and dimethyl sulfoxide (99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Fig. 4a,b displays the FTIR spectra of CS, SCS, SPEEK, and its composite membrane. The comparison between the FTIR spectra of CS and SCS membranes is shown in Fig. 4a. Both spectra indicate a broad stretching of the hydroxyl group (O-H) that appears in the range of 3200–3400 cm⁻¹ [28,29]. The vibration of the primary and secondary amine salts group can be seen on stretches that occur at 2700–3100 cm⁻¹ and 2100 cm⁻¹. In lower wavenumbers, primary amine salts are represented by two peaks at 1650 and 1563 cm⁻¹, which are respectively asymmetric and symmetric. For SCS, sulfonic acid groups are shown as an asymmetric bend at 1350 cm⁻¹ and a symmetric bend at 1150 cm⁻¹ [30]. In comparison, the peaks observed in SCS are noticeably broader than those in CS, which can likely be attributed to the sulfonation process undergone by CS. Thus, the introduction of additional functional groups results in overlapping functional group peaks in the SCS spectrum. Therefore, it could be concluded that the sulfonation process of CS was a success.

As for the SPEEK membrane spectrum in Fig. 4b, an asymmetric sodium sulfonate (O=S=O) bend can be pinpointed at 1076 cm⁻¹ while its symmetric bend is at 1014 cm⁻¹ [31]. There is also a distinct broad stretch of hydroxide (O-H) group at 3403 cm⁻¹. Moreover, aryl carbonyl (C=O) groups were detected at 1645 cm⁻¹. However, it could also be ketones that conjugate between two aromatic rings to be in the range of 1670–1600 cm⁻¹ [30]. Therefore, the synthesis process of SPEEK was validated using FTIR spectra. The S2 spectrum shows a bend of the sulfonate group (S-O) at 705 cm⁻¹. Another asymmetric sulfonate group was detected at 1016 cm⁻¹_, whereas the symmetric bend vibration was at 1074 cm⁻¹ [16]. In addition, there is an alkyl-substituted ether (C-O) vibration at 1155 cm⁻¹ [32]. The amine group is found at 1644 cm⁻¹ which resulted from vibration absorption of the NH-SO₃H group [28]. There is also amine group located at 3440 and 3472 cm⁻¹. These two peaks indicate the presence of primary amine in the S2 from the SCS [30].

Fig. 5 shows XRD patterns of (a) CS, (b) SCS, (c) S0, and (d) S2. Patterns of CS and SCS show distinct differences, such as shifting of CS and SCS peaks at 2θ = 9° and 2θ = 12°, respectively. Moreover, a CS peak at 2θ = 19° was broader compared to the corresponding peak in the SCS pattern. This phenomenon could be the effect of the sulfonation process, where the sulfonated polymer was not able to absorb more water due to the loss of hydrogen bonds or the disorder in the polymer structure.

Through the deconvolution technique, sharp peaks at 11.9°, 18.9°, and 24.6° from the XRD pattern of the SCS membrane in Fig. 5b were detected. These peaks indicate an increased crystallinity domain of SCS under the sulfuric acid treatment. This could be explained by the interaction between the −NH3+ groups in the chitosan and anions of SO42− which influenced the changes in crystallinity [24]. Fig. 6b shows the graph of deconvolution for SCS, which was used to calculate the degree of crystallinity of the SCS. The average degree of crystallinity (X_c) for SCS is 33.33%, with the highest X_c as 85%. This value can be calculated using Eq. (8), where A_c represents the area of crystalline peaks of diffraction and A_a refers to the area of amorphous peaks of diffraction [33].(8)Xc=AcAc+Aa

In Fig. 5c, a broad peak was observed at 2θ = 20°, which implies an amorphous characteristic in SPEEK. As for S2 in Fig. 5d, the pattern exhibits the same peak as the SPEEK pattern, where the broadening of the peak corresponds to its amorphous state [34]. The similarity of patterns between SPEEK and S2 was due to the higher composition of SPEEK in S2 rather than SCS.

Water uptake is a critical characteristic and a fundamental component for maintaining proton conduction from the anode to the cathode [35]. Fig. 7a presents a graph of water uptake in relation to SCS concentration, which suggests that water uptake increases with higher SCS concentration in the membrane. Table 2 presents the data obtained from this characterization. Although the degree of swelling impacts both the diameter and thickness of the membranes, the change in diameter is used in this study for its simplicity to quantify. The expansion of the membranes in the plane dimension (diameter) is less restricted than its thickness, which is confined by the structure of the membranes and the fuel cell assembly.

The presence of sulfonic and hydroxyl groups in the membrane promotes the development of hydrophilic ion domains, which are crucial for water uptake and proton transport. As the membrane absorbs water, the ionic clusters expand and interconnect to form continuous hydrated channels that enable efficient ion migration. Hence, higher water uptake enhances proton conductivity by improving the connectivity of these hydrated pathways [36]. Notably, vehicle mechanisms are one of the proton conductive mechanisms in polymer membranes that involve the association of protons with water molecules to form hydronium ions such as H3O+,H5O2+,H9O4+, or similar complexes, which then diffuse across the membrane. Fig. 8 illustrates the proton transport mechanism in the vehicle-type model in SPEEK/SCS membranes.

Moreover, water could hydrate the proton exchange membranes to facilitate the proton transport, however the excess swelling can result in the deterioration of the membrane-catalyst interface, severe water drag or fuel drag and possible performance failure of the fuel cell [37]. The membranes with a high degree of swelling have a high potential to lead towards poor mechanical stability, low durability and thus decrease the fuel cell performance. Fig. 7b indicates the degree of swelling trend for the membranes, which shows that the degrees of swelling decrease as the SCS concentration in the membrane increases. Notably, an increase of SCS concentration in the membranes promotes greater water uptake and enhances the capacity of the membranes to resist swelling. This behaviour is due to a higher degree of crystallinity in SCS compared to SPEEK, which is demonstrated by the XRD patterns in Fig. 5. In the composite membranes, SCS serves as a filler to the SPEEK base. As water uptake increases, the polymer chains of SPEEK tend to expand but are still obstructed by SCS. As SCS content increases, the membranes become more compact, which restricts the polymer chain mobility. Therefore, an increase in SCS concentration leads to a decrease in the degree of swelling, resulting in improved dimensional stability of the membranes.

Ionic Exchange Capacity (IEC) is defined as the number of moles of fixed Sulfonate anion (SO3−) sites per gram of polymer [38]. Measurement of the IEC was conducted using the back titration method, which provides information on charge density in the membranes. The IEC also reflects the concentration of exchangeable hydrophilic groups, which corresponds to the content of fixed SO3− sites in the composite membranes, thus explaining its charged nature [39]. This factor is significant for conductivity due to its indication towards the quantity of exchangeable ion groups available for the proton transfer process [28]. Fig. 9 illustrates the presence of SCS in the membrane, resulting in an increase in the IEC values. The low IEC of S0 was primarily due to the absence of SCS, resulting in less formation of sulfonic acid groups (−SO3H) within the membrane [40]. Consequently, fewer functional sites were available for ion exchange, thereby reducing ion transport through the membrane [41]. The increase of IEC values in the composite membranes suggests that the number of SO3− sites in the membranes has increased thus water molecules present were able to facilitate proton transport from one site to another, particularly through Grotthuss mechanisms that provide proton carriers and establish hydrogen bond networks [16,42]. The degree of sulfonation (DS) of pure SPEEK and SCS was calculated using IEC and is shown in Table 3. The DS for pure SPEEK and SCS were measured at 74% and 2.1%, respectively, which were crucial as DS affected the IEC, proton conductivity, and degree of swelling. The DS of the membranes can be modified by the reaction conditions during the membrane synthesis, including acid concentration, temperature, and duration of the sulfonation process [4].

Fig. 10 depicts the Grotthuss-model transfer mechanism in SPEEK/SCS membranes conveying three possible manners for proton hopping that exist simultaneously within the membrane, which are M₁, M₂ and M₃. M₁ is proton hopping via base groups that traverses along sulfonic chains, M₂ is proton hopping via acid groups and traverses along amine chains, and M₃ is proton hopping via acid-base pairs and traverses along the sulfonic-amine interface [43].

Furthermore, IEC influences the membrane’s ability to absorb water, with higher IEC leading to greater water uptake as a result of enhanced hydrophilicity [25]. Water first solvates the fixed ionic groups and counter-ions, followed by filling the hydrophilic zone of the membrane for water uptake, as outlined in the successive steps of membrane hydration [44]. The elevated water uptake and IEC suggest that protons are able to traverse through a concentrated ionic network found within completely water-saturated sulfonated polymer membranes [45]. Thus, the sulfonic acid content significantly influences IEC, which is associated with water uptake.

In the vehicle mechanism, the proton did not migrate as a single H⁺ particle but bound to a vehicle, such as O2−,H2O, and NH3, forming OH−,OH3+, and NH4+ [46]. In this process, a proton was donated to a vehicle molecule, such as water, on one side of the membrane and formed a larger charged complex, such as a hydronium ion (H3O+) [47]. This protonated vehicle then diffused through the membrane. Once the vehicle reached the other side of the membrane, it released the proton, often by transferring it to another molecule or into the solution. The uncharged vehicle molecule could then diffuse back to the original side of the membrane to pick up another proton and complete the cycle. This mechanism was particularly dominant in membranes with high water content [48].

In comparison with the Grotthuss mechanism, the proton migrated through a hopping process that involved the sequential formation and breaking of hydrogen bonds between molecules [48]. The proton directly hopped between adjacent sites along a chain of hydrogen-bonded molecules without the entire molecule moving [47]. This process began when an excess proton was transferred into water in the form of a hydronium ion (H3O+). Hydrogen atoms of the water molecule carried a net positive charge and a sulfonic species (SO3−), which carried a net negative charge. The positively charged hydrogen atoms were attracted to the negatively charged sulfonate groups. Conversely, if a positively charged counter-ion was present, such as a proton or a metal cation, the negatively charged oxygen end of water molecules was attracted to the counter-ion. These ion-dipole interactions facilitated the formation of a network of hydrogen bonds between water molecules and the sulfonate group, as well as among the water molecules themselves. The proton then hopped from a donor molecule to an acceptor molecule by breaking and reforming a hydrogen bond. This transfer was accompanied by a reorientation of the water molecules in the chain. This cycle repeated, with each proton transfer creating a new relay point in the chain, allowing the proton to move rapidly through the network. Due to protonation and deprotonation steps being part of the Grotthuss mechanism, the increase in SO3− species made the number of proton-hopping events increase [46]. At the same time, the protonated vehicle that diffused through the membrane also increased, affecting the water absorption capacity of the membrane. As a result, this interaction promoted the development of hydrophilic ion domains and increased water uptake and IEC.

The PEM with high proton conductivity above 10⁻² S cm⁻¹ is essential, particularly for fuel cell application [49]. Two primary factors that influence proton conductivity are mobility of protons and development of ionic cluster channels in the membrane. Generally, protons were transported through ionic cluster channels, which significantly affected water uptake and resulted in well-developed ionic cluster channels in the presence of an adequate quantity of water molecules [36].

Furthermore, water uptake significantly influences proton conductivity in the membrane by establishing a continuous proton conduction pathway through the Grotthuss mechanism. It has been suggested that protons can be transported together with hydrated ionic domains (cluster network channel) and have a major impact on the connectivity of these hydrated ionic domains [36]. These proton transfers between ionic domains containing polar groups such as −SO3H are well established in the literature [50]. Hence, the formation of hydrogen-bonding networks between sulfonic groups and water molecules facilitates proton mobility, which lowers the energy barrier for conduction.

The proton conductivity of composite membranes of SPEEK/SCS is illustrated in Fig. 11. The graph demonstrates the properly dispersed SCS within the SPEEK membranes, which preserve ideal ionic pathways and membrane homogeneity. The diagram indicates the potential of the composite SPEEK with SCS as a proton conductor within the 10⁻² S cm⁻¹ range, as well as a minor reduction in conductivity as the composition of SCS increases. The graph also shows that conductivity did not increase as the SCS content increased. Addition of SCS increases the density of sulfonic acid groups, enhancing the number of proton exchange sites and water uptake as depicted in Fig. 7a. However, under excessive hydration conditions, the conductivity has continued to decrease. The decline in conductivity could mostly be due to SCS being more crystalline than SPEEK membranes, as shown in Fig. 6b,c. Increasing the concentration caused the structure to be more compact and limit the space, which restricted the polymer chain mobility and decreased the swelling but was good for dimensional stability. This statement agrees with the degree of swelling result as depicted in Fig. 7b. Limited space in the membranes reduced development of ionic cluster channels, which led to the reduction of the mobility of H⁺ and a decrease in retention area for water around sulfonic acid groups [51]. An increase of SCS also enhanced IEC values, indicating the successful incorporation of additional sulfonic acid sites from SCS. However, a further increase in SCS resulted in a slight decline and subsequent plateau of IEC values. This trend is likely due to microphase separation or aggregation of SCS within the SPEEK matrix, which reduces the accessibility of ionizable sites to the surrounding water molecules [52]. In addition, excessive SCS may introduce tortuous diffusion pathways that hinder proton accessibility and lower the effective IEC. Therefore, limited polymer chain mobility and reduction in the accessibility of ionizable sites to the surrounding water molecules might limit protons to move easily and hence contribute to the decrease in proton conductivity.