Novel Nafion/Graphitic Carbon Nitride Nanosheets Composite Membrane for Steam Electrolysis at 110 °C

Hydrogen is expected to have an important role in future energy systems; however, further research is required to ensure the commercial viability of hydrogen generation. Proton exchange membrane steam electrolysis above 100 °C has attracted significant research interest owing to its high electrolytic efficiency and the potential to reduce the use of electrical energy through waste heat utilization. This study developed a novel composite membrane fabricated from graphitic carbon nitride (g-C3N4) and Nafion and applied it to steam electrolysis with excellent results. g-C3N4 is uniformly dispersed among the non−homogeneous functionalized particles of the polymer, and it improves the thermostability of the membranes. The amino and imino active sites on the nanosheet surface enhance the proton conductivity. In ultrapure water at 90 °C, the proton conductivity of the Nafion/0.4 wt.% g-C3N4 membrane is 287.71 mS cm−1. Above 100 °C, the modified membranes still exhibit high conductivity, and no sudden decreases in conductivity were observed. The Nafion/g-C3N4 membranes exhibit excellent performance when utilized as a steam electrolyzer. Compared with that of previous studies, this approach achieves better electrolytic behavior with a relatively low catalyst loading. Steam electrolysis using a Nafion/0.4 wt.% g-C3N4 membranes achieves a current density of 2260 mA cm−2 at 2 V, which is approximately 69% higher than the current density achieved using pure Nafion membranes under the same conditions.


Introduction
Hydrogen energy is expected to be of significant importance over the next 30 years, and the global hydrogen industry is projected to be worth 10 trillion dollars by 2050 [1]. Wind and solar power are distributed energy sources, which are difficult to deploy on the grid. This is due to the intermittent and unstable nature of renewable energy, which leads to a huge gap between energy supply and consumption [2]. However, they can be used to produce hydrogen, which is an efficient energy carrier, via economical and environmentally friendly methods. Hydrogen can be produced via water electrolysis, which is nonpolluting, does not require separation operations, and can be adjusted to suit the required hydrogen−production capacity [3].
Recent studies on water electrolysis have concentrated on proton exchange membrane (PEM) electrolysis for alkaline electrolytic cells, which produce high−purity hydrogen, operate at high electrical densities, exhibit transient responses, and utilize neutral water feeds [4][5][6][7][8][9][10]. However, the performance of PEM electrolysis must be improved to facilitate widespread commercial applications. To exploit the kinetics under high−temperature conditions, PEM electrolysis should be conducted at temperatures higher than 100 • C [11][12][13][14][15][16][17]. As the temperature increases, the ohmic losses of high−temperature PEM water electrolyzers (HT−PEMWEs) and the activated losses of the electrodes are reduced. Therefore, high However, no studies have applied g−C3N4 to PEM steam electrolysis. Therefore, this study aimed to prepare Nafion/g−C3N4 modified membranes by solution casting with g−C3N4 as the modified filler. The structure and physical properties of the membranes were characterized, and the mechanisms underlying the enhanced proton conductivity were investigated.

Preparation of g−C3N4 Nanosheets
g−C3N4 was produced as shown in Figure 1. Dicyandiamide (5 g) was heated in a tube furnace from room temperature at a heating rate of 5 °C min −1 and calcined at 550 °C for 4 h under an air condition. After cooling naturally, a yellow solid was obtained. Thereafter, g−C3N4 was produced by pulverizing the resulting product into a powder [38].

Fabrication of the Nafion/g−C3N4 Membrane
The Nafion/g−C3N4 composite membranes were produced using the solvent−molding technique, as shown in Figure 1. Firstly, 100 g of Nafion D520 ionomer was vaporized at 80 °C . Thereafter, 95 g of DMSO was used to dissolve the resin to obtain a 5 wt.% Nafion solution. g−C3N4 was dispersed in 16 g of the Nafion solution and subjected to vigorous sonication. The g−C3N4 mass fraction (relative to Nafion) in the composite membranes varied from 0-1.0%. The Nafion/g−C3N4 mixtures were inverted onto recess plates and dried in an oven at 80 °C for 24 h and at 120 °C for 12 h. The samples were naturally cooled to 25 °C , and then, the modified membranes were obtained by peeling them from the plates. The membranes were immersed in 0.5 mol L −1 H2SO4 at 80 °C for 3 h, and then heated in ultrapure water at 80 °C for 1 h, which was repeated for three times. Finally, the membranes were air−dried for further study. The membranes were labeled Nafion/X wt.% g−C3N4 depending on the mass of g−C3N4 doping. The thicknesses of the composite membranes were 49−53 μm, which was similar to that of the Nafion 212 membranes (50.8 μm).

Physical Characterization
The micromorphologies of the g−C3N4 nanosheets and modified membranes were observed using transmission electron microscopy (TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, SU8020). The crystal structures of g−C3N4 and the modified membranes was examined using X−ray diffraction (XRD, Empyrean−100). The chemical moieties of the g−C3N4 and modified membranes were examined using Fourier−transform

Fabrication of the Nafion/g-C 3 N 4 Membrane
The Nafion/g-C 3 N 4 composite membranes were produced using the solvent−molding technique, as shown in Figure 1. Firstly, 100 g of Nafion D520 ionomer was vaporized at 80 • C. Thereafter, 95 g of DMSO was used to dissolve the resin to obtain a 5 wt.% Nafion solution. g-C 3 N 4 was dispersed in 16 g of the Nafion solution and subjected to vigorous sonication. The g-C 3 N 4 mass fraction (relative to Nafion) in the composite membranes varied from 0-1.0%. The Nafion/g-C 3 N 4 mixtures were inverted onto recess plates and dried in an oven at 80 • C for 24 h and at 120 • C for 12 h. The samples were naturally cooled to 25 • C, and then, the modified membranes were obtained by peeling them from the plates. The membranes were immersed in 0.5 mol L −1 H 2 SO 4 at 80 • C for 3 h, and then heated in ultrapure water at 80 • C for 1 h, which was repeated for three times. Finally, the membranes were air−dried for further study. The membranes were labeled Nafion/X wt.% g-C 3 N 4 depending on the mass of g-C 3 N 4 doping. The thicknesses of the composite membranes were 49−53 µm, which was similar to that of the Nafion 212 membranes (50.8 µm).

Physical Characterization
The micromorphologies of the g-C 3 N 4 nanosheets and modified membranes were observed using transmission electron microscopy (TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, SU8020). The crystal structures of g-C 3 N 4 and the modified membranes was examined using X−ray diffraction (XRD, Empyrean−100). The chemical moieties of the g-C 3 N 4 and modified membranes were examined using Fourier−transform infrared (FTIR) spectroscopy (Nicolet iS50). The thermal stability was examined via the thermogravimetric analysis (TGA, STA449F5) with a warming rate of 10 • C min −1 under Membranes 2023, 13, 308 4 of 12 a N 2 atmosphere. The mechanical strengths of the modified membranes were examined using a universal testing machine (WDW−10).

Water Absorption and Degree of Swelling
The amount of water absorbed by a membrane can be determined by comparing its wet and dry masses. To determine the wet mass of the membrane, the membrane was first soaked in ultrapure water at 30 • C for 24 h; thereafter, the surface of the membrane was dried rapidly using filter paper before the sample was weighed. To determine the dry mass of the membrane, the membrane was dried in an oven at 80 • C for 24 h. The water absorption rate was calculated using the Equation (1): where M wet and M dry are the wet and dry masses of the membrane, respectively.
To determine the degree of swelling, the dry membranes were immersed in ultrapure water for 24 h. The degree of swelling was calculated by comparing the dry and wet areas of the membranes based on the Equation (2): where A wet and A dry are the wet and dry membrane areas, respectively.

Proton Conductivity Tests
The proton conductivities of the membranes were determined via electrochemical impedance spectroscopy (EIS) with a Gamry Interface 5000E with a frequency range of 1 MHz to 1 Hz and at a measurement potential amplitude of 10 mV.
For working temperatures of 30-90 • C, the membranes were maintained at the working temperature for 20 min before the proton conductivity tests (to reduce measurement errors), and then, they were soaked in ultrapure water. The proton conductivity (σ) of these samples were determined using the Equation (3): where L is the distance between the two electrodes; R is the resistance of the sample; W is the width of the sample; and d is the single membrane thickness. For working temperatures of 100-120 • C, the proton conductivity in the direction of membrane penetration was tested. The uncoated catalyst membrane was sandwiched between two gas diffusion layers (GDLs) and made into a membrane electrode assembly by hot pressing. The effective area of the sample was 5 cm 2 . Tests were conducted at ambient pressure using the HT−PEMWE setup with a water intake of 15 cm 3 min −1 . The proton conductivities of these samples were determined using the Equation (4): where L is the thickness of a single membrane; R is the resistance of the sample; and A is the area of the electrode.

Electrolysis Tests
The high−temperature PEM steam electrolysis polarization curves were recorded using a custom−built electrolysis platform, as shown in Figure 2a. Steam was supplied to the anode from an evaporator at 180 • C, and the inlet air path was insulated with heating tape at 140 • C. Water was pumped into the evaporator at a rate of 15 cm 3 min −1 , and it evaporated completely. The electrolyzer was warmed by heating bars to the working temperature for 30 min before steam was introduced into the electrolyzer. The electrolysis tests were conducted at 110 • C and ambient pressure. The effective area of the electrolyzer is 5 cm 2 . Ir black was used as the anode catalyst, 70 wt.% Pt/C was used as the cathode catalyst, and Nafion was used as the binder. The Ir loading was 1.5 mg cm −2 and the Pt loading was 0.5 mg cm −2 . The anode gas diffusion layer was Pt−plated Ti felt, and carbon paper was used as the cathode gas diffusion layer. Gas diffusion electrodes (GDEs) are prepared by covering the catalyst with GDLs using a spraying method. The PEM is placed between two GDEs and held in place by polyester frames, which is thermally pressed to form a membrane electrode assembly (MEA). The flow field of the bipolar plates is a parallel flow field. The cell is assembled in the structure shown in Figure 2b.
Membranes 2023, 13, x FOR PEER REVIEW 5 of 13 rated completely. The electrolyzer was warmed by heating bars to the working temperature for 30 min before steam was introduced into the electrolyzer. The electrolysis tests were conducted at 110 °C and ambient pressure. The effective area of the electrolyzer is 5 cm 2 . Ir black was used as the anode catalyst, 70 wt.% Pt/C was used as the cathode catalyst, and Nafion was used as the binder. The Ir loading was 1.5 mg cm −2 and the Pt loading was 0.5 mg cm −2 . The anode gas diffusion layer was Pt−plated Ti felt, and carbon paper was used as the cathode gas diffusion layer. Gas diffusion electrodes (GDEs) are prepared by covering the catalyst with GDLs using a spraying method. The PEM is placed between two GDEs and held in place by polyester frames, which is thermally pressed to form a membrane electrode assembly (MEA). The flow field of the bipolar plates is a parallel flow field. The cell is assembled in the structure shown in Figure 2b.

Characterization of g−C3N4
g−C3N4 has a clear two−dimensional sheet−like morphology with minimal aggregation (Figure 3a-c). Energy−dispersive spectroscopy (EDS) analysis indicates that N is uniformly distributed throughout the g−C3N4 nanosheets ( Figure 3d). The elemental composition of g−C3N4 is determined using EDS. g−C3N4 is composed mainly of C (42.98 at.%) and N (57.02 at.%) with a C/N ratio of 0.754 (approximately equal to the theoretical figure of 0.75). These results indicate the effective synthesis of g−C3N4.  rated completely. The electrolyzer was warmed by heating bars to the working temperature for 30 min before steam was introduced into the electrolyzer. The electrolysis tests were conducted at 110 °C and ambient pressure. The effective area of the electrolyzer is 5 cm 2 . Ir black was used as the anode catalyst, 70 wt.% Pt/C was used as the cathode catalyst, and Nafion was used as the binder. The Ir loading was 1.5 mg cm −2 and the Pt loading was 0.5 mg cm −2 . The anode gas diffusion layer was Pt−plated Ti felt, and carbon paper was used as the cathode gas diffusion layer. Gas diffusion electrodes (GDEs) are prepared by covering the catalyst with GDLs using a spraying method. The PEM is placed between two GDEs and held in place by polyester frames, which is thermally pressed to form a membrane electrode assembly (MEA). The flow field of the bipolar plates is a parallel flow field. The cell is assembled in the structure shown in Figure 2b.

Characterization of g−C3N4
g−C3N4 has a clear two−dimensional sheet−like morphology with minimal aggregation (Figure 3a-c). Energy−dispersive spectroscopy (EDS) analysis indicates that N is uniformly distributed throughout the g−C3N4 nanosheets ( Figure 3d). The elemental composition of g−C3N4 is determined using EDS. g−C3N4 is composed mainly of C (42.98 at.%) and N (57.02 at.%) with a C/N ratio of 0.754 (approximately equal to the theoretical figure of 0.75). These results indicate the effective synthesis of g−C3N4.  The crystal structure of g-C 3 N 4 is shown in Figure 4a. Two strong absorption peaks are observed in the g-C 3 N 4 spectrum at 27.43 • and 12.78 • , which corresponded to (002) and (100) planes, respectively. These results are in agreement with those reported by Yuan et al. [39], and they indicate that g-C 3 N 4 was successfully synthesized [35,36].
The crystal structure of g−C3N4 is shown in Figure 4a. Two strong absorption peaks are observed in the g−C3N4 spectrum at 27.43° and 12.78°, which corresponded to (002) and (100) planes, respectively. These results are in agreement with those reported by Yuan et al. [39], and they indicate that g−C3N4 was successfully synthesized [35,36]. Figure 4b shows the FT−IR spectrum of g−C3N4, confirming the production of g−C3N4. The strong peak at 803 cm −1 is the signature peak of the triazine ring, and the characteristic peaks between of 1250 and 1650 cm −1 correspond to the tensile peaks of N−(C)3 and C−NH−C [40,41]. The broad peak at approximately 3156 cm −1 is associated with the defective positions of the primary and secondary amines and their intermolecular hydrogen bonding interactions [42].

Properties of Nafion/X wt.% g−C3N4 Modified Membranes
The dispersion of g−C3N4 in Nafion is investigated using SEM, and elemental distribution is detected using EDS. g−C3N4 are evenly dispersed with negligible aggregation in Nafion, as shown in Figure 5, owing to the good surface interactions between g−C3N4 and Nafion and the large surface area of g−C3N4 [43]. Furthermore, the uniform g−C3N4 distribution enhances the tensile strength of the membrane, which was consistent with the test results for the modified membranes. Doping ratios higher than 1.0% have also been experimented. However, excessively high doping ratios can cause significant agglomeration of g−C3N4, which severely decreases the mechanical properties and homogeneity of the membrane, and even prevents the formation of the membrane. Two peaks at 2θ ≈ 17° and 2θ ≈ 39° were present in all the modified membranes (Figure 6a). The broad peak at 2θ ≈ 17° is composed of two peaks and can be decomposed into two peaks at 2θ = 16.6° and 2θ = 17.3°. Information about the morphology of the main chain in Nafion is represented by a peak at 2θ ≈ 16° and the crystallinity of the main chain is represented by a peak at 2θ ≈ 17.5° [44]. Therefore, the XRD patterns show that the addition of g−C3N4 had negligible effects on the crystallinity of the Nafion membranes. Figure 6b shows the FT−IR spectra of the modified membranes. For the Nafion (perfluorosulfonic acid) membranes, the typical absorption vibration peaks originate from the −SO3 group and the C−O−C groups. The stretching vibration absorption peak of −CF2−CF2 appeares at 1150 cm −1 and the typical peaks of −SO3 and C−O−C on the side chains are  Figure 4b shows the FT−IR spectrum of g-C 3 N 4 , confirming the production of g-C 3 N 4 . The strong peak at 803 cm −1 is the signature peak of the triazine ring, and the characteristic peaks between of 1250 and 1650 cm −1 correspond to the tensile peaks of N−(C) 3 and C−NH−C [40,41]. The broad peak at approximately 3156 cm −1 is associated with the defective positions of the primary and secondary amines and their intermolecular hydrogen bonding interactions [42].

Properties of Nafion/X wt.% g-C 3 N 4 Modified Membranes
The dispersion of g-C 3 N 4 in Nafion is investigated using SEM, and elemental distribution is detected using EDS. g-C 3 N 4 are evenly dispersed with negligible aggregation in Nafion, as shown in Figure 5, owing to the good surface interactions between g-C 3 N 4 and Nafion and the large surface area of g-C 3 N 4 [43]. Furthermore, the uniform g-C 3 N 4 distribution enhances the tensile strength of the membrane, which was consistent with the test results for the modified membranes. Doping ratios higher than 1.0% have also been experimented. However, excessively high doping ratios can cause significant agglomeration of g-C 3 N 4 , which severely decreases the mechanical properties and homogeneity of the membrane, and even prevents the formation of the membrane. 023, 13, x FOR PEER REVIEW 6 of 13 The crystal structure of g−C3N4 is shown in Figure 4a. Two strong absorption peaks are observed in the g−C3N4 spectrum at 27.43° and 12.78°, which corresponded to (002) and (100) planes, respectively. These results are in agreement with those reported by Yuan et al. [39], and they indicate that g−C3N4 was successfully synthesized [35,36]. Figure 4b shows the FT−IR spectrum of g−C3N4, confirming the production of g−C3N4. The strong peak at 803 cm −1 is the signature peak of the triazine ring, and the characteristic peaks between of 1250 and 1650 cm −1 correspond to the tensile peaks of N−(C)3 and C−NH−C [40,41]. The broad peak at approximately 3156 cm −1 is associated with the defective positions of the primary and secondary amines and their intermolecular hydrogen bonding interactions [42].

Properties of Nafion/X wt.% g−C3N4 Modified Membranes
The dispersion of g−C3N4 in Nafion is investigated using SEM, and elemental distribution is detected using EDS. g−C3N4 are evenly dispersed with negligible aggregation in Nafion, as shown in Figure 5, owing to the good surface interactions between g−C3N4 and Nafion and the large surface area of g−C3N4 [43]. Furthermore, the uniform g−C3N4 distribution enhances the tensile strength of the membrane, which was consistent with the test results for the modified membranes. Doping ratios higher than 1.0% have also been experimented. However, excessively high doping ratios can cause significant agglomeration of g−C3N4, which severely decreases the mechanical properties and homogeneity of the membrane, and even prevents the formation of the membrane. Two peaks at 2θ ≈ 17° and 2θ ≈ 39° were present in all the modified membranes (Figure 6a). The broad peak at 2θ ≈ 17° is composed of two peaks and can be decomposed into two peaks at 2θ = 16.6° and 2θ = 17.3°. Information about the morphology of the main chain in Nafion is represented by a peak at 2θ ≈ 16° and the crystallinity of the main chain is represented by a peak at 2θ ≈ 17.5° [44]. Therefore, the XRD patterns show that the addition of g−C3N4 had negligible effects on the crystallinity of the Nafion membranes. Figure 6b shows the FT−IR spectra of the modified membranes. For the Nafion (perfluorosulfonic acid) membranes, the typical absorption vibration peaks originate from the −SO3 group and the C−O−C groups. The stretching vibration absorption peak of −CF2−CF2 appeares at 1150 cm −1 and the typical peaks of −SO3 and C−O−C on the side chains are Two peaks at 2θ ≈ 17 • and 2θ ≈ 39 • were present in all the modified membranes ( Figure 6a). The broad peak at 2θ ≈ 17 • is composed of two peaks and can be decomposed into two peaks at 2θ = 16.6 • and 2θ = 17.3 • . Information about the morphology of the main chain in Nafion is represented by a peak at 2θ ≈ 16 • and the crystallinity of the main chain is represented by a peak at 2θ ≈ 17.5 • [44]. Therefore, the XRD patterns show that the addition of g-C 3 N 4 had negligible effects on the crystallinity of the Nafion membranes.  [45,46]. After doping with g−C3N4, the intensity of the peak between 1130 and 1250 cm −1 reduced, which may be caused by C−F···H−N hydrogen bonding. As shown in Figure 7, doping with g−C3N4 enhances the thermostability of the modified membrane. All the membranes exhibit a typical weight−reducing pattern [47,48]. The weight loss below 140 °C occurred as residual water was lost from the membranes. Between 140-250 °C, no significant weight loss was noted, and the membranes appeared to be stable. For pure Nafion membranes, the first loss at 250-350 °C occurred owing to the disconnection of the sulfonic acid group, whereas the second loss at 380-450 °C occurred owing to side−chain degradation. By contrast, for the composite membranes, the first loss interval did not begin until 340 °C. The mass loss at 340-400 °C is mainly caused by the decomposition of the side chains. As shown in Figure 7a, the main loss interval (accounting for approximately 70% of the total loss) in all the membranes occurs above 450 °C. This loss was caused by the thermal degradation of the Teflon backbone. Derivative thermogravimetric (DTG) analysis is shown in Figure 7b. In summary, the acid-base interaction between g−C3N4 and Nafion improves the thermostability of the modified membranes compared to that of the pure Nafion membrane [49].

Water Absorption, Swelling Degree, and Mechanical Properties
Appropriate water absorption can enhance the proton conductivity of the membranes and facilitate electrolysis at high temperatures. As presented in Table 1, g−C3N4 doping significantly increases the amount of water absorbed by the membranes, which occurs because of the strong attraction between g−C3N4 and the water molecules. The water absorption rate of the composite membrane with 0.8 wt.% doping is 30.65%, which is   [45,46]. After doping with g-C 3 N 4 , the intensity of the peak between 1130 and 1250 cm −1 reduced, which may be caused by C−F···H−N hydrogen bonding.
As shown in Figure 7, doping with g-C 3 N 4 enhances the thermostability of the modified membrane. All the membranes exhibit a typical weight−reducing pattern [47,48]. The weight loss below 140 • C occurred as residual water was lost from the membranes. Between 140-250 • C, no significant weight loss was noted, and the membranes appeared to be stable. For pure Nafion membranes, the first loss at 250-350 • C occurred owing to the disconnection of the sulfonic acid group, whereas the second loss at 380-450 • C occurred owing to side−chain degradation. By contrast, for the composite membranes, the first loss interval did not begin until 340 • C. The mass loss at 340-400 • C is mainly caused by the decomposition of the side chains. As shown in Figure 7a, the main loss interval (accounting for approximately 70% of the total loss) in all the membranes occurs above 450 • C. This loss was caused by the thermal degradation of the Teflon backbone. Derivative thermogravimetric (DTG) analysis is shown in Figure 7b. In summary, the acid-base interaction between g-C 3 N 4 and Nafion improves the thermostability of the modified membranes compared to that of the pure Nafion membrane [49].  [45,46]. After doping with g−C3N4, the intensity of the peak between 1130 and 1250 cm −1 reduced, which may be caused by C−F···H−N hydrogen bonding. As shown in Figure 7, doping with g−C3N4 enhances the thermostability of the modified membrane. All the membranes exhibit a typical weight−reducing pattern [47,48]. The weight loss below 140 °C occurred as residual water was lost from the membranes. Between 140-250 °C, no significant weight loss was noted, and the membranes appeared to be stable. For pure Nafion membranes, the first loss at 250-350 °C occurred owing to the disconnection of the sulfonic acid group, whereas the second loss at 380-450 °C occurred owing to side−chain degradation. By contrast, for the composite membranes, the first loss interval did not begin until 340 °C. The mass loss at 340-400 °C is mainly caused by the decomposition of the side chains. As shown in Figure 7a, the main loss interval (accounting for approximately 70% of the total loss) in all the membranes occurs above 450 °C. This loss was caused by the thermal degradation of the Teflon backbone. Derivative thermogravimetric (DTG) analysis is shown in Figure 7b. In summary, the acid-base interaction between g−C3N4 and Nafion improves the thermostability of the modified membranes compared to that of the pure Nafion membrane [49].

Water Absorption, Swelling Degree, and Mechanical Properties
Appropriate water absorption can enhance the proton conductivity of the membranes and facilitate electrolysis at high temperatures. As presented in Table 1, g−C3N4 doping significantly increases the amount of water absorbed by the membranes, which occurs because of the strong attraction between g−C3N4 and the water molecules. The water absorption rate of the composite membrane with 0.8 wt.% doping is 30.65%, which is

Water Absorption, Swelling Degree, and Mechanical Properties
Appropriate water absorption can enhance the proton conductivity of the membranes and facilitate electrolysis at high temperatures. As presented in Table 1, g-C 3 N 4 doping significantly increases the amount of water absorbed by the membranes, which occurs because of the strong attraction between g-C 3 N 4 and the water molecules. The water absorption rate of the composite membrane with 0.8 wt.% doping is 30.65%, which is approximately 74% higher than that of pure Nafion. The increased g-C 3 N 4 content is expected to form larger ion clusters and produce more hydrogen bonds, which can enhance proton transport. The mechanical properties of the membranes are listed in Table 1. The addition of g-C 3 N 4 increases the mechanical stability. As shown in Figure 8, the elongation at the breaking point of the pure Nafion membrane is 135.15%, and the tensile strength is 12.03 MPa. By comparison, the modified membranes had a higher tensile strength and lower elongation at the breaking point. The Nafion/0.8% g-C 3 N 4 membrane has the highest tensile strength of 14.63 MPa, which is approximately 22% higher than that of pure Nafion. The test results indicate that g-C 3 N 4 doping can be used to enhance the mechanical properties of the membrane. approximately 74% higher than that of pure Nafion. The increased g−C3N4 content is expected to form larger ion clusters and produce more hydrogen bonds, which can enhance proton transport. The mechanical properties of the membranes are listed in Table 1. The addition of g−C3N4 increases the mechanical stability. As shown in Figure 8, the elongation at the breaking point of the pure Nafion membrane is 135.15%, and the tensile strength is 12.03 MPa. By comparison, the modified membranes had a higher tensile strength and lower elongation at the breaking point. The Nafion/0.8% g−C3N4 membrane has the highest tensile strength of 14.63 MPa, which is approximately 22% higher than that of pure Nafion. The test results indicate that g−C3N4 doping can be used to enhance the mechanical properties of the membrane.

Proton Conductivity
Proton conductivity is an important characteristic that determines the electrolytic properties of the membranes; therefore, it is evaluated using EIS. The proton conductivities of the Nafion/g−C3N4 modified membranes are higher than those of pure Nafion (Figure 9a). In particular, the Nafion/0.4% g−C3N4 membrane has the highest proton conductivity of 287.71 mS cm −1 at 90 °C, which is approximately 89% higher than that of the pure Nafion. This may be because the modified membranes absorbed more water, which formed proton transport channels. However, the proton conductivity of the Nafion/0.8% g−C3N4 membrane decreased, probably because the excess g−C3N4 blocked some of the proton transport channels. Above 100 °C, the relative humidity is low, even when the water vapor feed was at its highest, which reduced the conductivity of the membrane. However, the modified membranes exhibited higher conductivity and did not exhibit abrupt decreases in conductivity at high temperatures (Figure 9b). This indicates that the strong water absorption of g−C3N4 effectively improves the membrane performance under low humidity conditions. The tests above 100 °C are performed at ambient pressure. Therefore, further improvements in the device to allow testing under pressurized conditions may reveal a significant increase in performance.

Proton Conductivity
Proton conductivity is an important characteristic that determines the electrolytic properties of the membranes; therefore, it is evaluated using EIS. The proton conductivities of the Nafion/g-C 3 N 4 modified membranes are higher than those of pure Nafion (Figure 9a). In particular, the Nafion/0.4% g-C 3 N 4 membrane has the highest proton conductivity of 287.71 mS cm −1 at 90 • C, which is approximately 89% higher than that of the pure Nafion. This may be because the modified membranes absorbed more water, which formed proton transport channels. However, the proton conductivity of the Nafion/0.8% g-C 3 N 4 membrane decreased, probably because the excess g-C 3 N 4 blocked some of the proton transport channels. Above 100 • C, the relative humidity is low, even when the water vapor feed was at its highest, which reduced the conductivity of the membrane. However, the modified membranes exhibited higher conductivity and did not exhibit abrupt decreases in conductivity at high temperatures (Figure 9b). This indicates that the strong water absorption of g-C 3 N 4 effectively improves the membrane performance under low humidity conditions. The tests above 100 • C are performed at ambient pressure. Therefore, further improvements in the device to allow testing under pressurized conditions may reveal a significant increase in performance. Membranes 2023, 13, x FOR PEER REVIEW 9 of 13

Electrolysis Performance
The steam electrolysis performances of the Nafion/g−C3N4 modified membranes were measured, and the results are shown in Figure 10. At low current densities, the electrolysis voltages of the different membranes were approximately the same, suggesting that they had the same catalytic properties and that the differences in their electrolytic performances were caused by their different conductivities. The commercial Nafion 212 membrane shows approximately the same results as the pure Nafion membrane that we produced. This indicates that the improvement in the electrolytic performance is derived from the addition of g−C3N4. At 2.0 V, the optimal peak current density of the Nafion/0.4% g−C3N4 and pure−Nafion membranes are 2260 and 1340 mA cm −2 , respectively. At a current density of 1000 mA cm −2 , the Nafion/0.4 %g−C3N4 membrane has the lowest electrolysis voltage of 1.684 V. The results suggest that doping with g−C3N4 facilitates electrolysis, which is attributed to the higher proton conductivity of the modified membranes than that of the pure−Nafion membranes. Table 2 lists the high−temperature PEM electrolysis performance of the Nafion/0.4% g−C3N4 membrane produced in this study compared to the results obtained in other studies. The Nafion/g−C3N4 membrane exhibits better electrolytic behavior with a relatively low catalyst loading, which demonstrates the superiority of Nafion/g−C3N4 modified membranes.

Electrolysis Performance
The steam electrolysis performances of the Nafion/g-C 3 N 4 modified membranes were measured, and the results are shown in Figure 10. At low current densities, the electrolysis voltages of the different membranes were approximately the same, suggesting that they had the same catalytic properties and that the differences in their electrolytic performances were caused by their different conductivities. The commercial Nafion 212 membrane shows approximately the same results as the pure Nafion membrane that we produced. This indicates that the improvement in the electrolytic performance is derived from the addition of g-C 3 N 4 . At 2.0 V, the optimal peak current density of the Nafion/0.4% g-C 3 N 4 and pure−Nafion membranes are 2260 and 1340 mA cm −2 , respectively. At a current density of 1000 mA cm −2 , the Nafion/0.4 %g-C 3 N 4 membrane has the lowest electrolysis voltage of 1.684 V. The results suggest that doping with g-C 3 N 4 facilitates electrolysis, which is attributed to the higher proton conductivity of the modified membranes than that of the pure−Nafion membranes. Table 2 lists the high−temperature PEM electrolysis performance of the Nafion/0.4% g-C 3 N 4 membrane produced in this study compared to the results obtained in other studies. The Nafion/g-C 3 N 4 membrane exhibits better electrolytic behavior with a relatively low catalyst loading, which demonstrates the superiority of Nafion/g-C 3 N 4 modified membranes.

Electrolysis Performance
The steam electrolysis performances of the Nafion/g−C3N4 modified membranes were measured, and the results are shown in Figure 10. At low current densities, the electrolysis voltages of the different membranes were approximately the same, suggesting that they had the same catalytic properties and that the differences in their electrolytic performances were caused by their different conductivities. The commercial Nafion 212 membrane shows approximately the same results as the pure Nafion membrane that we produced. This indicates that the improvement in the electrolytic performance is derived from the addition of g−C3N4. At 2.0 V, the optimal peak current density of the Nafion/0.4% g−C3N4 and pure−Nafion membranes are 2260 and 1340 mA cm −2 , respectively. At a current density of 1000 mA cm −2 , the Nafion/0.4 %g−C3N4 membrane has the lowest electrolysis voltage of 1.684 V. The results suggest that doping with g−C3N4 facilitates electrolysis, which is attributed to the higher proton conductivity of the modified membranes than that of the pure−Nafion membranes. Table 2 lists the high−temperature PEM electrolysis performance of the Nafion/0.4% g−C3N4 membrane produced in this study compared to the results obtained in other studies. The Nafion/g−C3N4 membrane exhibits better electrolytic behavior with a relatively low catalyst loading, which demonstrates the superiority of Nafion/g−C3N4 modified membranes.

Conclusions
In this study, a strategy for preparing Nafion−composite membranes using g-C 3 N 4 as a filler is developed. The prepared membranes achieve excellent results when utilized as steam electrolytic PEMs. Observations of the microscopic morphology reveal that the g-C 3 N 4 is uniformly distributed in the Nafion, which resulted in a high proton conductivity and good mechanical properties. TGA reveal that the addition of g-C 3 N 4 enhances the thermostability of the membranes. At 90 • C, the highest proton conductivity of the Nafion/0.4% g-C 3 N 4 membrane is 287.71 mS cm −1 compared to 151.86 mS cm −1 for pure Nafion. At temperatures higher than 100 • C, the modified membranes exhibit high conductivity, and no abrupt decreases in conductivity are observed. Compared to the materials developed in previous studies, the membranes in this study demonstrated a better electrolytic performance with relatively low catalyst loading. At 2.0 V, the optimum peak current density of the Nafion/0.4% g-C 3 N 4 membrane is 2260 mA cm −2 , which is approximately 69% higher than that of pure Nafion (1340 mA cm −2 ). Overall, these results indicate that Nafion/g-C 3 N 4 membranes have considerable potential for practical applications in PEM steam electrolysis.