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Article

Ion-Exchange Strategy Enabling Direct Reformation of Unreliable Perfluorinated Cationic Polymer for Robust Proton Exchange Membrane towards Hydrogen Fuel Cells

by
Xuqiu Xie
1,†,
Wenjing Jia
2,†,
Changyuan Liu
2,
Yongzhe Li
3,
Anhou Xu
1,* and
Xundao Liu
2,*
1
Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, Shandong Engineering Research Center for Fluorinated Materials, University of Jinan, Jinan 250022, China
2
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
3
State Key Laboratory of Fluorinated Functional Membrane Materials, Shandong Dongyue Future Hydrogen Energy Materials Company, Zibo 256401, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2024, 17(12), 2954; https://doi.org/10.3390/en17122954
Submission received: 24 May 2024 / Revised: 12 June 2024 / Accepted: 12 June 2024 / Published: 15 June 2024
(This article belongs to the Section D2: Electrochem: Batteries, Fuel Cells, Capacitors)
Browse Figures
Versions Notes

Abstract

:
Perfluorosulfonated anionic ionomers are known for their robust ion conductivity and chemical and mechanical stability. However, the structure and transport property degradation of perfluorinated cationic polymers (PfCPs) are not well understood. Herein, we propose an ion-exchange strategy to identify the structural degradation, ion transport mechanisms, and architectural reformation of PfCPs. Particularly, we demonstrate that the utility of a –SO2–N+ strategy employing the Menshutkin reaction cannot yield reliable PfCPs and anion-exchange membranes, but can yield an unreliable zwitterionic intermediate (cations–anions molar ratio is approximately 7.6%). Moreover, the degradation products were efficiently reformed as proton exchange membranes (PEMs), and the as-reformed PEMs achieved an ion-exchange capacity (IEC) value (0.89 mmol g−1), meanwhile retaining more than 94.7% of their initial capacity. Furthermore, the fuel cell assembled with reformed PEMs displayed a power density of 0.91 Wcm−2 at 2.32 A cm−2, which was 90.1% of that of the robust perfluorosulfonic acid PEMs. Our combined findings shed some fresh light on the state of understanding of the structure–property relationship in PfCPs.

1. Introduction

Perfluorosulfonated ionomers (PFSIs) have attracted considerable attention because of their unique Teflon-like structures and excellent ion conductivities (typically H+ and Na+). Among them, perfluorosulfonic acid (PfSO3H) is widely used as a solid electrolyte in energy-related electrochemical devices such as polymer–electrolyte fuel cells, proton exchange membrane water electrolysis devices, and chloro-alkali cells. The prominent physicochemical properties of PfSO3H are clearly associated with the self-assembled structure of a super-hydrophobic backbone and long hydrophilic side chains. Such unique phase-separation behavior plays a key role in ion transport processes, and this behavior is also found in fluorine-containing cationic polymers (CPs) and anion-exchange membranes (AEMs) [1,2,3,4]. However, the inferior ion conductivity and chemical stability of fluorinated AEMs, in contrast to those of their fully fluorinated PfSO3H counterpart, severely limits their further industrial application.
In the continuous pursuit of more “PfSO3H-like” structured CPs, much effort has been focused on the synthesis of perfluorinated cationic polymers (PfCPs), with the goal to enhance the OH− conductivity and alkaline stability. To date, traditional PfCPs have been mostly synthesized by two-step solid–liquid reactions (Scheme 1a): (1) perfluorinated sulfonyl fluoride precursors (PfSO2F) serve as the starting materials; (2) amination of PfSO2F forms pendant quaternized ammonium cations (QAs) with a tertiary amine reagent via the Menshutkin reaction; and (3) the F− counter-ion is converted to the hydroxide form for AEM construction under alkaline conditions. Among these reactions, various types of PfSO2F have been applied toward the synthesis of PfCPs, including Nafion-SO2F, 3M-SO2F, and Aquivion-SO2F. [5,6,7] Nevertheless, the utility of this method suffers from its “limited scope”, primarily due to the unreliable –S–N linkage, how stable and real is the proposed –SO2–N+ structure in PfCPs remained a question (Scheme 1a).
Although PfSO2F is uniquely suited for PfSO3H formation because of its strong ion-exchange ability, the unstable reactive –SO2F group, insolubility of PfSO2F, and vulnerable linkage between sulfone and cations remain severe challenges for the accurate synthesis of PfCPs. Recently, reliable PfCPs were synthesized for AEM construction and exhibited high ion conductivity and physical stability in an alkaline environment [7,8,9,10]. Nevertheless, unreliable PfCPs with zero anion-exchange capacities and no AEMs were generated when exposed to alkaline conditions based on PfSO2F because of the cleavage of the weak sulfone–cation linkage [11,12]. Although the –SO2–N+ group of PfCPs does exhibit instability in an alkaline environment, the degradation mechanism and degradation products of the hydrolysis still require identification.
Herein, we propose an efficient strategy to explore the structural degradation, ion transport mechanisms, and architectural reformation of PfCPs. Importantly, with the help of defining the chemistry assisted by the transport properties of perfluorosulfonate ionomers (PfSO3K) and PfSO3H (Scheme 1b), we directly proved that the –SO2–N+ linkage of the Re-PfCPs completely broke during the hydroxide exchange process and liberated the cationic groups, which exhibited zero OH− ion-exchange capacity (IEC) and OH− conductivity. Furthermore, the degradation products (Re-PfSO3K) were converted into H+ exchanged polymers (Re-PfSO3H) via a proton exchange reaction. The reformation of Re-PfSO3H is similar to that of traditional PfSO3H in proton exchange membrane (PEM) and proton exchange membrane fuel cell (PEMFC) applications.

2. Experimental Section

2.1. Materials

Perfluorinated sulfonyl fluoride (PfSO2F, IEC = 1.10 mmol g−1) was provided by Dongyue Shenzhou, Zibo, China. Ethanol, sodium chloride (NaCl), and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Sulfuric acid (≥95%, H2SO4) was purchased from Yantai Yuandong Fine Chemicals Co., Ltd. (Yantai, China) Pt/C catalysts (TKK, Tokyo, Japan) and gas diffusion layer (GDL; JNT) were supplied by JNTG Company (Hwaseong-si, Republic of Korea). Trimethylamine (TMA, 25 wt% in H2O) solution was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Synthesis of Re-PfCPs

The Menshutkin reaction between extruded-PfSO2F film and TMA was carried out at ambient temperature (25 °C) according to a published procedure [11,12]. The PfSO2F films were added to a solution of 30 mL of 45 wt% TMA in water and 10 mL of deionized (DI) water and reaction was allowed for 2 days to achieve a complete reaction. The resulting PfCPs polymer was washed with DI water and dried under vacuum at 60 °C for 24 h.

2.3. Synthesis of PfSO3K, PfSO3H and Re-PfSO3H

The ion exchange of PfSO2F films in 1 M KOH solution at 60 °C overnight provided the PfSO3K. Then, proton-exchanged PfSO3K in 6M H2SO4 produced the PfSO3H. Re-PfSO3H was synthesized via a similar process as PfSO3H, just with the exception of PfCPs instead of PfSO2F.

2.4. Membrane Preparation

Most of the traditional PFSIs used for PEMs were widely prepared by the solution casting method. Thus, we processed membrane formation by solution casting to display a similar process between the PfSO3H and the reformed PfCPs. In brief, the polymers were dissolved into a water–ethanol mixed solvent to form ~10 wt% solutions. Subsequently, the resulting solution was cast onto glass plates and dried in a vacuum oven at 130 °C and 150 °C for 2 h, respectively. Finally, the membranes were peeled off from the glass plates upon cooling in the oven. The thickness of the PfSO3H and Re-PfSO3H membranes were measured as follows: three locations of the membranes were taken from the edge to the middle to obtain an average thickness with a thickness measurement gauge. The similar thicknesses of PfSO3H and Re-PfSO3H membranes are 16.7 ± 1.0 μm and 15.3 ±1.0 μm, and the standard deviation from both membranes is 0.26.

2.5. Characterization and Measurements

The SAXS measurements were taken on a small-angle X-ray scattering station (BL16B1) with a long-slit collimation system in the Shanghai Synchrotron Radiation Facility (SSRF). The phase segregation structures of membranes were analyzed on a JEOL JEM-2010 transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS, Kα) analyses were carried out on an AXIS UltraDLD X-ray photoelectron spectrometer system equipped with Al radiation as a probe. The membranes were stained with Pb2+ (PbCl2/HCl) and then embedded in epoxy resin before the TEM measurements. Additional performance test steps are presented in the supporting information (including Figures S1–S4). Additional characterisation and measurement sections are detailed in Appendix A.

2.6. Fabrication of Membrane Electrode Assemblies (MEA)

Membrane electrode assemblies (MEA, 2.5 cm × 2.5 cm) were fabricated using the hot-pressing method. Catalyst ink was prepared by mixing the catalyst with the PfSO3H and Re-PfSO3H solutions (3%) in a mixture of ethanol and DI water (v/v = 8/2), respectively, sprayed on gas diffusion layers (GDL, JNT, Hwaseong-si, Republic of Korea), and dried for 3 h at 45 °C to prepare the Gas Diffusion Electrodes (GDEs). MEA of 2.5 cm × 2.5 cm was assembled by placing a GDE on either side of the dry PfSO3H and Re-PfSO3H membrane and pressing under a pressure of 1.5 MPa at 140 °C for 3 min. The Pt loading for both anode and cathode was 0.4 mg cm−2. The single cells were tested on a 100-WFC test station (850Es, Hephas Energy Co., Ltd., Taiwan, China). The polarization curves were obtained at the conditions of 80 °C and 95%RH, and the anode (H2) and cathode (O2) gas flow rates were 2 L min−1 and 5 L min−1, respectively, with a back pressure of 150 and 140 kPa for the anode and cathode.

3. Results and Discussion

3.1. Synthesis and Characterization of the Polymers

In this study, PfSO3K and PfSO3H were synthesized via the hydrolysis of PfSO2F as benchmark materials to assess the chemical structure and physicochemical properties of Re-PfCPs. As shown in Figure 1, the chemical structures of Re-PfCPs, PfSO3K, and PfSO3H were examined using FT-IR spectroscopy. The disappearance of the strong peak (–SO2F) for PfSO2F at 1467 cm−1 and the appearance of the peaks from the –SO3− group at ~1061 cm−1 and 1630 cm−1 in PfSO3K indicated chemical conversion, whereas the peak from –SO3− shifted to ~1056 cm−1 in PfSO3H via a proton exchange reaction [13,14]. Furthermore, the Re-PfCPs structure was confirmed by the appearance of peaks from the S–N group and –N+–(CH3)3 at ~1410 cm−1 and 1481 cm−1, while additional peaks at 2800–3082 cm−1 were attributed to the C–H bond (–CH2 and –CH3), suggesting the successful formation of –S–N+–(CH3)3 bonds in the reaction between PfSO2F and TMA. Notably, new characteristic peaks associated with the –SO3− stretching vibration appeared at 1052 cm−1, confirming the –SO3− bond of the Re-PfCPs, whereas the C–O–C (982 cm−1), –CF3 (1308 cm−1), and –CF2 (1197 and 1143 cm−1) bonds were similar to those in PfSO2F [15,16,17]. These FT-IR spectroscopy results demonstrate that the Re-PfCPs ionomer is a mixture of cationic and anionic intermediates. The area ratio of the typical S–N or –SO3− peaks to the C–O–C peak was close to 1:13.1, confirming that the cation–anion molar ratio was approximately 7.6%. The negative charge also suggested the greater –SO3− dissociation in Re-PfCPs, and the zeta potential was –0.83 mV (Figure S1).
Owing to the higher mobility of OH− versus those of other counter-ions (typically Cl− and F−), CPs often need to complete the ion conversion via the hydroxide exchange method. Accordingly, the structure of the Re-PfCPs after ion exchange was confirmed by FT-IR spectroscopy. All the characteristic signals of –S–N+–(CH3)3 completely disappeared in Re-PfSO3K upon exposure to the alkaline solution, whereas the –SO3− stretching vibration was still present and shifted to 1061 cm−1. Specifically, the FT-IR spectra of Re-PfSO3K were highly consistent with those of PfSO3K, confirming the complete cleavage of QAs in Re-PfCPs. Furthermore, extremely similar signals at 1056 cm−1 attributed to the –SO3H group were observed in Re-PfSO3H and PfSO3H after the proton exchange process, indicating that an efficient ion conversion reaction occurred in Re-PfSO3K. XPS was also employed to confirm the completion of the Menshutkin reaction and the thorough exchange of counter-ions in the ionomer mixtures, as shown in Figure S2. We can see clearly the characteristic N1s signal for the –S–N+–(CH3)3 in the Re-PfCPs after the reaction between PfSO2F and TMA at 402.7 eV, while the N1s peak completely disappeared through the exchange of counter-ions in Re-PfSO3H. Notably, the extremely similar signal characteristics for the F1s, O1s, and S2p of RePfSO3H and PfSO3H emerged, further confirming the structural degradation and architectural reformation of the Re-PfCPs.
Based on the FT-IR and XPS spectroscopy results, we propose a plausible degradation mechanism and reforming reaction of Re-PfCPs starting from (i) QAs covalently attached to the PfSO2F, followed by (ii) the formation of a zwitterionic intermediate (Re-PfCPs, OH− form) caused by nucleophilic OH− attack on the O=S=O group, (iii) the breakdown of the –SO2–N+ linkage down during the OH− exchange, and (iv) the final reformation of Re-PfSO3H. The presence of TMA activates this reaction, and its catalytic cycle is shown in Figure 2a.
To validate the proposed degradation reactions, we conducted 1H NMR spectroscopy to verify the architectural changes in the Re-PfCPs during the synthetic procedure. As shown in Figure 2b, the characteristic signal at 2.7 ppm originated from the –CH3 protons (A, B) in Re-PfCPs. The –CH3 signals disappeared completely in Re-PfSO3K, and –SO3H protons (D) appeared at 4.1 ppm in Re-PfSO3H. Moreover, –SO3H protons (C) were observed at 4.2 ppm in PfSO3H, providing further direct evidence for the structure degradation and architectural reformation of Re-PfCPs. Consistent with these results, the surface zeta potentials of Re-PfSO3K and Re-PfSO3H were lower than zero because of the presence of –SO3K or –SO3H (Figure S2). Based on the above results, PfSO3K and Re-PfSO3H were clearly proven to be anionic polymers rather than CPs.

3.2. Membrane Morphology and Crystalline Structure

A PEM derived from an anionic polymer (Re-PfSO3H) was fabricated to explore its structure and properties. Benefitting from the super-hydrophobic Teflon-like backbone and highly hydrophilic long-side chains, perfluorinated-type membranes often possess a microphase-separated structure with efficient ion transport channels [18,19,20,21,22]. We then used SAXS techniques to investigate the self-assembly of the Re-PfSO3H membranes at room temperature. As shown in Figure 3a, Re-PfSO3H and PfSO3H display significant ionomer peaks at q = 1.0~1.2 nm−1. These results indicated the formation of packed ionic clusters in Re-PfSO3H and PfSO3H. The cluster diameters (d) of Re-PfSO3H and PfSO3H calculated by Bragg’s law (d = 2π qmax−1) were 6.3 and 5.2 nm, respectively. A difference in the matrix peaks of Re-PfSO3H and PfSO3H was also observed, suggesting a different crystallite size or the crystallinity of the two membranes. In addition, the cross-sectional morphology of the membrane was observed by TEM. Notably, a quantity of ionic clusters (yellow dotted line, dark domains) of equal size (5~8 nm) were observed in the cross-sectional TEM images of Re-PfSO3H and PfSO3H (Figure 3b,c). The Re-PfSO3H membranes were uniformly dense and free of air holes as that of the PfSO3H membranes (Figure 4a,b).
Furthermore, X-ray diffraction (XRD) characterization found similar diffraction features located at 2θ = 12.2~22.0° and 2θ = 40.0° for Re-PfSO3H and PfSO3H [23]. The relative crystallinities calculated from the XRD spectra were 15.2% and 16.3% (Figure 5a). Accordingly, the normalized differential scanning calorimetry (DSC) profiles of Re-PfSO3H and PfSO3H showed that the phase transition of ionic clusters occurred at 70.1~84.9 °C (Figure 5b). All these results indicated that Re-PfSO3H exhibited a PfSO3H-like crystalline structure and self-assembly morphology.

3.3. Physical and Ion Transport Properties

The IEC value is critical for PEMs because it directly affects both the physical and ion transport properties [24,25,26]. As shown in Figure 6a, a high IEC value (0.89 mmol g−1) was retained in Re-PfSO3H after the reformation process, which was approximately 94.7% compared to that of PfSO3H. The decrease in the IEC value of Re-PfSO3H may be caused by the formation of an anhydride (–SO2–O–SO2–) crosslink in the sidechain during the OH−/H+ exchange process (Figure 6a) [27]. The water uptake (WU%) of Re-PfSO3H between 30 and 80 °C is shown in (Figure 6b). As anticipated, the WU% increased with temperature, and a significantly higher WU% was observed for the PfSO3H membrane because of its higher IEC value. The WU% was 9.2% at 30 °C and 34.7% at 80 °C for Re-PfSO3H. Meanwhile, the WU% was 10.6% at 30 °C and 37.5% at 80 °C for PfSO3H. The swelling ratio (SR%) of Re-PfSO3H (Figure 6c) followed a similar trend to that of the WU%. The SR% of PfSO3H was higher (11.4% at 30 °C and 23.9% at 80 °C) than that of Re-PfSO3H (10.5% at 30 °C and 21.9% at 80 °C) because of the greater water absorption.
The mechanical properties and thermal stability of the PEMs were also investigated [28,29,30,31]. As shown in Figure 7a, the PfSO3H membrane exhibited high tensile strength (19.9 MPa) and low elongation at break (64.6%) in the dry state, whereas Re-PfSO3H displayed a low strength (17.2 MPa) and high elongation (67.4%). The formation of an anhydride (–SO2–O–SO2–) crosslink in the side chain caused the lower crystallinity (15.2%) of Re-PfSO3H versus that of PfSO3H (16.3%) membrane, thus exhibiting lower mechanical strength. In addition, the presence of the anhydride structure was confirmed by the IR spectroscopy. As shown in Figure S3, the anhydride (–SO2–O–SO2–) was confirmed by the appearance of the IR absorption band at 1440 cm−1 for Re-PfSO3H, along with the absence of the sulfonic anhydride peaks for PfSO3H [32]. The thermal stabilities of the PEMs were investigated, and the results are shown in Figure 7b. The samples possessed similar two-stage degradation with the first weight loss of ~10%, occurring at 340 °C~382 °C (ascribed to the loss of the side chains) [33,34,35]. The second weight loss started after ~440 °C when the polymer backbone degraded (Figure 7b).
Proton conductivity is a dominant property that determines the performance of PEMs and PEMFCs [36,37,38,39]. Therefore, we investigated proton conductivity to evaluate the ion transport properties of the PEMs at 100% relative humidity in the range of 30~80 °C, as shown in Figure 8a. The proton conductivity of the Re-PfSO3H and PfSO3H membranes increased linearly with temperature, and these membranes achieved ion conductivities of 114.5 mS cm−1 and 137.3 mS cm−1 at 80 °C, respectively (Figure 8a). The activation energies (Ea) calculated using the Arrhenius equation were 11.64 and 10.48 kJ mol−1 for PfSO3H and Re-PfSO3H (Figure 8b) [40,41,42]. The lower Ea value for Re-PfSO3H may be due to the facilitated ion transport along larger ionic channels. As shown in Figure 8c, compared with the physical and ion transport properties in the PEMs, Re-PfSO3H exhibited ion transport properties as high as those of PfSO3H at the same IEC level. These findings indicated that the different physical properties (ion conductivity and phase separation) were associated with the IEC values of the PEMs, which directly influenced both their physical and ion transport characteristics.

3.4. Single Cell Performance

Importantly, Re-PfSO3H and PfSO3H were applied for H+ transportation from anode to cathode in catalyst layers (CLs) and membranes in MEA (Figure 9a). Furthermore, the H2/O2 PEMFC performance of the Re-PfSO3H and PfSO3H membranes were investigated and compared (Figure 9b). The PEMFCs assembled with Re-PfSO3H-MEA display a power density of 0.91 Wcm−2 at 2.32 A cm−2, which is 90.1% of that of the PEMFCs of PfSO3H-MEA (1.01 Wcm−2 at 2.46 A cm−2) (Figure 9b) and exceeding the performance of the state-of-the-art robust PEMs (Figure S4). The Re-PfSO3H-MEA displayed a higher ohmic resistance (Rohm, 0.025 Ω) evaluated by electrical impedance spectroscopy (EIS) measurements under 1 A cm−2, while the PfSO3H-MEA displayed a lower Rohm (0.022 Ω), consistent with the ion conductivity tendency (Figure 9c).

4. Conclusions

In summary, we revealed that no reliable PfCPs and AEMs were obtained via the –SO2–N+ linked strategy, while an unreliable zwitterionic intermediate was produced with accelerated degradation during hydroxide exchange. Importantly, combined with hydroxide and proton exchange reactions, the degradation products were converted directly into H+-exchanged polymers, and an anionic polymer was finally reformed. Remarkably, the as-reformed Re-PfSO3H membrane maintained a well-defined structure and robust physical and ion transport properties similar to those of PfSO3H at the same IEC level. Finally, high PEMFC performance was demonstrated, and a peak power density of 0.91 Wcm−2 was achieved, which is 90.1% of that of the PfSO3H. Therefore, we consider the ion-exchange strategy as a synthetic route to address the key challenges in PfCP and AEM synthesis.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en17122954/s1, Figure S1: Zeta potential of the samples; Figure S2: XPS survey scans of Re-PfCPs, orange line; Re-PfSO3H, pink line; and PfSO3H, olive line; Figure S3: FT-IR spectra of Re-PfSO3H, pink line; and PfSO3H, olive line; Figure S4: Comparison of the IEC and peak power density of this work with PEM reported in some literature [14,26,43,44,45,46,47,48,49,50].

Author Contributions

Credit: X.X. data curation (equal) and writing original draft (equal); W.J. data curation (equal) and writing original draft (equal); C.L. data curation and investigation; Y.L. data curation and investigation; A.X. supervision and writing—review and editing (lead); X.L. conceptualization (lead) and writing—review and editing (lead). All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Key R&D Program of Shandong Province, China 2022 CXGC020310, the National Natural Science Foundation of China, China (No. 51803073), and the Science and Technology Program of the University of Jinan (No.: XKY2105).

Data Availability Statement

Data will be made available upon request.

Acknowledgments

This work was financially supported by the University of Jinan.

Conflicts of Interest

Author Yongzhe Li was employed by the Shandong Dongyue Future Hydrogen Energy Materials Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Appendix A

Characterization and measurements
1H NMR spectra were conducted with a Bruker Avance III 600 M spectrometer operating (Bruker BioSpin AG, Fällanden, Switzerland) at 600 MHz for 1H NMR in DMSO-d6. FIR spectra in the range of 4000–400 cm−1 were obtained on a Thermofisher Scientific Nicolet iS10 spectrometer (USA). X-ray diffraction (XRD) data were recorded on a Smart Lab 9KW (Rigaku corporation, Japanese). The zeta potential value of the membrane was obtained from the nanometer particle size and Zeta potential analyzer (DLS, Malvern Zetasizer Nano ZS90, UK).
The universal tensile testing machine (WDW-05L, Jinan, China) was used to measure the mechanical strength of the membranes under ambient atmosphere at a speed of 100 mm min−1. A thermogravimetric analyzer (TGA, Perkin Elmer, Shelton, CT, USA) was used for the analysis of the thermal degradation of membranes. DSC curves were recorded on a Q-2000 DSC (TA Instruments, New Castle, DE, USA).
The IEC values of the membrane in the H+ form was measured by back-titration. The membranes were dried at 60 °C under vacuum for 24 h and then immersed into a 2 M NaCl standard solution for 48 h to release H+. The exchanged solution was collected and titrated with a 0.1 M standard NaOH solution to pH = 7 of the solution. The formula for IEC is as follows:
IEC = C NaOH V NaOH m
where CNaOH (mol L−1) and VNaOH (mL) are the concentration and volume of NaOH consumed, respectively, and m (g) is the mass of the dry membranes.
Proton conductivity (σ) can be determined by measuring the impedance of the membrane at 100% relative humidity at various temperatures using four-point probe AC impedance spectroscopy. The following formula is often used to calculate proton conductivity:
σ = L A × R
where σ is the proton conductivity, mS cm−1; L is the thickness of the membrane, cm; A is the cross-sectional area of the ionic membrane, cm2; and R is the resistance of the membrane (in KΩ) obtained from impedance measurements.
The membranes were dried in a vacuum oven for 24 h at 60 °C to obtain the mass (Mdry) and length (Ldry) of the dried membranes before measuring the water uptake and swelling ratio. Then, they were placed in DI water at different temperatures and their mass (Mwet) and length (Lwet) were measured after absorbing the surface water. The water uptake (WU%) and swelling ratio (SR%) were calculated using the following formulas:
WU % = M wet − M dry M dry × 100 %
where Mwet is the mass of the wet membrane sample in g; Mdry is the mass of the dry membrane sample in g.
SR % = L wet − L dry L dry × 100 %
where Lwet is the length of the wet (swollen) membrane; Ldry is the length of the dry membrane before immersion in water.

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Scheme 1. Traditional synthesized (a) and ion exchange-reformed (b) PfCPs and their membranes.
Scheme 1. Traditional synthesized (a) and ion exchange-reformed (b) PfCPs and their membranes.
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Figure 1. FT−IR spectra of RfSO2F, black line; Re-PfCPs, orange line; Re-PfSO3K, blue line; PfSO3K, violet line; Re-PfSO3H, pink line; and PfSO3H, olive line.
Figure 1. FT−IR spectra of RfSO2F, black line; Re-PfCPs, orange line; Re-PfSO3K, blue line; PfSO3K, violet line; Re-PfSO3H, pink line; and PfSO3H, olive line.
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Figure 2. (a) Proposed degradation mechanism and reforming reaction; (b) 1H NMR spectra of Re-PfCPs, orange line; Re-PfSO3K, blue line; PfSO3K, violet line; Re-PfSO3H, pink line; and PfSO3H, olive line. The colored letters in the figure correspond to the group structure on the diagram, and the black letters correspond to the signal peaks on the 1H NMR. The other * represents the signal peak of the solvent DMSO-d6.
Figure 2. (a) Proposed degradation mechanism and reforming reaction; (b) 1H NMR spectra of Re-PfCPs, orange line; Re-PfSO3K, blue line; PfSO3K, violet line; Re-PfSO3H, pink line; and PfSO3H, olive line. The colored letters in the figure correspond to the group structure on the diagram, and the black letters correspond to the signal peaks on the 1H NMR. The other * represents the signal peak of the solvent DMSO-d6.
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Figure 3. (a) SAXS patterns of Re-RfSO3H, pink line and RfSO3H, olive line; TEM images (cross-section) of (b) Re-RfSO3H and (c) RfSO3H.
Figure 3. (a) SAXS patterns of Re-RfSO3H, pink line and RfSO3H, olive line; TEM images (cross-section) of (b) Re-RfSO3H and (c) RfSO3H.
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Figure 4. Surface (a) and cross-section (b) SEM images of Re-PfSO3H and PfSO3H membranes.
Figure 4. Surface (a) and cross-section (b) SEM images of Re-PfSO3H and PfSO3H membranes.
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Figure 5. (a) XRD diffraction patterns and (b) DSC heating plots of PfSO3H and Re-PfSO3H membranes.
Figure 5. (a) XRD diffraction patterns and (b) DSC heating plots of PfSO3H and Re-PfSO3H membranes.
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Figure 6. (a) IEC value; (b) WU% and (c) SR% of PfSO3H and Re-PfSO3H membranes.
Figure 6. (a) IEC value; (b) WU% and (c) SR% of PfSO3H and Re-PfSO3H membranes.
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Figure 7. (a) The stress–strain and (b) TGA and DTG (inset) curves of the membranes.
Figure 7. (a) The stress–strain and (b) TGA and DTG (inset) curves of the membranes.
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Figure 8. (a) Proton conductivity (σ); (b) lnσ vs. temperature; (c) a radar plot comparing the water uptake (WU%), stress, proton conductivity (σ), and swelling ratio (SR%) of Re-PfSO3H and PfSO3H.
Figure 8. (a) Proton conductivity (σ); (b) lnσ vs. temperature; (c) a radar plot comparing the water uptake (WU%), stress, proton conductivity (σ), and swelling ratio (SR%) of Re-PfSO3H and PfSO3H.
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Figure 9. (a) Illustration of the assembly and reaction mechanism of PEMFCs; (b) polarization curves for the PEMFC cell; and (c) EIS measurement: the Nyquist plots of the MEAs performed at 1A cm−2.
Figure 9. (a) Illustration of the assembly and reaction mechanism of PEMFCs; (b) polarization curves for the PEMFC cell; and (c) EIS measurement: the Nyquist plots of the MEAs performed at 1A cm−2.
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Xie, X.; Jia, W.; Liu, C.; Li, Y.; Xu, A.; Liu, X. Ion-Exchange Strategy Enabling Direct Reformation of Unreliable Perfluorinated Cationic Polymer for Robust Proton Exchange Membrane towards Hydrogen Fuel Cells. Energies 2024, 17, 2954. https://doi.org/10.3390/en17122954

AMA Style

Xie X, Jia W, Liu C, Li Y, Xu A, Liu X. Ion-Exchange Strategy Enabling Direct Reformation of Unreliable Perfluorinated Cationic Polymer for Robust Proton Exchange Membrane towards Hydrogen Fuel Cells. Energies. 2024; 17(12):2954. https://doi.org/10.3390/en17122954

Chicago/Turabian Style

Xie, Xuqiu, Wenjing Jia, Changyuan Liu, Yongzhe Li, Anhou Xu, and Xundao Liu. 2024. "Ion-Exchange Strategy Enabling Direct Reformation of Unreliable Perfluorinated Cationic Polymer for Robust Proton Exchange Membrane towards Hydrogen Fuel Cells" Energies 17, no. 12: 2954. https://doi.org/10.3390/en17122954

APA Style

Xie, X., Jia, W., Liu, C., Li, Y., Xu, A., & Liu, X. (2024). Ion-Exchange Strategy Enabling Direct Reformation of Unreliable Perfluorinated Cationic Polymer for Robust Proton Exchange Membrane towards Hydrogen Fuel Cells. Energies, 17(12), 2954. https://doi.org/10.3390/en17122954

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