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Article

Impact of Cross-Linking-Monomer Characteristics on Pore-Filling-Membrane Performance and Durability in Anion-Exchange Water Electrolysis

by
Jong-Hyeok Park
1,2,
Yeri Park
3,
Tae-Seok Jeon
3,
Yuna Seo
3 and
Jin-Soo Park
1,2,3,*
1
Department of Civil, Environmental, and Biomedical Engineering, The Graduate School, Sangmyung University, Cheonan 31066, Republic of Korea
2
Future Environment and Energy Research Institute, Sangmyung University, Cheonan 31066, Republic of Korea
3
Department of Green Chemical Engineering, College of Engineering, Sangmyung University, Cheonan 31066, Republic of Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(3), 1495; https://doi.org/10.3390/app15031495
Submission received: 30 December 2024 / Revised: 28 January 2025 / Accepted: 30 January 2025 / Published: 1 February 2025
(This article belongs to the Section Energy Science and Technology)
Browse Figures
Versions Notes

Abstract

:
This study investigates the development of pore-filling anion-exchange membranes (PFAEMs) for water-electrolysis applications. Ionomers using two different cross-linking monomers, namely hydrophilic C10 and hydrophobic C11, along with a common electrolyte monomer, E3, were compared in terms of through-plane ion conductivity, hydrogen permeability, mechanical and chemical stability, I-V polarization, and water-electrolysis durability. The results revealed that the E3-C10 PFAEM exhibited 40% higher OH− conductivity (98.7 ± 7.0 mS cm−1) than the E3-C11 PFAEM with a similar ion-exchange capacity. This improvement was attributed to improved separation of hydrophobic and hydrophilic domains, creating well-connected ion channels by the hydrophilic C10. Alkaline stability tests demonstrated that the E3-C10 retained higher ion conductivity compared to E3-C11, due to the absence of ether linkages and increased resistance to nucleophilic attack. During water-electrolysis operations, the E3-C10 PFAEMs showed 10% better durability and 87% lower hydrogen permeability, confirming their suitability for anion-exchange-membrane water electrolysis (AEMWE). Despite the higher ion conductivity of the E3-C10 PFAEM, performance was limited by interfacial resistance. It is suggested that ionomer-coated electrodes could further enhance AEMWE performance by leveraging the higher ion conductivity of the E3-C10. Overall, this study provides valuable guidance on strategies for utilizing pore-filling membranes in water electrolysis.

1. Introduction

Hydrogen production through water electrolysis employs several key technologies: proton-exchange-membrane water electrolysis (PEMWE), alkaline water electrolysis (AWE), and anion-exchange-membrane water electrolysis (AEMWE) [1,2]. PEMWE is extensively commercialized due to its capability to generate high-purity hydrogen at elevated pressures, operating efficiently at high current densities with ultrapure water. However, it necessitates the use of precious metal catalysts, which are both expensive and scarce [3,4]. AWE, also commercialized, offers high durability under alkaline conditions using cost-effective catalysts. Nevertheless, it operates at lower current densities because of the significant resistance posed by porous diaphragms and is unable to produce high-pressure hydrogen [5]. AEMWE seeks to integrate the advantages of both PEMWE and AWE while mitigating their respective limitations. It allows for the utilization of inexpensive and non-precious metal catalysts and does not require costly Nafion membranes [6,7,8]. Unlike AWE, AEMWE can produce hydrogen at high pressures, and the expensive titanium components used in PEMWE can be substituted with more affordable stainless steel (SUS). Consequently, research is directed toward developing AEMWE systems that leverage economical materials to achieve high performance and durability for practical applications [9,10,11,12].
An AEMWE single cell comprises SUS end plates, current collectors, bipolar plates with flow channels, anode and cathode electrodes, and an anion-exchange membrane (AEM). Among these components, the AEM is pivotal as it separates the electrodes and facilitates hydroxide (OH−) ion transport [13]. The advancement of high-performance and durable AEMs is essential for the commercialization and industrial application of AEMWE, aiming to match the efficacy of PEMWE and AWE. However, challenges such as limited chemical stability and suboptimal performance of AEMs pose significant obstacles to the widespread adoption of AEMWE technology [14]. Therefore, developing AEMs with high alkaline stability and superior performance under conditions of high hydroxyl-ion concentration is crucial for the progression of this technology.
In general, the molar conductivity of OH− (198.3 mho·cm2 eq−1) is lower than that of H+ (349.8 mho·cm2 eq−1), resulting in lower conductivity of AEMs compared to proton-exchange membranes (PEMs) [15,16]. To enhance AEM performance to levels comparable with PEMs, the ion-exchange capacity (IEC) is typically increased by incorporating a higher content of functional groups, such as quaternary ammonium (QA) groups and other functional groups like imidazolium, pyridinium, piperidinium, and phosphonium. However, AEMs encounter challenges including limited chemical durability due to degradation mechanisms like Hofmann elimination, nucleophilic substitution, and E1 elimination in the presence of high OH− concentrations. Additionally, increased hydrophilicity associated with higher IEC can compromise mechanical stability [17,18,19,20,21]. To address these issues, numerous studies have focused on improving the performance, chemical durability, and mechanical stability of AEMs. Strategies include designing molecular architectures to disperse cationic functional groups for enhanced OH− conductivity [22], branching polymerization with comb-shaped structures [23], main-chain entanglement, and using chemically stable monomers. Other approaches involve improving chemical stability through ether-free designs and increased molecular weight (MW) [24,25], reinforcing composites with polytetrafluoroethylene (PTFE) [26,27], and developing pore-filling membranes using porous polyolefin substrates [28,29,30].
Various technologies have been employed to produce AEMs, such as solution casting and spin coating that utilize polymers dissolved in solvents, reinforcement using supports like porous PTFE or e-PTFE, and in situ cross-linked pore-filling technology utilizing porous polyolefin substrates [31,32,33]. Among these, the development of pore-filling type AEMs utilizing porous polyolefin substrates offers advantages in cost, high performance, chemical durability, dimensional stability, mechanical stability, and overcoming membrane production limitations with a streamlined process and low production cost [28,34,35,36,37,38]. However, due to the limited pore volume of the porous polyolefin substrate, it is necessary to identify the optimal combination of anion-conducting monomers and cross-linking monomers that can deliver outstanding performance, while also introducing monomers with structures that enhance chemical durability [39,40,41].
The fabrication of pore-filling membranes involves blending an electrolyte monomer, endowed with ion-exchange capacity, with a cross-linking monomer in an optimal ratio. This homogeneously mixed monomer solution is then introduced into a porous support and subjected to ultraviolet (UV)-induced cross-linking. The resulting ion-exchange membrane is designed to facilitate a high flux of specific ions along its thickness, a critical feature in applications like water electrolysis. To achieve this, the IEC of the ionomer must be substantial, while maintaining low water content to ensure the membrane’s dimensional stability. However, IEC and water content are inversely related, necessitating careful optimization. This balance can be attained by adjusting the types and ratios of the electrolyte monomer and cross-linking agent. Recent studies indicate that even with similar IEC and water content, variations in monomer structures can significantly influence ion conductivity, as well as the performance and durability of water electrolysis systems [29,30]. For instance, research has demonstrated that acrylamide-based pore-filling membranes outperform acrylate-based ones in current density, particularly at 1.8 V, when both lack methyl groups [29]. Additionally, hydrophilic acrylamide-based membranes have shown superior performance compared to hydrophobic benzyl-based membranes, exhibiting 25% and 41% higher ion conductivity when using cross-linkers with two and three cross-linking sites, respectively [30]. These findings underscore the importance of monomer selection and structural considerations in the design of pore-filling membranes for enhanced ion conductivity and electrolysis performance.
In this study, we diverged from the previous research that primarily focused on the hydrophilicity of electrolyte monomers. Instead, we employed two distinct cross-linkers, each varying in hydrophilicity, while utilizing a standard electrolyte monomer. This approach allowed us to observe how the properties of ionomers, integrated into polypropylene (PP) substrates, were affected. Additionally, ionomer-coated electrodes are introduced to overcome interfacial resistance between the pore-filling membrane and the electrode. Our ultimate goal was to assess the resulting water electrolysis performance and durability.

2. Materials and Methods

2.1. Preparation of Pore-Filling Membranes

AEMs with a thickness of 95 μm were fabricated via a pore-filling method. The proprietary material, PP, with a thickness of 95 μm and a porosity of 49.4% was used as the porous substrate and was provided by a confidential source. The pore size was analyzed by using field emission-scanning electron microscopy (FE-SEM, SIGMA500, Carl ZEISS, Oberkochen, Germany), exhibiting the pore size approximately 50–200 nm, as shown in Figure 1.
(Vinylbenzyl) trimethylammonium chloride (designated as E3; Sigma-Aldrich, St. Louis, MO, USA) served as the anion-conducting monomer. The cross-linking monomers employed were 1,3,5-triacryloylhexahydro-1,3,5-triazine (designated as C10; Sigma-Aldrich, St. Louis, MO, USA) and trimethylolpropane trimethacrylate (designated as C11; Sigma-Aldrich, St. Louis, MO, USA). Additionally, 2-hydroxy-2-methylpropiophenone (designated as P1; Sigma-Aldrich, St. Louis, MO, USA) was utilized as the photo-initiator. Table 1 summarizes the chemical information of the monomers.
Before filling various monomer solutions into PP substrates, the organic and inorganic contaminants in the porous PP substrates were eliminated by soaking in EtOH (99.5%, Sigma-Aldrich, St. Louis, MO, USA) for 15 min, followed by drying. After this, the cleaned hydrophobic porous PP substrate was pretreated with a 0.5 wt.% solution of sodium dodecylbenzenesulfonate (Sigma-Aldrich, St. Louis, MO, USA) in deionized water (D.I. water) to impart hydrophilic characteristics. To fabricate pore-filling anion-exchange membranes (PFAEMs) based on E3, C10, and C11 monomers, E3 monomers were first dissolved in a 50:50 D.I. water/EtOH solvents to prepare a 50 wt.% monomer solution. The electrolyte (E3) monomer solution was stirred for 1 h. Subsequently, C10 or C11 monomers were added to the E3 monomer solution. The resulting solutions, containing electrolyte and cross-linking (C10 or C11) monomers, were stirred for an additional hour. To initiate the polymerization reaction using a 1500 W wide-range UV light, 2 wt.% of P1 (relative to the total weight of electrolyte and cross-linker monomers) was added. The fabricated monomer solution for pore-filling had a 20:1 mole ratio of electrolyte to cross-linker monomer. The hydrophilic PP substrates were immersed in the fabricated monomer solution to allow penetration into the pores. The monomer solution-filled PP substrates were then sandwiched between silica-coated polyethylene terephthalate (PET) films. The excess solution was squeezed out to remove residual monomers and oxygen bubbles. The UV light was applied for 40 min to initiate in situ cross-linking. Afterward, the PET films were delaminated, and the PFAEMs were washed to remove any residual polymer. The procedures for membrane fabrication are detailed in Figure 2.

2.2. Membrane Characterization

To verify the polymerization of the electrolyte monomer and the cross-linking agent introduced into the porous substrate using the pore-filling membrane manufacturing technology applied in this study, Fourier transform infrared spectroscopy (FT-IR) was employed for analysis. For the FT-IR analysis (Spectrum 100, Perkinelmer, Waltham, MA, USA), the pore-filling membrane was cut into 1 × 1 cm2 sections, dried, and measured using the attenuated total reflectance method to obtain spectra. The characteristic peaks in the spectra were then analyzed. The OH− conductivity of the membranes was measured in 1.0 M KOH (85%, Junsei Chemical Co., Ltd., Tokyo, Japan) at 60 °C using a custom-made through-plane clip cell with a potentiostat/galvanostat (SP-150, Bio-Logic Science Instruments, Seyssinet-Pariset, France). Prior to measurement, the AEMs were soaked in 1.0 M KOH for 24 hr. Potentiostatic electrochemical impedance spectroscopy measurements were conducted at frequencies ranging from 100 kHz to 1 Hz with an amplitude of 10 mV. The procedure was repeated at least five times to obtain accurate OH− conductivity resistance values [29,30]. The OH− conductivity was calculated as:
I o n   c o n d u c t i v i t y :   σ   ( m S · c m − 1 ) = L R · A
where L is the thickness of the membranes, R is the measured impedance of the membranes, and A is the specific area of the membranes.
The IEC of the membranes was measured using the acid-base titration method. Prior to the IEC measurement, the fabricated membrane, with a size of 12 cm2, was immersed in a 1.0 M KOH solution for 24 h to exchange the Cl− ions in the QA functional groups of the membrane with OH− ions. After this, the membrane was rinsed several times with D.I. water and then immersed in a 0.01 M HCl (35−37%, Junsei Chemical Co., Ltd., Tokyo, Japan) solution for 24 h to convert the OH− ions in the membrane back to Cl− ions. The HCl solutions containing OH− ions were titrated using an automatic titrator (848 Titrino Plus, Metrohm, Herisau, Switzerland) with a 0.01 M NaOH solution. All procedures were conducted at room temperature (RT), and the process was repeated three times to obtain accurate IEC values [29,30]. The IEC was calculated as:
I o n   e x c h a n g e   c a p a c i t y   I E C :   m e q · g − 1 = C N a O H × ( V e q − V b l a n k ) W m e m b × X
where Wmemb is the dry weight of the membranes, CNaOH is the normal concentration of the NaOH solution, and Veq is the titrated volume of the NaOH solution for samples, Vblank is the titrated volume of the NaOH solution for blanks, and X is the ratio of total HCl volume to the sampling volume of HCl used for titration.
The hydrogen permeability of the PFAEM was evaluated using the membrane and a pair of electrodes. The electrodes were fabricated with Pt/C (Pt 46.4%, TKK TEC10F50E, TANAKA, Tokyo, Japan) and carbon paper (JNT20-A3, JNTG, Gyeonggi-do, Republic of Korea), with a Pt loading of 0.4 mg cm−2. A graphite single cell with an active area of 9 cm2 was assembled for the tests. Hydrogen permeability was measured under fully hydrated H2/N2 conditions (0.2/0.2 L min−1) at differential pressure using current measurement. The current was measured with an SP-150 potentiostat over a voltage range from 0.1 to 0.8 V at a scan rate of 0.01 V s−1 [29,30]. The hydrogen permeability coefficient (KH2) was calculated as:
H y d r o g e n   p e r m e a b i l i t y :   K H 2 ( m o l · c m − 1 · s − 1 · b a r − 1 ) = I L · L A · n · F · P
where IL (A) is the limiting current, L (cm) is the membrane thickness, A (cm2) is the active area of the single cell, n is the number of electrons in the hydrogen oxidation reaction, F is the Faraday constant (96,485 C mol−1), and P (bar) is the differential pressure.
To measure the stress–strain curve, OH−-equilibrated PFAEMs were cut into dimensions of 3 × 9 cm2. The stress–strain curve was measured using a universal testing machine (ST-1000, Salt, Incheon, Republic of Korea). The distance between the upper and lower clips was set to 7 cm, and the test was conducted following ASTM D882 [42], with a testing speed of 500 mm min−1.
The chemical stability of the membranes was assessed by immersing them in 4.0 M KOH at 60 °C for 960 h. To evaluate chemical stability, OH− conductivity was measured using 1.0 M KOH at 60 °C, and the loss in ion conductivity was calculated by comparing the beginning of the test (BOT) and the end of the test (EOT) [24,29]. The loss in OH− conductivity was calculated as:
L o s s   i n   h y d r o x i d e   i o n   c o n d u c t i v i t y :   %   ( m S · c m − 1 ) = B O T σ − E O T σ B O T σ × 100
where BOTσ is the OH− conductivity at the BOT, and EOTσ is the OH− conductivity at the EOT.

2.3. Fabrication of Porous Transport Layers

The porous transport layers (PTEs) were fabricated by coating the electrode onto the substrates. The anode, an oxygen evolution reaction electrode, was prepared using commercially available iridium oxide (IrO2, Ir 84.5% min, Alfa Aesar, Haverhill, MA, USA) nanoparticles, PiperION® dispersion (5 wt.%, Versogen, Newark, DE, USA) as anion-exchange ionomer (AEI), and a titanium substrate (2GDL09N-025, Bekaert, Zwevegem, Belgium). The anode catalyst ink was fabricated by mixing IrO2, PiperION®, D.I. water, and 2-propanol (99%, Carlo Erba Reagents, Cornaredo, Italy), with a total solid content of 4 wt.% and an ionomer content of 20 wt.%. The cathode, a hydrogen evolution reaction electrode, was prepared using commercially available Pt/C (Pt 46.4%, TKK TEC10F50E, TANAKA, Tokyo, Japan), PiperION® AEI, and carbon paper (JNT20-A3, JNTG, Gyeonggi-do, Republic of Korea). The cathode catalyst ink was fabricated by mixing Pt/C, PiperION®, D.I. water, 1-propanol (99%, Carlo Erba Reagents, Cornaredo, Italy), and 2-propanol, with a total solid content of 4 wt.% and an ionomer content of 30 wt.%. Both the anode and cathode PTEs were prepared by directly spraying the catalyst ink onto the substrates. The resulting PTEs had a loading of 1.0 mg IrO2 cm−2 for the anode and 0.4 mg Pt cm−2 for the cathode.

2.4. Single Cell Assembly and Testing

A single cell (SUS316, CNL Energy Corp., Seoul, Republic of Korea) based on SUS316 was used to evaluate the performance and durability of the AEMWE. The single cell hardware included a SUS316 end plate and an all-in-one SUS316 current collector and flow field, as shown in Figure 3. Prior to single cell assembly, the AEM, anode, and cathode PTEs were soaked in 1.0 M KOH for 24 h at RT to convert Cl− to the OH− form. The membrane-electrode assembly was prepared by sandwiching the AEM between the anode and cathode PTEs. The single cell was assembled with a torque of 7.5 N·m and incorporated into a custom-made AEMWE testing booth. The cell performance was evaluated using an SP-150 potentiostat with a VMP-3B current booster (Bio-Logic Science Instruments, Seyssinet-Pariset, France). Before evaluating the AEMWE, the single cell was preheated to 60 °C under a KOH flow rate of 30 mL min−1, and the temperature of the 1.0 M KOH solution was gradually increased from RT to 60 °C. After this, the AEMWE evaluation was conducted under asymmetric (dry cathode) 1.0 M KOH fed conditions. Current-voltage (I–V) polarizations were obtained by conducting linear sweep voltammetry in the range from 1.35 to 2.00 V at a scan rate of 1 mV s−1. PEIS measurements were conducted between 1.5 and 2.0 V (increasing by 0.1 V step−1), with frequencies ranging from 200 kHz to 0.1 Hz and an amplitude of 10 mV. The in situ durability evaluation of the AEMWE was conducted at a current of 500 mA cm−2 for 100 h. After the in situ durability evaluation, the AEMs were delaminated from both electrodes to measure the IEC in order to confirm the degradation of the QA functional group by Hofmann’s elimination and nucleophilic substitution under water electrolysis conditions.

3. Results and Discussion

To validate the formation of PFAEMs via the pore-filling technique, we conducted FE-SEM analyses on both cross-sectional and surface morphologies. As depicted in Figure 4a,b, the PP substrate exhibited an asymmetric and anisotropic pore distribution. In contrast, Figure 4c–e reveal that the E3-C10 and E3-C11 PFAEMs displayed a polymer-filled PP substrate devoid of visible pores. These observations confirm the successful and complete fabrication of void-free PFAEMs using both the E3-C10 and E3-C11 combinations. The successful fabrication of void-free PFAEMs is significant for applications in water-electrolysis devices, where membrane integrity, ion conductivity, and no gas permeation are crucial. The observed absence of visible pores in the FE-SEM images indicates a dense and uniform membrane structure.
To corroborate the successful in situ polymerization of PFAEMs, FT-IR analysis was conducted. As shown in Figure 5, the peaks corresponding to the polymerized structures of E3-C10 and E3-C11 include C-N stretching vibrations in quaternary ammonium (QA) groups at 1222 cm−1 and 1221 cm−1, symmetric C-H bending at 1383 cm−1 and 1385 cm−1, C-H scissoring vibrations at 1426 cm−1 and 1427 cm−1, and asymmetric C-H bending vibrations at 1489 cm−1 in both samples. Additionally, C=C stretching in aromatic rings is observed at 1614 cm−1 in both, while C=C stretching vibrations are identified at 1643 cm−1 (E3-C10) and 1636 cm−1 (E3-C11). C-H stretching vibrations are noted at 2928 cm−1 in E3-C10, and at 2929 cm−1 and 2969 cm−1 in E3-C11. Furthermore, C-H stretching vibrations are seen at 3022 cm−1 (E3-C10) and 3019 cm−1 (E3-C11). The major differences between polymerized E3-C10 and E3-C11 include the C-N stretching vibration in the triazine ring at 1243 cm−1 (E3-C10), C-O stretching vibrations from ester bonds at 1048 cm−1 and 1089 cm−1 (E3-C11), and the carbonyl (C=O) group in methacrylate esters at 1724 cm−1 (E3-C11). These results confirm the successful in situ polymerization, supporting the polymer-impregnated FE-SEM images shown in Figure 4.
C10 and C11 were chosen as cross-linkers due to their differing hydrophilic properties. C10 is slightly soluble in water and readily soluble in solvents like acetone, methanol, and tetrahydrofuran. In contrast, C11 is insoluble in water but soluble in acetone. By incorporating these cross-linkers with varying hydrophilicity into the ionomers within PP substrates, we investigated changes in the ionomers’ properties. The hydrophilic nature of the cross-linker influenced the ion transport channels and the overall morphology of the ionomer, which are critical factors in determining water electrolysis performance.
Table 2 presents the chemical structures, and theoretical and experimental IECs of two ionomers, E3-C10 and E3-C11. The theoretical IEC represents the value calculated based on the equivalent weight of ion-exchange groups in the polymer’s repeating unit, assuming complete reaction of the electrolyte and cross-linking monomers, relative to the dry weight of PFAEMs. In this study, we compared the theoretical IECs of PFAEMs incorporating the E3-C10 and E3-C11 ionomers. The results showed that E3-C10 and E3-C11 had similar IEC values of 2.20 and 2.16 meq g−1, respectively. This similarity suggests that disregarding differences in their chemical structures, the two ionomers may exhibit comparable physical properties. However, as previously discussed, the properties of E3-C10 and E3-C11, which utilize different cross-linkers—relatively hydrophilic C10 and hydrophobic C11—could vary. The IECs of E3-C10 and E3-C11 PFAEMs were measured at 1.57 ± 0.026 and 1.54 ± 0.038 meq g−1, respectively. As summarized in Table 2, although these measured IECs were slightly lower than the theoretical values, the trend remained consistent. Figure 6 illustrates the ion conductivity of PFAEMs containing these ionomers. PFAEMs fabricated by filling ionomers into a porous substrate have two different regions. The ionomer-filled region facilitates ion conduction, while the substrate region offers mechanical strength without contributing to ion conduction. The distinct separation between these regions promotes a physical hydrophilic–hydrophobic segregation effect. Typically, hydrophilic–hydrophobic segregation involves synthesizing polymers with significant differences in hydrophilicity between the main and side chains through multi-block or random co-polymerization. This structure enables more efficient ion conduction via ion-cluster channels within the polymer. PFAEMs adopt a strategy to enhance ion conductivity by physically forming such segregated structures. Examining the ion conductivity measurements of two ionomers with comparable theoretical and measured IECs, as shown in Figure 6, reveals that E3-C10 and E3-C11 exhibit conductivities of 98.7 ± 7.0 and 71.1 ± 4.0 mS cm−1, respectively, deviating from the IEC trend. In other words, despite similar IEC values, the ion conductivity of the E3-C10 ionomer, utilizing the hydrophilic C10, is 38% higher than that of E3-C11, which employs the hydrophobic C11. This indicates that the aforementioned hydrophilic–hydrophobic segregation effect is more pronounced in E3-C10. The enhanced performance of E3-C10 can be attributed to its increased hydrophilicity, which facilitates better ion transport in the ionomer region.
Ensuring low hydrogen permeability in AEMs is crucial for their application in AEMWEs, as it guarantees the production of high-purity hydrogen with enhanced efficiency. In this study, we measured hydrogen permeability under fully hydrated conditions with H2/N2 supplied under differential pressure, utilizing current measurement techniques. The hydrogen permeability was calculated based on crossover current, membrane thickness, and partial pressure. As depicted in Figure 7, the E3-C10 PFAEM exhibits lower hydrogen permeability compared to the E3-C11 PFAEM. This outcome is attributed to the triazine structure present in E3-C10, which effectively impedes hydrogen gas permeation, unlike the trimethylolpropane structure in E3-C11. The gas-impermeable nature of the E3-C10 structure contributes to the reduction in hydrogen permeability. Despite this difference, both E3-C10 and E3-C11 PFAEMs demonstrated exceptionally low hydrogen permeability values overall. Therefore, there are no concerns regarding the utilization of these PFAEMs in AEMWEs.
Mechanical-stability tests were conducted to investigate the effect of the structural formula of the cross-linking monomer. As shown in Figure 8a,b, the tensile strength of the PP substrate was measured at 86.94 MPa. Additionally, under both wet and dry conditions, the E3-C10 PFAEM exhibited tensile strengths of 100.2 and 183.9 MPa, respectively, which are higher than the 91.29 and 181.9 MPa recorded for the E3-C11 PFAEM. Based on the stress–strain curves, the higher mechanical strength of the PFAEMs compared to the PP substrate is attributed to the filling of the cross-linked ionomer into the PP substrate. Moreover, differences in mechanical strength between E3-C10 and E3-C11 PFAEMs exhibited based on the type of the cross-linking monomers. As shown in Table 1, C10 contains a triazine structural formula. It is well known that triazine-based compounds, including the C=N bonds in C10, enhance physical and mechanical properties when reacting with other components [43]. In other words, the triazine-containing C10 cross-linking-monomer-based PFAEM exhibited improved mechanical strength compared to the trimethylolpropane-containing C11 cross-linking-monomer-based PFAEM. These structural differences account for the observed variations in mechanical strength.
QA groups in the ionomer-filled region of PFAEMs can degrade, reducing their availability, through Hofmann elimination, nucleophilic substitution, and E1 elimination. Nonetheless, QA groups have been widely used in AEMs as OH− conduction pathways due to their high potential for OH− ion conduction in membranes. To overcome disadvantages such as Hofmann elimination, nucleophilic substitution, and E1 elimination, chemically stable polymer structures have been developed through structural improvements by selecting and utilizing highly stable monomers. To confirm the effect of the structural formula of the cross-linking monomer on the stability of AEMs, specifically PFAEMs, alkaline stability tests were conducted. These tests aimed to investigate the relationship between the structural formulas of the cross-linking monomers and the chemical stability of the fabricated PFAEMs. Herein, alkaline stability measurements of the triazine-structured E3-C10 and the trimethylolpropane-structured E3-C11 PFAEMs were conducted to investigate the structural effect of the cross-linking monomers. The PFAEMs were stored in 4 M KOH at 60 °C for up to 960 h to evaluate changes in OH− ion conductivity over time. As shown in Figure 9a, the E3-C10 PFAEM exhibited higher BOT and EOT OH− ion conductivities compared to the E3-C11 PFAEM. Furthermore, as illustrated in Figure 9b, the loss in ion conductivity was 5.35% for the E3-C10 PFAEM and 17.7% for the E3-C11 PFAEM. To analyze the degradation effects of the membranes based on the different cross-linking monomers, the structures of the monomers were compared. As shown in Figure 9b, the structural formula of C10 contains triazine groups without C–O bonds from ether or ester linkages. These features enhance chemical stability by suppressing degradation mechanisms such as Hofmann elimination, nucleophilic substitution, E1 elimination, and ring opening due to the bulky structure and ether-free linkage. In contrast, the C11 monomer contains ester linkages, which are susceptible to nucleophilic attack by water and OH− ions. Specifically, the C–O bonds in the ester group are vulnerable to hydroxyl radicals, leading to reduced chemical stability of the AEMs [44].
As shown in Figure 10a, the single cell performance increases in the order of E3-C10 (285.2 mA cm−2 at 1.9 V) < E3-C11 (338.7 mA cm−2 at 1.9 V). Additionally, as depicted in Figure 10b, the E3-C10 PFAEM-based single cell exhibited a Tafel slope of 153.3 mV dec−1, which is higher than that of the E3-C11 PFAEM-based single cell. Furthermore, as shown in Figure 10c, the E3-C10 PFAEM demonstrated higher ohmic overpotential and total resistance (RTotal, which includes the high frequency resistance (RHFR) + the charge transfer resistance (RCTR)). Specifically, the RTotal for E3-C10 was 1075 mOhm·cm2 at 1.9 V, compared to 856.6 mOhm·cm2 for E3-C11 at the same voltage. However, as illustrated in Figure 6, E3-C10 PFAEM exhibits higher OH− conductivity (98.7 ± 7.0 mS cm−1) and IEC (1.57 ± 0.026 meq g−1) than E3-C11 PFAEM (71.1 ± 4.0 mS cm−1 and 1.54 ± 0.038 meq g−1, respectively). In other words, it can be inferred that other resistance components, such as improper formation of the triple-phase boundary in electrode membrane-electrode interfacial resistance, contributed to the reduced current density observed in the single cell performance of E3-C10. Therefore, the material resistances, including membrane resistance, electrode resistance, and membrane-electrode interfacial resistance, were further investigated using separated resistance components (i.e., RHFR and RCTR), as illustrated in Figure 10d. Studies on optimizing the AEM-AEI combination have been conducted using various AEMs and AEIs, as the compatibility between AEM and AEI is crucial for achieving high performance in AEMWE [24,25]. In other words, the selection of an appropriate AEI as a binder in the electrode is critical due to the influence of the AEM-AEI combination. As shown in Figure 10d, the E3-C10 PFAEM-based single cell exhibited a lower RHFR compared to the E3-C11 PFAEM-based single cell, which aligns with membrane properties such as OH− ion conductivity and IEC. However, RCTR exhibited the opposite trend. In the context of RHFR, the PFAEMs’ performance in terms of OH− ion conductivity and IEC was consistent with their performance in water electrolysis. The reduced performance can be inferred as a result of interfacial resistance in RCTR, excluding electrode resistance, as the same electrode was used. Therefore, improvements to the membrane-electrode interfacial resistance should be considered to enhance overall cell performance.
The in situ durability of AEMWE with E3-C10 and E3-C11 was evaluated under constant-current conditions at 500 mA cm−2 with an asymmetric 1.0 M KOH-fed at 60 °C for 100 h. As shown in Figure 11a, the voltages increased during the constant-current operation. The degradation slopes, calculated from the time-dependent voltage variation, were used to compare membrane stability. The E3-C10 PFAEM exhibited a degradation slope of 1150 μV h−1, which is lower than that of the E3-C11 PFAEM. As mentioned in the alkaline stability section, when hydroxyl radicals are present, QA groups can degrade through Hofmann elimination, nucleophilic substitution, and E1 elimination, reducing their availability. Furthermore, polymers containing C–O and C=O bonds in ester groups are chemically unstable in AEMs. To investigate the degradation of AEMs, the PFAEMs used in the AEMWE in situ durability test were extracted and analyzed for their IEC. Prior to evaluating the IEC after the in situ durability test, the surfaces of the membranes that contacted both the anode and cathode electrodes were washed several times with D.I. water. Afterwards, the IEC of the PFAEMs was measured. As illustrated in Figure 11b, the E3-C10 and E3-C11 PFAEMs exhibited IEC values of 1.67 ± 0.038 and 0.93 ± 0.024 meq g−1, respectively. These results can be used only to observe the degradation trend of the PFAEMs under water-electrolysis operating conditions. This is because the PFAEMs used to evaluate the IEC at BOT and EOT were extracted from a single batch with a size of 12 × 12 cm2. In other words, the BOT and EOT PFAEMs were not identical. Nonetheless, the IEC of the E3-C11 PFAEM showed a significant and noticeable decrease. As shown in Figure 11b,c, the high loss in ion conductivity, as discussed in the alkaline stability section including factors such as the weakness of C–O and C=O bonds in ester groups, the amount of impregnated QA groups, and structural considerations, contributed to the high degradation slope. Moreover, the high loss in ion conductivity in the PFAEM properties correlated with the IEC decrease and the degradation slope under AEMWE operating conditions.
As shown in Figure 10a,d, and discussed in the AEMWE performance evaluation section, the OH− ion conductivity and IEC of PFAEM directly influence the performance of AEMWE, including RHFR. However, the current density was relatively low due to the high membrane-electrode interfacial resistance, as discussed in RCTR. To address this interfacial resistance, an ionomer layer-coated electrode was designed. The experiments were divided into four configurations: an ionomer layer-coated anode, an ionomer layer-uncoated anode, an ionomer layer-coated cathode, and an ionomer layer-uncoated cathode. Details are provided in Figure 12. As shown in Figure 12a, the AEMWE with both anode and cathode uncoated with the ionomer layer (An ×/Ca ×) exhibited the lowest performance, achieving a current density of 285.2 mA cm−2 at 1.9 V. However, coating the ionomer layer onto the anode and/or cathode improved current density, with sequential increases observed in the order of 397.9 mA cm−2 (An ×/Ca O), 428.9 mA cm−2 (An O/Ca ×), and 453.2 mA cm−2 (An O/Ca O). Furthermore, as illustrated in Figure 12b, RCTR sequentially decreased in the order of 1010 mOhm·cm2 (An ×/Ca ×), 508 mOhm·cm2 (An ×/Ca O), 443 mOhm·cm2 (An O/Ca ×), and 392 mOhm·cm2 (An O/Ca O) as the ionomer layer was applied. These results demonstrate that when the membrane-electrode interfacial resistance is high, it diminishes the usability of membranes with high ion conductivity, such as E3-C10 PFAEM. This issue can be mitigated by coating the ionomer layer onto the electrode.

4. Conclusions

This study explores the effects of cross-linkers with varying hydrophilicity on ionomers in PP substrates for water electrolysis applications. Unlike previous research focused on hydrophilic electrolyte monomers, two cross-linkers, E3-C10 (hydrophilic) and E3-C11 (hydrophobic), were utilized with a common electrolyte monomer to assess their impact on ionomer properties. Key findings include the successful creation of void-free PFAEMs, enhanced ion conductivity, and improved mechanical strength when hydrophilic and ester- and ether-free C10 was used. The PFAEMs demonstrated low hydrogen permeability and high chemical stability, particularly the E3-C10 PFAEM, which exhibited better alkaline stability. Performance tests showed that E3-C10 PFAEMs had higher ion conductivity and lower degradation compared to the E3-C11. However, membrane-electrode interfacial resistance affected the overall electrolysis performance, which was improved by coating electrodes with an ionomer layer. The study suggests that optimizing membrane-electrode interactions is crucial for improving the efficiency and durability of water electrolysis devices.
This study demonstrates that in energy conversion devices like water electrolysis, the electrolyte is assembled to the electrode layer. In this process, optimizing the compatibility of ion, electron, and gas transfer between the electrolyte and the electrode is essential to achieve optimal performance. Therefore, it is recommended that future research focus on identifying the specific physical properties of the electrolyte membrane under development.

Author Contributions

Conceptualization, J.-H.P. and J.-S.P.; methodology, J.-H.P.; validation, J.-S.P.; formal analysis, J.-H.P., Y.P., T.-S.J. and Y.S.; investigation, J.-H.P., Y.P., T.-S.J. and Y.S.; resources, J.-S.P.; data curation, J.-H.P., Y.P., T.-S.J. and Y.S.; writing—original draft preparation, J.-H.P.; writing—review and editing, J.-S.P.; supervision, J.-S.P.; project administration, J.-S.P.; funding acquisition, J.-S.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 01495 g001
Figure 1. Cross-sectional and surface FE-SEM image: (a) 95 μm thick, porous PP substrate utilized in this study; (b,c) pore size analysis.
Figure 1. Cross-sectional and surface FE-SEM image: (a) 95 μm thick, porous PP substrate utilized in this study; (b,c) pore size analysis.
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Figure 2. Schematic illustration for the preparation of PFAEMs.
Figure 2. Schematic illustration for the preparation of PFAEMs.
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Figure 3. Schematic illustration of single cell components for AEMWE.
Figure 3. Schematic illustration of single cell components for AEMWE.
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Figure 4. FE-SEM images of the porous PP substrate and PFAEMs: (a) cross-sectional view and (b) surface view of the PP substrate; (c) cross-sectional view of the E3-C10 PFAEM; (d) surface view of the E3-C10 PFAEM; (e) cross-sectional view of the E3-C11 PFAEM; (f) surface view of the E3-C11 PFAEM.
Figure 4. FE-SEM images of the porous PP substrate and PFAEMs: (a) cross-sectional view and (b) surface view of the PP substrate; (c) cross-sectional view of the E3-C10 PFAEM; (d) surface view of the E3-C10 PFAEM; (e) cross-sectional view of the E3-C11 PFAEM; (f) surface view of the E3-C11 PFAEM.
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Figure 5. FT-IR spectra of the PP substrate and fabricated PFAEMs.
Figure 5. FT-IR spectra of the PP substrate and fabricated PFAEMs.
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Figure 6. Through-plane ion conductivity of E3-C10 and E3-C11 PFAEMs in 1.0 M KOH at 60 °C.
Figure 6. Through-plane ion conductivity of E3-C10 and E3-C11 PFAEMs in 1.0 M KOH at 60 °C.
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Figure 7. Hydrogen permeability of E3-C10 and E3-C11 PFAEMs.
Figure 7. Hydrogen permeability of E3-C10 and E3-C11 PFAEMs.
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Figure 8. Tensile strength–strain curves of the porous PP substrate and the fabricated PFAEMs: (a) wet E3-C10 and E3-C11 PFAEMs; (b) dry E3-C10 and E3-C11 PFAEMs.
Figure 8. Tensile strength–strain curves of the porous PP substrate and the fabricated PFAEMs: (a) wet E3-C10 and E3-C11 PFAEMs; (b) dry E3-C10 and E3-C11 PFAEMs.
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Figure 9. Chemical stability of PFAEMs: (a) variation in ion conductivity of E3-C10 and E3-C11 PFAEMs stored at 4 M KOH and 60 °C as a function of storage time; (b) the decrement of OH− conductivity change between BOT and EOT.
Figure 9. Chemical stability of PFAEMs: (a) variation in ion conductivity of E3-C10 and E3-C11 PFAEMs stored at 4 M KOH and 60 °C as a function of storage time; (b) the decrement of OH− conductivity change between BOT and EOT.
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Figure 10. Performance of PFAEM-based AEMWE: (a) I-V polarization curves; (b) Tafel plots; (c) ohmic overpotentials and RTotal (= RHFR + RCTR); (d) RHFR and RCTR from 1.0 M KOH-fed AEMWE.
Figure 10. Performance of PFAEM-based AEMWE: (a) I-V polarization curves; (b) Tafel plots; (c) ohmic overpotentials and RTotal (= RHFR + RCTR); (d) RHFR and RCTR from 1.0 M KOH-fed AEMWE.
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Figure 11. In situ and ex situ durability of PFAEM-based AEMWE: (a) long-term durability of the asymmetric 1.0 M KOH-fed operation at 60 °C, 500 mA cm−2 of current density for 100 h; (b) BOT and EOT variations in IEC using AEMs of different samples; (c) relationship between the loss in ion conductivity of PFAEM and degradation slope in single cell durability evaluation.
Figure 11. In situ and ex situ durability of PFAEM-based AEMWE: (a) long-term durability of the asymmetric 1.0 M KOH-fed operation at 60 °C, 500 mA cm−2 of current density for 100 h; (b) BOT and EOT variations in IEC using AEMs of different samples; (c) relationship between the loss in ion conductivity of PFAEM and degradation slope in single cell durability evaluation.
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Figure 12. Performance of E3-C10 PFAEM-based AEMWE using ionomer layer-coated electrode from 1.0 M KOH-fed AEMWE at 60 °C: (a) I–V polarization curves; (b) Nyquist plots at 1.7 V.
Figure 12. Performance of E3-C10 PFAEM-based AEMWE using ionomer layer-coated electrode from 1.0 M KOH-fed AEMWE at 60 °C: (a) I–V polarization curves; (b) Nyquist plots at 1.7 V.
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Table 1. Chemical information of the monomers used in this study.
Table 1. Chemical information of the monomers used in this study.
CodeMonomerStructural Formula3D Model 1.MW (g mol−1)
E3(Vinylbenzyl) trimethylammonium chlorideApplsci 15 01495 i001Applsci 15 01495 i002211.73
C101,3,5-Triacryloylhexahydro-1,3,5-triazineApplsci 15 01495 i003Applsci 15 01495 i004249.27
C11Trimethylolpropane trimethacrylateApplsci 15 01495 i005Applsci 15 01495 i006338.40
1 Grey, red, blue, and white atoms represent carbon, oxygen, nitrogen, and hydrogen, respectively.
Table 2. Chemical structure, theoretical, experimental IEC, major bonds, and synthesis method of the cross-linked ionomers.
Table 2. Chemical structure, theoretical, experimental IEC, major bonds, and synthesis method of the cross-linked ionomers.
IonomerChemical
Structure		IEC (meq g−1)Major
Bonds		Synthesis
TheoreticalExperimental
E3-C10Applsci 15 01495 i0072.201.57 ± 0.026C-NUV
irradiation
E3-C11Applsci 15 01495 i0082.161.54 ± 0.038C-OC=OUV
irradiation
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Park, J.-H.; Park, Y.; Jeon, T.-S.; Seo, Y.; Park, J.-S. Impact of Cross-Linking-Monomer Characteristics on Pore-Filling-Membrane Performance and Durability in Anion-Exchange Water Electrolysis. Appl. Sci. 2025, 15, 1495. https://doi.org/10.3390/app15031495

AMA Style

Park J-H, Park Y, Jeon T-S, Seo Y, Park J-S. Impact of Cross-Linking-Monomer Characteristics on Pore-Filling-Membrane Performance and Durability in Anion-Exchange Water Electrolysis. Applied Sciences. 2025; 15(3):1495. https://doi.org/10.3390/app15031495

Chicago/Turabian Style

Park, Jong-Hyeok, Yeri Park, Tae-Seok Jeon, Yuna Seo, and Jin-Soo Park. 2025. "Impact of Cross-Linking-Monomer Characteristics on Pore-Filling-Membrane Performance and Durability in Anion-Exchange Water Electrolysis" Applied Sciences 15, no. 3: 1495. https://doi.org/10.3390/app15031495

APA Style

Park, J.-H., Park, Y., Jeon, T.-S., Seo, Y., & Park, J.-S. (2025). Impact of Cross-Linking-Monomer Characteristics on Pore-Filling-Membrane Performance and Durability in Anion-Exchange Water Electrolysis. Applied Sciences, 15(3), 1495. https://doi.org/10.3390/app15031495

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