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. 2025 Aug 1;5(5):425–434. doi: 10.1021/acsphyschemau.5c00009

Improved Mechanical Stability and Proton Conductivity of Reinforced Membranes for Proton Exchange Membrane Fuel Cells (PEMFCs)

Jae-Hun Kim †,‡,⬢, Min Su Noh ‡,§, Eun Jeong Shin ‡,∥, Soo Youn Lee ‡, Yuri Kim ‡,⊥, Hwi Jong Jung ‡,#, Hye Jin Lee ∇, Hae In Lee ○, Dong-Ha Lim ○, Yoo Seok Lee ◆,*, Hee Soo Kim ○,*, Sahng Hyuck Woo ‡,¶,*
PMCID: PMC12464781  PMID: 41019621

Abstract

As one of the electrochemical systems based on green chemistry, the fuel cell (FC) demonstrates strong sustainability in generating electricity without CO2 emissions. It operates primarily through the transportation of protons via a proton exchange membrane (PEM). However, the PEM requires high proton conductivity along with chemical and mechanical stability to improve FC performance. To develop PEMs at a low cost, researchers have explored various methods, including adding additives, cross-linking, and synthesizing new chemical structures. Among these methods, the reinforced composite membrane stands out as a promising technology due to its cost-effectiveness, low electrical resistance, and physical stability. However, their properties have not yet been fully summarized and organized in review articles, although reinforced membranes exhibit excellent performance. This article discusses the role and importance of the PEM in FCs and introduces significant characteristics and notable preparation strategies for reinforced composite membranes for enhancing FC performance.

Keywords: proton exchange membrane (PEM), cation exchange membrane (CEM), reinforced composite membrane, cross-linking, additive, hydrogen fuel cell, proton exchange membrane fuel cell (PEMFC), proton conductivity, chemical stability, mechanical stability

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1. Introduction

As global energy demand increases, the rising consumption of fossil fuels, the efficiency, reserve depletion, and development of hydrocarbon-free alternatives for environmentally friendly renewable energy production have garnered considerable attention over decades due to their eco-friendly characteristics. − Currently, over 80% of the world’s energy is consumed using fossil fuels, posing significant threats to global health and a sustainable future, primarily due to hazardous combustion product emissions. − Recently, several alternative green technologies have been explored to develop environmentally friendly and efficient systems using renewable energy resources. , Significant efforts are being directed toward cost-effective large-scale production, storage/transportation, and efficient green utilization of hydrogen, along with new technology development for sustainable energy. ,, Moreover, the significance of fuel cells, a crucial component of hydrogen-based energy technology, has greatly increased. Hydrogen fuel cells have emerged as an important solution to address global energy and environmental challenges. They offer high energy efficiency, produce only water as a byproduct, and can be powered by renewable hydrogen sources, making them a vital alternative to fossil fuels. Among them, PEMFCs are gaining attention as a promising next-generation energy source due to their high energy conversion efficiency and noncarbon emission characteristics, operating at low temperatures. , PEMs are particularly crucial as they significantly influence both performance and cost in PEMFCs. Although the well-known Nafion membrane offers exceptional performance, including low electrical resistance and mechanical and chemical stability, its high cost impedes its widespread application in PEMFCs. To address these challenges, developing reinforced membranes is an attractive alternative. This account discusses the role of PEMs, the characteristics of various reinforced membranes, and future perspectives for PEMFCs.

2. Role of Proton Exchange Membrane

In PEMFCs, protons migrate to the cathode through the PEMs under specific temperature and humidification conditions (Figure a). Enhancing electrical power requires the development of core materials, considering factors such as stack structure design, catalysts, and the conjugate moiety between the membrane and electrode. A key factor in improving PEMFC performance is the ionic transportation ability of the PEMs.

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(a) Operating principle of the PEMFC and (b) cationic (proton) migration in the ion channel of the PEM.

PEMs typically consist of a polymer backbone with negatively charged groups that attract cations (protons). Functional groups such as phosphonic acid (−PO3H2) and sulfonic acid (−SO3H) are prominent in PEMFCs. Consequently, PEMs form ion channels, which are nanosized pathways for proton migration (Figure b). − In these channels, protons migrate through two methods: (1) direct hopping through functional groups due to electric attraction, based on the Grotthuss-vehicle theory, and (2) migration surrounding water molecules in the ion channels.

Functional group usage varies according to the operating conditions in PEMFCs. For example, ion mobility and catalytic activity are enhanced while reducing catalyst poisoning effects above 100 °C. Phosphonic acid groups have been introduced for intermediate temperature operation (80–150 °C) due to their amphoteric structure and high dielectric constant. The sulfonic acid group is another key functional group in the PEMs. For instance, the widely used Nafion membrane comprises a PTFE backbone and side chain with sulfonic acid. The application of sulfonic groups stems from their high acidity, which facilitates the rapid transfer of hydrated acidic ions. Therefore, PEMs containing sulfonic groups generally exhibit high ion conductivity when humidity is present. Addressing both cost and performance concerns, researchers have developed various methods for constructing PEMs. Among these, reinforced membrane technologies stand out, offering specific factors that contribute to the enhancement of PEMFCs.

3. Reinforced Membrane for PEMFC

In this section, we present a detailed comparison of various reinforced membrane types and their corresponding performance enhancements. Table summarizes the key features, performance improvement mechanisms, and resulting enhancements in properties such as mechanical strength and proton conductivity. These improvements are critical for optimizing membrane performance in PEMFCs. The contents of the table are detailed in each section as follows.

1. Comparison of Reinforced Membrane Types and Their Performance Enhancement.

Reinforced membrane type Reason for the utilization of the developed polymers (features) Performance enhancement mechanism Performance improvement Reference
ePTFE + PFSA Provides high mechanical stability Prevents physical damage and enhances durability during PEMFC operation Tensile strength doubled from 15 to 30 MPa
PTFE + Catechol/PEI + SPEEK Enhances surface hydrophilicity Increases water retention, improving ion conductivity under humid conditions Proton conductivity improved from 5 to 180 mS/cm at 90 °C, 95% RH
SPEEK/PVDF + PFSA Enhances mechanical strength and surface energy Facilitates PFSA impregnation and durability enhancement Tensile strength increased up to 39 MPa (4 times higher than pristine PFSA)
SPPO + cBPPO Provides high surface area and stability Improves ionic conductivity through optimized pore filling Proton conductivity increased from 70 to 80 mS/cm (Room temperature)
PE film + Cross-linker Enhances thermal, mechanical, and oxidative stability Introduces cross-linked structure through UV curing Mechanical strength ∼130 MPa, thermal stability at 223.7 °C
SPAES + LCP nonwoven Maintains high proton conductivity while improving dimensional stability Improves water uptake and reduces methanol permeability Significant methanol crossover reduction
SPAES/DMAc + Silicate PPTA Enhances durability under low humidity Improves wettability and mechanical strength of substrate Maintains proton conductivity ∼118 mS/cm at 65 °C, 50% RH
THSPSA/DMAc + PI nonwoven Solves low conductivity issue in porous nonconductive substrates Maintains water retention and proton conductivity Proton conductivity 34.56 vs 35.70 mS/cm at 80 °C, 80% RH
ETFE + SAS Reduces cost while achieving high performance Enhances adhesion via acid–base hydrogen bonding Proton conductivity ∼104 mS/cm at 80 °C, 90% RH
PTFE + Aligned Nafion Aligns ion channels to enhance conductivity Facilitates direct proton pathways, reducing tortuosity Maintains proton conductivity above 120 °C (above T g)
PTFE + SPFBI Provides ultrathin, dense, and highly stable membrane Reduces swelling, enhances mechanical durability, and improves chemical stability Achieved 778 mW/cm2 power density; maintained OCV over 500 hat 90 °C, 30% RH
PTFE + SPP Enhances mechanical strength and reduces volume expansion Physical interlocking between PTFE and SPP ionomer improves toughness and dimensional stability 1.52 A/cm2 at 0.5 V for PEMFCs; 7.90 A/cm2 at 1.9 V for PEMWEs (32% and 16% higher than Nafion 212)
SiO2-enhanced pore-filling membrane Enhances ion conduction channel interconnectivity Capillary-driven impregnation of SiO2 nanoparticles into porous polyethylene substrate, followed by electrolyte impregnation and photopolymerization Improved perm-selectivity; IEC decreased from ∼1.6 to 1.1–1.4 mequiv/g; area resistance increased from ∼1.5 to 2–2.5 Ω·cm2
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3.1. ePTFE + PFSA

To ensure the durability of PEMFCs, high mechanical stability is essential in the PEM. During PEMFC operation, the membrane undergoes unfavorable in-plane tensile strength conditions due to alternating dehydration and saturation cycles. ePTFE provides excellent mechanical stability and low electrical resistance, minimizing physical damage during PEMFC operation. The introduction of ePTFE has emerged as an alternative to enhance mechanical stability, doubling that of Nafion 211 (30 vs 15 MPa). The membrane features a sandwich structure with a central ePTFE layer between PFSA layers. However, the degradation process of the membrane within the PEMFC system has not been thoroughly investigated. To examine the degradation process, morphological damage to the reinforced membrane was visualized through severity tests involving repeated supersaturation-dry phases (Figure a).

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(a) ePTFE + PFSA: initially, the membrane exhibits a uniform and defect-free structure. However, after repeated supersaturation/dry phase tests, membrane cracking was observed. Reproduced with permission from ref . Copyright 2020 Elsevier. (b) PTFE + catechol/PEI + SPEEK: the porous PTFE was modified into a hydrophilic substrate by immersing it in catechol/PEI. The PEM was fabricated by filling the SPEEK solution into the modified ePTFE. Due to the hydrophilicity of the modified PTFE, the optimized membrane showed enhanced conductivity and mechanical properties. It exhibited reduced degradation performance compared to that of the cast membrane in a single cell, particularly noticeable over 2000 wet/dry cycles. Reproduced with permission from ref . Copyright 2021 Elsevier. (c) SPEEK/PVDF + PFSA: the prepared substrate demonstrated a higher wettability compared to the porous substrate. The surface tension of the PFSA solution varied with the concentration. Optimizing the conditions between the SPEEK/PVDF substrate and the PFSA solution facilitated the creation of perfectly filled pores in the membrane. Reproduced with permission from ref . Copyright 2022 Elsevier. (d) SPPO + cBPPO: The substrate was fabricated via electrospinning to increase the surface-to-volume ratio. The polymer electrolyte solution of SPPO was pore-filled into the nanofiber mat. The optimized PEM displayed an improved power density, reaching up to 1050 mW/cm2. Reproduced with permission from ref . Copyright 2011 Elsevier.

3.2. PTFE + Catechol/PEI + SPEEK

The membrane was hydrophilized by forming a thin layer of catechol/PEI on top of the hydrophobic PTFE substrate, subsequently uniformly impregnated with SPEEK to enhance durability. Mechanical strength was improved to 28 MPa, as depicted in Figure b. Additionally, the hydrophilic modification of PTFE enhances water retention, contributing to higher ion conductivity even under low-humidity conditions. The proton conductivity of the prepared membrane improved significantly, from about 5 to 180 mS at 90 °C and 95% RH, attributable to the increased hydrophilicity. The phenomena of moisture-induced swelling and deswelling were minimized, thereby maintaining performance and preserving the membrane’s integrity even after extended cell operation.

3.3. SPEEK/PVDF + PFSA

PFSA was coated on the surface of the SPEEK/PVDF substrate. The fiber-reinforced SPEEK/PVDF substrate improves mechanical strength and surface energy, facilitating efficient PFSA impregnation and enhanced durability. A challenge in fabricating the composite membrane is the difficulty in impregnating PFSA into the polymer electrolyte due to the low surface energy of the substrate. To mitigate this, the substrate can be prepared by electrospinning a blend of PVDF and SPEEK, which enhances wettability. Furthermore, the concentration of the polymer electrolyte solution should be controlled by taking into account surface tension. A coating of 15 wt % PFSA on the SPEEK/PVDF surface results in a surface tension of 29 mN m–1 (Figure c), indicating that the reinforced membrane, with its high surface energy and low contact angle, offers a more wettable structure. Conversely, the PTFE film, with a contact angle (θ) ≥ 90° (eq ), poses challenges for impregnation with the PFSA solution.

δ=2γLV⁡cos⁡θrp 1

where γ LV, θ, and r p represent the surface tension of the ionomer solution, contact angle, and radius, respectively.

For this reason, using a low-concentration PFSA solution of 2.5 wt % or less can lead to excessive evaporation of the solvent and result in overcondensation of the PTFE substrate, forming a three-layer structure.

The reinforced membrane, composed of 0.52 mg cm–2 nanofiber-loaded SPEEK/PVDF, exhibited a stress–strain curve up to four times higher compared to that of pristine PFSA (reaching up to 39 MPa).

3.4. SPPO + cBPPO

The membrane was prepared by pore-filling SPPO into the nanofiber structure of cBPPO. The BPPO fiber was fabricated using an electrospinning deposition system, followed by a thorough cross-linking process involving ammonia treatment. The PEM was then prepared by infusing the synthesized SPPO into the cBPPO nanofiber. The cBPPO nanofiber offered distinct advantages, including a high surface-to-volume ratio and enhanced mechanical and chemical stability. As a result, the optimized membrane demonstrated an enhanced conductivity of up to 80 mS/cm, exceeding the room-temperature conductivity of the Nafion membrane (70 mS/cm).

3.5. PE Film + Cross-Linker (Three Cross-Linkable Sites or Two Cross-Linkable Sites)

Hydrocarbon-based PEMs exhibit several disadvantages such as low mechanical and electrochemical stability. To address these issues, the polymer electrolyte, infused into a porous substrate, was cross-linked using UV radiation. The porous substrate, made of PE, was immersed in monomer solutions mixed with ethylene sulfonic acid and either 1,3,5-triacryloylhexahydro-1,3,5-triazine (with three cross-linkable sites) or N,N′-ethylene bis-acrylamide (with two cross-linkable sites). The cross-linked membrane was then polymerized by applying UV radiation to the PE soaked between PET films. Both types of membranes showed improved thermal (223.7 °C), mechanical (130 MPa), and oxidative stability, attributable to the cross-linking structure and PE substrate. Notably, the membrane prepared with 1,3,5-triacryloylhexahydro-1,3,5-triazine exhibited a voltage of about 0.82 V after an accelerated lifetime test.

3.6. SPAES + LCP Nonwoven

As a hydrocarbon-type membrane, the SPAES membrane exhibits high proton conductivity and thermal stability. However, its application is limited due to low dimensional stability and increased water uptake with higher sulfonation degrees. To overcome these limitations, Yu et al. developed a reinforced membrane using LCP nonwovens. The membrane incorporated various types of SPAES synthesized from BP and HPF. Subsequently, the composite membrane was fabricated by filling SPAES into the porous substrate of the LCP nonwoven. This composite membrane not only improved mechanical properties but also enhanced water uptake. Moreover, the optimized composite membrane reduced methanol permeability by limiting methanol transportation. These improvements were attributed to the morphological differences in the SPAES within the substrate.

3.7. SPAES/DMAc + Silicate PPTA Substrate

To minimize proton conductivity reduction in the composite membrane, the porous substrate of PPTA was impregnated with a SiO2 solution using the sol–gel preparation method, as shown in Figure a. Mechanically, the silicate PPTA exhibited higher strength compared to the pristine PPTA (33 MPa vs 21.7 MPa). This improvement in the mechanical properties was accompanied by notably enhanced wettability. The composite membrane was prepared by infusing the SPAES proton exchange membrane solution into the silicate PPTA substrate. Consequently, the proton conductivity of the prepared membrane in low-humidity conditions demonstrated a smaller reduction rate compared to the pristine membrane, maintaining up to approximately 118 mS/cm over 15 min at 65 °C and 50% RH.

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(a) SPAES/DMAc + silicate PPTA substrate: the substrate prepared via the sol–gel reaction becomes more wettable due to SiO2 nanoparticles. Consequently, the composite PEM containing SiO2 shows improved proton conductivity at low humidity. Reproduced with permission from ref . Copyright 2012 Elsevier. (b) ETFE + SAS: the surface of the ETFE mesh is modified through hydroxylation, resulting in high adhesion of the SAS. The fabricated PEM demonstrates high proton migration. Reproduced with permission from ref . Copyright 2010 Elsevier.

3.8. THSPSA/DMAc + PI Nonwoven

The proton conductivity of the composite membrane is lower than that of the pristine membrane, primarily due to the use of a nonconductive porous substrate. To address this issue, THSPSA was coated on the PI nonwoven fiber through a sol–gel reaction. Subsequently, the cation exchange polymer solution was pore-filled into the sulfonated PI. The proton conductivity of the prepared membrane showed values similar to those of the pristine membrane (up to 34.56 mS/cm vs 35.70 mS/cm at 80 °C and 80% RH). This outcome was attributed to the retention of water molecules by the THSPSA coating on the PI fiber.

3.9. ETFE + SAS

Nafion 212, despite its high chemical and mechanical stability, is known for its high cost in PEMFC applications. To reduce costs while improving proton transportation, ETFE and a hydrocarbon-type membrane of SAS were utilized. Notably, the surface of the porous ETFE was chemically treated to function as a basic site, enhancing the adhesion between ETFE and SAS, as depicted in Figure b. This design leveraged the hydrogen bond between acid and base. In the modified ETFE mesh, now hydroxide, the synthesized SAS was easily impregnated using casting or dipping methods. This modification led to increased proton transportation (approximately 104 mS/cm at 80 °C and 90% RH) and improved water uptake (15% for the reinforced membrane vs 33% for the Nafion membrane), resulting in a membrane that demonstrated comparable performance to the Nafion 212 membrane.

3.10. PTFE + Aligned Nafion

In PEMs, protons transfer through tortuous ion channels that spontaneously form, following the second law of thermodynamic theory. However, this formation reduces ion conductivity, as protons must detour to migrate to the opposite side. The dipole groups (functional groups + counterions) in the PEM can react readily under an electric field. This composite membrane was prepared by applying a DC electric field and using the pore-filling method in porous PTFE. In the Nafion solution, ion channels were easily aligned at low electric field intensity within the porous substrate. The prepared membrane exhibited reduced amorphous regions and enhanced thermal stability. Notably, the aligned composite membrane maintained improved conductivity above 120 °C, which is the T g.

3.11. PTFE + SPFBI

Thuc et al. introduced a method for creating a poly­(tetrafluoroethylene) (PTFE)-reinforced composite membrane using sulfonated poly­(fluorenyl biphenyl) indole ionomer (PTFE-SPFBI) (Figure a). The resulting membrane is ultrathin and dense, demonstrating a reduced swelling ratio, high chemical stability, and exceptional mechanical durability, with over 200% elongation at break. The membrane’s performance in fuel cells is notable, achieving a maximum power density of 778 mW/cm2 and showing endurance in open-circuit voltage (OCV) tests for over 500 h at 90 °C and 30% relative humidity.

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(a) PTFE + SPFBI: a porous PTFE membrane is uniformly infiltrated with a sulfonated poly­(fluorenyl biphenyl) indole (SPFBI) ionomer, resulting in an ultrathin, dense composite membrane with improved mechanical durability and chemical stability. Reproduced with permission from ref . Copyright 2024 Elsevier. (b) PTFE + SPP: a five-layered composite membrane is fabricated by sequentially stacking sulfonated poly­(p-phenylene) (SPP) ionomer layers with n-propyl alcohol-treated porous PTFE substrates, significantly enhancing the mechanical strength and dimensional stability. Reproduced with permission from ref . Copyright 2023 Elsevier. (c) SiO2-enhanced pore-filling membrane: Nanoparticle-enhanced pore-filling ion exchange membranes (PIEMs) are prepared by impregnating porous polyethylene substrates with SiO2 dispersions followed by electrolyte filling and UV-induced photopolymerization, improving ion channel connectivity and membrane perm-selectivity. Reproduced with permission from ref . Copyright 2024 Elsevier.

3.12. PTFE + SPP

Noh et al. introduced a sulfonated poly­(p-phenylene) (SPP)/PTFE composite membrane for energy conversion devices (Figure b). The membrane is made by putting the SPP ionomer into a porous PTFE substrate. The composite membrane has a high current density of 1.52 A/cm2 at 0.5 V for PEMFCs and 7.90 A/cm2 at 1.9 V for PEMWEs. These values are 32% and 16% higher than those of Nafion 212, respectively. The use of PTFE improves mechanical properties and reduces volume expansion (1.7 times smaller) compared with pristine membranes, and the toughness of the composite membrane (6.0 ± 0.5 GPa) was 7 times higher than that of the pristine membrane (0.8 ± 0.1 GPa).

3.13. SiO2-Enhanced Pore-Filling Membrane

A new fabrication method for nanocomposite ion exchange membranes was developed by impregnating a porous polyethylene substrate (pore size ∼60 nm) with SiO2 nanoparticle dispersions (10–20 nm or 40–50 nm) via capillary forces, followed by electrolyte solution impregnation and photopolymerization (Figure c). The incorporation of nanoparticles enhanced ion conduction channel interconnectivity, improving perm-selectivity. However, it also reduced the electrolyte polymer content, leading to a decrease in ion exchange capacity (from ∼1.6 to 1.1–1.4 mequiv/g) and an increase in area resistance (from ∼1.5 to 2–2.5 Ω·cm2).

4. Future Perspectives for Pore-Filling Membranes in PEMFC Technology

Pore-filling membranes play a crucial role in advancing proton exchange membrane fuel cell technology by addressing issues related to durability and proton conductivity. Future research should focus on improving mechanical stability during hydration-dehydration processes to extend the fuel cell operating life. Simultaneously, it is essential to explore new materials and manufacturing methods to achieve a balance between high proton conductivity and robust mechanical strength. Optimizing the ionomer-to-catalyst ratio in the catalyst layer and enhancing the wettability of the porous support for uniform impregnation are critical for maximizing fuel cell performance. Furthermore, designing membranes tailored to specific performance requirements can be achieved by utilizing a variety of polymers, nanomaterials, and composite structures. Innovative manufacturing techniques, such as electrospinning and sol–gel processes, will aid in creating substrates with precisely controlled structures and enhanced functionality. Ultimately, future research should develop pore-filling membranes to address specific challenges, such as performance degradation under low-humidity conditions, thereby promoting the widespread application of high-performance and durable fuel cell technology.

5. Summary

The primary function of the PEM is proton migration through ion channels via the Grotthuss-vehicle mechanism. For cost reduction and performance improvement in PEMFCs, attributes such as low electrical resistance (or high ion conductivity) and chemical and mechanical stability are essential. Particularly, the membrane’s mechanical properties are critical for long-term operation, as repeated hydration/dehydration cycles during PEMFC operation can cause degradation and dimensional changes in the membrane. A significant advantage of reinforced membranes is the substantial improvement in mechanical properties due to the introduced substrate. Moreover, using a thin PEM with the substrate can reduce both the membrane’s electrical resistance and its cost. To address common drawbacks of reinforced membranes, several low-cost preparation methods have been introduced, showing performance fully comparable to that of Nafion. We believe that these novel approaches hold promise as effective membrane technologies for green electrochemistry applications.

Acknowledgments

This work was partially supported by the Korea Institute of Industrial Technology and the Commercialization Promotion Agency for R&D Outcomes (COMPA) grant funded by the Korean Government (Ministry of Science and ICT) (RS-2023-00304763). This work was supported by the Research and Development Program of the Korea Institute of Energy Research (KIER) (C5-2457). This research was supported by the Korea Ministry of Environment (RS-2021-KE001986), which is greatly appreciated. This study was supported by a research fund from Tech University of Korea, 2023. This work was supported by a research fund from Siheung Green Environment Center, 2025. This work was also supported by KITECH (UR250019).

Glossary

Abbreviations:

BP

4,4′-bisphenol

HPF

9,9′-bis­(4-hydroxyphenyl) fluorine

cBPPO

cross-linked bromomethylated poly­(2,6-dimethyl-1,4-phenylene oxide)

ePTFE

expanded polytetrafluorethylene

ETFE

ethylene tetrafluoroethylene

LCP

liquid crystal polymer

PE

polyethylene

PET

polyethylene terephthalate

PFSA

perfluorosulfonic acids

PEM

proton exchange membrane

PEMFC

proton exchange membrane fuel cell

FC

fuel cell

PEI

polyethylenimine

PI

polyimide

PPTA

poly­(paraphenylene terephthalamide)

PTFE

poly­(tetrafluoroethylene)

PVDF

polyvinylidene fluoride

SAS

styrene-acrylic-acid vinyl sulfonated polymer

SPAES

sulfonated poly­(arylene ether sulfones)

SPEEK

sulfonated polyether ether ketone

SPPO

sulfonated poly­(2,6-dimethyl-1,4-phenylene oxide)

Tg

glass transition temperature

THSPSA

3-trihydroxysilyl propane-1-sulfonic acid

UV

ultraviolet

J.-H.K.: writingoriginal draft, visualization, writingreview and editing; M.S.N.: writingrevised draft; E.J.S.: writingrevised draft; S.Y.L.: conceptualization, funding acquisition; Y.K.: writingrevised draft; H.J.J.: writingrevised draft; H.J.L.: conceptualization; H.I.L.: conceptualization; D.-H.L.: conceptualization, funding acquisition; Y.S.L.: conceptualization, writingreview and editing; H.S.K.: conceptualization, writingoriginal draft; S.H.W.: conceptualization, writingoriginal draft, writingreview and editing, supervision.

The authors declare no competing financial interest.

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