Recent advances in non-perfluorinated sulfonic acid proton exchange membranes in the energy field

Abstract

The proton exchange membrane (PEM) has become one of the most popular materials for membrane-based energy conversion and storage devices due to its robust proton transport performance and high selectivity. It holds significant promise in applications such as fuel cells and electrolytic hydrogen production. This review aims to explore the latest scientific research on the development of non-perfluorinated sulfonic acid proton exchange membranes (NPFSA-PEM). It analyzes the effects of NPFSA-PEM crosslinking methods on membrane performance, including hydrogen bond crosslinking, thermal crosslinking, and radiation crosslinking mechanisms, providing a comprehensive assessment of membrane enhancement. Additionally, the review summarizes the characteristics of NPFSA-PEM, including proton conduction capacity, mechanical stability, thermal stability, methanol barrier ability, and more. It highlights recent research progress, and enhancement pathways, and evaluates various optimization strategies. The review also delves into the research progress of NPFSA-PEM in applications such as fuel cells, flow batteries, and electrolytic hydrogen production. By discussing research prospects and associated challenges, the review provides insights into the future direction of NPFSA-PEM in these areas. Overall, it offers a comprehensive summary and evaluation, aiding researchers in understanding the latest advancements in NPFSA-PEM within the field of energy. It sheds light on the role of NPFSA-PEM development in advancing energy conversion and storage and is expected to provide new momentum for the development of clean energy technology.

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1 Introduction

With the continuous growth of global energy demand and the prominence of environmental problems, research on new clean energy, efficient energy conversion technology, and storage technology has become a hot topic in the energy field.¹ Among these, the proton exchange membrane fuel cell (PEMFC) stands out as a technology expected to replace traditional fossil fuels and meet the needs of sustainable development in the future. Due to its advantages of high energy conversion efficiency and low pollution,^2,3 PEMFC has become a widely used green power generation technology that has attracted significant attention in the field of new energy.^4,5 Additionally, non-perfluorinated sulfonic acid proton exchange membranes (NPFSA-PEM) have been extensively researched in areas such as electrolytic hydrogen production and flow batteries,^6,7 particularly in the field of electrolytic hydrogen production where efficient and stable proton exchange membranes are crucial for improving efficiency and hydrogen purity.⁸

As the core component of these devices, the performance of the proton exchange membrane directly determines the device's performance, lifespan, and reliability. NPFSA-PEM, characterized by excellent proton conduction performance,⁹ strong physical and chemical stability,¹⁰ and engineering application potential, has emerged as a standout among various proton exchange membrane materials. NPFSA-PEM is a proton exchange membrane that incorporates sulfonic acid groups but does not form perfluorosulfonic acid groups. It selectively allows proton permeation while blocking channels for other ions, electrons, and gas reactants to pass through the membrane structure and intramembrane groups.¹¹

The preparation process of NPFSA-PEM typically involves attaching the sulfonic acid functional group to the polymer substrate in a specific manner,^12–14 followed by post-treatments, such as increasing the sulfonic acid functional group content or adding additives,^11,15 to enhance proton conduction performance and stability before preparing the membrane using solution-casting method.^7,16 With the continuous development of interdisciplinary research in nanotechnology and polymer materials, the structure and performance of NPFSA-PEM have further been optimized and improved.¹⁷ New types of NPFSA-PEM have been prepared by enhancing proton conductivity and mechanical stability through methods such as group grafting,^18,19 nanomaterial composites and doping,^20–22 structure design,^23,24 and cross-linking optimization.²⁵ NPFSA-PEM doped with conductive polymers, nanofillers, and other special materials often exhibit improved proton conductivity but reduced mechanical strength and other properties of the membrane material.²⁶ Therefore, to ensure excellent performance of NPFSA-PEM, it is necessary to establish a stable and reliable connection among the polymer network, sulfonic acid functional groups, and doped materials, while ensuring phase uniformity of the membrane material. Moreover, to withstand harsh environments, such as extreme heat and heavily polluted conditions,²⁷ it is crucial to enhance the temperature stability and anti-fouling properties of NPFSA-PEM.²⁸ Considering the presence of liquids like water and methanol in the working environment, the swelling and structural stability of NPFSA-PEM are challenged,²⁹ making it important to control the hydrophilicity and group composition of NPFSA-PEM. Thus, there is an urgent need to further develop NPFSA-PEM with strong cycle performance and a wide operating temperature range.³⁰ Energy conversion efficiency is a vital indicator for membrane material devices, and the development of NPFSA-PEM materials with high conversion efficiency plays a significant role in promoting the advancement of energy conversion and storage devices.

NPFSA-PEM has been reviewed in some existing literature, Liu et al.³¹ have provided a comprehensive summary of the preparation methods, structural characteristics, stability, proton conduction performance, proton conduction mechanism, and application potential of different MOF-based PEMs. Neelakandan et al.³² conducted a review on the latest research progress and optimization strategies of branched-chain polymers as proton exchange membranes for low- and high-temperature fuel cells. Xu et al.³³ classified and summarized comments on SPAEK- and sulfonated poly aryl ether sulfone(SPAES)-based membranes, offering suggestions for improving organic–inorganic membranes. However, most literature focuses on the application of NPFSA-PEM in fuel cells, and there is little review on the development of NPFSA-PEM in the field of flow batteries and electrolysis. In addition, there is a lack of comprehensive and detailed review on the performance optimization of NPFSA-PEM and its application in the energy direction. Therefore, this work begins with the cross-linking methods commonly employed in the preparation of NPFSA-PEM, and combines the latest research results to review the performance optimization of SPAPEM in various aspects and its performance in many applications, aiming to provide new insights for promoting the development of NPFSA-PEM and energy conversion and storage. Firstly, we have summarized and evaluated the mechanisms of various crosslinking methods, such as hydrogen bond crosslinking and radiation crosslinking, in influencing the performance of membrane materials during the membrane preparation process. This effort aims to assist researchers in selecting the most suitable crosslinking method for the preparation of high-performance NPFSA-PEM. Secondly, we have outlined the mechanisms through which NPFSA-PEM influences proton conductivity, temperature stability, and oxidation resistance. We have explored enhancement pathways and proposed optimization strategies based on these mechanisms. Thirdly, we have summarized and discussed the typical applications of NPFSA-PEM in the energy sector, including proton exchange membrane fuel cells, electrolytic hydrogen production, and flow batteries. Furthermore, we have evaluated the potential feasibility of NPFSA-PEM applications in the energy field. Finally, we have discussed optimization strategies and outlined the future development trends of NPFSA-PEM. This article comprehensively analyzes the latest research on NPFSA-PEM in the energy field, aiming to further promote the development of membrane science and clean energy technology. It holds reference significance for advancing energy transformation and sustainable development.

2 Common crosslinking methods used in the preparation of NPFSA-PEM

NPFSA-PEM is typically composed of a polymer matrix, a functional group (usually a sulfonic acid group –SO₃H), and an additive (crosslinker or additive).²⁹ Currently, NPFSA-PEM is commonly prepared using methods such as sol–gel method (Fig. 1a) solution-casting method (Fig. 1b),^34,35 and others, with the process being relatively mature. In this section, we will analyze the commonly used crosslinking methods in the membrane preparation process, explore the strengthening mechanism of different crosslinking methods, and propose relevant optimization strategies.

Fig. 1 (a) Gel-PBI-PS/BS was prepared by the sol–gel method. Reproduced with permission.³⁴ Copyright 2023, Elsevier. (b) PBI-RGO/PPBI/PBI-RGO membranes were prepared by solution casting method. Reproduced with permission.³⁵ Copyright 2018, Elsevier.

2.1 Hydrogen bond crosslinking

In the process of hydrogen bond crosslinking, selecting the appropriate hydrogen bond donor and acceptor is crucial. When groups containing the hydrogen bond donor and acceptor are in close proximity, their lone pairs interact, forming hydrogen bonds. Consequently, a network of hydrogen bond crosslinking gradually forms between polymer chains through the establishment of multiple hydrogen bonds (Fig. 2).³⁶ Hydrogen bond crosslinking exhibits a strong correlation with the Grotthuss mechanism in the proton conduction process.³⁷ This crosslinking mechanism fosters the creation of proton conduction channels, significantly enhancing the rate of proton conduction. Additionally, the presence of hydrogen bond crosslinking in NPFSA-PEM heightens the membrane's structural strength and thermal stability at the molecular level. Furthermore, hydrogen bond crosslinking optimizes the membrane's methanol blocking ability.

Fig. 2 (a) The structure and interchain hydrogen bonds of the PIIK. Reproduced with permission.³⁸ Copyright 2015, Elsevier. (b) Self-crosslinking hydrogen bond network. Reproduced with permission.³⁹ Copyright 2021, The American Chemical Society.

Guo et al.³⁶ prepared a composite membrane of allyl-cyanuric trisulfonic (ACSA) and allyl-mPBI (APBI) that formed a tight network of hydrogen ion bonds through cross-linking. This microphase separation structure promoted the formation of proton conduction channels over a wide range of humidity. Kumar et al.⁴⁰ prepared SPES/SMWCNT/SPES composite membranes, which separated and interconnected the nanophases through hydrogen bonding. This improved the integration of hydrophilic and hydrophobic layers of SPES and SMWCNT, enhancing the tensile and mechanical properties of the membranes. Additionally, it promoted the formation of continuous proton conduction channels through the Grotthus mechanism. From these studies, it can be concluded that hydrogen bond crosslinking is beneficial for the formation of micro–nano composite structures in membranes, thereby enhancing membrane proton conductivity and mechanical stability. This provides an excellent example of the application of hydrogen bond crosslinking in membrane design. Zheng et al.⁴¹ prepared a Vr-SO₃H@Gly membrane that induced interaction between glycine (Gly) molecules through the sulfonic acid grafting channel, forming ordered acid–base pairs and a continuous hydrogen bonding network. Similarly, Duan et al.²⁹ introduced sulfonic acid groups into a metal–organic framework through grafting reactions, forming strong hydrogen bonds that promoted the formation of hydrogen bond networks. These studies demonstrated the feasibility of introducing sulfonic acid groups through grafting reactions to form a strong hydrogen bond crosslinking network, resulting in improved proton conduction performance. Liu et al.⁴² prepared GO/SL membranes embedded with sulfonated lignin (SL). The abundant sulfonic acid groups and ether bonds in SL provided additional proton skipping sites and formed a continuous hydrogen bonding network. By doping iHOFs and SL to enrich the hydrogen bond network in the membrane, they enhanced the degree of hydrogen bond cross-linking, thereby improving the proton conduction performance of the membrane. This reveals the mechanism of synergy between hydrogen bond cross-linking and material doping in enhancing membrane performance. Chang et al.³⁸ prepared a novel hydrogen-bonded cross-linked SPIEEK membrane, which exhibited low methanol permeability due to strong interchain interactions between imide and sulfonic acid groups. These research findings provide theoretical and experimental insights into improving methanol barrier performance through hydrogen bond cross-linking. Wang et al.⁴³ prepared a highly elastic, heatable, and durable membrane material (PU-IL-MX) through the complexation of poly(urea carbamate), ionic liquids (ILs), and MXene nanosheets. The ultra-high density hydrogen bonding cross-linking network inside the membrane endows the PU-IL-MX membrane with excellent IL retention performance and strong self-healing ability. During the self-healing process of two separate membrane materials in contact with each other, the tensile strength of PU-IL-MX membrane increases with the increase of healing time, and after 10 hours of healing time, it recovers about 90% of its original mechanical strength. The healed PU-IL-MX membrane can also fully restore its original proton conductivity. Attributed to the high mobility of PU chains, the mutual diffusion and entanglement of PU chains on the fracture surface, as well as the reorganization of hydrogen bonds between chains, enable the fracture membrane to heal. This proves that hydrogen bonding crosslinking is an important method to enhance the self-healing ability of the membrane.

These studies highlight the key role of hydrogen bond cross-linking in proton exchange membranes (PEMs). Hydrogen bond cross-linking can be achieved by introducing ionic hydrogen bond organic frameworks, sulfonated lignin, graphene oxide, and other materials, resulting in significant improvements in proton conductivity and structural stability. Consequently, these findings deepen our understanding of the mechanism of hydrogen bond crosslinking and provide an experimental and theoretical basis for the design of more efficient and stable proton exchange membranes, ultimately improving the performance of fuel cells and advancing the development of membrane materials for practical applications.

2.2 Heat-induced crosslinking

During the process of thermal crosslinking, the material undergoes initial pretreatment under suitable preheating conditions. Subsequently, the chemical bonds of the material are broken and recombined under high-temperature conditions, resulting in the formation of a new cross-linked structure.^44,45 The impact of thermal crosslinking on membrane properties is typically manifested in physicochemical stability and proton conduction properties (Fig. 3a).^46,47

Fig. 3 (a) Thermal self-crosslinking process in membrane preparation. Reproduced with permission.⁴⁷ Copyright 2022, The American Chemical Society. (b) Proposed proton transfer mechanism in the thermally crosslinked membranes. Reproduced with permission.⁴⁸ Copyright 2021, Elsevier.

Haragirimana et al.⁴⁸ prepared a NPFSA-PEM blend through thermal crosslinking in a mixed sulfonated poly(ether ketone)SPEEK/SPAES system (Fig. 3b), obtaining a hot-crosslinked membrane with excellent mechanical properties, strong proton conductivity, and a stable swelling ratio. The introduced cross-linked structure at high temperatures gives the membrane material good thermal stability, slowing down decomposition or structural damage under high-temperature operating conditions. This provides an experimental and theoretical basis for studying high-temperature proton exchange membranes. Nor et al.⁴⁹ significantly increased the water absorption of the nanocomposite membrane by incorporating imogolite into a highly sulfonated SPPSU polymer matrix through thermal crosslinking, thereby enhancing the proton conductivity of the membrane. Joseph et al.⁵⁰ prepared PBI membranes through thermal crosslinking, where the thermal crosslinking mechanism includes the Friedel–Crafts reaction between the hydrolysis of imidazole groups and the phenyl ring of PBI. The study of thermal crosslinking by adding sulfonated components and electron-rich aromatics to avoid hydrolytic chain break events at lower temperatures or shorter reaction times showed good thermal stability, hydrolytic stability, and air tightness. The above two results illustrate the effect of thermal crosslinking on the water stability of membrane materials and provide an optimization strategy for controlling thermal crosslinking conditions and, consequently, the stability of membrane water. Yagizatli et al.⁵¹ treated the SPEEK–PVA blend membrane at 180 °C for 48 hours to reduce the water solubility of the membrane material. The hydroxyl and sulfonic acid groups in the membrane were thermally crosslinked, making the membrane more oxidative stable. Han et al.⁵² prepared a novel SPEEK nanofiber composite membrane, cross-linked in the acidic SPEEK matrix after heat treatment with dimethyl sulfoxide and 180 °C. The oxidation stability of the composite membrane was significantly improved after thermal cross-linking treatment. Mohammed et al.⁵³ prepared a cross-linked PVA-sPTA membrane, incorporated with sPTA to improve the thermal stability of the membrane. Proton conductivity increased with the workload, and improved thermal stability promoted proton conductivity. Additionally, thermal crosslinking improved the methanol blocking performance of the membrane material. Guo et al.⁴⁴ studied the high-density crosslinking network formed by thermal crosslinking in the tqPBI-dBPEI/CePMP membrane, enhancing the air permeability of the membrane material and inhibiting methanol permeation. In both studies, the mechanism of thermal crosslinking action to improve the methanol barrier performance of membranes was investigated, providing an effective strengthening method.

As an important modification method, thermal crosslinking plays a key role in the physical and chemical stability, proton conduction performance, thermal stability, and methanol blocking performance of proton exchange membrane materials. These studies provide a useful reference and enlightenment for the design and preparation of high-performance and high-stability proton exchange membranes.

2.3 Initiator crosslinking

Initiator cross-linking refers to the process in which the initiator induces a cross-linking reaction between polymer chains, resulting in the formation of a cross-linked structure (Fig. 4). The selection of initiators and trigger conditions needs to be strictly controlled in this process, and the temperature and rate control during the reaction will also impact the structure and properties of the polymer.^54,55

Fig. 4 (a) Mechanism of the surface-initiated free radical polymerization of AMPS and SSA onto the amino-propylated silica nanoparticles initiated by the Ce(IV)-based redox initiation system. Reproduced with permission.²⁰ Copyright 2013, Elsevier. (b) ClNH₂ membrane crosslinking protocol with SO₂Cl-PES as crosslinker. Reproduced with permission.⁵⁶ Copyright 2014, Elsevier.

Initiator crosslinking enhances the thermal stability, proton conductivity, and mechanical properties of membrane materials.^57,58 The properties of membrane materials can be optimized and controlled through the selection of crosslinkers and reaction conditions.⁵⁹ However, compatibility with membrane materials must be considered to avoid degradation and deterioration caused by blind introduction.⁶⁰ Kumar et al.⁶¹ sulfonated polyvinylidene fluoride (SPVDF) and nano-alumina (Al₂O₃) with aniline-2-sulfonic acid (A2S) in a new interpenetrating polymer network (IPN) membrane material, using the crosslinker divinylbenzene (DVB). Different A2S concentrations and crosslinker DVB were used to determine the water absorption, swelling behavior, and ion exchange capacity of the cast membrane, revealing the influence mechanism of the crosslinker on the membrane performance. Nederstedt et al.⁶² prepared a large initiator with a poly(p-terphenyl)alkyl backbone, increasing backbone stiffness through cross-linking to replace the biphenyl. The synthesized membrane material exhibited an obvious phase separation structure, demonstrating high proton conductivity, moderate water absorption, and low temperature dependence. The study revealed the mechanism of enhancing the stiffness of the membrane structure and the structural phase structure in the process of crosslinking. Liu et al.⁶³ utilized initiator crosslinking in the synthesis of SPEEK/SHNT nanocomposite membranes. The structural similarity and hydrogen bond interaction of the surface grafting of the fillers achieved excellent interfacial compatibility. The introduction of SHNTs with a high aspect ratio improved connectivity along the long-dimensional ion conductive domain of the nanotubes, enhancing proton conductivity and reducing the activation energy of proton transport. Wong et al.⁶⁴ revealed that sulfosuccinic acid (SSA) as a crosslinker for CS/SPVA composite membranes had a positive effect on the proton conductivity of membrane materials. The alkyl group attached to the –SO₃ group in SSA provided a larger proton transport site in the membrane. These studies provide excellent research cases for the compatibility of crosslinkers and doped materials to synergistically enhance the proton conduction properties of membranes. Erkartal et al.⁶⁵ prepared a novel ternary composite membrane PVA/PAMPS/ZIF-8, and the in situ chemical crosslinking of glutaraldehyde (GA) strengthened the moisture management of the membrane materials, improving proton conductivity through moisture management. This indicates that initiator crosslinking can control the moisture management performance of the membrane.

In the above research process, the crosslinker enhances the proton conduction performance, temperature stability, and water management performance of the membrane through various initiation methods to reorganize the structure and composition of the membrane material and design the structure. This fully demonstrates that initiator crosslinking is an effective means to enhance the performance of NPFSA-PEM. It also provides an important reference for the design and preparation of proton exchange membranes with excellent performance. Research on initiator crosslinking is helpful to optimize the performance of membrane materials and promote the application of proton exchange membranes in fuel cells and other fields.

2.4 Radiation crosslinking

Radiation crosslinking initiates a crosslinking reaction between polymer chains by emitting high-energy particles or photons from a suitable radiation source (Fig. 5a).⁶⁶ This energy is then transferred through the polymer material, causing interactions among the polymer chains. Within this process, polymer chains break and rearrange, ultimately forming a cross-linked structure. Covalent bonds are established between the polymer chains, resulting in the improved stability of the three-dimensional cross-linking structure.⁶⁷

Fig. 5 (a) Radiation-induced styrene graft polymerization into PMP membranes to form polystyrene-graft-polymethylpentene copolymers. Reproduced with permission.⁶⁶ Copyright 2021, Elsevier. (b) Synthesis of UV-crosslinkable SCKs. Reproduced with permission.⁶⁸ Copyright 2018, WILEY.

Yan et al.⁶⁸ synthesized a novel sulfonated poly(arylene ether ketone)s (SCKs) containing photosensitive chalcone units in the main chain by condensation polymerization of 4,4′-dihydroxylchalcone, perfluorobiphenyl, and sodium 5,5′-carbonyl bis(2-fluorobenzenesulfonate) (Fig. 5b). The photocrosslinking of hydrated SCK membranes is achieved under UV irradiation, and the cross-linking structure formed in the hydrated membrane endows the crosslinked SCK membrane with stronger performance, greatly improving mechanical and chemical stability, as well as proton conductivity. This work indicates that the crosslinked membrane obtained through UV irradiation in a hydrated state is a promising candidate for PEM materials in fuel cells. Benavides et al.⁶⁹ synthesized styrene-co-acrylic acid copolymers using the free primitive copolymerization method and treated them with sulfonation and gamma radiation using sulfuric acid and silver sulfate. This treatment optimized the mechanical and thermal stability of the membrane materials. However, the use of a sulfonic acid group cross-linked copolymer matrix was found to reduce the proton conduction properties of the membrane materials. In this study, the mechanism of radiation cross-linking in enhancing the thermal stability of the membrane was elucidated, emphasizing the need to carefully control conditions to balance other membrane properties during the radiation process. Balasubramanian et al.⁷⁰ synthesized a stilbene membrane containing a sulfonated polyimide membrane, doubling the hydrolytic stability and further enhancing the oxidation stability of the membrane material after photo-crosslinking. Sohn et al.⁷¹ synthesized ETFE-based proton exchange membranes through synchrotron radiation method, exhibiting high proton conductivity, lower water absorption and size change, and higher chemical stability compared to non-crosslinked membranes. The studies by Balasubramanian⁷⁰ and Sohn⁷¹ highlighted the influential mechanisms of radiation cross-linking on membrane properties, such as oxidation resistance, proton conductivity, and water absorption. They also provided crucial insights and improvement strategies for incorporating radiation cross-linking into high-performance membrane materials, signifying its strong guiding significance.

2.5 Composite crosslinking

Composite crosslinking is currently the most widely used method, combining two or more materials to synthesize composite materials with superior properties.⁷² This technique allows for the organic combination of different materials, resulting in enhanced overall properties. PEM can be tailored for specific properties by introducing materials with different characteristics (Fig. 6a).^73,74 However, it is crucial to strictly control reaction parameters, including preparation conditions, temperature, and time, during the composite crosslinking process. The performance of membrane materials can be optimized through careful control and adjustment of these parameters.

Fig. 6 (a) GPS membranes were prepared by composite crosslinking. Reproduced with permission.⁷⁴ Copyright 2021, Elsevier. (b) Schematic diagram of the PDA-ADPS process by composite crosslinking. Reproduced with permission.⁷⁵ Copyright 2024, Elsevier.

Kamjornsupamitr et al.⁷³ utilized chitosan, polyvinyl alcohol, and sulfonic acid-functionalized silica nanoparticles as raw materials. The composite proton-conductive membrane was synthesized with SSA and glutaraldehyde as double crosslinkers. The introduction of SSA promoted the formation of a tighter structure, improving water absorption. Simultaneously, an internal network structure was formed between PVA and SSA, further optimizing the membrane properties. Shabanpanah et al.⁷⁶ incorporated diphenylamine-4-sulfonic acid sodium salt and silica nanoparticles into a polyvinyl alcohol (PVA) matrix, cross-linked with glutaraldehyde (GA) under acidic conditions. They synthesized novel cross-linked PVA with varying glutaraldehyde concentrations using the solution casting method. With increasing GA and silica nanoparticle content, the thermal and mechanical stability of the membrane material improved. However, the rise in crosslinker content simultaneously resulted in a reduction in proton conductivity. Cai et al.⁷⁴ prepared a nacre lamellar PEM (GPS-X) based on SPVA and PDA cross-linked GO sheets. PDA served as an interfacial crosslinker to enhance the interfacial strength of the GO substrate membrane (Fig. 6a). GPS-X exhibited excellent mechanical properties and proton conductivity, successfully improving the mechanical properties and proton conductivity of the composite membrane. Fan et al.⁷⁵ synthesized PDA-ADPS through a Michael addition reaction and incorporated it into a CS matrix to prepare CS/PDA-ADPS composite membranes (Fig. 6b). PDA-ADPS nanoparticles exhibited good compatibility and dispersion in the CS matrix. Acid–base ion pairs formed in the CS matrix, constructing new proton transport channels, thereby enhancing proton conductivity and mechanical properties. This provided an effective material doping method for improving the proton conduction performance of composite membranes. Bosson et al.⁷⁷ employed covalent crosslinking of fluorothiol modification of PPFS unit by (ethyleneedioxy)diethyl mercaptan and ionic crosslinking by mixing sulfonated anionic copolymer with polybenzimidazole (PBI-OO) for ionic crosslinking. This improved the mechanical stability of the membrane material, reduced the water absorption rate, and promoted the establishment of an efficient proton transport channel across the membrane. Qian et al.⁷⁸ prepared an amphoteric composite membrane composed of SPEEK containing an imidazole chain (SPEEK–IM) and side-chain sulfonated poly ether sulfone (CSPF) with covalently cross-linked perfluoroalkyl chain. This formed a covalent cross-linked structure, a semi-interpenetrating network, and a hydrogen bond network in the membrane, promoting proton transport. It had a certain effect on the mechanical properties and chemical stability of the membrane, providing a beneficial optimization method for improving the comprehensive performance of the composite membrane.

In summary, these studies establish the groundwork for further optimization and the broadened applications of proton exchange membranes. They have achieved multifaceted optimization of membrane material properties through composite crosslinking methods, offering a pivotal material foundation for the advancement of energy conversion and storage technologies. However, it's important to note that the direction of optimization and the trade-offs may differ among various studies. Therefore, making a judicious choice aligned with specific practical application needs becomes essential.

3 Properties of NPFSA-PEM

As a crucial component in various electrochemical processes, such as fuel cells and electrolytic hydrogen production, the performance of NPFSA-PEM is vital for the efficiency and reliability of the entire system. To enhance membrane material properties, including proton conductivity, temperature stability, mechanical properties, and oxidation stability, researchers have employed various methods, including sulfonation, phosphoric acid impregnation, blending, cross-linking, and the addition of inorganic fillers or metal–organic frameworks, as well as electro-treatment, among others.^79,80 In this section, we will summarize, compare, and discuss the proton conductivity, physicochemical properties, and other aspects of NPFSA-PEM, and propose relevant optimization strategies.

3.1 Proton conductivity

As a crucial membrane medium in the fields of fuel cells and electrolytic hydrogen production,^81,82 the strength of proton conduction performance directly influences the rate of energy conversion. Therefore, the development of NPFSA-PEM with high proton conduction performance is crucial to advancing the development of fuel cells and flow batteries.⁸³ The mechanism of proton conductivity testing is shown in the Fig. 7.

Fig. 7 The testing apparatus of conductivity. Reproduced with permission.⁶⁷ Copyright 2016, Elsevier.

In general, the proton conduction performance of membrane materials is directly linked to the conductive groups in the membrane and the properties of the membrane material. Researchers have approached the development of high-performance NPFSA-PEM from this perspective.^84,85 Sánchez-Ballester et al.⁸⁶ sulfonated polyvinyl alcohol (PVA) and doped it with graphene oxide (GO) in a two-step method to increase proton conductivity to 1.95 mS cm⁻¹ (Fig. 8a). This study reveals the effects of sulfonation degree and graphene oxide content on proton conduction performance, providing valuable insights for designing new proton exchange membranes. Abaci et al.⁸⁷ prepared an SPEEK–GO composite electrolyte, enhancing proton conductivity to 4.79 mS cm⁻¹ through the Grotthus mechanism. This is four times higher than that of the traditional SPEEK polymer electrolyte, promising improved performance in electrochemical devices like fuel cells and laying the foundation for practical applications. Muhmed et al.⁸⁸ demonstrated a maximum proton conductivity of 16.1 mS cm⁻¹ in the prepared NCC/PVA-SHGO-1.0 (Fig. 8b). The introduction of sulfonic acid groups into the graphene oxide (GO) membrane through sulfonation reactions promoted the hole effect of SHGO, enhancing the generation of proton junction channels and significantly improving proton conduction performance. However, excessive SHGO doping led to agglomeration and reduced proton conductivity, providing insight into designing more efficient membrane materials. Ramly et al.⁸⁹ prepared SPEEK–MC membranes based on SPEEK and methylcellulose (MC), achieving a conductivity of 8.7 mS cm⁻¹ (30 °C, 80% RH) through ultraviolet radiation cross-linking. Internal stress between SPEEK and MC enhanced the movement of charge carriers, and the addition of MC strengthened water absorption (Fig. 8c), promoting proton conduction. This study provides a simple and effective approach to improving membrane properties. Ahmadian-Alam et al.⁹⁰ fabricated a ternary nanocomposite polymer dielectric membrane, with 5% mMOF/Si-SO₃H exhibiting the highest proton conductivity of 17 mS cm⁻¹ (70 °C). With increasing nanoparticle SiO₂, proton conductivity quality improved, revealing the mechanisms of doped MOF and SiO₂ on membrane materials. Maiti et al.⁹¹ prepared XSPEEK/SPBI/PrSTO nanocomposite membranes, enhancing proton conductivity to 17 mS cm⁻¹ (90 °C, 100% RH) (Fig. 8d). This enhancement was attributed to the formation of additional proton transport channels on the surface of the polymer PrSGO packing and the introduction of sulfonic acid groups in the matrix to reduce the proton transfer barrier. However, the agglomeration of PrSGO at high temperatures reduced proton conduction performance. This study illustrates various material doping effects on enhancing membrane properties, providing an excellent example of membrane strengthening. Wu et al.⁹² prepared the S-DMEAx-BPT/PA hybrid membrane. The synergistic effect of the phosphate ion group and the sulfonic acid group enhanced proton conductivity, reaching the highest value of 295.4 mS cm⁻¹ (80 °C, 100% RH). Hydrogen bond channels formation, including those from the incorporated phosphate ion group and the sulfonic acid group, along with the negative charge effect of the phosphate group itself, contributed to the enhanced proton conductivity of the membrane material (Fig. 8e). This has significant engineering application value. However, attention should be paid to the specific gravity of the doped material during group incorporation; otherwise, proton conduction may be hindered. The SP/OA-POSS-g-GO-2 membrane prepared by Guan et al.¹⁵ exhibited the highest proton conductivity of 209.6 mS cm⁻¹ under conditions of 80 °C and 95% RH, attributed to the good dispersion of OA-POSS on GO and the formation of ordered acid–base pairs with SPEEK, which is conducive to the construction of continuous ion channels. GO, serving as a carrier, promotes the formation of long-range proton conduction pathways within the membrane (Fig. 8f). The improvement of the microstructure within the composite membrane and the increase in the bound water ratio enhance proton conduction under high-temperature and low-humidity conditions, offering a new approach for the development of high-performance PEMs.

Fig. 8 (a) The composite membrane sPVA/30SSA/GO was prepared by two-step method. Reproduced with permission.⁸⁶ Copyright 2020, Elsevier. (b) Suggested proton transfer mechanism of NCC/PVA-SHGO nanocomposite membrane. Reproduced with permission.⁸⁸ Copyright 2023, Elsevier. (c) Formation of channels in PEM with increasing water uptake. Reproduced with permission.⁸⁹ Copyright 2017, Elsevier. (d) Proton conductivity of XSPEEK/SPBI/PrSGO composite membranes. Reproduced with permission.⁹¹ Copyright 2023, Elsevier. (e) Possible proton transport mechanisms in S-DMEAx-BPT/PA hybrid membranes. Reproduced with permission.⁹² Copyright 2023, Elsevier. (f) Schematic diagram of proton conduction in the SP/OA-POSS-g-GO membrane. Reproduced with permission.¹⁵ Copyright 2023, Elsevier.

In addition, the proton conduction performance of NPFSA-PEM can also be optimized by increasing and optimizing the backbone microstructure of membrane materials and the doping of organic and inorganic materials, but it should be noted that the aggregation of pores reduces the mechanical properties of the membrane (Table 1).^93,94

Table 1 Proton conductivity and experimental conditions of doped membranes

Membrane	Doped materials	Doping amount (wt%)	Proton conductivity (mS cm^−1)	Temperature (°C)	Relative humidity (%)	Ref.
SPEEK–MC	MC	6	4.78	30	80	89
SPEEK–GO-(10.0)	GO	10	4.79	126.85	—	87
NCC/PVA-SHGO-1.0	SHGO	1.0	16.1	120	100	88
PSU/sPSU/NH2MIL-53(Al)	mMOF/Si-SO3H	5	17	70	100	90
SPAES/PCN-7.5	PCN	7.5	250	90	100	100
MSP-1% ZIF-67/GO	ZIF-67/GO	1	142.6	100	100	96
XSPEEK/SPBI/PrSGO	PrSGO	4	170	90	100	91
SPEEK/UNCS-3	UNCS	3	186.4	75	100	97
MNCS@SNF-PAEK	MNCS	1.5	188	80	100	29
S-C-SPAEKS/0.5% Im-MOF-801	Im-MOF-801	0.5	199	80	100	99
0.25PPy@ZIF-67/SPI	PPy@ZIF-67	1.5	233.7	80	100	102
GO-PDA-SPVA-60(GPS-60)	95% GO-5% PDA	40	336	80	100	74

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Yan et al.⁹⁵ successfully sulfonated the MXene material and introduced hydroxide to achieve a membrane material with excellent proton conductivity at high temperatures, and the OPBI@MXene with proton conductivity of 70.98 mS cm⁻¹ (160 °C) at high temperatures (Fig. 9a). It is shown that –OH and –SO₃H loaded on MXene promote the formation of new proton transport channels and MXene exhibits excellent water retention, which provides a potential application for the development of proton exchange membranes under high temperature conditions. Wang et al.²⁶ prepared a composite membrane based on SPEEK, PEEK-g-PVP, and H₃PW₁₂O₄₀ (PW), and constructed multiple proton transport channels in the membrane to achieve enhanced proton transport (Fig. 9b), and the composite membrane SPGP2-4 exhibited a high proton conductivity of up to 130 mS cm⁻¹, which was 2.2 times that of the original SPEEK. Çelebi et al.¹⁰¹ synthesized a novel sulfonic acid and trifluoromethylphenol functional polymerization aryl oxyphospholipids, which were characterized by their high hydrophobicity and porous structure, which contributed to the improvement of proton conductivity, and the synergistic effect of the Grotthuss mechanism and the carrier transport mechanism in the membrane to improve the proton conductivity of the membrane, and the proton conductivity of the blended membrane samples with more than 60% aryl oxyphosphazene composition of sulfonic acid functional agglomeration more than 60% of the sulfonic acid at 50 °C hydration exceeded 140 mS cm⁻¹, providing a new material design strategy. Xu et al.⁹⁶ prepared a hybrid membrane MSP-1% ZIF-67/GO with the highest conductivity of 142.6 mS cm⁻¹, forming a complex hydrogen bond network between oxygen-containing functional groups (including carboxyl groups, hydroxyl groups, and epoxides) in the metal–organic framework (ZIF-67/GO) to form proton transport channels (Fig. 9c). The proton conductivity of the hybrid membrane first increased and then decreased with the increase of the filler, which was due to the agglomeration of the added organic–inorganic filler and hindered the normal proton transport. The SPEEK/UNCS-3, studied by Long et al.⁹⁷ demonstrated the highest proton conductivity of 186.4 mS cm⁻¹. UNCS acts as a bridge between the proton donor and acceptor, thereby reducing activation energy, shortening proton conduction distance, and consequently enhancing proton conductivity (Fig. 9d). Both Xu⁹⁶ and Long⁹⁷ highlighted in their studies that excessive doping could lead to nanofiller accumulation in the membrane, impeding proton conduction. This insight serves as a valuable reference for selecting appropriate doping materials and specific gravity. Wu et al.⁹⁸ prepared a novel viscose-based PEM by cationic modification and staining with the reactive dye KE-7B1, which formed a complex internal three-dimensional network to promote the formation of efficient proton transport channels (Fig. 9e), making the proton conductivity of KE-7B1 reach up to 44.19 mS cm⁻¹, providing a feasible new method for improving the proton conduction performance of membrane materials. Duan et al.²⁹ developed a membrane-forming MNCS@SNF-PAEK using a one-step modification method, resulting in MNCS@SNF-PAEK-1.5 displaying the highest proton conductivity of 188 mS cm⁻¹ (Fig. 9f). The membrane's sulfonic acid groups aggregated, forming a proton transport channel and a hydrophilic–hydrophobic phase separation structure. This structure, coupled with the Grotthuss mechanism, reduced proton transport activation energy. Additionally, the interaction between amino and sulfonic acid groups in the membrane formed acid–base pairs, generating extra proton transport channels via hydrogen bonds. This in-depth exploration elucidates the influence of acid–base pairs, proton transport mechanisms, and phase separation structures on proton conductivity, offering new theoretical and experimental insights into enhancing proton conduction.

Fig. 9 (a) Proton conductivity of pristine membranes and OPBI@MXene in the temperature range of 40–160 °C in an anhydrous environment. Reproduced with permission.⁹⁵ Copyright 2023, Elsevier. (b) Preparation strategy of the nanocomposite membranes with multiplex proton transport channels. Reproduced with permission.²⁶ Copyright 2023, Elsevier. (c) The proton transport pathway of MSP-X% ZIF-67/GO. Reproduced with permission.⁹⁶ Copyright 2023, Elsevier. (d) Schematic illustration of the proton transport of SPEEK/UNCS composite membranes. Reproduced with permission.⁹⁷ Copyright 2023, Elsevier. (e) Internal structure of the DVM and H⁺ conductive mechanism. Reproduced with permission.⁹⁸ Copyright 2023, Elsevier. (f) Proton conductivity of MNCS@SNF-PAEK membranes. Reproduced with permission.²⁹ Copyright 2022, Elsevier. (g) Water uptake of the S-C-SPAEKS and hybrid membranes. Reproduced with permission.⁹⁹ Copyright 2022, Elsevier. (h) Schematic illustration of proton transport process in the SPAES/PCN membranes. Reproduced with permission.¹⁰⁰ Copyright 2024, Elsevier.

Chen et al.⁹⁹ developed a novel organic–inorganic hybrid membrane, S-C-SPAEKS/0.5% Im-MOF-801 exhibited the highest proton conductivity of 199 mS cm⁻¹. Compared to the pure S-C-SPAEKS membrane under identical conditions, the proton conductivity of the pure membrane was 1.5 times higher. The imidazole in the membrane acted as both a proton acceptor and donor, facilitating proton transport. Furthermore, the varied long and short sulfonic acid side chains on the polymer backbone significantly improved water absorption and expanded the proton transport pathway (Fig. 9g), presenting an innovative material doping strategy. The SPDI-100 prepared by Wang et al.⁷ achieved the highest proton conductivity of 235.5 mS cm⁻¹ (80 °C), which was due to the structure formed by the backbone, which was conducive to the aggregation of sulfonic acid groups and the formation of proton channels, and the proton conductivity increased with the increase of sulfonic acid group content. Liu et al.¹⁰⁰ introduced nanohybrids (PCNs) into SPAES polymers, creating composite membranes SPAES/PCN-x with robust proton conductivity of 250 mS cm⁻¹. The multi-level interaction between PCN and SPAES induced ion/water cluster adjustment along the two-dimensional interface, fostering unique phase separation morphology and continuous long-distance proton transport channels (Fig. 9h).

The Grotthuss mechanism and the Vehicle mechanism are widely recognized proton exchange membrane conduction mechanisms (Fig. 10). The exploration of ways to enhance these conduction mechanisms through modifications in membrane structure and composition is a hot topic among researchers.^31,103

Fig. 10 Mechanism for proton conduction in cCM-mPBI/SPNPP membranes. Reproduced with permission.⁴⁷ Copyright 2022, The American Chemical Society.

Cai et al.⁷⁴ developed the new GO-based substrate GPS-60 membrane, displaying high proton conductivity of 336 mS cm⁻¹. The membrane's high concentration of sulfonic acid groups within the continuous 2D channel enhanced proton conduction via the Grotthuss mechanism, accelerating proton conduction rates with increasing relative humidity. This serves as a pivotal reference for high-performance membrane preparation. Zhang et al.¹⁰⁴ employed concentrated sulfuric acid for direct sulfonation of SPEEK and utilized a solution casting method to produce a membrane. This membrane demonstrated the highest conductivity of 384 mS cm⁻¹ at 80 °C, exhibiting low activation energy (<38.5 kJ mol⁻¹). In this context, the Grotthuss mechanism dominated proton conduction when the activation energy was low, while the vector transport mechanism prevailed at high activation energies, revealing a synergistic mechanism for the two proton conduction pathways. In another study, Yang et al.¹⁰² incorporated PPy nanotubes and zeolite imidazolate backbone material-67 (ZIF-67) into a PPy@ZIF-67/sulfonated polyimide (SPI) membrane material. This composite material, made of SPI, demonstrated a proton conductivity of 233.7 mS cm⁻¹. The nanofiber metal–organic framework (MOF) structure in the membrane provided ample transition points for proton conduction through the interfacial conduction pathway between the PPy@ZIF-67 packing and the substrate. Consequently, this enhanced both the Grotthuss mechanism and carrier transport mechanism of proton transport in the membrane. Li et al.¹⁰⁵ prepared PVP-UiO-66-NH-SO₃H nanoparticles (PUNSNPs) filled with SPEEK and doped with phosphoric acid (PA) groups, resulting in PA/PUNSNPs/SPEEK = 20%, exhibiting the highest proton conductivity of 350 mS cm⁻¹. The high conductivity is attributed to the membrane material's unique continuous nanopore structure, which facilitates high absorbance of phosphoric acid (PA) and promotes inter-chain acid–base pairing interactions. Liu et al.⁶³ prepared SPEEK/SHNT membrane displayed the highest proton conductivity of 43 mS cm⁻¹, two orders of magnitude higher than that of the control SPEEK membrane. The improved proton conductivity was attributed to the introduction of sulfonic acid groups on the surface of the nanotubes, as well as the similarity of chemical structures and hydrogen bonds on the nanotube surfaces, leading to enhanced phase separation and a larger hydrophilic domain. Liu et al.¹⁰⁶ prepared an N-heterocyclic polyaryl ether (SPBPEK-Ps) membrane containing a sulfonated side phenyl group. The multi-level interaction between the sulfonic acid group and the heterocyclic structure in the SPBPEK-Ps membrane, coupled with the tunable microphase structure derived from the side proton-conductive group regulated by the copolymer composition, enhanced the proton conductivity to 125.0 mS cm⁻¹. The aforementioned studies by Yang¹⁰² and Li¹⁰⁵et al. successfully enhanced the conduction mechanism of proton-conductive membranes through structural and compositional modifications, providing valuable guidance and reference for the development of high-performance proton exchange membranes.

The enhancement of proton conductivity through the Grotthuss mechanism and the carrier transport mechanism is closely tied to the material's resistance value, the combination of various group types, and the structure of the membrane material. The proton conductivity of NPFSA-PEM can be effectively improved by adjusting the membrane material's structure, varying the group composition, and incorporating composite nanomaterials with high conductivity. However, the challenge lies in the cost of materials, making it difficult to widely apply NPFSA-PEM with high proton conductivity in industrial settings. The pursuit of cost-effective, high-performance NPFSA-PEM remains a focal point in current research.

3.2 Physical stability

Physical stability refers to the ability of a membrane to maintain its integrity and structure under mechanical stress, loading, and high temperatures.^107,108 Energy conversion storage devices typically operate under variable environmental conditions, and the physical stability of the membrane is directly related to the life and performance of the membrane.

3.2.1 Tensile strength. Tensile strength is a crucial indicator for evaluating the physical stability of membrane materials. The tensile strength of membrane materials can be significantly enhanced by modifying the membrane composition and structure through the doping of nanofillers and the addition of related additives.¹⁰⁹ However, it is essential to control the amount of material incorporation during the process; otherwise, the mechanical properties of the membrane may be compromised.¹⁰⁵ Duan et al.²⁹ utilized a one-step modification method to incorporate inorganic nano-filled MNCS into SNF-PAEK, resulting in the formation of MNCSS@NF-PAEK membranes (Fig. 11a). They achieved a maximum tensile strength of 36.49 MPa for MNCS@SNF, providing a viable approach to improve membrane tensile strength through doping and cross-linking mixing. Wu et al.⁹⁸ developed a novel viscose-based PEM through cationic modification and staining with the reactive dye KE-7B1, exhibiting a tensile strength of 42.12 MPa (Fig. 11b). The introduction of PVA and intramembrane forces (hydrogen bonding and covalent bonding) significantly enhanced the membrane's tensile strength, offering a new method for synthesizing low-cost and high-performance membranes.

Fig. 11 (a) The elemental analysis of NH₂-MIL-101 and MNCS. Reproduced with permission.²⁹ Copyright 2022, Elsevier. (b) Stress–strain curves of PVM, DVM-15, 1% PDVM-15 and 5% PDVM-15. Reproduced with permission.⁹⁸ Copyright 2023, Elsevier.

The tensile performance of the membrane can also be effectively improved through the design and control of the interchain and interlayer structures of the membrane material. The tensile strength can be enhanced by achieving a good chain entanglement state and fostering electrostatic interactions between sulfonic acid groups. Imaan et al.¹¹⁰ produced SPVDF–ZWP–PSSA membranes with a tensile strength of up to 44 MPa. The semi-crystallinity of the membrane decreases, resulting in a reduction in the crystallization area of the PVDF polymer and a transition towards the amorphous state (Fig. 12a). However, despite the decreased semi-crystallinity, the presence of the SPVDF skeleton in the synthesized membrane enables it to maintain high tensile strength. This highlights the relationship between tensile strength, internal skeleton structure, and crystallinity. Wang et al.⁷ synthesized SPDI-x with varying degrees of sulfonation through sulfonation. The tensile properties of the membrane materials exceeded 35 MPa, attributed to the increased sulfonic acid groups and elongation at the membrane break (Fig. 12b). Qian et al.⁷⁸ demonstrated an amphoteric composite membrane, SPEEK–IM/CSPF-15, composed of SPEEK–IM with imidazole chains and side-chain sulfonated poly ether sulfone with a covalently cross-linked perfluoroalkyl chain (CSPF). This proved to be an effective approach to improve the mechanical properties of PEM, with a maximum tensile strength of 114.7 MPa due to its local semi-interpenetrating network structure (Fig. 12c), providing an excellent example for the design of interlaminar structures to enhance tensile strength. Wang et al.²⁶ prepared the SPEEK/PGP/PW blended membrane, enhancing the mechanical strength through the bicontinuous structure and electrostatic crosslinking effect of PW nanoclusters. The membrane exhibited a tensile strength of 31.2 MPa and a high modulus of 4.3 GPa, offering a theoretical and experimental method for the crosslinking effect and the synergistic effect of nanocluster bicontinuous structure to promote the improvement of membrane performance. Dong et al.¹¹¹ incorporated phosphorylated polyethylene-co-vinyl alcohol (PEVOH) into SPAEK membranes, improving the mechanical properties with an increase in PEVOH content. SPAEK-8% PEVOH demonstrated the best mechanical properties, with a Young's modulus of 1.92 GPa, a maximum tensile stress of 80.2 MPa, and an elongation of 111.7%, thanks to the high crystallinity of the C=O group in the polymer backbone and the enhancement and toughening effect of PEVOH (Fig. 12d). This study provides practical experience for the customized design of membrane materials. Han et al.¹¹² cross-linked C-SP90/SP90NF by crosslinking high-sulfonated salt-type SPEEK nanofibers and acid-type SPEEK matrix composite membranes, forming a high-rigidity network during thermal crosslinking. The membrane exhibited excellent mechanical properties with a tensile stress of 68 MPa, Young's modulus of 1.31 GPa, and an elongation at break of 20%. The work of these researchers has made important contributions to improving the tensile strength of membrane materials. They have explored different methods, including incorporating nanofillers, adjusting the membrane structure, and adding additives, to enhance the tensile strength of membranes and improve the physical properties of membrane materials. This has important guiding significance for future research and applications of membrane materials.

Fig. 12 (a) Defect-free surface of SPVDF–ZWP–PSSA membrane. Reproduced with permission.¹¹⁰ Copyright 2022, Elsevier. (b) Diagram of the SPDI-x. Reproduced with permission.⁷ Copyright 2023, Elsevier. (c) Composite membrane SPEEK–IM/CSPF with local semi-interpenetrating network structure. Reproduced with permission.⁷⁸ Copyright 2022, Elsevier. (d) Schematic diagram of the synthesis process and internal structure of SPAEK-PEVOH. Reproduced with permission.¹¹¹ Copyright 2024, Elsevier.

The bonding between groups, such as chemical coordination bonds and hydrogen bonds, can significantly improve the tensile strength of membrane materials.⁹⁶ The new composite membrane GPS-30 prepared by Cai et al.,⁷⁴ exhibits a tensile strength of 256.2 MPa and a Young's modulus of 13.3 GPa. The enhancement of mechanical properties is attributed to the delicate layered solid structure and the complex interfacial interaction between the 2D GO sheet and the polymer (covalent bonds and hydrogen bonds). Michel et al.¹¹³ prepared a composite with 0.1 wt% boron nitride doping, exhibiting a maximum mechanical strength of 43.3 MPa due to stress transfer from the polymer matrix to boron nitride. Çelebi et al.¹⁰¹ developed a novel sulfonic acid-functionalized aryloxy-phosphor-phosphine with varying amounts of hydrophobic 4-trifluoromethylphenol and hydrophilic 4-hydroxybenzene sulfonic acid co-substituents on the polymer backbone. The tensile strength of the membrane material increased by 36.4 MPa, and this enhancement could be attributed to the incorporation of polyvinylidene fluoride (PVDF), forming an organic fluorine interaction within the membrane structure. The above-mentioned research by Cai,⁷⁴ Michel¹¹³et al. provides important empirical and theoretical support for improving the tensile strength of membrane materials by manipulating interfacial interactions and material structures. This not only deepens the understanding of the properties of membrane materials but also offers useful guidance for future research and applications.

3.2.2 Swelling resistance. Swelling resistance is another crucial criterion for evaluating the physical properties of membranes. It refers to the ability of membranes to effectively resist volume expansion caused by the absorption of liquids during use. Proton exchange membranes must maintain dimensional stability during operation to ensure the normal function and reliability of batteries or electrolytic cells.^114,115 Swelling resistance is highly correlated with membrane structure, as well as the adsorption and retention of water molecules by the porous structure of the membrane material.⁹⁶ Cross-linked or interpenetrating structures formed by different methods can optimize the structure of the membrane material and, consequently, improve swelling resistance. The addition of additives can enhance the swelling resistance of membrane materials, such as hard particles, polymer nanocomposites, and inorganic–organometallic frameworks. These enhancements are highly correlated with their material properties, including unique anti-swelling structures and phase distributions.¹¹⁶ The DSPAEK-CZ-x prepared by Xie et al.¹¹⁶ is significant for understanding the role of nanophase separation in improving the anti-swelling performance of the membrane. This improvement is achieved through the hydrophobic fluorination of the polymer backbone and the polarity difference between the side sedimentation thick sulfonic acid groups (Fig. 13a). Zeng et al.¹¹⁷ prepared PPO-g-PSSA with the introduction of a hydrophobic/hydrophilic nanophase separation morphology. The hybrid membrane exhibited good water resistance and swelling resistance without a loss of proton conductivity, contributing to the development of membranes with improved anti-swelling properties. Munavalli et al.¹⁰ incorporated zeolite into the membrane to create SPEES-SA/SMZ. With an increase in zeolite incorporation, the hydrophobicity of the membrane dominated, hindering water absorption and swelling (Fig. 13b). This led to an improvement in the anti-swelling of the membrane material. However, excessive restriction of water absorption and swelling may result in a decline in the proton conduction performance of the membrane. This provides a feasible strategy to control the swelling and proton conduction properties of materials in the membrane. Wang et al.¹¹⁸ inhibited excessive swelling by incorporating hydrophobic side chains into the membrane to enhance the polymer's hydrophobicity, thereby inhibiting excessive swelling. This made the sTBmT-1.88 membrane exhibit low swelling (Fig. 13c). The presence of more chain entanglements in the flexible sTBmT polymer reduced the space for water molecules, providing a strategy to control swelling by introducing hydrophobic side chains. Zhou et al.¹¹⁴ demonstrated the excellent anti-swelling ability of SP@PQD-15.1 membranes prepared by the in situ molecular level hybridization method of PQDs. The strong electrostatic interaction between the –NH₂/–NH– group and –SO₃H in the membrane kept the swelling rate unchanged when the test temperature increased (Fig. 13d and e). This method provides a simple and feasible preparation method for synthesizing high tensile membrane materials. The composition of the inner groups within the membrane material, including oxygen functional groups such as hydroxyl, carboxyl, and epoxide, significantly influences the membrane's swelling resistance.

Fig. 13 (a) TEM images of DSPAEK-CZ-25. Reproduced with permission.¹¹⁶ Copyright 2017, Elsevier. (b) SEM and contact angle images of SPEES-SA/SMZ-8. Reproduced with permission.¹⁰ Copyright 2019, Elsevier. (c) Length and thickness swelling of sTBmT-1.88. Reproduced with permission.¹¹⁸ Copyright 2023, Elsevier. (d) FTIR spectra of SP@PQD-15.1. Reproduced with permission.¹¹⁴ Copyright 2021, Elsevier. (e) Optical images of SP and SP@PQD-15.1 in water at 45 °C and 95 °C for 1 h. Reproduced with permission.¹¹⁴ Copyright 2021, Elsevier. (f) XRD of CS and CS/PDA-ADPS composite membranes. Reproduced with permission.⁷⁵ Copyright 2024, Elsevier. (g) Schematic diagram of the microphase structure of a SPEEK membrane. Reproduced with permission.¹⁰⁴ Copyright 2024, Elsevier.

However, water absorption decreases due to the emergence and strengthening of the agglomeration phenomenon within the membrane.¹¹⁹ Fan et al.⁷⁵ prepared the CS/PDA-ADPS composite membrane, which exhibited high water absorption and low swelling. This can be attributed to the uniformly dispersed PDA-ADPS in the matrix, reducing the membrane's crystallinity and consequently increasing its water storage capacity. The composite membrane, containing hydrophilic groups (–OH and –SO₃H) in PDA-ADPS (Fig. 13f), absorbs more water. Additionally, ion pairing between the quaternary ammonium and sulfonic acid groups in PDA-ADPS contributes to improving the dimensional stability of the membrane. In another study, Zhang et al.¹⁰⁴ utilized the Langmuir–Blodgett self-assembly process to fabricate the SPEEK proton exchange membrane. This membrane exhibited good water absorption and a controlled swelling rate. The membrane exhibited a microscopic phase separation mechanism (Fig. 13g), where the hydrophobic phase consisted of the main chain and the hydrophilic phase comprised side chains containing sulfonic acid groups. This balanced water absorption and swelling within the membrane, preventing the deformation of the fluorphlogopite when absorbing water and swelling. These studies offer profound insights into designing and fabricating membrane materials with excellent anti-swelling properties. They establish a foundation for further advancements in applications, such as proton exchange membranes.

3.2.3 Temperature stability. Temperature stability is a crucial indicator for assessing the physical properties of membranes. It refers to the ability of membranes to maintain their structure, performance, and stability under varying temperature conditions,^120–122 ensuring the reliable operation of equipment across different temperature ranges. Temperature stability is closely linked to phenomena such as water evaporation, group detachment, and the breakage of backbone and side chains in membrane materials during temperature escalation.

Zhu et al.³⁴ fabricated a Gel-PBI-PS membrane with a wide operating temperature range from 25 °C to 240 °C (Fig. 14a). This membrane exhibited minimal impact on performance during the initial stage of the first weight loss at 200 °C and did not show a significant decrease in performance until the sulfonic acid chain decomposed in a hot, low-moisture environment. The improvement in temperature stability could be attributed to the presence of flexible alkyl sulfonic acid side chains in addition to the main polymer chain. Mirfarsi et al.¹²³ conducted thermal stability tests on Pemion hydrocarbon-based membranes, finding that they did not undergo any phase transformation or decomposition below 300 °C. These membranes exhibited lower polymer chain mobility with increasing temperature compared to reinforced perfluoro sulfonic acid (r-PFSA) membranes. Wu et al.⁹⁸ developed a novel viscose-based PEM through cationic modification and staining with the reactive dye KE-7B1, showcasing excellent temperature stability at 200 °C. Mirfarsi,¹²³ Wu,⁹⁸ and others present a feasible experimental method for the preparation of high-temperature-stable membrane materials. The SPEEK/PGP/PW blends prepared by Wang et al.²⁶ all exhibited good temperature stability, with decomposition temperatures exceeding 250 °C (Fig. 14b). In contrast, the stability of graphene oxide (GO) at high temperatures was poor, limiting the thermal stability of the GO base membrane and revealing the influence mechanism of different materials on thermal stability. Tan et al.¹²⁴ introduced hydrophilic flexible side chains into the PFC-TF-SI membrane, promoting increased chain entanglement and resulting in better thermal stability than the original membrane material PFC. Chu et al.¹²⁵ incorporated PDA/PEI-modified PVDF nanofibers into an SPEEK base membrane to create an SPEEK–PDA/PEI@PVDF composite membrane that exhibited better thermal stability than pure PVDF and SPEEK membrane (Fig. 14c). The enhanced thermal stability of the membrane was attributed to the effective interaction between PDA/PEI@PVDF nanofibers and SPEEK molecular chains, as well as the inherent good thermal stability of PVDF as the substrate. This study provides a robust experimental and theoretical foundation for modifying materials to enhance the thermal stability of membranes. The XSPEEK/SPBI/PrSGO nanocomposite membranes, prepared by Maiti et al.,⁹¹ exhibited excellent thermal stability. The incorporation of SPEEK and PrSGO into SPBI significantly increased the glass transition temperature of the membrane (T_g = 588.91 K) (Fig. 14d). This enhancement can be attributed to the formation of strong hydrogen bonds between PrSGO and the SPEEK/SPBI matrix, robust dipole–dipole interactions, and strong interactions between fillers and polymer chains, reducing the segmental motion of polymer chains. The influence mechanism and enhancement path of intramembrane interaction forces on the thermal stability of the membrane are revealed, providing a solid theoretical and experimental basis. The SPEEK/0.05 BN hybrid membrane, as prepared by Michel et al.,¹¹³ exhibited the lowest weight loss compared to other membrane materials (such as Nafion, SPEEK, etc.) (Fig. 14e). The improved thermal stability of the membrane material is attributed to the formation of intermolecular hydrogen bonds between SPEEK and boron nitride. The SPEEK/0.05 BN hybrid membrane demonstrated better performance than Nafion in flow battery applications, indicating its excellent feasibility in practical applications. The BNC/LIG membrane synthesized by Souza et al.⁸⁴ demonstrated stronger thermal stability than the pure BNC membrane. While the BNC exhibited rapid weight loss between 250 and 350 °C, with a weight loss of 90 wt% at temperatures above 380 °C, the BNC/LIG membrane maintained a 64% weight loss after rapid weight loss at 290 °C to 480 °C (Fig. 14f). This difference is attributed to the BNC/LIG membranes requiring higher temperatures for decomposition. This provides a model for the preparation of highly thermally stable microbial fuel cells and presents a new idea for the development of more efficient and environmentally friendly membrane materials.

Fig. 14 (a) TGA curves of PA undoped Gel-PBI, Gel-PBI-PS-1, Gel-PBI-PS-2 and Gel-PBI-PS-3 membranes. Reproduced with permission.³⁴ Copyright 2023, Elsevier. (b) TGA curves of SPGP1-4, SPGP2-4, and SPGP3-4. Reproduced with permission.²⁶ Copyright 2023, Elsevier. (c) TG curves of SPEEK, PVDF, PDA/PEI@PVDF and SPEEK–PDA/PEI@PVDF. Reproduced with permission.¹²⁵ Copyright 2024, Elsevier. (d) TGA of PrSGO, GO fillers, SPEEK, XSPEEK, XSPEEK/SPBI, XSPEEK/SPBI/PrSGO-4, XSPEEK/SPBI/PrSGO-6 membrane samples. Reproduced with permission.⁹¹ Copyright 2023, Elsevier. (e) TGA of Nafion, SPEEK, SPEEK/0.05 BN, SPEEK/0.05 OH-BN and SPEEK/0.1 OH-BN. Reproduced with permission.¹¹³ Copyright 2024, Elsevier. (f) Thermogravimetric curves of pure BNC and BNC/LIG membranes, with the corresponding derivatives (DTG). Reproduced with permission.⁸⁴ Copyright 2023, Elsevier.

In summary, the above studies highlight temperature stability as a crucial evaluation index for the physical properties of membranes. This property is closely related to the ability of membranes to maintain structure, performance, and stability under different temperature conditions, ensuring the stable operation of equipment across various temperature ranges. These studies have made significant contributions to the advancement and innovation of membrane technology, laying a solid foundation for the development of more efficient and stable membrane materials in the future.

3.3 Chemical stability

Chemical stability refers to the membrane's ability to maintain its structure and properties in environments exposed to conditions such as acid–alkali and oxidation. This characteristic is of utmost importance for membrane materials used in applications like proton exchange membrane fuel cells (PEMFCs), where strong acidity, alkalinity, and oxidation are commonly encountered.^126,127

3.3.1 Anti-oxidation. Oxidation is an important factor in reducing the life of NPFSA-PEM, and research on anti-oxidation properties is an important way to improve the life of membrane materials.^128,129

Wu et al.⁹⁸ prepared a new viscose-based PEM through cationic modification and dyeing with the reactive dye KE-7B1. After soaking in a 30% H₂O₂ solution for 120 h, the mass loss was less than 10%, indicating its good antioxidant properties (Fig. 15a). Çelebi et al.¹⁰¹ incorporated polyvinylidene fluoride (PVDF) to prepare a new sulfonic acid-functional polymerized aryloxy phosphine, which improved the oxidation stability of the membrane material to 98.1% (in Fenton's reagent (3% H₂O₂ containing 2 ppm) in the aqueous solution of FeSO₄ at 80 °C for 12 hours). The enhanced oxidative stability is attributed to the establishment of a synergistic effect between PDVF and fluorinated sulfonic acid functionalized agglomerated phosphagens. The CS/PDA-ADPS composite membrane prepared by Fan et al.⁷⁵ improved the oxidative stability of the membrane material by incorporating PDA and adjusting the distribution of PDA-ADPS in the CS matrix (Fig. 15b). PDA has good antioxidant activity, and PDA-ADPS' high dispersion and compatibility in the CS matrix limit the movement of CS segments, further enhancing the stability of the membrane material. The hybrid membrane S-C-SPAEKS/0.5% Im-MOF-801 prepared by Chen et al.⁹⁹ still maintained 94.25% of the remaining weight after being treated with Fenton's reagent at 80 °C for 1 h, showing good oxidation stability. The doped organic–inorganic filler (Im-MOF-801) improves the density of the polymer matrix, but excessive doping can lead to agglomeration, thereby reducing the surface density of the membrane and making the polymer molecular chains more susceptible to damage from Fenton's attack. This, in turn, leads to a decrease in the oxidative stability of the membrane. Zhang et al.¹⁰⁴ used the Langmuir–Blodgett self-assembly process to prepare SPEEK proton exchange membranes that showed excellent oxidation stability. The antioxidant capacity of the membrane was evaluated by the residual mass of the membrane treated in Fenton's reagent for 1 h at 80 °C. The membrane material has always maintained outstanding antioxidant properties, with a mass residual rate as high as 95%, which is attributed to the orderly arrangement of molecules within the membrane inhibiting membrane degradation (Fig. 15c). Xu et al.¹³⁰ synthesized sulfonated fluorinated poly arylene ether SFPAEs membrane by chemical post-modification of fluorinated poly arylene ether precursor, and the membrane material was prepared by immersing the membrane in Fenton's reagent (3 wt% H₂O₂, 2 ppm FeSO₄) at 80 °C; the time for the membrane to begin to rupture is close to 5 hours. At low IEC levels, it has stronger oxidation stability than other membrane materials (SPAEs, SPPEKs) (Fig. 15d). That is, modification of membrane materials can effectively improve the oxidation stability of membranes, providing a feasible path to enhance the oxidation stability of membrane materials. The SP@PQD-15.1 membrane, as prepared by Zhou et al.¹¹⁴ underwent testing with Fenton's reagent and retained its structural integrity even after immersion for 1 hour, with a residual weight percentage of over 90%. Throughout this process, the decomposition of the SPEEK chain occurred slowly, preserving its structure. This resilience stems from the suppressed chain mobility and heightened resistance to free radical attacks due to strong electrostatic interactions, endowing the SP@PQD-15.1 membrane with robust oxidative stability.

Fig. 15 (a) Time-course of PVM, DVM-15, 1% PDVM-15 in H₂O₂ (30%, in mass) at room temperature. Reproduced with permission.⁹⁸ Copyright 2023, Elsevier. (b) The oxidation stability of CS and CS/PDA-ADPS composite membranes. Reproduced with permission.⁷⁵ Copyright 2024, Elsevier. (c) Oxidative stabilities of membranes. Reproduced with permission.¹⁰⁴ Copyright 2024, Elsevier. (d) The oxidative stabilities of SFPAEs with comparison to other membranes. Reproduced with permission.¹³⁰ Copyright 2017, Elsevier. (e) wt remaining after Fenton test of OPBI and IMOPBI membranes with different grafting degrees. Reproduced with permission.¹⁶ Copyright 2023, Elsevier.

In contrast, the SPBPEK-Ps membrane, developed by Liu et al.¹⁰⁶ exhibited superior antioxidant properties compared to traditional SPAEs membranes. This improvement is attributed to the reinforced molecular chain stemming from weaker connecting units (ether bonds and carbonyl groups in the double halide unit). The pendant benzene sulfonic acid group's steric effect mitigates free radical attacks on the main chain to a certain extent, enhancing free radical tolerance. These studies employed diverse strategies and materials—such as cationic modification, optimized membrane design, and organic–inorganic filler doping to enhance the oxidation stability of NPFSA-PEM. The research has significantly advanced the anti-oxidation performance of membrane materials, offering valuable insights for extending the lifespan of NPFSA-PEM, particularly in applications like fuel cells. The membrane prepared by Wang et al.¹⁶ showed excellent stability and maintained mechanical integrity in the Fenton test at 80 °C for 312 hours. However, the original OPBI membrane demonstrated better oxidation stability than the IMOPBI membrane because the grafting of imidazole cation side chains disrupted the hydrogen bonds between OPBI molecules, and the introduction of hydrophilic imidazole cations promoted penetration of the Fenton reagent into the membrane (Fig. 15e). Therefore, when introducing new functional groups and materials, attention should be paid to their impact on the comprehensive performance of the membrane, in order to enhance its feasibility in practical applications. This comprehensive series of studies delves deeply into systematically improving the oxidative stability of NPFSA-PEM, focusing on oxidation as the primary factor limiting their lifespan. Through the introduction of varied strategies and materials, these studies have achieved notable advancements across multiple dimensions, serving as a valuable reference for fortifying the oxidation resistance of membrane materials.

Additionally, incorporating inorganic free radical scavengers or organic antioxidants into proton exchange membranes represents an effective strategy for enhancing the oxidation resistance of these materials. The redox cycle of ferrocyanide in the composite proton exchange membrane CP4VP, prepared by Liu et al.¹³¹ continuously consumes the free radicals generated during PEMFC operation. It inhibits free radical attacks and greatly limits the chemical degradation of PEM, demonstrating strong antioxidant performance. This provides a feasible approach and beneficial insight for the development of more stable PEM. Xu et al.¹²⁹ grew thin porous polyaniline (PANI) membranes in situ on proton conductive sulfonated poly (biphenyl indole) membranes (SBPIM), encapsulated free radical scavengers (ceria oxide, CeO_x) inside, and suppressed the aggregation or diffusion loss of CeO_x through porous PANI membranes. In non in situ Fenton reagents and in situ durability experiments, it was shown that PANI/CeO_x thin layers can significantly prevent free radical attacks on PEM without sacrificing fuel cell performance, thereby improving the antioxidant performance of the membrane and further enhancing its durability in PEMFC, ensuring its long-term stable operation. Shen et al.¹³² prepared RQL-SPBPIMs by growing a polypyrrole/manganese dioxide (PPy/MnO_x) radical quenching layer on the surface of SPBPIMs. During this process, the free radical scavenger MnO_x was generated simultaneously with PPy. This synthesis method helps to fix and uniformly distribute MnO_x within the PPy porous layer, preventing free radical attacks within the membrane and reducing diffusion losses during fuel cell operation. Therefore, the RQL-SPBPIMs membrane exhibits better durability and antioxidant performance than the original membrane. Additionally, the effect of the porous free radical quenching layer on fuel cell performance is minimal, further improving the comprehensive performance of the membrane. Each research method demonstrated potential in mitigating free radical attacks, offering valuable insights and experimental support for the future development of robust, oxidation-stable proton exchange membranes.

3.3.2 Methanol barrier property. NPFSA-PEM is widely used in methanol fuel cells, and methanol tolerance and permeability are important indicators for evaluating the performance of membrane materials.¹³³ It is crucial to improve methanol resistance and reduce membrane permeability by doping materials with good membrane-forming properties and insoluble organic solvents.^134,135

The SPEEK/PGP/PW blend, prepared by Wang et al.,²⁶ effectively improved the oxidation stability of the membrane material due to the addition of PW. It exhibited low methanol permeability and long-term proton conduction stability in deionized water. The research by Wang²⁶ provides valuable insights into doping modifications to enhance the barrier performance against methanol. The cross-linked oxidative hybrid MPTMS-GPTMS membrane prepared by Mosa et al.¹³⁶ exhibited a methanol permeability of 1.72 × 10⁻⁶ cm² s⁻¹. The presence of MPTMS interferes with methanol permeability, possibly due to the grafting of sulfonic acid groups to the hybrid structure (Fig. 16a). This results in a stronger interaction with hydroxyl groups from the sol–gel hydrolysis reaction, creating a tortuous pathway at the molecular level. This forms a robust barrier against methanol molecules, providing experimental and theoretical support for exploring novel methods to reduce permeability. Liang et al.¹³⁷ synthesized a conjugated sulfonated poly phenyl quinoxaline (c-SPPQ) through a post-sulfonation process. It displayed good methanol permeability and high methanol tolerance. The length size change of c-SPPQ-3 in a solution with a concentration of 45 wt% methanol at 30 °C was 33%, attributed to the acid–base interaction between sulfonic acid in the membrane and the phenyl quinoxaline group. This offers a new approach to improving the methanol performance of the membrane through a specific chemical synthesis pathway. Xu et al.¹³⁸ developed a novel cross-linked sulfonated poly aryl ether ketone sulfone containing multiple sulfonic acid side chains. The S-Am-1.0/C membrane exhibited a minimum methanol permeability coefficient of 3.11 × 10⁻⁸. This reduction in permeability was attributed to the hydrophilic-SO₃H group positioned at the tail end of the flexible side chain, ensuring hydrophilicity. The formation of a hydrophobic phase separation structure impeded the crossover of methanol molecules. Additionally, the cross-linking network structure and hydrogen bond network effectively restrained methanol diffusion. Chu et al.¹²⁵ introduced PDA/PEI modified PVDF nanofibers into an SPEEK base membrane, resulting in the SPEEK–PDA/PEI@PVDF composite membrane showcasing notable methanol barrier capabilities (Fig. 16b). The enhancement in barrier performance stemmed from improved compatibility and a more compact structure within the composite membrane. This was attributed to the interfacial interaction between the inert three-dimensional network structure of PVDF and SPEEK, creating increased resistance and a more convoluted diffusion path for methanol.

Fig. 16 (a) Illustration of the hybrid structure simulating the protons transfers. Reproduced with permission.¹³⁶ Copyright 2015, Elsevier. (b) Polarization and power density curves of SPEEK–PDA/PEI@PVDF. Reproduced with permission.¹²⁵ Copyright 2024, Elsevier.

Collectively, these studies have delved deeply into the methanol tolerance and permeability of NPFSA-PEM in methanol fuel cells. They have enriched methods aimed at enhancing methanol tolerance and reducing NPFSA-PEM permeability, furnishing a solid theoretical and experimental foundation for enhancing methanol barrier performance and decreasing membrane permeability.

4 Applications of NPFSA-PEM

NPFSA-PEM is widely popular in the energy sector due to its excellent proton conductivity, chemical stability, and mechanical strength.^139,140 In this section, we will elaborate on some applications of NPFSA-PEM in the energy field, discussing both its benefits and challenges in practical applications, as well as outlining the direction for future optimization.

4.1 Proton exchange membrane fuel cells

As an advanced energy conversion technology,¹⁴¹ the proton exchange membrane fuel cell (PEMFC) offers the advantages of high energy density and low emissions.¹⁴² The development of PEMFC technology is of great significance in achieving carbon neutrality. PEMFC operates by splitting hydrogen at the anode to produce protons and electrons. Protons migrate from the anode to the cathode through a proton exchange membrane, while electrons flow through an external circuit, generating an electric current to the cathode. At the cathode, electrons recombine with the protons passing through the membrane and oxygen from the air.¹⁴³ Common types of PEMFCs include direct methanol fuel cells,¹³⁷ hydrogen fuel cells,¹⁴⁴ and microbial fuel cells.¹⁴⁵ NPFSA-PEM, as one of the most widely used membrane materials in fuel cells, will be the focus of our review on its applications in this section.

Sun et al.¹⁴⁶ demonstrated that the proton conductivity of the self-crosslinking composite membrane C-SPEEK/HPW/GO was significantly higher than that of C-SPEEK alone. At 80 °C, the proton conductivity of C-SPEEK/HPW/GO reached 119.04 mS cm⁻¹, which was 2.4 times higher than that of C-SPEEK under the same conditions. Additionally, the output power of H₂/O₂ cells using C-SPEEK/HPW/GO membrane was measured to be 876.80 mW cm⁻², whereas C-SPEEK achieved an output power of 776.72 mW cm⁻² under the same conditions (Fig. 17a). These findings highlight the promising potential of C-SPEEK and C-SPEEK/HPW/GO as fuel cell membrane materials. The GPS-50 membrane prepared by Cai et al.⁷⁴ demonstrated excellent performance with a high power density of 213 mW cm⁻² at 45 °C and 75% relative humidity (Fig. 17b). The improved power density can be attributed to the promotion of proton transport facilitated by the SPVA and PDA cross-linked GO sheets, which reduced membrane impedance and accelerated the cathodic reaction. However, it should be noted that the thermal stability of the GO sheets was found to be poor, resulting in reduced stability at high temperatures. Liang et al.¹³⁷ utilized a synthesized c-SPPQ-3 material in a methanol fuel cell and achieved a power density of up to 88 mW cm⁻² at 75 °C and with a methanol feed concentration of 20 wt%. This performance enhancement was attributed to the reduction in methanol permeation through acid–base crosslinking of c-SPPQ-3 and the enhanced proton conductivity facilitated by the modified material. However, it was observed that the performance of the fuel cell decreased with increasing methanol feed concentration due to methanol crossover. Han et al.¹¹² investigated cross-linked C-SP90/SP90NF membranes comprising high-sulfonated salt-based SPEEK nanofibers and acid-based SPEEK matrix composite membranes. These membranes were applied in single-cell H₂/air fuel cells, demonstrating a maximum power density of 485 mW cm⁻². They exhibited improved proton conductivity in wet conditions, low hydrogen permeability, and chemical stability during practical experiments. The PFC₂-TF-SI membrane examined by Tan et al.¹²⁴ displayed a remarkable power density of up to 1005 mW cm⁻² (Fig. 17c). The higher power density was attributed to the presence of bifunctional groups, which provided a higher proton concentration under high humidity, thus reducing proton dilution by the membrane. The dense flexible side chain and difunctional groups enhanced the hydrophobic/hydrophilic separation, making it significant for optimizing battery performance under low humidity conditions. The length of the hydrophilic oligomers in the side chain and the synergistic effect of the difunctional groups significantly reduced proton conduction resistance, providing a new research idea and experimental basis for the development of NPFSA-PEM suitable for a wide humidity range. Chu et al.¹²⁵ assembled the SPEEK–PDA/PEI@PVDF composite membrane into a methanol fuel cell and achieved a peak power density of 58.9 mW cm⁻² at 2 M methanol fuel concentration and 80 °C. The membrane maintained an initial open-circuit voltage of 97.39% and a peak power density of 54.3 mW cm⁻² after 100 hours of continuous operation under these conditions. The preparation process of this composite membrane exhibited superior comprehensive performance, providing a feasible method and experimental theoretical basis for creating high-performance NPFSA-PEM materials. The phosphoric acid doped IMOPBI membrane IM-60, prepared by Wang et al.¹⁶ maintained good conductivity stability (92.5% residual after 240 hours at 160 °C) in H₂/air cell tests under conditions of 200 kPa back pressure, 1 mg cm⁻² Pt loading, and 160 °C (Fig. 17d). It exhibited a maximum power density of 708 mW cm⁻², placing it at the forefront of current research on high-temperature proton exchange membrane fuel cells. Additionally, the membrane material exhibited good durability; after 100 hours of operation at a current density of 0.5 A cm⁻², the voltage of the HT-PEMFC assembled with the IM-60 membrane decreased from 623.53 to 620.57 mV, representing a loss of about 0.47% and a decay rate of 29.4 μV h⁻¹. The results reported in this work may provide a new technique for the preparation of quaternary ammonium groups grafted onto polybenzimidazole for high-performance, high-temperature proton exchange membrane fuel cells. Wang et al.¹⁴⁷ introduced optimized MWs PIM-1 into an aryl ether PBI matrix using solution casting technology to produce a series of PIM-1 containing alloy membranes. The membrane material exhibited excellent comprehensive properties, including high acid absorption performance, good mechanical strength, and high proton conductivity. Among them, the OPBI/L-PIM alloy membrane, specifically L-10 PA, showed 438 mW cm⁻² (at 160 °C, under non-humidification conditions) in H₂/O₂ battery testing, indicating its enormous potential for application in HT-PEMFC. This provides an effective strategy for preparing new polymer alloy membranes. The CS/PDA-ADPS-5% composite membrane, synthesized by Fan et al.,⁷⁵ when used in a methanol fuel cell, demonstrated a maximum output power density of 30.2 mW cm⁻² (2 M methanol, 2 M H₂SO₄, 70 °C) at 70 °C. Additionally, it exhibited an open-circuit voltage of 0.68 V at the same temperature. These results indicate that the composite membrane possesses satisfactory power density and OCV performance, making it a promising candidate as a NPFSA-PEM for methanol fuel cell applications. The study also proposed an effective strategy to optimize the proton conductivity and mechanical properties of the NPFSA-PEM composite membrane. Maiti et al.⁹¹ developed XSPEEK/SPBI/PrSGO nanocomposite membranes for fuel cell assembly. The fuel cell assembled using these nanocomposite membranes demonstrated a maximum power density of 820 mW cm⁻² (100% RH, 80 °C) under a 4 wt% PrSGO load (Fig. 17e). It displayed a high cell open-circuit voltage and low permeability, indicating that the incorporation of PrSGO into the cross-linked SPEEK membrane, evenly dispersed within the polymer matrix, effectively improved the performance of the fuel cell. These modifications resulted in higher proton conductivity, lower permeability, and better water retention characteristics. The comprehensive study of fuel cell membrane materials, along with the adoption of various modification methods and material designs, has significantly improved the proton conduction performance and overall performance of fuel cells. This progress provides a material basis for the development of fuel cell technology. The membrane materials show excellent power density, proton conductivity, and stability, presenting better material choices for the practical application of fuel cells in the future. Mondal et al.¹⁴⁸ synthesized a novel composite membrane consisting of sulfonated polybenzimidazole (SPBI) and sulfonated graphene oxide (SGO) with varying amounts of SGO fillers. This composite membrane was then employed in microbial fuel cells (MFCs). Results showed that the membrane achieved a remarkable power density of up to 472.46 mW m⁻² in MFCs, demonstrating enhanced current production with lower voltage drop. These findings suggest that the composite membrane exhibits superior power generation efficiency, making it a promising membrane material for the development of high-performance MFCs. Souza et al.⁸⁴ synthesized BNC/LIG membranes based on bacterial nanocellulose (BNC) and lignin (LIG) and applied them to microbial fuel cells (MFCs) (Fig. 17f). The MFCs utilized E. coli as microorganisms in the anode chamber and showed the maximum power density with stable operation for 66 hours before a decline in performance. This implies that the BNC/LIG membranes exhibit good biocompatibility, enabling the viability of bacteria during operation, while the lignin component acts as a barrier to biological contamination. Compared to other biodegradable membranes, the BNC/LIG membranes offer a longer service life. This study provides a practical theoretical and experimental basis for the development of green and efficient biopolymer membrane materials.

Fig. 17 (a) Fuel cell performance curves of C-SPEEK and C-SPEEK/HPW/GO membranes. Reproduced with permission.¹⁴⁶ Copyright 2020, Elsevier. (b) Polarization and power density curves of the GPS-50 and Nafion212 membranes. Reproduced with permission.⁷⁴ Copyright 2021, Elsevier. (c) Cell performance of the prepared membranes at 80 °C, 80% RH. Reproduced with permission.¹²⁴ Copyright 2023, Elsevier. (d) Durability of IM-60 membrane at 160 °C under 0.5 A cm⁻² with back pressures of 200 kPa, voltage change. Reproduced with permission.¹⁶ Copyright 2023, Elsevier. (e) Performance of single-cell with XSPEEK/SPBI/PrSGO nanocomposite membranes at 80 °C and 100% RH. Reproduced with permission.⁹¹ Copyright 2023, Elsevier. (f) Schematic diagram of a two-chamber microbial fuel cell highlighting the electrochemical processes and the morphology of the membrane composed of BNC/LIG. Reproduced with permission.⁸⁴ Copyright 2023, Elsevier. (g) Polarization curves of the semi-passive DMFCs using the SPEEK, SPEEK/GO/SPEEK, SPEEK/SGO/SPEEK membranes, and blend SPEEK/SGO membrane and Nafion112 in 1.0 M methanol at 65 °C. Reproduced with permission.¹⁴⁹ Copyright 2017, Elsevier. (h) Durability test curves of membranes. Reproduced with permission.¹⁰ Copyright 2019, Elsevier. (i) The polarization curves for cells loading SPBPEK-Ps membranes at 80 °C with the feed gas of H₂ and O₂. Reproduced with permission.¹⁰⁶ Copyright 2023, Elsevier.

Li et al.¹⁴⁹ prepared a sandwich SPEEK/SGO/SPEEK membrane which has shown promising potential for fuel cell applications due to its proton conductivity and excellent methanol blocking ability, comparable to Nafion112. It achieved the highest power density of 42.5 mW cm⁻² (Fig. 17g) in a semi-passive direct methanol fuel cell (DMFC) using 1 M methanol at 65 °C. This power density is 112% higher than that of Nafion112. Consequently, this membrane could serve as a potential replacement for fuel cell membrane materials due to its good stability. In their study, Munavalli et al.¹⁰ developed SPEES-SA/SMZ membrane materials, which exhibited a maximum power density of 450 mW cm⁻² (1.1 A cm⁻²) in fuel cells, with an OCV value of 0.951 V. The composite membrane displayed excellent fuel cell performance, and there were no significant changes in fuel cell voltage after stable operation for 100 hours on an ignition point battery (Fig. 17h). This indicates that the doping of zeolite derivatives in the membrane material greatly enhanced its durability within the fuel cell. The study offers a new concept and an excellent material and theoretical experimental basis for the application and development of fuel cells, providing valuable insights for the preparation of high-performance proton exchange membranes with novel structures. It also opens up new possibilities for developing other specific membrane types. The SPBPEK-Ps membrane developed by Liu et al.¹⁰⁶ demonstrated the highest power density of 1210 mW cm⁻² (80 °C, 100% RH) in fuel cells (Fig. 17i). This exceptional performance can be attributed to the multi-level interaction between the sulfonic acid groups and the heterocyclic structure within the membrane. Furthermore, the tunable microphase structure, derived from the side proton conductive groups regulated by the copolymer composition, contributes to excellent proton conductivity and mechanical dimensional stability. The membrane's discontinuous hydrophilic phase structure and microporous structure effectively enhance the binding between the SPBPEK-Ps membrane and water molecules, reducing its sensitivity to oxidation gas and providing excellent hydrogen barrier performance. It exhibits favorable battery performance and oxidation stability in hydrogen fuel cells, showing great potential for practical applications.

In conclusion, a comprehensive multi-faceted strategy can be employed to optimize NPFSA-PEM materials for fuel cells. These strategies include:

(1) Experimenting with material combinations and dopants such as carbon nanomaterials and phosphor-tungstic acid to enhance proton conduction performance and power density.

(2) Enhancing the mechanical strength and proton conduction properties through crosslinking technology and functionalization. Optimizing the type and concentration of crosslinker in the membrane preparation process. Designing mechanisms to prevent methanol permeation and minimize the negative effects of crossover on battery performance. Developing more effective isolation mechanisms.

(3) Improving material stability by considering thermal stability and chemical stability. Implementing structural optimization and incorporating components with enhanced stability to prevent membrane degradation.

(4) Gaining an in-depth understanding of the relationship between material structure and performance. Optimizing the comprehensive performance of NPFSA-PEM materials to strike a balance between proton conductivity, mechanical properties, and chemical stability, catering to various application requirements.

By implementing these integrated approaches, it is possible to systematically enhance the performance and stability of NPFSA-PEM materials for fuel cell applications.

4.2 Flow batteries

Proton exchange membrane flow batteries (PEMFBs) combine the characteristics of fuel cells and flow batteries, playing a crucial role in energy conversion and storage. The working mechanism of PEMFBs, as described by reference,¹⁵⁰ involves several steps: during the charging process, a current passes through the battery, oxidizing the positive electrolyte to generate protons. Simultaneously, the negative electrolyte undergoes a reduction reaction, producing electrons. Protons and electrons are then stored in the electrolyte, representing stored electrical energy. During discharge, the electrons combine with protons through external circuits for power generation. PEMFBs offer advantages such as safety and high energy efficiency (EE) and find applications in energy storage,^151,152 renewable energy integration,¹⁵³ and grid energy storage.

Wu et al.¹⁵⁴ introduced covalent organic frameworks (COFs) with sulfonic acid groups and uniformly arranged rigid nanochannels into aqueous organic redox flow batteries (AORFBs). These COFs provide advantages in terms of high safety and low cost, making them attractive for energy storage applications. Covalent organic frameworks (COFs) with uniformly arranged rigid nanochannels are suitable for fabricating membranes implemented in AORFBs and retain nearly 100% discharge capacity after 100 cycles of SPEEK@TpAzo-4 with no significant change in topography (Fig. 18a). Michel et al.¹¹³ prepared hybrid membranes with enhancement from 0.05 wt% hydroxylated boron nitride (OH-BN) in SPEEK polymers. These membranes exhibit the longest self-discharge time (14.21 hours) and the highest CE (∼97%@120 mA cm⁻²), as well as the highest ion selectivity of 52.629 × 10³ S cm⁻³ min. They also demonstrate notable stability in cycling tests. The SPDI-x membrane developed by Wang et al.⁷ exhibits good stability in terms of VO₂ acceptability and proper vanadium permeability. When used in vanadium redox flow batteries (VRFBs), the SPDI-x membrane outperforms the commercially available Nafion212 membrane (Fig. 18b), demonstrating improved coulombic efficiency and high ion selectivity. The assembled VRFB, utilizing the SPDI-x membrane, maintains stable operation for over 100 cycles without significant degradation. Zhang et al.¹⁵⁵ prepared SPSF-62 membranes, which exhibit membrane properties comparable to Nafion117 and SPEEK membranes when employed in VRFBs. These SPSF-62 membranes strike a balance between proton conductivity and vanadium ion permeability, resulting in high CE (98.8%) and EE (86.2%) at a current density of 100 mA cm⁻². They also demonstrate good cycling stability during 100 charge–discharge cycles, enhancing EE and physicochemical properties (Fig. 18c). This study provides valuable experimental and theoretical guidance for membrane selection and modification, presenting an effective research approach for developing efficient and low-cost membranes. The SPEEK/Him-pS composite membrane developed by Zhang et al.¹⁵⁶ was utilized in vanadium redox flow batteries (VRFBs) and demonstrated excellent CE of 98.8% and EE of 78.5% at a current density of 300 mA cm⁻². The CE and EE values did not show significant attenuation during 600 charge–discharge cycles, indicating the stability of the composite membrane (Fig. 18d). Yan et al.¹⁵⁷ investigated the PBI-PS 200% membrane in VRFBs, which exhibited outstanding electrochemical performance with VE of 90.73%, EE of 89.13%, and CE of 97.51% at a current density of 80 mA cm⁻². The open circuit voltage (OCV) of the PBI-PS 200% battery also lasted for 180 hours under the test conditions, significantly surpassing the 24 hours OCV of the Nafion212 membrane (Fig. 18e). The study highlights the high efficiency and performance of the PBI-PS 200% membrane and its promising application prospects in VRFBs. Aziz et al.¹⁵⁸ investigated the SPEKS/sGO composite membrane for all-vanadium redox flow batteries (VRBs). The composite membrane exhibited excellent CE of 99.4% and EE of 82.8% at a current density of 40 mA cm⁻². It maintained an OCV of 0.8 V for 438 hours and showed stable electrochemical properties over 300 cycles (Fig. 18f). The outstanding electrochemical performance and stability of the composite membrane position it as a compelling alternative to the expensive Nafion membranes, demonstrating the potential for the development of more economical membrane materials. Wang et al.¹⁵⁹ developed the S-PBI-100 membrane for iron–chromium redox flow batteries (ICRFBs). This membrane exhibited a minimum area resistance of 0.69 Ω cm⁻² and excellent CE of 98.2% and EE of 83.17% at a current density of 80 mA cm⁻². After undergoing 1370 cycles (800 hours) of testing at 80 mA cm⁻², the membrane retained 82.21% of its high EE. The S-PBI-100 membrane, with its comparable performance to the Nafion212 membrane and lower cost, possesses stronger economic value and practical feasibility for ICRFB applications. This study provides an experimental and theoretical basis for the development of low-cost and high-efficiency membrane materials. Chen et al.¹⁶⁰ investigated the SPEEK–PBI blend membrane for vanadium flow batteries (VFBs). Among the studied membranes, the SPEEK-15 membrane exhibited the best electrochemical performance with CE of 98.5%, VE of 91.1%, and EE of 89.8% at an extremely high current density of 180 mA cm⁻². The CE remained constant after 350 cycles, while VE and EE only exhibited slight decreases (Fig. 18g). The SPEEK-15 membrane achieved a good balance between high ionic selectivity and high proton conductivity, demonstrating excellent electrochemical properties and stability. This research greatly contributes to the commercial development of VFBs. Cai et al.¹⁶¹ developed the 8SPAES-12 membrane for VRFBs, exhibiting high cell efficiency with CE of 95.2%, VE of 91.7%, and EE of 87.3% at a current density of 60 mA cm⁻². The membrane showed good stability over 100 cycles, further demonstrating its suitability for VRFB applications. Ding et al.¹⁶² prepared the SPBI-30 membrane for VRFBs by controlling the degree of sulfonation. This membrane exhibited higher CE, VE, and EE in VRFBs compared to the Nafion115 membrane. After 500 cycles, it maintained a capacity retention rate of 54.95% (compared to 20.47% for the Nafion115 membrane) and exhibited a self-discharge time of 384 hours (compared to 56 hours for the Nafion115 membrane) (Fig. 19e). The SPBI-30 membrane shows excellent electrochemical performance and stability, making it a potential candidate for VRFB membranes.

Fig. 18 (a) Cross-section SEM images of TpAzo membrane before and after the charge–discharge cycles. Reproduced with permission.¹⁵⁴ Copyright 2023, Elsevier. (b) Cycling test results of VRFB single cells with Nafion212, SPDI-90 and SPDI-100 at 80 mA cm⁻². Reproduced with permission.⁷ Copyright 2023, Elsevier. (c) 100-time cycling stability of Nafion117 and SPSF-62 membrane at 50 mA cm⁻². Reproduced with permission.¹⁵⁵ Copyright 2019, Elsevier. (d) Cyclic efficiencies of VRFB assembled with the SPEEK/Him-pS-3 composite membrane varied with cycle numbers. Reproduced with permission.¹⁵⁶ Copyright 2021, The American Chemical Society. (e) Cycling performance of the cells using the PBI-PS 200%. Reproduced with permission.¹⁵⁷ Copyright 2020, Elsevier. (f) Cyclic efficiencies of the VRB assembled with the SPEKS/sGO (0.5%) composite membrane. Reproduced with permission.¹⁵⁸ Copyright 2018, The Royal Society of Chemistry. (g) The cycling performance of SPEEK-15 at 180 mA cm⁻². Reproduced with permission.¹⁶⁰ Copyright 2018, Elsevier. (h) Cell performance of the VRFBs with S-PAEK-40 and Nafion®117 membranes at 20 mA cm⁻² for the charge–discharge cycling test with 100 cycles. Reproduced with permission.¹⁶³ Copyright 2017, The Royal Society of Chemistry.

Fig. 19 The PEM electrolyzer internal structure. Reproduced with permission.¹⁶⁶ Copyright 2022, Elsevier.

Zhang et al.¹⁶⁴ conducted a study where they introduced DCNTs-HPW nanofillers to prepare S/DCNTs-HPW-1 membranes. These membranes exhibited excellent CE of 98.2% and EE of 89.5% at a current density of 50 mA cm⁻². They also demonstrated a self-discharge time of up to 576 hours without a significant decrease in efficiency over 100 cycles. The DCNTs-HPW hybrid membranes show great potential for applications in vanadium redox flow batteries (VRFBs), offering an innovative approach to the preparation of membrane materials with strong stability. In another study, Yang et al.¹⁶³ prepared S-PAEK membranes from amine-containing PAEK precursors. These membranes exhibited higher EE compared to Nafion117 membranes. Furthermore, their performance did not degrade significantly after 100 cycles (Fig. 18h), indicating that poly-aryl-ether-ketone with long aliphatic sulfonic acid groups provides excellent chemical stability and electrochemical properties for ion exchange membranes. The development of S-PAEK membranes provides valuable support for the advancement of higher-efficiency VRFBs. Overall, these studies highlight various membrane materials with excellent electrochemical properties, stability, and potential for cost-effectiveness, greatly enhancing the development and application of advanced membrane materials in energy storage systems.

In summary, the majority of the studies reviewed demonstrate good stability of the developed membrane materials in cyclic tests, confirming their reliability in practical applications. These findings offer a wider range of options and possibilities for the application and development of flow batteries. They also lay a strong foundation for the design of more efficient, stable, and economical membrane materials in the future. However, further research is needed to validate the performance, scalability, and exploration of lower-cost membrane materials for larger-scale commercial applications.

4.3 Electrolysis

Hydrogen production through water electrolysis is currently the primary method employed, and PEM electrolysis technology has emerged as a promising approach due to its environmental benefits and high efficiency.¹⁶⁵ The working principle of a PEM electrolyzer is depicted in the following Fig. 19.

The sTBmT-1.88 membrane, made by Wang et al.¹¹⁸ exhibited superior electrochemical performance, longer lifespan, and improved stability. Compared to Nafion212, sTBmT-1.88 demonstrated better I–V performance with a maximum current density of 4.6 A cm⁻²@2.0 V and lower ohmic resistance. Additionally, the sTBmT-1.88 membrane performed well in the cyclic accelerated stress test (AST), maintaining consistent performance for 80 hours and having a lifespan almost twice that of Nafion212 (Fig. 20a). Park et al.¹⁶⁷ introduced a hydrocarbon-based SPAES/PIN composite proton exchange membrane by incorporating sulfonated poly arylene ether sulfone (SPAES) as the proton conductor and embedding PINs in the SPAES polymer matrix. The composite membrane exhibited excellent mechanical and dimensional stability, along with a high degree of selectivity for protons. Compared to the Nafion-211 membrane, the SPAES/PIN membrane showed significant improvement in microbial electrolyzer performance and achieved a hydrogen purity of 97.2%, indicating its promising application potential (Fig. 20b). The SPAES50 membrane, developed by Park et al.¹⁶⁸ demonstrated excellent mechanical properties and superior proton conductivity compared to the Nafion211 membrane at operating temperature. Under PEMWE conditions, the SPAES50 membrane showcased an outstanding performance of up to 1069 mA cm⁻² at 1.6 V, surpassing Nafion212, which attained 900 mA cm⁻² under the same conditions (Fig. 20c). As a cost-effective hydrocarbon-based material, SPAES50 has emerged as a potentially efficient and economical membrane for PEMWE applications. Han et al.¹⁶⁹ synthesized a hydrocarbon-based BPSH membrane with different polymer structures, namely random and multi-block, as substitutes for the PFSA membrane used in PEMWE. The random BPSH membrane displayed superior performance during PEMWE operation compared to the multi-block variant. The block BPSH membrane exhibited enhanced proton conductivity, higher hydrogen permeability, and achieved an excellent performance of up to 5.3 A cm⁻² in PEMWE at 1.9 V. This performance surpassed that of Nafion 212, which achieved 4.8 A cm⁻² under the same conditions (Fig. 20d). These findings provide compelling research cases for the exploration of hydrocarbon-based ionomers. Klose et al.¹⁷⁰ utilized a fluorine-free MEA incorporating sPPS as the membrane and ionomer in the electrode. When compared to state-of-the-art Nafion N115-MEAs, the performance of sPPS-MEAs was significantly better, achieving 3.5 A cm⁻² as opposed to 1.5 A cm⁻² at 1.8 V. This performance improvement stemmed from the considerably lower high-frequency resistance of sPPS-MEA compared to Nafion-MEA, contributing to enhanced performance and lower material costs. Additionally, the sPPS membrane demonstrated a three times lower hydrogen crossover rate (0.3 mA cm⁻²) than the Nafion-N115 membrane (1.1 mA cm⁻²) and displayed higher conductivity properties, widening the operational window for sPPS-based PEM systems. Kim et al.¹⁷¹ reported the high-temperature water electrolysis performance of a cross-linked sulfonated polyphenylsulfone (CSPPSU) membrane. They used the CSPPSU membrane and IrO₂/Ti as a catalytic electrode for oxygen evolution in water electrolysis at 150 °C and 1.8 V. The current density achieved was 456 mA cm⁻², which provides new ideas for membrane research in high-temperature PEMWE. Choi et al.¹⁷² enhanced the mechanical adhesion and chemical stability of the hydrocarbon-based membrane at the PEMWE interface by embedding free radical scavengers into the interlocking interface layer (IIL) (Fig. 20e). This resulted in the formation of a ball-and-socket interlocking structure. Compared to Nafion-MEA, the Ce-IIL-MEA showed stable operation for up to 500 hours under polarization with lower polarizability. This study provides guidance for future development of hydrocarbon membranes for PEMWE systems. Ahn et al.¹⁷³ developed a highly sulfonated aromatic G-sPSS-X membrane, which demonstrated high proton conductivity and low hydrogen permeability. The membrane displayed excellent performance, achieving 6000 mA cm⁻² under a voltage condition of 1.9 V, which is 2.1 times higher than Nafion-212 (Fig. 20f). It also exhibited a loss-free endurance performance of 50 hours under a high current density of 1000 mA cm⁻². This research offers a feasible approach for designing high-performance PEM and provides a practical method for analyzing PEM in PEMWE.

Fig. 20 (a) Voltage profile during the AST at 0.02 and 2 A cm⁻². Reproduced with permission.¹¹⁸ Copyright 2023, Elsevier. (b) Gas composition comparisons between a SPAES/PIN composite membrane and Nafion-211. Reproduced with permission.¹⁶⁷ Copyright 2017, Elsevier. (c) Polarization curves at 1.6 V of SPAES-based and Nafion-based (Nafion211 and Nafion115) PEMWE. Reproduced with permission.¹⁶⁸ Copyright 2021, Elsevier. (d) The performance of BPSH membranes is compared to Nafion115 and Nafion212 (open symbols). Reproduced with permission.¹⁶⁹ Copyright 2021, Elsevier. (e) Schematic diagram of the structure and fabrication process for the radical scavenger-containing interlocking interfacial layer for PEMWE. Reproduced with permission.¹⁷² Copyright 2022, The Royal Society of Chemistry. (f) A 3D-plot comparing the current density at 1.9 V and 90 °C with reported values, as a function of IEC and the current density of compared Nafion. Reproduced with permission.¹⁷³ Copyright 2022, The American Chemical Society.

In summary, researchers have successfully improved the performance and stability of membrane materials used in PEMWE by modifying the membrane structure, adjusting material composition, and optimizing preparation methods. Optimization measures include incorporating new materials (such as embedding free radical scavengers), altering polymer structures, optimizing assembly methods, designing multi-layer structures, and introducing coating agents. These measures not only enhance the efficiency and stability of PEMWE systems but also reduce preparation costs in some cases, expanding the application prospects of proton exchange membrane technology in PEMWE. These studies contribute to the advancement of proton exchange membrane technology in PEMWE and offer practical guidance and support for commercial applications.

5 Conclusion and perspectives

5.1 Conclusion

In summary, high-performance NPFSA-PEM relies on key parameters such as proton conductivity, mechanical strength, thermal stability, and chemical stability. Researchers have delved into novel raw materials for membrane synthesis and optimized the membrane preparation process. Techniques such as doping organic–inorganic metal materials and refining cross-linking methods have been explored. These efforts aim to exert control over the composition and structure of functional groups within the membrane, leading to enhanced proton conductivity, elevated temperature resistance, and prolonged service life of NPFSA-PEM. These advancements validate the significant potential of NPFSA-PEM in applications such as fuel cells and liquid flow batteries. The exploration of new materials and optimization processes serves to pave the way for further development and industrial application of NPFSA-PEM. This review offers a comprehensive and detailed analysis of NPFSA-PEM research, elucidating its performance and potential applications in various energy fields such as flow batteries and electrolysis. It aims to provide researchers with a deeper understanding of the advancements in this area, shedding light on the development prospects of non-perfluorinated sulfonic acid proton exchange membrane technology in energy applications. This work holds significant reference value for advancing PEM materials, enhancing energy conversion and storage efficiency, facilitating the industrialization of NPFSA-PEM, driving energy structure transformation, and achieving the goal of sustainable green development.

5.2 Perspectives

5.2.1 Material innovation at NPFSA-PEM. New materials play a pivotal role in enhancing energy conversion efficiency and energy density. The material characteristics of NPFSA-PEM directly impact its energy output density in fuel cells, storage efficiency in flow batteries, and hydrogen production efficiency and purity in electrolysis hydrogen production. Doping, structural manipulation, nanocomposites, crosslinking optimization, and group control are crucial optimization strategies to improve NPFSA-PEM performance. NPFSA-PEM governs proton conduction, which, in turn, influences device performance, such as energy conversion rate and energy density. Incorporating doped nanoparticles (e.g., GO, MOFs), nano-porous materials, and other high proton-conducting materials, controlling the membrane's phase state, optimizing cross-linking methods, and managing the composition and density of groups in the membrane effectively enhance its proton conduction capacity. However, it is essential to balance and improve the interplay between proton conduction performance and physical and chemical property control. This prevents the excessive pursuit of high proton conduction capacity, which could compromise the membrane's thermal stability and durability. Ensuring a stable enhancement of the membrane's overall performance is crucial for promoting the large-scale application of NPFSA-PEM. Moreover, similar strategies can be employed to strengthen other properties. For instance, doping nanofibers can significantly improve the membrane's methanol barrier ability, while enhancing the cross-linking can boost its tensile strength.

5.2.2 Commercialization. Although the performance of NPFSA-PEM in fuel cells and other applications has surpassed some commercial Nafion membranes, its commercial application in energy conversion and storage devices still faces significant challenges. These challenges primarily revolve around cost efficiency, large-scale production, and long-term stability. Current applications of fuel cells, flow cells, and electrolytic hydrogen production are hampered by issues such as low energy density, poor durability, and economic constraints, hindering their widespread adoption. Technological breakthroughs are crucial, particularly in enhancing service life and proton conductivity. Continued exploration is necessary to develop more efficient, economical, and environmentally friendly NPFSA-PEM alternatives to replace perfluorosulfonic acid membranes, which currently contribute to significant environmental pollution.

Strengthening NPFSA-PEM based on application scenarios is a practical approach to drive industrialization. Consideration of thermal stability and anti-swelling properties becomes crucial in different temperature and humidity scenarios. For instance, under low-temperature and low-humidity conditions, where thermal stability and anti-swelling properties have relatively low performance requirements, other properties like proton conductivity and tensile strength can be prioritized to enhance the overall membrane performance. Strengthening mechanical properties at stress concentration points and improving self-healing capabilities are also essential to resist damage, extend lifespan, and enhance the economy, thereby facilitating the broader application of NPFSA-PEM.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was sponsored by the National Natural Science Foundation of China (NSFC 52106268) and the National Natural Science Foundation of Hubei Province, China (No. 2022CFB748).

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