Effect of Polyvinylpyrrolidone on the Structure and Performance of Composite Membranes for Alkaline Water Electrolysis

Effect of Polyvinylpyrrolidone on the Structure and Performance of Composite Membranes for Alkaline Water Electrolysis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of Polyvinylpyrrolidone on the Structure and Performance of Composite Membranes for Alkaline Water Electrolysis daiying sun, Jianyun He, Bo Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9285461/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Hydrogen energy is regarded as a promising clean energy source due to its high energy density, environmental friendliness, and wide range of applications. As a key component of alkaline electrolyzers, composite membranes play a crucial role in determining the efficiency and stability of alkaline water electrolysis systems. In this work, composite membranes for alkaline water electrolysis were prepared via an immersion precipitation phase inversion method using Polyphenylene sulfide mesh as the reinforcement layer, polysulfone as the polymer matrix, polyvinylpyrrolidone as the pore-forming additive, and Nano-zirconia particles as the inorganic filler. The effects of polyvinylpyrrolidone content and molecular weight on the microstructure and electrochemical properties of the membranes were systematically investigated by evaluating area resistance, alkali uptake, gas evolution current density, and membrane morphology. The results indicate that the introduction of polyvinylpyrrolidone significantly influences membrane formation, leading to the evolution of the internal structure from a dense morphology to a porous asymmetric structure with sponge-like, finger-like, and mixed pore configurations. Meanwhile, the area resistance of the membranes shows a pronounced dependence on both polyvinylpyrrolidone content and molecular weight, whereas the working current density exhibits relatively weak sensitivity to molecular weight variation. These findings demonstrate that tuning polyvinylpyrrolidone parameters provides an effective strategy for regulating pore structure and improving the performance of composite membranes for alkaline water electrolysis. hydrogen energy composite membrane phase transformation polyvinylpyrrolidone surface resistance micromorphology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction With the intensifying impacts of global climate change and the continuous advancement of carbon neutrality goals, the transition toward energy systems dominated by renewableand clean energy sources has become an inevitable trend. In recent years, renewable energy technologies such as wind and solar power have developed rapidly. However, their large-scale utilization still faces significant challenges. On the one hand, renewable energy resources are inherently intermittent and fluctuating, and their supply is unevenly distributed inboth time and space, which restricts the efficient utilization of renewable electricity [ 1 ] . On the other hand, renewable energy is mainly converted into electrical energy, while its direct application remains difficult in sectors that are hard to fully electrify, such as high-temperature industrial processes and certain building heating systems. Hydrogen energy, owing to its high energy density, environmental friendliness, and favorable storage and transportation characteristics, has been widely regarded as an important energy carrier linking renewable energy generation with end-use energy demand [ 2 ] . Hydrogen possesses a high heatingvalue of up to 142.35 MJ·kg⁻¹, exceeding that of most conventional fossil fuels, and its combustion product is primarily water, resulting in nearly zero environmental pollution [ 3 ] .Therefore, coupling renewable energy with hydrogen production through water electrolysis enables the conversion of surplus electricity into hydrogen [ 4 ] . This process not only facilitates the large-scale storage of renewable energy but also provides a clean energy carrier for transportation, industrial processes, and integrated energy systems [ 5 ] . Currently, waterelectrolysis technologies for hydrogen production mainly include alkaline water electrolysis(AWE) [ 6 ] , anion exchange membrane water electrolysis (AEMWE) [ 7 ] , and proton exchangemembrane water electrolysis (PEMWE) [ 8 ] . Among these technologies, AWE has become the most widely applied method for industrial hydrogen production due to its longdevelopment history, high technological maturity, and relatively simple system configuration [ 9 ] . In addition, AWE systems employ alkaline electrolytes and generally do not require noble metal catalysts for electrode materials [ 10 ] , which provides significant advantages in termsof both capital investment and operational costs. Despite its technological maturity, conventional AWE systems still suffer from relatively low energy efficiency. The electrolysis efficiency of typical alkaline electrolyzers is generally in the range of 52–62%, which is lower than that of emerging technologies such as AEMWE and PEMWE [ 11 ] . In the overall hydrogen production process based on alkalinewater electrolysis, electricity consumption is the dominant cost factor, accounting for as much as 87% of the total hydrogen production cost. Consequently, improving the efficiency of alkaline electrolyzers has become a critical objective for the further development of AWE technology [1213] . Within an alkaline electrolyzer, the diaphragm is a key functional component responsible for separating the anodic and cathodic compartments while allowingionic transport between the electrodes. The performance of the diaphragm directly affectsboth the purity of the produced hydrogen and oxygen and the overall energy consumptionof the electrolysis system [ 14 ] . Therefore, the development of high-performance diaphragmswith low resistance and high stability is of great importance for enhancing hydrogen production efficiency and reducing operational costs in alkaline water electrolysis systems. In the field of alkaline electrolysis diaphragm materials, polymer-based composite membranes have attracted considerable attention due to their excellent chemical stability, tunable microstructure, and relatively low fabrication cost. In recent years, polymer materials such as polysulfone (PSU), polyether ether ketone (PEEK), and their sulfonated derivatives have been widely used for the fabrication of alkaline electrolysis diaphragms [ 15 ] . Furthermore, inorganic fillers, such as ZrO₂ and SiO₂, are often incorporated to construct organic–inorganic composite structures, which can enhance the mechanical strength, chemical stability, and ionic transport performance of the membranes [ 16 ] . Meanwhile, the microstructureof the diaphragm, particularly its pore morphology and pore size distribution, is considereda key factor affecting ionic transport resistance and gas barrier performance [ 17 ] . Therefore, regulating membrane pore structure through the phase inversion process has become an important strategy for improving diaphragm performance. During membrane fabrication, theintroduction of suitable pore-forming additives can effectively modify the phase separation kinetics of the casting solution, thereby tailoring the membrane morphology and pore distribution. Among various additives, polyvinylpyrrolidone (PVP) is a widely used water-soluble polymer that plays multiple roles in membranes prepared via the phase inversion method.PVP can act as a pore-forming agent to regulate membrane morphology, modify the phaseseparation behavior of the casting solution, and suppress the aggregation of inorganic particles, thereby improving the dispersion of fillers within the membrane matrix [ 18 ] . In addition, as a hydrophilic polymer, PVP can enhance the surface hydrophilicity of membranes [ 19 ] , which is beneficial for improving electrolyte wettability and reducing membrane area resistance. However, the role of PVP in alkaline electrolysis composite diaphragms has not yet been systematically clarified. In particular, the synergistic effects of PVP content and molecular weight on membrane microstructure evolution and electrochemical performance remain insufficiently understood. Therefore, a systematic investigation of PVP-regulated membrane structure and performance is of great significance for optimizing the design of alkaline electrolysis diaphragms. In this work, Polyphenylene sulfide (PPS) mesh was employed as the reinforcement support layer, polysulfone (PSU) as the polymer matrix, and Nano-zirconia (ZrO₂) as the inorganic filler, while PVP was introduced as the pore-forming additive to preparealkaline electrolysis composite diaphragms via the immersion precipitation phase inversion method. The effects of PVP content and molecular weight on membrane microstructure evolution and electrochemical performance were systematically investigated. Membrane properties were evaluated by characterizing the surface morphology, area resistance, alkali uptake,and gas evolution current density. By optimizing the PVP parameters, the pore structure ofthe composite membrane can be effectively regulated, enabling the development of alkalineelectrolysis diaphragms with low area resistance, high porosity, and good structural stability.This study aims to provide theoretical guidance and experimental insights for the designand fabrication of high-performance diaphragms for alkaline water electrolysis. Experimental 1.1 Materials N,N-Dimethylacetamide (DMAC) was purchased from Beijing InnoChem Science & Technology Co., Ltd. (Beijing, China). PPS mesh was supplied by Shanghai Haifan Filtration Material Co., Ltd. (Shanghai, China). ZrO₂, (VK-R30) was obtained from Zhejiang Zhita Nano New Materials Co., Ltd. (Zhejiang, China). PSU was provided by BASF SE (Germany). PVP with different number-average molecular weights (Mn) was supplied by Guangdong Yuemei Chemical Co., Ltd. (Guangdong, China), including four grades: 3,000–16,000 g·mol⁻¹, 30,000–58,000 g·mol⁻¹, 170,000–500,000 g·mol⁻¹, and 800,000–1,600,000 g·mol⁻¹. 1.2 Preparation of composite diaphragm The fabrication of the composite diaphragm mainly involved three steps, including preparation of the casting solution, membrane casting, and post-treatment. The detailed preparation process is schematically shown in Fig. 1 . 1.2.1 Preparation of casting solution First, a certain amount of DMAC was added into a beaker equipped with a magnetic stir bar and placed on a magnetic stirrer. A predetermined amount of PVP was then added into the solvent. The beaker was sealed with plastic film and stirred at room temperature until the PVP was completely dissolved. Subsequently, an appropriate amount of PSU was added to the solution and the mixture was continuously stirred under sealed conditions until complete dissolution was achieved. The solution was then allowed to stand for 30 min to eliminate entrapped air bubbles. After no obvious bubbles were observed on the surface of the solution, a certain amount of ZrO₂ was slowly introduced into the mixture. The beaker was sealed again and vigorously stirred until a homogeneous milky casting solution was obtained. Finally, the casting solution was subjected to a degassing treatment by slow stirring on a thermostatic magnetic stirrer until no visible bubbles remained in the solution. 1.2.2 Membrane casting Before membrane casting, a casting knife with a gap of 750 µm and a clean glass plate were prepared, and the casting temperature was controlled at a predetermined value. The PPS mesh was first placed flat on the clean glass plate to ensure close contact between the mesh and the glass substrate. The prepared casting solution was slowly poured along the edge of the beaker onto one side of the PPS mesh while keeping the beaker close to the mesh surface to avoid the formation of bubbles. The casting solution was then rapidly spread over the mesh using the casting knife to obtain a uniform liquid film. After standing in air for a short period, the coated substrate was immediately immersed in a coagulation bath containing deionized water. During immersion, solvent–nonsolvent exchange occurred, resulting in phase separation. A rapid phase inversion occurred at the membrane surface, forming a dense skin layer, while delayed phase separation occurred inside the membrane, gradually generating a porous structure from the surface to the interior. After sufficient immersion, the membrane spontaneously detached from the glass plate, forming a PPS-supported composite diaphragm. 1.2.3 Post-treatment The obtained composite diaphragm was first immersed in deionized water and then repeatedly rinsed with fresh deionized water to remove residual solvent from the membrane. This washing process was repeated until no white precipitate was observed. Finally, the cleaned composite diaphragm was air-dried to remove surface water, cut into appropriate pieces, and stored for subsequent characterization. 1.3 Characterization of composite diaphragms 1.3.1 Thickness measurement The thickness of the diaphragm, defined as the distance between the upper and lower surfaces, was measured using a digital micrometer with an accuracy of 0.01 mm. 1.3.2 Porosity measurement The porosity of the composite diaphragm was defined as the ratio of pore volume to the total membrane volume and was determined using a gravimetric method. Membrane samples (20 mm × 20 mm) were prepared, and three specimens were used for each measurement. The samples were first immersed in deionized water for cleaning, followed by drying in an oven at 90°C for approximately 1 h until completely dry. After cooling to room temperature for 30 min, the dry membrane masses (M d₁ , M d₂ , M d₃ ) were recorded with an accuracy of 0.001 g. The thicknesses (T₁, T₂, T₃) of the dried membranes were then measured using a micrometer with an accuracy of 0.01 mm. Subsequently, the samples wereimmersed in deionized water for sufficient wetting. After removal from the water, each sample was suspended in air for 30 s to remove excess surface liquid, and the wet membrane masses (Mw₁, Mw₂, Mw₃) were measured. If residual droplets remained, they were gently removed before weighing. The average values of mass and thickness were used for calculation. The porosity (ε) was calculated using Eq. (1): $$\:\begin{array}{c}\text{ε}\text{=}\frac{\text{M}\text{w}\text{-M}\text{d}}{\text{ρ·S·T}}\times\:100\%\#（1）\end{array}$$ where M w is the wet membrane mass (g), M d is the dry membrane mass (g), \(\:\rho\:\:\) is the density of deionized water (g·mL⁻¹), \(\:S\) is the membrane area (cm²), and \(\:T\) is the membrane thickness (cm). 1.3.3 Alkali uptake measurement Alkali uptake was used to evaluate the compatibility between the membrane and electrolyte and was expressed as the percentage increase in mass after immersion in KOH solution. Membrane samples (20 mm × 20 mm) were prepared, and three specimens were tested. The samples were first washed with deionized water and then dried in a vacuum oven at 90°C until completely dry. The dry membrane masses (Mn₁, Mn₂, Mn₃) were recorded with an accuracy of 0.001 g. The dried samples were immersed in 30 wt.% KOH solution for more than 4 h to ensure full absorption. After removal, the samples were suspended in air for 30 s to remove excess solution, and the wet membrane masses (Ml₁, Ml₂, Ml₃) were measured. Any remaining droplets were carefully removed prior to weighing. The average values were used for calculation. The alkali uptake (A) was calculated using Eq. (2): $$\:\begin{array}{c}A=\frac{{\text{M}}_{\text{L}}}{{\text{M}}_{\text{N}}}\times\:100\%\#（2）\end{array}$$ where M l is the mass of the membrane after alkali absorption (g), and M n is the dry membrane mass (g). 1.3.4 Area resistance measurement The area resistance of the diaphragm was measured using electrochemical impedance spectroscopy (EIS). The membrane sample was placed in a testing cell consisting of two polytetrafluoroethylene (PTFE) plates and two platinum electrodes, and fixed with PTFE screws. The cell was immersed in a 30 wt.% KOH solution, and impedance measurements were performed using an electrochemical workstation. The frequency range was 0.01 Hz to 10 kHz with an AC perturbation amplitude of 5 mV. The impedance spectra were recorded, and the first intercept of the high-frequency region with the real axis (Z′) was taken as the total resistance (R₂). Under identical conditions (temperature, electrolyte, and electrode configuration), the resistance of the electrolyte without the membrane (R₁) was measured. The area resistance (R) was calculated using Eq. (3): $$\:\begin{array}{c}R=\:\left(\text{R}2-\text{R}1\right)\times\:S\#（3）\end{array}$$ where R is the membrane area resistance (Ω·cm²), \(\:{R}_{1}\) is the electrolyte resistance (Ω), R 2 is the total resistance (Ω), and S is the effective membrane area (cm²). 1.3.5 Current density measurement The current density–voltage (J–V) characteristics of the membrane were measured using an electrochemical workstation. An H-type electrolytic cell was employed, with the membrane placed between the two compartments. A glassy carbon electrode, a Hg/HgO electrode, and a carbon rod were used as the working, reference, and counter electrodes, respectively. The electrolyte was 30 wt.% KOH solution, and the test temperature was maintained at 25°C. The applied voltage range was 1.5–2.5 V with a scan rate of 5 mV·s⁻¹. 1.3.6 Morphology characterization The surface and cross-sectional morphologies of the membranes were observed using scanning electron microscopy (SEM). The membrane samples were first fractured in liquid nitrogen to obtain clean cross-sections, then mounted on sample holders and coated with a thin layer of gold prior to observation. SEM images of both surface and cross-sectional structures were subsequently recorded. Results and discussion 2.1 Effect of PVP content on membrane properties Composite diaphragms were prepared according to the procedure described above. Membranes with different PVP contents of 0%, 5%, 10%, 15%, and 20% were fabricated. In all cases, the ZrO₂ content and PSU content were fixed at 70 wt.% and 20 wt.%, respectively. The pre-evaporation time was set to 15 s, and the coagulation bath temperature was maintained at 25°C. The prepared composite diaphragms were systematically characterized, and the corresponding results are summarized in Table 1 . Table 1 Membrane properties prepared with different content of PVP PVP content/wt.% thickness/mm porosity/% Alkali uptake /% Resistance /Ω·cm² 0 0.48 54.3 105 0.572 5 0.51 60.7 112 0.238 10 0.47 64.6 118 0.224 15 0.49 68.3 123 0.204 20 0.48 63.3 115 0.328 2.1.1 Effect of PVP content on membrane morphology Figure 2 presents the cross-sectional morphologies of the membranes prepared with different PVP contents (0%, 5%, 10%, 15%, and 20%). As the PVP content increases from 0% to 20%, a systematic evolution in membrane structure can be observed. At a PVP content of 0%, the casting solution exhibits a significant increase in viscosity after standing for a certain period. This behavior can be attributed to the absence of PVP, which functions not only as a pore-forming agent but also as a dispersant. Without PVP, ZrO₂ nanoparticles tend to aggregate, while PSU molecular chains are more prone to entanglement [ 20 ] , resulting in increased solution viscosity. Consequently, the phase separation process is dominated by delayed demixing, leading to the formation of a dense membrane structure. When the PVP content is increased to 5%, sparse sponge-like pores begin to appear; however, the pore connectivity remains relatively poor. At approximately 10% PVP content, a continuous hydrophilic network is formed within the casting solution, which promotes rapid and controlled phase separation [ 21 ] . As a result, a uniform and highly interconnected sponge-like porous structure is obtained. This morphology is considered optimal for achieving a balance between high ionic conductivity and low gas permeability. With a further increase in PVP content to 15%, the membrane structure transitions to one dominated by finger-like pores. Although the area resistance may decrease under this condition, the gas permeability tends to increase accordingly. When the PVP content reaches 20%, the thermodynamic stability of the system is significantly reduced. Excessive PVP increases the viscosity of the casting solution and slows down the phase separation kinetics [ 22 ] , leading to the formation of a thicker dense skin layer, a reduced number of finger-like pores, and an increased proportion of sponge-like structures [ 23 ] . 2.1.2 Effect of PVP content on membrane area resistance Figure 3 shows the electrochemical impedance spectra and the corresponding variation in area resistance of the membranes prepared with different PVP contents. It can be observed that the area resistance is strongly correlated with the membrane cross-sectional structure. At 0% PVP content, the membrane exhibits a nearly dense and nonporous structure, resulting in severely hindered ion transport pathways and consequently a very high area resistance. When the PVP content is increased to 5%, sparse pores are formed. Although the pore connectivity remains limited, the ion transport resistance is significantly reduced, leading to a noticeable decrease in area resistance. When the PVP content reaches approximately 10%, an optimal balance between thermodynamic stability and phase separation kinetics is achieved [ 24 ] , resulting in a uniform and highly interconnected sponge-like porous structure. This morphology provides a high porosity with moderate tortuosity, thereby offering efficient ion transport pathways and further reducing the area resistance. At a PVP content of around 15%, the membrane structure becomes dominated by finger-like pores, which leads to a further decrease in area resistance due to the formation of more direct ion transport channels. However, the presence of these finger-like macrovoids may also increase gas permeability. When the PVP content is further increased to 20% or higher, excessive pore-forming agent induces structural deterioration [ 25 ] . A mixed structure consisting of finger-like pores and sponge-like pores, or even the formation of a dense surface layer, may occur. These structural changes increase the resistance to ion transport, resulting in a significant rise in area resistance. 2.2 Effect of PVP molecular weight on membrane properties Composite diaphragms with different PVP molecular weights were prepared. The PVP content was fixed at 10% for all samples, while the contents of ZrO₂ and PSU were maintained at 70 wt.% and 20 wt.%, respectively. The pre-evaporation time was set to 15 s, and the coagulation bath temperature was controlled at 25°C. The resulting composite diaphragms were systematically characterized, and the corresponding results are summarized in Table 2 . Table 2 Membrane properties prepared with different molecular weights of PVP PVP number PVP molecular weights thickness /mm porosity /% Alkali uptake /% Resistance /Ω·cm² ① 3000–16000 0.45 66.3 122 0.335 ② 30000–58000 0.47 62.5 115 0.117 ③ 350000–600000 0.50 64.6 117 0.139 ④ 800000–1600000 0.52 60.8 111 0.279 2.2.1 Effect of PVP molecular weight on membrane morphology Figure 4 shows the cross-sectional morphologies of the membranes prepared with different PVP molecular weights (PVP①–PVP④). It can be observed that the membrane structure is strongly influenced by the molecular weight of PVP, which governs the phase separation behavior during membrane formation. When low-molecular-weight PVP①is used, its high diffusion rate accelerates solvent–nonsolvent exchange, leading to rapid liquid–liquid demixing. As a result, large finger-like macrovoids with vertical penetration are easily formed, accompanied by relatively high porosity. Meanwhile, the formation of a relatively dense surface layer effectively suppresses gas permeation, resulting in a balanced performance in terms of safety and area resistance. For medium to high molecular weight PVP② and PVP③, the longer polymer chains and enhanced chain entanglement slow down the solvent–nonsolvent exchange process. This leads to a more stable thermodynamic transition during phase separation, resulting in the coexistence of sponge-like structures and well-developed finger-like pores with improved uniformity and connectivity. However, when the molecular weight is further increased to PVP④, the significantly elevated solution viscosity severely suppresses the phase separation kinetics. This often results in the formation of a dense skin layer on the membrane surface, while the internal structure evolves into a disordered combination of finger-like and sponge-like pores. Consequently, the overall porosity decreases and pore connectivity is deteriorated [ 26 ] . 2.2.2 Effect of PVP molecular weight on membrane area resistance Figure 5 presents the electrochemical impedance spectra and the corresponding variation in area resistance of the membranes prepared with different PVP molecular weights. It can be observed that the molecular weight of PVP has a significant influence on the area resistance of the composite diaphragms, which exhibits an overall trend of first decreasing and then increasing with increasing molecular weight. When low-molecular-weight PVP ① is used, the shorter polymer chains facilitate rapid phase separation during membrane formation, leading to the development of finger-like macrovoid structures. However, its limited ability to enhance membrane hydrophilicity restricts OH⁻ transport [ 27 ] , resulting in relatively high area resistance. For medium molecular weight PVP ② and PVP ③, the chain length is more suitable for regulating the phase inversion process. These conditions enable the formation of a uniform and highly interconnected porous structure, while simultaneously improving membrane hydrophilicity. As a result, efficient ion transport pathways are established, facilitating rapid migration of OH⁻ ions in the alkaline electrolyte, and the membrane exhibits the lowest area resistance. When the molecular weight is further increased to the high-molecular-weight range PVP ④, the long polymer chains tend to entangle and aggregate, significantly increasing the viscosity of the casting solution and slowing down the phase separation process [ 28 ] . This leads to the formation of a denser pore structure. In addition, the presence of long-chain segments further hinders OH⁻ transport, and the strong binding effect of high-molecular-weight PVP promotes structural densification of the membrane. Consequently, the area resistance increases again. (b) Influence curves of molecular weights of PVP on surface resistance of membranes 2.2.3 Effect of PVP molecular weight on current density Figure 6 illustrates the variation in current density of the membranes prepared with different PVP molecular weights. As shown, the change in PVP molecular weight has a negligible effect on the current density of the composite diaphragms, and no obvious trend can be observed. This indicates that, under the present system, the molecular weight of PVP is not the dominant factor governing the current density. Instead, PVP molecular weight primarily influences the membrane microstructure and hydrophilicity, thereby affecting area resistance of ion transport, while its impact on current density can be considered insignificant. Conclusions In this study, composite diaphragms for alkaline water electrolysis with a ZrO₂ content of 70% and a PSU content of 20% were investigated. The effects of PVP content and molecular weight on the membrane microstructure and electrochemical performance were systematically evaluated. In this study, composite diaphragms for alkaline water electrolysis with a ZrO₂ content of 70% and a PSU content of 20% were investigated. The effects of PVP content and molecular weight on the membrane microstructure and electrochemical performance were systematically evaluated. The results demonstrate that, as the PVP content increases from 0% to 20%, the membrane structure evolves from a dense, nonporous morphology to a highly interconnected sponge-like porous structure, and subsequently to a mixed structure dominated by finger-like pores with a dense asymmetric skin layer. Correspondingly, the area resistance exhibits a decreasing–increasing trend. The lowest area resistance is 0.204 Ω·cm² ,which is achieved at a PVP content of 15%, while a more balanced performance between ionic conductivity and gas barrier properties is obtained at 10% PVP content. Furthermore, with increasing PVP molecular weight, the area resistance also shows a similar trend of first decreasing and then increasing. The minimum area resistance is 0.117 Ω·cm² ,which is observed within the molecular weight range of 30,000–600,000. In contrast, the variation in PVP molecular weight has no significant effect on the operating current density of the membranes. Declarations Funding: This research received no external funding. Competing Interests: The authors declare no competing interests. Author Contributions: Daiying Sun performed the experiments and wrote the manuscript. Jianyun He and Bo Liu supervised the work and revised the manuscript. All authors approved the final version. References Zhu QQ (2025) Preparation and performance of polysulfone composite diaphragms for alkaline water electrolysis. Beijing University of Chemical Technology Song ZL Preparation of polysulfone diaphragms for alkaline water electrolyzers. Hunan University, 2025. DOI:CNKI:CDMD:2.1018.067958 Yu JN, Wang YN, Fu JJ et al (2023) Research progress on hydrogen production by water electrolysis and diaphragm materials. Contemp Chem Res, (20): 5–7 Liu YY, Shang YS, Cheng XB et al (2024) Optimization study of diaphragm materials for alkaline water electrolysis. 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Membrane Sci Technol, 2013(1): 6. 10.3969/j.issn.1007-8924.2013.01.002 Jung B, Yoon J, Kim B et al (2004) Effect of molecular weight of polymeric additives on formation and permeation properties of asymmetric polyacrylonitrile membranes. J Membr Sci 243(1–2):45–57. 10.1016/j.memsci.2004.06.011 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 13 Apr, 2026 Reviewers invited by journal 13 Apr, 2026 Editor invited by journal 08 Apr, 2026 Editor assigned by journal 07 Apr, 2026 First submitted to journal 06 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9285461","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":622285201,"identity":"afa96c97-1124-4fde-9b12-9864aa582184","order_by":0,"name":"daiying sun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBklEQVRIie3PMUvDQBTA8QsHL8ulWS8ozVd4JVARin4BP8RJoS6p4NatJ4G6FF1PEPwW4nhHINOha6CLTi4Ol13UOqsX3BzuN73h/eE9QoLgn9JuAcM8vnrpurcJS1PZW0RG2UExWlugmZztZkr3JrROVsNj2R4BSWQ9QSn8+7maa6NgHJ2rpHnK7h8ZEh25rvw9wfZUaMdmNN65PMEzu2F7VNLs+s6T8BKN4g1ENxo5hw3blxpo4klyVWLN8IORViBn8MBQC39C2q9EAN8OY56sdH+C9nV7mAYcrZtpwe2UZcpU3l/yi7Jw3Tssb+PKPLvFwWGaVsZ1vsN+EMm/7QdBEATffAI621nRDJ+XZQAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0009-0000-5839-9438","institution":"Beijing University of Chemical Technology","correspondingAuthor":true,"prefix":"","firstName":"daiying","middleName":"","lastName":"sun","suffix":""},{"id":622285202,"identity":"e612c855-44e4-4162-a699-37d6f540c42a","order_by":1,"name":"Jianyun He","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jianyun","middleName":"","lastName":"He","suffix":""},{"id":622285203,"identity":"e571677e-5f7c-456d-98b7-19f3bd2d9aab","order_by":2,"name":"Bo Liu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Bo","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2026-04-01 02:21:57","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9285461/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9285461/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107371447,"identity":"bfe7ccd7-f197-4d8d-89af-adbadd0dfbf6","added_by":"auto","created_at":"2026-04-20 22:16:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":248363,"visible":true,"origin":"","legend":"\u003cp\u003eProcess diagram of composite membrane preparation\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/39440d26fa629aae2a781ed3.png"},{"id":107371450,"identity":"591b2875-a7c3-4514-a016-532f3d3782db","added_by":"auto","created_at":"2026-04-20 22:16:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":495086,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional morphologies of membranes prepared with different PVP contents. （a）0%PVP（b）5%PVP（c）10%PVP(d)15%PVP（e）20%PVP\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/56c998a39494ea9b61a993cf.png"},{"id":107371448,"identity":"ec6eff41-6452-449a-a7d2-6d446263c3f9","added_by":"auto","created_at":"2026-04-20 22:16:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":129193,"visible":true,"origin":"","legend":"\u003cp\u003e(a) EIS of membranes prepared with different PVP contents; (b) Influence curves of different PVP content on surface resistance of membranes\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/a423920da6d291180f94388d.png"},{"id":107487014,"identity":"30052ff8-36d3-45ba-986f-9907a28256c4","added_by":"auto","created_at":"2026-04-22 02:39:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":536672,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional morphologies of membranes prepared with different PVP molecular weights. （a）PVP①（b）PVP②（c）PVP③（d）PVP④\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/a8586bf66a77dccd3aa79fe1.png"},{"id":107488146,"identity":"af63fc4a-87f3-439f-972d-6031035b7749","added_by":"auto","created_at":"2026-04-22 02:43:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":123448,"visible":true,"origin":"","legend":"\u003cp\u003e(a) EIS of membranes prepared with different molecular weights of PVP;\u003c/p\u003e\n\u003cp\u003e(b) Influence curves of molecular weights of PVP on surface resistance of membranes\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/3933d6dd94a1bdb41af00ef6.png"},{"id":107487575,"identity":"3c5f4026-2c6d-41b8-a173-9b0c60a74c77","added_by":"auto","created_at":"2026-04-22 02:42:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":88598,"visible":true,"origin":"","legend":"\u003cp\u003eThe influence curve of PVP molecular weight variation on the current density of the prepared membrane\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/8bf83454e7d2ff9116d3f257.png"},{"id":107705473,"identity":"a6d2d18c-7fa6-42ad-b77e-c91441aedb25","added_by":"auto","created_at":"2026-04-24 09:13:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2028172,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9285461/v1/44243be9-15b2-403c-aa65-16bd5aa0c598.pdf"}],"financialInterests":"","formattedTitle":"Effect of Polyvinylpyrrolidone on the Structure and Performance of Composite Membranes for Alkaline Water Electrolysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWith the intensifying impacts of global climate change and the continuous advancement of carbon neutrality goals, the transition toward energy systems dominated by renewableand clean energy sources has become an inevitable trend. In recent years, renewable energy technologies such as wind and solar power have developed rapidly. However, their large-scale utilization still faces significant challenges. On the one hand, renewable energy resources are inherently intermittent and fluctuating, and their supply is unevenly distributed inboth time and space, which restricts the efficient utilization of renewable electricity\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. On the other hand, renewable energy is mainly converted into electrical energy, while its direct application remains difficult in sectors that are hard to fully electrify, such as high-temperature industrial processes and certain building heating systems. Hydrogen energy, owing to its high energy density, environmental friendliness, and favorable storage and transportation characteristics, has been widely regarded as an important energy carrier linking renewable energy generation with end-use energy demand\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Hydrogen possesses a high heatingvalue of up to 142.35 MJ\u0026middot;kg⁻\u0026sup1;, exceeding that of most conventional fossil fuels, and its combustion product is primarily water, resulting in nearly zero environmental pollution\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e.Therefore, coupling renewable energy with hydrogen production through water electrolysis enables the conversion of surplus electricity into hydrogen\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. This process not only facilitates the large-scale storage of renewable energy but also provides a clean energy carrier for transportation, industrial processes, and integrated energy systems\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. Currently, waterelectrolysis technologies for hydrogen production mainly include alkaline water electrolysis(AWE)\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e, anion exchange membrane water electrolysis (AEMWE)\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e, and proton exchangemembrane water electrolysis (PEMWE)\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Among these technologies, AWE has become the most widely applied method for industrial hydrogen production due to its longdevelopment history, high technological maturity, and relatively simple system configuration\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. In addition, AWE systems employ alkaline electrolytes and generally do not require noble metal catalysts for electrode materials\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e, which provides significant advantages in termsof both capital investment and operational costs.\u003c/p\u003e \u003cp\u003eDespite its technological maturity, conventional AWE systems still suffer from relatively low energy efficiency. The electrolysis efficiency of typical alkaline electrolyzers is generally in the range of 52\u0026ndash;62%, which is lower than that of emerging technologies such as AEMWE and PEMWE\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. In the overall hydrogen production process based on alkalinewater electrolysis, electricity consumption is the dominant cost factor, accounting for as much as 87% of the total hydrogen production cost. Consequently, improving the efficiency of alkaline electrolyzers has become a critical objective for the further development of AWE technology\u003csup\u003e[1213]\u003c/sup\u003e. Within an alkaline electrolyzer, the diaphragm is a key functional component responsible for separating the anodic and cathodic compartments while allowingionic transport between the electrodes. The performance of the diaphragm directly affectsboth the purity of the produced hydrogen and oxygen and the overall energy consumptionof the electrolysis system\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. Therefore, the development of high-performance diaphragmswith low resistance and high stability is of great importance for enhancing hydrogen production efficiency and reducing operational costs in alkaline water electrolysis systems.\u003c/p\u003e \u003cp\u003eIn the field of alkaline electrolysis diaphragm materials, polymer-based composite membranes have attracted considerable attention due to their excellent chemical stability, tunable microstructure, and relatively low fabrication cost. In recent years, polymer materials such as polysulfone (PSU), polyether ether ketone (PEEK), and their sulfonated derivatives have been widely used for the fabrication of alkaline electrolysis diaphragms\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Furthermore, inorganic fillers, such as ZrO₂ and SiO₂, are often incorporated to construct organic\u0026ndash;inorganic composite structures, which can enhance the mechanical strength, chemical stability, and ionic transport performance of the membranes\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. Meanwhile, the microstructureof the diaphragm, particularly its pore morphology and pore size distribution, is considereda key factor affecting ionic transport resistance and gas barrier performance\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. Therefore, regulating membrane pore structure through the phase inversion process has become an important strategy for improving diaphragm performance. During membrane fabrication, theintroduction of suitable pore-forming additives can effectively modify the phase separation kinetics of the casting solution, thereby tailoring the membrane morphology and pore distribution. Among various additives, polyvinylpyrrolidone (PVP) is a widely used water-soluble polymer that plays multiple roles in membranes prepared via the phase inversion method.PVP can act as a pore-forming agent to regulate membrane morphology, modify the phaseseparation behavior of the casting solution, and suppress the aggregation of inorganic particles, thereby improving the dispersion of fillers within the membrane matrix\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. In addition, as a hydrophilic polymer, PVP can enhance the surface hydrophilicity of membranes\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, which is beneficial for improving electrolyte wettability and reducing membrane area resistance. However, the role of PVP in alkaline electrolysis composite diaphragms has not yet been systematically clarified. In particular, the synergistic effects of PVP content and molecular weight on membrane microstructure evolution and electrochemical performance remain insufficiently understood. Therefore, a systematic investigation of PVP-regulated membrane structure and performance is of great significance for optimizing the design of alkaline electrolysis diaphragms. In this work, Polyphenylene sulfide (PPS) mesh was employed as the reinforcement support layer, polysulfone (PSU) as the polymer matrix, and Nano-zirconia (ZrO₂) as the inorganic filler, while PVP was introduced as the pore-forming additive to preparealkaline electrolysis composite diaphragms via the immersion precipitation phase inversion method. The effects of PVP content and molecular weight on membrane microstructure evolution and electrochemical performance were systematically investigated. Membrane properties were evaluated by characterizing the surface morphology, area resistance, alkali uptake,and gas evolution current density. By optimizing the PVP parameters, the pore structure ofthe composite membrane can be effectively regulated, enabling the development of alkalineelectrolysis diaphragms with low area resistance, high porosity, and good structural stability.This study aims to provide theoretical guidance and experimental insights for the designand fabrication of high-performance diaphragms for alkaline water electrolysis.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e1.1 Materials\u003c/h2\u003e \u003cp\u003eN,N-Dimethylacetamide (DMAC) was purchased from Beijing InnoChem Science \u0026amp; Technology Co., Ltd. (Beijing, China). PPS mesh was supplied by Shanghai Haifan Filtration Material Co., Ltd. (Shanghai, China). ZrO₂, (VK-R30) was obtained from Zhejiang Zhita Nano New Materials Co., Ltd. (Zhejiang, China). PSU was provided by BASF SE (Germany). PVP with different number-average molecular weights (Mn) was supplied by Guangdong Yuemei Chemical Co., Ltd. (Guangdong, China), including four grades: 3,000\u0026ndash;16,000 g\u0026middot;mol⁻\u0026sup1;, 30,000\u0026ndash;58,000 g\u0026middot;mol⁻\u0026sup1;, 170,000\u0026ndash;500,000 g\u0026middot;mol⁻\u0026sup1;, and 800,000\u0026ndash;1,600,000 g\u0026middot;mol⁻\u0026sup1;.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e1.2 Preparation of composite diaphragm\u003c/h2\u003e \u003cp\u003eThe fabrication of the composite diaphragm mainly involved three steps, including preparation of the casting solution, membrane casting, and post-treatment. The detailed preparation process is schematically shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e1.2.1 Preparation of casting solution\u003c/h2\u003e \u003cp\u003eFirst, a certain amount of DMAC was added into a beaker equipped with a magnetic stir bar and placed on a magnetic stirrer. A predetermined amount of PVP was then added into the solvent. The beaker was sealed with plastic film and stirred at room temperature until the PVP was completely dissolved. Subsequently, an appropriate amount of PSU was added to the solution and the mixture was continuously stirred under sealed conditions until complete dissolution was achieved. The solution was then allowed to stand for 30 min to eliminate entrapped air bubbles. After no obvious bubbles were observed on the surface of the solution, a certain amount of ZrO₂ was slowly introduced into the mixture. The beaker was sealed again and vigorously stirred until a homogeneous milky casting solution was obtained. Finally, the casting solution was subjected to a degassing treatment by slow stirring on a thermostatic magnetic stirrer until no visible bubbles remained in the solution.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e1.2.2 Membrane casting\u003c/h2\u003e \u003cp\u003eBefore membrane casting, a casting knife with a gap of 750 \u0026micro;m and a clean glass plate were prepared, and the casting temperature was controlled at a predetermined value. The PPS mesh was first placed flat on the clean glass plate to ensure close contact between the mesh and the glass substrate. The prepared casting solution was slowly poured along the edge of the beaker onto one side of the PPS mesh while keeping the beaker close to the mesh surface to avoid the formation of bubbles. The casting solution was then rapidly spread over the mesh using the casting knife to obtain a uniform liquid film. After standing in air for a short period, the coated substrate was immediately immersed in a coagulation bath containing deionized water. During immersion, solvent\u0026ndash;nonsolvent exchange occurred, resulting in phase separation. A rapid phase inversion occurred at the membrane surface, forming a dense skin layer, while delayed phase separation occurred inside the membrane, gradually generating a porous structure from the surface to the interior. After sufficient immersion, the membrane spontaneously detached from the glass plate, forming a PPS-supported composite diaphragm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e1.2.3 Post-treatment\u003c/h2\u003e \u003cp\u003eThe obtained composite diaphragm was first immersed in deionized water and then repeatedly rinsed with fresh deionized water to remove residual solvent from the membrane. This washing process was repeated until no white precipitate was observed. Finally, the cleaned composite diaphragm was air-dried to remove surface water, cut into appropriate pieces, and stored for subsequent characterization.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e1.3 Characterization of composite diaphragms\u003c/h2\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e1.3.1 Thickness measurement\u003c/h2\u003e \u003cp\u003eThe thickness of the diaphragm, defined as the distance between the upper and lower surfaces, was measured using a digital micrometer with an accuracy of 0.01 mm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e\u003cb\u003e1.3.2 Porosity measurement\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eThe porosity of the composite diaphragm was defined as the ratio of pore volume to the total membrane volume and was determined using a gravimetric method. Membrane samples (20 mm \u0026times; 20 mm) were prepared, and three specimens were used for each measurement. The samples were first immersed in deionized water for cleaning, followed by drying in an oven at 90\u0026deg;C for approximately 1 h until completely dry. After cooling to room temperature for 30 min, the dry membrane masses (M\u003csub\u003ed₁\u003c/sub\u003e, M\u003csub\u003ed₂\u003c/sub\u003e, M\u003csub\u003ed₃\u003c/sub\u003e) were recorded with an accuracy of 0.001 g. The thicknesses (T₁, T₂, T₃) of the dried membranes were then measured using a micrometer with an accuracy of 0.01 mm. Subsequently, the samples wereimmersed in deionized water for sufficient wetting. After removal from the water, each sample was suspended in air for 30 s to remove excess surface liquid, and the wet membrane masses (Mw₁, Mw₂, Mw₃) were measured. If residual droplets remained, they were gently removed before weighing. The average values of mass and thickness were used for calculation. The porosity (ε) was calculated using Eq.\u0026nbsp;(1):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\text{\u0026epsilon;}\\text{=}\\frac{\\text{M}\\text{w}\\text{-M}\\text{d}}{\\text{\u0026rho;\u0026middot;S\u0026middot;T}}\\times\\:100\\%\\#（1）\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere M\u003csub\u003ew\u003c/sub\u003e is the wet membrane mass (g), M\u003csub\u003ed\u003c/sub\u003e is the dry membrane mass (g), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\:\\)\u003c/span\u003e\u003c/span\u003eis the density of deionized water (g\u0026middot;mL⁻\u0026sup1;), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:S\\)\u003c/span\u003e\u003c/span\u003e is the membrane area (cm\u0026sup2;), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:T\\)\u003c/span\u003e\u003c/span\u003e is the membrane thickness (cm).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e1.3.3 Alkali uptake measurement\u003c/h2\u003e \u003cp\u003eAlkali uptake was used to evaluate the compatibility between the membrane and electrolyte and was expressed as the percentage increase in mass after immersion in KOH solution. Membrane samples (20 mm \u0026times; 20 mm) were prepared, and three specimens were tested. The samples were first washed with deionized water and then dried in a vacuum oven at 90\u0026deg;C until completely dry. The dry membrane masses (Mn₁, Mn₂, Mn₃) were recorded with an accuracy of 0.001 g. The dried samples were immersed in 30 wt.% KOH solution for more than 4 h to ensure full absorption. After removal, the samples were suspended in air for 30 s to remove excess solution, and the wet membrane masses (Ml₁, Ml₂, Ml₃) were measured. Any remaining droplets were carefully removed prior to weighing. The average values were used for calculation. The alkali uptake (A) was calculated using Eq.\u0026nbsp;(2):\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}A=\\frac{{\\text{M}}_{\\text{L}}}{{\\text{M}}_{\\text{N}}}\\times\\:100\\%\\#（2）\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere M\u003csub\u003el\u003c/sub\u003e is the mass of the membrane after alkali absorption (g), and M\u003csub\u003en\u003c/sub\u003e is the dry membrane mass (g).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e1.3.4 Area resistance measurement\u003c/h2\u003e \u003cp\u003eThe area resistance of the diaphragm was measured using electrochemical impedance spectroscopy (EIS). The membrane sample was placed in a testing cell consisting of two polytetrafluoroethylene (PTFE) plates and two platinum electrodes, and fixed with PTFE screws. The cell was immersed in a 30 wt.% KOH solution, and impedance measurements were performed using an electrochemical workstation. The frequency range was 0.01 Hz to 10 kHz with an AC perturbation amplitude of 5 mV. The impedance spectra were recorded, and the first intercept of the high-frequency region with the real axis (Z\u0026prime;) was taken as the total resistance (R₂). Under identical conditions (temperature, electrolyte, and electrode configuration), the resistance of the electrolyte without the membrane (R₁) was measured. The area resistance (R) was calculated using Eq.\u0026nbsp;(3):\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}R=\\:\\left(\\text{R}2-\\text{R}1\\right)\\times\\:S\\#（3）\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere R is the membrane area resistance (Ω\u0026middot;cm\u0026sup2;), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{1}\\)\u003c/span\u003e\u003c/span\u003e is the electrolyte resistance (Ω), R\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eis the total resistance (Ω), and S is the effective membrane area (cm\u0026sup2;).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e1.3.5 Current density measurement\u003c/h2\u003e \u003cp\u003eThe current density\u0026ndash;voltage (J\u0026ndash;V) characteristics of the membrane were measured using an electrochemical workstation. An H-type electrolytic cell was employed, with the membrane placed between the two compartments. A glassy carbon electrode, a Hg/HgO electrode, and a carbon rod were used as the working, reference, and counter electrodes, respectively. The electrolyte was 30 wt.% KOH solution, and the test temperature was maintained at 25\u0026deg;C. The applied voltage range was 1.5\u0026ndash;2.5 V with a scan rate of 5 mV\u0026middot;s⁻\u0026sup1;.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e1.3.6 Morphology characterization\u003c/h2\u003e \u003cp\u003eThe surface and cross-sectional morphologies of the membranes were observed using scanning electron microscopy (SEM). The membrane samples were first fractured in liquid nitrogen to obtain clean cross-sections, then mounted on sample holders and coated with a thin layer of gold prior to observation. SEM images of both surface and cross-sectional structures were subsequently recorded.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Effect of PVP content on membrane properties\u003c/h2\u003e \u003cp\u003eComposite diaphragms were prepared according to the procedure described above. Membranes with different PVP contents of 0%, 5%, 10%, 15%, and 20% were fabricated. In all cases, the ZrO₂ content and PSU content were fixed at 70 wt.% and 20 wt.%, respectively. The pre-evaporation time was set to 15 s, and the coagulation bath temperature was maintained at 25\u0026deg;C. The prepared composite diaphragms were systematically characterized, and the corresponding results are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMembrane properties prepared with different content of PVP\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePVP content/wt.%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ethickness/mm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eporosity/%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAlkali uptake /%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eResistance /Ω\u0026middot;cm\u0026sup2;\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e54.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.572\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e112\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.238\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e64.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e118\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.224\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e68.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.204\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e63.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.328\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e2.1.1 Effect of PVP content on membrane morphology\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the cross-sectional morphologies of the membranes prepared with different PVP contents (0%, 5%, 10%, 15%, and 20%). As the PVP content increases from 0% to 20%, a systematic evolution in membrane structure can be observed. At a PVP content of 0%, the casting solution exhibits a significant increase in viscosity after standing for a certain period. This behavior can be attributed to the absence of PVP, which functions not only as a pore-forming agent but also as a dispersant. Without PVP, ZrO₂ nanoparticles tend to aggregate, while PSU molecular chains are more prone to entanglement\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, resulting in increased solution viscosity. Consequently, the phase separation process is dominated by delayed demixing, leading to the formation of a dense membrane structure. When the PVP content is increased to 5%, sparse sponge-like pores begin to appear; however, the pore connectivity remains relatively poor. At approximately 10% PVP content, a continuous hydrophilic network is formed within the casting solution, which promotes rapid and controlled phase separation\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. As a result, a uniform and highly interconnected sponge-like porous structure is obtained. This morphology is considered optimal for achieving a balance between high ionic conductivity and low gas permeability. With a further increase in PVP content to 15%, the membrane structure transitions to one dominated by finger-like pores. Although the area resistance may decrease under this condition, the gas permeability tends to increase accordingly. When the PVP content reaches 20%, the thermodynamic stability of the system is significantly reduced. Excessive PVP increases the viscosity of the casting solution and slows down the phase separation kinetics\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e, leading to the formation of a thicker dense skin layer, a reduced number of finger-like pores, and an increased proportion of sponge-like structures\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e2.1.2 Effect of PVP content on membrane area resistance\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the electrochemical impedance spectra and the corresponding variation in area resistance of the membranes prepared with different PVP contents. It can be observed that the area resistance is strongly correlated with the membrane cross-sectional structure. At 0% PVP content, the membrane exhibits a nearly dense and nonporous structure, resulting in severely hindered ion transport pathways and consequently a very high area resistance. When the PVP content is increased to 5%, sparse pores are formed. Although the pore connectivity remains limited, the ion transport resistance is significantly reduced, leading to a noticeable decrease in area resistance. When the PVP content reaches approximately 10%, an optimal balance between thermodynamic stability and phase separation kinetics is achieved\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e, resulting in a uniform and highly interconnected sponge-like porous structure. This morphology provides a high porosity with moderate tortuosity, thereby offering efficient ion transport pathways and further reducing the area resistance. At a PVP content of around 15%, the membrane structure becomes dominated by finger-like pores, which leads to a further decrease in area resistance due to the formation of more direct ion transport channels. However, the presence of these finger-like macrovoids may also increase gas permeability. When the PVP content is further increased to 20% or higher, excessive pore-forming agent induces structural deterioration\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. A mixed structure consisting of finger-like pores and sponge-like pores, or even the formation of a dense surface layer, may occur. These structural changes increase the resistance to ion transport, resulting in a significant rise in area resistance.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Effect of PVP molecular weight on membrane properties\u003c/h2\u003e \u003cp\u003eComposite diaphragms with different PVP molecular weights were prepared. The PVP content was fixed at 10% for all samples, while the contents of ZrO₂ and PSU were maintained at 70 wt.% and 20 wt.%, respectively. The pre-evaporation time was set to 15 s, and the coagulation bath temperature was controlled at 25\u0026deg;C. The resulting composite diaphragms were systematically characterized, and the corresponding results are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMembrane properties prepared with different molecular weights of PVP\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePVP number\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePVP molecular weights\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ethickness /mm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eporosity /%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAlkali uptake /%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eResistance /Ω\u0026middot;cm\u0026sup2;\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e①\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3000\u0026ndash;16000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e66.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e122\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.335\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e②\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e30000\u0026ndash;58000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e62.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.117\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e③\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e350000\u0026ndash;600000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e117\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.139\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e④\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e800000\u0026ndash;1600000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e111\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.279\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Effect of PVP molecular weight on membrane morphology\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the cross-sectional morphologies of the membranes prepared with different PVP molecular weights (PVP①\u0026ndash;PVP④). It can be observed that the membrane structure is strongly influenced by the molecular weight of PVP, which governs the phase separation behavior during membrane formation. When low-molecular-weight PVP①is used, its high diffusion rate accelerates solvent\u0026ndash;nonsolvent exchange, leading to rapid liquid\u0026ndash;liquid demixing. As a result, large finger-like macrovoids with vertical penetration are easily formed, accompanied by relatively high porosity. Meanwhile, the formation of a relatively dense surface layer effectively suppresses gas permeation, resulting in a balanced performance in terms of safety and area resistance. For medium to high molecular weight PVP② and PVP③, the longer polymer chains and enhanced chain entanglement slow down the solvent\u0026ndash;nonsolvent exchange process. This leads to a more stable thermodynamic transition during phase separation, resulting in the coexistence of sponge-like structures and well-developed finger-like pores with improved uniformity and connectivity. However, when the molecular weight is further increased to PVP④, the significantly elevated solution viscosity severely suppresses the phase separation kinetics. This often results in the formation of a dense skin layer on the membrane surface, while the internal structure evolves into a disordered combination of finger-like and sponge-like pores. Consequently, the overall porosity decreases and pore connectivity is deteriorated\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 Effect of PVP molecular weight on membrane area resistance\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents the electrochemical impedance spectra and the corresponding variation in area resistance of the membranes prepared with different PVP molecular weights. It can be observed that the molecular weight of PVP has a significant influence on the area resistance of the composite diaphragms, which exhibits an overall trend of first decreasing and then increasing with increasing molecular weight. When low-molecular-weight PVP ① is used, the shorter polymer chains facilitate rapid phase separation during membrane formation, leading to the development of finger-like macrovoid structures. However, its limited ability to enhance membrane hydrophilicity restricts OH⁻ transport\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, resulting in relatively high area resistance. For medium molecular weight PVP ② and PVP ③, the chain length is more suitable for regulating the phase inversion process. These conditions enable the formation of a uniform and highly interconnected porous structure, while simultaneously improving membrane hydrophilicity. As a result, efficient ion transport pathways are established, facilitating rapid migration of OH⁻ ions in the alkaline electrolyte, and the membrane exhibits the lowest area resistance. When the molecular weight is further increased to the high-molecular-weight range PVP ④, the long polymer chains tend to entangle and aggregate, significantly increasing the viscosity of the casting solution and slowing down the phase separation process\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. This leads to the formation of a denser pore structure. In addition, the presence of long-chain segments further hinders OH⁻ transport, and the strong binding effect of high-molecular-weight PVP promotes structural densification of the membrane. Consequently, the area resistance increases again.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e(b) Influence curves of molecular weights of PVP on surface resistance of membranes\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 Effect of PVP molecular weight on current density\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e illustrates the variation in current density of the membranes prepared with different PVP molecular weights. As shown, the change in PVP molecular weight has a negligible effect on the current density of the composite diaphragms, and no obvious trend can be observed. This indicates that, under the present system, the molecular weight of PVP is not the dominant factor governing the current density. Instead, PVP molecular weight primarily influences the membrane microstructure and hydrophilicity, thereby affecting area resistance of ion transport, while its impact on current density can be considered insignificant.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this study, composite diaphragms for alkaline water electrolysis with a ZrO₂ content of 70% and a PSU content of 20% were investigated. The effects of PVP content and molecular weight on the membrane microstructure and electrochemical performance were systematically evaluated. In this study, composite diaphragms for alkaline water electrolysis with a ZrO₂ content of 70% and a PSU content of 20% were investigated. The effects of PVP content and molecular weight on the membrane microstructure and electrochemical performance were systematically evaluated. The results demonstrate that, as the PVP content increases from 0% to 20%, the membrane structure evolves from a dense, nonporous morphology to a highly interconnected sponge-like porous structure, and subsequently to a mixed structure dominated by finger-like pores with a dense asymmetric skin layer. Correspondingly, the area resistance exhibits a decreasing\u0026ndash;increasing trend. The lowest area resistance is 0.204 Ω\u0026middot;cm\u0026sup2; ,which is achieved at a PVP content of 15%, while a more balanced performance between ionic conductivity and gas barrier properties is obtained at 10% PVP content. Furthermore, with increasing PVP molecular weight, the area resistance also shows a similar trend of first decreasing and then increasing. The minimum area resistance is 0.117 Ω\u0026middot;cm\u0026sup2; ,which is observed within the molecular weight range of 30,000\u0026ndash;600,000. In contrast, the variation in PVP molecular weight has no significant effect on the operating current density of the membranes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This research received no external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e Daiying Sun performed the experiments and wrote the manuscript. Jianyun He and Bo Liu supervised the work and revised the manuscript. All authors approved the final version.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZhu QQ (2025) Preparation and performance of polysulfone composite diaphragms for alkaline water electrolysis. 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J Membr Sci 243(1\u0026ndash;2):45\u0026ndash;57. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.memsci.2004.06.011\u003c/span\u003e\u003cspan address=\"10.1016/j.memsci.2004.06.011\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"hydrogen energy, composite membrane, phase transformation, polyvinylpyrrolidone, surface resistance, micromorphology","lastPublishedDoi":"10.21203/rs.3.rs-9285461/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9285461/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHydrogen energy is regarded as a promising clean energy source due to its high energy density, environmental friendliness, and wide range of applications. As a key component of alkaline electrolyzers, composite membranes play a crucial role in determining the efficiency and stability of alkaline water electrolysis systems. In this work, composite membranes for alkaline water electrolysis were prepared via an immersion precipitation phase inversion method using Polyphenylene sulfide mesh as the reinforcement layer, polysulfone as the polymer matrix, polyvinylpyrrolidone as the pore-forming additive, and Nano-zirconia particles as the inorganic filler. The effects of polyvinylpyrrolidone content and molecular weight on the microstructure and electrochemical properties of the membranes were systematically investigated by evaluating area resistance, alkali uptake, gas evolution current density, and membrane morphology. The results indicate that the introduction of polyvinylpyrrolidone significantly influences membrane formation, leading to the evolution of the internal structure from a dense morphology to a porous asymmetric structure with sponge-like, finger-like, and mixed pore configurations. Meanwhile, the area resistance of the membranes shows a pronounced dependence on both polyvinylpyrrolidone content and molecular weight, whereas the working current density exhibits relatively weak sensitivity to molecular weight variation. These findings demonstrate that tuning polyvinylpyrrolidone parameters provides an effective strategy for regulating pore structure and improving the performance of composite membranes for alkaline water electrolysis.\u003c/p\u003e","manuscriptTitle":"Effect of Polyvinylpyrrolidone on the Structure and Performance of Composite Membranes for Alkaline Water Electrolysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-20 22:15:52","doi":"10.21203/rs.3.rs-9285461/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-04-13T13:09:54+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-13T12:30:22+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Polymer Research","date":"2026-04-08T16:16:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-08T03:25:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Polymer Research","date":"2026-04-06T22:46:09+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a2aefce8-268b-4598-a771-1f44fb17c6d1","owner":[],"postedDate":"April 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T22:15:52+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-20 22:15:52","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9285461","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9285461","identity":"rs-9285461","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}