Performance enhancement of supercapacitors using zinc oxide/reduced graphene oxide nanocomposites and Nafion-117 based hybrid electrolytes

Performance enhancement of supercapacitors using zinc oxide/reduced graphene oxide nanocomposites and Nafion-117 based hybrid electrolytes | 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 Performance enhancement of supercapacitors using zinc oxide/reduced graphene oxide nanocomposites and Nafion-117 based hybrid electrolytes Santi Rattanaveeranon, Knavoot Jiamwattanapong, Rudeerat Suntako This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6927421/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Nov, 2025 Read the published version in Journal of Materials Science: Materials in Engineering → Version 1 posted You are reading this latest preprint version Abstract This study presents a cost-effective strategy to enhance supercapacitor performance using ZnO/reduced graphene oxide (ZnO/rGO) nanocomposites synthesized via a microwave-assisted method. The nanocomposites exhibit pseudocapacitive behavior, enabling improved charge storage. A PVA/KOH gel polymer electrolyte and a Nafion-117 film were integrated to enhance ionic conductivity and structural stability. Structural and morphological characterizations (XRD, FTIR, SEM, and TGA) confirmed the successful formation of uniformly distributed ZnO nanoparticles (average size: 22.44 ± 0.09 nm) on rGO sheets. Electrochemical testing demonstrated a specific capacitance of 812.23 F·g⁻¹, an energy density of 28.20 Wh·kg⁻¹, and a power density of 4,060.80 W·kg⁻¹. The composite also retained 99.97% capacitance after 5,000 cycles. These results demonstrate the potential of ZnO/rGO-Nafion hybrid electrodes for next-generation high-performance supercapacitors. Supercapacitor ZnO/rGO nanocomposite microwave-assisted synthesis Nafion-117 gel polymer electrolyte Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Supercapacitors, also known as electrochemical capacitors, have attracted significant attention in recent years due to their high power density, rapid charge–discharge capabilities, long cycle life, wide operating temperature range, and environmental friendliness (Wang et al., 2016 ). Despite these advantages, their relatively low energy density compared to conventional batteries limits their practical application as primary energy storage devices. To address this limitation, research has focused on developing advanced electrode materials with enhanced specific capacitance, particularly through the use of nanostructured materials (Zhang et al., 2009 ). Among the materials investigated, carbon-based materials—such as activated carbon, carbon nanotubes (CNTs), and carbon aerogels—have gained considerable attention due to their excellent electrical conductivity and large surface area, both essential for enhancing electric double-layer capacitance (EDLC) (Yu et al., 2012 ). In EDLCs, energy is stored through electrostatic charge accumulation at the electrode–electrolyte interface, a non-Faradaic process. Achieving high specific capacitance in EDLCs requires increasing both the surface area and the conductivity of the electrode material. Graphene, in particular, has emerged as a promising candidate due to its outstanding electrical conductivity, chemical stability, mechanical strength, and large surface-to-volume ratio (Song et al., 2012 ). Supercapacitors are typically classified into two types based on their charge storage mechanisms: EDLCs and pseudocapacitors. Pseudocapacitors store energy through fast, reversible Faradaic redox reactions at the electrode surface, resulting in higher specific capacitance and energy density compared to EDLCs. Materials such as conducting polymers and transition metal oxides—including NiO, RuO 2 , MnO 2 , Co 3 O 4 , and V 2 O 5 —exhibit pseudocapacitive behavior (Wang et al., 2012 ). Among these, Co 3 O 4 has demonstrated excellent electrochemical performance for supercapacitor applications (Meher et al., 2011). However, their relatively low electrical conductivity and structural instability often require the integration of conductive carbon materials to enhance charge transport and overall stability. To leverage the advantages of both EDLCs and pseudocapacitors, hybrid supercapacitors combining carbon materials with conducting polymers or metal oxides have been developed. These systems utilize both electrostatic and Faradaic charge storage mechanisms, thereby improving overall capacitance and energy density. The integration of graphene with polymers or metal oxides has been shown to enhance electrochemical performance due to the synergistic effects of high conductivity and active redox behavior (Yu et al., 2012 ). However, graphene sheets tend to restack during fabrication, reducing their accessible surface area and overall performance. This limitation can be mitigated by forming composites with other functional materials (Song et al., 2012 ). For example, Wu et al. demonstrated that a graphene–polyaniline nanocomposite achieved a high specific capacitance of 210 Fg⁻¹, outperforming its individual components (Wu et al., 2010 ). Among various metal oxides, RuO₂ exhibits excellent capacitance, but its practical use is constrained by high cost and environmental concerns (Shen et al., 2013 ). As a result, low-cost and environmentally friendly alternatives are being explored. Zinc oxide (ZnO) has emerged as a promising candidate due to its favorable optical and electrical properties, abundance, and environmental compatibility (Purushothaman et al., 2011 ). ZnO is widely used in optoelectronics, sensing, solar cells, and energy storage. However, ZnO/graphene composites reported to date have shown only modest specific capacitance values (Zhang et al., 2009 ). This research explores the integration of ZnO/reduced graphene oxide (ZnO/rGO) nanocomposites, which exhibit pseudocapacitive behavior, as advanced electrode materials for high-performance supercapacitors. A thin Nafion-117 film is employed as a proton-conducting layer to facilitate efficient ion transport, while a PVA/KOH gel polymer electrolyte serves as a solid-state ion-selective membrane. This configuration significantly enhances electrochemical performance, leading to improved charge storage capacity, superior cycling stability, higher specific energy, and increased power density. The synergistic combination of ZnO/rGO, Nafion-117, and gel electrolytes presents a promising, cost-effective solution for next-generation energy storage applications. 2. Materials and Method 2.1 Materials Graphite powder (99.99% purity; Lianyungang Jinli Carbon) was used as the carbon source. The chemical reagents sodium nitrate (NaNO 3 ), potassium permanganate (KMnO 4 ), sulfuric acid (H 2 SO 4 , 97 wt%), hydrogen peroxide (H₂O₂, 30 wt%), and hydrazine monohydrate (NH 2 NH 2 ·H 2 O) were used for the synthesis of graphene oxide and its reduction. Additional reagents included zinc chloride (ZnCl 2 ), sodium hydroxide (NaOH), polyvinyl alcohol (PVA), Nafion-117 membrane, and distilled water. All chemicals were of analytical grade and used without further purification. 2.2 Graphene oxide synthesis Graphene oxide (GO) was synthesized following the Hummers’ method as described by Chen (2013). Initially, 3 g of graphite powder was preheated at 300°C for 2 hours to remove moisture and impurities. The dried graphite was then mixed with 1.5 g of sodium nitrate (NaNO 3 ) and 9 g of potassium permanganate (KMnO 4 ) in a 1000 mL beaker. While continuously stirring in an ice bath maintained below 20°C, 69 mL of concentrated sulfuric acid (H 2 SO 4 ) was slowly added over 5 minutes. The mixture was subsequently heated in an oil bath at 35 ± 5°C for 30 minutes. Afterwards, 138 mL of deionized water was added, and the temperature was increased to 80 ± 5°C for another 30 minutes. To further dilute the reaction mixture and cool it to approximately 50°C, 420 mL of deionized water was added, followed by 3 mL of 30 wt% hydrogen peroxide (H 2 O 2 ) under continuous stirring. The suspension was stirred at room temperature for 30 minutes, then vacuum filtered and thoroughly rinsed with distilled water until the filtrate reached a neutral pH. Finally, the GO precipitate was washed once more with distilled water and redispersed in 30 mL of distilled water (Chen et al., 2013 ). 2.3 Preparation of ZnO/rGO Hybrid Nanocomposite Materials ZnO/rGO nanocomposites were synthesized by first dispersing graphene oxide (GO) in deionized water (30 mL, 0.2 mg/mL) via ultrasonication for 20 minutes to obtain a uniform colloidal solution. Subsequently, a 0.2 M solution of zinc chloride (ZnCl 2 ) was added, followed by 30 mL of 0.1 M sodium hydroxide (NaOH) under continuous magnetic stirring. The mixture was then subjected to microwave-assisted synthesis using irradiation pulses of 30 seconds at 420 W, alternating with 20-second cooling intervals, for a total duration of 10 minutes. This process facilitated controlled nucleation and growth of ZnO nanoparticles. After microwave treatment, the pH of the suspension was adjusted to neutral (~ pH 7), and the solution was ultrasonicated for an additional 20 minutes to enhance nanoparticle dispersion. The mixture was then thermally treated at 80°C for 2 hours to ensure complete reaction and stabilization of the material. The resulting dark grey precipitate was separated by vacuum filtration and washed repeatedly with deionized water until the filtrate reached neutral pH. This synthesis strategy was adapted and optimized based on prior reports on ZnO/rGO nanocomposites (Chong et al., 2013 ). Finally, the collected material was annealed in a tube furnace under an argon atmosphere at 300°C for 2 hours. This heat treatment promoted crystallization of ZnO nanoparticles on the reduced graphene oxide matrix, yielding the hybrid ZnO/rGO nanocomposite. The synthesis route was optimized to precisely control nanoparticle formation while preserving the structural integrity of the graphene support. 2.4 Material Characterization The synthesized ZnO/rGO nanocomposite materials were comprehensively characterized using several advanced analytical techniques. Field Emission Scanning Electron Microscopy (FESEM) was employed to examine the surface morphology and particle distribution of the samples. Fourier Transform Infrared Spectroscopy (FTIR) analysis was conducted to identify chemical bonds and functional groups present in the composites. For structural analysis, the composite materials were dispersed in water and deposited onto clean glass substrates by a drop-casting method. The films were dried on a hotplate at 60°C to ensure uniform coverage and strong adhesion to the substrate. X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.540 Å). Scans were conducted in a θ–2θ configuration at a rate of 0.021°/s over a 2θ range from 10° to 85°, providing high-resolution diffraction data. Thermogravimetric analysis (TGA) was also utilized to evaluate the thermal decomposition behavior and thermal stability of the composite materials. 2.5 Preparation of Coated Electrodes A 5 wt% Nafion-117 solution in ethanol was first ground in a mortar to achieve a homogeneous and viscous consistency. This solution was then used to form a uniform Nafion-117 polymer film, approximately 0.10 mm thick, which was coated onto screen-printed electrodes (SPEs) using a film applicator. Next, 3 mg of ZnO/rGO nanocomposite powder was mixed with 20 µL of the 5 wt% Nafion-117/ethanol solution to prepare a well-dispersed slurry. This slurry was drop-cast onto the Nafion-coated SPEs and left to dry at room temperature under ambient conditions. The prepared electrodes were then subjected to electrochemical testing to evaluate their performance. 3. Results and Discussion The thermogravimetric analysis (TGA) curves of pure ZnO and ZnO/rGO nanocomposites under a nitrogen atmosphere at a heating rate of 10°C/min are shown in Fig. 1 The ZnO sample exhibits minimal weight loss across the entire temperature range, confirming its high thermal stability. In contrast, the ZnO/rGO composite displays three distinct stages of weight loss: an initial loss around 200°C due to evaporation of moisture and volatile compounds; a second stage near 400°C corresponding to the decomposition of oxygen-containing functional groups; and a major degradation around 600°C related to the thermal decomposition of the rGO framework. The residual weight at the end of the analysis indicates that the ZnO content in the composite is approximately 7%, reflecting the inorganic phase remaining after carbon removal (Wu et al., 2010 ). Figure 2 shows the FTIR spectra of GO, rGO, ZnO/rGO, and Nafion-117, illustrating the functional groups present in each material. The broad absorption band around 3414 cm⁻¹ in the GO and ZnO/rGO spectra corresponds to the O–H stretching vibration, indicating the presence of hydroxyl groups. Peaks near 2859–2959 cm⁻¹ in ZnO/rGO are attributed to C–H stretching vibrations. The GO spectrum also exhibits characteristic peaks at 1730 cm⁻¹ (C = O stretching of carboxyl groups), 1620–1400 cm⁻¹ (aromatic C = C), and 1228 cm⁻¹ (C–O stretching), which are significantly reduced in rGO, indicating successful reduction. Strong absorption bands at 435–764 cm⁻¹ in ZnO/rGO correspond to Zn–O vibrations, confirming the incorporation of ZnO nanoparticles. Additionally, the Nafion-117 spectrum shows distinct peaks in the fingerprint region (1200–500 cm⁻¹), associated with sulfonic acid groups (–SO₃H) and C–F bonds, confirming its structural integrity (Wang et al., 2009 ). Figure 3 shows the X-ray diffraction (XRD) patterns of graphene oxide (GO), reduced graphene oxide (rGO), zinc oxide (ZnO), and ZnO/rGO nanocomposites. The diffraction peak observed at around 2θ ≈ 11° in the GO sample corresponds to the (001) plane, characteristic of the layered structure of graphene oxide containing oxygenated functional groups. This peak disappears in the rGO sample, replaced by a broad diffraction peak centered at approximately 24°, indexed to the (002) plane, indicating the reduction of GO and partial restoration of graphitic domains in rGO. The ZnO pattern exhibits sharp peaks at 2θ ≈ 31.8°, 34.5°, 36.3°, 47.5°, 56.6°, 62.8°, 66.3°, 67.9°, and 69.0°, corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) planes, respectively. These peaks align well with the hexagonal wurtzite structure of ZnO (JCPDS Card No. 36-1451), indicating high crystallinity. In the ZnO/rGO composite, all prominent ZnO peaks are preserved, although slight peak broadening and intensity variations are observed, reflecting the nanoscale nature of ZnO and its interaction with the rGO sheets. The disappearance or weakening of the rGO peak in the composite confirms the successful incorporation of ZnO nanoparticles onto the rGO surface, likely via electrostatic interactions or chemical bonding Zhang et al., 2009 ). These results confirm the formation of a ZnO/rGO nanocomposite with retained ZnO crystallinity and a reduced graphene oxide support structure, potentially beneficial for enhanced electronic or photocatalytic applications. Figure 4 (a) shows the morphology of graphene oxide (GO), exhibiting typical thin, wrinkled, sheet-like structures with transparent features, indicating successful exfoliation of graphite oxide into GO nanosheets. Figure 4 (b) illustrates reduced graphene oxide (rGO), which maintains the wrinkled morphology but appears more crumpled and thicker than GO, likely due to partial restacking during the reduction process. Figure 4 (c) displays the ZnO nanoparticles (ZnO NPs), observed as densely packed, spherical particles with a narrow size distribution. The inset shows the particle size distribution, with an average diameter of approximately 21.56 ± 0.17 nm. Figure 4 (d) reveals the morphology of ZnO/rGO nanocomposites, where ZnO nanoparticles are uniformly distributed on the rGO sheets, suggesting successful decoration of ZnO NPs onto the rGO surface. The presence of rGO may help prevent ZnO agglomeration and enhance the composite’s structural stability. Figure 5 (a)–(e) display the electrochemical performance of the ZnO/rGO/Nafion-117 composite, evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and cycling stability tests to assess its suitability as a supercapacitor electrode. As shown in Fig. 5 (a), the CV curves at a scan rate of 100 mV s⁻¹ reveal that ZnO/rGO/Nafion-117 exhibits a much larger enclosed area compared to GO, rGO, and ZnO/rGO, indicating significantly higher charge storage capability. The presence of redox peaks suggests pseudocapacitive behavior arising from faradaic reactions of ZnO, while the high conductivity and large surface area of rGO facilitate efficient electron transport. Nafion-117, acting as an ionic conductor and binder, enhances ionic mobility and structural stability. In Fig. 5 (b), CV curves at various scan rates (100–500 mVs⁻¹) demonstrate that the ZnO/rGO/Nafion-117 electrode maintains a nearly symmetrical shape even at higher scan rates, indicating good rate capability and rapid ion diffusion. Long-term cycling performance in Fig. 5 (c) shows excellent electrochemical stability, with the specific capacitance remaining at approximately 812.2 F g⁻¹ after 5000 cycles and capacitance retention exceeding 99%, confirming the durability of the electrode. The inset in Fig. 5 (d) further supports this, showing almost overlapping CV curves from the 1st and 5000th cycles. Additionally, the symmetric and linear GCD curves in Fig. 5 (e) reflect ideal capacitive behavior with high coulombic efficiency. These impressive results are attributed to the synergistic effects between ZnO, rGO, and Nafion-117, where ZnO provides redox activity, rGO offers excellent conductivity, and Nafion-117 improves ion transport and mechanical integrity. Morphological analysis revealed that the ZnO particles are spherical with an average diameter of 22.44 ± 0.09 nm, uniformly anchored onto the rGO matrix surface. This structure contributes to the good thermal stability of the ZnO/rGO nanocomposite. The electrochemical properties were found to be temperature-dependent. At a ZnO loading of 7 wt% and using a screen-printed electrode, the nanocomposite exhibited an exceptional specific capacitance of 812.23 F g⁻¹, a high specific energy of 28.20 Wh kg⁻¹, and a specific power up to 4.06 kW kg⁻¹. Capacitance retention decreased by only 0.032% over more than 5,000 charge–discharge cycles, demonstrating excellent stability. Overall, the ZnO/rGO/Nafion-117 composite shows superior capacitive performance, stability, and reversibility, making it a promising material for high-performance supercapacitor applications (Zhai et al., 2011 and Conway et al., 1999]. Figure 6 (a) shows the galvanostatic charge–discharge (GCD) profiles of the ZnO/rGO/Nafion-117 composite at different current densities. The nearly symmetrical triangular shapes confirm ideal capacitive behavior, while the minimal IR drop indicates low internal resistance, attributed to the conductive network formed by Nafion-117 that enhances charge transfer (Wang et al., 2018 ). Figure 6 (b) presents the Nyquist plots, where the ZnO/rGO/Nafion-117 electrode exhibits a smaller semicircle—corresponding to lower charge-transfer resistance—and a steeper slope in the low-frequency region, indicating faster ion diffusion compared to the unmodified ZnO/rGO electrode. This synergy between the pseudocapacitive ZnO/rGO and the ion-conductive Nafion-117 aligns with previous studies on hybrid supercapacitor materials (Li et al., 2021 ). Batteries are characterized by high energy storage capacity and low self-discharge, whereas supercapacitors provide advantages such as high power output and rapid charge–discharge capability. These complementary properties enable their combined application in large-scale energy systems. Although supercapacitor technologies continue to advance, they are unlikely to fully replace batteries in the near future (Dutta et al., 2023 ). 4. Conclusion In this study, a ZnO/rGO nanocomposite was successfully synthesized using a microwave-assisted method and further integrated with Nafion-117 and a PVA/KOH gel polymer electrolyte to enhance the electrochemical performance of supercapacitor electrodes. Comprehensive characterization confirmed the formation of a structurally robust and thermally stable nanocomposite, with ZnO nanoparticles uniformly anchored onto the rGO matrix. Electrochemical analyses demonstrated that the ZnO/rGO/Nafion-117 configuration exhibited excellent pseudocapacitive behavior, high specific capacitance, impressive energy and power densities, and outstanding cycling stability, with over 99% capacitance retention after 5,000 cycles. The synergistic effect of ZnO’s redox activity, rGO’s high conductivity, and Nafion-117’s efficient ion transport substantially improved the overall performance compared to the individual components. These results present a cost-effective and scalable strategy for developing high-performance supercapacitors, offering strong potential for next-generation energy storage applications. Declarations Competing Interests The corresponding author declares, on behalf of all authors, that there are no conflicts of interest related to this work. Funding This research was financially supported by the Rajamangala University of Technology Rattanakosin (RMUTR), Nakhon Pathom, Thailand, under Grant No. FRD6721/2567. Authors’ Contributions Dr. Santi Rattanaveeranon conceptualized the study, designed and conducted the experiments, synthesized the materials, carried out the characterization, and drafted the manuscript. Dr. Knavoot Jiamwattanapong contributed to the data analysis, interpretation of results, and provided input on manuscript refinement. Dr. Rudeerat Suntako was responsible for the graphical processing and formatting of figures and assisted in manuscript editing. All authors have reviewed and approved the final version of the manuscript. Acknowledgment The authors sincerely acknowledge the financial support provided by Rajamangala University of Technology Rattanakosin (RMUTR), Nakhon Pathom, Thailand. Data Availability The data supporting the findings of this study are available from the corresponding author upon reasonable request. References Wang, H., Lin, J., & Shen, Z. X. (2016). Polyaniline (PANi) based electrode materials for energy storage and conversion. J Sci : Adv Mater Devices , 1 , 225–255. Zhang, Y., Sun, X., Pan, L., Li, H., Sun, Z., Sun, C., & Tay, B. K. (2009). Carbon nanotube–ZnO nanocomposite electrodes for supercapacitors. Solid State Ionics , 180 , 1525–1528. 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Nafion-coated carbon electrodes for enhanced charge storage. J Energy Storage , 72 , e339. Cite Share Download PDF Status: Published Journal Publication published 10 Nov, 2025 Read the published version in Journal of Materials Science: Materials in Engineering → Version 1 posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-6927421","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":476709132,"identity":"3d41a75b-c22c-49b7-8ecf-eb10e473a3da","order_by":0,"name":"Santi Rattanaveeranon","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYLCCBAYGOSQuD0ENjA1ALcZgBpjPRowWIJHYQLQW+fazxx88YLiXvn3aGfPHFQx28gzyvQfwajE4k5cIdFhx7pzbOYaNZxiSDRvY+BLwa2HIMQRqScidIQ3U0sDAnAB0mAF+h/W/AWtJl4BoqSesheEGxJYEqJbDhLUY3HhjOCPBIMFwhnRa4cwGg+OGbWw5hByWY/DxR0WCvIR08oaPDRXV8vzMZwg4DGIXEoONCPWjYBSMglEwCggAAA5DO6VgPxuCAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0005-8674-2754","institution":"RMUTR: Rajamangala University of Technology Rattanakosin","correspondingAuthor":true,"prefix":"","firstName":"Santi","middleName":"","lastName":"Rattanaveeranon","suffix":""},{"id":476709133,"identity":"de458515-debd-43c0-a366-db4908b5b99f","order_by":1,"name":"Knavoot Jiamwattanapong","email":"","orcid":"","institution":"RMUTR: Rajamangala University of Technology Rattanakosin","correspondingAuthor":false,"prefix":"","firstName":"Knavoot","middleName":"","lastName":"Jiamwattanapong","suffix":""},{"id":476709134,"identity":"6023a00b-ddbf-4155-b00b-dd4fdd619c81","order_by":2,"name":"Rudeerat Suntako","email":"","orcid":"","institution":"Kasetsart University","correspondingAuthor":false,"prefix":"","firstName":"Rudeerat","middleName":"","lastName":"Suntako","suffix":""}],"badges":[],"createdAt":"2025-06-19 04:39:38","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6927421/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6927421/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40712-025-00354-0","type":"published","date":"2025-11-10T15:58:37+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85774323,"identity":"29a88d1f-b85a-4ce7-90c4-893c66aed948","added_by":"auto","created_at":"2025-07-01 14:09:11","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":277253,"visible":true,"origin":"","legend":"\u003cp\u003eThermogravimetric analysis (TGA) curves of rGO and ZnO/rGO composites, recorded at a heating rate of 10 °C/min under a nitrogen atmosphere.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/bebd7e751be5a9c3c689aa3a.png"},{"id":85774326,"identity":"352368d2-aca0-44af-84fe-c929a0b2a597","added_by":"auto","created_at":"2025-07-01 14:09:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":474260,"visible":true,"origin":"","legend":"\u003cp\u003eCharacteristic FTIR spectra of GO, rGO, and ZnO/rGO samples showing key functional group vibrations.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/a5034d56b3a57e7f6dfe72f8.png"},{"id":85775215,"identity":"c84a56a7-cc3a-4613-bb42-18d5e537f3e3","added_by":"auto","created_at":"2025-07-01 14:17:11","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":414201,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction (XRD) patterns of GO, rGO, ZnO, and ZnO/rGO nanocomposites.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/093cc97cea3ecfd5cb79ca37.png"},{"id":85775216,"identity":"cf77305a-d916-4d87-83fe-4aaf87313782","added_by":"auto","created_at":"2025-07-01 14:17:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":308307,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) graphene oxide (GO), (b) reduced graphene oxide (rGO), (c) zinc oxide nanoparticles (ZnO NPs), and (d) ZnO/rGO nanocomposites. The inset in (c) shows the particle size distribution of ZnO NPs, with an average diameter of 21.56 ± 0.17 nm.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/0c56f35ee8618b35e76eb99c.png"},{"id":85774329,"identity":"a1a7f518-8b67-432b-bcbf-c5024a69e3c3","added_by":"auto","created_at":"2025-07-01 14:09:11","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":589219,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cyclic voltammetry (CV) curves of the samples measured within a potential window of 0 to 0.5 V at a scan rate of 50 mVs⁻¹ in 2 M KOH electrolyte. (b) CV profiles recorded at various scan rates. (c) Variation of specific capacitance as a function of cycle number, monitored from the 50th to the 5000th cycle during cyclic stability testing. (d) Inset showing the CV curves at the 1st and 5000th cycles. (e) Charge–discharge stability of the ZnO/rGO/Nafion-117 sample.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/2f28a01bf829f12d0ead066d.jpeg"},{"id":85774320,"identity":"2a6573e8-c39e-4769-9a4c-f869df2b9e58","added_by":"auto","created_at":"2025-07-01 14:09:11","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":407492,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Galvanostatic charge–discharge (GCD) curves demonstrating excellent capacitive behavior of the ZnO/rGO/Nafion-117 composite. (b) Nyquist plots showing reduced charge-transfer resistance and improved ion diffusion in the ZnO/rGO/Nafion-117 composite compared to ZnO/rGO.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/12cd0bdadfbe628f3f7e1b10.png"},{"id":96105165,"identity":"ada44483-bc62-48ff-84eb-bbed5798a979","added_by":"auto","created_at":"2025-11-17 16:09:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2549380,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6927421/v1/8d5916fe-c2f9-49ac-b4de-ae8bd6ceba76.pdf"}],"financialInterests":"","formattedTitle":"Performance enhancement of supercapacitors using zinc oxide/reduced graphene oxide nanocomposites and Nafion-117 based hybrid electrolytes","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSupercapacitors, also known as electrochemical capacitors, have attracted significant attention in recent years due to their high power density, rapid charge\u0026ndash;discharge capabilities, long cycle life, wide operating temperature range, and environmental friendliness (Wang et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Despite these advantages, their relatively low energy density compared to conventional batteries limits their practical application as primary energy storage devices. To address this limitation, research has focused on developing advanced electrode materials with enhanced specific capacitance, particularly through the use of nanostructured materials (Zhang et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAmong the materials investigated, carbon-based materials\u0026mdash;such as activated carbon, carbon nanotubes (CNTs), and carbon aerogels\u0026mdash;have gained considerable attention due to their excellent electrical conductivity and large surface area, both essential for enhancing electric double-layer capacitance (EDLC) (Yu et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In EDLCs, energy is stored through electrostatic charge accumulation at the electrode\u0026ndash;electrolyte interface, a non-Faradaic process. Achieving high specific capacitance in EDLCs requires increasing both the surface area and the conductivity of the electrode material. Graphene, in particular, has emerged as a promising candidate due to its outstanding electrical conductivity, chemical stability, mechanical strength, and large surface-to-volume ratio (Song et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSupercapacitors are typically classified into two types based on their charge storage mechanisms: EDLCs and pseudocapacitors. Pseudocapacitors store energy through fast, reversible Faradaic redox reactions at the electrode surface, resulting in higher specific capacitance and energy density compared to EDLCs. Materials such as conducting polymers and transition metal oxides\u0026mdash;including NiO, RuO\u003csub\u003e2\u003c/sub\u003e, MnO\u003csub\u003e2\u003c/sub\u003e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e\u0026mdash;exhibit pseudocapacitive behavior (Wang et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Among these, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e has demonstrated excellent electrochemical performance for supercapacitor applications (Meher et al., 2011). However, their relatively low electrical conductivity and structural instability often require the integration of conductive carbon materials to enhance charge transport and overall stability.\u003c/p\u003e \u003cp\u003eTo leverage the advantages of both EDLCs and pseudocapacitors, hybrid supercapacitors combining carbon materials with conducting polymers or metal oxides have been developed. These systems utilize both electrostatic and Faradaic charge storage mechanisms, thereby improving overall capacitance and energy density. The integration of graphene with polymers or metal oxides has been shown to enhance electrochemical performance due to the synergistic effects of high conductivity and active redox behavior (Yu et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, graphene sheets tend to restack during fabrication, reducing their accessible surface area and overall performance. This limitation can be mitigated by forming composites with other functional materials (Song et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). For example, Wu et al. demonstrated that a graphene\u0026ndash;polyaniline nanocomposite achieved a high specific capacitance of 210 Fg⁻\u0026sup1;, outperforming its individual components (Wu et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAmong various metal oxides, RuO₂ exhibits excellent capacitance, but its practical use is constrained by high cost and environmental concerns (Shen et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). As a result, low-cost and environmentally friendly alternatives are being explored. Zinc oxide (ZnO) has emerged as a promising candidate due to its favorable optical and electrical properties, abundance, and environmental compatibility (Purushothaman et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). ZnO is widely used in optoelectronics, sensing, solar cells, and energy storage. However, ZnO/graphene composites reported to date have shown only modest specific capacitance values (Zhang et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis research explores the integration of ZnO/reduced graphene oxide (ZnO/rGO) nanocomposites, which exhibit pseudocapacitive behavior, as advanced electrode materials for high-performance supercapacitors. A thin Nafion-117 film is employed as a proton-conducting layer to facilitate efficient ion transport, while a PVA/KOH gel polymer electrolyte serves as a solid-state ion-selective membrane. This configuration significantly enhances electrochemical performance, leading to improved charge storage capacity, superior cycling stability, higher specific energy, and increased power density. The synergistic combination of ZnO/rGO, Nafion-117, and gel electrolytes presents a promising, cost-effective solution for next-generation energy storage applications.\u003c/p\u003e"},{"header":"2. Materials and Method","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 \u003cem\u003eMaterials\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eGraphite powder (99.99% purity; Lianyungang Jinli Carbon) was used as the carbon source. The chemical reagents sodium nitrate (NaNO\u003csub\u003e3\u003c/sub\u003e), potassium permanganate (KMnO\u003csub\u003e4\u003c/sub\u003e), sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 97 wt%), hydrogen peroxide (H₂O₂, 30 wt%), and hydrazine monohydrate (NH\u003csub\u003e2\u003c/sub\u003eNH\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO) were used for the synthesis of graphene oxide and its reduction. Additional reagents included zinc chloride (ZnCl\u003csub\u003e2\u003c/sub\u003e), sodium hydroxide (NaOH), polyvinyl alcohol (PVA), Nafion-117 membrane, and distilled water. All chemicals were of analytical grade and used without further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 \u003cem\u003eGraphene oxide synthesis\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eGraphene oxide (GO) was synthesized following the Hummers\u0026rsquo; method as described by Chen (2013). Initially, 3 g of graphite powder was preheated at 300\u0026deg;C for 2 hours to remove moisture and impurities. The dried graphite was then mixed with 1.5 g of sodium nitrate (NaNO\u003csub\u003e3\u003c/sub\u003e) and 9 g of potassium permanganate (KMnO\u003csub\u003e4\u003c/sub\u003e) in a 1000 mL beaker. While continuously stirring in an ice bath maintained below 20\u0026deg;C, 69 mL of concentrated sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) was slowly added over 5 minutes. The mixture was subsequently heated in an oil bath at 35\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C for 30 minutes.\u003c/p\u003e \u003cp\u003eAfterwards, 138 mL of deionized water was added, and the temperature was increased to 80\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C for another 30 minutes. To further dilute the reaction mixture and cool it to approximately 50\u0026deg;C, 420 mL of deionized water was added, followed by 3 mL of 30 wt% hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) under continuous stirring. The suspension was stirred at room temperature for 30 minutes, then vacuum filtered and thoroughly rinsed with distilled water until the filtrate reached a neutral pH. Finally, the GO precipitate was washed once more with distilled water and redispersed in 30 mL of distilled water (Chen et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 \u003cem\u003ePreparation of ZnO/rGO Hybrid Nanocomposite Materials\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eZnO/rGO nanocomposites were synthesized by first dispersing graphene oxide (GO) in deionized water (30 mL, 0.2 mg/mL) via ultrasonication for 20 minutes to obtain a uniform colloidal solution. Subsequently, a 0.2 M solution of zinc chloride (ZnCl\u003csub\u003e2\u003c/sub\u003e) was added, followed by 30 mL of 0.1 M sodium hydroxide (NaOH) under continuous magnetic stirring. The mixture was then subjected to microwave-assisted synthesis using irradiation pulses of 30 seconds at 420 W, alternating with 20-second cooling intervals, for a total duration of 10 minutes. This process facilitated controlled nucleation and growth of ZnO nanoparticles.\u003c/p\u003e \u003cp\u003eAfter microwave treatment, the pH of the suspension was adjusted to neutral (~\u0026thinsp;pH 7), and the solution was ultrasonicated for an additional 20 minutes to enhance nanoparticle dispersion. The mixture was then thermally treated at 80\u0026deg;C for 2 hours to ensure complete reaction and stabilization of the material. The resulting dark grey precipitate was separated by vacuum filtration and washed repeatedly with deionized water until the filtrate reached neutral pH. This synthesis strategy was adapted and optimized based on prior reports on ZnO/rGO nanocomposites (Chong et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFinally, the collected material was annealed in a tube furnace under an argon atmosphere at 300\u0026deg;C for 2 hours. This heat treatment promoted crystallization of ZnO nanoparticles on the reduced graphene oxide matrix, yielding the hybrid ZnO/rGO nanocomposite. The synthesis route was optimized to precisely control nanoparticle formation while preserving the structural integrity of the graphene support.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 \u003cem\u003eMaterial Characterization\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eThe synthesized ZnO/rGO nanocomposite materials were comprehensively characterized using several advanced analytical techniques. Field Emission Scanning Electron Microscopy (FESEM) was employed to examine the surface morphology and particle distribution of the samples. Fourier Transform Infrared Spectroscopy (FTIR) analysis was conducted to identify chemical bonds and functional groups present in the composites.\u003c/p\u003e \u003cp\u003eFor structural analysis, the composite materials were dispersed in water and deposited onto clean glass substrates by a drop-casting method. The films were dried on a hotplate at 60\u0026deg;C to ensure uniform coverage and strong adhesion to the substrate. X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.540 \u0026Aring;). Scans were conducted in a θ\u0026ndash;2θ configuration at a rate of 0.021\u0026deg;/s over a 2θ range from 10\u0026deg; to 85\u0026deg;, providing high-resolution diffraction data. Thermogravimetric analysis (TGA) was also utilized to evaluate the thermal decomposition behavior and thermal stability of the composite materials.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 \u003cem\u003ePreparation of Coated Electrodes\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eA 5 wt% Nafion-117 solution in ethanol was first ground in a mortar to achieve a homogeneous and viscous consistency. This solution was then used to form a uniform Nafion-117 polymer film, approximately 0.10 mm thick, which was coated onto screen-printed electrodes (SPEs) using a film applicator. Next, 3 mg of ZnO/rGO nanocomposite powder was mixed with 20 \u0026micro;L of the 5 wt% Nafion-117/ethanol solution to prepare a well-dispersed slurry. This slurry was drop-cast onto the Nafion-coated SPEs and left to dry at room temperature under ambient conditions. The prepared electrodes were then subjected to electrochemical testing to evaluate their performance.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eThe thermogravimetric analysis (TGA) curves of pure ZnO and ZnO/rGO nanocomposites under a nitrogen atmosphere at a heating rate of 10\u0026deg;C/min are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e The ZnO sample exhibits minimal weight loss across the entire temperature range, confirming its high thermal stability. In contrast, the ZnO/rGO composite displays three distinct stages of weight loss: an initial loss around 200\u0026deg;C due to evaporation of moisture and volatile compounds; a second stage near 400\u0026deg;C corresponding to the decomposition of oxygen-containing functional groups; and a major degradation around 600\u0026deg;C related to the thermal decomposition of the rGO framework. The residual weight at the end of the analysis indicates that the ZnO content in the composite is approximately 7%, reflecting the inorganic phase remaining after carbon removal (Wu et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the FTIR spectra of GO, rGO, ZnO/rGO, and Nafion-117, illustrating the functional groups present in each material. The broad absorption band around 3414 cm⁻\u0026sup1; in the GO and ZnO/rGO spectra corresponds to the O\u0026ndash;H stretching vibration, indicating the presence of hydroxyl groups. Peaks near 2859\u0026ndash;2959 cm⁻\u0026sup1; in ZnO/rGO are attributed to C\u0026ndash;H stretching vibrations. The GO spectrum also exhibits characteristic peaks at 1730 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O stretching of carboxyl groups), 1620\u0026ndash;1400 cm⁻\u0026sup1; (aromatic C\u0026thinsp;=\u0026thinsp;C), and 1228 cm⁻\u0026sup1; (C\u0026ndash;O stretching), which are significantly reduced in rGO, indicating successful reduction. Strong absorption bands at 435\u0026ndash;764 cm⁻\u0026sup1; in ZnO/rGO correspond to Zn\u0026ndash;O vibrations, confirming the incorporation of ZnO nanoparticles. Additionally, the Nafion-117 spectrum shows distinct peaks in the fingerprint region (1200\u0026ndash;500 cm⁻\u0026sup1;), associated with sulfonic acid groups (\u0026ndash;SO₃H) and C\u0026ndash;F bonds, confirming its structural integrity (Wang et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the X-ray diffraction (XRD) patterns of graphene oxide (GO), reduced graphene oxide (rGO), zinc oxide (ZnO), and ZnO/rGO nanocomposites. The diffraction peak observed at around 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;11\u0026deg; in the GO sample corresponds to the (001) plane, characteristic of the layered structure of graphene oxide containing oxygenated functional groups. This peak disappears in the rGO sample, replaced by a broad diffraction peak centered at approximately 24\u0026deg;, indexed to the (002) plane, indicating the reduction of GO and partial restoration of graphitic domains in rGO.\u003c/p\u003e \u003cp\u003eThe ZnO pattern exhibits sharp peaks at 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;31.8\u0026deg;, 34.5\u0026deg;, 36.3\u0026deg;, 47.5\u0026deg;, 56.6\u0026deg;, 62.8\u0026deg;, 66.3\u0026deg;, 67.9\u0026deg;, and 69.0\u0026deg;, corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) planes, respectively. These peaks align well with the hexagonal wurtzite structure of ZnO (JCPDS Card No. 36-1451), indicating high crystallinity.\u003c/p\u003e \u003cp\u003eIn the ZnO/rGO composite, all prominent ZnO peaks are preserved, although slight peak broadening and intensity variations are observed, reflecting the nanoscale nature of ZnO and its interaction with the rGO sheets. The disappearance or weakening of the rGO peak in the composite confirms the successful incorporation of ZnO nanoparticles onto the rGO surface, likely via electrostatic interactions or chemical bonding Zhang et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThese results confirm the formation of a ZnO/rGO nanocomposite with retained ZnO crystallinity and a reduced graphene oxide support structure, potentially beneficial for enhanced electronic or photocatalytic applications.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) shows the morphology of graphene oxide (GO), exhibiting typical thin, wrinkled, sheet-like structures with transparent features, indicating successful exfoliation of graphite oxide into GO nanosheets. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) illustrates reduced graphene oxide (rGO), which maintains the wrinkled morphology but appears more crumpled and thicker than GO, likely due to partial restacking during the reduction process. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c) displays the ZnO nanoparticles (ZnO NPs), observed as densely packed, spherical particles with a narrow size distribution. The inset shows the particle size distribution, with an average diameter of approximately 21.56\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 nm. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) reveals the morphology of ZnO/rGO nanocomposites, where ZnO nanoparticles are uniformly distributed on the rGO sheets, suggesting successful decoration of ZnO NPs onto the rGO surface. The presence of rGO may help prevent ZnO agglomeration and enhance the composite\u0026rsquo;s structural stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)\u0026ndash;(e) display the electrochemical performance of the ZnO/rGO/Nafion-117 composite, evaluated using cyclic voltammetry (CV), galvanostatic charge\u0026ndash;discharge (GCD), and cycling stability tests to assess its suitability as a supercapacitor electrode. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), the CV curves at a scan rate of 100 mV s⁻\u0026sup1; reveal that ZnO/rGO/Nafion-117 exhibits a much larger enclosed area compared to GO, rGO, and ZnO/rGO, indicating significantly higher charge storage capability. The presence of redox peaks suggests pseudocapacitive behavior arising from faradaic reactions of ZnO, while the high conductivity and large surface area of rGO facilitate efficient electron transport. Nafion-117, acting as an ionic conductor and binder, enhances ionic mobility and structural stability. In Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), CV curves at various scan rates (100\u0026ndash;500 mVs⁻\u0026sup1;) demonstrate that the ZnO/rGO/Nafion-117 electrode maintains a nearly symmetrical shape even at higher scan rates, indicating good rate capability and rapid ion diffusion.\u003c/p\u003e \u003cp\u003eLong-term cycling performance in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) shows excellent electrochemical stability, with the specific capacitance remaining at approximately 812.2 F g⁻\u0026sup1; after 5000 cycles and capacitance retention exceeding 99%, confirming the durability of the electrode. The inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d) further supports this, showing almost overlapping CV curves from the 1st and 5000th cycles. Additionally, the symmetric and linear GCD curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(e) reflect ideal capacitive behavior with high coulombic efficiency. These impressive results are attributed to the synergistic effects between ZnO, rGO, and Nafion-117, where ZnO provides redox activity, rGO offers excellent conductivity, and Nafion-117 improves ion transport and mechanical integrity.\u003c/p\u003e \u003cp\u003eMorphological analysis revealed that the ZnO particles are spherical with an average diameter of 22.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 nm, uniformly anchored onto the rGO matrix surface. This structure contributes to the good thermal stability of the ZnO/rGO nanocomposite. The electrochemical properties were found to be temperature-dependent. At a ZnO loading of 7 wt% and using a screen-printed electrode, the nanocomposite exhibited an exceptional specific capacitance of 812.23 F g⁻\u0026sup1;, a high specific energy of 28.20 Wh kg⁻\u0026sup1;, and a specific power up to 4.06 kW kg⁻\u0026sup1;. Capacitance retention decreased by only 0.032% over more than 5,000 charge\u0026ndash;discharge cycles, demonstrating excellent stability.\u003c/p\u003e \u003cp\u003eOverall, the ZnO/rGO/Nafion-117 composite shows superior capacitive performance, stability, and reversibility, making it a promising material for high-performance supercapacitor applications (Zhai et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2011\u003c/span\u003e and Conway et al., 1999].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) shows the galvanostatic charge\u0026ndash;discharge (GCD) profiles of the ZnO/rGO/Nafion-117 composite at different current densities. The nearly symmetrical triangular shapes confirm ideal capacitive behavior, while the minimal IR drop indicates low internal resistance, attributed to the conductive network formed by Nafion-117 that enhances charge transfer (Wang et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) presents the Nyquist plots, where the ZnO/rGO/Nafion-117 electrode exhibits a smaller semicircle\u0026mdash;corresponding to lower charge-transfer resistance\u0026mdash;and a steeper slope in the low-frequency region, indicating faster ion diffusion compared to the unmodified ZnO/rGO electrode. This synergy between the pseudocapacitive ZnO/rGO and the ion-conductive Nafion-117 aligns with previous studies on hybrid supercapacitor materials (Li et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBatteries are characterized by high energy storage capacity and low self-discharge, whereas supercapacitors provide advantages such as high power output and rapid charge\u0026ndash;discharge capability. These complementary properties enable their combined application in large-scale energy systems. Although supercapacitor technologies continue to advance, they are unlikely to fully replace batteries in the near future (Dutta et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, a ZnO/rGO nanocomposite was successfully synthesized using a microwave-assisted method and further integrated with Nafion-117 and a PVA/KOH gel polymer electrolyte to enhance the electrochemical performance of supercapacitor electrodes. Comprehensive characterization confirmed the formation of a structurally robust and thermally stable nanocomposite, with ZnO nanoparticles uniformly anchored onto the rGO matrix. Electrochemical analyses demonstrated that the ZnO/rGO/Nafion-117 configuration exhibited excellent pseudocapacitive behavior, high specific capacitance, impressive energy and power densities, and outstanding cycling stability, with over 99% capacitance retention after 5,000 cycles. The synergistic effect of ZnO\u0026rsquo;s redox activity, rGO\u0026rsquo;s high conductivity, and Nafion-117\u0026rsquo;s efficient ion transport substantially improved the overall performance compared to the individual components. These results present a cost-effective and scalable strategy for developing high-performance supercapacitors, offering strong potential for next-generation energy storage applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe corresponding author declares, on behalf of all authors, that there are no conflicts of interest related to this work.\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis research was financially supported by the Rajamangala University of Technology Rattanakosin (RMUTR), Nakhon Pathom, Thailand, under Grant No. FRD6721/2567.\u003c/p\u003e\u003ch2\u003eAuthors\u0026rsquo; Contributions\u003c/h2\u003e \u003cp\u003eDr. Santi Rattanaveeranon conceptualized the study, designed and conducted the experiments, synthesized the materials, carried out the characterization, and drafted the manuscript. Dr. Knavoot Jiamwattanapong contributed to the data analysis, interpretation of results, and provided input on manuscript refinement. Dr. Rudeerat Suntako was responsible for the graphical processing and formatting of figures and assisted in manuscript editing.\u003c/p\u003e \u003cp\u003eAll authors have reviewed and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eThe authors sincerely acknowledge the financial support provided by Rajamangala University of Technology Rattanakosin (RMUTR), Nakhon Pathom, Thailand.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eThe data supporting the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWang, H., Lin, J., \u0026amp; Shen, Z. X. (2016). 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(1999). \u003cem\u003eElectrochemical Supercapacitors: Scientific Fundamentals and Technological Applications\u003c/em\u003e. Springer.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, Y., Zhang, H., Liu, J., \u0026amp; Chen, W. (2018). Nafion as a binder for carbon-based supercapacitor electrodes: effects on conductivity and stability. \u003cem\u003eAcs Applied Materials \u0026amp; Interfaces\u003c/em\u003e, \u003cem\u003e10\u003c/em\u003e, 12308\u0026ndash;12315.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, H., Wang, X., Chen, J., Zhang, Y., \u0026amp; Liu, M. (2021). Enhancing the rate capability of graphene-based supercapacitors using Nafion binders. \u003cem\u003eJournal Of Power Sources\u003c/em\u003e, \u003cem\u003e482\u003c/em\u003e, 228939.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDutta, A., Mitra, S., Basak, M., \u0026amp; Banerjee, T. (2023). Nafion-coated carbon electrodes for enhanced charge storage. \u003cem\u003eJ Energy Storage\u003c/em\u003e, \u003cem\u003e72\u003c/em\u003e, e339.\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":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Supercapacitor, ZnO/rGO nanocomposite, microwave-assisted synthesis, Nafion-117, gel polymer electrolyte","lastPublishedDoi":"10.21203/rs.3.rs-6927421/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6927421/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study presents a cost-effective strategy to enhance supercapacitor performance using ZnO/reduced graphene oxide (ZnO/rGO) nanocomposites synthesized via a microwave-assisted method. The nanocomposites exhibit pseudocapacitive behavior, enabling improved charge storage. A PVA/KOH gel polymer electrolyte and a Nafion-117 film were integrated to enhance ionic conductivity and structural stability. Structural and morphological characterizations (XRD, FTIR, SEM, and TGA) confirmed the successful formation of uniformly distributed ZnO nanoparticles (average size: 22.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 nm) on rGO sheets. Electrochemical testing demonstrated a specific capacitance of 812.23 F\u0026middot;g⁻\u0026sup1;, an energy density of 28.20 Wh\u0026middot;kg⁻\u0026sup1;, and a power density of 4,060.80 W\u0026middot;kg⁻\u0026sup1;. The composite also retained 99.97% capacitance after 5,000 cycles. These results demonstrate the potential of ZnO/rGO-Nafion hybrid electrodes for next-generation high-performance supercapacitors.\u003c/p\u003e","manuscriptTitle":"Performance enhancement of supercapacitors using zinc oxide/reduced graphene oxide nanocomposites and Nafion-117 based hybrid electrolytes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-01 14:09:06","doi":"10.21203/rs.3.rs-6927421/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"043f9d2d-7375-421a-bed9-fed56c703cae","owner":[],"postedDate":"July 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-11-17T16:03:46+00:00","versionOfRecord":{"articleIdentity":"rs-6927421","link":"https://doi.org/10.1186/s40712-025-00354-0","journal":{"identity":"journal-of-materials-science-materials-in-engineering","isVorOnly":true,"title":"Journal of Materials Science: Materials in Engineering"},"publishedOn":"2025-11-10 15:58:37","publishedOnDateReadable":"November 10th, 2025"},"versionCreatedAt":"2025-07-01 14:09:06","video":"","vorDoi":"10.1186/s40712-025-00354-0","vorDoiUrl":"https://doi.org/10.1186/s40712-025-00354-0","workflowStages":[]},"version":"v1","identity":"rs-6927421","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6927421","identity":"rs-6927421","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}