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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.33 No.4 pp.211-221
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2023.33.4.211

Preparation of Dual-functionalized Polymeric Membrane Electrolyte and Ni, Co-based Nanowire/MOF Array on Carbon Cloth for High-performance Supercapacitor

Hye Jeong Son, Bong Seok Kim, Ji Min Kwon, Yu Bin Kang, Chang Soo Lee†
Department of Polymer Science and Engineering, Kumoh National Institute of Technology, Gumi 39177, Korea

1 These authors contributed equally


†Corresponding author(e-mail: cslee21@kumoh.ac.kr; http://orcid.org/0000-0002-8375-4793)
July 27, 2023 ; ; August 17, 2023

Abstract


This study presents a comprehensive study on the synthesis and characterization of PVI-PGMA/LiTFSI polymeric membrane electrolytes and CxNy-C flexible electrodes for energy storage applications. The dual-functional PVI-PGMA copolymer exhibited excellent ionic conductivity, with the PVI-PGMA73/LiTFSI200 membrane electrolyte achieving the highest conductivity of 1.0 × 10-3 S cm-1. The electrochemical performance of the CxNy-C electrodes was systematically investigated, with C3N2-C demonstrating superior performance, achieving the highest specific capacitance of 958 F g-1 and lowest charge transfer resistance (Rct) due to its highly interconnected hybrid structure comprising nanowires and polyhedrons, along with binary Co/Ni oxides, which provided abundant redox-active sites and facilitated ion diffusion. The presence of a graphitic carbon shell further contributed to the enhanced electrochemical stability during charge-discharge cycles. These results highlight the potential of PVI-PGMA/LiTFSI polymeric membrane electrolytes and CxNy-C electrodes for advanced energy storage devices, such as supercapacitors and lithium-ion batteries, paving the way for further advancements in sustainable and high-performance energy storage technologies.


polymer , block copolymer , membrane , electrolyte , metal-organic frameworks (MOFs)

이중 기능 고분자 전해질 막의 제조 및 탄소 섬유에 니켈, 코발트 기반의 나노와이어/MOF 배열을 통한 고성능 슈퍼커패시터 연구

손 혜 정, 김 봉 석, 권 지 민, 강 유 빈, 이 창 수†
금오공과대학교 고분자공학과

초록


본 연구는 에너지 저장 응용을 위한 PVI-PGMA/LiTFSI 고분자 막 전해질 및 CxNy-C 유연 전극의 합성 및 특성 에 관한 연구이다. 이중 기능을 갖는 PVI-PGMA 공중합체는 우수한 이온 전도성을 나타내었으며, PVI-GMA73/LiTFSI200 막 전해질은 1.0 × 10-3 S cm-1의 최고 전도도를 달성하였다. CxNy-C 전극의 전기화학적 성능을 체계적으로 분석하였으며, C3N2-C는 나노와이어와 다면체로 구성된 높은 연결성을 갖는 하이브리드 구조와 이중 Co/Ni 산화물을 포함하여 풍부한 산 화환원 활성 부위와 이온 확산을 용이하게 하는 특징으로 인해 958 F g-1의 최고용량 및 최소한의 전하 전달 저항(Rct)을 달성 하였다. 흑연 탄소 껍질의 존재는 충전-방전 동안 높은 전기화학적 안정성에 기여하였다. 이러한 결과들은 고성능 에너지 저 장 장치인 슈퍼커패시터 및 리튬 이온 전지와 같은 첨단 에너지 저장 장비에 PVI-PGMA/LiTFSI 고분자 막 전해질과 CxNy-C 전극을 활용하는 잠재력을 보여주었으며, 지속 가능하고 고성능의 에너지 저장 기술을 더욱 발전시키는 길을 열어가 고 있다.


    1. Introduction

    The escalating need for sustainable energy storage solutions, coupled with the rapid expansion of renewable energy sources, has intensified the search for highperformance energy storage devices[1]. Supercapacitors, also known as electrochemical capacitors, have emerged as promising candidates due to their superior power density, rapid charge-discharge rates, and long cycle life compared to traditional batteries[2-4]. Central to the operation of supercapacitors is the electrolyte, which plays a pivotal role in facilitating ion transport between the positive and negative electrodes[5]. In this context, polymeric electrolytes have garnered increasing attention as they offer advantages over traditional liquid electrolytes, such as improved safety, flexibility in design, and enhanced electrochemical stability[6].

    Most of researchers focuses on the recent developments in supercapacitor polymeric electrolytes, aiming to provide a comprehensive understanding of their synthesis, properties, and performance metrics[7,8]. The design and selection of polymer materials have a profound impact on the ionic conductivity, mechanical strength, and thermal stability of the electrolyte, ultimately influencing the overall efficiency of the supercapacitor device[9-11]. Recent advancements in polymer chemistry, nanotechnology, and composite materials have enabled the development of novel polymeric electrolytes with tailored properties to meet the specific requirements of supercapacitor applications[12-15]. The investigation of these cutting-edge polymeric electrolytes offers exciting opportunities to enhance the energy storage capacity, power output, and overall sustainability of supercapacitor devices[16,17]. By addressing critical challenges, such as improving energy density and device efficiency, and highlighting the potential for large-scale energy storage applications, this study seeks to contribute to the realization of a more sustainable and energy-efficient future[18]. As supercapacitors continue to evolve and advance, the integration of high-performance polymeric electrolytes represents a significant step towards meeting the energy demands of modern society and accelerating the transition towards a cleaner and greener energy landscape.

    Central to the success of supercapacitors is the development of high-performance electrode materials that can efficiently store and deliver energy. In recent years, metal-organic frameworks (MOFs) have emerged as an intriguing candidate for supercapacitor electrodes, owing to their unique properties, including high surface areas, tunable structures, and diverse metal coordination environments. Most of research article focuses on the exciting progress in the utilization of Ni and Co-based MOF electrodes for supercapacitor applications. These transition metal-based MOFs have exhibited exceptional electrochemical performance, making them attractive alternatives to conventional carbon-based electrodes. The tunable nature of MOFs allows for tailored design and optimization to achieve specific electrochemical properties, such as enhanced specific capacitance, improved rate capability, and excellent cycling stability[19]. By thoroughly investigating the synthesis methodologies, structural characteristics, and electrochemical behavior of these MOF electrodes, this study seeks to shed light on their potential to revolutionize the landscape of energy storage materials and contribute to the development of high-performance, eco-friendly supercapacitors.

    2. Materials and Methods

    2.1. Materials

    Glycidyl methacrylate (GMA, ≥ 97%), 1-vinylimidazole (VI, ≥ 99%), bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, 99.95%), 2-methylimidazole (2MI, 99%), cobalt(II) nitrate hexahydrate (Co(NO3)2⋅6H2O, ≥ 98%), nickel(II) nitrate hexahydrate (Ni(NO3)2⋅6H2O, ≥ 97.0%) and potassium hydroxide (KOH, ≥ 85%) were all purchased from Sigma-Aldrich and directly used without any further purification. 2,2'-azobis (2- methylpropionitrile) (AIBN, 98%) was purchased from Acros Organics. Ethanol (EtOH, 99.9%), N,N-dimethylformamide (DMF, 99.5%), ethyl acetate (EtOAc, 99.9%), n-hexane (99.9%) and deionized H2O (HPLC grade) were all purchased from Daejung Chemicals. Carbon cloth was purchased from Nara Cell Tech.

    2.2. Preparation of PVI-PGMA/LiTFSI polymeric membrane electrolyte

    A total of 10 g of VI and GMA monomers were dissolved in a mixture of EtOH and DMF with a 7:3 weight ratio. Subsequently, 0.05 g of AIBN was introduced into the solution, and the mixture was purged with N2 for 1 h under mild stirring. The polymerization process was carried out at 65°C for 18 h. The resulting polymer was precipitated in a mixture of n-hexane and EtOAc and washed three times with the same solvent. Afterward, it was dried in a conventional oven overnight and subjected to vacuum drying at room temperature for 24 h.

    To prepare the PVI-PGMA/LiTFSI polymeric membrane, the PVI-PGMA polymer and LiTFSI were dissolved in DMF. The LiTFSI/PVI-POEM weight ratio was maintained at 50, 100, 150, and 200. The polymeric solution was poured into a glass petri dish and dried at 120°C for 48 h, followed by vacuum drying at 80°C overnight to ensure complete evaporation of residual solvents. The resulting polymeric membrane electrolyte was carefully peeled off from the petri dish, resulting in a clear membrane ready for direct use in conductivity testing.

    2.3. Preparation of CxNy-C electrodes

    A specific amount of Co(NO3)2⋅6H2O and 2-MeIm methanolic solutions (40 mL) were mixed, and a piece of carbon cloth was immersed in the solution. After stirring for an additional 10 seconds, Ni(NO3)2⋅6H2O methanolic solution was added to the mixture. The resulting solution was then transferred directly into a Teflon-lined stainless steel autoclave and maintained at 110°C for 12 hours for the synthesis process. Following the reaction, the electrodes were washed three times with excess methanol and subsequently dried in a 50°C oven overnight, yielding the CxNy electrodes.

    For the preparation of CxNy-C flexible electrodes, the as-prepared electrodes underwent annealing at 250°C under ambient air conditions to oxidize the cobalt and nickel metals. This was followed by post-annealing at 600°C for 1 hour under an Ar atmosphere. The weight content of the active material in CxNy-C was utilized to characterize the electrochemical behavior of the electrodes.

    2.4. Characterization

    The morphology of the electrode materials was analyzed using field-emission scanning electron microscopy (FE-SEM) with models 7610F-plus and JEOL- 7800F (JEOL, Japan), as well as transmission electron microscopy (TEM) with model JEM-F200 (JEOL, Japan). Fourier-transform infrared spectroscopy (FT-IR) was conducted on a Spectrum 100 instrument (PerkinElmer, USA) to investigate the chemical structures of both the polymeric membrane and electrode materials. The crystallinity of the Co/Ni compounds was assessed using high-resolution X-ray diffraction (HR-XRD) on a SmartLab instrument (Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) was performed using Thermo U.K. equipment to analyze the elemental composition of the electrode materials. Furthermore, thermogravimetric analysis (TGA) was carried out on a Discovery TGA instrument (TA Instruments, USA) to characterize the degradation behavior of the electrodes.

    For evaluating the electrochemical performance of the electrodes, an electrochemical analyzer (ZIVE MP1, WonATech, Korea) was employed with a three-electrode configuration in a 6 M KOH aqueous solution. A Hg/HgO electrode in 1 M NaOH and a platinum mesh served as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) measurements were performed within a potential window of 0–0.6 V at specific scan rates, while galvanostatic charge-discharge (GCD) results were collected at specific current densities. Electrochemical impedance spectroscopy was conducted at 50 mV in the frequency range of 10 kHz to 0.001 Hz.

    The specific capacitance (Cs, F g−1) was calculated from the CV curves using the following equation:

    C s = ∫ I d V m ⋅ S ⋅ 2 Δ V
    (1)

    where I (A) is the current, m (g) is the weight of the active materials, S (V s−1) is the scan rate of CV, and ΔV (V) is the potential range of the CV curves. The specific capacitance from the GCD curve was calculated from the discharge curve using the following equation:

    C s = I ⋅ t m ⋅ Δ V
    (2)

    where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the weight of the active materials, and ΔV (V) is the discharge potential window.

    3. Results and Discussion

    3.1. PVI-PGMAxy polymeric ion conductive membrane electrolyte

    Fig. 1a depicts the successful synthesis of the dual- functional PVI-PGMAxy copolymer through freeradical polymerization. This copolymer contains both imidazole and epoxy groups, with potential applications in modifying the imidazole group into an ionic liquid and forming covalent bonds with functional groups like -NH2 or -COOH. The FT-IR spectra in Fig. 1b confirm the successful synthesis of PVI-PGMA37, 55, and 73 copolymers, with the spectral peaks aligning well with the expected weight content of monomers during the synthesis process. The presence of the C=N stretching at around 1250 cm-1 and C-O from the epoxy group at 1295 cm-1 confirms the presence of both imidazole and epoxy groups in the copolymer[20]. Additionally, the observation of a C-O bond after polymerization suggests partial ring-opening reactions during the process[21].

    The conductivity of PVI-PGMA73/LiTFSI membrane electrolytes was investigated using the Nyquist plot, and the results are summarized in Table 1 and depicted in Fig. 1c. Among the different LiTFSI weight fractions, PVI-PGMA73/LiTFSI200 exhibited the lowest charge transfer resistance (Rct) value of 47.9 Ω and the highest conductivity of 1.0 × 10-3 S cm-1, which is ten times higher than that of LiTFSI50. Notably, the inset images in Fig. 1c visually demonstrate the physical stability of the PVI-PGMA/LiTFSI200 membrane electrolyte, showing its ability to form a free-standing membrane similar to PVI-PGMA/LiTFSI50. The higher conductivity and favorable membrane formation characteristics of PVI-PGMA/LiTFSI200 indicate its potential as a promising polymeric membrane electrolyte for various electrochemical applications, especially in lithium- ion batteries and related energy storage devices[22]. The dual-functional nature of the copolymer offers versatility in tailoring and optimizing its performance for specific applications, enhancing its potential for practical utilization.

    Fig. 2 represents the synthesis process of NxCy electrode materials on the carbon cloth support. The solvothermal reaction successfully facilitated the growth of ZIF-67 nanostructures on the carbon fiber surface. By employing Co/Ni co-precursors in a one-step solvothermal reaction, Co/Ni nanowires and Ni-doped ZIF-67 hybrid structures were formed. This approach offers the advantage of enhancing the specific surface area, leading to an increase in active sites for the electrode materials. The combination of Co/Ni nanowires and Ni-doped ZIF-67 hybrid structures holds promise for improving the overall performance of the electrodes, making them highly attractive for electrochemical applications.

    FE-SEM images of the C5N0, C4N1, C3N2, and C2N3 electrodes were examined, as shown in Fig. 3. The C5N0 and C4N1 electrodes exhibited well-grown ZIF-67 nanoparticles on the carbon fiber support. Interestingly, the coverage of C4N1 was significantly higher than that of C5N0, suggesting that the co-existence of the Ni precursor enhanced the surface coating and growth on the carbon fiber[23]. The C3N2 electrode displayed Co/Ni nanowires, attributable to the presence of 40% Ni precursor during synthesis, with 2-MeIm serving as a structure directing agent to generate the nanowire or nanoweb structure. Particularly noteworthy is the presence of Co/Ni-ZIF-67 nanoparticles impregnated within the nanowire construction, leading to a highly interconnected structure even after the annealing process. In contrast, the C2N3 electrode exhibited a decrease in octahedral ZIF-67 structure due to the shortage of Co precursor necessary for nucleus formation during the solvothermal reaction. Consequently, the Co/Ni-based nanowire/ZIF-67 hybrid structure demonstrated the most favorable growth in the C3N2 electrode, making it a promising candidate for high-performance supercapacitors[24].

    The FT-IR spectra of the CxNy electrodes are shown in Fig. 4a, providing insights into the chemical bonds formed in these materials. The C5N0 spectrum displayed Co-N, C-N, and C=N stretching vibrations at 424, 1142, and 1417 cm-1, respectively, indicating the formation of ZIF-67 polyhedrons on the carbon cloth[25]. With increasing Ni content, the Ni-N stretching appeared at 450 cm-1, resulting from the exchange of Co to Ni in ZIF-67. Furthermore, a Ni-O stretching band was observed with higher Ni precursor content, attributable to the nanowire structure associated with the formation of Ni/Co-double hydroxides. Consequently, C2N3 exhibited a Ni-O bond but not a Ni-N bond due to the preferred formation of hydroxides. The TGA curves shown in Fig. 4b demonstrated that while C5N0 experienced a slight decrease in active materials due to the carbonization of ZIF-67 on the carbon cloth, C3N2 and C2N3 underwent sharp decreases, indicating the oxidation and degradation of hydroxides, leading to the generation of Co/Ni oxides[26]. Furthermore, compared to the physically adsorbed ZIF-67 particles in C5N0, C3N2, and C2N3 exhibited more active materials grown on the carbon fiber due to the introduction of the Ni precursor, resulting in higher weight loading on the carbon cloth. The XRD spectra illustrated clear observation of ZIF-67 crystals in C5N0 and C4N1. As the Ni content increased, the crystal facets of double hydroxides dedicated to the nanowire structure became apparent.

    XPS spectra of the CxNy electrodes were also analyzed to study the elemental bond energy of the electrode materials (Fig. 4d). The O 1s spectra indicated that the relative content of Ni is positively related to the oxygen-deficient sites compared to that of Co-O and O-H bonds, suggesting that the Ni precursor acts as a generator of redox sites (Fig. 4e). The Co 2p region in Fig. 4f revealed significant changes in the relative intensity of Co 2p3/2 and Co 2p1/2 with respect to Ni content[27]. This result suggests that the oxidation states of Co and Ni are influenced by the weight ratio of the Co and Ni precursors. Additionally, the relative content of Co2+ and Co3+ at 780.0 and 779.8 eV, respectively, was altered due to factors such as switching Co with Ni in the ZIF-67 polycrystals to modify crystal facets, the formation of Co/Ni hydroxides, and an excessive amount of Ni precursor contributing to the formation of ZIF-67 nanocrystals[28]. Collectively, based on the results from FT-IR, TGA, XRD, and XPS analyses, it can be concluded that the C3N2-C electrode holds great promise for achieving high energy storage performance.

    Following the annealing process to form oxides with the carbon shell, FE-SEM images in Fig. 5 were examined to identify the morphology of the electrodes. The C5N0-C electrode displayed cracking behavior, attributed to the poor adhesion of ZIF-67 crystals resulting from the formation of nuclei outside the surface. In contrast, the surface of the C4N1-C electrode was fully covered with ZIF-67 crystals, showing no significant cracks. Notably, the ZIF-67 underwent conversion to highly porous Co3O4 polyhedrons with a pore size of approximately 10 nm without disrupting the original nanostructure. Both the C3N2-C and C2N3-C electrodes exhibited finely coated nanowires with Co3O4 polyhedrons, although the Co3O4 polyhedrons were rarely observed in the C2N3-C electrode. The sizes of the nanowires and polyhedrons were slightly reduced due to the degradation of organics during the annealing under air conditions.

    TEM images of the CxNy-C active materials were examined to analyze the formation of the carbon shell and cobalt and nickel oxides (Fig. 6). All CxNy-C samples exhibited a nanoporous architecture adorned with a 1~3 nm-thick graphitic carbon shell, offering enhanced electrochemical stability during the chargedischarge process in energy storage devices. Notably, the C5N0-C, C4N1-C, and C3N2-C maintained their hollow polyhedron structures, providing a significant advantage in terms of offering numerous active sites for redox reactions and facilitating fast electrically conductive pathways[29]. The presence of a nanowire region was also confirmed in the C2N3-C sample, and after calcination, the nanoporous nanowire was observed without disrupting the original hierarchical morphology. However, the polyhedron structure was rarely observed in C2N3-C, indicating that the crucial advantages offered by the ZIF-67 nanostructure may not be fully utilized.

    N2 isotherms were plotted in Fig. 7a to study the porous architecture of carbon cloth, C3N2-C, and C2N3-C using BET and BJH analyses, and the calculated BET specific surface area and pore volume are provided in Table 2. The specific surface area and pore volume of carbon cloth are notably low due to the absence of micro/mesoporous structures. In contrast, the C3N2-C on carbon cloth exhibited the highest BET specific surface area of 6.54 m2 g-1 and pore volume of 1.50 cm3 g-1, attributed to the generation of a porous architecture in the nanowire/polyhedron hybrids. However, the C2N3-C on carbon cloth showed a slight decrease in specific surface area and pore volume, as the polyhedron structure almost disappeared due to the excess amount of Ni precursor. In Fig. 7b, the BJH pore size distribution result indicates the highly porous architecture of the C3N2-C electrode, with multi-pores generated due to the hybrid nanostructure. In conclusion, the C3N2-C on carbon cloth electrode offers significant advantages in terms of high specific surface area, interconnected morphology, carbon shell-decorated metal oxides, and good surface coverage on carbon cloth.

    The electrochemical redox reaction and capacitive performance of the CxNy-C electrodes were evaluated using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and Nyquist plots as shown in Fig. 8. The CV curves in Fig. 8a demonstrate that C3N2-C exhibited the highest electrochemical reaction, as evidenced by the highest redox peak among all the electrodes. The calculated specific capacitances for C5N0-C, C4N1-C, C3N2-C, and C2N3-C were found to be 342.9, 281.2, 847.6, and 604.0 F g-1, respectively. Notably, the superior energy storage performance of C3N2-C, achieving the highest specific capacitance, can be attributed to its highly porous architecture and binary metal oxides, providing abundant redox-active sites. The CV curves of C3N2-C at different scan rates (100, 50, 40, 30, 20, and 10 mV s-1) were shown in Fig. 8b, revealing specific capacitances of 333.2, 499.3, 592.8, 713.1, 847.5, and 1076 F g-1, respectively. Remarkably, C3N2-C achieved a capacitance exceeding 1000 F g-1, making it a promising candidate for high-performance energy storage supercapacitors. GCD curves in Fig. 8c demonstrated that C3N2-C achieved the highest specific capacitance of 958 F g-1, significantly surpassing the values for C5N0-C (406 F g-1), C4N1-C (274 F g-1), and C2N3-C (722 F g-1). Additionally, in Fig. 8d, C3N2-C exhibited the lowest charge transfer resistance (Rct) among the other electrodes. Notably, the highly interconnected hybrid structure of C3N2-C, coupled with its good coverage of carbon cloth, facilitated ion diffusion, contributing to its exceptional performance. Thus, the coexistence of nanowire/polyhedron morphology and binary Co/Ni oxides in C3N2-C plays a pivotal role in enhancing its energy storage performance.

    4. Conclusion

    In conclusion, this study successfully synthesized and characterized PVI-PGMA/LiTFSI polymeric membrane electrolytes and CxNy-C flexible electrodes for energy storage applications. The dual-functional PVI-PGMA copolymer with imidazole and epoxy groups exhibited promising ionic conductivity, with the PVI-PGMA73/ LiTFSI200 membrane electrolyte achieving the highest conductivity of 1.0 × 10-3 S cm-1. The electrochemical performance of the CxNy-C electrodes was systematically investigated, with C3N2-C demonstrating superior performance among all samples. The C3N2-C electrode exhibited the highest specific capacitance of 958 F g-1 and lowest charge transfer resistance (Rct) due to its highly interconnected hybrid structure comprising nanowires and polyhedrons, along with binary Co/Ni oxides, which provided abundant redox-active sites and facilitated ion diffusion. The presence of a graphitic carbon shell further contributed to the enhanced electrochemical stability during charge-discharge cycles. The synthesis and characterization of the PVI-PGMA /LiTFSI polymeric membrane electrolyte and CxNy-C electrodes have paved the way for further advancements in energy storage devices, such as supercapacitors and lithium-ion batteries. The dual-functionality and tunability of the copolymer offer great potential for tailoring its properties to suit specific applications. Moreover, the promising electrochemical performance of the C3N2-C electrode demonstrates the significance of its unique hybrid morphology, indicating it as a highly promising candidate for practical energy storage applications. The combination of innovative polymeric electrolytes and advanced electrode materials is expected to contribute significantly to the development of high-performance and environmentally friendly energy storage technologies.

    Acknowledgements

    This research was supported by Kumoh National Institute of Technology (2022~2023).

    Figures

    MEMBRANE_JOURNAL-33-4-211_F1.gif

    (a) Synthesis route of PVI-PGMAxy (x: weight fraction of PVI, y: weight content of PGMA) (b) FT-IR spectra of VI, GMA, and PVI-PGMAxy copolymers (c) Nyquist plot of PVI-PGMA73/LiTFSI50, 100, 150, and 200 polymeric membrane electrolyte (inset image displays PVI-PGMA50 and PVI-PGMA200 electrolyte membrane).

    MEMBRANE_JOURNAL-33-4-211_F2.gif

    Schematic illustration of C5N0-C and C3N2-C synthesis pathways. (C4N1-C and C2N3-C follow similar synthesis routes as C3N2-C, with varying Co:Ni ratios).

    MEMBRANE_JOURNAL-33-4-211_F3.gif

    FE-SEM images of (a, b) C5N0, (c, d) C4N1, (e, f) C3N2 and (g, h) C2N3 electrodes.

    MEMBRANE_JOURNAL-33-4-211_F4.gif

    Characterization of C5N0, C4N1, C3N2, and C2N3 electrodes: (a) FT-IR spectra, (b) TGA curves, and (c) HR-XRD spectra. (d) XPS survey spectra and deconvoluted (e) O 1s, (f) Co 2p, and (g) Ni 2p region of C5N0-C, C4N1-C, C3N2-C, and C2N3-C using Gaussian fitting.

    MEMBRANE_JOURNAL-33-4-211_F5.gif

    FE-SEM images of (a, b) C5N0-C, (c, d) C4N1-C, (e, f) C3N2-C and (g, h) C2N3-C electrodes.

    MEMBRANE_JOURNAL-33-4-211_F6.gif

    HR-TEM images of (a, b) C5N0-C, (c, d) C4N1-C, (e, f) C3N2-C and (g, h) C2N3-C samples.

    MEMBRANE_JOURNAL-33-4-211_F7.gif

    (a) N2 adsorption/desorption isotherm plots and (b) BJH pore size distributions of carbon cloth, C3N2-C and C2N3-C electrodes.

    MEMBRANE_JOURNAL-33-4-211_F8.gif

    Electrochemical characterization of C5N0-C, C4N1-C, C3N2-C, and C2N3-C electrodes in 6M KOH solution: (a) cyclic voltammetry (CV) curves of the four electrodes at a scan rate of 20 mV s-1. (b) CV curves of C3N2-C electrode at specific scan rates (100, 50, 40, 30, 20, 10 mV s-1). (c) galvanostatic charge discharge (GCD) curves of C5N0-C, C4N1-C, C3N2-C, and C2N3-C electrodes at 10 A g-1. (d) Nyquist plots of the four electrodes.

    Tables

    Nyquist Plot Analysis of Conductivity for PVI-PGMA73/LiTFSI Membrane Electrolytes with Various LiTFSI Weight Fractions (50, 100, 150 and 200)

    Calculated BET Specific Surface Area and Pore Volume of Carbon Cloth, C3N2-C and C2N3-C Electrodes from the N2 Sorption Data

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