超支化聚合物模板法改性碳纳米管薄膜原位构筑多孔NiCo₂Se₄柔性非对称超级电容器

Hierarchical porous NiCo₂Se₄ grown on carbon nanotube film via a hyperbranched polymer template for flexible asymmetric supercapacitors

摘要

轻质碳纳米管薄膜在柔性电子领域备受关注，但如何同时提升其高电容与能量密度仍面临挑战.报道了一种以环氧端基超支化聚合物作为反应性模板，在原始碳纳米管薄膜上原位制备多孔NiCo₂Se₄的方法.所得多孔NiCo₂Se₄具有核壳纳米结构，与未使用碳纳米管薄膜制备的NiCo₂Se₄相比，展现出更高的比表面积和电化学性能.作为电极材料时，CNT@NiCo₂Se₄薄膜表现出281.7 F‧cm^-3的高体积比电容.以其组装的柔性非对称超级电容器具有高体积电容（23.3 F‧cm^-3）、5000次充放电循环后95.6%的电容保持率以及良好的弯曲性能，同时实现了3.2 mWh‧cm^-3的高能量密度.此外，该器件在不同弯曲状态下仍能保持稳定性能，显示出其在柔性和可穿戴电子器件中的应用潜力.在该体系中，碳纳米管薄膜既作为柔性集流体，又提供了良好的支撑，多孔NiCo₂Se₄则提供了丰富的活性位点，从而实现优异的电化学性能.利用环氧端基超支化聚合物作为反应性模板制备多孔NiCo₂Se₄的反应机制并通过FT-IR、XRD和TG进行了分析.结果表明：EHP模板策略能够有效促进CNT薄膜上多孔NiCo₂Se₄纳米网络结构的形成，从而提高电导率、提供丰富的活性位点，并改善离子/电子传输，进而显著提升柔性非对称超级电容器的整体电化学性能和储能能力.

Abstract

Lightweight carbon nanotubes （CNTs） have garnered significant interest in the realm of flexible electronics； however， achieving both high capacitance and high energy density simultaneously remains a formidable challenge. The application of epoxy-terminated hyperbranched polymer （EHP） is presented as a reactive template for the in-situ synthesis of porous NiCo₂Se₄ on a pristine CNT film. The as-prepared porous NiCo₂Se₄ exhibits a core-shell nanostructure and demonstrates a larger surface area and superior electrochemical properties compared to NiCo₂Se₄ synthesized without the CNT film. When employed as an electrode material， the CNT@NiCo₂Se₄ film achieves a high specific volume capacitance of 281.7 F‧cm^-3. The flexible symmetric supercapacitor （SC） fabricated from the CNT@NiCo₂Se₄ film exhibits a high specific volume capacitance （23.3 F‧cm^-3）， retains 95.6% capacitance retention after 5000 charge-discharge cycles， displays excellent bending resilience， and achieves a high energy density （3.2 mWh‧cm^-3）. Moreover， the device sustains stable performance across diverse bending conditions， underscoring its promise for flexible and wearable electronic applications. Within this system， the CNT film functions as a flexible， conductive current collector， while the porous NiCo₂Se₄ provides ample electroactive sites， collectively facilitating outstanding electrochemical performance. The reaction mechanism underlying the use of epoxy-terminated hyperbranched polymer as a reactive template in the preparation of porous NiCo₂Se₄ was elucidated through FT-IR， XRD， and TG analyses. The findings reveal that the EHP-templated approach effectively facilitates the formation of a porous NiCo₂Se₄ nanonetworks on CNT films， which improves electrical conductivity， creates abundant active sites， and improves ion/electron transport. Consequently， this strategy significantly enhances the overall electrochemical performance and energy-storage capability of the assembled flexible asymmetric supercapacitor.

Graphical abstract

关键词

超支化聚合物 / 碳纳米管薄膜 / 柔性非对称超级电容器 / 模板法 / NiCo₂Se₄纳米网络

Key words

hyperbranched polymers / carbon nanotubes film / flexible asymmetric supercapacitor / template method / NiCo₂Se₄ nanonetwork

Amid growing global attention on the development of sustainable energy， rechargeable batteries and supercapacitors （SCs） have been broadly investigated and deemed as promising highly efficient energy storage systems^［1-2］. SCs have gained great interests as a power source for their high power density， fast charge-discharge rate， long discharge time and high cycle life. Although SCs can deliver high power density， their practical applications are restricted by the low energy density. According to the equation E=1/2 CV²， two promising approaches to address this issue are to synthesize high capacitive electrode materials and construct asymmetric SCs （ASCs）^［3］. ASC can combine a battery-type faradaic electrode and a capacitance-type electrode to increase the output voltage^［4］. Thus， developing appropriate electrode materials has become the essential part to boost the energy densities.

Recently， electrochemically active transition metal selenides have attracted considerable interest as promising electrode candidates for supercapacitors owing to their superior electrical conductivity and high theoretical capacitance. NiCo₂Se₄ is considered to be a promising electrode material. Compared with a single nickel and cobalt selenide， NiCo₂Se₄ has high conductivity and abundant redox reaction sites which can provide better electrochemical performance^［5］. Moreover， rational design of electrode structures， such as three-dimensional core-shell architectures， can further enhance electrochemical performance by providing abundant active sites and facilitating ion/electron transport^［6］. Rational design of morphology and/or structure provides one of the most feasible ways to create high performance SCs. Currently， the hydrothermal method is widely employed for the preparation of NiCo₂Se₄^［7-9］. For instance， Akbar et al^［10］ successfully synthesized NiCo₂Se₄ microspheres via a two-step hydrothermal approach. The electrode obtained in this work showed a remarkable specific capacitance of 2112.30 F‧g^-1 along with excellent cycling stability. Furthermore， an asymmetric device assembled with FeS₂ demonstrated exceptional overall performance， realizing a specific capacitance of 221.30 F‧g^-1 and a high energy density of 78.68 Wh‧kg^-1. Compared with the above processes， a new strategy is proposed to construct NiCo₂Se₄ using epoxy-terminated hyperbranched polymer （EHP） as both a template and electrode additive. Hyperbranched polymers are superior soft template materials because of their low-viscosity and large number of terminal functional groups^［11］， which offer active sites to nucleate and grow nanomaterials. Such a method could significantly enhance the kinetic performance and long-term cycling stability. Our group have previously prepared Cu₉S₅， NiCo₂O₄ and CuS using an amino-ended hyperbranched polyamide and a hydroxyl hyperbranched polyamide as template， respectively^［12-14］.

Herein， we synthesized unique hierarchical NiCo₂Se₄ nanonets on carbon nanotube （CNT） film by using EHP as a template， The resultant CNT@NiCo₂Se₄ composite exhibited high electrochemical performance and was employed as the positive electrode in asymmetric devices. As a result， the designed ASC based on CNT@NiCo₂Se₄//CNT@PPy yielded an extended voltage window up to 1.5 V. At the power density of 101.35 mW‧cm^-3， the highest energy density of 7.90 mWh‧cm^-3 can be reached， and the energy density of 5.41 mWh‧cm^-3 was still maintained at the maximum power density of 1013.51 mWh‧cm^-3. After 6000 cycles， the ASC device still had 85.98% of the original capacitance.

1 Experimental section

1.1 Materials and instruments

All chemicals were of analytical reagent grade and used without further purification. Sodium selenate （Na₂SeO₄）， hydrazine hydrate （N₂H₄·H₂O）， and potassium hydroxide （KOH） were purchased from Sinopharm Chemical Reagent Co.， Ltd. Multi-walled carbon nanotubes （MWCNTs） were supplied by Aladdin Industrial Corporation. Deionized water was used throughout the experiment.

The experiments were carried out using the following instruments： an electronic analytical balance （model BSAA224S， Beijing Sartorius Instruments Co.， Ltd.）， an electrochemical workstation （model CHI760E， Shanghai Chenhua Instrument Co.， Ltd.）， a coin cell testing system （model CT-4008T-5V50mA， Shenzhen Neware Electronics Co.， Ltd.）， a vacuum drying oven （model DZF-6020， Shanghai Sopu Instrument Co.， Ltd.）， a high-speed centrifuge （model TG16-WS， Changsha Xiangzhi Centrifuge Instrument Co.， Ltd.）， and a thermostatic heating magnetic stirrer （model DF-101S， Wuhan Keer Instrument Equipment Co.， Ltd.）.

1.2 Preparation of NiCo₂Se₄ positive electrode

NiCo₂Se₄ nanonets were prepared by a two-step process. The NiCo₂O₄ was obtained according to our previous report^［15］. Then， NiCo₂O₄ was converted to NiCo₂Se₄ by hydrothermal treatment in a selenide solution. Typically， Na₂SeO₄ （0.2 g） and hydrazine hydrate （10 mL） were first mixed under vigorous stirring and then sealed in a Teflon-lined stainless-steel autoclave， followed by heating at 120 ℃ for 10 h. Finally， the converted sample was washed with distilled water and dried at 50 ℃ under vacuum.

1.3 Preparation of PPy negative electrode

Electrochemical characterization was performed in a three-electrode setup consisting of a CNT film as the working electrode， an Ag/AgCl electrode as the reference， and a platinum foil as the counter electrode. The PPy deposited with a constant voltage of 0.8 V in deionized water （57 mL） solution including NaClO₄ and pyrrole （3 mL） monomer for 2 h.

1.4 Fabrication of asymmetric symmetric SCs

The NiCo₂Se₄ nanocomposite film and PPy was coated with PVA/KOH gel electrolyte several times to ensure uniformity and dried at 50 ℃ for 1 h. The identical coated NiCo₂Se₄ and PPy were placed face to face to form an ASC for electrochemical characterization.

2 Results and discussion

2.1 Fabrication and structural characterization of electrode materials

The fabrication process of the NiCo₂Se₄ and assembly of the ASC are illustrated in Fig.1. In detail， the EHP was used as a reactive template， metal ions （Ni²⁺， Co²⁺） attack the oxygen atoms on the terminal epoxy groups. The oxygen atoms after ring opening form a coordination covalent bond with the metal ions， which provides a directed point to prepare a metal salt precursor with EHP. Verified in previous work， EHP acts as the template and nucleophilic reagent， and its terminal epoxy functional group becomes a more active site for nucleation and growth of nanomaterials.

Scanning Electron Microscope images （Fig.2（a）-（c）） reveal that CNT@NiCo₂Se₄ maintains a three-dimensional porous network structure after selenization， in which CNTs are uniformly covered by NiCo₂Se₄ nanoflakes， forming a core-shell architecture. This structure facilitates electrolyte accessibility and enhances electrochemical performance. Transmission Electron Microscope observations （Fig.2（d）） further confirm that NiCo₂Se₄ is coated on the CNT surface， consistent with the SEM results. The HRTEM image （Fig.2（e）） shows a lattice spacing of 0.278 nm， corresponding to the （101） plane of NiCo₂Se₄， while the SAED pattern （Fig.2（f）） indicates its polycrystalline nature. In addition， CNT@PPy exhibits a dense yet porous network structure （Fig.2（g），（h））， where PPy is uniformly coated on individual CNTs， providing efficient ion transport pathways. The FT-IR （Fig.2（i）） shows characteristic peaks corresponding to N―H， C\stackrel{}{̿}C， and C—N vibrations， confirming the successful formation of conductive PPy.

The sample was analyzed by X-Ray Diffraction （Fig.3（a））， the main diffraction peaks in the spectrum correspond to the NiCo₂Se₄ （JCPDS 04-006-5241）. X-ray photoelectron spectroscopy （XPS） was conducted to examine the chemical bonding states of the surface elements in NiCo₂Se₄. The electron-bonding energies of Ni 2p_3/2 and Ni 2p_1/2 at 856.2 and 873.8 eV correspond to Ni²⁺ cations^［16］（Fig.3（b））. The Co 2p spectrum （Fig.3（c）） exhibits characteristic peaks corresponding to Co²⁺ and Co³⁺ along with satellite peaks， confirming the coexistence of multiple valence states^［17-18］. The Se 3d spectrum （Fig.3（d）） shows two characteristic peaks assigned to Se 3d_5/2 and Se 3d_3/2， indicating the formation of metal-selenium bonds. These results further confirm the successful synthesis of NiCo₂Se₄.

2.2 Electrochemical performance of CNT@NiCo₂Se₄ positive electrode

The electrochemical test of CNT@NiCo₂Se₄ was carried out with 3 mol‧L^-1 KOH aqueous solution as the electrolyte， and the voltage window was determined to be -0.15~0.47 V. As shown in Fig.4（a）， with the increasing sweep rate， the current response increases without any obvious change in the shape of the CV curves， indicating that CNT@NiCo₂Se₄ has better rate performance as an electrode material. A pair of redox peaks can be clearly observed at 2 mV‧s^-1， which corresponds to Co²⁺/Co³⁺ at 0.06 V， and corresponds to the conversion of Ni²⁺/Ni³⁺ at 0.35 V. Fig.4（b） shows the galvanostatic charge-discharge （GCD） curve of CNT@NiCo₂Se₄， which shows obvious charge-discharge platform due to its pseudo-capacitance characteristics. Meanwhile， all GCD curves at different currents appear to have a relatively symmetric shape with visible potential plateaus. Fig.4（c） is a comparison of CNT@NiCo₂Se₄ and CNT@NiCo₂O₄ under 10 mV‧s^-1. The CV curve of CNT@NiCo₂O₄ is completely surrounded by CNT@NiCo₂Se₄ which shows that the performance of CNT@NiCo₂Se₄ has been significantly improved. Fig.4（d） is a GCD comparison of CNT@NiCo₂Se₄ and CNT@NiCo₂O₄ at 1 mA‧cm^-2. The discharge time of CNT@NiCo₂Se₄ is twice than that of NiCo₂O₄. Fig.4（e） is a comparison of capacitance under different current densities of the two samples， which confirms that CNT@NiCo₂Se₄ delivers a higher area specific capacitance. Furthermore， the CNT@NiCo₂Se₄ delivers a lower impedance than CNT@NiCo₂O₄ （Fig.4（f））. In the Nyquist plot， the high-frequency semicircle represents the charge transfer resistance （R_ct）， and the low-frequency slope reflects ion diffusion kinetics. The smaller semicircle of CNT@NiCo₂Se₄ confirms its superior conductivity and faster reaction kinetics via the EHP template method.

2.3 Electrochemical performance of CNT@PPy negative electrode

Fig.5（a） is the CV curve of CNT@PPy. Obvious redox peaks can be observed at low scan rates. With increasing the scan rates， there are no obvious changes in the curve， indicating that CNT@PPy has better rate performance. Fig.5（b） is the GCD curve of the electrode under different currents. Obvious charge and discharge platforms can be observed， corresponding to the redox peaks. Fig.5（c） shows the specific capacitance calculated from the GCD curve. At a current density of 0.33 mA‧cm^-2， the electrode delivers an areal capacitance of 1526.38 mF‧cm^-2， reflecting its superior electrochemical storage capacity. Fig.5（d） shows an impedance value of 18.2 Ω， indicating low internal resistance. In addition， the slope is close to 1 in the low frequency region， indicating excellent capacitance performance.

2.4 Assembly and voltage window of the asymmetric supercapacitor

Based on the positive and negative materials that have been successfully prepared， CNT@NiCo₂Se₄ and CNT@PPy were employed as the positive and negative electrodes， respectively， and an ASC device was assembled using KOH/PVA gel as the electrolyte. The mass ratio was optimized based on the charge balance principle （q⁺ = q^-） using q = C × ΔV × m.​As shown in Fig.6（a）， the ratio of CV area at 30 mV s^-1 is 1∶1. Fig.6（b） and 6（c） show the CV and GCD curves of the ASC devices. As the voltage window increases， these CV and GCD curves still maintain similar shapes， indicating that 1.5 V is a stable operating voltage for devices.

2.5 Electrochemical Performance of the Asymmetric Supercapacitor

As illustrated in Fig.7（a）， the CV profiles recorded at various scan rates over a potential window of 0-1.5 V exhibit two distinct redox peaks， which can be attributed to reversible faradaic reactions， in agreement with the results shown in Fig.7（b）. The ASC device delivers a high areal capacitance of approximately 48 F‧cm^-2 at a current density of 0.2 mA‧cm^-2， which retains about 38 F‧cm^-2 even at a high current density of 500 mA‧cm^-2， demonstrating excellent rate capability （Fig.7（c））. The impedance spectrum （Fig.7（d）） reveals a relatively low internal resistance （46.4 Ω）， indicating efficient charge transfer in the ASC device. The According to the Ragone plot （Fig.7（e））， when operated at a power density of 101.35 mW‧cm^-3， the device exhibits an energy density reaching 7.90 mWh‧cm^-3， and still maintains 5.41 mWh‧cm^-3 even when the power density increases to 1013.51 mW‧cm^-3， reflecting its superior energy-power characteristics. Furthermore， after undergoing 6000 charge-discharge cycles， the ASC still maintains 85.98% of its original capacitance， indicating reliable long-term cycling stability. （Fig.7（f））. The flexible ASC is evaluated under different deformation conditions and continuous deformation tests. It can be observed that the capacity remains almost unchanged at 0.2 mA （Fig.7（g））. Due to the limited power of a single ASC， it is difficult to apply it alone in real life. Therefore， ASCs are connected in series or parallel to meet specific requirements. From Fig.7（h）， the CD curve of two ASCs in series shows a voltage range that is twice of a single device and the charge and discharge time is not much different from that of a single device. The parallel connection of two ASCs has the same voltage window as a single device （Fig.7（i））， but the discharge time is about twice that of a single device. Thus， our solid-state ASC as a reliable and powerful energy storage system can be used potentially in various practical applications.

3 Conclusion

In summary， CNT@NiCo₂O₄ was successfully prepared on a carbon film using hyperbranched as a template， followed by a simple one-step selenization to prepare a CNT@NiCo₂Se₄ composite film， which exhibits higher specific capacitance and lower impedance than CNT@NiCo₂O₄. The ASC device is assembled based on CNT@NiCo₂Se₄ positive and CNT@PPy negative electrode. The voltage window of the asymmetric device can be as high as 1.5 V， and the ASC delivers a high energy density of 7.8 F‧cm^-3. After 6000 charge and discharge cycles， it still exhibits a high capacitance retention of 85.98% at 0.5 mA.

参考文献

[1] Wang Q， Tian X， Zhang D. Flexible asymmetric supercapacitors and electrocatalytic water splitting based on CoNiSe2/CoNiSe₂ nanoflowers［J］. Materials Letters， 2020， 276： 128245.

[2] Ali Khan B， Haider F， Zhang T， et al. Revealing the potential of MXene composites in advanced energy storage： A review of supercapacitor applications［J］. Journal of Alloys and Compounds， 2026， 1052： 186100.

[3] Shao Y， El-Kady M F， Sun J， et al. Design and mechanisms of asymmetric supercapacitors［J］. Chemical Reviews， 2018， 118（18）： 9233-9280.

[4] Maurya R K， Agarwal A， Gupta R K， et al. Recent technological advancement in asymmetric supercapacitors： A comprehensive review［J］. Journal of Power Sources， 2025， 640： 236818.

[5] Wang M， Li Y， Yao S， et al. Conversion mechanism of NiCo₂Se₄ nanotube sphere anodes for potassium-ion batteries［J］. Energy Materials and Devices， 2023， 1（1）： 9370001.

[6] 邵雯柯， 张道洪， 王秋凡. 基于核壳花状电极的高性能可伸缩超级电容器与光电探测器的集成（英文）［J］. 中南民族大学学报（自然科学版）， 2022， 41（2）： 129-137.

[7] Amin B G， Masud J， Nath M. Facile one-pot synthesis of NiCo2Se4-rGO on Ni foam for high performance hybrid supercapacitors［J］. RSC Advances， 2019， 9（65）： 37939-37946.

[8] Yan Y， Tian L， Xue K， et al. Synthesis and electrochemical performance study of NiCo₂Se₄/Co₃O₄ nanocomposite materials［J］. Journal of Electroanalytical Chemistry， 2025， 996： 119340.

[9] Wan L， Zhang Y， Chen J， et al. Three-dimensional tree-like NiCo₂Se₄@ZnNi layered double hydroxide core-shell heterojunctions for supercapacitors［J］. Applied Surface Science， 2024， 655： 159631.

[10] Mohammadi Zardkhoshoui A， Hosseiny Davarani S S， Asgharinezhad A A. Designing graphene-wrapped NiCo₂Se₄ microspheres with petal-like FeS₂ toward flexible asymmetric all-solid-state supercapacitors［J］. Dalton Transactions， 2019， 48（13）： 4274-4282.

[11] Maksimov A， Yarullin B， Kharlampidi K， et al. Advances in hyperbranched polymer chemistry［J］. Iranian Polymer Journal， 2025， 34（3）： 433-452.

[12] Qin A， Wu H， Chen J， et al. Constructing hyperbranched polymers as a stable elastic framework for copper sulfide nanoplates for enhancing sodium-storage performance［J］. Nanoscale， 2019， 11（15）： 7188-7198.

[13] Wu H， Li T， Li H， et al. Sodium-storage performance of CuS microspheres with hydroxyl hyperbranched polyamide additive［J］. Materials Letters， 2020， 262： 127181.

[14] 胡燕茹. 模板法制备纳米镍钴化合物及柔性超级电容器的组装和性能研究［D］. 武汉： 中南民族大学， 2020.

[15] 凌丹丹， 王琪， 张道洪， 等. Kirkendall效应诱导空心结构材料在储能领域的应用（英文）［J］. 中南民族大学学报（自然科学版）， 2025， 44（6）： 721-737

[16] Guo J， Hu Y， Li J， et al. Balancing high performance with synthesis complexity： one-step hydrothermal growth of self-supported NiCo₂Se₄ nanoflowers on carbon cloth for advanced asymmetric supercapacitors［J］. RSC advances， 2026， 16（14）： 12215-12225.

[17] Wang M， Ge H， Jin Z， et al. Hollow NiCo2Se4 microspheres composed of nanoparticles as multifunctional electrocatalysts for unassisted artificial photosynthesis［J］. Electrochimica Acta， 2018， 283： 628-637.

[18] Zhang Y， Li T， Cao S A， et al. NiCo2Se4 hierarchical microflowers of nanosheets and nanorods as pseudocapacitive Mg-storage materials［J］. ACS Sustainable Chemistry & Engineering， 2020， 8（7）： 2964-2972.

基金资助

湖北省自然科学基金联合基金资助项目(2024AFD103)