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Fig. 27 CV and GCD curves of ASC bended and twisted at 10 mV s-1 and 4 A g-1 Fig. 26a is the image of gel electrolytes. The smooth surface and transparent feature suggest its good properties. As shown in Fig. 26b, NiCo2S4@MnS grown on carbon cloth as positive electrode and AC sprayed on carbon cloth as negative electrode is separated by PVA/KOH gel electrolyte, and they were packed together.
The assembled device can be twisted and bended randomly, indicating its good flexibility. The corresponding CV and GCD curves were recorded and presented in Fig.
27. The ignorable change of CV and GCD shape indicates the toughness-enabled properties[40-41].
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General conclusions
This thesis focuses on the improvement of electrochemical performance of supercapacitor, especially the energy density. The main idea is to explore kinds of materials which have effectively accessible surface and good conductivity as well as relatively cheap and friendly-to-environment. Great efforts have been made to get nanostructured materials with designed morphology, which is relevant to the specific capacitance. Furthermore, the composite has been fabricated and asymmetric supercapacitors or even solid-state flexible asymmetric supercapacitors have been assembled to apply in the energy storage systems. The involved design and methodology have guidance for fabricating electrode materials and devices.
In Chapter 1, we introduced the fundaments and structure of supercapacitors.
The merits and demerits were also summarized, accompanied with the application. In Chapter 2, the equipment and reagents used in the synthesis process were briefly summarized. Furthermore, the principles of electrochemical test were mainly descripted from aspects of CV, GCD and EIS.
In Chapter 3, a series of NiCo2S4 with different morphologies were prepared.
The chemical composition and morphology were characterized by XRD and FESEM.
Consequently, a growth mechanism of NiCo2S4 was deduced. After electrochemical test, it demonstrated the morphology has a great influence on the electrochemical performance. Among them, the 3D honeycomb-like NiCo2S4 exhibited a high specific capacitance and good stability.
In Chapter 4, based on the results in Chapter 3, NiCo2S4@VS2 was prepared by two-step method. Due to the good conductivity and capacitive feature of both NiCo2S4 and VS2, the composite presented better electrochemical performance than the corresponding counterparts. Finally, asymmetric supercapacitors were assembled using NiCo2S4@VS2 and AC as positive and negative electrodes based on the charge balance theory. The as-fabricated asymmetric device showed a high energy density.
The good rate capability under different current densities was obtained.
In Chapter 5, considering the practical application, solid state flexible
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asymmetric supercapacitors were assembled. Firstly, the ratio of NiCo2S4@MnS/CC was optimized and confirmed by FESEM and electrochemical test. The optimized NiCo2S4@MnS exhibited higher capacitance than the corresponding single parts.
Secondly, the negative electrode was prepared by spray method, which was very easy and controllable. The preparation parameters of PVA/KOH gel were explored. At last, solid-state flexible asymmetric supercapacitors were assembled based on NiCo2S4@MnS/CC and AC/CC as positive electrode and negative electrode as well as PVA/KOH gel. It showed a high energy density in a potential range of 0-1.45 V. The device also demonstrated excellent flexibility. Compared with carbon cloth, Ni foam substrate has better conductivity and loading rate, contributing to wider operating potential and high mass-loading.
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Future prospects
Our work indicated that NiCo2S4 and its composite showed good electrochemical performance, but the performance should be further improved by preparing more reasonable structure and using green electrolytes with broad voltage.
The future prospects of supercapacitors may involve several parts: higher energy density, portable, wearable, miniaturization and compatibility with other energy storage system. According to this thesis and previous research, the intrinsic nature, morphology and working potential play determinative roles on the energy density.
Exploring and study new materials or composites with good conductivity and designing novel structure can increase the specific capacitance and rate capability.
Moreover, developing a kind of stable electrolyte in wide operating window with green and benign property is another effective way to improve the energy density.
Given the convenience, portable and wearable properties become the requirements.
Therefore, looking for flexible material or flexible substrate is essential to achieve this goal. Not only in supercapacitor, but also in other many fields, miniaturization has been the hot sports. Consequently, it brings a big challenge for the technology.
Additionally, the compatibility with other energy equipment is also significant. For example, they can connect with perovskite solar cells or batteries to provide the required energy. We believe supercapacitors have a great potential to be as important as batteries for future energy storage systems.
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Achievements
Publication
[1] Zhiguo Zhang, Xiao Huang, Huan Li, Yingyuan Zhao, Tingli Ma. 3-D honeycomb NiCo2S4 with high electrochemical performance used for supercapacitor electrodes, Applied Surface Science 400 (2017) 238-244.
[2] Zhiguo Zhang, Xiao Huang, Huan Li, Hongxia Wang, Yingyuan Zhao, Tingli Ma. All-solid-state flexible asymmetric supercapacitors with high energy and power densities based on NiCo2S4@MnS and active carbon, Journal of Energy Chemistry 26 (2017) 1260-1266.
[3] Zhiguo Zhang, Xiao Huang, Tingli Ma. Free-standing NiCo2S4@VS2
nanorod array composite electrode for high-performance asymmetric supercapacitors, Electrochimica Acta (Revise).
[4] Xiao Huang, Zhiguo Zhang, Huan Li, Yingyuan Zhao, Hongxia Wang, Tingli Ma. Novel fabrication of Ni3S2/MnS composite as high performance supercapacitor electrode, Journal of Alloys and Compounds 722 (2017) 662-668.
[5] Huan Li, Zhiguo Zhang, Xiao Huang, Tongbin Lan, Mingdeng Wei, Tingli Ma. Metal-organic framework hierarchical porous TiO2 nanopills as a super stable anode for Na-ion batteries, Journal of Energy Chemistry 26 (2017) 667-672.
[6] Xiao Huang, Zhiguo Zhang, Huan Li, Hongxia Wang, Tingli Ma. In-situ growth of nanowire WO2.72 on carbon cloth as a binder-free electrode for flexible asymmetric supercapacitors with high performance, Journal of Energy Chemistry (accepted)
Conference presentation
[1] Zhiguo Zhang, Xiao Huang, Tingli Ma*: “All-solid-state asymmetric supercapacitors with high energy and power densities based on NiCo2S4@MnS and active carbon”; presented at the fall meeting of Electrochemical Society in Japan, 2017.