新型水性聚氨酯-聚丙烯酸鉀之多功能高分子於可撓式超級電容器之製備與特性

Preparation and Characterization of Novel Bifunctional Waterborne Polyurethane-Potassium Poly(acrylate) Polymer for Flexible Supercapacitors

王政安(Jeng-An Wang)

指導教授 ： 馬振基 ； 胡啟章

國立清華大學/工學院/化學工程學系/博士(2019年)

摘要

本論文旨在研究且開發新型且多功能之吸水性高分子材料(waterborne polyurethane - potassium poly(acrylate), WPU-PAAK)應用於可撓式超級電容器(flexible supercapacitor)之中。本論文的研究可分為三個部分：
(一) 應用於超級電容器之新型多功能鹼性高分子電解質WPU-PAAK之製備與性能探討
(二) 多功能高分子WPU-PAAK應用於鹼性離子通道與電極間之黏著劑探討
(三) 高性能之非對稱式類固態超級電容器NaxMnO2@CNT/WPU-PAAK/AC-CNT之製備與性能研究

1. 應用於超級電容器之新型多功能鹼性高分子電解質WPU-PAAK之製備與性能探討
本論文第一部分(第三章)旨在開發一種新型黏性具網狀結構之聚氨酯-聚丙烯酸鉀聚合物(WPU-PAAK)應用於膠態電解質中，此特殊的交聯網狀結構不僅可增強高分子之保水性，亦提供了良好的可撓曲性，且其可避免膠態電解質由於環境因素所造成含水量上升或下降。
本研究所製備之鹼性膠態電解質(WPU-PAAK-M，M：Li、Na與K)除了可以當作超級電容器的電解質層外，亦可充當黏著劑緊密的貼合正、負兩電極，避免撓曲過程電極剝離或接觸所造成的阻抗或內短路。其中聚丙烯酸鉀(PAAK)由與側鏈羧基具有良好的吸水與保水能力，有利於電解質離子的擴散，並且使用水性聚氨脂(WPU)交聯後，可避免PAA高分子的結晶性，進而提升其離子導電度。再者，WPU優異的黏著性質亦反應於膠態電解質之上，使其可緊密塗佈黏貼於電極之表面。在性質探討部分，WPU-PAAK-M鹼性膠態電解質具有良好之離子導電度，其值可達10-2 S/cm。而本研究所使用之電極材料為經過酸洗處理之氧化商用碳紙( acidic treatment carbon paper, ACP)，此酸洗碳紙搭配WPU-PAAK-K膠態電解質使用後，其在循環伏安法掃描速率為10 mV/s時，比電容值可達211.6 mF/cm2。且於交流阻抗測試中，由於WPU優異的黏著性質，可有效地降低裝置的等效串聯電阻，其值僅為0.44 Ω，明顯低於使用商用膠態電解質PVA與PAAK之6.65與5.14 Ω。而在類固態式超級電容器ACP/WPU-PAAK-K/ACP的測試中，在恆電流充放電測試中且電流密度為0.5 mA/cm2時，其裝置之比電容值可達35.5 mF/cm2。並且在彎曲測試中，保有良好的比電容維持率，甚至當其彎曲角度達到180o時，其依舊保有90 %的電容維持率。

2. 多功能高分子WPU-PAAK應用於鹼性離子通道與電極間之黏著劑探討
本論文第二部分(第四章)的目的是通過WPU-PAAK高分子在電極材料內部形成離子通道，進而提升可撓式超級電容器之性能。本研究採用中鋼碳素公司所提供之高表面積之活性碳(activated carbon, ACS 25 - 2500 m2/g)，進行超級電容器之電容值的提升，並且使用本研究所製備之黏性WPU-PAAK高分子取代電極材料中一般常用之黏著劑聚偏二氟乙烯(polyvinylidene difluoride, PVDF)。
應用於電極活性材料中的WPU-PAAK黏著劑，除了可以固定且黏住活性材料之顆粒外，其經由電解液膨潤後可形成深入於電極層之離子通道，可顯著的提升電解質層與電極層的接觸面積且優化活性材料的利用率，達到更高的比電容值。從研究結果得知，使用WPU-PAAK取代PVDF當作電極材料之間的黏著劑，將有助於比電容值的提升，在恆電流充放電的情況下，當電流密度為1 A/g時，其提升率可達64 %，甚至當電流密度提高到10 A/g，其提升率可超過100 %。並且使用WPU-PAAK當作電極層之黏著劑時，隨著電極活性材料AC-CNT沉積量的提升，由於離子通道的作用，在單位面積之下比電容值亦與沉積量呈現正比的提升。再者，在全電池的測試中，經過電荷平衡之活性碳AC/WPU-PAAK-K/AC對稱式超級電容器其電位窗可達1.4 V，且其裝置比電容值在電流密度1 mA/cm2時可達122.43 mF/cm2，而能量密度與功率密度則可達33.33 Wh cm-2 與0.7 mW cm-2。此外，此可撓式超級電容器亦展現出良好的循環穩定性以及撓取穩定性。當其循環圈數達到10,000圈時，電容維持率可達87 .5 %；當彎曲角度達到100o時，其電容維持率可達95.6 %。

3. 高性能之對稱式類固態超級電容器NaxMnO2@CNT /WPU-PAAK/AC-CNT之製備與性能研究
本論文第三部分(第五章)的目的是透過Na+的預嵌入用以提高MnO2的比電容值，並且進行NaxMnO2@CNT/AC-CNT非對稱式的組裝以提高電位窗範圍。藉由裝置電容值與電位窗範圍的優化用以提升整個儲能元件的電化學性能。
從實驗室的先前研究得知，Na+的預嵌可以優化二氧化錳的氧化還原反應，進而提升比電容值；再者，CNT的使用將有益於改善二氧化錳的電子導電度。因此，本研究將對CNT與NaxMnO2沉積量進行性能最適化之探討。從研究結果得知，NaxMnO2@CNT21具有最佳之電化學性能。在1 M Na2SO4電解液中進行恆電流充放電試驗可發現，當電流密度為1 A/g時，其比電容值可達150 F/g，甚至當電流密度提高到20 A/g，其比電容值可達130 F/g。在使用WPU-PAAK膠態電解質時，其比電容值在1與20 A/g時，可更進一步的提升到330.2與140.4 F/g。在全電池的測試中，經過電荷平衡之NaxMnO2@CNT21/WPU-PAAK-1M Na2SO4/AC-CNT｢非｣對稱式超級電容器其電位窗可達1.53 V，且其在電流密度1 mA/cm2時，裝置比電容值可達301.9 mF/cm2，而能量密度與功率密度則可達130.51 Wh cm-2 與1.03 mW cm-2。並且在循環穩定性以及可撓性測試中可發現，當充放電圈數達到10,000時，其電容維持率約為72.2 %；彎曲角度達到180o時，其電容維持率依舊可達92.4 %。

關鍵字

超級電容器 ； 高分子膠態電解質 ； 離子通道

並列摘要

This study focuses on the preparation and the performance of the supercapacitor for energy storage devices. The research topics of this dissertation are related to the preparation and properties of novel absorbent polymer, polyurethane - potassium poly(acrylate) (WPU-PAAK), which not only forms the gel polymer electrolyte but also substitutes the commercial binder between electrode materials. There are three parts in this study:

(1) Synthesis and characterization of the novel polymer for the electrolyte and adhesive in flexible all-solid-state electrical double-layer capacitors.
(2) Mechanism of bi-functional WPU-PAAK polymer in both electrodes and electrolyte.
(3) High performance asymmetric supercapacitor NaxMnO2@CNT /WPU-PAAK/AC-CNT

The objective of the first part (chapter 3) of this dissertation is to develop a sticky network copolymer, WPU-PAAK. The cross-linked structure is believed to not only enhance the water retention but also provide the mechanical strength in gel polymer electrolyte (GPE). This interesting polymer can avoid the swelling or drying of GPE due to its naturally adsorption/desorption of moisture from the ambient environment.
This polymer neutralized with 1 M KOH and soaked with various alkaline solutions (denoted as WPU-PAAK-M, M: Li, Na, K) which can act not only as an electrolyte but also as an adhesive for both positive and negative electrodes for flexible quasi solid-state electrical double-layer capacitors (EDLCs). The PAA backbone chains in the copolymer increase the amount of carboxyl groups and promote the segmental motion. The carboxyl groups enhance the water-uptake capacity which facilitates the ion transport and therefore improves the ionic conductivity. The cross-linked agent, WPU chains, effectively keeps the water content and provides the unique stickiness to serve as a binder for electrodes. The WPU-PAAK soaked with alkaline solutions exhibits an ionic conductivity which is greater than 10-2 S cm-1. A commercial available carbon paper treated with acidic solutions (denoted as ACP) demonstrates excellent capacitive behavior using the WPU-PAAK-K polymer electrolyte. From the cyclic voltammetric test, this ACP shows a high area capacitance of 211.6 mF cm-2 at 10 mV s-1. In the electrochemical impedance spectroscopic analysis, a full cell of ACP/WPU-PAAK-K/ACP displays a low equivalent series resistance of 0.44 Ω in comparison with the other cells using commercial available polymer electrolytes. A quasi solid-state ACP/WPU-PAAK-K/ACP EDLC provides an excellent specific capacitance of 35.5 mF cm-2 at 0.5 mA cm-2. This device with over 90 % capacitance retention under 180o bending angle shows an outstanding flexibility.

The objective of the second part (chapter 4) is to develop a high performance supercapacitor by generating the ionic tunnel in the electrode material. To further enhance the performance of flexible supercapacitor, a high surface area (~2500 m2/g) activated carbon, namely ACS 25, was used as electrode material in this study. And the commercial binder of poly(vinylidene fluoride) (PVDF) was substituted by WPU-PAAK in electrode materials. This hydrogel of WPU-PAAK not only acted as the adhesive between each particle of electrode active materials, but also formed the ionic tunnel in the electrode materials, which can more deeply bring the electrolyte ions into the inner site of active materials to enhance the effective area in the interface of electrode and electrolyte.
The results show that WPU-PAAK binder in substitution for PVDF binder can enhance the specific capacitance of active electrode about 64 % in current density of 1 A/g. In the high current density of 10 A/g, it can even enhance over 100 % in specific capacitance. Furthermore, the areal specific capacitance of active electrode, which used the WPU-PAAK binder, was increased with the increasing of mass loading in the same ratio. The quasi solid-state device of the sandwich type demonstrates a potential window of 1.4 V and a high device-areal specific capacitance of 122.43 mF cm-2 at 1 mA cm-2. This highly flexible electrical double-layer capacitor (over 95.6 % areal specific capacitance retention at a bending angle of 180o) also delivers an energy density of 33.33 Wh cm-2 at a power density of 0.7 mW cm-2 with an excellent cycle life of 87.5% retention in the 10,000-cycle test.

The objective of the third part (chapter 5) is to develop a high performance supercapacitor by enhancing the specific capacitance and potential window. Therefore, this study tries to use the pre-intercalation of Na+ in MnO2 to improve the pseudocapacitance, and also building NaxMnO2@CNT/AC-CNT asymmetrical assembly to enlarge the potential window.
According to our previous research, the pre-intercalution of Na+ can optimize the redox reaction in MnO2 to increase the specific capacitance. In addition, the use of CNTs could be beneficial to optimize the electronic conductivity in MnO2. Therefore, this study will explore the performance optimization in the ratio of CNT and NaxMnO2. From the results, NaxMnO2@CNT21 shows the best electrochemical performance. Which exhibits the highest specific capancitance of 150 and 130 F/g at current density of 1 and 20 A/g in 1 M Na2SO4 electrolyte. Furthermore, the specific capacitance of NaxMnO2@CNT21 can be further increased to 330.2 and 140.4 F/g at different current densities in WPU-PAAK-1M Na2SO4 gel polymer electrolyte. This quasi solid-state asymmetrical device, NaxMnO2@CNT21/WPU-PAAK-1M Na2SO4/AC-CNT, shows a potential window of 1.53 V and a high device-areal specific capacitance, specific energy density and specific power density of 301.9 mF cm-2, 130.51 uWh cm-2 and 1.03 mW cm-2 at 1 mA cm-2, respetively. This highly flexible asymmetrical supercapacitor (over 92.4 % areal specific capacitance retention at a bending angle of 180o) also exhibits an excellent cycle life of 90 % and 72.2 % retention in the 5,000-cycle and 10,000-cycle test.

並列關鍵字

supercapacitor ； gel polymer electrolyte ； ionictunnel

參考文獻

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