應用於電化學儲能與轉換之石墨烯系電極材料的設計與製備

Design and Preparation of Graphene-based Electrode Materials for Electrochemical Storage and Conversion

楊士億

指導教授 ： 馬振基

國立清華大學/工學院/化學工程學系/博士(2012年)

摘要

石墨烯(graphene)在2004年由曼徹斯特大學A.K.Geim及K.S. Novoselov領導研究組發現，石墨烯是由六邊形單層碳原子緊密堆積成二維蜂窩狀晶格結構碳質新材料，由於其特殊的二維奈米結構、超高理論比表面積、高孔隙度及優異的電、熱和機械特性，石墨烯被視為下世代應用於電化學能儲存及轉換之電極材，在第一章的文獻回顧中會詳細介紹其石墨烯系材料的性質及目前的合成途徑對其性質上之影響，而在第二章則會介紹觸媒和超級電容器的理論基礎及相關文獻回顧，基於過去相關研究與發展回顧，使我們有了設計的方向與準則，本文的主要研究課題，則是去設計與製備兼具環境友善與高電化學活性之石墨烯系電極材料以應用於電化學能儲存與轉換上。而本文的研究流程與詳細實驗方法將說明於第三章中。
　　由於聚集且再堆疊的石墨烯其特性會類似於石墨，其實際的性質表現會因為此現象而大幅地衰退，在第五章中，本文系統性地探討不同石墨烯與碳奈米管組裝之重量比例對於其微結構及電容特性之影響，可得知在適當量的碳奈米管導入下，長且彎曲的一維碳奈米結構能夠插入石墨烯層與層間以抑制石墨烯再推疊現象發生，使得石墨烯可利用表面積大幅提升，並且能夠填補石墨烯層間的間隙以形成石墨烯層間的導電通路，進一步形成三維導電碳結構，此外，這特殊三維階級式的石墨烯－碳奈米管結構能夠提供更高的孔隙度及通道給電解液離子快速地移動，減少其離子的擴散距離。GS-CNTs-9-1的比電容可以達到326.5 F/g at 20 mV/s，大幅度地高於純GS　(83 F/g at 20 mV/s)，並且GS-CNTs-9-1更能兼具高能量及功率密度(21.74 Wh/kg and 78.29 kW/kg)，此結果表示本研究成功利用二維的氧化石墨烯與一維碳奈米管在溶液相中自組裝形成特殊三維奈米碳結構，並且在超級電容器之電極材料設計上，具有極大的潛力。
延續著第四章的設計概念，在本文的第五章更進一步去設計與製備出兼具高能量與高功率密度之電極材料以應用於超級電容器上，本章節利用電化學陽極沉積法以控制過渡金屬氧化物(氧化錳)均勻地以花朵形貌成長在三維石墨烯-碳奈米管奈米結構上，而氧化錳/石墨烯-碳奈米管複合電極的比電容值(1200 F g1)也遠大於純氧化錳電極(233 F g1)，這是因為三維石墨烯-碳奈米管奈米結構的高孔隙度、大比表面積、潤濕表面特性佳及優異的導電性質能夠成為以理想平台來讓氧化錳做沉積，進一步改善氧化錳由於其低導電性所導致整體活性物質利用率低的問題， 由結果可知，沉積在此三維平台之氧化錳的實際電容值(~1200 F/g)非常接近理論值，證實本研究成功設計出一兼具高能量及功率密度之氧化錳/石墨烯-碳奈米管複合電極材料 (46.2 Wh/kg and 33.2 kW/kg)，而這三維平台的設計概念也能延伸去乘載其他活性物質，以達到製備出更高表現之下世代超級電容器。
延續著前兩章的設計概念，在第六章中，本研究設計了一種石墨烯系擔體以提升白金催化活性之方式，利用二維的氧化石墨烯(GS)與一維碳奈米管(CNTs)在溶液相中自組裝形成特殊三維奈米碳結構，並在溶液相中同步還原白金及氧化石墨烯，碳奈米管能夠作為有用的奈米間格者，有效地抑制石墨烯層與層間之堆疊，進一步提升其有效表面積，此三維階級式碳奈米結構觸媒之白金有效電化學活性表面積(~127.9 m2/g)明顯大於Pt/GS (~105.4 m2/g)和Pt/CNTs (~51.5 m2/g)。結果證明本研究所設計之三維碳奈結構具有極佳之電化學特性。　　　
　　近年來，在電催化觸媒領域最熱門的研究方向是發展可替代昂貴白金貴金屬的替代材料。在目前具取代性觸媒材料的選擇中，由於官能化的碳材料具有低成本、環境友善、穩定性高及多功能特性，其被視為最有潛力的替代材料。在本文的第七章，本研究設計了一種簡單、有效且整合型的途徑去合成高比表面積且具有特定氮摻雜結構的石墨烯，本途徑整合了自組裝、分子合成、共價傳換及快速熱膨脹還原之概念，由本研究的成果可知，經由此途經合成的氮摻雜石墨烯能夠具有比白金更優異的氧氣還原反應之催化性，主要原因如下：(1) 本方式合成的氮摻雜石墨烯具有高空隙度及大比表面積，使得氮摻雜活性位置可以有效展現。 (2) 在高溫退火的過程中，含氮的苯環小分子可以藉由共價轉換及分子合成的方式沿著石墨烯原生缺陷上做填補之功能，進而達到極佳的結晶性質。(3) 此方式可藉由選擇氮源分子來控制合成出的氮參摻雜石墨烯之氮摻雜官能基型態，在本研究主要以pyridinic- (49.8%) 和graphitic-N sites (38.2%)型態為主，其可以在氧氣還原反應中展現四電子轉移之機制，綜合以上結果，可以證實本研究提出了一低成本且有效的氮摻雜石墨烯合成方式，並在取代白金系電催化觸媒應用上，具有極大的潛力。
　　最後，本文的總結與個人簡歷整理於第八章中。

關鍵字

石墨烯 ； 超級電容器 ； 電催化觸媒 ； 電化學儲能與轉換

並列摘要

The detail introduction of graphene-based materials and their fabricating routes are summarized in Chapter 1. Chapter 2 introduces the basic theory and literature review about proton exchange membrane fuel cells and supercapacitors. The experimental methods and procedures are descripted in Chapter 3.
The physicochemical properties of aggregated GS are similar to that of graphite, their performances are significantly worse than expected. In Chapter 4, a simple approach was designed to enhance the utilization (i.e., enlarging electrolyte-accessible surface area) of GS and to improve the capacitive performances by combining 1D CNTs and 2D GS. In addition, dependence of the microstructure and capacitive performance on the composition of GS-CNT composites is systematically investigated by varying the weight ratio of reduced GO to CNTs. The specific capacitance of GS-CNTs-9-1 (326.5 F/g at 20 mV/s) is much higher than that of GS material (83 F/g). Furthermore, the energy and power densities of GS-CNTs-9-1 are as high as 21.74 Wh/kg and 78.29 kW/kg, respectively exhibiting that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites.
In Chapter 5, a simple, efficient and practical approach to fabricate a novel hierarchical a-MnOx/GS-CNT composite which contains a homogeneous ultrathin a-MnOx nanoflower film with slender nanopetals was designed to approach the full utilization of electroactive materials through combing a simple solution-assembled process and the cost-effective electrochemical deposition technology. The utilization of a-MnOx on the desired 3D hierarchical a-MnOx/GS-CNT composite (specific capacitance of MnOx, CS,Mn = 1200 F g1) is much-higher than that of a pure a-MnOx electrode (CS,Mn = 233 F g1). Futhermore, at the CV scan rate = 200 mV s1, the specific energy and specific power of a-MnOx/GS-CNT are respectively as high as 46.2 Wh kg1 and 33.2 kW kg1, revealing that this 3D hierarchical a-MnOx/GS-CNT electrode delivers energy and power densities well above those of electrochemical capacitors (ECs) in current state-of-the-art
In chapter 6, an approach was designed to enhance the utilization of GS-based materials and to improve the electrocatalytic performances of Pt nanoclusters by combining 1-D CNTs and 2-D GS. Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes to form a 3-D hierarchical structure which enhances the utilization of GS-based composites. The specific electrochemically active surface area (SECSA) and specific double-layer capacitance (CS,DL) of Pt/GS–CNTs (127.9m2/g, 171.3 F/g) are much higher than those of Pt/GS (105.4m2/g, 104.7 F/g) and Pt/CNTs (51.5m2/g, 37.1 F/g), respectively, revealing the synergistic effects between GS and CNTs on enhancing the electrochemical activity of Pt nanoparticles and electrolyte-accessible surface area.
In Chapter 7, an integrated approach is proposed to fabricate N-doped GS with pyridinic-N and graphitic-N as the main functional groups (ca. 88% in nitrogen structures) and high surface area through combining molecular functionalization, ultrafast thermal expansion-exfoliation, and covalent transformation steps. The exceptional performances of N-doped GS for the oxygen reduction reaction are attributable to the following reasons: (i) The highly porous architectures of N-doped GS possess higher electrolyte-accessible surface area, especially the nitrogen active sites, leading to a much higher electrochemical activity. (ii) Annealing at high temperatures thermally reduces GO and induces bond formation between aromatic nitrogen molecules and GS, resulting in the formation of highly crystalline N-doped GS. (iii) The N-doped GS prepared in this work contains high percentages of pyridinic- (49.8%) and graphitic-N sites (38.2%), is promised for the 4-electron transfer ORR. Hence, this work offers a promising route for fabricating N-doped GS, a typical metal-free electrocatalyst.
Finally, the main contributions of this dissertation and further feasibility works are summarized in Chapter 8.

並列關鍵字

graphene ； supercapacitor ； electrocatalyst ； electrochemical energy storage and conversion

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