Over the past several decades, porous carbon has been recognized as a promising electrode material due to its low density, high electrical conductivity, mechanical stability, high surface area, chemical inertness, and relatively low cost. Previously, porous carbon was prepared from organic precursors, either natural or synthetic, which were carbonized and activated through a gasification process. However, these materials consisted of complex, intricate assemblies of distorted graphite-like nanocrystallites, with the consequent disadvantage that pore size uniformity and shape were typically difficult to control. An ideal electrochemical electrode requires both a large surface area for charge accumulation and an interconnected porous network with pores that are sufficiently accessible for electrolyte wetting and rapid ionic transport. Unfortunately, current commercially available activated carbon electrodes are microporous (less than 2 nm in pore diameter) and are not easily accessed by electrolyte ions.
While microporous structures contribute to the charge storage capacity, mesoporous structures are more accessible to electrolytes and allow greater ion mobility. The mesoporous structure therefore allows for the delivery of a large quantity of energy at a high rate. Template-based methods have been considered the best for the synthesis of porous carbon materials with designed pore architecture and control over the pore size distribution. Activated carbon materials produced using this technique exhibit well ordered mesoporous structures with large specific volumes. However, the aggressive chemicals employed limit this approach to the production of stable graphitic carbon.
Therefore, my research objective is a fabrication of highly mesoporous carbon nanofibers reinforced with carbon nanotubes and their applications for supercapacitor and Li-ion battery electrodes with high capacities.
The strategy to obtain mesoporous carbon materials without using template method is using natural polymer, starch. Starch has a natural ability to assemble into a nanoscale lamellar structure consisting of crystalline and amorphous regions. Starch was firstly electrospun into the starch nanofibers with diameters ranging from 150 to 200 nm with the aid of spinning agent (PVA) and used as a carbon source material of the electrochemical electrodes for applications such as supercapacitor and Li-ion battery. By using the natural ability of starch lamellar structure and controlling the carbonization temperature, we successfully fabricated binder-free electrochemical electrode material consisting of highly mesoporous carbon nanofibers reinforced with CNTs. In addition, we controlled the porous carbon structure through activation process using $CO_2$ gas. Electrochemical properties of these CNT/Carbon nanocomposite fibers with tunable porous structure were characterized for applications of supercapacitor and Li-ion battery.
In application for supercapacitor, CNT/Carbon nanocomposite fiber with highly mesoporous structure has extraordinarily higher specific capacitance ($105uF/cm^{2}$) than other carbon electrodes derived from synthetic polymers and free-standing CNT electrodes. The high specific capacitance of highly mesoporous carbon nanofibers electrode reinforced with CNTs comes from the moderate specific surface area and the sufficient pore distributions at the effective mesoporous sizes of 3 ~ 5nm. In addition, CNTs are successfully added as reinforcements playing a role of enhancing the mechanical strength and electrical conductivity as well as providing effective surface area. In addition, CNT/Carbon nanocomposite fibers showed very high energy and power density by changing liquid electrolyte (1M $H_2SO_4$) into liquid organic electrolyte (1M $LiPF_6$ in EC/DMC) which voltage can be applied up to 3 V.
In application for Li-ion battery anode, the first charge capacity of 1wt.% CNT/activated carbon showed very high value of 743 mAh/g and good cycle performance after 30 cycles having specific capacity of 510mAh/g, which is significantly higher than that of graphite (\lt 372 mAh/g). This high specific capacity is mainly due to its larger specific surface area which can offer many adsorption sites of Li ions and higher electrical conductivity compared with carbon nanofibers without CNTs. From these results, we can derive that CNTs play a major role to enhance specific capacity of carbon nanofibers by increasing both specific surface area and electrical conductivity. However, larger irreversible capacity is observed in the first discharge and charge process of 1wt.% CNT/activated carbon. From this result, we can know that when Li is extracted from CNT/Carbon nanocomposite fibers (charge), it is trapped and cannot be extracted from CNTs.
Thereby, this research reveals that starch which has advantages in aspects of low cost and environmental-friendly material is an ideal material as the carbon source of electrochemical electrodes and can provide a simple and cheap approach for the fabrication of binder-free electrochemical electrodes.
본 연구논문에서는 최초로 녹말의 전기방사를 이용하여 탄소섬유를 제조하였다. 제조된 탄소섬유를 이용하여 초고용량 커패시터 및 리튬이차전지 음극으로의 응용을 위해 산화안정화 공정 및 탄화공정을 거쳐 탄소섬유 전극을 제조하였다. 여기에 탄소나노튜브는 전극의 비표면적 향상과 전기전도도 향상을 위해 탄소섬유내에 강화되었다.
탄소섬유의 탄화공정 온도를 제어함으로써 메조기공을 다량으로 함유하는 탄소섬유가 제조되었다. 이는 녹말 고유의 비정질과 결정질이 일정한 간격으로 적층되어 있는 분자구조로 인해 특정 온도 이상에서 비정질이 완전 분해됨으로써 탄소섬유 표면에 다량의 메조기공이 형성된 것이다. 더하여 활성화 공정을 도입함으로써 탄소섬유내에 있는 무질서한 탄소들을 이산화탄소 가스로 분해시킴으로써 마이크로 기공 및 메조 기공을 다량으로 형성시켜 비표면적을 최대한 증가시켰다.
제조된 탄소나노튜브가 강화된 탄소섬유를 이용하여 초고용량커패시터 전극 응용을 위한 특성평가를 실시하였다. 다량의 메조기공을 함유하고 있는 탄소섬유는 이온의 원활한 입.출입 및 넓은 비표면적으로 인하여 아주 높은 비용량값을 나타내었다. 그리고 초고용량 커패시터내의 전해질을 액체 전해질에서 유기 전해질로 바꿈으로써 메조기공을 다량으로 함유하고 있는 탄소섬유는 아주 높은 용량 밀도 및 출력 밀도값을 나타내었다.
제조된 탄소나노튜브가 강화된 탄소섬유를 이용하여 리튬이차전지 음극으로의 응용을 위한 특성평가를 실시하였다. 리튬이차전지 음극의 경우에는 리튬이 흡착할 수 있는 비표면적이 큰 활성화된 탄소섬유의 경우에 가장 큰 용량값을 나타내었다. 이는 XRD 분석을 통하여 활성화된 탄소섬유내에 그래핀 단일층이 가장 많이 함유되어 있다는 것이 밝혀졌으며, 이로 인해 리튬이 보다 많이 흡착될 수 있었다. 그리고 수명평가에서 30 싸이클 이후에도 현재 상용화되고 있는 흑연 전극보다 훨씬 높은 용량값을 유지하고 있었다.
이상의 결과들을 종합해 볼 때 본 연구결과에서 사용한 녹말은 환경친화적이며 아주 저렴한 재료로써 전기화학용 전극재료 응용을 위하여 아주 이상적이라고 판단된다.
