The work presented in this thesis addressed the synthesis, characterization and electrochemical application of carbon materials possessing a mesopore-micropore hierarchy.
Nanoporous carbons, a class of non-oxide porous materials, have received a great deal of scientific and technological interest due to their large surface area and tunable porosity as well as the superior physical and chemical properties, such as electrical conductivity, thermal conductivity, chemical stability and light-weight. These advantages make nanoporous carbons significantly desirable for use as electrode materials in energy conversion and storage devices including fuel cells, Li-ion batteries and electrical double layer capacitors (EDLC). In particular, with growing need of the devices capable of supplying high power, extensive studies related to the carbon applications have focused on design of suitable carbon electrodes for high performance of EDLC. In EDLC, where the energy stores by the adsorption of ions on the electrode surfaces, the performance is dramatically affected by the pore size of electrodes. Microporous carbons exhibit higher capacitance at low discharge current density than mesoporous carbons, due to the high surface area. In high current conditions, on the other hand, mesoporous carbons show better performance due to low diffusion resistance of electrolyte. However, neither of these materials can achieve high energy density and high power density simultaneously. To achieve the both, several attempts to synthesize carbon materials with a hierarchically meso-/microporous structure were made in the past.
In the present work, a hierarchically meso-/microporous carbon was synthesized via templating the intracrystalline microporous texture of beta zeolite nanocrystals and their intercrystalline mesoporous texture. Various compounds were evaluated as the carbon precursors in the carbon synthesis and resultant carbon products were rigorously characterized by X-ray diffraction analysis, gas adsorption-desorption isotherm, transmission electron microscopy. From the analysis results, it was founded that the use of ethanol as a carbon precursor gives a more selective deposition of carbon inside the nanocrystalline zeolite frameworks rather than at the exterior, compared with propylene and acetylene. The advantage of ethanol was attributed to the formation of water from the ethanol decomposition during carbonization process. The water vapor inhibited the deposition of carbons at the exterior (or the mesopore walls) and hence assisted in formulating a sp2-boned carbon framework selectively inside the template micropores. More interestingly, such an effect of water vapor on the carbonization results was consistently applied during a synthesis using various types of carbon precursors. As a result of using acetonitrile with steam for the carbon synthesis, N-doped hierarchical carbons having a highly ordered micropore structure were successfully synthesized. The selective carbon depositions inside the zeolite micropores and subsequent removal of the zeolite template result in the formation of hierarchically-porous carbon that is composed of highly ordered micropores (1 nm in diameter) and irregular mesopores (10-30 nm) arranged in a hierarchical manner. The pores were three-dimensionally connected with each other. Brunauer-Emmett-Teller surface area up to 2500 m2 g-1 could be achieved. The resulting carbon exhibited a very high EDLC capacitance both at low and high discharge current density in galvanostatic measurement, compared to solely microporous or solely mesoporous carbons. This result can be attributed to the facile transport of electrolytes to and from high-surface micropores via mesopores. Solely microporous carbons with a comparable surface area showed high capacitance at low current density, but exhibited dramatically reduced capacitance at high current density due to the hindered diffusion of electrolytes. Solely mesoporous carbon exhibited stable capacitance behavior even at high current density, but its absolute capacitance value was relatively small due to the low surface area of the solely mesoporous structure. Such phenomena became more pronounced when a bulky organic electrolyte was used.
본 연구에서는 주형합성법을 이용하여 메조기공과 마이크로기공을 동시에 갖는 위계적 나노다공성 탄소물질을 합성하였다. 이러한 탄소물질은 나노결정성 제올라이트 골격으로 이뤄진 메조다공성 주형물질의 마이크로기공에만 선택적으로 탄소를 형성시킨 후, 주형물질을 HF나 NaOH로 제거함으로서 얻어진다. 성공적인 합성을 위해 여러 가지 유기 화합물을 주형합성의 탄소 전구물질로 사용하였고, X-선 회절법, 질소흡착-탈착 등온선, 전자현미경 등을 사용하여 얻어지는 탄소의 구조적 특성을 분석하였다. 합성된 탄소의 구조분석결과, 에탄올은 주형합성법을 통해 위계적 나노다공성 탄소물질을 합성하는데 있어서 가장 적합한 탄소 전구물질임을 밝혀냈다. 에탄올을 사용해서 얻어지는 탄소는 규칙적인 마이크로기공을 가지고 있으며, 동시에 주형이 가지고 있던 메조기공의 크기와 형태를 그대로 보존하여 가지고 있었다. 에탄올 사용시의 장점은 에탄올이 탄화되는 과정 중에 분해되면서 생성되는 물 분자에서 기인하며, 물 분자는 탄화과정에서 제올라이트 결정 외부에서 탄소가 생성되는 것을 억제하는 역할을 하여 제올라이트 마이크로 기공 안으로 지속적인 탄소 전구물질의 확산이 용이하도록 도와준다. 이는 제올라이트 나노결정 내부에 균일하게 탄소골격이 만들어지는데 기여한다. 이러한 물의 효과를 질소원자가 포함된 탄소 전구물질인 아세토나이트릴을 이용하는 탄소합성에 적용하여 질소원자가 포함된 위계적 나노다공성 탄소 또한 합성할 수 있었다.
이렇게 주형합성법을 통하여 얻어지는 위계적 나노다공성 탄소는 전기화학 이중층 캐패시터 (EDLC)의 전극물질로 응용했을 경우 메조다공성 탄소와 마이크로다공성 탄소에 비하여 매우 높은 성능을 나타내었다. 정전류 측정법으로 탄소의 전기용량을 측정한 결과, 넓은 비표면적과 메조기공구조를 동시에 가지고 있는 위계적 나노다공성 탄소는 매우 높은 전기용량을 높은 전류밀도에서까지도 계속 유지하였다. 이러한 장점 때문에 위계적 나노다공성 탄소는 높은 에너지밀도와 파워밀도를 동시에 갖춘 고성능 EDLC 전극 물질로서 전기자동차나 무정전 전원장치, 휴대용 전자제품 등에 널리 활용 될 수 있을 것으로 보여진다.
