슈퍼캐패시터(supercapacitor)는 배터리와 함께 많은 양의 전기에너지를 저장 및 공급하 는 중요한 에너지 저장 장치이다. 특히 슈퍼캐 패시터는 고출력이 가능하고 크기와 형태가 조절가능하여 전자기기 부터 자동차 까지 그 사용분야가 매우 넓다 [1-3]. 최근에 웨어러블 (wearable) 디바이스와 플렉서블(flexible) 전 자기기의 발달과 함께 구부릴 수 있고, 당길 수 있는(stretchable) 슈퍼캐패시터의 개발 또한 활발히 진행되고 있다 [4-8]. 슈퍼캐패시터의 작동원리에 따라 전기이중층 캐패시터(electric double-layer capacitor, EDLC)와 의사캐패시터 (pseudocapacitor)로 나뉜다. EDLC는 전하분 리현상을 이용하기 때문에 넓은 표면적을 갖 는 활성탄(activated carbon)과 같은 전극 재료 를 사용하며 의사캐패시터는 전극 재료의 산 화·환원반응(reduction and oxidation, redox) 을 이용하므로 redox반응을 잘 일으키면서 넓 은 표면적을 갖는 전도성 고분자와 금속산화 물 등의 전극 재료를 사용하게된다 [9]. 슈퍼캐 패시터의 전극 재료로서 높은 에너지 저장능 력 및 성능을 갖으려면, 일반적으로 높은 표면 적을 갖도록 해야하며, 슈퍼캐패시터의 성능 은 전극 활물질의 모폴로지(morphology), 기 공크기분포(pore size distribution), 전기전도 도(electrical conductivity), 표면 특성, 열 특성 등의 다양한 성질에 의해 결정되며, 이를 최적 화 했을 때 높은 성능의 슈퍼캐패시터의 제조 가 가능하다 [1]. 일반적으로 다공성 구조의 카 본 및 금속산화물을 만들기 위해서 그 재료의 전구체를 계면활성제(surfactant)를 이용하여 모폴러지 및 다공성을 조절하였다. 계면활성 제의 사용은 다양한 모양과 구조의 활물질제 조를 가능하게 하였지만, 많은 양의 계면활성 제의 사용은 시약의 가격, 후처리, 환경적인 측 면에서 단점을 가진다. 금속유기구조체(metal-organic frameworks, MOFs)는 금속이온과 유기물 연결체(organic linker)로 만들어진 조성물로서, 합성 시 이러한 추가적인 계면활성제의 사용없이 매우 높은 표 면적을 갖는 금속유기 조성물을 만들 수 있다 (그림 1). 이러한 MOF는 사용되는 금속이온, 유 기연결체, 결정구조 등에 따라 MOF-N, HKUSTN, ZIF-N 등 (N: number)으로 구분되어 명명된 다 (그림 1(b)) [10, 11] . 또한, 사용하는 금속이 온과 유기물 연결체의 종류에 따라 다공성 특 성을 조절할 수 있고, 이들의 열처리를 통해서 다공성 카본체 및 금속산화물의 제조가 가능하 다 [12]. 더욱이, MOF는 기존의 다양한 재료에 적용이 가능하여 다양한 에너지저장 재료로 만 들어 질 수 있으며, 나노기술 및 다양한 접근 방 법을 통해 나노구조체 및 조성물의 합성이 가 능하다 [13]. 이러한 장점으로 인해 최근 많은 종류의 MOF 물질들이 슈퍼캐패시터 및 2차전 지의 에너지 저장시스템(energy storage systems, ESSs) 에 응용되고 있다 (그림 2). MOF 중 이미다졸(imidazole) 유도체를 유기연결체로

Metal–Organic Frameworks (MOFs)

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Oxygen electrocatalysis is of great importance for many energy storage and conversion technologies, including fuel cells, metal–air batteries and water electrolysis. Replacing noble metal-based electrocatalysts with highly efficient and inexpensive non-noble metal-based oxygen electrocatalysts is critical for the practical applications of these technologies. Here we report a general approach for the synthesis of hollow frameworks of nitrogen-doped carbon nanotubes derived from metal–organic frameworks, which exhibit higher electrocatalytic activity and stability for oxygen reduction and evolution than commercial Pt/C electrocatalysts. The remarkable electrochemical properties are mainly attributed to the synergistic effect from chemical compositions and the robust hollow structure composed of interconnected crystalline nitrogen-doped carbon nanotubes. The presented strategy for controlled design and synthesis of metal–organic framework-derived functional nanomaterials offers prospects in developing highly active electrocatalysts in electrochemical energy devices. Precious metals are efficient oxygen electrocatalysts but suffer from poor stability and high cost. Now, nitrogen-doped carbon nanotubes derived from metal–organic frameworks are shown to have activity and durability comparable to that of Pt/C catalysts.

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A metal–organic framework-derived bifunctional oxygen electrocatalyst

Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

Metal–organic frameworks (MOFs) have received a lot of attention because of their diverse structures, tunable properties and multiple applications such as gas storage, catalysis and magnetism. Recently, there has been a rapidly growing interest in developing MOF-based materials for electrochemical energy storage. MOFs have proved to be particularly suitable for electrochemical applications because of their tunable chemical composition that can be designed at the molecular level and their highly porous framework in which fast mass transportation of the related species is favorable. In this review, the recent progress in fabricating MOFs and MOF-derived nanostructures for electrochemical applications is presented. The review starts with an introduction of the principles and strategies for designing targeted MOFs followed by a discussion of some novel MOF-derived structures and their potential applications in electrochemical energy storage and conversion. Finally, major challenges in electrochemical energy storage are highlighted and prospective solutions from current progress in MOF-based nanostructure research are given.

Metal–organic frameworks and their derived nanostructures for electrochemical energy storage and conversion

We have successfully prepared nanoporous Carbon-L and -S materials by using ZIF-7 as a precursor and glucose as an additional carbon source. Results indicate that Carbon-L and -S show an appropriate nitrogen content, high surface area, robust pore structure and excellent graphitization degree. The addition of an environmentally friendly carbon source – glucose – not only improves the graphitization degree of samples, but also plays a key role in removing residual Zn metal and zinc compound impurities, which makes the resulting materials metal-free in situ nitrogen-doped porous carbons. By further investigating the electrocatalytic performance of these nitrogen-doped porous carbons for oxygen reduction reaction (ORR), we find that Carbon-L, as a metal-free electrocatalyst, shows excellent electrocatalytic activity (the onset and half-wave potentials are 0.86 and 0.70 V vs. RHE, respectively) and nearly four electron selectivity (the electron transfer number is 3.68 at 0.3 V), which is close to commercial 20% Pt/C. Moreover, when methanol was added, the Pt/C catalyst would be poisoned while the Carbon-L and -S would be unaffected. By exploring the current-time chronoamperometric response in 25 000 s, we found that the duration stability of Carbon-L is much better than the commercial 20% Pt/C. Thus, both Carbon-L and -S exhibit excellent ability to avoid methanol crossover effects, and long-term operation stability superior to the Pt/C catalyst. This work provides a new strategy for in situ synthesis of N-doped porous carbons as metal-free electrocatalysts for ORR in fuel cells.

ZIF-derived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction

A series of ZIF-derived porous carbon materials are prepared via co-carbonization of ZIF-7 and additional carbon sources, such as glucose, ethylene glycol, glycerol and furfuryl alcohol. Results indicate that ZIF-7/glucose composite-derived Carbon-L-950 as an electrode for the electrochemical capacitor exhibits a high specific capacitance of 228 F g−1 in 6 M KOH at a current density of 0.1 A g−1, even 178 F g−1 at a high current of 10 A g−1 and good stability over 5000 cycles. Moreover, the conductive agent (like acetylene black) is not required in the preparation process of the working electrode, which not only lowers the preparation costs but also is favorable for stability and performance. This facile fabrication of ZIF-derived porous carbon materials may open up a new avenue for producing a new family of porous carbon materials for advanced energy storage devices, such as fuel cells, supercapacitors and lithium batteries.

ZIF-derived porous carbon: a promising supercapacitor electrode material

Micronization of atorvastatin calcium by antisolvent precipitation process.

Preparation of amorphous cefuroxime axetil nanoparticles by controlled nanoprecipitation method without surfactants.

Micromixing plays a crucial role in fast chemical reactions. The micromixing efficiency in a rotating packed bed (RPB) was first studied via a throughway method with specially designed RPB structur...

Zhigang Shen

Papers

ZIF-derived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction

ZIF-derived porous carbon: a promising supercapacitor electrode material

Micronization of atorvastatin calcium by antisolvent precipitation process.

Preparation of amorphous cefuroxime axetil nanoparticles by controlled nanoprecipitation method without surfactants.

Micromixing Efficiency in a Rotating Packed Bed: Experiments and Simulation