Ketjenblack (KB), which is a mesoporous carbon black, can be used as electrode active material for an electric double layer capacitor (EDLC). This paper addresses the capacitance properties of the activated KB with a high specific surface area as the electrode active material. The mesopore volume in KB was decreased by KOH-activation, while the micropores were developed to increase the specific surface area. As a result, the KOH-activated KB showed a higher volumetric capacitance as an EDLC electrode compared with the pristine KB. Additionally, it was also shown that the nitrogen-doping of the KOH-activated KB by nitrogen-monoxide produces a better volumetric capacitance and an ability to tolerate high voltages compared to the pristine KB electrode.
The linear thermal expansion of two artificial graphite products, an isotropic graphite and an extruded nipple electrode, was measured during heating up to 2400 °C by a newly developed device with a non-contact laser micro-gage. The temperature dependence of the coefficient of thermal expansion (CTE) of two graphites was investigated. The test piece was 100 mm long a round rod of 20 mm diameter, which is large enough to reflect the different textures of these products. In the nipple electrode, two specimens taken from parallel or perpendicular to the extrusion direction were used. For the isotropic graphite and the parallel specimen of the nipple electrode, temperature dependences of the CTE below 500 °C were different. In the case of the parallel nipple specimen, a negative value of CTE was found in the temperature region from 150 to 400 °C, probably reflecting the negative CTE along the basal planes of a graphite crystal. In situ measurement of the dimensional changes of the specimens before graphitization process of these products during heat treatment up to 2400 °C was also attempted. For the isotropic product, two-step of thermal shrinkage was observed, but for the nipple electrode, growth above 1400 °C was clearly observed, resulting from the “puffing” of calcined needle cokes. For the puffing, it was found that there was clear anisotropy in the extruded direction of the nipple electrode.
In this review, the authors address the method for evaluating the durability of activated carbon electrodes for electric double layer capacitors (EDLCs). The durability of the EDLC can be evaluated by the charge-discharge cycle or the float charging test. The latter is not only the more aggressive method, but is also very relevant from the viewpoint of actual operation. The development of high-performance carbon electrodes for a EDLC should seek to optimize the surface condition, the pore structure, and the three-dimensional structure of the electrode, based on an analysis of results using pristine and tested electrodes.
The crystal structure of monolayer graphene is a honeycomb structure of carbon atoms with each unit cell containing two carbon atoms; one π electron is associated with each carbon atom as a conduction electron. The first Brillouin zone is derived as a hexagon containing one electron per atom. The wave function in the tight binding approximation is written as the sum of two component wave functions through an undetermined coefficient. Each component wave function is given by the linear combination of the normalized carbon orbitals centered at each of the same type of carbon atom. The elements of the Hamiltonian matrix are defined with the overlap integral or the exchange integral by component wave functions. With the elimination of the undetermined coefficient, and since only nearest neighbor hopping is allowed in the tight binding approximation, the final Hamiltonian matrix elements become zero for diagonal elements and hopping energies ℏvF (κx−iκy) and ℏvF (κx+iκy), respectively, for off-diagonal elements. κ is the wave vector measured from the corner points of the first Brillouin zone, vF=3a0γ0/2ℏ=0.874×106 [m·s−1]∼c/300 is the Fermi velocity, i.e. the velocity of the massless particle after the supposed π electron conversion; a0 is the distance between nearest neighbor carbon atoms; −γ0 is the hopping integral; ℏ is the Planck constant divided by 2π; and c is the velocity of light. The Hamiltonian matrix is obtained as ℏvFσ̂·κ; and the energy dispersion relation as ε=±ℏvFκ. σ̂ is the Pauli matrix.
グラフェンはsp2炭素から成る1枚のシートであり，1原子分の薄さから高い比表面積(2630 m2/g)や機械的柔軟性をもち，またその化学的な安定性は非常に高く，導電性も有する魅力的な材料である。しかし，グラフェンは二次元のシートであるために，π–π相互作用により容易に積層してしまい，比表面積の低下や機械的柔軟性の損失が起きる。そこで，グラフェンを多孔質化あるいは三次元化し，自立構造をもたせる研究が行われている。本研究室ではゼオライトまたはアルミナナノ粒子といった金属酸化物を鋳型として用いることで数nm以下のサイズの細孔を有する，1∼2層のグラフェン骨格から成る多孔体の合成に成功している。ゼオライトを鋳型として合成される多孔体はゼオライト鋳型炭素(ZTC)と呼ばれ，ゼオライト由来の規則的なミクロ孔を有するほか，積層の無いグラフェン骨格が大量のエッジサイト（炭素六角網面の端）を有するために単層グラフェンを上回る比表面積を有する。アルミナナノ粒子を鋳型にして合成される多孔体はグラフェンメソスポンジ(GMS)と呼ばれ，3∼8 nmの細孔を有しており，エッジサイトが非常に少ないという特徴をもつ。本研究では，これらグラフェンから成る材料の機械的特性と電気化学的な安定性に着目し，新たな応用の可能性を探索する。