Satoshi Mitani

Metal-free aqueous redox capacitor via proton rocking-chair system in an organic-based couple

Safe and inexpensive energy storage devices with long cycle lifetimes and high power and energy densities are mandatory for the development of electrical power grids that connect with renewable energy sources. In this study, we demonstrated metal-free aqueous redox capacitors using couples comprising low-molecular-weight organic compounds. In addition to the electric double layer formation, proton insertion/extraction reactions between a couple consisting of inexpensive quinones/hydroquinones contributed to the energy storage. This energy storage mechanism, in which protons are shuttled back and forth between two electrodes upon charge and discharge, can be regarded as a proton rocking-chair system. The fabricated capacitor showed a large capacity (>20 Wh/kg), even in the applied potential range between 0–1 V, and high power capability (>5 A/g). The support of the organic compounds in nanoporous carbon facilitated the efficient use of the organic compounds with a lifetime of thousands of cycles.

MnO2 assisted oxidative polymerization of aniline on graphene sheets: Superior nanocomposite electrodes for electrochemical supercapacitors

Graphene-polyaniline nanocomposite electrodes have been developed via oxidative polymerization of aniline by MnO2 on a graphene surface for electrochemical supercapacitor applications. The formation mechanism of the above nanocomposite indicates that the MnO2 undergoes oxidative disintegration and results in porous polyaniline (PANI) nanofibers formation on the graphene surface. The electron microscopic (SEM and TEM) images clearly showed the presence of porous PANI nanofiber formation on the graphene-PANI nanocomposites. The XRD, SEM-EDX and TG analysis confirmed the complete removal/degradation of MnO2 during the oxidative polymerization of aniline. For comparison, graphene-PANI nanocomposites have also been prepared via conventional polymerization using (NH4)2S2O8 as oxidant in acidic medium and the effect of carbon nanotube (CNT) addition has been studied. The electrochemical properties of different graphene-PANI nanocomposites have been investigated using galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy measurements. A superior supercapacitive performance (∼15 to 40% more than the reported capacitance) has been observed for the graphene-PANI nanocomposite electrode obtained via oxidative polymerization of aniline by MnO2. Addition of CNT into the graphene-PANI nanocomposite prepared via the conventional polymerization method showed improved specific capacitance and stability. Carbon nanofibers and graphite have been used as the carbon source and the effect of the carbon source on the specific capacitance has been investigated.

Supercritical fluid assisted synthesis of N-doped graphene nanosheets and their capacitance behavior in ionic liquid and aqueous electrolytes

N-doped graphene nanosheets (N-doped GNS) were obtained by a single step supercritical fluid assisted reaction of N-containing organic compounds with graphene oxide (GO) solution. A N-doped GNS sample shows capacitances of 280 F g−1 in aqueous 1 M H2SO4 (0.9 V) and 104 F g−1 in ionic liquid EMI-TFSA (3.6 V). A NE-GNS electrode shows energy densities of 8 W h kg−1 and 40 W h kg−1 in 1 M H2SO4 and EMI-TFSA, respectively. The nature of chemical bonding and the amount of N doping in the graphene samples were estimated by XPS spectroscopy. The amount of N-doping varies with the nature of the N-containing organic compounds and the supercapacitance behaviour depends on the amount of N-doping as well as the nature of N-doping in the graphene. TEM, FE-SEM images and Raman spectroscopic characterization reveals the presence of few-layer N-doped GNS. FT-IR spectra exhibit the presence of various functional groups on N-doped GNS. XRD diffraction analysis showed the weakly stacked N-doped GNS due to the N-doping and the presence of N-containing functional groups on N-doped GNS. The cyclic voltammetry studies showed the capacitance behaviour of N-doped GNS electrodes at a high potential window of 4.25 V in ionic liquids. The charge–discharge profile showed the stable charge–discharge behaviour of the N-doped GNS electrodes.