To realize a prosperous and sustainable society in the future, continuous progress in electrical energy storage technology is essential. Carbon materials have played a pivotal role in the global economy since electrical power was harnessed in the 19th century. To improve electrode performance, it is vital to develop carbon electrodes with designed porous structures over multiple length scales. In this regard, a self-standing, binder-free carbon electrode has many advantages that are not available in conventional composite electrodes involving binders and other additives. This review highlights porous monolithic carbon electrodes derived from crosslinked organic gels in terms of their synthesis and the control of their pore structure by sol–gel techniques. Special focus is given to porous carbon monoliths with three-dimensionally interconnected macropores prepared by a phase separation strategy. Electrochemical studies on energy storage devices using the monolithic carbon electrodes are also overviewed.
The key features of pulverized graphite for use in electric double layer capacitors (EDLCs) are highlighted. Pulverized graphite is produced by mechanochemical processing of natural graphite using a planetary ball mill. The crystallinities and porosities of the obtained carbon materials can be controlled by tuning the milling conditions. Because of their high electrical conductivities and specific surface areas, pulverized graphites are expected to be suitable as electrode materials in EDLCs. In addition, because of their mesopore-rich structures and high electrode densities, they have better volumetric capacitances and rate capabilities than those of conventional microporous activated carbons. Their long-term stability at high voltages is comparable or superior to those of the latter materials.
On the basis of many reported experimental results, the exact differentiations of graphene from graphite, and also of graphene oxide from graphite oxide, using the abbreviations GnO and GtO, respectively, are proposed. In addition we suggest identifying GnOs synthesized by the Hummers and Brodie methods by the notations HGnO, BGnO, respectively. We also strongly recommend differentiating graphenes synthesized (a) by CVD or peeling from a graphite crystal, and (b) from reduced graphene oxide involving the oxidation, exfoliation and reduction of graphite.
Due to their high specific surface areas, the properties of nanomaterials can be controlled by appropriate surface functionalization. However, precise control has been rarely achieved due to the lack of a reliable quantitative methodology for their functionalization and characterization. We have found that the polyglycerol (PG) functionalization can be applied to various nanomaterials to give them high aqueous dispersibility. The PG functionalization also enabled homogenous conditions in the reactions, purification and characterization of nanomaterials, allowing us to apply methodologies and/or principles in organic chemistry to nanomaterials. Among the many nanomaterials, nanodiamond (ND) is chosen as a typical carbonaceous nanomaterial, because it has been attracting considerable attention, especially in the fields of biology and medicine. We summarize the 1) controlled functionalization of ND with PG and the production of derivatives from it, 2) quantitative characterization of its structures by thermogravimetric analysis as well as solution-phase nuclear magnetic resonance spectroscopy and combustion elemental analysis, which are conventional analytical methods in organic chemistry, and 3) relationships between its structures and physical and biochemical properties such as zeta potential and protein affinity.
At the nanoscale, the electronic properties of graphene vary significantly depending on its shape and chemical modification. Although many experimental and theoretical studies have been conducted on the electronic properties of nanographene, experimental characterization has been insufficient. This is mainly due to the difficulty in fabricating graphene with precisely controlled shapes and chemical structures and in accurately identifying the nanographene structure at the atomic level. In this paper, we describe the electronic properties of graphene nanostructures with different geometries and chemical structures, which were characterized by scanning probe microscopy (SPM). SPM showed that the atomic-level shape and chemical modifications significantly impacted the electronic properties of graphene. Furthermore, small π-conjugated molecules associated with fullerenes and aromatic molecules between metal electrodes can be regarded as nanographene. Additional electronic and transport properties can be derived by modulating the intermolecular and metal–molecule interactions of nanographene molecules. Studies on the control of the intermolecular and metal–molecule interactions of chemically modified aromatic molecules and the associated modulation of the electronic structure and charge transport properties are presented.
We investigated a generalized strategy for synthesizing an iodine-based composite electrode by the electrochemical encapsulation of iodine inside single-wall carbon nanotubes (SWCNTs). We performed the encapsulation reaction in aqueous electrolytes in the pH range of 4–12, and investigated the iodine redox reaction conditions in the hollow cores of the SWCNTs and the properties of the obtained materials using electrochemical measurements and spectroscopic techniques such as Raman spectroscopy, and X-ray diffraction. Raman scattering showed the possibility of encapsulating iodine in SWCNTs by the electro-oxidation of iodide ions in both acidic and alkaline aqueous solutions, and cyclic voltammetry of the iodine redox reactions at the SWCNT electrode in both electrolytes resulted in almost the same voltammogram regardless of the pH value of the electrolyte. This indicates that the insertion and extraction of iodine molecules into and from SWCNTs occurred reversibly in the same way regardless of the electrolyte pH. In contrast, the energy storage performance of the obtained composite electrodes showed a dependence on the pH of the electrolytic solution, as shown by the charge–discharge experiments of a full-cell zinc-iodine battery, which showed that the batteries worked well only in neutral and alkaline electrolytes.