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Carbon materials, such as graphite and activated carbon, have been widely used as electrodes in batteries and electric double-layer capacitors (EDLCs). Graphene, which has an extremely thin sheet-like structure, is considered as a fundamental carbon material. However, it was less investigated as an electrode material than graphite and activated carbons. This is because graphene is a relatively new material and is difficult to handle. However, using graphene electrodes can enhance the performance of nanodevices. Here, the performance of EDLCs based on single-layer and bilayer graphene electrodes in LiCl, NaCl, and KCl aqueous electrolyte solutions was evaluated using cyclic voltammetry, and the charging mechanism was evaluated using molecular dynamics simulations. KCl aqueous solution provided the highest capacitance compared to LiCl and NaCl aqueous solutions in the case of single-layer graphene electrodes. In contrast, the dependence of the capacitance on the ion species was hardly observed in the case of bilayer graphene. This indicates that Li and Na ions also contributed to the capacitances. The high EDLC performance can be attributed to the fast ion transfer promoted by the dehydration and modification of the second hydration shell on the bilayer graphene because of the relatively strong interaction of ions with the bilayer graphene. [ABSTRACT FROM AUTHOR]

Activated carbons have been well-used for separation, removal, and storage for various gases. Pore width and surface functional groups are primal factors of interactions with adsorbed molecules. Surface functional groups especially influence adsorptions of acidic and basic molecules, although all molecules are considerably influenced by pore width. Controlling optimal pore width and surface functional group is however very difficult, because both factors of pore width and surface functional groups are simultaneously changed in preparation process. We here attempted to prepare oxygen-introduced activated carbon with different surface oxides into activated carbons and with the similar pore structure. NH3, CO2, acetaldehyde, isoprene, and water were used for evaluating the influence of surface oxides in carbon pores. As the surface oxides increased, NH3 adsorbed amounts were considerably increased accompanying an irreversible adsorption and a threshold pressure of water adsorption was shifted to lower relative pressure, while adsorption isotherms of the other molecules were rarely changed. A comparison of interaction between any surface oxides and adsorbed molecules indicated that NH3 and water molecules having the electron donor were strongly adsorbed on the acid surface oxides. The oxygen-introduced activated carbons without any destruction of the pore structure exhibited a considerable adsorption potential for molecules having the electron donor, and maintained an adsorption potential for molecules having the nonpolar and electron acceptor, proposing effective removal and separation with high adsorption capacity. [ABSTRACT FROM AUTHOR]

Separation of CO2 based on adsorption, absorption, and membrane techniques is a crucial technology necessary to address current global warming issues. Porous media are essential for all these approaches and understanding the nature of the porous structure is important for achieving highly efficient CO2 adsorption. Porous carbon is considered to be a suitable porous media for investigating the fundamental mechanisms of CO2 adsorption, because of its simple morphology and its availability in a wide range of well-defined pore sizes. In this study, we investigated the dependence of CO2 adsorption on pore structures such as pore size, volume, and specific surface area. We also studied slit-shaped and cylindrical pore morphologies based on activated carbon fibers of 0.6–1.7 nm and carbon nanotubes of 1–5 nm, respectively, with relatively uniform structures. Porous media with larger specific surface areas gave higher CO2 adsorption densities than those of media having larger pore volumes. Narrower pores gave higher adsorption densities because of deep adsorption potential wells. However, at a higher pressure CO2 adsorption densities increased again in nanopores including micropores and small mesopores. The optimal pore size ranges of CO2 adsorption in the slit-shaped and cylindrical carbon pores were 0.4–1.2 and 1.0–2.0 nm, respectively, although a high adsorption density was only expected for the narrow carbon nanopores from adsorption potentials. The wider nanopore ranges than expected nanopore ranges are reasonable when considering intermolecular interactions in addition to CO2–carbon pore interactions. Therefore, cooperative adsorption among CO2 in relatively narrow nanopores can allow for high density and high capacity adsorption. [ABSTRACT FROM AUTHOR]

In this study, we investigated the possibilities of using activated carbon fibre (ACF) as a carbon capture and storage (CCS) technology. The CO2 adsorption isotherms of ACFs with different porosities were systematically examined at 273 and 298 K under ambient pressure conditions. The porosities of the ACFs were characterized by the adsorption of nitrogen at 77 K. We analyzed the adsorption capabilities of three types of ACFs (A5, A10 and A20) having different slit-shaped pore widths, specific surface areas and micropore volumes. Our results reveal that A5 had ultramicropores and achieved a higher adsorption of CO2 at low relative pressure (<0.015) at 273 K. However, A10, which had an average pore width of 0.9 nm, exhibited the highest adsorption capacity of 195 mg g-1 at a higher pressure of about 100 kPa, which is a relatively high value compared with that of conventional activated carbon. By establishing the temperature dependence of CO2 adsorptivity and using Dubinin-Radushkevich analysis, we characterized the interaction energy between pores and CO2 molecules. Our results shed light on the fundamental aspects of CO2 adsorption of ACFs, moving them towards being a viable CCS. [ABSTRACT FROM AUTHOR]

Water adsorption isotherms of hydrophobic activated carbon fibres (ACFs) having average pore widths, w, of 0.7 and 1.1 nm at 303 K were greatly different from each other; the adsorption isotherm of ACF with w = 0.7 nm had no clear hysteresis loop, while that of ACF with w = 1.1 nm had an explicit hysteresis loop. The structures of water adsorbed in both nanopores were studied using adsorption measurement, grand canonical Monte Carlo (GCMC) simulation, and in situ small angle X-ray scattering (SAXS). The in situ SAXS study of water in the 1.1 nm pores with the aid of GCMC simulation showed that adsorption proceeds through isolated cluster formation, whereas a mixed structure of a partial monolayer array and isolated clusters are formed on the desorption near the fractional filling, φ = 0.4. This adsorbed structure difference is the origin of the adsorption hysteresis. In contrast, both adsorption and desorption processes have the isolated cluster structure near φ = 0.4 in the case of 0.7 nm pores, agreeing with the absence of a clear hysteresis loop. [ABSTRACT FROM AUTHOR]

Water has unique properties in the nanopores of activated carbons and carbon nanotubes, which are apparently different from those of typical adsorbed molecules such as and Ar. Here the mechanism of water vapor adsorption in carbon nanopores and the adsorbed structures are reviewed using adsorption isotherms, X-ray scattering, and molecular simulations. Water cluster formation in hydrophobic carbon nanopores promoted self-adsorption of water by gaining stability. Water stabilization is a result of anomalously strong hydrogen bonding between water molecules in such carbon nanopores. On the other hand, in the case of an aqueous electrolyte solution, the formation of a hydration shell around ions is dominant in carbon nanopores. These findings promote our understanding of water nanoscience and nanotechnology as well as electrochemical nanoscience. [ABSTRACT FROM AUTHOR]

Superuniform nanosized gates (nanogates; 0.41 ± 0.02 nm in size) were donated on graphene sheets of single wall carbon nanohorn (SWCNH) particles by controlled oxidation. Super-uniform nanogate donation confers high molecular selectivity and large adsorption capacity upon SWCNH. The molecular selectivity of CO2 over CH4 attains 17 below 0.01 MPa by the nanogate separation. [ABSTRACT FROM AUTHOR]

Superuniform nanosized gates (nanogates; 0.41 ± 0.02 nm in size) were donated on graphene sheets of single wall carbon nanohorn (SWCNH) particles by controlled oxidation. Super-uniform nanogate donation confers high molecular selectivity and large adsorption capacity upon SWCNH. The molecular selectivity of CO2 over CH4 attains 17 below 0.01 MPa by the nanogate separation. [ABSTRACT FROM AUTHOR]

Abstract: The role of surface functionality on silica and carbonaceous materials for adsorption of cadmium(II) was examined using various mesoporous silica and activated carbon. Silica surfaces were principally functionalized by mono-amino- and mercapto-groups, while carboxylic group was introduced to the activated carbons by oxidation. Functional groups on silica surface were formed using grafting and co-condensation techniques in their preparation. Mono-amino group was found more effective than di- and tri-amino groups for cadmium(II) adsorption on the grafted silica. Mono-amino groups prepared by co-condensation adsorbed cadmium(II) as much as 0.25mmol/g compared to mercapto- and carboxyl-groups which adsorbed around 0.12mmol/g, whereas Langmuir adsorption affinities were as strong as 50–60L/mmol for all of the three functions. The working pH range was wider for mercapto- and carboxyl-functions than for amino-group. Basic site could be an adsorption center for amino-functional groups while ion exchange sites were found to work for the mercapto- and carboxyl-functions to adsorb cadmium(II) from aqueous phase. Based on the experimental results, surface functional groups rather than structure of silica and carbon seemed to play a decisive role for cadmium(II) adsorption. [Copyright &y& Elsevier]

Abstract: A new catalyst for efficient production of H2 from CH4 without CO2 emission has been requested. We prepared highly-dispersed Pd nanoparticles on single wall carbon nanohorn (Pd-SWCNH) and on oxidized SWCNH (Pd-oxSWCNH) without an anti-aggregation agent. The Pd nanoparticle size on SWCNH and oxSWCNH determined by electron microscopy are around 2.5 and 2.7nm, respectively. Each sample provides efficiently H2 and hollow carbon nanofibers through CH4 decomposition. The H2 release over the Pd-dispersed SWCNH samples starts from ca. 820K and is quite large amount compared with a commercial Pd-activated carbon. [Copyright &y& Elsevier]