Carbon Nanospaces Research Articles

Direct Evidence of Reversible SnO2-Li Reactions in Carbon Nanospaces.

We present herein that carbon nanospaces are the key reaction space to improve the reversibility of the reaction of SnO2 with Li-ions for lithium-ion batteries, demonstrated by both ex situ and in situ observations using high-resolution scanning transmission electron microscopy with electron energy loss spectroscopy. Conversion-type electrode materials, such as SnO2, undergo large volume changes and phase separation during the charge-discharge process, which lead to degradation in the battery performance. By confining the SnO2-Li reaction within carbon nanopores, the battery performance is improved. However, the exact phase changes of SnO2 in the nanospaces are unclear. By directly observing the electrodes during the charge-discharge process, the carbon walls are capable of preventing the expansion of SnO2 particles and minimizing the conversion-induced phase separation of Sn and Li2O on the sub-nanometer scale. Thus, nanoconfinement structures can effectively improve the reversibility performance of conversion-type electrode materials.

Polyiodide Production Triggered by Acidic Phase of Aqueous Solution Confined in Carbon Nanospace

Abstract Polyiodide species were synthesized by the acceleration of an acidic environment in the nanospace of single-walled carbon nanotubes (SWCNT) with light irradiation. Raman and EXAFS results strongly support the production of polyiodide species after the adsorption of CsI on SWCNT from aqueous solution. Interestingly, the reaction was initiated by the nano-confined acidic phase formed in a basic environment. The acidic phase plays an essential role as an oxidant for the production of the diiodine that is a source of polyiodide.

Low-Temperature Synthesis of Lithium Lanthanum Titanate/Carbon Nanowires for Fast-Charging Li-Ion Batteries.

Due to the lower working voltage and higher capacity, the Li-rich lithium lanthanum titanate perovskite (LLTO) anode is becoming a potential candidate for the commercial Li4Ti5O12 (LTO) Li-ion battery anode [Zhang, L. Lithium Lanthanum Titanate Perovskite as an Anode for Lithium Ion Batteries. Nat. Commun. 2020, 11, 3490]. However, a high temperature of 1250 °C is required to fabricate pure LLTO particles by the conventional solid-phase calcination method, limiting their further practical applications. Here, an in situ carbon nanospace confined method is developed to synthesize the pure LLTO with sub-nanometer grain size at an extremely low temperature of 800 °C. The LLTO precursor is confined in the in situ formed carbon nanowire matrix during heating, resulting in a shorter solid-phase diffusion distance and subsequently lower energy required for the formation of the pure LLTO phase. The low-temperature-synthesized pure LLTO/carbon composite nanowires (P-LLTO/C NWs) exhibit improved lithium storage performances than the traditionally prepared LLTO due to the fast electronic conduction of carbon and the stable carbon surface. In addition, the working potentials of P-LLTO/C||LiFePO4 and P-LLTO/C||LiCoO2 full cells are all 0.7 V higher than that of the corresponding commercial full cells with LTO as an anode, meaning much higher power energy densities (307.6 W kg-1 at 2C and 342.4 W kg-1 at 1C vs 198.4 W kg-1 and 275.2 W kg-1 for LTO||LiFePO4 and LTO||LiCoO2 full cells based on electrode materials, respectively). This low-temperature synthesis method can extend to other solid-state ionic materials and electrode materials for electrochemical devices.

Direct Evidence of Black Phosphorus Formation in Carbon Nanospaces by Quasi-high Pressure Effect

Abstract Black phosphorus (BP) has attracted attention as an anode material for lithium or sodium ion batteries due to its high electrical conductivity and theoretical specific capacity (2596 mAh/g). However, the synthesis of BP requires high-pressure conditions at >1 GPa. We directly demonstrated that carbon nanospaces with the pore size around 4 nm are effective to synthesize BP without an external high-pressure apparatus.

Silicon Anode Confined in Carbon Nano-Spaces for Efficient Lithium Storage

Commercial Li-ion batteries employ graphite and other carbonaceous materials as anodes, which has excellent stability and low cost, but has a theoretical capacity limit at 372 mAh/g by forming a fully intercalated LiC6 compound. For anode materials, silicon is also known to have the highest theoretical specific capacity (4200 mAh/g), and is considered to be the active materials for the next generation lithium ion batteries. However it has the large volume change (>400%) during lithiation and delithiation causes pulverization, resulting in capacity loss in a high number of cycles. Here, this study reports the use of engineering nanocarbon film with confined silicon nanoparticles as an anode material for lithium-ion batteries. The unique “concave” graphitic nanostructure, prepared in porous templates with morphology of interconnected arrays, makes nanocarbon electrodes a good lithium-ion intercalation medium and, more importantly, robust nanocontainers to effectively confine high-capacity silicon nanoparticles for lithium-ion storage. Our unique anode, which is based the concave graphitic film and silicon particles confined in its nanospace, have three notable features for the use as the effective electrode material in a lithium-ion battery. First, Li ion diffusion into the interior of the nanocarbons through the opened structures can shorten the diffusion length of the redox couple and provide rapid charge transport. Second, the exposed silicon active materials in the direction of Li ion diffusion increase the accessibility of the electrode interior to the electrolyte, which ensures a high charge/discharge capacity. Finally, the nanoconfined silicon system inside the carbon nanospace improves the cycle life by preventing volume expansion during electrochemical Li insertion/extraction. As a result, high performance Li storage with a mass-specific capacity of 2500 mAh/g was obtained at 0.5A/g. The discharge capacity was retained constantly during 200 cycles with coulombic efficiency of above 98%. These results suggest that the novel structure can lead to high storage capacities and rate capability as anode material in future high performance Li-ion batteries.

Surplus adsorption of bromide ion into π-conjugated carbon nanospaces assisted by proton coadsorption

Nanoporous carbons can preferentially adsorb bromide ions from an aqueous solution of alkali metal bromides, even on π-conjugated surfaces. Our results show a new adsorption mechanism whereby coadsorption of protons enhances the adsorption of the anions onto the carbons.

Unusually Stable Plastic Crystal Phase of CCl4 Confined in Graphitic Slit-shaped Micropores from Neutron Diffraction

Structural changes with temperature of CCl4 confined in a 1.1 nm slit-shaped carbon nanospace were investigated by neutron diffraction (ND) measurements and radial distribution function fdrdf analy...

Nanospace Control of SnO2Nanocrystallites-Embedded Nanoporous Carbon for Reversible Electrochemical Charge–Discharge Reactions

The electrochemical charge–discharge reactions of SnO2 with Li ions, which are composed of SnO2–Sn conversion and Sn–Li alloying/dealloying reactions, are generally irreversible, causing severe capacity fading during cycling. As a novel material overcoming the problem, SnO2 nanocrystallites-embedded nanoporous carbons, of which SnO2 loading amount, nanopore size and nanopore volume were systematically tuned, were successfully synthesized by in situ synthesis using vaporized SnCl2. Effectiveness of carbon nanospace confinement as well as key factors of appropriate pore filling fraction and dispersive deposition of SnO2 nanocrystallites in carbon nanospace for reversible electrochemical reactions were revealed by a detailed investigation on the relation between the structural parameters and charge–discharge properties. Consequently, the nanoporous composites with SnO2 loading amount below 69 wt % showed high capacities of 734–855 mAh g–1 and high capacity retentions of 70–86% at the 30th charge–discharge cy...

Effects of Hydrophobic Nanospaces on Structures of Lysozyme

Adsorption of lysozyme on carbon nanospaces was studied by measurement of structures and biological activities of the adsorbed lysozyme. Carbon aerogel (CA) was used as the carbon material. Several kinds of CAs with different pore sizes were prepared. Porosities of CAs were characterized by a nitrogen adsorption measurement at 77 K. Adsorption measurements showed that mesopores with diameters between 15 and 25 nm were the most suitable for lysozyme adsorption. However, because the size of lysozyme is less than 5 nm in length, this adsorption behaviour suggests that lysozyme molecules may associate with one another to form clusters on mesopores of CAs. This is a unique behaviour because it is well-known that lysozyme indicates monolayer adsorption on the flat surface. The X-ray diffraction patterns of lysozyme-adsorbed CA showed a few peaks originating from the structure of lysozyme. The strongest peak of a bulk species of lysozyme was shifted by heating because of changes in its secondary structures. Lyso...

Charge–Discharge Property of Si and SiOx Nanoparticles Produced in Regulated Carbon Nanospace

Abstract Si and SiOx nanoparticle-embedded nanoporous carbons were successfully synthesized by reducing a SiO2 opal–carbon composite precursor with Mg, and their interstitial nanospace in carbon pores was tunable by partly etching the SiO2 particles in the precursor. It was revealed that dispersive loading of Si and SiOx nanoparticles in carbon nanospace is necessary to enhance the charge–discharge performance in addition to providing the interstitial nanospace as a buffer space for the volume change associated with Si–Li reactions.

Development of MEAs with Pt/Mesoporous Carbon as a Cathode Catalyst

Polymer electrolyte fuel cells (PEFCs) have recently been paid great attention owing to commercialization of fuel cell vehicles (FCVs) since 2015. However, PEFCs still have some problems to be overcome, such as improvement of cost and durability for the next generation. In our study, improvement on durability of electrocatalysts is targeted by reducing the mobility of Pt particles on the carbon support. Particularly, Pt particles are introduced into the nano-space of Mesoporous Carbon (MC), which has the nano-channel structure with a diameter of ca. 10 nm. Such encapsulation of Pt in the carbon nano-space leads to successfully reducing the mobility of Pt. MEAs have been prepared with such electrocatalysts as cathode materials, and the sufficient performance has been confirmed, so far. In this study, we have particularly focused on the triple phase boundary formed inside the carbon nano-space. The methods of Nafion introduction were controlled in order to study how they influence on IV performance and durability.Electrocatalysts were synthesized from platinum (Ⅱ) acetylacetonate as a precursor by supporting ca. 30 wt% Pt on our original MC, and named as Pt/MC. Pt/MC was used only for the cathode. For the anode, a commercial Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) was in use. Then, MEAs were prepared by spraying the slurry of electrocatalysts and Nafion on the both sides of Nafion membrane, followed by hot-pressing. The solvents for introducing Nafion into carbon nano-space were controlled by ethanol or hexafluoropropanol.Based on our previous research work, introduction of Nafion into carbon nano-space is known to depend on the kind of solvents[1]. Therefore, in this study, MEAs were made using hexafluoropropanol, which was thought to be more suitable for introduction of Nafion than regular ethanol. Besides the initial performance of MEAs, their durability was also evaluated, using the cycle test at the potential range where the dissolution and re-precipitation of Pt particles occurs[2]. As a result, the initial activity was higher for MEAs made with ethanol, whereas durability was higher for MEAs made with hexafluoropropanol. Successful formation of the triple phase boundary has led to the increase in durability. Further, we are now working on improving the initial activity by optimizing introduction methods of Nafion.

A new route to nanoscale ceramics in asymmetric reaction fields of carbon nanospaces

We fabricated nanoscale BaTiO3 and its asymmetric crystal structure was obtained using an asymmetric reaction field in carbon nanospaces. The nano-crystal phases changed from symmetric to asymmetric crystals with decreasing crystallite size. The fabrication of an asymmetric crystal in a nanospace can be adapted for various ceramic fabrications.

Enhanced charge–discharge properties of SnO2 nanocrystallites in confined carbon nanospace

Almost perfect embedding of SnO2 nanocrystallites in carbon nanopores was achieved by in situ synthesis using vaporized SnCl2 and silica opal-derived nanoporous carbons. The reversibility of SnO2-Sn conversion and Sn-Li alloying/de-alloying reactions was greatly enhanced by the confinement in regulated carbon nanospace.

Temperature dependence of water structure in carbon nanotubes

Water vapor adsorption on porous carbons has been actively studied because of its importance in various applications and basic science. However, the unique behavior and structure of water in carbon nanospaces have still been remained unclear. Carbon nanotubes inherently have rather restricted one-dimensional nanospaces and thus, force the adsorbed water to be aligned in them. The structure of water adsorbed in those nanospaces is ice-like even at ambient temperature. In this paper, we show the water structures in two-different single wall carbon nanotubes (CNTs) with average CNT diameters of 1.1 (narrow) and 2.5 (wide) nm at 260–300 K, which were evaluated by using in situ synchrotron X-ray diffraction (XRD). The water structures in the both CNTs were nano-ice-like forms at 300 K rather than a liquid-like. The ice-like structure in the wide CNTs was developed with decreasing temperature, although even at 260 K, sharp ice peaks were not observed in the XRD pattern. On the other hand, in the narrow CNTs, the water structure was gradually transformed from an ice-like form to a liquid-like form with decreasing temperature to 260 K. The structure transformation of solid-to-liquid with decreasing temperature is due to the fact that the highly restricted nanospaces obstruct the formation of ice-like clusters and/or hydrogen bonds. The anomalous structure transformation in both CNTs could be a clue to reveal one-dimensional water behavior in CNTs. [TANSO 2013 (No. 258) 195–200.]

極小カーボンナノチューブ中の水構造の温度変化

Water vapor adsorption on porous carbons has been actively studied because of its importance in various applications and basic science. However, the unique behavior and structure of water in carbon nanospaces have still been remained unclear. Carbon nanotubes inherently have rather restricted one-dimensional nanospaces and thus, force the adsorbed water to be aligned in them. The structure of water adsorbed in those nanospaces is ice-like even at ambient temperature. In this paper, we show the water structures in two-different single wall carbon nanotubes (CNTs) with average CNT diameters of 1.1 (narrow) and 2.5 (wide) nm at 260-300 K, which were evaluated by using in situ synchrotron X-ray diffraction (XRD). The water structures in the both CNTs were nano-ice-like forms at 300 K rather than a liquid-like. The ice-like structure in the wide CNTs was developed with decreasing temperature, although even at 260 K, sharp ice peaks were not observed in the XRD pattern. On the other hand, in the narrow CNTs, the water structure was gradually transformed from an ice-like form to a liquid-like form with decreasing temperature to 260 K. The structure transformation of solid-to-liquid with decreasing temperature is due to the fact that the highly restricted nanospaces obstruct the formation of ice-like clusters and/or hydrogen bonds. The anomalous structure transformation in both CNTs could be a clue to reveal one-dimensional water behavior in CNTs.

Highly compressed nanosolution restricted in cylindrical carbon nanospaces

We shed light on the specific hydration structure around a zinc ion of nanosolution restricted in a cylindrical micropore of single-wall carbon nanotube (SWNT) by comparison with the structure restricted in a cylindrical mesopore of multi-wall carbon nanotube (MWNT) and that of bulk aqueous solution. The average micropore width of open-pore SWNT was 0.87 nm which is equivalent to the size of a hydrated zinc ion having 6-hydrated water molecules. We could impregnate the zinc ions into the micropore of SWNT with negligible amounts of ion-exchanged species on surface functional groups by the appropriate oxidation followed by heat treatment under an inert condition. The results of X-ray absorption fine structure (XAFS) spectra confirmed that the proportion of dissolved species in nanospaces against the total adsorbed amounts of zinc ions on the open-pore SWNT and MWNT were 44 and 61%, respectively, indicating the formation of a dehydrated structure in narrower nanospaces. The structure parameters obtained by the analysis of XAFS spectra also indicate that the dehydrated and highly compressed hydration structure can be stably formed inside the cylindrical micropore of SWNT where the structure is different from that inside the slit-shaped micropore whose pore width is less than 1 nm. Such a unique structure needs not only a narrow micropore geometry which is equivalent to the size of a hydrated ion but also the cylindrical nature of the pore.

Co-adsorption of N2 in the presence of CH4 within carbon nanospaces: evidence from molecular simulations

Molecular simulations were performed to study the separation of CH4 and N2 from mixtures of composition xCH4/xN2 = 5/95 and xCH4/xN2 = 10/90 at 50 bar and 298 K on prototype carbon materials with different pore structures. The studied carbon structures include a slit and a tubular pore, that represent the simplest form of activated carbon and carbon nanotubes, respectively, in addition to a realistic porous carbon model with disordered pore structure and a recently introduced carbon foam model, which has a three-dimensional pore structure. The results indicate that, depending on the pressure and composition, the pore structure influences both the CH4/N2 selectivity and the adsorption behaviour of the fluid molecules. The selectivity was decided by the interactions between CH4 and N2 molecules within the pore structure, in addition to the solid–fluid interactions. The simulation results indicate that, at least for the case of activated carbons (slit and random pores), it would not be appropriate to predict the binary adsorption behaviour of methane and nitrogen by means of pure component information. Regardless of the pore structure, the simulation results indicate that carbon materials show a CH4/N2 (thermodynamic) selectivity of only 2–3 up to 2 bar at 298 K, and above this pressure, at equilibrium, none of the carbon materials is adequate for the efficient separation of this mixture.

Displacement of Methane by Coadsorbed Carbon Dioxide Is Facilitated In Narrow Carbon Nanopores

By using simulation methods, we studied the adsorption of binary CO2–CH4 mixtures on various CH4 preadsorbed carbonaceous materials (e.g., triply periodic carbon minimal surfaces, slit-shaped carbon micropores, and Harris’s virtual porous carbons) at 293 K. Regardless of the different micropore geometry, two-stage mechanism of CH4 displacement from carbon nanospaces by coadsorbed CO2 has been proposed. In the first stage, the coadsorbed CO2 molecules induced the enhancement of CH4 adsorbed amount. In the second stage, the stronger affinity of CO2 to flat/curved graphitic surfaces as well as CO2–CO2 interactions cause the displacement of CH4 molecules from carbonaceous materials. The operating conditions of CO2-induced cleaning of the adsorbed phase from CH4 mixture component strongly depend on the size of the carbon micropores, but, in general, the enhanced adsorption field in narrow carbon ultramicropores facilitates the nonreactive displacement of CH4 by coadsorbed CO2. This is because in narrow carbon ultramicropores the equilibrium CO2/CH4 selectivity (i.e., preferential adsorption toward CO2) increased significantly. The adsorption field in wider micropores (i.e., the overall surface energy) for both CO2 and CH4 is very similar, which decreases the preferential CO2 adsorption. This suppresses the displacement of CH4 by coadsorbed CO2 and assists further adsorption of CH4 from the bulk mixture (i.e., CO2/CH4 mixing in adsorbed phase).

Electrochemistry in Nano-Space of Structurally Controlled Carbon Materials

For the purpose of studying electrochemical reactions in nano-space of structurally controlled carbon, electrochemistry inside the hydrophobic 8 nm pores with platinum deposited mesoporous carbon (Pt/MC) was investigated, and reaction selectivity within carbon nano-space was demonstrated. Distinctive activities toward hydrogen evolution/oxidation and oxygen evolution reactions were observed, comparing to reactions in larger pores (>30 nm). Only small amount of water is expected to exist within such small nano-space even though a proton pass is effectively maintained. Consequently, selectivity in reactions, such as activation of proton-related reactions and suppression of water-related reactions, is achieved by Pt/MC electrocalatysts.

Electrocatalytic Reactions Occurring at Pt/Cu Catalysts Within the Carbon Nano-Space

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