Alfred Kleinhammes

Electrochemical intercalation of single-walled carbon nanotubes with lithium

Abstract Single-walled carbon nanotubes (SWNT) synthesized by laser ablation were electrochemically intercalated with lithium. As-grown SWNTs showed a reversible saturation composition of Li 1.2 C 6 (450 mAh g −1 ). After removing the impurity phases by filtration, the reversible saturation composition increased to Li 1.6 C 6 (600 mAh g −1 ), significantly higher than the ideal value of LiC 6 (372 mAh g −1 ) for graphite. All the SWNT materials showed large irreversible capacities and voltage hysteresis. Upon processing the nanotubes by ball-milling, the reversible Li capacity increased to 1000 mAh g −1 (Li 2.7 C 6 ) while the irreversible capacity decreased to 650 mAh g −1 .

Enhanced saturation lithium composition in ball-milled single-walled carbon nanotubes

Abstract The effects of processing on the structure and morphology of single-walled carbon nanotubes (SWNT) and their electrochemical intercalation with lithium were investigated. Purified SWNTs were processed by impact ball-milling and were electrochemically intercalated with lithium. The reversible saturation Li composition increased from Li 1.7 C 6 in purified SWNTs to Li 2.7 C 6 after 10 min of milling. The irreversible capacity decreased from Li 3.2 C 6 to Li 1.3 C 6 . Electron microscopy, Raman and X-ray diffraction measurements indicated that ball-milling induced disorder within the bundles and fractured the nanotubes.

Intercalation and partial exfoliation of single-walled carbon nanotubes by nitric acid

Abstract Single-walled carbon nanotubes (SWNTs) were reacted with HNO 3 solution. An expansion of the inter-nanotube spacing and increase in the amount of hydrogen in the material were observed by X-ray diffraction and proton NMR measurements after the SWNTs were immersed into the HNO 3 solution for less than 2 hours. The change in diffraction pattern was reversible, indicating intercalation of HNO 3 into the SWNT bundles. When the SWNTs were immersed in the HNO 3 solution for a longer period of time, the bundles became disordered and partially exfoliated. Isolated SWNTs and large nano-particles were often observed.

Temperature-Induced Hydrophobic-Hydrophilic Transition Observed by Water Adsorption

The properties of nanoconfined and interfacial water in the proximity of hydrophobic surfaces play a pivotal role in a variety of important phenomena such as protein folding. Water inside single-walled carbon nanotubes (SWNTs) can provide an ideal system for investigating such nanoconfined interfacial water on hydrophobic surfaces, provided that the nanotubes can be opened without introducing excess defects. Here, we report a hydrophobic-hydrophilic transition upon cooling from 22°C to 8°C via the observation of water adsorption isotherms in SWNTs measured by nuclear magnetic resonance. A considerable slowdown in molecular reorientation of such adsorbed water was also detected. The observed transition demonstrates that the structure of interfacial water could depend sensitively on temperature, which could lead to intriguing temperature dependences involving interfacial water on hydrophobic surfaces.

Synthesis of Microporous Boron-Substituted Carbon (B/C) Materials Using Polymeric Precursors for Hydrogen Physisorption

This paper discusses a new synthesis route to prepare microporous boron substituted carbon (B/C) materials that show a significantly higher hydrogen binding energy and physisorption capacity, compared with the corresponding carbonaceous (C) materials. The chemistry involves a pyrolysis of the designed boron-containing polymeric precursors, which are the polyaddition and polycondensation adducts between BCl3 and phenylene diacetylene and lithiated phenylene diacetylene, respectively. During pyrolysis, most of the boron moieties were transformed into a B-substituted C structure, and the in situ formed LiCl byproduct created a microporous structure. The microporous B/C material with B content > 7% and surface area > 700 m2/g has been prepared, which shows a reversible hydrogen physisorption capacity of 0.6 and 3.2 wt % at 293 and 77 K, respectively, under 40 bar of hydrogen pressure. The physisorption results were further warranted by absorption isotherms indicating a binding energy of hydrogen molecules of approximately 11 kJ/mol, significantly higher than the 4 kJ/mol reported on most graphitic surfaces.

Lithium intercalation into etched single-wall carbon nanotubes

Abstract The effects of structure and morphology on lithium storage in single-wall carbon nanotubes (SWNTs) were studied by electrochemistry. SWNTs were chemically etched and were intercalated with Li. The reversible Li storage capacity increased from LiC6 in close-end SWNTs to LiC3 after etching, which is twice the value observed in Li intercalated graphite. The enhanced capacity is attributed to Li diffusion into the interior of the SWNTs through the opened ends and sidewall defects.

NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons

(1)H NMR spectroscopy is used to investigate a series of microporous activated carbons derived from a poly(ether ether ketone) (PEEK) precursor with varying amounts of burnoff (BO). In particular, properties relevant to hydrogen storage are evaluated such as pore structure, average pore size, uptake, and binding energy. High-pressure NMR with in situ H(2) loading is employed with H(2) pressure ranging from 100 Pa to 10 MPa. An N(2)-cooled cryostat allows for NMR isotherm measurements at both room temperature ( approximately 290 K) and 100 K. Two distinct (1)H NMR peaks appear in the spectra which represent the gaseous H(2) in intergranular pores and the H(2) residing in micropores. The chemical shift of the micropore peak is observed to evolve with changing pressure, the magnitude of this effect being correlated to the amount of BO and therefore the structure. This is attributed to the different pressure dependence of the amount of adsorbed and non-adsorbed molecules within micropores, which experience significantly different chemical shifts due to the strong distance dependence of the ring current effect. In pores with a critical diameter of 1.2 nm or less, no pressure dependence is observed because they are not wide enough to host non-adsorbed molecules; this is the case for samples with less than 35% BO. The largest estimated pore size that can contribute to the micropore peak is estimated to be around 2.4 nm. The total H(2) uptake associated with pores of this size or smaller is evaluated via a calibration of the isotherms, with the highest amount being observed at 59% BO. Two binding energies are present in the micropores, with the lower, more dominant one being on the order of 5 kJ mol(-1) and the higher one ranging from 7 to 9 kJ mol(-1).

Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

Ion distribution in aqueous electrolytes near the interface plays a critical role in electrochemical, biological and colloidal systems, and is expected to be particularly significant inside nanoconfined regions. Electroneutrality of the total charge inside nanoconfined regions is commonly assumed a priori in solving ion distribution of aqueous electrolytes nanoconfined by uncharged hydrophobic surfaces with no direct experimental validation. Here, we use a quantitative nuclear magnetic resonance approach to investigate the properties of aqueous electrolytes nanoconfined in graphitic-like nanoporous carbon. Substantial electroneutrality breakdown in nanoconfined regions and very asymmetric responses of cations and anions to the charging of nanoconfining surfaces are observed. The electroneutrality breakdown is shown to depend strongly on the propensity of anions towards the water-carbon interface and such ion-specific response follows, generally, the anion ranking of the Hofmeister series. The experimental observations are further supported by numerical evaluation using the generalized Poisson-Boltzmann equation.

Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film

A nuclear magnetic resonance technique for precisely measuring the bulk magnetic susceptibility of micron-thick hydrogenated amorphous silicon (a-Si:H) film is introduced. The large disorder-induced diamagnetic enhancement exhibited by a-Si:H is shown to provide a sensitive bulk measurement for detecting variations in structural order in a-Si:H films. Furthermore, this approach is shown to be effective in revealing the details of microstructure in a-Si:H, including the presence of microstructural anisotropy.

Dehydration of Ions in Voltage-Gated Carbon Nanopores Observed by in Situ NMR

Ion transport through nanochannels is of fundamental importance in voltage-gated protein ion channels and energy storage devices. Direct microscopic observations are critical for understanding the intricacy of ionic processes in nanoconfinement. Here we report an in situ nuclear magnetic resonance study of ion hydration in voltage-gated carbon nanopores. Nucleus-independent chemical shift was employed to monitor the ionic processes of NaF aqueous electrolyte in nanopores of carbon supercapacitors. The state of ion hydration was revealed by the chemical shift, which is sensitive to the hydration number. A large energy barrier was observed for ions to enter nanopores smaller than the hydrated ion size. Increasing the gating voltage above 0.4 V overcomes this barrier and brings F(-) into the nanopores without dehydration. Partial dehydration of F(-) occurs only at gating voltage above 0.7 V. No dehydration was observed for Na(+) cations, in agreement with their strong ion hydration.