Anode materials of nanostructured silicon have been prepared by physical vapor deposition and characterized using electrochemical methods. The electrodes were prepared in thin-film form as nanocrystalline particles ͑12 nm mean diameter͒ and as continuous amorphous thin films ͑100 nm thick͒. The nanocrystalline silicon exhibited specific capacities of around 1100 mAh/g with a 50% capacity retention after 50 cycles. The amorphous thin-film electrodes exhibited initial capacities of 3500 mAh/g with a stable capacity of 2000 mAh/g over 50 cycles. We suggest that the nanoscale dimensions of the silicon circumvents conventional mechanisms of mechanical deterioration, permitting good cycle life. © 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1596917͔ All rights reserved. There is intense interest in developing new materials for anodes and cathodes that store higher densities of lithium. From the criterion of lithium density, the ultimate anode would be lithium metal itself. Unfortunately, issues of safety have confined lithium anodes to small rechargeable cells. During charging, lithium metal is electroplated onto the anode surface, but there is no thermodynamic tendency to stop the formation of shape asperities such as dendrites that can cause short circuits across the electrolyte.1 A framework material seems necessary for preserving the anode shape, and metallic alloys can provide this function. The Li-Si system has the potential for one of the highest gravimetric capacities. Electrochemical alloying of lithium with silicon to form alloys of Li x Si has shown stable crystalline phases up to Li 4.4 Si.2-6 For a range of x from 0 to 4.4 the theoretical specific capacity of pure silicon is 4200 mAh/g, far greater than the theoretical capacity of 372 mAh/g for graphitic carbons. Furthermore, lithium alloys do not suffer from the solvent cointercalation that can occur in graphitic carbons, and may degrade the storage capacity.7-11 These advantages of silicon anodes are well known, but it is also well known that a 300% volume dilatation is associated with alloying 4.4 lithium atoms per silicon atom. This generates enormous mechanical stresses within the brittle material, which pulverizes during the first few charge/discharge cycles and electrical integrity is lost.11-14 Li et al. have recently shown that bulk silicon loses approximately 90% of its initial capacity after five cycles at ambient temperature. 15 The properties of nanostructured materials have also received intense interest over the last decade. When dimensions in a material are tens of nanometers, the conventional mechanisms for deformation and fracture are expected to be altered. There is some evidence that thin films of silicon ͑1.2 m͒ prepared by chemical vapor deposition undergo smaller capacity losses during cycling than bulk silicon.14 Sayama et al. have shown discharge capacities of up to 4000 mAh/g over 10 cycles in chemical vapor deposition ͑CVD͒ silicon electrodes.16 Evaporated thin films ͑40 nm͒ have shown stable capacities of up to 3000 mAh/g ...

Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes ͑SWNT͒ was found to exceed 8 wt. %, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes.

Germanium nanocrystals ͑12 nm mean diam͒ and amorphous thin films ͑60-250 nm thick͒ were prepared as anodes for lithium secondary cells. Amorphous thin film electrodes prepared on planar nickel substrates showed stable capacities of 1700 mAh/g over 60 cycles. Germanium nanocrystals showed reversible gravimetric capacities of up to 1400 mAh/g with 60% capacity retention after 50 cycles. Both electrodes were found to be crystalline in the fully lithiated state. The enhanced capacity, rate capability ͑1000C͒, and cycle life of nanophase germanium over bulk crystalline germanium is attributed to the high surface area and short diffusion lengths of the active material and the absence of defects in nanophase materials.

B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH 4 , Mg(BH 4 ) 2 , LiSc(BH 4 ) 4 , and the mixed Ca(AlH 4 ) 2 -LiBH 4 system. The 11 B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K 2 B 12 H 12 that contains the [B 12 H 12 ] 2anion. The presence of [B 12 H 12 ] 2in the partially decomposed borohydrides was further confirmed by high-resolution solution 11 B and 1 H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.

We used electron-energy-loss spectrometry to measure the intensities of the white lines found at the onsets of the L2 and L3 absorption edges for most of the 3d and 4d transition metals. The intensities of the white lines, normalized to the trailing background, decreased nearly linearly with increasing atomic number, rejecting the filling of the d states. One-electron Hartree-Slater calculations of the white-line intensities were in good agreement with observed spectra. Empirical correlations between normalized white-line intensity and d-state occupancy provide a method for measuring changes in d-state occupancy due to alloying.

Alloying with Si is shown to destabilize the strongly bound hydrides LiH and MgH 2 . For the LiH/Si system, a Li 2.35 Si alloy forms upon dehydrogenation, causing the equilibrium hydrogen pressure at 490°C to increase from approximately 5 × 10 -5 to 1 bar. For the MgH 2 /Si system, Mg 2 Si forms upon dehydrogenation, causing the equilibrium pressure at 300°C to increase from 1.8 to >7.5 bar. Thermodynamic calculations indicate equilibrium pressures of 1 bar at approximately 20°C and 100 bar at approximately 150°C. These conditions indicate that the MgH 2 /Si system, which has a hydrogen capacity of 5.0 wt %, could be practical for hydrogen storage at reduced temperatures. The LiH/Si system is reversible and can be cycled without degradation. Absorption/desorption isotherms, obtained at 400-500°C, exhibited two distinct flat plateaus with little hysteresis. The plateaus correspond to formation and decomposition of various Li silicides. The MgH 2 /Si system was not readily reversible. Hydrogenation of Mg 2 Si appears to be kinetically limited because of the relatively low temperature, <150°C, required for hydrogenation at 100 bar. These two alloy systems show how hydride destabilization through alloy formation upon dehydrogenation can be used to design and control equilibrium pressures of strongly bound hydrides.

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.

Recent technological advancements in wearable sensors have made it easier to detect sweat components, but our limited understanding of sweat restricts its application. A critical bottleneck for temporal and regional sweat analysis is achieving uniform, high-throughput fabrication of sweat sensor components, including microfluidic chip and sensing electrodes. To overcome this challenge, we introduce microfluidic sensing patches mass fabricated via roll-to-roll (R2R) processes. The patch allows sweat capture within a spiral microfluidic for real-time measurement of sweat parameters including [Na+], [K+], [glucose], and sweat rate in exercise and chemically induced sweat. The patch is demonstrated for investigating regional sweat composition, predicting whole-body fluid/electrolyte loss during exercise, uncovering relationships between sweat metrics, and tracking glucose dynamics to explore sweat-to-blood correlations in healthy and diabetic individuals. By enabling a comprehensive sweat analysis, the presented device is a crucial tool for advancing sweat testing beyond the research stage for point-of-care medical and athletic applications.