Electrochemical and Solid-State Letters

The structural changes in silicon electrochemically lithiated and delithiated at room temperature were studied by X-ray powder diffraction. Crystalline silicon becomes amorphous during lithium insertion, confirming previous studies. Highly lithiated amorphous silicon suddenly crystallizes at 50 mV to form a new lithium-silicon phase, identified as This phase is the fully lithiated phase for silicon at room temperature, not as is widely believed. Delithiation of the phase results in the formation of amorphous silicon. Cycling silicon anodes above 50 mV avoids the formation of crystallized phases completely and results in better cycling performance. © 2004 The Electrochemical Society. All rights reserved.

This article reports the development and characterization of a microfluidic reactor for the electrochemical reduction of carbon dioxide. The use of gas diffusion electrodes enables better control of the three-phase interface where the reactions take place. Furthermore, the versatility of the microfluidic reactor enables rapid evaluation of catalysts under different operating conditions. Several catalysts as well as the effects of electrolyte pH on reactor efficiency for reduction of to formic acid were tested. Operating at acidic pH resulted in a significant increase in performance: faradaic and energetic efficiencies of 89 and 45%, respectively, and current density of .

On-board hydrogen storage remains a big challenge for fuel cell powered electric vehicles. Ammonia contains 17.6 wt % hydrogen and has been recognized as a potential on-board vehicular hydrogen media. Direct ammonia fuel cells are interesting because they do not require an ammonia cracking process to produce hydrogen, whereas conventional proton exchange membrane fuel cells based on acidic membranes such as Nafion are not compatible with . Here we report the operation of direct ammonia alkaline anion-exchange fuel cells based on low cost membrane and non-noble catalysts with potential use in transportation and other applications.

The structure, synthesis, and electrochemical behavior of layered for 5/12, and 1/2 are reported for the first time. is derived from or by substitution of and by while maintaining all the remaining Mn atoms in the 4+ oxidation state. with can deliver steady capacities of 150 and 160 mAh/g at 30 and 55°C, respectively, between 3.0 and 4.4 V using a current density of 30 mA/g. Differential scanning calorimetry experiments on charged electrodes of for indicate that this material should be safer than with 5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, respectively, at 30°C. with gives a capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a current density of 30 mA/g. © 2001 The Electrochemical Society. All rights reserved.

The reversible intercalation of ions into manganese dioxide was first reported in an aqueous system and a large capacity was measured. A cycle life test was performed at , and after , no capacity fading was found, which indicates the good cycling properties of manganese dioxide toward the insertion of zinc ions. X-ray photoelectron spectrum measurements indicated that ions in the electrolyte are involved in the charge storage process of manganese dioxide. X-ray diffraction results exhibited the kinetic stability of the host structure in allowing high-capacity, single-phase, and reversible zinc-ion intercalation.

A quantitative approach is used to identify sources of contribution of capacity fade in commercial rechargeable lithium battery cells in laboratory evaluations. Our approach comprises measurements of close-to-equilibrium open-circuit voltage (cte-OCV) of the cell after relaxation at the end of the charging and discharging regimes and an incremental capacity analysis, in addition to conventional cycle-life test protocols using the dynamic stress test schedule. This approach allows us to separate attributes to capacity fade due to intrinsic and extrinsic origins.

The most accurate values to date were determined for conductivity of water from 0-100°C, permitting new determination of high-temperature hydroxide ion equivalent conductance. These values were incorporated into a fundamental water coefficient table including hydroxide and hydrogen ion mobilities, water ionization constant, density, conductivity, and resistivity. The conductivity/resistivity values were measured with a multiple-pass, closed, recirculating flow conductivity system, with improved multiple resistance temperature device measurement, and improved analysis of temperature and impurity effects. An accurate conductivity knowledge is necessary to understand water-limiting processes and to facilitate the analysis of trace ionic impurities in water. © 2004 The Electrochemical Society. All rights reserved.

A mechanism that may cause accelerated performance decay of fuel cells is presented. The mechanism is explained using a one-dimensional model of the potential profile. The analysis indicates that the electrolyte potential drops from 0 to (vs. RHE) when the anode is partially exposed to hydrogen and partially exposed to oxygen. This causes flow of current opposite to normal fuel cell mode at the oxygen-exposed region and raises the cathode interfacial potential difference to 1.44 V, causing carbon corrosion, which decreases performance. The decay mechanism was validated using two different experimental setups which reproduced the carbon-corrosion phenomenon.

The results of a first principles investigation of lithium diffusion within the layered form of are presented. Kinetic Monte Carlo simulations predict that lithium diffusion is mediated through a divacancy mechanism between and and with isolated vacancies at infinite vacancy dilution. The activation barrier for the divacancy migration mechanism depends strongly on lithium concentration resulting in a diffusion coefficient that varies within several orders of magnitude. We also argue that the thermodynamic factor in the expression of the chemical diffusion coefficient plays an important role at high lithium concentration. ©2000 The Electrochemical Society

The electrochemical performance of negative electrodes based on commercially available crystalline Si powder and sodium carboxymethyl cellulose (CMC) binder was investigated. Compared to the conventional binder, polyvinylidene fluoride, Si electrodes using CMC binder show vastly improved cycling performance. A high specific capacity of about for has been achieved with a lower cutoff potential of vs . Si electrodes made using CMC binder have better capacity retention than those using a binder consisting of CMC and styrene butadiene rubber. CMC is an extremely stiff and brittle polymer, so it is surprising that it functions well as a binder in electrodes where the volume change of the active material particles is about 100%.