RESUMEN
Lithium-sulfur batteries discharge via the transformation of solid sulfur to solid lithium sulfide via the formation of several polysulfide species that have only been observed in solution. Reported here is the first experimental phase diagram of a S8-Li2S-electrolyte system, which is shown to be a practical tool to determine the solution composition and formation of solid (S8 and Li2S) phases in lithium-sulfur batteries. The phase diagram is constructed by the combination of measurements of the total sulfur concentration [S]T and average oxidation state (Sm-) of polysulfide solutions prepared by reaction of S8 and Li2S. The phase diagram is used to predict the equilibrium discharge/charge profile of lithium-sulfur batteries as a function of the amount of electrolyte and the onset of precipitation and dissolution of solid products. High energy batteries should operate with a minimum amount of electrolyte, where both solid S8 and Li2S will be present during most of the charge and discharge of the cell, in which case we predict the observation of only one voltage plateau, instead of the two voltage plateaus commonly reported.
RESUMEN
Lithium-oxygen battery development is hampered by degradation reactions initiated by superoxide, which is formed in the pathway of oxygen reduction to peroxide. This work demonstrates that the superoxide lifetime is drastically decreased upon addition of ethyl viologen, which catalyses the reduction of superoxide to peroxide.
RESUMEN
Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect (i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.
RESUMEN
Previous work demonstrated that the Frumkin isotherm is inadequate to model the competitive coadsorption of species with different saturation coverages, such as hydrogen and bromide coadsorption on Pt(100) [N. Garcia-Araez et al., J. Electroanal. Chem., 2006, 588, 1]. Therefore, Monte Carlo simulations were necessary to determine meaningful values of the microscopic parameters (namely, energies of adsorption and interaction). In the present work, an alternative analytical isotherm is developed, by taking into account the occupation of two sublattices, which together compose the whole lattice of adsorption sites. Despite its relatively simple mathematical form, this isotherm presents, under certain conditions, a significant improvement over the classical Frumkin isotherm for the modeling of competitive adsorption processes, thus providing a closer agreement with results from Monte Carlo simulations. Finally, it is demonstrated that the sublattice-model isotherm will be generally applicable to systems in which the formation of segregated adlayers, whose structure is not explicitly taken into account in the model, is energetically unfavorable.
RESUMEN
The effect of bismuth adsorption on the entropy of formation of the double layer on Pt(111) electrodes has been studied with the laser-induced temperature jump method. The coulostatic response to the temperature change induced by pulsed laser illumination allows the estimation of the sign and magnitude of the thermal coefficient of the potential drop at the interphase. This is related to the entropy of formation of the double layer, and the particular potential where this thermal coefficient becomes zero can be identified with the potential of maximum entropy of double-layer formation (pme). The effect of bismuth adsorption on the pme depends on the adatom coverage. At high coverages, a marked decrease of the pme is observed. This trend follows the change of the potential of zero charge expected from work function measurements, and it is likely due to the change in the orientation of solvent molecules induced by surface dipoles originated between the adatom and the substrate. At low coverage, the pme increases with the bismuth coverage. The disruption of the water structure due to the presence of the bismuth adatoms is tentatively proposed as the most likely explanation for this behavior.
