There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.
et al.2 studied the kinetics of °2 reduction on Pt electrodes covered with thin films of solid polymer [recast polybenzimidazole (PBI) or Nafion] in 0_i M aqueous solutions of H3P04, H2S04, and HC1O4. These authors determined the ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-04-12 to IP
The kinetics of 0, reduction on both oxidized and reduced Pt rotating disk surfaces covered with thin films (1 to 15 m) of recast polybenzimidazole (PBI) or Nafion have been examined in 0.1 M aqueous solutions of HPO 4 , H,SO 4 , and HC10 4 at 25.0'C. Polymer films were deposited on a Pt disk electrode by a spin-coating technique. Experiments were performed at different rotation rates and potential scan rates. This investigation was undertaken to establish to what extent the solid polymer electrolyte influences the mechanism and kinetics of 0, reduction. The number of electrons exchanged per 0, molecule and kinetic parameters for 0, reduction (reaction order with respect to 02, Tafel slope/transfer coefficient, and exchange current density) are not changed in the presence of the polymer films, indicating that the reaction path and kinetics are not altered in the presence of recast PBI and Nafion films. The diffusion coefficient (D) and concentration (C) of 02 in recast films were determined by means of linear sweep voltammetry measurements in quiescent solution. The D and C values in polymer films immersed in acidic solutions were found to have different values than those in the solutions. D(O,) is about 2.5 times lower in the recast films than in the solutions, whereas C(O,) is slightly higher in the film. The largest concentration increase, by a factor of two, was found in PBI film in 0.1 M HC10 4 . Cyclic voltammetric measurements under argon atmosphere have shown that PBI and Nafion recast films are electrochemically stable in the potential range between the onsets of H, and 0, evolution. A decrease in the effective electrocatalyst surface area due to deposition of solid polymer films was 15 to 20%.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see ABSTRACT Niobium oxide electrochromic thin films were prepared by reactive dc magnetron sputtering and their electrochromic properties for Li intercalation and durability were studied. Chronoamperometric analyses revealed that the extended space-charge limited model by Zhang et al. is applicable to Nb 2 O 5 films. Crystallized Nb 2 O films showed excellent electrochromism and stability over many coloration-bleaching cycles. The best performance was obtained for films with a substrate temperature of 500°C and an oxygen flow rate of 10 seem.
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