Covalently Interlinked Graphene Sheets with Sulfur-Chains Enable Superior Lithium–Sulfur Battery Cathodes at Full-Mass Level

Sulfur represents a low‑cost, sustainable, and high theoretical capacity cathode material for lithium–sulfur batteries, which can meet the growing demand in portable power sources, such as in electric vehicles and mobile information technologies. However, the shuttling effect of the formed lithium polysulfides, as well as their low conductivity, compromise the electrochemical performance of lithium–sulfur cells. To tackle this challenge, a so far unexplored cathode, composed of sulfur covalently bonded directly on graphene is developed. This is achieved by leveraging the nucleophilicity of polysulfide chains, which react readily with the electrophilic centers in fluorographene, as experimental and theoretical data unveil. The reaction leads to the formation of carbon–sulfur covalent bonds and a particularly high sulfur content of 80 mass%. Owing to these features, the developed cathode exhibits excellent performance with only 5 mass% of conductive carbon additive, delivering very high full‑cathode‑mass capacities and rate capability, combined with superior cycling stability. In combination with a fluorinated ether as electrolyte additive, the capacity persists at ≈700 mAh g⁻¹ after 100 cycles at 0.1 C, and at ≈644 mAh g⁻¹ after 250 cycles at 0.2 C, keeping ≈470 mAh g⁻¹ even after 500 cycles.     

Kinetics of polyesterification and models, 3. Kinetics and mechanism of pentaerythritol esterification by long-chain carboxylic acids at technological conditions

Kinetics of the esterification of pentaerythritol by long-chain carboxylic acids (C₇ – C₁₈) has been followed by potentiometric titrimetry. A simplified approach for the estimation of the rate constant values and the rate constants of the consecutive processes was developed according to the previous established kinetic law for similar systems. A computer simulation of the component concentrations variation has been carried out in order to prove the selected kinetic model.     

Oxidation of multiwalled carbon nanotubes by nitric acid

The oxidation of MWCNTs in nitric acid was monitored using sample weight, Raman spectrum, solubility, morphology and alignment. The influence of the acid concentration, temperature and oxidation duration on the monitored parameters was assessed. A new method, based on optical microscopy is proposed for the determination of MWCNT solubility in concentrated aqueous-suspensions. The investigations revealed that the solubility is determined not only by the functional groups on the MWCNT, but also by the functionalized amorphous carbon generated during the digestion of the nanotubes. High solubility (20–40 mg/ml) is obtained only after prolonged exposure (24–48 h) in concentrated acid (60%). But in these conditions 60–90% of the MWCNTs are lost. Furthermore the MWCNTs are strongly fragmented and covered by amorphous carbon after 48 h of oxidation. It was found that the solubility correlates well with the area ratio of the G and D bands from the Raman spectrum. SEM examination of the MWCNT films showed extended alignment after 24 h of oxidation.     

Hybrid Coordination Networks for Removal of Pollutants from Wastewater

The adsorption properties of two coordination polymers, resulting from the reaction of divalent metal (Ca²⁺ or Co²⁺) salts with (2-carboxyethyl)(phenyl)phosphinic acid, are presented in this paper. The structural and textural characterization before and after adsorption experiments is presented. The adsorbent materials were prepared using the hydrothermal procedure. The compound Ca[O₂P(CH₂CH₂COOH)(C₆H₅)]₂ (CaCEPPA) has a layered topology, with the phenyl groups oriented into the interlayer space and crystallizes in the monoclinic system. Compound Co₂[(O₂P(CH₂CH₂COO)(C₆H₅)(H₂O)]₂·2H₂O (CoCEPPA) has a 1D structure composed of zig-zag chains. The adsorption performances of CaCEPPA and CoCEPPA materials were tested in the removal of cadmium and lead from aqueous solutions. The optimum pH of ions adsorption was found to be five for both adsorbent materials. Pseudo-first and second-order kinetic models were used for fitting kinetic experimental data, and Langmuir and Freundlich isotherms were used for modeling the equilibrium experimental data. The pseudo-second-order kinetic model and Langmuir isotherm best described the adsorption of Cd and Pb ions onto the studied materials, judging from the results of the error function (correlation coefficient, sum of square error, chi-square test, and average relative error) analysis. The studied materials present a higher affinity for Cd ions compared with the adsorption capacity developed for the removal of Pb ions from aqueous solutions. CoCEPPA showed the highest adsorption performance in the removal process of metal ions from aqueous solutions compared with CaCEPPA (q_m = 54.9 mg Cd²⁺/g of CoCEPPA, q_m = 36.5 mg Cd²⁺/g of CaCEPPA).     

Optical determination of differential coverage-potential relations of redox-active species immobilized on electrode surfaces

Single-wavelength (HeNe laser, lambda = 633 nm), normal incidence UV-visible reflectance spectroscopy was used to monitor the optical properties of the glassy carbon (GC)|0.2 M NaOH(aq) interface as a function of the applied potential, E. Whereas the electroreflectance coefficient for bare GC was found to be small and potential independent, surface functionalization by an irreversibly adsorbed layer of tetrasulfonated cobalt phthalocyanine (CoTSPc) yielded a clearly defined sigmoidally shaped normalized reflectance change (DeltaR/R) vs E curve over the potential region in which the adsorbate displayed redox peaks. Assuming DeltaR/R is proportional to the extent of redox conversion, as has been reported for macrocycles adsorbed on other types of carbon (e.g., Kim, S.; Xu, X.; Bae, I. T.; Wang, Z.; Scherson, D. A. Anal. Chem. 1990, 62, 2647-2650), differential coverage-potential relations were determined based purely on the optical data collected. A similar optical behavior was found for irreversible adsorbed CoPc and tetraamino CoPc (CoTAPc) adsorbed on GC, for which the voltammetric peaks were ill-defined, too small for coulometric analyses to be reliably performed, or both. No detectable changes in the DeltaR/R vs E profiles of either bare or macrocyclic-functionalized surfaces were observed upon addition of hydrazine to the neat 0.2 M NaOH solution at potentials at which these surfaces display electrocatalytic properties for its oxidation. Possible factors responsible for this behavior are discussed.     

Statistics of specular points at a randomly rough surface

Statistical parameters are obtained for an ensemble of specular points at a randomly rough Gaussian statistically isotropic surface at normal incidence. The joint probability density functions (PDFs) of specular point heights and total curvatures are derived separately for maxima, minima, and saddle points. The joint PDFs of brightness and surface elevations of specular points of different types are obtained analytically in an explicit form.     

Automata-based verification of programs with tree updates

This paper describes a verification framework for Hoare-style pre- and post-conditions of programs manipulating balanced tree-like data structures. Since the considered verification problem is undecidable, we appeal to the standard semi-algorithmic approach in which the user has to provide loop invariants, which are then automatically checked, together with the program pre- and post-conditions. We specify sets of program states, representing tree-like memory configurations, using Tree Automata with Size Constraints (TASC). The main advantage of this new class of tree automata is that they recognise tree languages based on arithmetic reasoning about the lengths of various (possibly all) paths in trees, like, e.g., in AVL trees or red–black trees. TASCs are closed under union, intersection, and complement, and their emptiness problem is decidable. Thus we obtain a class of automata which are an interesting theoretical contribution by itself. Further, we show that, under few restrictions, one can automatically compute the effect of tree-updating program statements on the set of configurations represented by a TASC, which makes TASC a practical verification tool. We tried out our approach on the insertion procedure for red–black trees, for which we verified that the output on an arbitrary balanced red–black tree is also a balanced red–black tree.     

Phonetic segmentation using multiple speech features

In this paper we propose a method for improving the performance of the segmentation of speech waveforms to phonetic units. The proposed method is based on the well known Viterbi time-alignment algorithm and utilizes the phonetic boundary predictions from multiple speech parameterization techniques. Specifically, we utilize the most appropriate, with respect to boundary type, phone transition position prediction as initial point to start Viterbi time-alignment for the prediction of the successor phonetic boundary. The proposed method was evaluated on the TIMIT database, with the exploitation of several, well known in the area of speech processing, Fourier-based and wavelet-based speech parameterization algorithms. The experimental results for the tolerance of 20 milliseconds indicated an improvement of the absolute segmentation accuracy of approximately 0.70%, when compared to the baseline speech segmentation scheme.     

Two-stage phone duration modelling with feature construction and feature vector extension for the needs of speech synthesis

We propose a two-stage phone duration modelling scheme, which can be applied for the improvement of prosody modelling in speech synthesis systems. This scheme builds on a number of independent feature constructors (FCs) employed in the first stage, and a phone duration model (PDM) which operates on an extended feature vector in the second stage. The feature vector, which acts as input to the first stage, consists of numerical and non-numerical linguistic features extracted from text. The extended feature vector is obtained by appending the phone duration predictions estimated by the FCs to the initial feature vector. Experiments on the American-English KED TIMIT and on the Modern Greek WCL-1 databases validated the advantage of the proposed two-stage scheme, improving prediction accuracy over the best individual predictor, and over a two-stage scheme which just fuses the first-stage outputs. Specifically, when compared to the best individual predictor, a relative reduction in the mean absolute error and the root mean square error of 3.9% and 3.9% on the KED TIMIT, and of 4.8% and 4.6% on the WCL-1 database, respectively, is observed.