Aqueous Polysulfide Solutions Research Articles

Improved Formulation and Optimization of Sodium Polysulfide/Bromine Electrolytes for Redox Flow Battery

Sodium polysulfide/bromine RFBs are one the most promising electrolyte couples for scale-up technology. Sodium polysulfide/bromine RFB utilizes aqueous sodium polysulfide and sodium bromide solution as anolyte and catholyte, respectively, separated by a sodium ion exchange membrane.Polysulfide electrolyte of different concentrations ranging from S:Na ratio of 0.5 that is without the addition of elemental sulfur, up to saturation limit having S:Na ratio of 2.4 with the addition of elemental sulfur are prepared.Studies of electrolyte composition have been essential to establishing the optimum conditions for practical use. These involve various parameters such as sulfide concentration, the ratio of dissolved sulfur to sulfide, speciation, pH, and the effect of temperature on battery stability and lifetime. Different experimental methodologies were used to investigate the speciation in these concentrated solutions. A titration method was developed to find the concentration of HS- and OH- species in any sodium sulfide and polysulfide solution. Sulfide and polysulfide chemistry is presented in more detail by comparing the UV-ATR spectra. It was found that polysulfide electrolyte shows very complex system containing species such as HS-, OH-, H+ and Sn 2- where n = 2-5 and sulphur oxoanions.Furthermore, we have established that controlled speciation of polysulfide solution can substantially impact on the electrochemistry in these electrolytes. Controlling the pH, concentrations of HS- and OH- ions in polysulfide electrolytes is extremely important to overcome many challenges of the polysulfide/bromine RFB system.Therefore, we have developed an improved polysulfide electrolytes with high charge densities but more soluble species in the presence of excessive HS- species. Charging and discharging data on our improved and well optimized polysulfide electrolyte will be presented.

Continuous-Flow Synthesis of Thioureas, Enabled by Aqueous Polysulfide Solution.

We have developed the continuous-flow synthesis of thioureas in a multicomponent reaction starting from isocyanides, amidines, or amines and sulfur. The aqueous polysulfide solution enabled the application of sulfur under homogeneous and mild conditions. The crystallized products were isolated by simple filtration after the removal of the co-solvent, and the sulfur retained in the mother liquid. Presenting a wide range of thioureas synthesized by this procedure confirms the utility of the convenient continuous-flow application of sulfur.

Chromatography-Free Multicomponent Synthesis of Thioureas Enabled by Aqueous Solution of Elemental Sulfur.

The development of a new three‐component chromatography‐free reaction of isocyanides, amines and elemental sulfur allowed us the straightforward synthesis of thioureas in water. Considering a large pool of organic and inorganic bases, we first optimized the preparation of aqueous polysulfide solution from elemental sulfur. Using polysulfide solution, we were able to omit the otherwise mandatory chromatography, and to isolate the crystalline products directly from the reaction mixture by a simple filtration, retaining the sulfur in the solution phase. A wide range of thioureas synthesized in this way confirmed the reasonable substrate and functional group tolerance of our protocol.

Electrochemical Reactions and Failure Mechanism Study of Sodium‐Aqueous Polysulfide Conversion Reactions in Redox Flow Batteries

Low‐cost aqueous polysulfide (APS) chemistry has recently attracted intensive research as it is a promising candidate for redox flow batteries. However, the intrinsic properties, electrochemical stability, and compatibility with sodium super ionic conductor (NASICON) membrane have been rarely demonstrated to clarify the challenges on the road to practical application. Herein, a detailed study on the electrochemical reactions and failure mechanism of aqueous polysulfides is presented. Starting from investigating the intrinsic stability, the optimized concentration of NaOH is determined. Through constructing a hybrid Na‐APS cell to avoid the effect from the counterelectrode, the cycling behavior is found to be stable under the capacity cut‐off condition, while severe decay occurs after ≈80 cycles. Comprehensive analysis of the decay mechanism shows the decomposition and loss of active materials during the long‐term cycles. Meanwhile, the NASICON separator surface is found to be corrosive after electrochemical exposure with APS solution under a high‐pH environment. It is anticipated that the systematic study of the APSs’ electrochemical reactions and decay mechanism could be beneficial for their further application in redox flow batteries.

Band engineered ternary solid solution CdSxSe1−x-sensitized mesoscopic TiO2 solar cells

The optical band gap of the light absorber and the alignment of its bands with the underlying wide band gap metal oxide are critical for efficient light harvesting and charge separation in semiconductor-sensitized solar cells (SSCs). In practice, these two requirements are however not always fulfilled concomitantly in SSCs. Favourable band alignment in CdSe-sensitized TiO2 requires utilization of quantum sized CdSe, which causes great losses in the harvesting of long wavelength photons due to quantum confinement effects. In the present study, ternary cadmium sulfoselenide (CdSxSe1-x), which has a tunable band gap between those of CdSe and CdS without reducing the dimension, was proposed as a sensitizer for TiO2. CdSxSe1-x was successfully synthesized by alternately depositing CdS and CdSe layers under ambient conditions. SSCs utilizing CdSxSe1-x-sensitized TiO2 yielded a power conversion efficiency of 4.05% under simulated AM1.5 100 mW cm(-2) illumination, rivalling the well-studied cascaded CdS/CdSe electrodes when an aqueous polysulfide solution was used as the electrolyte and Cu2S as the counter electrode. The findings of the present study provide an alternative and viable approach for optimizing the energetics of semiconductor sensitizers for efficient charge separation, while also maintaining good light harvesting.

Raman spectroscopic determination of oxoanions in aqueous polysulfide electrolyte solutions

The equilibrium distribution of aqueous sodium polysulfide solutions is investigated over a wide concentration range using Raman spectroscopy technique. A total of eighteen bands were determined in the spectra recorded for aqueous electrolyte solutions of different S:Na ratios. Beyond the presence of four polysulfide (S n 2−) species, where “ n” varied between 2 and 5 as reported in previous literature, the originality of this study is to show the presence of oxidised sulfur species in these aqueous polysulfide solutions. A detailed comparison of all the bands for polysulfide and oxoanion species determined in this study with those cited in literature is summarised.

Equilibrium Distribution of Polysulfide Ions in Aqueous Solutions at Different Temperatures by Rapid Single Phase Derivatization

Polysulfides are abundant form of reduced sulfur compounds whose distribution in aquatic systems continues to pose environmental challenge. The Gibbs free-energy of formation, enthalpy of formation, and standard entropy of inorganic polysulfides were derived based on measurements of the temperature-dependent distribution of inorganic polysulfides in supersaturated aqueous polysulfide solutions. The data complements the relevant Gibbs free-energy data that were derived in our recent publication. The thermodynamic data show that the average polysulfide length is increased and polysulfides dissolve better at elevated temperatures, though the extent of this increase is pH dependent. At high pH (pH > 10) increasing the temperature from 25 to 80 degrees C results in a 5.6% increase in the concentration of polysulfide bound sulfur (i.e., dissolved zerovalent sulfur) and increases the average chain length (n) by 0.2 sulfur atoms, whereas at pH 8.2 the n increases by 0.25, and the dissolved polysulfide sulfur increases threefold.

Synthesis of nanostructured Cd–Se–Te films through periodic voltammetry for photoelectrochemical applications

In the present investigation, polycrystalline semiconductor Cd–Se–Te films have been electrodeposited at room temperature on conducting glass substrates using cyclic voltammetric technique under controlled periodic scans. The successive anodic and cathodic scans were recorded within the potential range, from 0 to −1.0 V, and over the range of periodic cycles, from 250 to 2,000, in an acidic bath containing respective reducible precursor ions like Cd2+, Se4+, Te4+, and 1 vol.% Triton X-100 as the surface-active reagent. Thin composite films were produced having variable thickness and composition and grain size of the order of 80–100 nm. The film properties were determined by focused ion beam analysis, energy dispersive analysis of x-rays, x-ray diffraction studies, atomic force microscopy, and scanning electron microscopy. Thickness of the semiconductor films was found to increase linearly with the number of voltammetric cycles. Band gap energies of the films as derived from the reflectance spectra were found to lie between 1.4 and 1.7 eV. The composite films of the Cd–Se–Te ternary system when electrochemically characterized in aqueous polysulfide solution exhibited n-type semiconducting properties and photoconversion efficiency more than 0.4%.

Kinetics of the chemical oxidation of polysulfide anions in aqueous solution

The kinetic properties of the chemical oxidation of aqueous polysulfide solutions have been studied in phosphate-buffered systems at pH 7–12, at temperatures between 20 and 40 °C, and ionic strength between 0.05 and 0.50 M. Polysulfide solutions were mixed with a buffer solution of known dissolved oxygen concentration, after which the decrease in the oxygen concentration of the solution was measured in time. The rate of oxygen consumption can be described by the empirical relation d [ O 2 ] / d t = - k [ S x 2 - ] [ O 2 ] 0.59 . The reaction rate constant k is moderately dependent on pH and goes through a maximum at pH 10. The rate of oxygen consumption for polysulfide solutions is approximately four times higher than for sulfide solutions. At pH values below 9, reaction products were formed according to S x 2 - + 3 / 2 O 2 → S 2 O 3 2 - + ( x - 2 ) S 0 . At pH values higher than 9, more thiosulfate and additional sulfide were formed, which is attributed to the low chemical stability of the sulfur of oxidation state zero, formed upon polysulfide oxidation. Our results strongly suggest that hydrolysis of this ‘nascent’ elemental sulfur to HS − and S 2O 3 2− is faster than hydrolysis of crystalline inorganic sulfur or colloidal particles of biologically produced sulfur, and has a significant contribution already at 30 °C and pH 10.

Aluminum Anodic Behavior in Aqueous Sulfur Electrolytes

We report on an unexpected domain of high Coulombic efficiency for electrochemical oxidation of aluminum in aqueous polysulfide solutions at high current density for the process: Al + 3OH- → Al(OH)3 + 3e-. This high-efficiency domain, of importance to battery processes, includes aluminum oxidation in a wide range of solutions containing concentrated dissolved zerovalent sulfur. As expected at lower concentrations of dissolved sulfur, aluminum electrochemical oxidation is inefficient, due to various exothermic parasitic reactions, including: Al(c) + ySx2-(aq) + yH2O(l) ⇄ 1/2Al2S3(c) + yOH-(aq) + yHS-(aq), y = 1.5/(x − 1), and Al(c) + 1.5S22-(aq) + 3H2O(1) ⇄ Al(OH)3(amorph) + 3HS-(aq). However, at high polysulfide and sulfur concentrations, the Coulombic efficiency can approach 100%. This domain of high efficiency is correlated to an observed cathodic shift with increasing sulfur concentration, leading to improved chemical passivation at the aluminum surface.

Disproportionation of Aqueous Sulfur and Sulfide: Kinetics of Polysulfide Decomposition

Thermodynamically, aqueous polysulfide solutions are unstable, and sulfur, dissolved as polysulfide species, Sx2-, x = 2−5, will decompose to thiosulfate, S2O32- in accord with the net consumption ...

Synthesis and characterization of a novel oxo-disulfidotetravanadate(V) anion with a bridging tetrasulfido ligand [{V(O)(S2)2}2(µ2-S4){V(O)(S2)2}2]6–

NH4[VO3] reacts with aqueous polysulfide solutions in the presence of 4,4′-bipyridine to yield the tetranuclear anion [V4S20O4]6– which contains four bipyramids connected via a zigzag S42– bridge.

The interface chemistry of HgCdTe passivated with native sulfide layers grown from nonaqueous and aqueous polysulfide solutions

We have grown anodic sulfide passivation layers on HgCdTe from both nonaqueous and aqueous polysulfide solutions. In both cases CdS layers are nominally obtained. The growth behavior of the sulfide layers is found to be quite different in the different electrolytes. Capacitance–voltage measurements on metal-insulator semiconductor device structures that incorporated a ZnS cap and Pd gate metal over the CdS have been used to compare the electrical properties of the interfaces produced. Preliminary evidence suggests that aqueous anodic sulfide layers may be more stable than nonaqueous ones, but contain more positive fixed charge and mobile ion charge. Auger electron spectroscopy depth profiles indicate that the aqueous anodic sulfides are oxide free, but appear to contain more Hg and Te contamination than nonaqueous anodic sulfides.

Response to “Comment on ‘Licht's Comment on Kinetics of Aqueous Polysulfide Solutions, Part II’” [J. Electrochem. Soc., 133, 2517

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Response to “Comment on ‘Licht's Comment on Kinetics of Aqueous Polysulfide Solutions, Part I’” [J. Electrochem. Soc., 133, 2510

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Comment on “Kinetics of Aqueous Polysulfide Solutions” [J. Electrochem. Soc., 133, 2517]: II . Electrochemical Measurement of the Rates of Coupled Electrochemical and Chemical Reactions by the Potential Step Method

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Response to “Comment on ‘Kinetics of Aqueous Polysulfide Solutions’” [J. Electrochem. Soc., 133, 2517]: II . Electrochemical Measurement of the Rates of Coupled Electrochemical and Chemical Reactions by the Potential Step Method

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Response to “Comment on ‘Kinetics of Aqueous Polysulfide Solutions’” [J. Electrochem. Soc., 133, 2510]: I . Theory of Coupled Electrochemical and Chemical Reactions, Response to a Potential Step

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Kinetics of Aqueous Polysulfide Solutions: Part III . Investigation of Homogeneous and Electrode Kinetics by the Rotating Disk Method

The steady‐state current density‐overpotential relationships for cobalt and platinum rotating disk electrodes in aqueous polysulfide electrolyte (1.3M total S2−, 1M S0, and ) were determined over a temperature range of 25°–77°C. A model developed to take into account the rate of the homogeneous dissociation of to as well as a finite heterogeneous electrochemical reaction rate yielded parameters consistent with the results of previous transient experiments (in the low overpotential region). At higher overpotentials in the cathodic region a second cathodic reaction was evident. This second cathodic reaction was less prominent when the current densities were lower (as on Pt electrodes) or when the concentrations were higher (as temperature increased). The addition of 13 mole percent dimethylformamide (DMF) to the aqueous polysulfide solution was found to increase significantly the current density, at a given temperature and overpotential, by increasing the equilibrium concentration of . In the range of overpotentials studied, additions of DMF also caused the first cathodic reaction wave to become much more prominent. Although independent quantitative data are lacking for the DMF‐water mixtures, the use of reasonable values for and allows the quantitative prediction of the observed cathodic behavior.

Kinetics of Aqueous Polysulfide Solutions: II . Electrochemical Measurement of the Rates of Coupled Electrochemical and Chemical Reactions by the Potential Step Method

The kinetics of the aqueous polysulfide redox couple were investigated using a potential step technique in a solution 1M in S0 and 1.3M in total S2−, at pH 12. The low overpotential region (−20 to +20 mV) on cobalt and platinum electrodes was investigated from 25° to 80°C. The active species concentration was found to be several orders of magnitude lower than those of any of the major (poly)sulfide ions, and its temperature dependence corresponded to an of about 10 kcal/ mol. These results are consistent, with the active species being . Deviations were found at temperatures above 45°C between experimental data and the simple analytical form for the current response for an electrode reaction with one rate‐determining step. The results agree with a new model that considers two rate‐determining steps, one of which is the reaction . The rate constants for this reaction determined using this new model agree with those measured for similar reactions.