Aqueous Polysulfide Research Articles

High-Valent Thiosulfate Redox Electrochemistry for Advanced Sulfur-Based Aqueous Batteries.

Sulfur-based aqueous batteries (SABs) are promising for safe, low-cost, and high-capacity energy storage. However, the low output voltage of sulfur cannot meet the demands of high-energy cathode applications due to its intrinsic negative potential (E0 = -0.51 V vs SHE) of low-valent polysulfide redox (S2-/S0). Here, instead of relying on traditional aqueous polysulfide redox, for the first time, we demonstrate a high-valent thiosulfate redox (S2O32-/S4O62-) electrochemistry, exhibiting positive redox potential (E0 > 0 V vs SHE) and reversible cation storage in aqueous environment. Operando X-ray absorption fine structure spectroscopy, in situ Raman spectroscopy, and density functional theory calculations reveal the high reversibility and dynamic charge transfer process of high-valent thiosulfate redox. Significantly, the aqueous thiosulfate redox exhibits a high operating voltage of approximately 1.4 V, a reversible capacity of 193 Ah L-1, and a long cycling life of over 1000 cycles (99.6% capacity retention). This work provides new insights into the high-valent S-based electrochemistry and opens a new pathway to achieve energetic aqueous batteries.

An aqueous polysulfide redox flow battery with a semi-fluorinated cation exchange membrane

An economically viable thermally annealed cation exchange membrane for polysulfide-based redox flow batteries.

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.

Quantification of Polysulfide Species in Aqueous Sulfur Thermocell

Polysulfide is one of the candidates for the redox pairs for thermocell devices; however, its aqueous solution has never been investigated. Polysulfides are in complex equilibria in an aqueous system, and the open-circuit potential of the solution is made of their mixed potential; thus, the dominant redox equilibrium determines its thermoelectric property. In this paper, the aqueous polysulfide thermocell was fabricated, and the underlying mechanism of the polysulfide thermocell was investigated by electrochemical studies, UV-vis spectroscopy at various temperatures, and thermodynamic calculations.

Spectroelectrochemistry of next-generation redox flow battery electrolytes: A survey of active species from four representative classes

The spectroelectrochemistry of five active electrolyte species from four representative classes of compounds used in aqueous, near-neutral pH redox flow batteries (RFBs) is reported. The electrolytes include two anolytes, di-sulfonatopropyl viologen ((SPr)2V) and polysulfide (PS), and three catholytes, the nitroxyl radical 4-hydroxy-TEMPO (TEMPO-OH), ammonium-substituted ferrocene ([FcN2]2+), and iron 2,2′-tris-bipyridine ([Fe(bpy)3]2+). Time-resolved UV–vis absorbance spectra monitor redox transformations of these species in situ during cyclic voltammetry and chronoamperometry experiments. Molar attenuation coefficients at peak wavelengths are reported for both redox forms, representing the charged and discharged species in RFBs. The method is validated using the well-studied ferr-cyanide redox couple, also with relevance to RFBs. Anomalous and as-yet unreported observations on the reduced viologen species implicate formation of a pimer complex and new insights are presented on aqueous polysulfide speciation, both with the aid of time-dependent density functional theory (TD-DFT) calculations. The findings are applicable fundamentally – most of the charged-form spectral data have not been reported – as well as practically, for the in and ex situ monitoring of electrolyte chemistry to track performance, state of charge, and degradation in redox flow batteries.

Electrochemical Stability and Reversibility of Aqueous Polysulfide Electrodes Cycled Beyond the Solubility Limit

Batteries which use dissolved redox-active species, such as redox flow batteries (RFBs), are often considered to be constrained in their operation and energy density by the solubility limit of the redox species. Here, we show that soluble redox active electrolytes can be reversibly cycled deeply into the precipitation regime, permitting higher effective concentrations, energy densities, and lower costs. Using aqueous sodium polysulfide negative electrolytes cycled in the nominal Na2S2 to Na2S4 capacity range as an example, we show that the effective solubility can be increased from 5 M in the fully-dissolved state to as much as 10 M using the precipitation strategy. Stable cycling was observed at 8 M concentration over more than 1600h at room temperature. We also analyze the range of polysulfide electrochemical stability, and characterize the precipitate composition. This enhanced effective concentration approach may be generalized to other redox chemistries that utilize solubilized reactants, and may be especially useful for long-duration storage applications where slow charge-discharge rates allow equilibration of precipitated species with the redox-active solution.

Atomic-level insights into the activation of nitrogen via hydrogen-bond interaction toward nitrogen photofixation

Summary As the activation of N2 molecules is generally regarded as the bottleneck in N2 reduction, it is of significant importance to develop promising strategies to efficiently activate N2 molecules. Herein, we discovered an innovative approach to activate N2 molecules via hydrogen-bond interaction in N2 photofixation. Porous Cu with surface modification of S atoms (3%-S/Cu) exhibited remarkable catalytic activity and stability in N2 photofixation. Further mechanistic studies revealed that surface S atoms directly participated in the catalytic process by accepting and donating H atoms. In addition, the forming S–H bond significantly promoted the activation of N2 molecules via hydrogen-bond interaction, contributing to the enhanced catalytic activity.

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.

Nano-decocted ferrous polysulfide coordinates ferroptosis-like death in bacteria for anti-infection therapy

Abstract Antibacterial nanomaterials provide promising alternative strategies to combat the global challenge of bacterial infection due to deteriorating resistance. However, few have been applied for intracellular bacteria killing or in vivo therapy due to potential cytotoxicity and poor biocompatibility. Here we present a strategy to generate formula suitable for in vivo anti-infective therapy by decocting antibacterial nanomaterial. An aqueous formula containing ferrous iron and polysulfide (Fe(II)Snaq) is prepared by a decoction procedure using nano-iron sulfide (nFeS) as raw substance. Theoretical calculation indicated that replacement of a sulfur atom by an oxygen atom occurred, resulting in polysulfide release and iron dissolution. The Fe(II)Snaq decocted from nFeS induced bacterial death with ferroptosis-like hallmarks, including iron enrichment, lipid peroxidation, and glutathione (GSH) depletion. The antibacterial action of Fe(II)Snaq was dependent on ferrous iron, which could be inhibited by iron chelators, ferroptosis inhibitors, and GSH, while the polysulfide prevented ferrous iron oxidation and counteracted GSH. Furthermore, Fe(II)Snaq not only killed up to 99 % of planktonic bacteria within 5 min, but also suppressed up to 90 % of intracellular Staphylococcus aureus without triggering cytotoxicity to host cell. Administration of Fe(II)Snaq achieved equivalent therapeutic effect to vancomycin for infectious pneumonia treatment and significantly prolonged the survival period of septic mice. These results indicate that aqueous ferrous polysulfide can induce ferroptosis- like death in bacteria and may be formulated as an antibacterial alternative for anti-infection therapy.

Control of the interfacial charge transfer resistance to improve the performance of quantum dot sensitized solar cells with highly electrocatalytic Cu-doped SnS counter electrodes

High charge transfer resistance at the interface between counter electrodes (CEs) and sulfide/polysulfide based electrolytes is responsible for the low photoelectrochemical performance of quantum dot sensitized solar cells (QDSSCs). Here, for the first time, we fabricated highly electrocatalytic and stable Cu-doped SnS CEs by inexpensive chemical bath deposition (CBD) method, to control the interfacial charge transfer resistance in QDSSCs and increase their photovoltaic performance. TiO2/CdS/CdSe/ZnS photoanodes and an aqueous polysulfide electrolyte were used to fabricate the QDSSCs with CEs. The QDSSCs with a 10% Cu-doped SnS CE achieved an outstanding conversion efficiency of 4.07% under standard simulated AM 1.5 illumination, exceeding that of a cell with an undoped SnS CE (2.80%). The surface morphologies of SnS CEs with different Cu contents were significantly different. The voids between SnS nanoparticles deposited on fluorine-doped tin oxide (FTO) substrate were reduced upon the uniform distribution of Cu dopants. The Cu-doped SnS CE had significantly improved electrocatalytic activity for catalysis during the reduction of electrolyte, low charge transfer resistance at the CE/electrolyte interface, and fast electron transport from SnS surface to the electrolyte to regenerate the redox couple. These findings may inspire the design of efficient CEs for the large-scale production of next-generation QDSSCs.

Development of low-cost sodium-aqueous polysulfide hybrid batteries

There has been increasing interest in sodium-sulfur (Na-S) batteries as an option for low-cost grid-scale energy storage. However, traditional Na-S batteries operate at high temperatures, raising concerns about long-term maintenance costs and safety. On the other hand, room-temperature Na-S batteries have their own limitations, including Na dendrite formation, polysulfide shuttling, and low utilization of active material. To overcome these issues, we present here a novel low-cost room-temperature sodium-aqueous polysulfide (Na-APS) hybrid battery system with a Na-metal anode, Na+-ion solid electrolyte separator, and an aqueous polysulfide catholyte. The solid electrolyte blocks Na dendrites and polysulfide shuttling while simultaneously protecting the reactive Na metal anode from the aqueous catholyte. The redox kinetics of the APS catholyte is improved with a rational development of a freestanding CuS-CNT catalytic electrode. The improvements with the Na-APS hybrid battery compared to the traditional Na-S battery systems are discussed. The Na-APS hybrid battery displays excellent performance under a 418 mA h g−1 of sulfur capacity-restricted cycling. The battery achieves an energy efficiency of 90% over 100 cycles at 0.5 mA cm−2 current density.

Aqueous Polysulfide–Air Battery with a Mediator-Ion Solid Electrolyte and a Copper Sulfide Catalyst for Polysulfide Redox

Aqueous sodium polysulfide–air batteries are a promising system for enabling low-cost, high energy density, environmentally benign batteries for grid-scale deployment. However, the sluggish redox kinetics of aqueous polysulfide requires the use of a catalyst. Cobalt sulfide is a traditional catalyst with good activity and stability, but it uses expensive and toxic cobalt. This study demonstrates for the first time the long-term cycling performance of a sodium polysulfide–air battery with a low-cost copper sulfide catalyst for polysulfide redox. Alkaline sodium polysulfide anolyte is paired with an acidic sodium phosphate buffered air catholyte by means of a Na+-ion solid-state electrolyte, which eliminates chemical crossover between the two electrodes while serving as a mediator-ion solid electrolyte. Long-term cycling is achieved by means of a decoupled charge and discharge air electrode. Furthermore, the effect of CuS redox activity in the polysulfide–air voltage window is tested by pre-discharging CuS ...

In-situ tracking of NaFePO4 formation in aqueous electrolytes and its electrochemical performances in Na-ion/polysulfide batteries

In-situ formation of pure olivine NaFePO4 from chemically synthesized LiFePO4 nanoparticles via an electrochemical ion-exchange route in sodium salt containing aqueous electrolyte is reported. Both in-situ electrochemical quartz crystal microbalance (EQCM) and in-situ X-ray diffraction (XRD) measurements are performed to monitor the formation of NaFePO4. Subsequently, a rechargeable Na-ion aqueous polysulfide battery is demonstrated where NaFePO4 and dissolved Na2S5 solution are used as cathode and anolyte, respectively; and are separated from each other by an ion-exchange polymeric membrane. In order to prevent diffusion of Na2S5 polysulfide from the anode to the cathode side, salt concentration at both sides of the NaFePO4||Na2S5 full cell is finely tuned resulting a 45 mAh g−1 cycling capacity over 200 cycles.

Electrochemical Stability and Reversibility upon Cycling of Aqueous Polysulfide Electrodes Beyond the Solubility Limit

The low cost, high solubility, and high specific capacity of aqueous polysulfide electrodes are attractive in battery applications. Our group recently developed a redox flow battery system that used an air (oxygen) cathode and an aqueous polysulfide anode, aiming at low-cost large scale grid storage[1]. Surprisingly, for low cost electrochemically active materials such as aqueous polysulfide, the cost of auxiliary components including the supporting electrolytes and the tanks is ~2 times that of the active materials themselves. Accordingly, to further decrease the energy cost, it is imperative to increase the volumetric capacity (Ah/L) of aqueous polysulfide electrolytes. One natural way to increase the capacity of the redox active solution is to increase the “effective” concentration of the active species, which is bounded by the solubility of the active species. However, the solubility of aqueous polysulfides in the reduced form (Na2S2) at room temperature is ~2.5M (5M total sulfur), equivalent to 67 Ah/L when utilizing capacity range between Na2S2 and Na2S4. One strategy to overcome this limitation is to leverage the redox activity of the precipitated polysulfide hydrate (Na2S2 xH2O) during the reduction reaction, which subsequently can be oxidized to the fully soluble higher-order polysulfide (Na2S4). Using this approach, we doubled the capacity to 134 Ah/L by using 10M total sulfur in the reversible cycling between Na2S2 and Na2S4. Long-term stability of this “precipitation-solution” conversion is studied using galvanostatic cycling for more than 1600 hours. In addition, XPS and SEM are utilized to measure the overall oxidation state of the precipitates and to understand the fast dissolution of the polysulfide hydrate. The overall oxidation state of the precipitates is particularly important in inferring the stability of the polysulfide species remained in the solution as the overall nominal solution composition needs to remain between Na2S2 to Na2S4 to avoid capacity fading associated with chemical decays. Furthermore, aqueous polysulfide using carbon suspension electrodes is demonstrated to alleviate large overpotential associated with insulating precipitate formation. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. [1] Li et al., Joule 1, 306-327 (2017).

Rechargeable Zinc-Aqueous Polysulfide Battery with a Mediator-Ion Solid Electrolyte.

Large-scale energy storage for the electric grid will require low-cost and high-energy-density solutions. We demonstrate in this letter a rechargeable zinc-aqueous polysulfide battery in which a metallic zinc anode is separated from a liquid aqueous polysulfide catholyte by an alkali-metal-ion solid-state electrolyte, wherein the alkali metal ion is Na+ or Li+. The solid-state electrolyte effectively separates the anode from the reactive polysulfide catholyte, preventing the crossover of the polysulfide species and maintaining good electrochemical performance with a reversible discharge capacity of 822 mA h g-1 with nearly 100% Coulombic efficiency.

Arrested precipitation assembly of nanosheets Cu2ZnCd (S, Se)3 thin film for solar cell performance: Novel skilful synthesis

In the present investigation, a high-quality nanosheet-like hexagonal Cu2ZnCd (S, Se)3 thin film has been synthesized via facile, simple and cost effective arrested precipitation technique (APT) through the complex stability, and release of metal ions. The surface morphology, chemical, optical, compositional and electrochemical properties of the resultant Cu2ZnCd (S, Se)3 thin film studied systematically. X-ray photoelectronic results show that stoichiometric composition. EIS reveals low value of resistance, power conversion efficiency of photoanode obtained was 0.62% in an aqueous polysulfide electrolyte.

Quantum dot sensitized solar cell based on TiO2/CdS/Ag2S heterostructure

Quantum dot sensitized solar cell (QDSSC) is fabricated based on a stepwise band structure of TiO2/CdS/Ag2S to improve the photoconversion efficiency of TiO2/CdS system by incorporating a low band gap Ag2S QDs. Vertically aligned TiO2 nanorods assembly is prepared by a simple hydrothermal technique. The formation of CdS and Ag2S QDs over TiO2 nanorods assembly as a photoanode is carried out by successive ionic layer adsorption and reaction (SILAR) technique. The synthesized electrode materials are characterized by XRD, XPS, field emission scanning electron microscopy (FE-SEM), Optical, solar cell and electrochemical performances. The results designate that the QDs of CdS and Ag2S have efficiently covered exterior surfaces of TiO2 nanorods assembly. A cautious evaluation between TiO2/CdS and TiO2/CdS/Ag2S sensitized cells tells that CdS and Ag2S synergetically helps to enhance the light harvesting ability. Under AM 1.5G illumination, the photoanodes show an improved power conversion efficiency of 1.87%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 7.03 mA cm−2 which is four fold higher than that of a TiO2/CdS system.

Chemical synthesis of CdS onto TiO2 nanorods for quantum dot sensitized solar cells

A quantum dot sensitized solar cell (QDSSC) is fabricated using hydrothermally grown TiO2 nanorods and successive ionic layer adsorption and reaction (SILAR) deposited CdS. Surface morphology of the TiO2 films coated with different SILAR cycles of CdS is examined by Scanning Electron Microscopy which revealed aggregated CdS QDs coverage grow on increasing onto the TiO2 nanorods with respect to cycle number. Under AM 1.5G illumination, we found the TiO2/CdS QDSSC photoelectrode shows a power conversion efficiency of 1.75%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.04 mA/cm2 which is higher than that of a bare TiO2 nanorods array.