Establishing reaction networks in the 16-electron sulfur reduction reaction.

The sulfur reduction reaction (SRR) plays a central role in high-capacity lithium sulfur (Li-S) batteries. The SRR involves an intricate, 16-electron conversion process featuring multiple lithium polysulfide intermediates and reaction branches1-3. Establishing the complex reaction network is essential for rational tailoring of the SRR for improved Li-S batteries, but represents a daunting challenge4-6. Herein we systematically investigate the electrocatalytic SRR to decipher its network using the nitrogen, sulfur, dual-doped holey graphene framework as a model electrode to understand the role of electrocatalysts in acceleration of conversion kinetics. Combining cyclic voltammetry, in situ Raman spectroscopy and density functional theory calculations, we identify and directly profile the key intermediates (S8, Li2S8, Li2S6, Li2S4 and Li2S) at varying potentials and elucidate their conversion pathways. Li2S4 and Li2S6 were predominantly observed, in which Li2S4 represents the key electrochemical intermediate dictating the overall SRR kinetics. Li2S6, generated (consumed) through a comproportionation (disproportionation) reaction, does not directly participate in electrochemical reactions but significantly contributes to the polysulfide shuttling process. We found that the nitrogen, sulfur dual-doped holey graphene framework catalyst could help accelerate polysulfide conversion kinetics, leading to faster depletion of soluble lithium polysulfides at higher potential and hence mitigating the polysulfide shuttling effect and boosting output potential. These results highlight the electrocatalytic approach as a promising strategy for tackling the fundamental challenges regarding Li-S batteries.

Development of flow battery technologies using the principles of sustainable chemistry.

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

Eutectic Electrolytes as a Promising Platform for Next-Generation Electrochemical Energy Storage.

ConspectusThe rising global energy demand and environmental challenges have spurred intensive interest in renewable energy and advanced electrochemical energy storage (EES), including redox flow batteries (RFBs), metal-based rechargeable batteries, and supercapacitors. While many researchers focus on the design of new chemistry and structures for high-capacity and stable electrode materials, the electrolyte also plays a significant role in enabling the successful function of these new electrode materials and chemistries. Discovery of new electrolytes is urgently needed to keep up with the rapid growth of EES. Benefiting from the strong intermolecular interaction between different components, eutectic electrolytes possess various specific functionalities that conventional electrolytes do not have, such as highly concentrated systems, non-flammability, high degrees of structural flexibility, and good thermal and chemical stability, thereby leading researchers to consider them as a new class of ionic fluids for EES applications.In this Account, we aim to provide a mechanistic understanding of this energy chemistry and an overview of recent progress in the development of eutectic electrolytes for next-generation EES. First, we describe different mechanisms that guide the formation of eutectic electrolytes and discuss the structure-property relations, electron transfer and ion transport mechanisms, and interfacial chemistry in eutectic electrolytes. Generally, three main intermolecular interactions, namely hydrogen-bond interactions, Lewis acid-base interactions, and van der Waals interactions, control the formation of eutectic electrolytes and determine their unique characters in terms of electrochemical, thermal, ion transport, and interfacial properties. These versatile intermolecular interactions can be further modified by tailoring the functional moieties of organic molecules and/or selecting suitable compositions of mixtures. The solvent-free eutectic electrolyte can maximize the molar ratio of redox-active materials, thus increasing the energy density of RFBs. We discuss the relationships between eutectic parameters (viscosity, polarity, ionic conductivity, surface tension, and coordination environment) and the molar ratio, stability, utilization, and electrochemical reversibility of redox-active materials, RFB power, and energy density. We then introduce the application of both metal- and organic-based eutectic electrolytes in the RFB field, along with the relevant perspective for future study in this field. The highly concentrated eutectic electrolytes show attractive features at electrolyte/electrode interfaces to expand the electrochemical window and meanwhile inhibit metal dendrite formation in metal-based rechargeable batteries, supercapacitors, and hybrids of these. The remaining challenges and potential research directions in these areas are also discussed. Eutectic electrolytes offer enormous opportunities and open appealing prospects as redox reaction and charge transport media for EES. We hope this Account provide guidance for the future design of advanced eutectic electrolytes toward next-generation EES systems.

Hybrid Electrolyte Engineering Enables Safe and Wide-Temperature Redox Flow Batteries.

Electrolyte is an important component in redox flow batteries (RFBs) that determines the current capability, potential window, and safety, but both aqueous and nonaqueous electrolytes have their intrinsic limits. Here, we develop the proof-of-concept hybrid electrolyte chemistry to enable the design of safe and wide-temperature RFBs. In addition to the non-flammable characteristics, the hybrid electrolyte also inherits the high electrochemical stability and wide operational temperature range. It can show a potential window of 2.5 V and maintain high ion conductivities at low temperatures. It also enables LiI to achieve high Coulombic efficiencies of >99.9 %, showing long cycling stability over 800 cycles. Moreover, it enables the successful operation of Zn/LiI RFBs at -20 °C for 150 cycles with nearly no capacity loss. This study highlights the great potential of hybrid electrolyte chemistry for the approach of safe and high-performance large-scale energy storage systems in wide temperature ranges.

Insights into the Redox Chemistry of Organosulfides Towards Stable Molecule Design in Nonaqueous Energy Storage Systems.

Nonaqueous redox flow batteries (RFBs) have great potential to achieve high-energy storage systems. However, they have been limited by low solubility and poor stability of active materials. Here we demonstrate organosulfides as a new-type model material system to explore the rational design of redox-active molecules in nonaqueous systems. The tetraethylthiuram disulfide (TETD) molecule shows high solubility in various common organic solvents and achieves a high reversible capacity of ca. 50 Ah L-1 at a high concentration of 1 M. The resonance structures in the reduced product endow the molecule with high electrochemical stability in different organic electrolytes. The underlying mechanism in redox chemistry of organodisulfides involving the cleavage and reformation of disulfide bonds is revealed by material/structural characterizations. This study provides a new perspective of molecule designs for the development of redox-active materials for high-performance nonaqueous RFBs.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries.

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Architecting a Stable High-Energy Aqueous Al-Ion Battery.

Aqueous Al-ion batteries (AAIBs) are the subject of great interest due to the inherent safety and high theoretical capacity of aluminum. The high abundancy and easy accessibility of aluminum raw materials further make AAIBs appealing for grid-scale energy storage. However, the passivating oxide film formation and hydrogen side reactions at the aluminum anode as well as limited availability of the cathode lead to low discharge voltage and poor cycling stability. Here, we proposed a new AAIB system consisting of an AlxMnO2 cathode, a zinc substrate-supported Zn-Al alloy anode, and an Al(OTF)3 aqueous electrolyte. Through the in situ electrochemical activation of MnO, the cathode was synthesized to incorporate a two-electron reaction, thus enabling its high theoretical capacity. The anode was realized by a simple deposition process of Al3+ onto Zn foil substrate. The featured alloy interface layer can effectively alleviate the passivation and suppress the dendrite growth, ensuring ultralong-term stable aluminum stripping/plating. The architected cell delivers a record-high discharge voltage plateau near 1.6 V and specific capacity of 460 mAh g-1 for over 80 cycles. This work provides new opportunities for the development of high-performance and low-cost AAIBs for practical applications.

Molecular Engineering of Azobenzene-Based Anolytes Towards High-Capacity Aqueous Redox Flow Batteries.

Aqueous redox flow batteries (RFBs) are promising alternatives for large-scale energy storage. However, new organic redox-active molecules with good chemical stability and high solubility are still desired for high-performance aqueous RFBs due to their low crossover capability and high abundance. We report azobenzene-based molecules with hydrophilic groups as new active materials for aqueous RFBs by utilizing the reversible redox activity of azo groups. By rationally tailoring the molecular structure of azobenzene, the solubility is favorably improved from near zero to 2 M due to the highly charged asymmetric structure formed in alkaline environment. DFT simulations suggest that the concentrated solution stability can be enhanced by adding hydrotropic agent to form intermolecular hydrogen bonds. The demonstrated RFB exhibits long cycling stability with a capacity retention of 99.95 % per cycle over 500 cycles. It presents a viable chemical design route towards advanced aqueous RFBs.

In Situ Formation of Liquid Metals via Galvanic Replacement Reaction to Build Dendrite-Free Alkali-Metal-Ion Batteries.

Galvanic replacement reactions have been studied as a versatile route to synthesize nanostructured alloys. However, the galvanic replacement chemistry of alkali metals has rarely been explored. A protective interphase layer will be formed outside templates when the redox potential exceeds the potential windows of nonaqueous solutions, and the complex interfacial chemistry remains elusive. Here, we demonstrate the formation of room-temperature liquid metal alloys of Na and K via galvanic replacement reaction. The fundamentals of the reaction at such low potentials are investigated via a combined experimental and computational method, which uncovers the critical role of solid-electrolyte interphase in regulating the migration of Na ions and thus the alloying reaction kinetics. With in situ formed NaK liquid alloys as an anode, the dendritic growth of alkali metals can be eliminated thanks to the deformable and self-healing features of liquid metals. The proof-of-concept battery delivers reasonable electrochemical performance, confirming the generality of this in situ approach and design principle for next-generation dendrite-free batteries.

Molecular engineering of organic electroactive materials for redox flow batteries.

With high scalability and independent control over energy and power, redox flow batteries (RFBs) stand out as an important large-scale energy storage system. However, the widespread application of conventional RFBs is limited by the uncompetitive performance, as well as the high cost and environmental concerns associated with the use of metal-based redox species. In consideration of advantageous features such as potentially low cost, vast molecular diversity, and highly tailorable properties, organic and organometallic molecules emerge as promising alternative electroactive species for building sustainable RFBs. This review presents a systematic molecular engineering scheme for designing these novel redox species. We provide detailed synthetic strategies for modifying the organic and organometallic redox species in terms of solubility, redox potential, and molecular size. Recent advances are then introduced covering the reaction mechanisms, specific functionalization methods, and electrochemical performances of redox species classified by their molecular structures. Finally, we conclude with an analysis of the current challenges and perspectives on future directions in this emerging research field.

Biredox Eutectic Electrolytes Derived from Organic Redox-Active Molecules: High-Energy Storage Systems.

One promising candidate for high-energy storage systems is the nonaqueous redox flow battery (NARFB). However, their application is limited by low solubility of redox-active materials and poor performance at high current density. Reported here is a new strategy, a biredox eutectic, as the sole electrolyte for NARFB to achieve a significantly higher concentration of redox-active materials and enhance the cell performance. Without other auxiliary solvents, the biredox eutectic electrolyte is formed directly by the molecular interactions between two different redox-active molecules. Such a unique electrolyte possesses high concentration with low viscosity (3.5 m, for N-butylphthalimide and 1,1-dimethylferrocene system) and a relatively high working voltage of 1.8 V, enabling high capacity and energy density of NARFB. The resulting high-performance NARFB demonstrates that the biredox eutectic based strategy is potentially promising for low-cost and high-energy storage systems.

Highly Efficient Photoelectrochemical Water Splitting from Hierarchical WO3/BiVO4 Nanoporous Sphere Arrays.

Nanoarchitecture of bismuth vanadate (BiVO4) photoanodes for effectively increasing light harvesting efficiency and simultaneously achieving high charge separation efficiency is the key to approaching their theoretic performance of solar-driven water splitting. Here, we developed hierarchical BiVO4 nanoporous sphere arrays, which are composed of small nanoparticles and sufficient voids for offering high capability of charge separation. Significantly, multiple light scattering in the sphere arrays and voids along with the large effective thickness of the BiVO4 photoanode induce efficient light harvesting. In addition, attributed to ultrathin two-dimensional Bi2WO6 nanosheets as the precursor, the synergy of various enhancement strategies including WO3/BiVO4 nanojunction formation, W-doping, and oxygen vacancy creation can be directly incorporated into such a unique hierarchical architecture during the one-step synthesis of BiVO4 without complex pre- or post-treatment. The as-obtained photoanode exhibits a water splitting photocurrent of 5.5 mA cm-2 at 1.23 V versus RHE under 1-sun illumination, among the best values reported up-to-date in the field.

A Sustainable Redox-Flow Battery with an Aluminum-Based, Deep-Eutectic-Solvent Anolyte.

Nonaqueous redox-flow batteries are an emerging energy storage technology for grid storage systems, but the development of anolytes has lagged far behind that of catholytes due to the major limitations of the redox species, which exhibit relatively low solubility and inadequate redox potentials. Herein, an aluminum-based deep-eutectic-solvent is investigated as an anolyte for redox-flow batteries. The aluminum-based deep-eutectic solvent demonstrated a significantly enhanced concentration of circa 3.2 m in the anolyte and a relatively low redox potential of 2.2 V vs. Li+ /Li. The electrochemical measurements highlight that a reversible volumetric capacity of 145 Ah L-1 and an energy density of 189 Wh L-1 or 165 Wh kg-1 have been achieved when coupled with a I3- /I- catholyte. The prototype cell has also been extended to the use of a Br2 -based catholyte, exhibiting a higher cell voltage with a theoretical energy density of over 200 Wh L-1 . The synergy of highly abundant, dendrite-free, multi-electron-reaction aluminum anodes and environmentally benign deep-eutectic-solvent anolytes reveals great potential towards cost-effective, sustainable redox-flow batteries.

Preparation of MoS2@PDA-Modified Polyimide Films with High Mechanical Performance and Improved Electrical Insulation.

Polyimide (PI) has been widely used in cable insulation, thermal insulation, wind power protection, and other fields due to its high chemical stability and excellent electrical insulation and mechanical properties. In this research, a modified PI composite film (MoS2@PDA/PI) was obtained by using polydopamine (PDA)-coated molybdenum disulfide (MoS2) as a filler. The low interlayer friction characteristics and high elastic modulus of MoS2 provide a theoretical basis for enhancing the flexible mechanical properties of the PI matrix. The formation of a cross-linking structure between a large number of active sites on the surface of the PDA and the PI molecular chain can effectively enhance the breakdown field strength of the film. Consequently, the tensile strength of the final sample MoS2@PDA/PI film increased by 44.7% in comparison with pure PI film, and the breakdown voltage strength reached 1.23 times that of the original film. It can be seen that the strategy of utilizing two-dimensional (2D) MoS2@PDA nanosheets filled with PI provides a new modification idea to enhance the mechanical and electrical insulation properties of PI films.

Emerging chemistries and molecular designs for flow batteries.

Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.

High-efficiency two-dimensional separation of natural products based on ß-cyclodextrin stationary phase working in both hydrophilic and reversed hydrophobic modes.

Natural products are rather complex samples containing a large number of compounds ranging from polar to nonpolar, small molecules to macromolecules, as well as numerous homologs/analogs with quite similar structures, which create great opportunities and treasures for discovery of bioactive drugs. For the purpose of better understanding the complex natural products and controlling their qualities, powerful analytical techniques for adequately separating chemical constituents and tracking potentially bioactive components are quite essential. Here, a design concept of bidirectional ß-cyclodextrin (ß-CD)-modified chromatographic stationary phase toward separation of bufadienolides extracted from toad is proposed. Bufadienolides could be divided into two classes: toad venom ligand (AAUBs) and toad toxin (AACBs) with remarkable differences in structures and polarity. The hydrophobic cavity of ß-CD can encapsulate the steroid part of AACBs while the hydroxyls exposed on the ß-CD surface have strong hydrophilic interactions with the arginine part of AACBs. Isothermal titration calorimetry and hydrogen nuclear magnetic resonance titration experiment further validate the rationality of this design. Furthermore, the ß-CD-based stationary phase can be used as a hydrophilic material to construct a HILIC × RPLC 2D separation mode for the separation of AACBs, also works as a reverse phase material to construct a RPLC × RPLC 2D separation mode for the separation of AAUBs with good orthogonality. This study will open an avenue and provide a novel insight for high-efficiency two-dimensional separation of natural products.

Mechanism of antifungal activity and therapeutic action of ß-ionone on Aspergillus fumigatus keratitis via suppressing LOX1 and JNK/p38 MAPK activation.

PURPOSE: To investigate the anti-inflammatory and antifungal role of ß-ionone (BI) in fungal keratitis (FK). METHODS: In vitro antifungal activity of BI against Aspergillus fumigatus (A. fumigatus) was evaluated by using minimum inhibitory concentration (MIC), crystal violet staining, biofilm biomass measurement, propidium iodide uptake test, and adherence assay. And RT-PCR was carried out to measure the levels of RodA, RodB, Rho, FKs, CshA-D, RlmA, Cyp51A-B and Cdr1B. Network pharmacology analysis was applied to predict the relationship between BI and FK. Cell Count Kit-8 (CCK8) assay was utilized to detect the cytotoxicity of BI to RAW264.7 and immortalized human corneal epithelial cells (HCECs). The underlying mechanism of BI at regulating the level of inflammatory factors in FK was assessed by RT-PCR, ELISA and Western blot in vitro and in vivo. The therapeutic effect of BI has investigated in A. fumigatus keratitis by employing the clinical score, pathological examination, plate count, immunofluorescence and myeloperoxidase (MPO) assay. We also used the slit-lamp microscopy, clinical scores, and HE staining to assess the effect of natamycin compared with BI treatment in vivo. RESULTS: BI suppressed the growth of A. fumigatus and had a significant effect on A. fumigatus biofilms and membrane permeability. RT-PCR demonstrated that exposure of A. fumigatus to BI inhibited the expression of genes that function in hydrophobin (RodA, RodB), cell wall integrity (Rho, FKs, CshA-D, RlmA), azole susceptibility (Cyp51A-B, Cdr1B). Network pharmacology showed that the effects of BI in FK implicate with C-type lectin receptor signaling pathway. In vivo, after A. fumigatus infection, BI treatment markedly reduced the severity of FK by decreasing clinical score, neutrophil recruitment, and fungal load. And BI treatment also obviously reduced the expression of inflammatory cytokines, Lectin-like oxidized LDL receptor (LOX-1), phosphorylation of p38MAPK and p-JNK versus the DMSO-treated group. BI and natamycin both significantly increased corneal transparency and decreased inflammatory cell recruitment in the FK in the mice model. CONCLUSION: These results indicated that BI had fungicidal activities against A. fumigatus. It also ameliorated FK in mice by reducing inflammation, which was regulated by LOX-1, p-p38MAPK and p-JNK.

Reversible Deposition of Lithium Particles Enabled by Ultraconformal and Stretchable Graphene Film for Lithium Metal Batteries.

Originating from inhomogeneous Li deposition and dissolution, the formation of dendritic and/or dead Li lies as a fundamental barrier to the practical implementation of Li metal anodes for high-energy Li-ion batteries. Here, an ultraconformal and stretchable solid-electrolyte interphase (SEI) composed of parallelly stacked few-layer defect-free graphene nanosheets, which can deform to remain ultraconformal during the expansion and shrinkage of micro-sized Li metal particles is reported. The shape-adaptive graphene protective skin is prepared via a facile mechanical method followed by Li stripping, which enables fast Li-ion diffusion, and inhibits Li dendrites and Li pulverization. The interlayer slips and wrinkles of the graphene film endow the robust protective skin with high stretchability. This work represents a unique strategy of building ultraconformal and stretchable 2D-materials-based protective skins on the surface of Li metal toward high-energy, long-life, and safe Li metal batteries.

Reversible redox chemistry in azobenzene-based organic molecules for high-capacity and long-life nonaqueous redox flow batteries.

Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability. Here we introduce azobenzene-based organic compounds as new active materials to realize high-performance nonaqueous RFBs with long cycling life and high capacity. It is capable to achieve a stable long cycling with a low capacity decay of 0.014% per cycle and 0.16% per day over 1000 cycles. The stable cycling under a high concentration of 1 M is also realized, delivering a high reversible capacity of ~46 Ah L-1. The unique lithium-coupled redox chemistry accompanied with a voltage increase is observed and revealed by experimental characterization and theoretical simulation. With the reversible redox activity of azo group in π-conjugated structures, azobenzene-based molecules represent a class of promising redox-active organics for potential grid-scale energy storage systems.

Phenothiazine-Based Organic Catholyte for High-Capacity and Long-Life Aqueous Redox Flow Batteries.

Redox-active organic materials have been considered as one of the most promising "green" candidates for aqueous redox flow batteries (RFBs) due to the natural abundance, structural diversity, and high tailorability. However, many reported organic molecules are employed in the anode, and molecules with highly reversible capacity for the cathode are limited. Here, a class of heteroaromatic phenothiazine derivatives is reported as promising positive materials for aqueous RFBs. Among these derivatives, methylene blue (MB) possesses high reversibility with extremely fast redox kinetics (electron-transfer rate constant of 0.32 cm s-1 ), excellent stability in both neutral and reduced states, and high solubility in an acetic-acid-water solvent, leading to a high reversible capacity of ≈71 Ah L-1 . Symmetric RFBs based on MB electrolyte demonstrate remarkable stability with no capacity decay over 1200 cycles. Even concentrated MB catholyte (1.5 m) is still able to deliver stable capacity over hundreds of cycles in a full cell system. The impressive cell performance validates the practicability of MB for large-scale electrical energy storage.