Redox Reaction Mechanism Research Articles

Recent Advances in Pristine Iron Triad Metal-Organic Framework Cathodes for Alkali Metal-Ion Batteries.

Pristine iron triad metal-organic frameworks (MOFs), i.e., Fe-MOFs, Co-MOFs, Ni-MOFs, and heterometallic iron triad MOFs, are utilized as versatile and promising cathodes for alkali metal-ion batteries, owing to their distinctive structure characteristics, including modifiable and designable composition, multi-electron redox-active sites, exceptional porosity, and stable construction facilitating rapid ion diffusion. Notably, pristine iron triad MOFs cathodes have recently achieved significant milestones in electrochemical energy storage due to their exceptional electrochemical properties. Here, the recent advances in pristine iron triad MOFs cathodes for alkali metal-ion batteries are summarized. The redox reaction mechanisms and essential strategies to boost the electrochemical behaviors in associated electrochemical energy storage devices are also explored. Furthermore, insights into the future prospects related to pristine iron triad MOFs cathodes for lithium-ion, sodium-ion, and potassium-ion batteries are also delivered.

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives.

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

Adsorption and recovery of Cr(vi) from wastewater by Chitosan–Urushiol composite nanofiber membrane

Abstract Chitosan (CS) is widely used in the treatment of wastewater containing metal ions. However, the poor stability in acidic aqueous solutions severely limits its application in many practical scenarios. In this work, a CS-based composite nanofiber membrane was prepared by electrospinning using urushiol, a natural biomaterial, as the cross-linking agent. The application of the CS-urushiol (CS-U) membrane in the adsorption and recovery of Cr(vi) in wastewater was systematically studied. The CS-U membrane showed great resistance to strongly acidic and oxidative environments, and the adsorption process combined two mechanisms of electrostatic attraction and redox reaction. Due to the nanoscale fibers, porous structure, and strong acid resistance, the CS-U membrane adsorbed Cr(vi) rapidly and efficiently in both batch and continuous modes. Moreover, the adsorption capacity and selectivity of the CS-U membrane for Cr(vi) could be maximized simultaneously by adjusting the solution pH, promoting the recovery of high-purity Cr2O3.

A Tetramethylurea-Electrolyte-Based Lithium-Oxygen Battery with High Performance and Alternative Mechanism

The electrolyte is a critical component in electrochemical energy storage devices as it creates the reaction environment for the electrode process. In lithium–oxygen (Li–O2) batteries, the oxygen electrode reaction involves a complex multiphase transition (e.g., gas, liquid, solid) and a variety of intermediates (e.g., O2 –, O2 2–, 1O2), making the role of the electrolyte system more prominent. As an ideal electrolyte for Li–O2 batteries, it should not only resist the attack of superoxide intermediate, but also provide a kinetically favorable O2 redox reaction mechanism. However, currently, no electrolyte solvent is perfectly suitable for Li–O2 batteries, which greatly limited their practical applications. Here, we present a new electrolyte solvent, 1,1,3,3-tetramethylurea, for Li–O2 batteries that exhibits high performance and an alternative mechanism. Specifically, it enables a kinetically favorable one-electron Li2O2 oxidation pathway upon charging. Furthermore, due to the absence of alpha-hydrogen atoms in its molecular structure, it also exhibits good stability in the presence of superoxide intermediates, significantly suppressing the related hydrogen extraction side reactions. Consequently, Li–O2 batteries utilizing this electrolyte show a much lower charge overpotential and a four-fold longer cycle life compared to those with the benchmark electrolyte. Acknowledgments X.L. acknowledges financial support from the Marie Skłodowska-Curie Actions (grant agreement No. 101064286) for his postdoctoral fellowship.

Conformational states of NADH in water–alcohol solutions studied by molecular dynamics simulations

Redox reactions in living cells involving a coenzyme nicotinamide-adenine-dinucleotide (NAD) are stereospecific and depend on NAD conformation states and on the structure of enzyme binding sites. Therefore, information on NAD conformation states in various environments is important for understanding of the mechanisms of redox reactions. In this paper the structural dynamics of reduced nicotinamide-adenine-dinucleotide (NADH) in water–methanol and water–ethanol solutions was investigated by means of molecular dynamics simulations. The trajectory analysis revealed three stable conformation groups characterized by the distance RNA−AD between adenine and nicotinamide centers of mass: folded (RNA−AD< 5.5 Å), intermediate (5.5 Å <RNA−AD< 12 Å) and unfolded (RNA−AD> 12 Å). It was shown that conformation equilibrium shifted from the folded conformation group at low alcohol concentrations towards the intermediate conformation group at high alcohol concentrations, while the fractional concentration of unfolded conformation group remained almost constant. The nicotinamide and adenine rings were found to be almost parallel in folded conformations and perpendicular in intermediate conformations, while no preferable positions of the rings were found in unfolded conformations. The results obtained were used for the analysis of experimentally determined rotational diffusion times τrexp in NADH in water–methanol and water–ethanol mixtures. Comparison between the calculated and experimental τr values using the generalized Stokes–Einstein–Debye equation allowed for determination of the wettability coefficient C and effective molecular volume Vm as a function of alcohol concentration. The coefficient C was found to be always less than unity and have a minimum at 40% of alcohol which was more pronounced for ethanol than for methanol mixtures. This dependence was attributed to the influence of solution viscosity that has a maximum at 40% of alcohol. The effective molecular volume Vm was found to be practically constant at alcohol concentrations below 60% and rise dramatically at higher alcohol concentrations. This dependence was attributed to the solvation of NADH at high alcohol concentrations.

Dual storage mechanism of charge adsorption desorption and Faraday redox reaction enables aqueous symmetric supercapacitor with 1.4 V output voltage

Preparation of heterostructure electrode materials with dual storage mechanisms of charge adsorption desorption (electric double-layer capacitance) and Faraday redox reaction (pseudo-capacitance) remains a great challenge for supercapacitors with wide operating voltage and high energy density. Herein, the heterostructure ZnO nanoparticles decorated NiFe/CNTs/rGO (ZnO-FeNi/CG) was rational designed and synthesized via a straightforward two-step hydrothermal process for supercapacitor application. The rational designed flower-like heterostructure ZnO-FeNi/CG reveals outstanding specific capacitance of 1265F/g at 1 A/g and 88.5 % capacitance retention after 10,000 cycles even at 30 A/g in three-electrode test. Benefiting from the designed unique heterostructure and the effective anti-catalytic strategy, the Faradaic redox reactions of ZnO-FeNi/CG electrode were well coordinated by HER and OER, so that water splitting process was greatly suppressed under high operating voltage in aqueous alkaline electrolytes. Thus, the fabricated symmetric supercapacitor based on the dual storage mechanisms of electric double-layer capacitance and pseudo-capacitance displays 1.4 V output voltage, ultrahigh capacitance of 227F/g (1 A/g), and exceptional energy density of 62 Wh kg−1 at power density of 1400 W kg−1 as well as extraordinary capacitance retention 84.7 % for 10,000 cycles, outperforming the previous spinel-type metal oxides/carbon-based supercapacitors in aqueous alkaline electrolytes. The dual storage mechanism of charge adsorption desorption and Faraday redox reaction can provide a pathway to design next-generation aqueous symmetric supercapacitors with wide output voltage and ultrahigh energy density for advanced energy storage systems.

Unveiling the outstanding full-cell performance of P2-type Na0.67(Mn0.44Ni0.06Fe0.43Ti0.07)O2 cathode active material for Na-ion batteries

In this study, we unravel the effect of Ni doping on the half-cell and full-cell performances of the Na0.67Mn0.5-xNixFe0.43Ti0.07O2 cathode materials where x varies between 0.02 and 0.1. The cyclic voltammetry (CV) analysis of the half-cells is performed at 10 °C, room temperature (RT), and 50 °C to elucidate the redox reaction mechanisms at different temperatures. Among the studied cathodes, the highest specific capacity is obtained fox = 0.06 which delivered a specific capacity of 186 mAh g−1 at C/3-rate. The full cell of Na0.67Mn0.44Ni0.06Fe0.43Ti0.07O2/hard carbon couple is assembled in coin cell format and the specific capacity of the cell at C/2, 1C, and 2C rates are found as 153 mAh g−1, 125 mAh g−1 and 120 mAh g−1, respectively. At the C/2-rate, the excellent capacity retention of the full cell is around 70% after 500 cycles delivering a specific capacity of 103 mAh g−1. Along with the conventional physicochemical characterization methods such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Fourier-transform Infrared Spectroscopies (FTIR), we also utilize X-ray photoelectron spectroscopy (XPS) to bridge the nexus between the performance and the structure properties of the studied materials. Furthermore, we also employ synchrotron-based X-ray Absorption (XAS) to understand the local geometry of the optimized cathode materials in operando.

Understanding Lattice Oxygen Redox Behavior in Lithium‐Rich Manganese‐Based Layered Oxides for Lithium‐Ion and Lithium‐Metal Batteries from Reaction Mechanisms to Regulation Strategies

AbstractLithium‐rich manganese‐based layered oxides (LMLOs) are considered to be one type of the most promising materials for next‐generation cathodes of lithium batteries due to their distinctive anionic redox processes contributing ultrahigh capacity and energy density. Unfortunately, their practical applications are still plagued by several challenges such as undesirable interfacial reactions and structural evolution, as well as voltage hysteresis/recession, in which irreversible anionic redox behavior bears the brunt as the primacy factor. Undoubtedly, a deep understanding of anionic redox reaction mechanisms and irreversible behavior of oxygen species is crucial in order to provide essential guidance for modification strategies for LMLOs. In this paper, the fundamental understanding of intricate anionic redox reaction mechanisms from thermodynamics models to kinetic anionic redox reaction pathways is comprehensively reviewed, and the existing challenges of LMLOs related with irreversible oxygen reaction behavior are analyzed. Furthermore, numerous representative modification strategies for overcoming these challenges, coupled with their underlying mechanisms for regulating anionic redox reversibility are summarized. In addition, the aspects of multi‐scale structural modifications, integration of interdisciplinary technologies, and application in quasi‐/all‐solid‐state battery systems are given some emphasis in terms of further improvement of LMLOs‐based cathode materials for advanced lithium batteries‐based energy storage systems.

Nanostructured Porous Polymer with Low Volume Expansion, Structural Distortion, and Gradual Activation for High and Durable Lithium Storage.

Organic compounds exhibit great potential as sustainable, tailorable, and environmentally friendly electrode materials for rechargeable batteries. However, the intrinsic defects of organic electrodes, including solubility, low ionic conductivity, and restricted electroactivity sites, will inevitably decrease the cycling life and capacity. We herein designed and prepared nanostructured porous polymers (NPP) with a simple one-pot method to overcome the above defects. Theoretical calculations and experimental results demonstrate that the as-synthesized NPP exhibited low volume expansion, molecular-structural distortion, and a gradual function activation process during cycling, thus exhibiting superior, high, and durable lithium storage. The gradual molecular distortion during the lithium storage processes provides more redox-active sites for Li storage, increasing the Li-storage capacity. Ex situ spectrum studies reveal the redox reaction mechanism of Li storage and demonstrate a gradual activation process during the repeated charging/discharging until the full storage of 18 Li ions is achieved. Additionally, a real-time observation on the NPP anode by in situ transmission electron microscope reveals a slight volume expansion during the repeating lithiation and delithiation processes, ensuring its structural integrity during cycling. This quantitative work for high-durability lithium storage could be of immediate benefit for designing organic electrode materials.

Chemical looping preferential oxidation of CO over ceria-supported γ-Fe2O3

A novel chemical looping preferential oxidation (CL-PROX) process is proposed and demonstrated for eliminating trace CO from a H2-rich stream. The proposed process exhibited an excellent CO removal performance (CO concentration < 100 ppm) and a high H2 recovery (>∼96 %) with a ceria-supported γ-Fe2O3 oxygen carrier. It was obtained by activating the fresh sample that was synthesized through a co-precipitation method and the innovative activation process comprising a partial reduction step at 400 °C followed by an oxidation process at 260 °C. The structure and evolution of iron species in the oxygen carrier were investigated through experimental characterizations during activation and redox cycles. TPR characterizations illustrated a significant difference in the reducibility of the active species, γ-Fe2O3, under H2 and CO atmospheres, which could be the evidence for its relatively high selectivity to CO oxidation. In-situ FTIR characterization also pointed out that the desorption of reaction intermediates to CO2 is facilitated via the reduction of the surface γ-Fe2O3, which might contribute to the outstanding CO oxidation performance of the oxygen carrier. The successful application of chemical looping process to selective oxidation of CO in H2-rich gas not only illustrates a new strategy for H2 purification, but also provides insights into the redox reaction mechanism with the newly-prepared γ-Fe2O3-based oxygen carrier.

Dual-Functional S-Scheme Fe3O4/TiO2/g-C3N4 double-heterostructure bridged by TiO2 for collaborative removal of U(VI) and Sb(III)

Exploiting dual-functional photocatalysts to reduce U(VI) and oxidize Sb(III) is a challenging but efficient way of achieving simultaneous purification. Herein, Fe3O4/TiO2/g-C3N4 (FTC) double-heterostructures with customized stacking order were designed as a dual-functional photocatalyst to collaborative remove U(VI) and Sb(III). When introducing TiO2/g-C3N4 heterojunction and bridging it with Fe3O4/TiO2 using TiO2 as the intermediate layer, FTC showed outstanding photocatalytic activity and can simultaneously achieve 96.5% U(VI) reduction and 88.5% Sb(III) oxidation under sunlight, achieving 90.9% and 85.8% in natural wastewater. The constructed FTC architecture of an S-scheme framework can achieve efficient co-removals of U(VI) and Sb(III) by enhancing the separation and transferal of photoexcited charge carriers with outstanding redox ability employing different reaction sites. TiO2 and g-C3N4 accumulated electrons and holes, respectively, and completed concurrent redox reactions, following the path determined by N(g−C3N4) and O(TiO2), which enhanced the separation of photoexcited carriers. The two-way effect of O2• ⁻ activated by FTC supported positive feedback coupling relationships between the reduction of U(VI) and Sb(III) oxidation. This study not only fulfills the co-removal of U(VI) and Sb (III) for the first time with a capacity of 170.6 and 15.9 mg/g in engineering applications, but also proposes the reaction mechanism of a two-step continuous concerted redox reaction causing high removal determined by the electronic structure, so it lays a foundation for further development of highly efficient and stable heterojunction photocatalysts with concerted effects applied in complex wastewater treatment.

Insights into electrogenerated intermediates and rapid screening of electrochemical reactions by surface-modified carbon fiber paper redox spray ionization mass spectrometry

The combination of electrochemistry and mass spectrometry is a powerful analytical tool for studying redox reaction mechanisms and identifying products or intermediates. However, the previously reported devices all require bespoke fabrication and are too complicated to be assembled and used by others. Crucially, the long ion transport distance and small spray volumes make it difficult to capture the short-lived intermediates. We present a practical mass spectrometric method in which surface-modified carbon fiber paper is innovatively applied to detect electrogenerated intermediates. Treating carbon fiber paper with dilute nitric acid removes its surface impurities, enhancing the capability of electro-redox. Electrospray ionization and redox reaction occur simultaneously on the tip of the paper. Transient electro-redox species generate and transfer into gas phase as soon as the appearance of spray. Rapid transport of quantities of electrogenerated ions to the mass spectrometer inlet makes it possible for mass spectrometric identification on the millisecond scale. The short-lived radical cations and iminium ions were successfully captured, reflecting the starting step of the cross-dehydrogenation coupling reaction. The real-time oxidation and online functionalization reactions of tertiary amines were achieved using this device without additional oxidants and electrolytes. In this way we could achieve in-depth mechanistic understanding and rapid screening of serial reactions.

Cationic–Anionic Redox Chemistry in Multivalent Metal‐Ion Batteries: Recent Advances, Reaction Mechanism, Advanced Characterization Techniques, and Prospects

AbstractRechargeable multivalent metal‐ion batteries (MMIBs) have garnered a surge of attention as competitive candidates for large‐scale energy storage applications owing to their high capacity, abundant resources, and good security. However, their practical implementation is still stuck at the prototype stage, mainly plagued with the lack of suitable cathode materials capable of reversible insertion/extraction of multivalent ions and the intrinsically complicated redox reaction mechanism. Recently, anionic redox chemistry has shown to be an effective strategy to increase energy density, providing a new research direction for the next generation of high‐energy rechargeable batteries. Unfortunately, anion redox chemistry has not received sufficient attention in MMIBs so far. Here, the fundamental principle and mechanism of anionic redox reactions in MMIBs are discussed and the recent advances regarding cathode materials based on cooperative cationic–anionic redox (CCAR) mechanism are summarized. Additionally, various advanced characterization techniques for studying the anionic redox process are highlighted, aiming to effectively illustrate the underlying reaction mechanism. Finally, challenges and perspectives for the future research on cationic–anionic redox chemistry in MMIBs are proposed. Insight into the significance of CCAR chemistry is provided here in MMIBs, presenting a new avenue for the development of high‐energy‐density cathode materials for MMIBs.

Effect of different carbon materials on the structure and glass crystallization temperature, wettability, and expansion coefficient properties of glass

Bismuthate glasses have garnered increasing attention in the fields of optics, packaging, and electronic pastes due to their remarkable stability and high visible light transmission. Nevertheless, high sealing temperature, high expansion coefficient, and poor wettability remain significant challenges in their application. Herein, we report a carefully designed approach that begins with bismuthate glasses of well-defined composition and coats them with various carbon materials to form different C@aminated glass composites by hydrothermal synthesis. This work investigates the effects of different carbon materials on the glass structure and properties, such as melting temperature, crystallinity, wettability, and expansion coefficient, and establishes the redox reaction mechanism involved in glass crystallization formation. Graphene oxide, with its different number of functional distributions and oxygen content, exhibits the largest decrease in softening point and softening-to-melting point temperature difference, the highest increase in wettability, and the greatest reduction in expansion coefficient compared to other carbon materials. In addition, different carbon materials promote a greater variety and strength of crystallization. GO@glass exhibits the most moderate reactivity and generates a significant amount of crystallization at the interface when co-sintered with Si substrates, indicating that the inclusion of crystalline phases can improve the bond strength of the glass. This work provides new insights into designing high-performance bismuthate glasses and promoting their applications.

A Nickel-Based Coordination Compound with Tunable Morphology for High-Performance Anode and the Lithium Storage Mechanism

Metal-organic coordination compounds (MCCs) have received a lot of attention as anodes for lithium-ion batteries (LIBs) due to their abundant structural configuration, tunable morphology, high surface area, and low cost, but the lithium storage mechanism of MCCs is still a mystery. Herein, we synthesized a kind of nickel-based coordination compound (marked as Ni-PP-x, x = 1, 2, or 3) with tunable morphologies and different solvent ratios via a microwave irradiation solvothermal method and then applied them as anodes for LIBs. Among them, the Ni-PP-2 electrode, with a hollow and urchin-like structure, showed the longest lifespan and maintained a high capacity of 713 mAh g−1 at 2.0 A g−1 after 800 cycles. Measured by ex situ X-ray photoelectron spectroscopy (XPS) and ex situ Fourier transform infrared spectroscopy (FT-IR), the Ni-PP-2 electrode was confirmed by a redox reaction mechanism of Li+ cations with a benzene ring and O-Ni2+/O-Ni0 coordination bonds, and the cyclic voltammetry curves have exhibited a capacitive dominated lithium storage behavior. This work provides a new type of Ni-based coordination compound and an in-depth understanding of their lithium storage mechanism, paving the way for the application of MCC compounds in the future.

Dual electrochemical quantitative analysis of dopamine based on flow injection amperometry with double glassy carbon electrodes

A simple, rapid and stable electrochemical flow injection amperometry (FI-Amp) system based on a flow cell with two glassy carbon electrodes arranged in series was constructed and the redox mechanism of dopamine (DA) was investigated as a research model. Electrochemical oxidation of DA occurs at the surface of the first working electrode (WE1, +0.24 V working potential), and then its oxidation product undergoes electrochemical reduction when it flows through the second working electrode (WE2, +0.05 V working potential). The source of the reduction currents on the WE2 was discussed and finally identified as dopaquinone (DAQ). In addition, dual quantitative analysis of DA was achieved by the oxidation current of DA on WE1 and the reduction current of DAQ on WE2. Under optimized conditions, the dual linear response ranges for DA were from 0.5 µM to 250 µM and 2 µM to 250 µM, and the dual limits of detection were found to be 0.20 µM and 0.68 µM (S/N = 3), respectively. Moreover, the RSDs of the amperometric responses for 5 µM, 50 µM and 200 µM DA were all less than 10% over 50 repeated injections, showing that the FI-Amp system has excellent repeatability and stability. The approach is universal and superior to the traditional three-electrode system. It can be used to study the mechanisms of molecular redox reactions and has great potential for application in the electrochemical identification of other electroactive molecules.

Thiosulfate in the Upper Part of the Black Sea Anoxic Water Column

Thiosulfate is a sulfur compound in an intermediate oxidation state between sulfate and sulfide. It emerges during processes of sulfide oxidation in bacterial and chemical reactions. Thiosulfate plays a prominent role in oxidation of sulfide and its presence may clear the mechanism of redox reactions in the biogeochemical cycles of sulfur and carbon. This work presents the distribution of thiosulfate, determined by derivatization technique with reagent 2,2'-Dithiobis(5-nitropyridine), in the upper part of the Black Sea anoxic water column in 2018–2021. Observations were carried out annually at the station located seaward of Gelendzhik in the area of continental slope. Independently of observation time the concentration of thiosulfate rises depth wards simultaneously with increase of sulfide. The thiosulfate concentration above limit of detection 0.01 µM are found at water depth with density (σθ) of 16.3 kg/m3 and sulfide concentration of 8–11 µM. Thiosulfate reaches its maximum values of 0.30 µM at maximal studied depth of 600 m. We suggest that in the absence of oxidants in the anoxic waters the thiosulfate appearance is attributed to the process of sulfate reduction in reductive part of sulfur cycle. The thiosulfate disappears in the upper part of anoxic water column before the sulfide.

Kinetic Role of Reactive Intermediates in Controlling the Formation of Chlorine Dioxide in the Hypochlorous Acid–Chlorite Ion Reaction

An advanced experimental protocol is reported for studying the kinetics and mechanism of the complex redox reaction between chlorite ion and hypochlorous acid under acidic condition. The formation of ClO2 is followed directly by the classical two-component stopped-flow method. In sequential stopped-flow experiments, the target reaction is chemically quenched using NaI solution and the concentration of each reactant and product is monitored as a function of time by utilizing the principles of kinetic discrimination. Thus, in contrast to earlier studies, not only the formation of one of the products but the decay of the reactants was also directly followed. This approach provides a firm basis for postulating a detailed mechanism for the interpretation of the experimental results under a variety of conditions. The intimate details of the reaction are explored by simultaneously fitting 78 kinetic traces, i.e., the concentration vs. time profiles of ClO2-, HOCl, and ClO2, to an 11-step kinetic model. The most important reaction steps were identified, and it was shown that two reactive intermediates have a pivotal role in the mechanism. While chlorate ion predominantly forms via the reaction of Cl2O, chlorine dioxide is exclusively produced in reaction steps involving Cl2O2. This study leads to clear conclusions on how to control the stoichiometry of the reaction and achieve optimum conditions to produce chlorine dioxide and to reduce the formation of the toxic chlorate ion in practical applications.

Development of organic redox‐active materials in aqueous flow batteries: Current strategies and future perspectives

AbstractAqueous redox flow batteries, by using redox‐active molecules dissolved in nonflammable water solutions as electrolytes, are a promising technology for grid‐scale energy storage. Organic redox‐active materials offer a new opportunity for the construction of advanced flow batteries due to their advantages of potentially low cost, extensive structural diversity, tunable electrochemical properties, and high natural abundance. In this review, we present the emergence and development of organic redox‐active materials for aqueous organic redox flow batteries (AORFBs), in particular, molecular engineering concepts and strategies of organic redox‐active molecules. The typical design strategies based on organic redox species for high‐capacity, high‐stability, and high‐voltage AORFBs are outlined and discussed. Molecular engineering of organic redox‐active molecules for high aqueous solubility, high chemical/electrochemical stability, and multiple electron numbers as well as satisfactory redox potential gap between the redox pair is essential to realizing high‐performance AORFBs. Beyond molecular engineering, the redox‐targeting strategy is an effective way to obtain high‐capacity AORFBs. We further discuss and analyze the redox reaction mechanisms of organic redox species based on a series of electrochemical and spectroscopic approaches, and succinctly summarize the capacity degradation mechanisms of AORFBs. Furthermore, the current challenges, opportunities, and future directions of organic redox‐active materials for AORFBs are presented in detail.

Modulation of Local Charge Distribution Stabilized the Anionic Redox Process in Mn-Based P2-Type Layered Oxides.

An anionic redox reaction is an extraordinary method for obtaining high-energy-density cathode materials for sodium-ion batteries (SIBs). The commonly used inactive-element-doped strategies can effectively trigger the O redox activity in several layered cathode materials. However, the anionic redox reaction process is usually accompanied by unfavorable structural changes, large voltage hysteresis, and irreversible O2 loss, which hinders its practical application to a large extent. In the present work, we take the doping of Li elements into Mn-based oxide as an example and reveal the local charge trap around the Li dopant will severely impede O charge transfer upon cycling. To overcome this obstacle, additional Zn2+ codoping is introduced into the system. Theoretical and experimental studies show that Zn2+ doping can effectively release the charge around Li+ and homogeneously distribute it on Mn and O atoms, thus reducing the overoxidation of O and improving the stability of the structure. Furthermore, this change in the microstructure makes the phase transition more reversible. This study aimed to provide a theoretical framework for further improve the electrochemical performance of similar anionic redox systems and provide insights into the activation mechanism of the anionic redox reaction.