Colloidal Supercapattery: Redox Ions in Electrode and Electrolyte

Redox chemistry is the cornerstone of various electrochemical energy conversion and storage systems, associated with ion diffusion process. To actualize both high energy and power density in energy storage devices, both multiple electron transfer reaction and fast ion diffusion occurred in one electrode material are prerequisite. The existence forms of redox ions can lead to different electrochemical thermodynamic and kinetic properties. Here, we introduce novel colloid system, which includes multiple varying ion forms, multi‐interaction and abundant redox active sites. Unlike redox cations in solution and crystal materials, colloid system has specific reactivity‐structure relationship. In the colloidal ionic electrode, the occurrence of multiple‐electron redox reactions and fast ion diffusion leaded to ultrahigh specific capacitance and fast charge rate. The colloidal ionic supercapattery coupled with redox electrolyte provides a new potential technique for the comprehensive use of redox ions including cations and anions in electrode and electrolyte and a guiding design for the development of next‐generation high performance energy storage devices.     

Magnetic Cobalt Ferrite Nanocrystals For an Energy Storage Concentration Cell

Energy‐storage concentration cells are based on the concentration gradient of redox‐active reactants; the increased entropy is transformed into electric energy as the concentration gradient reaches equilibrium between two half cells. A recyclable and flow‐controlled magnetic electrolyte concentration cell is now presented. The hybrid inorganic–organic nanocrystal‐based electrolyte, consisting of molecular redox‐active ligands adsorbed on the surface of magnetic nanocrystals, leads to a magnetic‐field‐driven concentration gradient of redox molecules. The energy storage performance of concentration cells is dictated by magnetic characteristics of cobalt ferrite nanocrystal carriers. The enhanced conductivity and kinetics of redox‐active electrolytes could further induce a sharp concentration gradient to improve the energy density and voltage switching of magnetic electrolyte concentration cells.     

Anthraquinone with tailored structure for a nonaqueous metal-organic redox flow battery

A nonaqueous, hybrid metal-organic redox flow battery based on tailored anthraquinone structure is demonstrated to have an energy efficiency of ~82% and a specific discharge energy density similar to those of aqueous redox flow batteries, which is due to the significantly improved solubility of anthraquinone in supporting electrolytes.     

Concurrently Increasing Specific Energy and Suppressing Self-Discharge of Electrochemical Capacitors by Complexing Carbon Nanotubes with Redox Active Units-Containing Charged Copolymers

Specific energy and self-discharge are two important performances of electrochemical capacitors. In this work, we have fabricated the composite electrodes by complexing the negatively charged carboxylated multi-walled carbon nanotubes (cMWCNT) with the redox active units-containing positively charged random copolymers. 2, 2, 6, 6-Tetramethylpiperidinyl-N-oxyl and viologen are employed as model redox active units to exemplify the strategy of the concurrent increase of specific energy and suppression of self-discharge of a two-electrode device. The slower hydrogen and oxygen evolution reactions compared with the reactions of the redox active units lead to an increased electrolyte decomposition window, thereby giving rise to an increase in specific energy. On the other hand, the complexation between the cMWCNT and the copolymers suppresses both the redox shuttling and the cross-diffusion of the redox active units-containing polymer chains, leading to an improved performance of self-discharge. Based on the complexation between carbon nanotubes and redox active units-containing charged copolymers, this work provides a convenient and universal strategy to concurrently increase specific energy and suppress self-discharge of electrochemical capacitors.     

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.     

Radical Covalent Organic Frameworks: A General Strategy to Immobilize Open‐Accessible Polyradicals for High‐Performance Capacitive Energy Storage

Ordered π‐columns and open nanochannels found in covalent organic frameworks (COFs) could render them able to store electric energy. However, the synthetic difficulty in achieving redox‐active skeletons has thus far restricted their potential for energy storage. A general strategy is presented for converting a conventional COF into an outstanding platform for energy storage through post‐synthetic functionalization with organic radicals. The radical frameworks with openly accessible polyradicals immobilized on the pore walls undergo rapid and reversible redox reactions, leading to capacitive energy storage with high capacitance, high‐rate kinetics, and robust cycle stability. The results suggest that channel‐wall functional engineering with redox‐active species will be a facile and versatile strategy to explore COFs for energy storage.     

Sustainable Electrical Energy Storage through the Ferrocene/Ferrocenium Redox Reaction in Aprotic Electrolyte

The large‐scale, cost‐effective storage of electrical energy obtained from the growing deployment of wind and solar power is critically needed for the integration into the grid of these renewable energy sources. Rechargeable batteries having a redox‐flow cathode represent a viable solution for either a Li‐ion or a Na‐ion battery provided a suitable low‐cost redox molecule soluble in an aprotic electrolyte can be identified that is stable for repeated cycling and does not cross the separator membrane to the anode. Here we demonstrate an environmentally friendly, low‐cost ferrocene/ferrocenium molecular redox couple that shows about 95 % energy efficiency and about 90 % capacity retention after 250 full charge/discharge cycles.     

Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

Redox flow batteries (RFBs) are an increasingly attractive option for renewable energy storage, thus providing flexibility for the supply of electrical energy. In recent years, research in this type of battery storage has been shifted from metal-ion based electrolytes to soluble organic redox-active compounds. Aqueous-based organic electrolytes are considered as more promising electrolytes to achieve “green”, safe, and low-cost energy storage. Many organic compounds and their derivatives have recently been intensively examined for application to redox flow batteries. This work presents an up-to-date overview of the redox organic compound groups tested for application in aqueous RFB. In the initial part, the most relevant requirements for technical electrolytes are described and discussed. The importance of supporting electrolytes selection, the limits for the aqueous system, and potential synthetic strategies for redox molecules are highlighted. The different organic redox couples described in the literature are grouped in a “family tree” for organic redox couples. This article is designed to be an introduction to the field of organic redox flow batteries and aims to provide an overview of current achievements as well as helping synthetic chemists to understand the basic concepts of the technical requirements for next-generation energy storage materials.     

An Organodiselenide Comediator to Facilitate Sulfur Redox Kinetics in Lithium–Sulfur Batteries with Encapsulating Lithium Polysulfide Electrolyte

Lithium–sulfur (Li–S) batteries are regarded as promising high-energy-density energy storage devices. However, the cycling stability of Li–S batteries is restricted by the parasitic reactions between Li metal anodes and soluble lithium polysulfides (LiPSs). Encapsulating LiPS electrolyte (EPSE) can efficiently suppress the parasitic reactions but inevitably sacrifices the cathode sulfur redox kinetics. To address the above dilemma, a redox comediation strategy for EPSE is proposed to realize high-energy-density and long-cycling Li–S batteries. Concretely, dimethyl diselenide (DMDSe) is employed as an efficient redox comediator to facilitate the sulfur redox kinetics in Li–S batteries with EPSE. DMDSe enhances the liquid–liquid and liquid–solid conversion kinetics of LiPS in EPSE while maintains the ability to alleviate the anode parasitic reactions from LiPSs. Consequently, a Li–S pouch cell with a high energy density of 359 Wh kg⁻¹ at cell level and stable 37 cycles is realized. This work provides an effective redox comediation strategy for EPSE to simultaneously achieve high energy density and long cycling stability in Li–S batteries and inspires rational integration of multi-strategies for practical working batteries.     

Reversible Oxygen Redox Chemistry in Aqueous Zinc‐Ion Batteries

Rechargeable aqueous zinc‐ion batteries (ZIBs) are promising energy‐storage devices owing to their low cost and high safety. However, their energy‐storage mechanisms are complex and not well established. Recent energy‐storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO₄ cathodes and a water‐in‐salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high‐voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal‐structure evolution of VOPO₄ during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.     

离子的胶体状态与其电化学储能极限

阳离子氧化还原化学是电化学储能技术中最核心的储能机理,如何高效快速利用氧化还原活性阳离子是发展兼具高功率密度与高能量密度储电技术的关键。 处于胶体状态的阳离子可形成热力学平衡态和非平衡态,具有高反应活性和低离子迁移势垒,展现出独特的电化学特性。 本文着重介绍氧化还原活性阳离子的胶体状态与其在电化学储能上的应用,并从热力学和动力学方面阐述其储能机理,以及活性胶体离子电极和超级电容电池的构筑。 利用胶体的高比表面积、高离子吸附能力和荷电离子梯度分布等特性,创造性地构筑胶体超级电容电池,解决了现有电化学储能电极材料体系中高容量与高功率不能兼具的问题,同时开拓了胶体体系新的应用方向。     

高比能电池新材料与安全性新技术研究进展 Ⅱ.基于多电子反应的高能量密度电极材料

正在崛起的新能源技术为化学电源的发展提供了巨大机遇,同时也提出了巨大的技术挑战:即在现有基础上大幅度提升能量和功率密度,以满足各个层次高效储电的要求.利用多电子反应电池体系是成倍提高化学电源能量密度的有效途径.本文以作者所在课题组的研究工作为主,简要介绍了几类典型的多电子电极反应,包括金属硼化物多电子氧化反应、合金储锂反应、高价金属化合物结构转化反应等,以及这些反应体系用于构建高能量密度电池的关键问题,并试图分析解决这些问题的可能技术途径.     

A Redox‐Active 2D Metal–Organic Framework for Efficient Lithium Storage with Extraordinary High Capacity

Metal–organic framework cathodes usually exhibit low capacity and poor electrochemical performance for Li‐ion storage owing to intrinsic low conductivity and inferior redox activity. Now a redox‐active 2D copper–benzoquinoid (Cu‐THQ) MOF has been synthesized by a simple solvothermal method. The abundant porosity and intrinsic redox character endow the 2D Cu‐THQ MOF with promising electrochemical activity. Superior performance is achieved as a Li‐ion battery cathode with a high reversible capacity (387 mA h g^?1), large specific energy density (775 Wh kg^?1), and good cycling stability. The reaction mechanism is unveiled by comprehensive spectroscopic techniques: a three‐electron redox reaction per coordination unit and one‐electron redox reaction per copper ion mechanism is demonstrated. This elucidatory understanding sheds new light on future rational design of high‐performance MOF‐based cathode materials for efficient energy storage and conversion.     

A Nitrogen Battery Electrode involving Eight-Electron Transfer per Nitrogen for Energy Storage

Redox flow batteries have been discussed as scalable and simple stationary energy storage devices. However, currently developed systems encounter less competitive energy density and high costs, restricting their wider application. There is a lack of appropriate redox chemistry, preferably based on active materials that are abundant in nature and show high solubility in aqueous electrolytes. A nitrogen-centered redox cycle operating between the limiting species ammonia and nitrate via an eight-electron redox reaction stayed practically unnoticed, albeit its ubiquity in biological processes. Ammonia or nitrate are world-scale chemicals with high aqueous solubility, and are then comparably safe. We demonstrate here the successful implementation of such a nitrogen-based redox cycle between ammonia and nitrate with eight-electron transfer as a catholyte for Zn-based flow batteries, which continuously worked for 12.9 days with 930 charging-discharging cycles. A very competitive energy density of 577 Wh L⁻¹ can be reached, which is well above most reported flow batteries (e.g. 8 times the standard Zn-bromide battery), demonstrating that the nitrogen cycle with eight-electron transfer can offer promising cathodic redox chemistry for safe, affordable, and scalable high-energy-density storage devices.     

Direct Solar Charging of an Organic–Inorganic,Stable, and Aqueous Alkaline Redox Flow Battery with a Hematite Photoanode

The intermittent nature of the sunlight and its increasing contribution to electricity generation is fostering the energy storage research. Direct solar charging of an auspicious type of redox flow battery could make solar energy directly and efficiently dispatchable. The first solar aqueous alkaline redox flow battery using low cost and environmentally safe materials is demonstrated. The electrolytes consist of the redox couples ferrocyanide and anthraquinone‐2,7‐disulphonate in sodium hydroxide solution, yielding a standard cell potential of 0.74 V. Photovoltage enhancement strategies are demonstrated for the ferrocyanide‐hematite junction by employing an annealing treatment and growing a layer of a conductive polyaniline polymer on the electrode surface, which decreases electron–hole recombination.     

High electro-catalytic graphite felt/Mn O2composite electrodes for vanadium redox flow batteries

A mild and simple synthesis process for large-scale vanadium redox flow batteries(VRFBs)energy storage systems is desirable.A graphite felt/Mn O_2(GF-MNO)composite electrode with excellent electrocatalytic activity towards VO~(2+)/VO_2~+redox couples in a VRFB was synthesized by a one-step hydrothermal process.The resulting GF-MNO electrodes possess improved electrochemical kinetic reversibility of the vanadium redox reactions compared to pristine GF electrodes,and the corresponding energy efficiency and discharge capacity at 150 m A cm~(-2)are increased by 12.5%and 40%,respectively.The discharge capacity is maintained at 4.8 A h L~(-1)at the ultrahigh current density of 250 m A cm~(-2).Above all,80%of the energy efficiency of the GF-MNO composite electrodes is retained after 120 charge-discharge cycles at 150 m A cm~(-2).Furthermore,these electrodes demonstrated that more evenly distributed catalytic active sites were obtained from the Mn O_2particles under acidic conditions.The proposed synthetic route is facile,and the raw materials are low cost and environmentally friendly.Therefore,these novel GF-MNO electrodes hold great promise in large-scale vanadium redox flow battery energy storage systems.     

Oxygen equilibrium pressure as a measure of oxygen binding energy in the V2O5−x system

Using the redox mechanism of vanadia catalyst action as a starting point, oxygen binding energy and its measurement have been discussed. Experimental results on the equilibrium oxygen pressure are presented and two types of equilibrium are distinguished: chemisorbed and redox. It has been shown that univariant redox equilibrium is appropriate for characterizing vanadia catalysts.     

A comprehensive understanding of the anionic redox chemistry in layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries(SIBs) have demonstrated great application prospects in large-scale energy storage systems and low-speed electric vehicles due to the cost effectiveness and abundant resources. Layered transition-metal oxides are recognized as one of the most attractive sodium-ion storage cathode candidates by virtue of their high compositional diversity, environmental friendliness, ease of synthesis, and promising theoretical capacities. The practicability, however, is still limited by the fact that the energy densities of most Na-storage layered oxide cathodes solely using the conventional cationic redox are not comparable to those of the lithium-ion storage counterparts. Recently, the strategy of activating anionic redox(O^2-/O^n-) which is popular in Li-rich layered materials has been successfully applied in oxide cathodes of SIBs to promote the energy density to a new level. It is interesting to note that excess Na is not the prerequisite to induce anionic redox in sodium oxides, indicating a new mechanism underlying Na-ion materials. Herein, the latest advances on the anionic redox chemistry in layered oxide cathodes for SIBs,including the fundamental theories, triggering strategies, and applicable cathode materials, are comprehensively reviewed.Moreover, the challenges(mainly O₂ release) facing anionic redox are discussed, and the possible remedies are outlined for future developments toward a highly reversible oxygen usage. We believe that this review can provide a valuable guidance for the exploration of high-energy layered oxide cathode materials of SIBs.     

A Redox-Active 2D Metal–Organic Framework for Efficient Lithium Storage with Extraordinary High Capacity

Metal–organic framework cathodes usually exhibit low capacity and poor electrochemical performance for Li-ion storage owing to intrinsic low conductivity and inferior redox activity. Now a redox-active 2D copper–benzoquinoid (Cu-THQ) MOF has been synthesized by a simple solvothermal method. The abundant porosity and intrinsic redox character endow the 2D Cu-THQ MOF with promising electrochemical activity. Superior performance is achieved as a Li-ion battery cathode with a high reversible capacity (387 mA h g⁻¹), large specific energy density (775 Wh kg⁻¹), and good cycling stability. The reaction mechanism is unveiled by comprehensive spectroscopic techniques: a three-electron redox reaction per coordination unit and one-electron redox reaction per copper ion mechanism is demonstrated. This elucidatory understanding sheds new light on future rational design of high-performance MOF-based cathode materials for efficient energy storage and conversion.     

Direct Solar Charging of an Organic–Inorganic,Stable, and Aqueous Alkaline Redox Flow Battery with a Hematite Photoanode

The intermittent nature of the sunlight and its increasing contribution to electricity generation is fostering the energy storage research. Direct solar charging of an auspicious type of redox flow battery could make solar energy directly and efficiently dispatchable. The first solar aqueous alkaline redox flow battery using low cost and environmentally safe materials is demonstrated. The electrolytes consist of the redox couples ferrocyanide and anthraquinone‐2,7‐disulphonate in sodium hydroxide solution, yielding a standard cell potential of 0.74 V. Photovoltage enhancement strategies are demonstrated for the ferrocyanide‐hematite junction by employing an annealing treatment and growing a layer of a conductive polyaniline polymer on the electrode surface, which decreases electron–hole recombination.