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Redox flow batteries (RFBs) are promising candidates to establish a grid-scale energy storage system for intermittent energy sources. While the current technology of vanadium RFBs has been widely exploited across the world, the rise in the price of vanadium and its limited volumetric energy density have necessitated the development of new kinds of redox active molecules. Organic molecules can be used as new and economical redox couples in RFBs to address these issues. In addition, the redox organic species also provide ample advantages to increase the voltage and solubility, provide multiple numbers of electron transfer, and ensure electrochemical/chemical stability by molecular engineering through simple synthetic methods. This review focuses on the recent developments in aqueous organic RFBs, including the molecular design and the corresponding cycling performance as these organic redox molecules are employed as either the negolyte or posolyte. Various strategies for tuning the electrochemical/chemical characteristics of organic molecules have improved their solubility, redox potential, cycling stability, and crossover issue across a separating membrane. We also put forward new strategies using nanotechnology and our perspective for the future development of this rapidly growing field.

The electrification of heavy-duty transport and aviation will require new strategies to increase the energy density of electrode materials 1 , 2 . The use of anionic redox represents one possible approach to meeting this ambitious target. However, questions remain regarding the validity of the O 2− /O − oxygen redox paradigm, and alternative explanations for the origin of the anionic capacity have been proposed 3 , because the electronic orbitals associated with redox reactions cannot be measured by standard experiments. Here, using high-energy X-ray Compton measurements together with first-principles modelling, we show how the electronic orbital that lies at the heart of the reversible and stable anionic redox activity can be imaged and visualized, and its character and symmetry determined. We find that differential changes in the Compton profile with lithium-ion concentration are sensitive to the phase of the electronic wave function, and carry signatures of electrostatic and covalent bonding effects 4 . Our study not only provides a picture of the workings of a lithium-rich battery at the atomic scale, but also suggests pathways to improving existing battery materials and designing new ones. High-energy X-ray Compton measurements and first-principles modelling reveal how the electronic orbital responsible for the reversible anionic redox activity can be imaged and visualized, and its character and symmetry determined.

Redox biology is at the core of life sciences, accompanied by the close correlation of redox processes with biological activities. Redox homeostasis is a prerequisite for human health, in which the physiological levels of nonradical reactive oxygen species (ROS) function as the primary second messengers to modulate physiological redox signaling by orchestrating multiple redox sensors. However, excessive ROS accumulation, termed oxidative stress (OS), leads to biomolecule damage and subsequent occurrence of various diseases such as type 2 diabetes, atherosclerosis, and cancer. Herein, starting with the evolution of redox biology, we reveal the roles of ROS as multifaceted physiological modulators to mediate redox signaling and sustain redox homeostasis. In addition, we also emphasize the detailed OS mechanisms involved in the initiation and development of several important diseases. ROS as a double‐edged sword in disease progression suggest two different therapeutic strategies to treat redox‐relevant diseases, in which targeting ROS sources and redox‐related effectors to manipulate redox homeostasis will largely promote precision medicine. Therefore, a comprehensive understanding of the redox signaling networks under physiological and pathological conditions will facilitate the development of redox medicine and benefit patients with redox‐relevant diseases. Redox homeostasis is essential for human health, supported by multiple functional signaling pathways including the Keap1‐Nrf2, FOXO, HIF and NF‐κB pathways. However, redox imbalance causes multiple serious diseases such as type 2 diabetes, atherosclerosis, chronic obstructive pulmonary disease, Alzheimer's disease, cancer, and aging. Our review emphasizes the significance of redox manipulation in clinical therapeutics and points out the existing challenges involved in this field.

Photoredox catalysis, especially in combination with transition metal catalysis, can produce redox states of transition metal catalysts to facilitate challenging bond formations that are not readily accessible in conventional redox catalysis. For arene functionalization, metallophotoredox catalysis has successfully made use of the same leaving groups as those valuable in conventional cross-coupling catalysis, such as bromide. Yet the redox potentials of common photoredox catalysts are not sufficient to reduce most aryl bromides, so synthetically useful aryl radicals are often not directly available. Therefore, the development of a distinct leaving group more appropriately matched in redox potential could enable new reactivity manifolds for metallophotoredox catalysis, especially if arylcopper(iii) complexes are accessible, from which the most challenging bond-forming reactions can occur. Here we show the conceptual advantages of aryl thianthrenium salts for metallophotoredox catalysis, and their utility in site-selective late-stage aromatic fluorination.

Aqueous 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. This review presents the development of organic redox‐active materials (ORAMs) in aqueous organic redox flow batteries (AORFBs). Smart strategies, particularly molecular engineering of ORAMs in terms of high aqueous solubility, high chemical/electrochemical stability, multiple electron numbers, and satisfactory redox potential, play a critical role in establishing advanced AORFBs. Furthermore, the challenges, opportunities, and future directions are presented.

• A highly negative glutathione redox potential (EGSH ) is maintained in the cytosol, plastids and mitochondria of plant cells to support fundamental processes, including antioxidant defence, redox regulation and ironasulfur cluster biogenesis. Out of two glutathione reductase (GR) proteins in Arabidopsis, GR2 is predicted to be dual-targeted to plastids and mitochondria, but its differential roles in these organelles remain unclear. • We dissected the role of GR2 in organelle glutathione redox homeostasis and plant development using a combination of genetic complementation and stacked mutants, biochemical activity studies, immunogold labelling and in vivo biosensing. • Our data demonstrate that GR2 is dual-targeted to plastids and mitochondria, but embryo lethality of gr2 null mutants is caused specifically in plastids. Whereas lack of mitochondrial GR2 leads to a partially oxidised glutathione pool in the matrix, the ATP-binding cassette (ABC) transporter ATM3 and the mitochondrial thioredoxin system provide functional backup and maintain plant viability. • We identify GR2 as essential in the plastid stroma, where it counters GSSG accumulation and developmental arrest. By contrast a functional triad of GR2, ATM3 and the thioredoxin system in the mitochondria provides resilience to excessive glutathione oxidation.

Oxidation–reduction (redox) ladders are a fundamental biogeochemical concept that explain basic and applied phenomena including microbial metabolic evolution and chemical pollution remediation. However, it is difficult to measure and predict which redox couples dominate an environment. This discrepancy manifests a redox paradox: while redox ladders are a fundamental biogeochemical concept, it is difficult to use existing measurements to predict which redox couples dominate in an environment or use redox to predict biogeochemical state changes. Additionally, there is often confusion around the concept due to the differential emphasis in biological vs. geochemical sub-disciplines in explanations of the: (1) roles of actual vs. standard free energy calculations, (2) thermodynamics vs. enzymatic kinetics in reactions, (3) ubiquity of alternative electron donors as opposed to assuming processes are solely driven by organic carbon, and (4) difference between redox intensity and redox buffering capacity. Redox buffering capacity is a particularly underdeveloped concept within biogeochemical studies, which have largely used redox potential, a measurement with myriad analytical concerns. We review the small number of studies that have quantified redox buffering capacity. We highlight four challenges to overcome before redox buffering capacity can be a useful concept. Measuring redox buffering capacity may be key to understanding the ability of the ecosystems to resist change in redox conditions. Biogeochemists are often most interested in predicting these state changes to understand hot spot/hot moment phenomena such as greenhouse gas fluxes; thus, incorporating redox buffering capacity into our predictive frameworks and methodological toolbox may be key to unlocking the redox paradox. We postulate that the well-known ecological resilience and resistance framework constitutes the needed theoretical foundation on which measurements of redox buffering capacity can be incorporated.

Alloxazines are a promising class of organic electroactive compounds for application in aqueous redox flow batteries (ARFBs), whose redox properties need to be tuned further for higher performance. High-throughput computational screening (HTCS) enables rational and time-efficient study of energy storage compounds. We compared the performance of computational chemistry methods, including the force field based molecular mechanics, semi-empirical quantum mechanics, density functional tight binding, and density functional theory, on the basis of their accuracy and computational cost in predicting the redox potentials of alloxazines. Various energy-based descriptors, including the redox reaction energies and the frontier orbital energies of the reactant and product molecules, were considered. We found that the lowest unoccupied molecular orbital (LUMO) energy of the reactant molecules is the best performing chemical descriptor for alloxazines, which is in contrast to other classes of energy storage compounds, such as quinones that we reported earlier. Notably, we present a flexible in silico approach to accelerate both the singly and the HTCS studies, therewithal considering the level of accuracy versus measured electrochemical data, which is readily applicable for the discovery of alloxazine-derived organic compounds for energy storage in ARFBs.

ABSTRACT Many bacteria and archaea produce the redox cofactor F420. F420 is structurally similar to the cofactors FAD and FMN but is catalytically more similar to NAD and NADP. These properties allow F420 to catalyze challenging redox reactions, including key steps in methanogenesis, antibiotic biosynthesis and xenobiotic biodegradation. In the last 5 years, there has been much progress in understanding its distribution, biosynthesis, role and applications. Whereas F420 was previously thought to be confined to Actinobacteria and Euryarchaeota, new evidence indicates it is synthesized across the bacterial and archaeal domains, as a result of extensive horizontal and vertical biosynthetic gene transfer. F420 was thought to be synthesized through one biosynthetic pathway; however, recent advances have revealed variants of this pathway and have resolved their key biosynthetic steps. In parallel, new F420-dependent biosynthetic and metabolic processes have been discovered. These advances have enabled the heterologous production of F420 and identified enantioselective F420H2-dependent reductases for biocatalysis. New research has also helped resolve how microorganisms use F420 to influence human and environmental health, providing opportunities for tuberculosis treatment and methane mitigation. A total of 50 years since its discovery, multiple paradigms associated with F420 have shifted, and new F420-dependent organisms and processes continue to be discovered. This review provides a comprehensive description of the distribution and biosynthesis of the redox cofactor F420, as well as its enzymology, physiological roles and biotechnological applications.

Over the past decade, photoredox catalysis has harnessed light energy to accelerate bond-forming reactions. We postulated that a complementary method for the redox-activation of small organic molecules in response to applied mechanical energy could be developed through the piezoelectric effect. Here, we report that agitation of piezoelectric materials via ball milling reduces aryl diazonium salts. This mechanoredox system can be applied to arylation and borylation reactions under mechanochemical conditions.