Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. However, widespread implementation of traditional RFBs, such as vanadium and Zn-Br RFBs, is limited due to a number of challenges related to materials, including low abundance and high costs of redox-active metals, expensive separators, active material crossover, and corrosive and hazardous electrolytes. To address these challenges, we demonstrate a neutral aqueous organic redox flow battery (AORFB) technology utilizing a newly designed cathode electrolyte containing a highly water-soluble ferrocene molecule. Specifically, water-soluble (ferrocenylmethyl)trimethylammonium chloride (FcNCl, 4.0 M in HO, 107.2 Ah/L, and 3.0 M in 2.0 NaCl, 80.4 Ah/L) and N-ferrocenylmethyl-N,N,N,N,N-pentamethylpropane-1,2-diaminium dibromide, (FcNBr, 3.1 M in HO, 83.1 Ah/L, and 2.0 M in 2.0 M NaCl, 53.5 Ah/L) were synthesized through structural decoration of hydrophobic ferrocene with synergetic hydrophilic functionalities including an ammonium cation group and a halide anion. When paired with methyl viologen (MV) as an anolyte, resulting FcNCl/MV and FcNBr/MV AORFBs were operated in noncorrosive neutral NaCl supporting electrolytes using a low-cost anion-exchange membrane. These ferrocene/MV AORFBs are characterized as having high theoretical energy density (45.5 Wh/L) and excellent cycling performance from 40 to 100 mA/cm. Notably, the FcNCl/MV AORFBs (demonstrated at 7.0 and 9.9 Wh/L) exhibited unprecedented long cycling performance, 700 cycles at 60 mA/cm with 99.99% capacity retention per cycle, and delivered power density up to 125 mW/cm. These AORFBs are built from earth-abundant elements and are environmentally benign, thus representing a promising choice for sustainable and safe energy storage.

The state-of-the-art advances of non-aqueous organic redox flow batteries for grid-scale energy storage were evaluated and summarized.

Cobalt complexes have shown great promise as electrocatalysts in applications ranging from hydrogen evolution to C−H functionalization. However, the use of such complexes often requires polydentate, bulky ligands to stabilize the catalytically active Co(I) oxidation state from deleterious disproportionation reactions to enable the desired reactivity. Herein, we describe the use of bidentate electronically asymmetric ligands as an alternative approach to stabilizing transient Co(I) species. Using disproportionation rates of electrochemically generated Co(I) complexes as a model for stability, we measured the relative stability of complexes prepared with a series of N,N-bidentate ligands. While the stability of Co(I)Cl complexes demonstrates a correlation with experimentally measured thermodynamic properties, consistent with an outer-sphere electron transfer process, the set of ligated Co(I)Br complexes evaluated was found to be preferentially stabilized by electronically asymmetric ligands, demonstrating an alternative disproportionation mechanism. These results allow a greater understanding of the fundamental processes involved in the disproportionation of organometallic complexes and have allowed the identification of cobalt complexes that show promise for the development of novel electrocatalytic reactions.

Establishing an efficient extracellular electron transfer (EET) process between photoelectroactive microorganisms and an electrode surface is critical for the development of photobioelectrocatalysis. Soluble and immobilized redox mediators have been applied with the purple bacterium Rhodobacter capsulatus for this purpose. However, detailed information on its EET with an electrode surface is not available and, therefore, choice of mediators has been by trial and error. Herein, we experimentally evaluated the capability of different soluble, quinone-based redox mediators to support EET and compared the experimental data with a computational model based on density functional theory calculations. We show that computed electrochemical redox properties of redox mediators in a lipophilic environment correlate to EET processes of Rhodobacter capsulatus, suggesting that intermembrane mediator characteristics are more diagnostic than redox properties of the mediators in an aqueous solution, and that the limiting electron transfer step takes place in the lipophilic membrane of the bacterial cells. This knowledge provides critical insight into designing future mediated bioelectrocatalysis systems.

The development of electrochemical catalytic conversion of 5‐hydroxymethylfurfural (HMF) has recently gained attention as a potentially scalable approach for both oxidation and reduction processes yielding value‐added products. While the possibility of electrocatalytic HMF transformations has been demonstrated, this growing research area is in its initial stages. Additionally, its practical applications remain limited due to low catalytic activity and product selectivity. Understanding the catalytic processes and design of electrocatalysts are important in achieving a selective and complete conversion into the desired highly valuable products. In this Minireview, an overview of the most recent status, advances, and challenges of oxidation and reduction processes of HMF was provided. Discussion and summary of voltammetric studies and important reaction factors (e. g., catalyst type, electrode material) were included. Finally, biocatalysts (e. g., enzymes, whole cells) were introduced for HMF modification, and future opportunities to combine biocatalysts with electrochemical methods for the production of high‐value chemicals from HMF were discussed.

Electrochemistry has made a significant impact on scientific discovery and industrial development throughout recent history. One of the most important contributions of the field, the battery, has provided much of the energy storage for this progress. Recently, redox flow batteries have emerged as a promising modern battery technology toward grid‐scale energy storage. Through the employment of non‐aqueous electrolytes and optimization of redox‐active organic molecules as catholyte and anolyte, these batteries have the potential to offer affordable, environmentally‐friendly energy storage without sacrificing desirable high energy densities and long cycling lifetimes. These developments are ongoing, and the associated computational tools have expanded the capabilities and scope of redox flow batteries and shown a path toward the eventual commercialization of this technology to continue to provide power to humanity into a bright future.

Redox flow batteries (RFBs) are a promising solution to grid-scale energy storage that utilize solvated redox-active species to store charge. These electrolytes are flowed over stationary electrodes during charge and discharge cycling. However, solubilizing the charge storage species allows for their crossover through the separating membrane, causing electrolyte mixing, and leads to capacity fade and battery failure. Herein, we employ a series of trimethylammonium-functionalized polyethylene membranes in RFB cells to address membrane swelling in organic solvent while maintaining high counterion (PF6 –) conduction. We show unprecedented results with 99.99% average capacity retention per cycle and 88% total capacity retention through 1000 charge/discharge cycles with low crossover, as compared to a commercial membrane often used in nonaqueous RFB (NARFB) studies which retained 36% capacity. Our results represent a critical step in developing and understanding anion-exchange membranes (AEMs) as separators for NARFBs and other electrochemical systems employing organic solvents.

Summary Phenazines are redox-active nitrogen-containing heterocyclic compounds that can be produced by either bacteria or synthetic approaches. As an electron shuttles (mediators), phenazines are involved in several biological processes facilitating extracellular electron transfer (EET). Therefore, it is of great importance to understand the structural and electronic properties of phenazines that promote EET in microbial electrochemical systems. Our previous study experimentally investigated a phenazine-based library as an exogenous mediator system to facilitate EET in Escherichia coli . Herein, we combine our experimental data with density functional theory (DFT) calculations and multivariate linear regression modeling to understand the structure-function relationships in phenazine-based mediated EET. These calculations demonstrate that the computed redox properties of phenazines in lipophilic environments (e.g., cell membrane) correlate to experimental mediated current densities. Additional DFT-derived molecular properties were considered to develop a predictive model, which could be used in metabolic engineering approaches to introduce phenazines as endogenous mediators into bacteria.