Biological anolyte regeneration system for redox flow batteries

Redox flow battery (RFB) electrolyte degradation is a common failure mechanism in RFBs. We report an RFB using genetically engineered, phenazine-producing Escherichia coli to serve as an anolyte regeneration system capable of repairing the degraded/decomposed redox-active phenazines. This work represents a new strategy for improving the stability of RFB systems because, under the influence of genetically engineered microbes, the anolyte species does not display degradation after battery cycling.

Photoactivation Properties of Self-n-Doped Perylene Diimides: Concentration-dependent Radical Anion and Dianion Formation

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R^•–) and dianions (D^••2–). We observe R^•– formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D^••2– formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R^•– to the strongly reducing doublet excited state ²[R^•–]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state ¹[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R^•– and D^••2– formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

Using structure-function relationships to understand the mechanism of phenazine-mediated extracellular electron transfer in Escherichia coli

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.

Unbranched Hybrid Conducting Redox Polymers for Intact Chloroplast-Based Photobioelectrocatalysis

Photobioelectrocatalysis (PBEC) adopts the sophistication and sustainability of photosynthetic units to convert solar energy into electrical energy. However, the electrically insulating outer membranes of photosynthetic units hinder efficient extracellular electron transfer from photosynthetic redox centers to an electrode in photobioelectrocatalytic systems. Among the artificial redox-mediating approaches used to enhance electrochemical communication at this biohybrid interface, conducting redox polymers (CRPs) are characterized by high intrinsic electric conductivities for efficient charge transfer. A majority of these CRPs constitute peripheral redox pendants attached to a conducting backbone by a linker. The consequently branched CRPs necessitate maintaining synergistic interactions between the pendant, linker, and backbone for optimal mediator performance. Herein, an unbranched, metal-free CRP, polydihydroxy aniline (PDHA), which has its redox moiety embedded in the polymer mainchain, is used as an exogenous redox mediator and an immobilization matrix at the biohybrid interface. As a proof of concept, the relatively complex membrane system of spinach chloroplasts is used as the photobioelectrocatalyst of choice. A "mixed" deposition of chloroplasts and PDHA generated a 2.4-fold photocurrent density increment. An alternative "layered" PDHA-chloroplast deposition, which was used to control panchromatic light absorbance by the intensely colored PDHA competing with the photoactivity of chloroplasts, generated a 4.2-fold photocurrent density increment. The highest photocurrent density recorded with intact chloroplasts was achieved by the "layered" deposition when used in conjunction with the diffusible redox mediator 2,6-dichlorobenzoquinone (-48 ± 3 μA cm-2). Our study effectively expands the scope of germane CRPs in PBEC, emphasizing the significance of the rational selection of CRPs for electrically insulating photobioelectrocatalysts and of the holistic modulation of the CRP-mediated biohybrids for optimal performance.

Advances in Electrochemical Modification Strategies of 5-Hydroxymethylfurfural

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.

Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) toward Sustainable and Safe Energy Storage

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-Br₂ 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 H₂O, 107.2 Ah/L, and 3.0 M in 2.0 NaCl, 80.4 Ah/L) and N¹-ferrocenylmethyl-N¹,N¹,N²,N²,N²-pentamethylpropane-1,2-diaminium dibromide, (FcN₂Br₂, 3.1 M in H₂O, 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 FcN₂Br₂/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/cm². Notably, the FcNCl/MV AORFBs (demonstrated at 7.0 and 9.9 Wh/L) exhibited unprecedented long cycling performance, 700 cycles at 60 mA/cm² with 99.99% capacity retention per cycle, and delivered power density up to 125 mW/cm². These AORFBs are built from earth-abundant elements and are environmentally benign, thus representing a promising choice for sustainable and safe energy storage.