Bio-inspired Molecular Redesign of a Multi-redox Catholyte for High-Energy Non-aqueous Organic Redox Flow Batteries

Redox-active organic materials (ROMs) have recently attracted significant attention for redox flow batteries (RFBs) to achieve green and cost-efficient energy storage. In particular, multi-redox ROMs have shown great promise, and further tailoring of these ROMs would yield RFB technologies with the highest possible energy density. Here, we present a phenazine-based catholyte material, 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ), that undergoes two single-electron redox reactions at high redox potentials (-0.29 and 0.50 V versus Fc/Fc(+)) with enhanced solubility (0.5 M in acetonitrile), remarkable chemical stability, and fast kinetics. Moreover, an all-organic flow battery exhibits cell voltages of 1.2 and 2.0 V when coupled with 9-fluorenone (FL) as an anolyte. It shows capacity retention of 99.94% per cycle over 200 cycles and 99.3% per cycle with 0.1 M and 0.4 M BMEPZ catholyte, respectively. Notably, the BMEPZ/FL couple results in the highest energy density (similar to 17 Wh L-1) among the non-aqueous all- organic RFBs reported to date

Versatile Redox-Active Organic Materials for Rechargeable Energy Storage

CONSPECTUS: With the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metalbased electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions. In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal-air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.N

Pyrrolinium-substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications

Organic batteries for a greener rechargeable world

The emergence of electric mobility has placed high demands on lithium-ion batteries, inevitably requiring a substantial consumption of transition-metal resources. The use of this resource raises concerns about the limited supply of transition metals along with the associated environmental footprint. Organic rechargeable batteries, which are transition-metal-free, eco-friendly and cost-effective, are promising alternatives to current lithium-ion batteries that could alleviate these mounting concerns. In this Review, we present an overview of the efforts to implement transition-metal-free organic materials as the redox-active component in diverse types of organic rechargeable batteries. In addition, we critically evaluate the current status of organic rechargeable batteries from a practical viewpoint and assess the feasibility of their use in various energy-storage applications with respect to environmental and economic aspects. We believe this Review provides a timely evaluation of organic rechargeable batteries from a real-world perspective, and we hope it will spur more intensive efforts towards a greener energy future. Redox-active organic materials are a promising electrode material for next-generation batteries, owing to their potential cost-effectiveness and eco-friendliness. This Review compares the performance of redox-active organic materials from a practical viewpoint and discusses their potential in various post-lithium-ion-battery platforms.N

Pyrrolinium-Substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and Fe-57 Mossbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the pyrrolinium-substituted ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications.11Nsciescopu

Tunable Redox-Active Triazenyl-Carbene Platforms: A New Class of Anolytes for Non-Aqueous Organic Redox Flow Batteries

Non-aqueous all organic redox flow batteries (NORFBs) are one of the promising options for large-scale renewable energy storage systems owing to their scalability with energy and power along with the affordability. The discovery of new redox-active organic molecules (ROMs) for the anolyte/catholyte would bring them one step closer to the practical application, thus it is highly demanded. Here, we report a new class of ROMs based on cationic triazenyl systems supported by N-heterocyclic carbenes (NHCs) and demonstrate, for the first time, that the triazenyl can serve as a new redox motif for ROMs and could be significantly stabilized for the use in NORFBs by the coupling with NHCs even at radical states. A series of NHC-triazenyl ROM families were successfully synthesized via the reaction of a synthon, N-heterocyclic carbene azido cation, with various Lewis bases including NHCs. Remarkably, it is revealed that NHCs substituted on the triazenyl fragments can serve as a versatile platform for tailoring the electrochemical activity and stability of triazenyl-based compounds, introducing various ROMs exploiting triazenyl redox motif, as demonstrated in the full cell of NORFBs for an anolyte.

Phenoxazine as a high-voltage p-type redox center for organic battery cathode materials: small structural reorganization for faster charging and narrow operating voltage

Although organic p-type cathode materials with high redox potential (>3.5 V vs. Li/Li+) are sustainable alternatives to transition metal oxide cathodes for lithium-ion batteries, only a limited number of these materials have been investigated to date. Therefore, the discovery of new p-type redox centers is essential for further development of successful organic cathodes. Herein, we report phenoxazine (PXZ) as a new p-type redox center for high-voltage cathode materials. Negligible structural reorganization of this PXZ center facilitates a kinetically faster electrochemical pathway, leading to a narrow voltage plateau, full utilization of the capacity, and superior rate capability in a new PXZbased cathode material, PXZ trimer (3PXZ). The 3PXZ cathode delivered a specific capacity of 112 mA h g(-1) at 1C with a high average discharge voltage of 3.7 V vs. Li/Li+ in a Li-organic cell; moreover, even at a high rate of 20C, 73% capacity retention (76 mA h g(-1)) was achieved. In addition, a 3PXZ composite with mesoporous carbon CMK-3 exhibited a capacity of 100 mA h g(-1) with high stability, losing only 0.044% capacity per cycle over 500 cycles at 5C. As 3PXZ outperforms most reported p-type cathodes in terms of both rate capability and stability, we suggest the adoption of the PXZ unit as a novel and promising redox center for high-performance and sustainable energy storage systems.

Exploiting Biological Systems: Toward Eco-Friendly and High-Efficiency Rechargeable Batteries

To meet the ever-increasing energy demands and sustainability requirements, next-generation battery systems must provide superior energy densities while employing eco-friendly components. Transition metal oxide-based materials have served as important high-energy-density battery electrodes over the past few decades; however, their further development is challenging as we approach the theoretical limits arising from their crystal structures and constituting elements. Exploiting materials from biological systems, or bio-inspiration, offers an alternative strategy to overcome the conventional energy storage mechanism through the chemical diversity, highly efficient biochemistry, sustainability, and natural abundance provided by these materials. Here, we overview recent progress in biomimetic research focused on novel electrode material design for rechargeable batteries, exploiting redox-active molecules involved in the biometabolism and diverse bioderived materials with various morphologies. Successful demonstrations of energy storage using biomimetic materials that simultaneously exhibit outstanding performance and sustainability would provide insight toward the development of an eco-friendly and high-efficiency energy storage system.11scopu

In operando visualization of redox flow battery in membrane-free microfluidic platform

Copyright © 2022 the Author(s). Published by PNAS.Redox flow batteries (RFBs) are attractive large-scale energy storage techniques, achieving remarkable progress in performance enhancement for the last decades. Nevertheless, an in-depth understanding of the reaction mechanism still remains challenging due to its unique operation mechanism, where electrochemistry and hydrodynamics simultaneously govern battery performance. Thus, to elucidate the precise reactions occurring in RFB systems, an appropriate analysis technique that enables the real-time observation of electrokinetic phenomena is indispensable. Herein, we report in operando visualization and analytical study of RFBs by employing a membrane-free microfluidic platform, that is, a membrane-free microfluidic RFB. Using this platform, the electrokinetic investigations were carried out for the 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ) catholyte, which has been recently proposed as a high-performance multiredox organic molecule. Taking advantage of the inherent colorimetric property of BMEPZ, we unravel the intrinsic electrochemical properties in terms of charge and mass transfer kinetics during the multiredox reaction through in operando visualization, which enables theoretical study of physicochemical hydrodynamics in electrochemical systems. Based on insights on the electrokinetic limitations in RFBs, we verify the validity of electrode geometry design that can suppress the range of the depletion region, leading to enhanced cell performance.11Nsciescopu