Redox-active Materials Research Articles

Advances in Zinc and Magnesium Battery Polymer Cathode Materials

Electrical energy storage is an ever growing and important area of research in a modern technological world. The quest for energy storage materials is always in the limelight of research for the replacement of conventional environmentally toxic metal-based redox-active materials by organic molecules which provides an alternative for rechargeable batteries. Zinc/magnesium-based conducting polymer batteries attracted significant attention due to their high abundance, safety, and cost-effectiveness compared with lithium ion batteries (LIBs). This Review lays out an extensive overview of metal anodes like zinc/magnesium with conducting polymer cathode materials that possess high conductivity and theoretical capacitance. In addition, the complete redox behavior of polymer cathodes, the mechanism, the anode behavior in acidic and alkaline media, the effect of different electrolyte uses and drawbacks, the binders, and the housing of these batteries have been reviewed in detail. The socioeconomic impact, problems associated with dendrite, and passive layer formation with zinc/magnesium polymer cathode batteries, as well as future perspectives, will give a complete overview for the general reader as well as for experts working in these fields.

Surficial amide-enabled integrated organic anode–binder electrode for electrochemical reversibility and fast redox kinetics in lithium–ion batteries

Binders have been focused on the improvement of electrode stability. However, they have great potential to enhance the electrochemical properties by introducing additional interaction with functional groups of organic electrodes. In this study, we propose an integrated organic anode–binder system from small molecule to polymer. It comprises a redox-active benzene group for highly stable and fast lithium-ion batteries via chemical cross-linking of the aromatic redox-active molecule with the poly(acrylic acid) binder. The surficial amide linkage not only suppresses electrolyte dissolution but also increases the lithium-ion reaction kinetics of the benzene ring with high reversible capacity. The lowered lowest unoccupied molecular orbital level and lithium-ion induction effect reduce the charge transfer and interface resistance, significantly outperforming the traditional electrode system with a poly(vinylidene fluoride) binder. This strategy is successfully expanded to polymeric system of polyimide microparticles possessing additional redox-active imide moiety along with benzene rings in their backbone. This work suggests a new strategy of providing additional capacity through reaction kinetics enhancement for multiscale redox-active organic materials.

Nanostructured Poly(hydroquinonyl-benzoquinonyl sulfide)/Multiwalled Carbon Nanotube Composite Cathodes: Improved Synthesis and Performance for Rechargeable Li and Mg Organic Batteries

Cost-effective, simple, and easily reproducible synthesis methods of polymers are of profound significance when it comes to extracting high battery performance metrics from polymeric redox-active materials. This work reports a procedure for the solvothermal synthesis of a poly(hydroquinonyl-benzoquinonyl sulfide) (PHBQS) polymer and the development of its nanostructured composites with multiwalled carbon nanotubes (MWCNTs). Polymers are tested as high-performance cathode materials for Li+ and Mg2+ batteries. In configurations, compared to neat PHBQS, the PHBQS@5%MWCNT cathode exhibits superior electrochemical performance with high active material utilization owing to improved ion/electron transport pathways. Galvanostatic characterization of the PHBQS@5%MWCNT cathode in lithium batteries exhibited peak capacity up to 358 mAh g–1 at a current density of 50 mA g–1 (C/8) and excellent rate performance with a discharge capacity of 236 mAh g–1 maintained even at high current density of 10C. The galvanostatic characterization in Mg batteries reveals more sluggish kinetics with a stable capacity of 200 mAh g–1 at 50 mA g–1.

(General Student Poster Session Award Winner - 1st Place) Effect of Surface Chemistry and Morphology on Polyluminol-Carbon Redox Active Composites

The rising demand for clean energy and environmental sustainability has led to the growing interest in redox-active organic-carbon composite electrodes for high power and long-lasting energy storage, especially for electrochemical capacitors (EC)[1]. Nitrogen containing organic redox active materials such as the conducting polymer polyluminol chemically polymerized (CpLum) on multiwalled carbon nanotubes (CNT) showed increased capacitive charge storage properties in the composite (CpLum-CNT)[2]. With merely a few nm CpLum, the composite electrodes exhibited reversible faradaic redox reactions and stored c.a. 3x higher volumetric charge than that of bare CNT. Predictions using DFT showed π interactions between the polymers and the CNT substrate. These interactions are thought to have stabilized the composite and contributed to the strong electrochemical performance. In addition to CNT substrates, activated carbons (ACs) offer high specific surface areas, diverse pore size distributions and native surface functionalities owing to their sources. Biomass-derived ACs can serve as low cost, sustainable alternative substrates from feedstocks as pinecone and waste tea [3]–[6].In this work, we investigate the effects of different surface chemistry, porosity and graphitization on CNT and porous activated carbon surfaces towards the redox activity of the CpLum-carbon composite. The surface functionalities of interest include hydroxyl and carboxyl groups owing to their known contributions toward surface wettability and pseudocapacitance while being present on naturally derived ACs. Varying porosity and graphitization also offer insight into the effects of differing specific surface areas, pore sizes, wettability, and conductivity on the CpLum-carbon composites. Figure 1 a. shows a schematic of the groups of interest and graphitization to investigate the surface interaction of luminol on CNT and Figure 1 b. shows the redox active behavior of CpLum-CNT. The insights from these studies will be used to engineer the surface of carbons such as CNTs and ACs to improve the interfacial properties for redox active materials.

Dicationic Heteroaryl Pyridinium As a Highly Stable, Soluble, and Crossover-Resistant Anolyte for Nonaqueous Redox Flow Batteries

Recently, redox flow batteries (RFBs) have emerged as one of the promising candidates for safe and economical grid-scale energy storage system (ESS) that store the intermittent energies such as wind or solar energy. Unlike the conventional aqueous vanadium RFBs, the design of new molecules in nonaqueous RFB (NRFB) has been in the spotlight for the high energy density, cycling stability, and low cost. Among the redox active materials, in particular, organic molecule has various structures through functionalization, and thus its redox potential, solubility, and chemical stability can be controlled. However, there are only a few viable negative-side redox electrolyte, called a negolyte (or anolyte) to date, and even the reported negolytes have suffered from low solubility and stability, and severe crossover problems for long-term cycling.Here we show the systematic design strategies for pyridinium-based organic redox molecule to enhance the its stability and solubility and suppress the crossover rate for NRFB. A benzo[d]thiazole ring, which provides an electron-withdrawing effect, was introduced at the C4 position of pyridinium core by Pd catalyzed C-H arylation. The addition of the π-conjugation system to the pyridinium redox core was key to enhance chemical and electrochemical stability, resulting in negatively low redox potential of -1.19 ~ 1.21 V vs. Fc/Fc+. The solubility of pyridinium derivatives was significantly enhanced from 0.26 M to 1.00 M in acetonitrile by simple anion exchange from tetrafluoroborate (BF4 -) or hexafluorophosphate (PF6 -) anion to bis(trifluoromethanesulfonyl)imide (TFSI-) anion. Moreover, exchanging the functional group on N of pyridinium from methyl group to cationic 3-(trimethylammonio)propyl (TMAP) group suppressed the crossover rate with an anion-exchange membrane in the NRFB. 4-(benzo[d]thaizol-2-yl)-1-(3-(trimethylammonio)propyl)pyridin-1-ium (TMAP-BTP) led to electrochemical stability in symmetric cell, showing a capacity decay rate of 0.0083% per cycle. Contrary to the results of only 60% capacity retention after 100 cycle in full cell with 4-(benzo[d]thaizol-2-yl)-1-methylpyridin-1-ium (BTP) as negolyte, in the case of full cell with TMAP-BTP as negolyte, the capacity retention was significantly increased, showing 89.8 % after 100 cycle, which is ~0.08% capacity decay rate per cycle. In this presentation, I will demonstrate the more detailed design strategies, cycling performances, and electrolyte analysis. (Figure 1).1 References Ahn, J. H. Jang, J. Kang, M. Na, J. Seo, V. Singh, J. M. Joo and H. R. Byon, ACS Energy Lett., 6, 3390 (2021). Figure 1

(Digital Presentation) Facile Synthesis of Pillar[6]Quinone and Investigation of Its Electrochemical Properties

Pillar[n]arene (n: number of units), macrocyclic para-arylene methylene molecules, have attracted attention owing to their symmetrical structures, host-guest properties, and original supramolecular assembly characteristics.1 Similarly, their quinone counterparts, pillar[n]quinone (P[Q]n), are also fascinating macrocycles containing electron-deficient quinone units, and therefore have potential application for novel host-guest chemistry and redox active materials. A hexagonal molecule, pillar[6]quinone (P[Q]6), is expected to form densely packed structures and a candidate for organic active material but the synthesis of P[Q]6 still remains a challenge.We previously demonstrated that the electrochemical oxidation (1.0 V vs. SCE) of 1,4-dihydroxypillar[6]arene (P[HQ]6) in methanol afforded micrometer scale hexagonal cylindrical depositions on electrode surfaces, which were composed of partially oxidized P[HQ]6-m[Q]m (composed of both hydroquinone and benzoquinone units) aggregating via quinhydrone formation.2 In this work, we successfully synthesized P[Q]6 for the first time by oxidation of its hydroquinone precursor P[HQ]6. Electrochemical oxidation (1.2 V vs. SCE) of P[HQ]6 in methanol gave the similar hexagonal cylindrical crystal of P[Q]6 evidenced by single crystal X-ray diffraction, NMR and HRMS analyses. In the crystallographic data, all quinone moieties seem to have intermolecular CH-O interaction between adjacent macrocycles, resulting in the formation of a hexagonal packing structure. In addition, we also found that scalable synthesis of P[Q]6 was possible by chemical oxidation of P[HQ]6 with phenyliodine(III)bis(trifluoroacetate) in 1,1,1,3,3,3-hexafluoro-2-propanol.To understand the electrochemical properties and electron-transfer behavior of P[Q]6, various voltametric studies were carried out. We revealed that three electrons are injected first, followed by stepwise three one-electron reductions due to the electrostatic repulsion in the latter electron-transfer process.Reference T. Ogoshi, T. Yamagishi, Y. Nakamoto, Chem. Rev., 116, 7937 (2016).C. Tsuneishi, Y. Koizumi, R. Sueto, H. Nishiyama, K. Yasuhara, T. Yamagishi, T. Ogoshi, Tomita and S. Inagi, Chem. Commun., 53, 7454 (2017).T. Hirohata, N. Shida, H. Uekusa, N. Yasuda, H. Nishihara, T. Ogoshi, I. Tomita, S. Inagi, Chem. Commun., 57, 6360 (2021). Figure 1

High-Capacity Bio-Inspired Redox Flow Batteries with Insoluble Redox Boosters

Redox flow batteries (RFBs) are considered to be one of the key technologies for addressing the intermittency problem in renewable energy sources, due to their unique architecture that allows for unprecedented scalability and flexibility, and cost-effectiveness in long-duration storage [1]. Despite their promise, typical RFBs suffer from low energy density because of the limit imposed by the solubility of the redox active species in the electrolyte [2]. One promising approach to bypass the solubility limitation and drastically improve the energy density is the addition of insoluble charge storage materials into the electrolyte tanks. These redox active materials are reversibly oxidized/reduced in the tanks upon interaction with the redox active species dissolved in the electrolyte. Thus, the capacity is no longer dependent on the concentration of soluble species but rather the quantity of solids in the tanks [3-4]. This new concept is called redox-mediated flow battery (RMFB).In this presentation, we demonstrate a high energy density RMFB using a highly stable bio-inspired active material (vanadium(IV/V)bis-hydroxyiminodiacetate (VBH)) and a suitable solid storage material. VBH, as a high-stability active material [5], provides an ideal scaffold to investigate the complex electrochemical processes that occur during RMFB operation, which remain virtually unexplored [6]. For the solid storage material, a Prussian Blue Analogue (PBA) is synthesized and coupled with VBH mediators. Symmetric cell cycling experiments are performed with and without the solid energy booster material to provide evidence that the addition of a compatible solid material greatly improves the energy density. Compatibility between the PBA and VBH is investigated using cyclic voltammetry (CV) experiments. Optimization of the critical parameters such as the amount of solid storage material, the operating current density, and the electrolyte flow rate is presented.

Half-Cell Flow Batteries: A Powerful Approach to Evaluating Cycling Stability of a Redox Active Electrolyte

Aqueous redox flow batteries (ARFBs) represent one promising energy storage technology for integration of renewable energy and balancing the electricity grids because of their technical merits of decoupled energy and power, sustainable and tunable redox active materials, and non-flammable and low cost aqueous supporting electrolytes. Despite numerous new flow battery chemistries reported in the last decade, the cycling life of ARFBs is still primarily limited by the chemical stability of redox active electrolytes. This presentation discusses the proper use and data interpretation of a half-cell flow battery to evaluate the cycling stability of a single redox active electrolyte. Specifically, the half-cell flow battery studies of K4[Fe(CN)6]/K3[Fe(CN)6] at alkaline conditions using balanced and unbalanced cell configurations will be discussed and compared. Our results reveal that the capacity loss of the K4[Fe(CN)6]/K3[Fe(CN)6] half-cell is attributed to cyanide ligand dissociation and then subsequent redox degradation. The reported half-cell flow battery methodology can be widely applied to develop new redox active electrolyte materials.

(Invited) Accelerating Material Design for Aqueous Organic Redox Flow Batteries

Aqueous soluble organic (ASO) redox-active materials have recently shown great promise as alternatives to transition metal ions to be employed as energy-bearing active materials in redox flow batteries for large-scale energy storage because of their structural tunability, cost-effectiveness, availability, and safety features. Development so far however has been limited to a small palette of organics that are aqueous soluble. How to quickly identify and design organic molecules for the targeted properties became a critical challenge in accelerating the aqueous organic flow battery development.In this presentation, a data-driven material design process is discussed. This process incorporates database building, machine learning modeling for properties prediction, and the subsequent material synthesis and testing. Examples including Fluorenone and Phenizane derivative development for the flow battery application will be discussed.Reference: Feng et al., Science 372, 836–840 (2021)

(Invited) Development and Characterization of Polyoxometallate-Based Systems for Aqueous Redox Flow Batteries

Redox flow batteries are of potential importance to both large scale energy storage and powering the electrical vehicle. It is now accepted that flow batteries are the battery technology with the greatest potential to be one of the key elements in the energy transition to a sustainable electricity supply. Among critical challenges are low volumetric energy density of redox electrolytes, high cost and the maintenance limitations that greatly impede the wide application of conventional flow batteries. In this respect, the redox-active charge-storage material has a significant impact on the performance of a flow battery. The concentration of redox centers and their reaction kinetics have an influence on the available current densities and, thus, the power of the device.Many inorganic and organic electroactive systems have been proposed as alternatives to vanadium species in redox flow batteries. In the study, we explore the concept that highly concentrated solutions of the polyoxometallates of molybdenum and tungsten can serve as model examples of multi-electron systems for all-liquid redox flow batteries and related fundamental investigations. Polyoxometallates are polynuclear inorganic materials with well-defined multi-electron reversible electrochemistry and electrocatalytic properties [1]. Among other important characteristics of heteropolyacids are that they exhibit very strong Brønsted acidity, act as proton conductors, and undergo fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (e.g. tungsten(VI,V) or molybdenum(VI,V) heteropoly blue) compounds. The polyoxometallate-based redox electrolytes have different chemical identities, and they could be considered as anolytes or catholytes, depending on their redox potentials but, typically, their use would require formation of an asymmetric system with different-type redox species. The scope of existing inorganic and organic electroactive materials can be expanded due the possibility of their functionalization and structural modification. Recent developments in the area of the transition-metal-derivatized polyoxometallates are also promising [2] because they imply the feasibility of formation of the bi-redox polyoxmetallate-based electrolytes. In particular, the systems containing such metals as copper, iron, ruthenium, nickel or tin could of interest. Such features as the feasibility of reversible multi-electron redox processes, the improved potential output and cycling performance, sufficiently high solubility and reasonable stability will be examined here. An alternative approach can be based on the preparation of robust zeolite-type cesium salts of polyoxometallates, Cs2.5H0.5PMo12O40, Cs2.5H1.5SiMo12O40, Cs2.5H0.5PW12O40, and Cs2.5H1.5SiW12O40. It is noteworthy that, at certain contents of cesium (or rubidium), these porous salts are characterized by fast charge propagation. They can be considered for application in a form of colloidal suspensions.While kinetics of electrochemical processes has an influence on the systems’ current densities, the viscosity of the electrolyte and the mass transport dynamics are also affected by the choice of the redox-active material and its concentration. Trying to develop useful electroanalytical diagnostic criteria, we are going to extend the historical concepts of charge propagation in semi-solid or semi-liquid systems developed for mixed-valence redox polymers and polynuclear materials to the development of redox electrolytes. Fundamental electroanalytical approaches utilizing ultramicrodisk electrodes and interdigitated arrays will be adapted to characterization of solid suspensions synthesized in a form of stable colloidal solutions utilizing redox active centers capable of exhibiting fast electron transfers according to electron hopping mechanism. Of additional interest is the dynamics of electron transfer at the electrolyte/electrode interface, considered at the molecular and monolayer scales.[1] I.A. Rutkowska, P.J. Kulesza, “Metal Oxide Cluster and Polyoxometallate Supports for Noble Metal Nanoparticles in Efficient Electrocatalysis” in Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, vol. 5, pp 207–216, 2018.[2] J. Goura, B.S. Bassil, J.K. Bindra, I.A. Rutkowska, P.J. Kulesza, N.S. Dalal, U. Kortz, Chemistry - A European Journal 26 (2020) 15821 – 15824.

(Invited) Redox Flow Battery Innovation

Flow battery innovations should offer significant improvements in performance, without compromising the durability / lifetime, and be cost-effective and scalable. The presentation will review some of the progress that has been made to enhance flow battery performance, and will discuss a number of recent innovations that aim to deliver these characteristics. These will include: Magnetic flowable electrodes applied in a polysulfide-iodide flow battery.Using flow through the current feeder to enhance mass transport and enable dendrite free zinc deposition in the zinc-iodide flow battery.Graphene modified membrane for enhanced power density. Flowable electrodes have emerged as a novel concept for high energy density batteries. To date, in most cases the flowable solid phase includes a redox active energy storage material, for example in zinc-nickel, sodium-sulfur, and lithium-sulfur systems [1-3]. In contrast, we have demonstrated the use of a carbon – magnetite nanocomposite which acts as an electrocatalyst but is not redox active [4,5]. This nanomaterial can be dispersed in the electrolyte and circulated through the battery to enhance the performance of a conventional static electrode. The magnetic characteristics of the nanocomposite can also be exploited, by using a magnetic field to assemble a high surface area electrode comprising a percolating network of the nanomaterial on the current feeder. The electrode also can be removed by releasing the magnetic field at the current feeder, and after being washed out of the cell the nanocomposite can be separated in a magnetic field. This enables replacement of the active electrode without the need to dismantle the cell.Zinc-iodide flow batteries offer high energy density due to the high aqueous solubility of the ZnI2. However, the power density that can be achieved is limited by potential for the dendritic growth of zinc deposits, and as zinc metal builds up in the cell the areal capacity is limited. We have found that by drawing some of the electrolyte through the current feeder, improved performance can be obtained [6]. This enables operation at higher power density and the denser uniform deposit should enable increased areal capacity.We attempted to reduce crossover in the all-vanadium redox flow battery by using a graphene modified nafion membrane. However, we found that the addition of the graphene reduced the losses in the battery and enabling a significant increase in the power density and discharge capacity. Currently we are working to optimize and scale up the membrane modification process, and to explore the mechanism of performance enhancement. References G. Zhu et al. (2020) High-energy and high-power Zn–Ni flow batteries with semi-solid electrodes. Sustainable Energy Fuels, 4, 4076-4085.Yang et al. (2018) Sodium–Sulfur Flow Battery for Low-Cost Electrical Storage. Advanced Energy Materials, 11, 1711991.Suo et al. (2015) Carbon cage encapsulating nano-cluster Li2S by ionic liquid polymerization and pyrolysis for high performance Li–S batteries. Nano Energy, 13, 467-473.Rahimi, A.M. Dehkordi, E.P.L. Roberts (2021) Magnetic nanofluidic electrolyte for enhancing the performance of polysulfide/iodide redox flow batteries. Electrochimica Acta, 309, 137687.Rahimi, A.M. Dehkordi, H. Gharibi, E.P.L. Roberts (2021) Novel Magnetic Flowable Electrode for Redox Flow Batteries: A Polysulfide/Iodide Case Study. Ind. Eng. Chem. Res., 60, 824-841.F. ShakeriHosseinabad et al. (2021) Influence of Flow Field Design on Zinc Deposition and Performance in a Zinc-Iodide Flow Battery. ACS Applied Mat. & Interfa

(Keynote) Opportunities and Challenges of Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) represent a promising energy storage technology for the integration of renewable energy and balancing electricity grids because of their technical merits of decoupled energy and power, sustainable and tunable redox active materials, and non-flammable and low cost aqueous supporting electrolytes. This presentation will review recent efforts in developing new flow battery chemistries, including viologen (anolyte), quinone (anolyte), TEMPO (catholyte), ferrocene (catholyte), and ferrocyanide (catholyte). The merits and drawbacks of representative acidic, pH neutral, and alkaline AORFBs will be discussed and compared. A number of research strategies will be discussed to develop high-performance redox active electrolytes to enable energy dense, durable, low-cost flow battery technologies. Particularly, this presentation emphasizes that in-depth mechanistic understandings of redox active electrolytes play crucial roles in developing advanced organic redox flow batteries towards sustainable, long-duration energy storage.

A Critical Approach to Multi-Electron Materials for High Energy Density Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are very promising storage systems in the transition towards renewable energy sources. They can be broadly classified in aqueous and non-aqueous systems. Last-named operate with organic solvents, which allow for a much broader potential window (up to three times higher) compared to water. Combination of organic solvents with organic redox active materials could pave the way for all carbon-based RFBs with superior energy densities compared to aqueous systems.In this contribution, newly developed organic electrolytes will be presented, focusing on their performance as RFB materials in acetonitrile with quaternary ammonium electrolyte salts. To push the limits of energy density, we investigated multi-electron reduction and oxidation reactions on a single molecule.As catholyte, tetrathiafulvalene (TTF) as a core structure was used. The unsubstituted TTF exhibits two reversible oxidation events (-0.04 and +0.34 V vs Fc/Fc+) but a poor solubility in acetonitrile. To optimize this and to achieve a higher oxidation potential a series of molecules with different side chains was synthesized. The resulting solubilities led to volumetric capacities of up to 71 Ah/L for the newly designed compounds. Electrochemical cycling stability was evaluated in bulk electrolysis, UV-Vis-NIR and flow cycling experiments. Current state-of-the-art anolytes based on N-methylphthalimide compounds exhibit one reversible reduction pair at a desirable low reduction potential (-1.87 V vs Fc/Fc+) and good cycling stability in bulk electrolysis experiments.[1] Expanding on this structure, new derivatives with one or two additional functional imide groups per phthalimide core were synthesized. Consequently, molecules with a single (-1.87 V vs Fc/Fc+), double (-1.26 and -1.88 V vs Fc/Fc+) and triple reduction event (-1.02, -1.65 and -2.37 V vs Fc/Fc+) can be obtained. To optimize their solubility in acetonitrile, a series of molecules with different side chains was synthesized for each of the three core structures. Determination of the solubility led to volumetric capacities of up to 66 Ah/L for the newly developed compounds. Additionally, we tested the electrochemical cycling stability in bulk electrolysis, UV-Vis-NIR, coin cell and flow cycling experiments. In the final flow battery, a high energy density of 24 Wh/L was achieved (at 1 M of transferred electrons).[2] All of this led to a critical comparison between the different molecular designs, cumulating in design rules that will influence future designs of the organic redox active compounds for organic RFBs. [1] Wei, X.; Duan, W.; Huang, J.; Zhang, L.; Li, B.; Reed, D.; Xu, W.; Sprenkle, V.; Wang, W. A High-Current, Stable Nonaqueous Organic Redox Flow Battery. ACS Energy Lett. 2016, 1, 705−711 [2] Daub, N.; Janssen, R. A. J.; Hendriks, K. H. Imide-Based Multielectron Anolytes as High-Performance Materials in Nonaqueous Redox Flow Batteries. ACS Appl. Energy Mater. 2021, 4, 9, 9248–9257

Solubility Trends in Pyridinium Salts for Non-Aqueous Organic Redox Flow Batteries

Redox flow batteries (RFBs) are a strong candidate for grid-scale energy storage applications. Recent pursuits for chemical systems involve focus on organic species, due to their chemical abundance, and non-aqueous solvent systems, due to an expanded electrochemical stability window. Currently, RFBs suffer from limitations that prevent them from being economically competitive when scaled. Among the critical properties hindering RFB expansion, one limitation is low system energy densities. The energy density of a RFB system is dependent on voltage, electrons transferred, and concentration of the anolyte and catholyte. Electrolyte advancements have focused on optimizing energy density by targeting species that maximize divergence of anolyte/catholyte redox potentials, increase species solubility, and feature reversible multi-electron transfer. Strategic structural engineering of redox-active materials is necessary to tune these distinct qualities. Understanding the relationship between molecular design and these variables, and then developing strategies to predict structures with optimal characteristics could help identify promising electrolyte candidates. Our work is focused on understanding and predicting the solubility trends of pyridinium anolyte materials. The pyridiniums in this series all feature low reduction potentials and a broad range of solubility in acetonitrile. By carefully aligning experimental data to DFT and other modeled parameters we are investigating the predictive parameters involved in controlling electrolyte solubility.

New organic electrode materials for lithium batteries produced by condensation of cyclohexanehexone with p-phenylenediamine

The redox-active materials based on the condensation product of cyclohexanehexone with p-phenylenediamine (PTDA) were synthesized. The effect of the reactant ratios (1: n, n = 1, 2, 3) on the morphology and electrochemical properties of the resulting polymers PTDA1-PTDA3 was analyzed. In the case of PTDA3, a regular network crosslinked by CN-C bonds is formed resulting in a significant increase in the practical capacity of this material from 160 to 285 mAh g−1 corresponding to a three-electron redox process. Quantum-chemical modeling was performed to clarify the peculiarities of the lithiation mechanism. It is shown that the best sites for lithium cation coordination are adjacent oxygen and nitrogen atoms or two nitrogen atoms. For all polymer structures, sequential redox lithiation-delithiation processes have been established.

Emerging chemistries and molecular designs for flow batteries.

Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.

Fluorination Enables Simultaneous Improvements of a Dialkoxybenzene-Based Redoxmer for Nonaqueous Redox Flow Batteries.

Redoxmers or redox-active organic materials, are one critical component for nonaqueous redox flow batteries (RFBs), which hold high promise in enabling the time domain of the grid. While tuning redox potentials of redoxmers is a very effective way to enhance energy densities of NRFBs, those improvements often accompany accelerated kinetics of the charged species, undermining stability and cycling performance. Herein, a strategy for designing redoxmers with simultaneous improvements in redox potential and stability is proposed. Specifically, the redoxmer 1,4-di-tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene (ANL-C46) is developed by incorporating fluorinated substitutions into the dialkoxybenzene-based platform. Compared to the non-fluorinated analogue, ANL-C46 demonstrates not only an increased (∼0.41 V) redox potential but also much enhanced stability (1.6 times) and cyclability (4 times) evidenced by electron paramagnetic resonance kinetic study, H-cell and flow cell cycling. In fact, the cycling performance of ANL-C46 is among the best of high potential (>1.0 V vs Ag/Ag+) redoxmers ever reported. Density functional theory calculations suggest that while the introduced fluorine substitutions elevate the redox potentials, they also help to depress the decomposition reactions of the charged redoxmers, affording excellent stability. The findings represent an interesting strategy for simultaneously improving energy density and stability, which could further prompt the development of high-performance redoxmers.

Redox-active anomalous electrochemical performance of mesoporous nickel manganese sulfide nanomaterial as an anode material for supercapattery devices

In this work, we are reporting nickel manganese sulfide hierarchical redox-active nanostructured material synthesized using a facile one-step hydrothermal technique to investigate its potential for supercapattery devices. The surface morphology, crystallinity, elemental analysis/composition surface area, porosity, and homogeneity were investigated through X-ray diffraction (XRD), Energy dispersive X-ray (EDX) spectra, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET). The electrochemical characterizations were performed in a three-electrode standard cell whereas the electrolyte used was 1 M potassium hydroxide. These characterizations predict that sample S-0.4 is exhibiting superior performance over all other electrodes and therefore it was paired with activated carbon for the assembling of supercapattery (Ni–Mn–S//AC). This supercapattery was probed electrochemically with CV, GCD, EIS, and stability tests which reveals superb performance by delivering a high value of capacity (420.10 C/g) with a maximum energy density of 75.96 Wh/kg. The device was able to deliver the power density of 2865 W/kg, along with an outstanding cyclic life by sustaining 85% of capacity even after 5000 GCD cycles. Our analysis for this electrode material suggests that our synthesized material can be applied for future high-performance supercapattery devices.

An Energy-Dense, Powerful, Robust Bipolar Zinc-Ferrocene Redox-Flow Battery.

Zinc metal represents a low-cost, high-capacity anode material to develop energy-dense aqueous redox-flow batteries (RFB). However, the energy-storage applications of traditional inorganic Zn halide flow batteries are primarily plagued by the material challenges of traditional halide cathode electrolytes (e.g., bromine), including corrosion, toxicity, and severe crossover. Herein, we report a bipolar zinc-ferrocene salt compound, zinc 1,1'-bis(3-sulfonatopropyl)ferrocene, Zn[Fc(SPr)2 ] (1.80 M solubility or 48.2 Ah L- charge storage capacity)-a robust, energy-dense, bipolar redox-active electrolyte material for RFBs. Zn[Fc(SPr)2 ]-based redox-flow batteries operated at high current densities of up to 200 mA cm-2 and delivered an energy efficiency of up to 81.5 % and a power density of up to 270.5 mW cm-2 . A Zn[Fc(SPr)2 ] flow battery demonstrated an energy density of 20.2 Wh L-1 and displayed nearly 100 % capacity retention for 2000 cycles (1284 h or 53.5 days).

Unraveling Synergistic Redox Interactions in Tetraphenylporphyrin-Polyluminol-Carbon Nanotube Composite for Capacitive Charge Storage.

Organic redox-active materials, combined with high-surface-area carbonaceous substrates, form sustainable and low-cost composites with greatly enhanced electrochemical charge storage capacities. The electrochemical capacitive behavior of a composite electrode containing tetraphenylporphyrin sulfonate (TPPS), Chemically polymerized luminol (CpLum), and carbon nanotubes (TPPS-CpLum-CNT) was studied and compared with individual TPPS-CNT and CpLum-CNT composites. The dual-layer TPPS-CpLum had a combined contribution to the electrochemical charge storage, which led to an increased volumetric capacitance over the bare CNT and individual TPPS-CNT and CpLum-CNT composites. The synergistic interactions in the composite enabled faster charge storage kinetics and great stability. Spectroscopic analyses revealed that TPPS and CpLum interact electronically through noncovalent π-π and van der Waals bonds, which facilitates the transfer of electrons during charge and discharge. The synergy in charge storage was confirmed by density functional theory computational analysis, which suggested favorable physisorption and interfacial electronic interactions for TPPS adsorbed to a CpLum-carbon substrate. The combined insights from experimental and computational characterizations show that superimposing redox-active organic layers can be an effective and sustainable approach to design and engineer the surface of carbonaceous materials for capacitive charge storage.