Redox-active Organics Research Articles

Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review.

Aqueous organic redox flow batteries (RFBs) could enable widespread integration of renewable energy, but only if costs are sufficiently low. Because the levelized cost of storage for an RFB is a function of electrolyte lifetime, understanding and improving the chemical stability of active reactants in RFBs is a critical research challenge. We review known or hypothesized molecular decomposition mechanisms for all five classes of aqueous redox-active organics and organometallics for which cycling lifetime results have been reported: quinones, viologens, aza-aromatics, iron coordination complexes, and nitroxide radicals. We collect, analyze, and compare capacity fade rates from all aqueous organic electrolytes that have been utilized in the capacity-limiting side of flow or hybrid flow/nonflow cells, noting also their redox potentials and demonstrated concentrations of transferrable electrons. We categorize capacity fade rates as being "high" (>1%/day), "moderate" (0.1-1%/day), "low" (0.02-0.1%/day), and "extremely low" (≤0.02%/day) and discuss the degree to which the fade rates have been linked to decomposition mechanisms. Capacity fade is observed to be time-denominated rather than cycle-denominated, with a temporal rate that can depend on molecular concentrations and electrolyte state of charge through, e.g., bimolecular decomposition mechanisms. We then review measurement methods for capacity fade rate and find that simple galvanostatic charge-discharge cycling is inadequate for assessing capacity fade when fade rates are low or extremely low and recommend refining methods to include potential holds for accurately assessing molecular lifetimes under such circumstances. We consider separately symmetric cell cycling results, the interpretation of which is simplified by the absence of a different counter-electrolyte. We point out the chemistries with low or extremely low established fade rates that also exhibit open circuit potentials of 1.0 V or higher and transferrable electron concentrations of 1.0 M or higher, which are promising performance characteristics for RFB commercialization. We point out important directions for future research.

Tribute to Michel Armand: from Rocking Chair – Li-ion to Solid-State Lithium Batteries

Professor Michel Armand is one of the world's leading scientists in the R&D of modern energy storage systems. His scientific works have been devoted to the concepts and technologies of today's batteries, in particular, mastering research on electrodes and electrolytes. With respect to innovative discoveries, Michel had several scientific breakthroughs from the description of the solid-solution electrode and rocking-chair battery to the applications of novel materials utilized in all-solid-state batteries (SSBs); these refer to Li-ion, Na-ion, Li-S, and Li-air cells. At a young age, he formalized the concept of electrochemical intercalation and fabricated the first SSB with sodium-intercalated graphite as the electrode in 1972. Subsequently, he led the use of solid polymer electrolytes and developed lithium-metal-polymer batteries with vanadium oxide as the cathode in 1978. Michel successfully conducted research in new salts topics based on delocalized anions of the sulfonimide family in 1986; his research areas included 1) organic electrode materials, i.e., innovations in redox-active organics, which included poly quinones and aromatic dicarboxylates, and 2) carbon “nano painting” process that has made LiFePO4@C the safest cathode in 2002. Because Michel has co-authored over 500 publications and many patents, it is impractical to fully review his outstanding contributions in electrochemistry in this work. This work is limited to a few of his contributions pertaining to the evolution of electrochemical energy storage. We further discussed different routes envisioned for further progress in rechargeable batteries.

Ionic Liquids-Promoted Utilization of Redox-Active Organic Materials for Flow Batteries

Redox flow batteries (RFBs) are regarded as technique-of-choice for load levelling and peak shaving for the utilization of renewable energy sources, and are key component in smart grid network.[1,2] Electrolyte chemistry is a key consideration for the performance enhancement.[3-9] The conventional RFBs using non-organic redox species in aqueous H2SO4, KOH and NaCl, with limited operating voltage and volumetric energy density. Development of new high-performance electrolytes is highly important for speeding up the market penetration of RFBs. Herein, we explore the feasibility of using “water-in-ionic liquid” supporting electrolytes for tailoring the properties of electrolytes, including the electrochemical stability window, temperature adaptability, solubility of redox-active organics and chemical/electrochemical stability and reversibility of active materials.

Predicting the Solubility of Potential Redox Flow Battery Materials Using a Quantitative Structure-Property Relationship Models

Predicting the solubility and stability of redox-active organics across all states of charge is a challenging task. A trained chemist can often predict trends, but when it comes to numerical values, even the best computer software cannot be counted upon. In screening active materials for redox flow batteries (RFBs) – in which both properties are important – we are slave to trial and error, modifying organic molecules in attempt to optimize (increase) solubility and stability without compromising other important properties such as redox potential.1,2We hope to change our approach to materials development by better predicting properties in advance. Using quantitative structure-property relationship (QSPR), we have shown that properties of unknown organic molecules can be predicted when experimental properties of a training set of related compounds is provided.3Here we sought to expand this approach to a different redox core, phenothiazines, which we have evaluated as posolyte candidates for RFBs. We measured the solubility of about a dozen phenothiazine derivatives – in both relevant states of charge (neutral and radical cation) – in a nonaqueous electrolyte. The structure of these derivatives included variations in the number, type, and position of substituents to offer a diverse training set. We used our experimental values and the results of density functional theory calculations to develop a QSPR model for the phenothiazine core, following which we synthesized and measured the solubility of a new set of phenothiazines. Here we will report on the degree of success of our model for solubility prediction. References Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries.” J. Mater. Chem. A 2017, 5, 24371.Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries.” Energy Environ. Sci. 2016, 9, 3531.Sevov, C. S.; Hickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.*; Sigman, M. S.; Sanford, M. S. "Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications." J. Am. Chem. Soc. 2017, 139, 2924.

A Quinone-Based Redox Flow Battery at Near-Neutral pH with Record Capacity Retention Rate

A highly stable functionalized anthraquinone is introduced as the redox-active material in a negative potential electrolyte (negolyte) for aqueous redox flow batteries operating at nearly neutral pH. The design and synthesis of the molecule, which has a high solubility at pH 9 and above, is described. Chemical stability studies demonstrate high stability at both pH 9 and 12. By pairing it with a potassium ferri/ferrocyanide positive electrolyte across an inexpensive, non-fluorinated permselective polymer membrane, this near-neutral quinone-based aqueous flow battery exhibits an open-circuit voltage of 1.0 V and a capacity fade rate of 0.00036% / cycle and 0.014% / day, which is the lowest ever reported for any flow battery in the absence of rebalancing processes. It is further demonstrated that the negolyte pH drifts upward upon atmospheric oxygen penetration but, when oxygen is excluded, oscillates reversibly between 9 and 12 during cycling. These results enhance the suitability of aqueous-soluble redox-active organics for use in large-scale energy storage, potentially enabling massive penetration of intermittent renewable electricity.

Organic Eutectic Electrolytes for Future Flow Batteries

In this issue of Chem, Yu and coworkers report phthalimide-based eutectic anolytes, which achieved a high concentration and enhanced redox reversibility. The organic-molecule-based eutectic electrolytes take advantages of both the superior tunability of the organic molecule and the high molar concentration of the redox-active molecules of the eutectic solvents. With combined computational and experimental analysis, this work demonstrates that forming eutectic electrolytes with self-containing redox-active organics is a promising strategy for the future development of high-energy-density redox-flow batteries.

Macromolecular Design Strategies for Preventing Active‐Material Crossover in Non‐Aqueous All‐Organic Redox‐Flow Batteries

AbstractIntermittent energy sources, including solar and wind, require scalable, low‐cost, multi‐hour energy storage solutions in order to be effectively incorporated into the grid. All‐Organic non‐aqueous redox‐flow batteries offer a solution, but suffer from rapid capacity fade and low Coulombic efficiency due to the high permeability of redox‐active species across the battery's membrane. Here we show that active‐species crossover is arrested by scaling the membrane's pore size to molecular dimensions and in turn increasing the size of the active material above the membrane's pore‐size exclusion limit. When oligomeric redox‐active organics (RAOs) were paired with microporous polymer membranes, the rate of active‐material crossover was reduced more than 9000‐fold compared to traditional separators at minimal cost to ionic conductivity. This corresponds to an absolute rate of RAO crossover of less than 3 μmol cm−2 day−1 (for a 1.0 m concentration gradient), which exceeds performance targets recently set forth by the battery industry. This strategy was generalizable to both high and low‐potential RAOs in a variety of non‐aqueous electrolytes, highlighting the versatility of macromolecular design in implementing next‐generation redox‐flow batteries.

Macromolecular Design Strategies for Preventing Active-Material Crossover in Non-Aqueous All-Organic Redox-Flow Batteries.

Intermittent energy sources, including solar and wind, require scalable, low-cost, multi-hour energy storage solutions in order to be effectively incorporated into the grid. All-Organic non-aqueous redox-flow batteries offer a solution, but suffer from rapid capacity fade and low Coulombic efficiency due to the high permeability of redox-active species across the battery's membrane. Here we show that active-species crossover is arrested by scaling the membrane's pore size to molecular dimensions and in turn increasing the size of the active material above the membrane's pore-size exclusion limit. When oligomeric redox-active organics (RAOs) were paired with microporous polymer membranes, the rate of active-material crossover was reduced more than 9000-fold compared to traditional separators at minimal cost to ionic conductivity. This corresponds to an absolute rate of RAO crossover of less than 3 μmol cm-2 day-1 (for a 1.0 m concentration gradient), which exceeds performance targets recently set forth by the battery industry. This strategy was generalizable to both high and low-potential RAOs in a variety of non-aqueous electrolytes, highlighting the versatility of macromolecular design in implementing next-generation redox-flow batteries.

Cyclopropenium Salts as Cyclable, High‐Potential Catholytes in Nonaqueous Media

A series of small redox-active organics based on cyclopropenium salts are identified as promising nonaqueous catholytes. Synthetic modification provides access to highly soluble catholytes at all redox states (>0.7 m) and that undergo >200 charge–discharge cycles at high potentials (>0.8 V vs Fc/Fc+). The oxidized catholyte is isolable and stable at high concentration for days without degradation.

Recent Progress in Aqueous Organic Flow Batteries

Aqueous-soluble redox-active organic molecules such as quinones are potential low-cost alternatives to metal-based redox flow battery (RFB) chemistries, as demonstrated by a high-performance quinone-hydrobromic acid RFB [1-3]. Here we report the performance of new RFBs based on alkaline chemistries that are non-toxic and less corrosive than the acidic chemistry reported earlier. One cell utilizes 2,6-dihydroxy anthraquinone, which is stable and soluble in KOH, as the negolyte, coupled with ferri/ferrocyanide as the posolyte (Fig. 1, top) [4]. Others introduce RFBs to new classes of redox-active organics other than quinones, including one with a simple, single-step low-cost synthesis with good yield, which exhibits extremely high solubility and operates in a cell (Fig. 1, bottom) with >99.98% capacity retention per cycle. Fig. 1. Top: CV and cell performance for 2,6-DHAQ vs ferri/ferrocyanide alkaline RFB, from [4]. Bottom: CVs and cell performance for very inexpensive non-quinone RFB. [1] B. Huskinson et al., Nature 505, 195 (2014). [2] Q. Chen et al., J. Electrochem. Soc. 163, A5010 (2016). [3] Q. Chen et al., J. Electrochem. Soc. 163, A5057 (2016). [4] K. Lin et al., Science 349, 1529 (2015). Figure 1

A redox-flow battery with an alloxazine-based organic electrolyte

Redox-flow batteries (RFBs) can store large amounts of electrical energy from variable sources, such as solar and wind. Recently, redox-active organic molecules in aqueous RFBs have drawn substantial attention due to their rapid kinetics and low membrane crossover rates. Drawing inspiration from nature, here we report a high-performance aqueous RFB utilizing an organic redox compound, alloxazine, which is a tautomer of the isoalloxazine backbone of vitamin B2. It can be synthesized in high yield at room temperature by single-step coupling of inexpensive o-phenylenediamine derivatives and alloxan. The highly alkaline-soluble alloxazine 7/8-carboxylic acid produces a RFB exhibiting open-circuit voltage approaching 1.2 V and current efficiency and capacity retention exceeding 99.7% and 99.98% per cycle, respectively. Theoretical studies indicate that structural modification of alloxazine with electron-donating groups should allow further increases in battery voltage. As an aza-aromatic molecule that undergoes reversible redox cycling in aqueous electrolyte, alloxazine represents a class of radical-free redox-active organics for use in large-scale energy storage. Redox-flow batteries with organic-based electrolytes hold many advantages over conventional-flow batteries. Here the authors report a high-performance flow battery based on alloxazine, an aqueous-stable and soluble redox-active organic molecule resembling the backbone structure of vitamin B2.

Reverse Engineering To Suggest Biologically Relevant Redox Activities of Phenolic Materials

Phenolics are among the most abundant redox-active organics in nature, but the intractability of phenolic materials (e.g., melanin) has precluded study of their biological activities and functions. Previous studies demonstrated that a model abiotic catecholic matrix can rapidly exchange electrons with biological oxidants and reductants without the need for enzymes. Here, a novel electrochemically based reverse engineering approach was employed to probe redox interactions between this model matrix and a population of bacteria. Specifically, this method employs redox-active natural products (e.g., pyocyanin) to shuttle electrons between the bacteria and the abiotic matrix, and imposed oscillating potential inputs to engage redox-cycling mechanisms that switch the matrix's redox state. The oscillating output currents were observed to be amplified, gated, and partially rectified, while the overall magnitude and direction of electron flow across the matrix depended on the biological and environmental context. These response characteristics support hypotheses that natural phenolic materials may be integral to extracellular electron transport for processes that include anaerobic respiration, redox signaling, and redox-effector action.