Novel, Stable Catholyte for Aqueous Organic Redox Flow Batteries: Symmetric Cell Study of Hydroquinones with High Accessible Capacity.

Owing to their broad range of redox potential, quinones/hydroquinones can be utilized for energy storage in redox flow batteries. In terms of stability, organic catholytes are more challenging than anolytes. The two-electron transfer feature adds value when building all-quinone flow battery systems. However, the dimerization of quinones/hydroquinones usually makes it difficult to achieve a full two-electron transfer in practical redox flow battery applications. In this work, we designed and synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring to probe molecular stability upon battery cycling. The redox potential of the four molecules were investigated, followed by long-term stability tests using different supporting electrolytes and cell cycling methods in a symmetric flow cell. The derivative with two unoccupied ortho positions was found highly unstable, the cell of which exhibited a capacity decay rate of ~50% per day. Fully substituted hydroquinones turned out to be more stable. In particular, 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol (asym-O-5) displayed a capacity decay of only 0.45%/day with four-week potentiostatic cycling at 0.1 M in 1 M H3PO4. In addition, the three fully substituted hydroquinones displayed good accessible capacity of over 82%, much higher than those of conventional quinone derivatives.

Study of Anion Exchange Membrane Properties Incorporating N-spirocyclic Quaternary Ammonium Cations and Aqueous Organic Redox Flow Battery Performance.

Flexible cross-linked anion exchange membranes (AEMs) based on poly (p-phenylene oxide) grafted with N-spirocyclic quaternary ammonium cations were synthesized via UV-induced free-radical polymerization by using diallylpiperidinium chloride as an ionic monomer. Five membranes with ion exchange capacity (IEC) varying between 1.5 to 2.8 mmol Cl-·g-1 polymer were obtained and the correlation between IEC, water uptake, state of water in the membrane and ionic conductivity was studied. In the second part of this study, the influence of properties of four of these membranes on cell cycling stability and performance was investigated in an aqueous organic redox flow battery (AORFB) employing dimethyl viologen (MV) and N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TMA-TEMPO). The influence of membrane properties on cell cycling stability and performance was studied. At low-current density (20 mA·cm-2), the best capacity retention was obtained with lower IEC membranes for which the water uptake, freezable water and TMA-TEMPO and MV crossover are low. However, at a high current density (80 mA·cm-2), membrane resistance plays an important role and a membrane with moderate IEC, more precisely, moderate ion conductivity and water uptake was found to maintain the best overall cell performance. The results in this work contribute to the basic understanding of the relationship between membrane properties and cell performance, providing insights guiding the development of advanced membranes to improve the efficiency and power capability for AORFB systems.

An Approach Toward Replacing Vanadium: A Single Organic Molecule for the Anode and Cathode of an Aqueous Redox-Flow Battery.

By combining a viologen unit and a 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) radical in one single combi-molecule, an artificial bipolar redox-active material, 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-1'-methyl-[4,4'-bipyridine]-1,1'-diium-chloride (VIOTEMP), was created that can serve as both the anode (-0.49 V) and cathode (0.67 V vs. Ag/AgCl) in a water-based redox-flow battery. While it mimics the redox states of flow battery metals like vanadium, the novel aqueous electrolyte does not require strongly acidic media and is best operated at pH 4. The electrochemical properties of VIOTEMP were investigated by using cyclic voltammetry, rotating disc electrode experiments, and spectroelectrochemical methods. A redox-flow battery was built and the suitability of the material for both electrodes was demonstrated through a polarity-inversion experiment. Thus, an organic aqueous electrolyte system being safe in case of cross contamination is presented.

Redox-Flow Batteries: From Metals to Organic Redox-Active Materials.

Research on redox-flow batteries (RFBs) is currently experiencing a significant upturn, stimulated by the growing need to store increasing quantities of sustainably generated electrical energy. RFBs are promising candidates for the creation of smart grids, particularly when combined with photovoltaics and wind farms. To achieve the goal of "green", safe, and cost-efficient energy storage, research has shifted from metal-based materials to organic active materials in recent years. This Review presents an overview of various flow-battery systems. Relevant studies concerning their history are discussed as well as their development over the last few years from the classical inorganic, to organic/inorganic, to RFBs with organic redox-active cathode and anode materials. Available technologies are analyzed in terms of their technical, economic, and environmental aspects; the advantages and limitations of these systems are also discussed. Further technological challenges and prospective research possibilities are highlighted.

An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System.

Redox-flow batteries (RFB) can easily store large amounts of electric energy and thereby mitigate the fluctuating output of renewable power plants. They are widely discussed as energy-storage solutions for wind and solar farms to improve the stability of the electrical grid. Most common RFB concepts are based on strongly acidic metal-salt solutions or poorly performing organics. Herein we present a battery which employs the highly soluble N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TEMPTMA) and the viologen derivative N,N'-dimethyl-4,4-bipyridinium dichloride (MV) in a simple and safe aqueous solution as redox-active materials. The resulting battery using these electrolyte solutions has capacities of 54 Ah L-1 , giving a total energy density of 38 Wh L-1 at a cell voltage of 1.4 V. With peak current densities of up to 200 mA cm-2 the TEMPTMA/MV system is a suitable candidate for compact high-capacity and high-power applications.

Polymer-Based Organic Batteries.

The storage of electric energy is of ever growing importance for our modern, technology-based society, and novel battery systems are in the focus of research. The substitution of conventional metals as redox-active material by organic materials offers a promising alternative for the next generation of rechargeable batteries since these organic batteries are excelling in charging speed and cycling stability. This review provides a comprehensive overview of these systems and discusses the numerous classes of organic, polymer-based active materials as well as auxiliary components of the battery, like additives or electrolytes. Moreover, a definition of important cell characteristics and an introduction to selected characterization techniques is provided, completed by the discussion of potential socio-economic impacts.

Investigation of Ice-Templated Porous Electrodes for Application in Organic Batteries.

Application and investigation of porous composite electrodes for organic batteries fabricated by an ice-templating method are reported for the first time. The possibility to produce polymer composite electrodes with highly aligned, parallel pores is demonstrated and electrochemical investigations are presented to examine their suitability for application in organic batteries. The performance of such ice-templated porous electrodes is experimentally compared with planar electrodes of similar composition against zinc and lithium counter electrodes, respectively. Fundamental properties limiting the performance of ice-templated porous electrodes are discussed and further means to overcome those limitations are proposed.

Poly(TEMPO)/Zinc Hybrid-Flow Battery: A Novel, "Green," High Voltage, and Safe Energy Storage System.

The combination of a polymer-based 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) catholyte and a zinc anode, together with a cost-efficient size-exclusion membrane, builds a new type of semi-organic, "green," hybrid-flow battery, which features a high potential range of up to 2 V, high efficiencies, and a long life time.

An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials.

For renewable energy sources such as solar, wind, and hydroelectric to be effectively used in the grid of the future, flexible and scalable energy-storage solutions are necessary to mitigate output fluctuations. Redox-flow batteries (RFBs) were first built in the 1940s and are considered a promising large-scale energy-storage technology. A limited number of redox-active materials--mainly metal salts, corrosive halogens, and low-molar-mass organic compounds--have been investigated as active materials, and only a few membrane materials, such as Nafion, have been considered for RFBs. However, for systems that are intended for both domestic and large-scale use, safety and cost must be taken into account as well as energy density and capacity, particularly regarding long-term access to metal resources, which places limits on the lithium-ion-based and vanadium-based RFB development. Here we describe an affordable, safe, and scalable battery system, which uses organic polymers as the charge-storage material in combination with inexpensive dialysis membranes, which separate the anode and the cathode by the retention of the non-metallic, active (macro-molecular) species, and an aqueous sodium chloride solution as the electrolyte. This water- and polymer-based RFB has an energy density of 10 watt hours per litre, current densities of up to 100 milliamperes per square centimetre, and stable long-term cycling capability. The polymer-based RFB we present uses an environmentally benign sodium chloride solution and cheap, commercially available filter membranes instead of highly corrosive acid electrolytes and expensive membrane materials.

Poly(exTTF): a novel redox-active polymer as active material for li-organic batteries.

The first polymer bearing exTTF units intended for the use in electrical charge storage is presented. The polymer undergoes a redox reaction involving two electrons at -0.20 V vs Fc/Fc(+) and is applied as active cathode material in a Li-organic battery. The received coin cells feature a theoretical capacity of 132 mAh g(-1) , a cell potential of 3.5 V, and a lifetime exceeding more than 250 cycles.

Polymers based on stable phenoxyl radicals for the use in organic radical batteries.

Polymers with pendant phenoxyl radicals are synthesized and the electrochemical properties are investigated in detail. The monomers are polymerized using ring-opening metathesis polymerization (ROMP) or free-radical polymerization methods. The monomers and polymers, respectively, are oxidized to the radical either before or after the polymerization. These phenoxyl radicals containing polymers reveal a reversible redox behavior at a potential of -0.6 V (vs Ag/AgCl). Such materials can be used as anode-active material in organic radical batteries (ORBs).

Powering up the future: radical polymers for battery applications.

Our society's dependency on portable electric energy, i.e., rechargeable batteries, which permit power consumption at any place and in any time, will eventually culminate in resource wars on limited commodities like lithium, cobalt, and rare earth metals. The substitution of conventional metals as means of electric charge storage by organic and polymeric materials, which may ultimately be derived from renewable resources, appears to be the only feasible way out. In this context, the novel class of organic radical batteries (ORBs) excelling in rate capability (i.e., charging speed) and cycling stability (>1000 cycles) sets new standards in battery research. This review examines stable nitroxide radical bearing polymers, their processing to battery systems, and their promising performance.