Browsing by Subject "Redox flow batteries"

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Direct measurement of vanadium cross-over in an operating redox flow battery
(2013-05) Sing, David Charles; Manthiram, Arumugam; Meyers, Jeremy P.
A redox flow battery (RFB) is an electrochemical energy storage device in which the storage medium is in the form of liquid electrolyte, which is stored in external reservoirs separate from the cell stack. The storage capacity of such systems is limited by the size of the external tanks, making the RFB an ideal technology for grid level energy storage. The vanadium redox flow battery (VRB) is a particularly attractive variant of the RFB, due to its use of a single transition-metal element in both the positive and negative electrolytes. However, the performance of the VRB is affected by the cross-over of electrolytes through the ion-exchange membrane which separates the positive and negative electrolytes. Cross-over causes degradation of energy storage efficiency and long term capacity loss. Previous studies of ion cross-over have focused primarily on the measurement of ion diffusion across ion exchange membranes in the absence of electrical current. In this work a novel VRB cell is described in which ion cross-over can be measured directly in the presence and absence of electrical current. Measurements are made of cross-over using this cell with three different types of ion exchange membrane in both charge and discharge modes. The results reported in this work show that the rate of ion cross-over can be greatly enhanced or suppressed depending upon the magnitude of the current flow and its direction relative to the ion concentration gradient.
Highly concentrated electrolyte design for high-energy and high-power redox flow batteries
(2021-01-22) Zhang, Leyuan; Yu, Guihua (Assistant professor); Goodenough, John Bannister; Manthiram, Arumugam; Mullins, Charles Buddie
Redox flow batteries (RFBs) have attracted immense research interests as one of the most promising energy storage devices for grid-scale energy storage. However, designing cost-effective systems with high energy and power density as well as long cycle life is still a big challenge for the development of next-generation RFBs. Eutectic electrolytes as a novel class of electrolytes have been recently explored to enhance the energy density of RFBs as they offer advantageous features such as low cost, ease of preparation, and high concentration of active materials. On the other hand, promising organic molecules with high solubility are also considered as potential candidates for next-generation RFBs due to the high tunability of redox potential, solubility, and stability. Furthermore, it is also necessary to design low-cost and high-performance membranes to realize the long-term stable cycling of RFBs. Here, the eutectic concept has been proposed as a new strategy to enable the design of highly concentrated electrolytes, thus boosting the energy density. The metal-based eutectic electrolytes are mainly formed by mixing anhydrous/hydrated metal halides with hydrogen bond donors (HBDs), such as urea or acetamide. Two metal-based eutectic electrolytes (Al & Fe) are mainly synthesized and studied and their redox reactions and physicochemical properties can be highly tuned. In the end, an all eutectic-based RFBs with high energy density is demonstrated. Besides, by incorporating the eutectic concept with the advantageous features of organic molecules, it becomes an alternative strategy to maximize the molar fraction of active species in organic-based eutectic electrolytes. Organic redox species can be further adopted to develop promising electrolytes with high concentration. By screening possible molecular structures integrated with molecular engineering and fundamental understanding, azobenzene- and organodisulfide-based molecules are found as promising redox species for high-energy and long-life RFBs. They show high solubility in supporting electrolytes and their electrochemical properties, stability, and redox chemistry are systematically studied via detailed electrochemical characterization and advanced calculation methods. Last but not least, we also explored the design of new membranes for RFBs. By utilizing the lamellar structure of stacked graphene oxide sheets or metal-organic frameworks, the designed membranes have the potential to provide high ionic conductivity and high selectivity for RFB applications. By integrating the highly concentrated electrolytes and new membrane design, we aim to provide an effective strategy to design the next-generation RFBs for grid-scale applications.
A multicomponent membrane model for the vanadium redox flow battery
(2012-08) Michael, Philip Henry; Meyers, Jeremy P.; Chen, Dongmei, Ph. D.
With its long cycle life and scalable design, the vanadium redox flow battery (VRB) is a promising technology for grid energy storage. However, high materials costs have impeded its commercialization. An essential but costly component of the VRB is the ion-exchange membrane. The ideal VRB membrane provides a highly conductive path for protons, prevents crossover of reactive species, and is tolerant of the acidic and oxidizing chemical environment of the cell. In order to study membrane performance and optimize cell design, mathematical models of the separator membrane have been developed. Where previous VRB membrane models considered minimal details of membrane transport, generally focusing on conductivity or self-discharge at zero current, the model presented here considers coupled interactions between each of the major species by way of rigorous material balances and concentrated solution theory. The model describes uptake and transport of sulfuric acid, water, and vanadium ions in Nafion membranes, focusing on operation at high current density. Governing equations for membrane transport are solved in finite difference form using the Newton-Raphson method. Model capabilities were explored, leading to predictions of Ohmic losses, vanadium crossover, and electro-osmotic drag. Experimental methods were presented for validating the model and for further improving estimates of uptake parameters and transport coefficients.