(148d) Large-Scale Energy Storage: New Generation Vanadium Mixed Acid Redox Flow Battery

Electrical energy storage is considered as a key enabler to significant penetration of intermittent renewable power from solar and wind, as well as implementation of the Smart Grid. Among the most promising technologies is the all-vanadium redox flow battery (VRFB), which is capable of storing electricity on a MW/MWh scale in the form of two pairs of reduced and oxidized vanadium species (V²⁺/V³⁺ and V⁴⁺/V⁵⁺) dissolved in two separate liquid electrolytes, for a discharge duration of hours or even longer. VRFBs offer the flexibility of separate design of power (kW) and energy (kWh): power depends on stack design, while energy depends on the volume and vanadium concentrations in electrolytes. With electrodes free of repetitive mechanical or structural stresses, VFRBs potentially deliver virtually unlimited cycle life. In addition, VRFBs are inherently safe as flowing electrolytes carry away the heat generated during operation. Other advantages include quick response, deep discharge ability, capability to withstand a fluctuating power supply, tolerance to over-charge and over-discharge, and fast charge and discharge. Substantial advancement has been made in VRFBs using the traditional vanadium sulfate electrolytes in the past years. However this technology cannot meet the economic and performance requirements for broad market penetration. While there is much room to improve the economy and performance by engineering and system integration, the current technologies are inherently limited by the current electrolyte chemistries. To prevent irreversible vanadium oxide precipitation, active control of operating temperatures in the range 10-35^oC is essential, as is vanadium concentrations (i.e. energy capacity) ≤ 1.7M (1.5M in practical systems). Many impurities have to be controlled down to ppm levels to maintain the stability of the electrolytes. 

Recently, new vanadium redox chemistry with SO₄^2-/Cl^- mixed supporting electrolytes was invented.¹ To stabilize the vanadium electrolytes, chloride was added into the aqueous sulfate electrolyte solution to form new complexes of vanadium species. The new complexes were proved to be much more stable over the current vanadium sulfate chemistries, leading to: i) an extension of operation temperatures to a range of -5 to 60^oC, potentially eliminating the need for external heat management; ii) an increase in concentrations of vanadium species (V⁵⁺, V⁴⁺, V³⁺, V²⁺) to 2.5M, representing an energy capacity increase of 70% when compared to the all-vanadium sulfate chemistry; iii) achievement of nearly 90% energy efficiency at 50mA.cm^-2, with negligible degradation over a few hundred cycles. In this presentation, the structure of the vanadium species in the new electrolyte solutions and the performance and large-scale implementation potential of this advanced VRFB system will be discussed.

Acknowledgements

Part of the work was performed at Pacific Northwest National Laboratory, and supported by US Department of Energy.

 References

1. Li^*, Kim, Wang, Vijayakumar, Nie, Chen, Zhang, Xia, Hu, Graff, Liu, Yang^*, Advanced Energy Materials, 1, 394~400, 2011.8th International Vanadium Symposium: Chemistry, Biological Chemistry, & Toxicology (vanadium8.org)