Hybrid flow batteries are similar to traditional liquid/liquid flow batteries in that they store electrolytes separately from the power conversion cells. Unlike traditional flow batteries, hybrid flow batteries involve the deposition of a solid layer on at least one of the electrodes. The Zn/Fe hybrid flow battery negative electrolyte utilizes a complexed zinc anion. Zinc metal is deposited during charge and released back into solution on discharge. The positive electrolyte is comprised of an iron salt which changes oxidation states during charge and discharge. These electrolytes are composed of commercially available commodity chemicals that can be sourced for well under $100/kWh. A cation exchange membrane is used to separate the positive and negative electrolytes in the power conversion cell. Low molecular weight cations are shuttled back and forth across the membrane on charge and discharge to balance the reactions. Electrodes are made using non-precious metals and cell housings are composed of common injection moldable plastics.
Most redox flow battery systems utilize acidic electrolytes. The ViZn Zn/Fe battery system utilizes basic electrolytes. Therefore, the redox species in the Zn/Fe system are anionic. Utilizing anionic redox species separated by a cationic exchange membrane significantly reduces susceptibility to cross over contamination. The relatively large anion species are unable to cross the membrane due to the principal of Gibbs-Donnan exclusion. This results in a battery system with high coulombic efficiency and the ability to use thin highly conductive cation exchange membranes. These membranes, when coupled with a precision engineered cell, create a hybrid flow battery system with high power and high energy efficiency.
Inexpensive electrolytes coupled with high performance operation has allowed ViZn Energy Systems, Inc. to produce a robust four-hour battery system with a near term installed battery cost of under $350/kWh for use in the MW scale grid storage and regulation markets.