All Vanadium Redox Flow Battery Development for Stationary Energy Storage Applications

The DOE Office of Electricity’s – Energy Storage program at Pacific Northwest National Laboratory (PNNL) has been focused on the development of cost effective energy storage technologies to improve the grid reliability, power quality, and adoption of the intermittent renewable energy. Among the most promising large-scale energy storage technologies are redox flow batteries (RFBs). RFB’s are electrochemical devices capable of storing up to multi-megawatt-hours (MWhs) of electrical energy via a reversible electrochemical energy conversion. Unlike traditional batteries in which the electrodes serve as the supply/sink of ionic charge, RFB’s store electricity in liquid electrolytes that flow on either side of a membrane. As such, the power and energy of a RFB can be separately designed, offering greater flexibility for a greater selection of stationary applications.

All-vanadium RFB system enlists the same element, vanadium, in both the catholyte and anolyte. Researchers at PNNL have revisited the standard vanadium sulfate electrolyte chemistry used in this system and developed an novel mixed acid electrolyte (sulfate and chloride) that can stabilize all four vanadium cations at concentrations ≥ 2.5M within a temperature range of 5~50^oC. Recently, PNNL recently demonstrated a kW/kWh scale prototype battery utilizing this electrolyte system in order to better understand the system benefits of the mixed acid electrolyte.  The system consisted of ~ 20 cell stack with 780cm² of active area/cell, ~ 30 liter electrolyte tanks, pumps and associated piping. The stack incorporated new interdigitated flow fields for reduced pressure drop and thinner Nafion membranes for improved performance. Results of stack testing under a wide range of current densities and temperature conditions will be reported along with resulting energy efficiencies.