Invited: High Efficiency V/V Redox Flow Battery Stack Utilizing Mixed Acid Electrolyte Chemistry

The demand for large-scale electrical energy storage (EES) devices has been growing for both improved efficiency and flexibility of the current grid infrastructure and to enable a higher penetration of stochastic renewable sources such like solar and wind onto the grid.  Among the most promising technologies for the grid-scale EES are redox flow batteries (RFBs), which are capable of storing a large quantity of electricity (multi-MWs/MWhs) in a relatively simple and straightforward design. With support from the Department of Energy-Office of Electricity’s Energy Storage Program, PNNL developed a mixed-acid electrolyte (hydrochloric and sulfuric acid) for the all vanadium redox flow battery that offers significant improvements in thermal stability and solubility over the conventional, sulfate only system.

PNNL has incorporated the mixed acid electrolyte into kW scale systems in order to determine the optimal system efficiency at various flow, temperature, and current density conditions.  A 15-cell stack utilizing Nafion® 115 was successfully operated at more 160 mA/cm² over a 10 % - 90% SOC range with a stack energy efficiency of 74 %.  At a flow rate of 6 lpm for both the catholyte and anolyte, the system obtained an overall efficiency (stack - pumping power) of 65%.  With no active cooling the system reached a maximum temperature of 47°C.   Replacing the 5 mil Nafion® 115 membrane with a 2 mil Nafion® 212, the energy efficiency increased to 79% while the operating temperature decreased to 43°C for the same 6 lpm flowrate. Further development was done on the catholyte and anolyte flow field to reduce the pressure drop through the stack.  A new interdigitated flow field was designed and incorporated into the 15-cell stack.  With the interdigitated flow field and the Nafion® 212 membrane, the stack efficiency remained at 79% but the system efficiency improved to 72% due to the lower pressure drop in the system.  At 6 lpm flow rate, the interdigitated design resulted in ~ 3X decrease in the pressure drop across the cell (from an average of 6.5 psi to 2.5 psi for the interdigitated design). The operating temperature under these conditions was 40°C. Further testing of the interdigitated flow field and the Nafion® 212 membrane at 240mA/cm² and the same electrolyte flow rate netted a stack energy efficiency of 71%, a system efficiency of 67%, and an operating temperature of 46°C.