PNNL

Abstract

The utilization of aqueous organic redox flow batteries holds great promise for large-scale and sustainable energy storage. The development of low-cost and high-efficient aqueous redox flow batteries lies in a comprehensive understanding of the electrochemical behaviors of redox-active compounds. An alkaline organic redox battery with dihydroxyphenazine sulfonate (DHPS) anolyte and ferro-/ferricyanide (Fe(CN)6) catholyte is investigated as a typical example of aqueous organic redox flow batteries. The electrochemical kinetics of DHPS and Fe(CN)6 are separately characterized using the symmetrical cell design. The resistance components are calculated directly from the experimental measurement. The key kinetic parameters are extracted and compared for DHPS and Fe(CN)6 electrolytes. The extracted parameters are validated with symmetrical and full flow cell simulations at different operating conditions. Key parameters and internal loss are also compared with all-vanadium redox flow battery, representing current state-of-the-art. The redox kinetics of four reaction-active species are ordered and it shows sluggish reaction for DHPS. Their combined effect in full flow cell is also compared. It shows high ohmic loss and low reaction-related loss (activation and concentration loss) for DHPS-based flow batteries. The high membrane resistance obstacles further improvement of DHPS-based flow battery. In addition, our extracted key parameters from symmetrical flow cell are compared with the measured key parameters by cyclic voltammetry, a widely deployed electroanalytical technique. The reaction rate measured from cyclic voltammetry is several orders of magnitude larger than our extracted one. It is mainly caused by dissimilar testing conditions since cyclic voltammetry is ex-situ measurement and symmetrical flow cell is in-situ measurement. Therefore, precautions must be taken to directly use kinetic parameters from cyclic voltammetry. The cell performance prediction of DHPS anolyte on a 780 cm2 interdigitated cell is made and found the power density is peaked at 475 mW/cm2 at our measurement condition.