Impact of Electrolyte Viscosity on Redox Flow Battery Performance

Due to their inherent scalability and long operational lifetimes, redox flow batteries (RFBs) have shown promise as grid-level energy storage systems that enable the integration of intermittent renewable energy sources and the improvement of non-renewable energy processes on the grid. 1 However, additional cost reductions are required to reach proposed capital cost targets for widespread commercial implementation. 2 While multiple efforts have focused on discovery and development of next-generation materials for RFBs, fewer studies have systematically investigated the impact of the physical properties of electrolytes on cell performance. This is particularly relevant as many new electrolyte formulations push towards higher concentrations and incorporate new redox structures (e.g., macromolecules), which improve capacity and reduce cost, but also result in higher viscosities and greater property variation as a function of state-of-charge. These factors are expected to strongly influence cell performance. Specifically, engineering an efficient, cost-effective reactor depends on mitigation of resistive losses, several of which depend on electrolyte viscosity, and this dependence has yet to be fully characterized within a RFB. Here, we use a single-electrolyte flow cell configuration 3 , coupled with a model iron-based electrolyte, to probe the impact of electrolyte viscosity on RFB losses. Through the use of glucose as a chemically and electrochemically inert solution thickener, we investigate polarization as a function of electrolyte viscosity, electrolyte flowrate, and flow field geometry. Experimental data is combined with an one dimensional porous electrode polarization model to extract ohmic, kinetic, and mass transfer contributions to cell resistance. Of particular interest are mass transfer rates in RFBs, which are rarely quantified, thus the observed trends in mass transfer coefficient are correlated in terms of dimensionless numbers (e.g., Reynolds, Schmidt) in a traditional power-law format. This study aims to link electrolyte properties and cell performance and to provide a scalable descriptions of mass transfer in RFBs. Acknowledgments The authors acknowledge the financial support of the Joint Center for Energy Storage Research, which was formed under the Office of Basic Energy Sciences within the Department of Energy. We thank A. Helal and G. H. McKinley for rheological guidance and aid in viscosity measurements. We also thank M. Z. Bazant for use of milling equipment to fabricate flow fields used in this work. References 1. Weber, A. Z. et al. Redox flow batteries : a review. J. Appl. Electrochem. 41, 1137–1164 (2011). 2. Dmello, R., Milshtein, J. D., Brushett, F. R. & Smith, K. C. Cost-driven materials selection criteria for redox flow battery electrolytes. J. Power Sources 330, 261–272 (2016). 3. Darling, R. M. & Perry, M. L. The Influence of Electrode and Channel Configurations on Flow Battery Performance. J. Electrochem. Soc. 161, A1381–A1387 (2014). Figure 1