Stabilizing Redox Active Molecules in Aqueous Electrolytes: A Path through Molecular Spectroscopy

Abstract

Redox flow batteries (RFB) are promising Large scale energy storage device (>kWh) which are compatible with intermittent energy conversion devices such as solar and wind energy. The efficiency of RFB critically depends on the chemical and thermal stability of redox active species (such as vanadium–bromide, iron–chromium, zinc–bromine, and all-vanadium) dissolved in aqueous electrolyte medium. Evidently, research efforts are fueled towards designing better electrolytes which can host stable redox active species through wide temperature and electrochemical window. In this aspect, our research group recently reported mixed acid based electrolyte system which offers better energy density through stabilizing vanadium (V) species in the aqueous solution [1]. This novel electrolyte design mainly arises from the synergetic effort between electrolyte synthesis efforts and fundamental studies about the molecular structure and stability of redox active species. A comprehensive analysis of chemistry behind redox active species in aqueous electrolyte system is done through combining experimental molecular spectroscopic studies (optical, magnetic resonance and X-ray spectroscopy) with density functional theory (DFT)-based simulations [2-4]. In this presentation, a detailed molecular level insight about the structure and dynamics of redox species in aqueous electrolytes gained through molecular spectroscopy will be presented. This fundamental insight will enhance the ability to predict the chemical stability of redox active molecules within aqueous electrolyte which is crucial in designing better aqueous electrolyte for RFB systems. Acknowledgements : The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery and Energy Reliability (OE) (under Contract No. 57558). We also are grateful for insightful discussions with Dr. Imre Gyuk of the DOE-OE Grid Storage Program. The spectroscopic work was conducted in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at PNNL. Pacific Northwest National Laboratory is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830. References :Li, L., Kim, S., Wang, W., Vijayakumar, M., Nie, Z., Chen, B., Zhang, J., Xia, G., Hu, J., Graff, G., Liu, J. and Yang, Z., Adv. Energy Mater., 1(2011), 394–400M. Vijayakumar, L. Li, G. Graff, J. Liu, H. Zhang, Z. Yang, J.Z. Hu, J. Power Sources, 196 (2011) 3669-3672.M. Vijayakumar, L. Li, Z. Nie, Z. Yang, J. Hu, Phys. Chem. Chem. Phys, 14 (2012) 10233-10242.M. Vijayakumar, W. Wang, Z. Nie, V. Sprenkle, J. Hu, J. of Power Sources, 241 (2013) 173-177.