Session: Innovative Chemistry and Materials for Electrochemical Energy Storage:

Nonaqueous redox flow batteries (NRFB) are a promising technology for future energy storage systems at utility scale. Inexpensive redox-active species that can yield high energy density over extended cycling are key to their implementation. Low solubility and poor stability of redox-active species, which limit the performance and energy density of NRFB, overshadow the numerous advantages offered by this technology. The calcium salt of VBH displays poor solubility in battery-relevant solvents. Here we report a theoretical and experimental approach to optimize the solubility of VBH-based active material by modifying the counter cation. A series of tetraalkylammonium (C =1-4) VBH were synthesized and probed as a tool to tune thermodynamic properties, including solvation free energy and lattice enthalpy. (1) We present evidence that an increase in chain length of alkyl group in cation facilitates the weakening of crystal lattice thereby increasing the solubility, despite less favorable solvation energy. Fundamental aspects of these thermodynamic underpinnings will be provided, based on computational chemistry results. Finally, we report cycling of this active material in a symmetric cell and a full cell at high concentration for up to 200 hrs by coupling the VBH based catholyte with an anthraquinone-based anolyte.

Patrick Cappillino

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