(Digital Presentation) Tailoring Solvation and Counterion Complexation in the Electrolyte for Enhanced Titanium Redox Flow Battery Performance

Abstract

The use of titanium (Ti) in redox flow batteries (RFBs) is of increasing interest as Ti is 100x as abundant[1] worldwide as vanadium (V) with a 10x lower cost of production. The half-cell potential of the Ti4+/Ti3+ redox couple is 0.1 V (vs. SHE) as compared to -0.26 V (vs. SHE) for V3+/V2+, which makes the Ti4+/Ti3+ redox couple less prone to parasitic hydrogen evolution reactions. Aiming to optimize electrolyte formulation for Ti-based RFBs, we measured the transport and kinetic parameters for the Ti3+/Ti4+ redox couple in various acidic supporting electrolytes [2, 3, 4] such as H2SO4, HCl, HNO3 and CH3SO3H (methanesulfonic acid). The solubility and electrochemical reversibility of the Ti3+/Ti4+ redox couple was examined with a view toward increasing the energy density and energy efficiency of Ti-based RFBs. The solvation sphere around the redox active species was manipulated by varying the counterion and by careful control of the redox active species to counterion ratio. The solvation and complexation behavior were correlated to the transport and kinetic properties through the impact of the solvent reorganization energy () on the electrochemical Thiele Modulus[5]. The kinetics of the Ti3+/Ti4+ redox couple diverged by >2x between the forward and reverse reactions and the diffusion coefficients were found to vary by an order of magnitude with varying complexation and solvation. Finally, we also demonstrate the predictive ability of the electrochemical Thiele Modulus by correlating the predictions based on the effectiveness factor to polarization performance of Ti symmetric cells.