Electrochemical and Spectroscopic Investigation of Solvation and Complexation Effects on Titanium Redox Flow Battery Electrolytes

Abstract

Titanium (Ti) is a promising elemental redox active species for redox flow batteries (RFBs) as an alternative to the commercially advanced vanadium RFB (V-RFB) due to its 100x availability in the Earth crust1, and 10x lower cost1 (both compared to elemental vanadium). The half-cell potential of the Ti4+/Ti3+ (0.1 V vs SHE) redox couple is closer to the H+/H2 (0V vs SHE) redox couple compared to V3+/V2+ (-0.26V vs SHE)2 thereby mitigating parasitic hydrogen evolution reactions. Thus, the development of Ti-based RFBs (by coupling it with another economical and abundant elemental active) is a promising pathway towards cost effective, grid-scale energy storage and the present work aims to optimize the Ti electrolyte to enable this integration.Ti electrolytes (typically as the anolyte) have been coupled withFe3, 4, Mn5 and Ce6 based catholytes to yield a number of relatively (compared to other aqueous systems) high-potential and high energy density RFBs7. The Ti-Fe, Ti-Mn, and Ti-Ce RFBs have theoretical energy densities of 9 Wh.L-1, 18.9 Wh.L-1, and 19.4 Wh.L-1 respectively7. Electrolyte design with an eye towards improving actives solubility (thereby increasing energy density) and decreasing side-reactions has proven crucial to realizing these systems. For example, deleterious Cl2 evolution reduced the coulombic efficiency of the earliest Ti-Fe RFBs3 and has been overcome by the use of H2SO4 as an alternate supporting electrolyte4. In Ti-Mn RFBs, Mn3+ is highly unstable and inclined to form MnO2. A highly acidic environment or increasing the amount of Mn2+ reduces the disproportionation of Mn3+. Alternatively, the addition of an equal concentration of TiOSO4 with MnSO4 at the catholyte and reducing the maximum operational state of charge to 50%5 reduces MnO2 precipitation. In Ti-Ce RFBs, Ce is the limiting element in terms of solubility as the solubility of Ce is lower (0.5M in H2SO4 and 0.9M in CH3SO3H)6, 8, 9 whereas Ti is highly soluble (up to 5M TiOSO4 in 4M H2SO4). On the other hand, kinetically, Ti is the limiting element with 3x lower rate constant than the Ce redox couple6.Seeking to harness the higher solubility (and hence energy density) of the Ti electrolyte while overcoming the kinetic limitations, we investigated the solubility and the electrochemical reversibility of Ti4+/Ti3+. We characterized the behavior of Ti ions in various supporting electrolytes namely, H2SO4, HCl, HNO3, CH3SO3H by varying the ratio of Ti redox active species to counterion. The diffusion coefficients of the Ti3+ and Ti4+ ions were measured and the impact of the Tix+ to solvating ligand ratio was examined (see example in Fig.1(a)). Spectroscopically determining the coordination structures around solvated Tix+ ions10, we identified electrolyte compositions that result in increasing ionic conductivity (Fig.1(b)). The effect (or lack thereof) of solvation structure on the Ti3+/Ti4+ redox rate constants were examined and correlated to the calculated solvation energy (hence distinguishing between inner- and outer-sphere processes) and the role of catalysts was addressed. Finally, utilizing the electrochemical Thiele modulus framework11, 12, the best (highest energy density coupled with optimal transport and kinetic properties) Ti electrolyte compositions for Ti-Fe, Ti-Mn and Ti-Ce RFBs has been identified.