Low pH Titanium Electrochemistry in the Presence of Sulfuric Acid and its Implications for Redox Flow Battery Applications

Titanium (Ti) is a promising elemental redox active species for redox flow batteries (RFBs) due to its 100x availability in the Earth crust, and 10x lower cost (compared to elemental vanadium). Furthermore, Ti salts are highly soluble in water and concentrations >5 M can be easily obtained. Seeking to harness the higher solubility (and hence energy density) of the Ti electrolyte for flow battery applications, the Ti⁴⁺/Ti³⁺ redox couple was investigated at high concentrations (up to 5 M) relevant to RFB applications. The behavior of Ti ions in H₂SO₄ supported electrolytes was investigated by varying the ratio of Ti redox active species to counterion. The electrochemical characteristics, transport properties, and redox kinetics of the Ti⁴⁺/Ti³⁺ redox couple were measured and the impact of the Ti^x+ to solvating ligand ratio was examined. The coordination structures around solvated Ti^x+ ions were spectroscopically determined and the effect 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. The Ti electrolyte development guidelines presented herein will advance the development of Ti-based RFBs as a promising pathway towards cost effective, grid-scale energy storage.