Local structures of titanium-ion complexes in redox flow battery electrolytes as revealed by X-ray scattering with difference analysis

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

“…23 Ti 4+ ions can exist as either electrically neutral (e.g., [Ti(OH) 2 SO 4 (H 2 O) 3 ] 0 ) or anionic (e.g., [Ti(OH) 2 (SO 4 ) 2 (H 2 O) 2 ] 2− ) complexes under high pH conditions, which can lead to nucleation and precipitation of TiO 2 . 3,16,24 Precipitation was observed in electrolytes containing 0.5 M TiOSO 4 in DIW after a few days, consistent with previous findings. The precipitation may not depend solely on pH values but also on the SO 4 2− to H + ratio, as evidenced by the precipitation of 5 M TiOSO 4 in 4 M H 2 SO 4 (with a SO 4 2− to H + ratio of 9 to 4) after several months.…”

“…23 Mononuclear complexes, such as [Ti(OH) 2 SO 4 (H 2 O) 3 ]°, [Ti(OH) 2 (SO 4 ) 2 (H 2 O) 2 ] 2− , and [Ti=O(OH) 2 (H 2 O) 3 ]°, are formed through the coordination of Ti 4+ ions with SO 4 2− . Multinuclear complexes result from the polymerization of Ti 4+ complexes with oxygen atoms, leading to -Ti-O-Ti-O-zigzag structures 24 or the formation of [Ti 2 O 2 (H 2 O) 5 (OH) 2 SO 4 ]. 25 Ti 4+ complexes in the Ti-O-SO 4 system are either electrically neutral or anionic in high pH conditions.…”