Aqueous titanium redox flow batteries—State-of-the-art and future potential

Abstract

Market-driven deployment of inexpensive (but intermittent) renewable energy sources, such as wind and solar, in the electric power grid necessitates grid-stabilization through energy storage systems Redox flow batteries (RFBs), with their rated power and energy decoupled (resulting in a sub-linear scaling of cost), are an inexpensive solution for the efficient electrochemical storage of large amounts of electrical energy. Titanium-based RFBs, first developed by NASA in the 1970s, are an interesting albeit less examined chemistry and are the focus of the present review. Ti, constituting 0.6% of the Earth’s crust and an ingredient in inexpensive white paints, is amongst the few elements (V and Mn being some others) which exhibit multiple soluble oxidation states in aqueous electrolytes. Further, the very high (approaching 10 M) solubility of Ti in low pH solutions suggests the possibility of developing exceptionally high energy density aqueous Redox Flow Batteries systems. With these advantages in mind, we present the state-of the-art in Ti-RFBs with a focus on Ti/Mn, Ti/Fe and Ti/Ce couples and systems that use Ti as an additive (such as Ti/V/Mn). The inherent advantages of inexpensive Ti actives and relatively high energy density is contrasted with potential side-reactions resulting in reduced energy efficiency. Technological pathways are presented with a view to overcoming critical bottlenecks and a vision is presented for the future development of Ti-RFBs.

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.…”

“…, Ti(OH) 2+ , TiO + , and Ti(OH) 2 + . 3,18,28 complexes. 19 Electrochemical reduction of Ti 4+ to Ti 3+ proceeds as a simple one-electron reaction in HCl, whereas an intermediate stage exists in H 2 SO 4 .…”

“…The 0.1 V half-cell potential of the Ti 4+ /Ti 3+ couple, which is close to H + /H 2 reduces concerns about hydrogen evolution reaction (HER) as an undesired side reaction The current roster of Ti-based RFBs includes systems with Fe, Mn, and Ce as the catholyte. [2][3][4][5][6][7][8][9][10] The greater abundance of Ti (in 2022, worldwide reserves and production of Ti were 7 × 10 11 kg and 9 × 10 9 kg; and for V, these were 2.4 × 10 10 kg and 1.1 × 10 8 kg respectively 3,11,12 ) and its lower cost (10 times less than vanadium) render it a potentially cost effective alternative active material for RFBs. 3 To put the proven reserves in context, if all available Ti were to be employed in electrochemical energy storage, Ti-Fe RFBs would be capable of storing 78.7 TWh of energy (limited by Ti reserves), which is approximately 11 times greater than all-vanadium RFBs (6.95 TWh).…”

“…[2][3][4][5][6][7][8][9][10] The greater abundance of Ti (in 2022, worldwide reserves and production of Ti were 7 × 10 11 kg and 9 × 10 9 kg; and for V, these were 2.4 × 10 10 kg and 1.1 × 10 8 kg respectively 3,11,12 ) and its lower cost (10 times less than vanadium) render it a potentially cost effective alternative active material for RFBs. 3 To put the proven reserves in context, if all available Ti were to be employed in electrochemical energy storage, Ti-Fe RFBs would be capable of storing 78.7 TWh of energy (limited by Ti reserves), which is approximately 11 times greater than all-vanadium RFBs (6.95 TWh). The Ti-Ce RFB, on the other hand, would be capable of 9.9 TWh of energy (limited by Ce reserves), approximately 1.5 times more than vanadium RFBs.…”

“…In various acidic environments (i.e., H 2 SO 4 , HCl, HNO 3 , HClO 4 , H 3 PO 4 ), Ti ions display a diverse range of complex formations, influenced by both the specific acidic medium and their oxidation states. 3,[14][15][16][17][18][19][20][21] The reversibility of the Ti 4+ /Ti 3+ couple is significantly impacted by different acidic conditions and their concentrations. For instance, polarography experiments have revealed that the Ti 4+ /Ti 3+ couple demonstrates high reversibility in H 3 PO 4 , irrespective of acid concentration.…”