A Negative-Based TiO2 Electrode for Aqueous Proton Batteries
Authors: Toorabally Milad , Bregiroux Damien , Krins Natacha , Singh Arvinder , Dambournet Damien , Laberty-Robert Christel 
Published: September 2023

Complementary solutions to Li-ion batteries must be studied for the growing need for energy demand. Accordingly, we have been interested in developing negative electrode materials of TiO2 capable of reversibly intercalating proton ions for aqueous batteries. One of these batteries’ main challenges comes from the low potential window available. Aqueous batteries cannot provide sufficient energy density with a thermodynamic working potential of only 1.23V. To do so, we have optimized the intrinsic transport properties of proton-ions can lead to the reduction of side reactions such as Hydrogen Evolution Reaction (HER).
Solvothermal route synthesis made at different temperatures (from 90°C to 150°C) gives access to different TiO2 structures. At 90°C, a lamellar type lepidocrocite is obtained, including sheets of “TiO2” and water in the inter-lamellar spaces. This inter-lamellar space allows proton conduction by the Grotthus mechanism [1]. But, lepidocrocite type TiO2has been shown to be a non-conductive ionic conductor. To make it conductive, different cations can be inserted into the inter-lamellar space during the solvothermal synthesis, thus allowing the reorganization of water molecules, then facilitating the intercalation, conduction, and diffusion of protons [2]. Several levels of Zn2+ were tested: from 10 to 50 mol% relative to Ti4
+. At 150°C, we have been able to stabilize a complete condensed anatase (one polymorph of TiO2) phase while a defect anatase phase has been synthesized at a temperature between 90°C and 150°C. Interestingly, the quantity of defect can be tuned by the temperature and the hydrolysis ratio, h=nH2O/nTi=3.33 [3]. These defects are mainly cationic vacancies, thanks to X-Ray Pair Distribution Function (PDF) analysis.
The electrochemical properties of these materials (shaped with carbon black as conductive support and Nafion as a binder) were studied in half-cell aqueous electrolyte buffered at pH 5 (CH3COOH/CH3COOK, pKa = 4.76, (1M)). Experimental capacities of more than 100 mAh/g, 80% of coulombic efficiency over 100 cycles have been obtained and potentials down to -1.4V (V vs Ag/AgCl, KCl saturated) have been achieved.
For defective anatase (synthesized at 110°C), the CV curves exhibit two distinct peaks that can be linked to the co-existence of two sites for proton intercalation. The proton can be intercalated either inside a vacancy or within the lattice. This behavior allows a promising working potential of -1.6V with a gravimetric capacity of 250 mAh.g-1 with a 90% of efficiency over 50 cycles. Finally, the relationship between structure and electrochemical properties will be discussed in this presentation with the objective of designing an efficient MnO2/TiO2 aqueous battery.
REFERENCE:
[1] Wu, X.; Hong, J. J.; Shin, W.; Ma, L.; Liu, T.; Bi, X.; Yuan, Y.; Qi, Y.; Surta, T. W.; Huang, W.; Neuefeind, J.; Wu, T.; Greaney, P. A.; Lu, J.; Ji, X. Nat. Energy 2019, 4 (2), 123–130

[2] Chimie, E. D.; Analytique, C.; Centre, D. P.; Kang, P. S. Sorbonne Université Design de Matériaux Lamellaires Par Chimie Douce Pour Batteries à Proton et Ion Multivalent. 2020.
[3] Kang, S; Singh. A; Badot, J-c; Reeves, K; Durand-Vidal, Serge, Legein, C; Body, Monique, Dubrunfaut, O; Borkiewez, O; Tremblay, B; Laberty-Robert, C; Dambournet, D Chemistry of Materials
2020
32 (21), 9458-9469

Figure 1