Rutile TiO2 As a Negative Electrode Material for Proton Rechargeable Batteries
Authors: Shimizu Masahiro , Nishida Daisuke , Kikuchi Ayaka , Arai Susumu 
Published: December 2024

With the increasing importance of rechargeable batteries, the use of monovalent alkali metal ions such as Na and K ions has been actively investigated. However, there have been relatively few research reports on proton rechargeable batteries. When using protons as a carrier ion, there are concerns about irreversible H2 evolution due to the poor electrochemical stability of aqueous electrolytes. If the fast proton conduction originating from Grötthuss mechanism can be utilized, the rechargeable batteries with water-based electrolytes including protons as carrier ions that combine safety and rapid charge–discharge performance is expected.1−3 On the other hand, it is required that negative electrodes operate at a lower potential to improve battery voltages. Although anatase TiO2 have been reported as the host material for protons to show a reversible capacity, H2 evolution remains an unavoidable issue. In this study, we focused on the energy level of the conduction band of rutile TiO2, which is more positively located than anatase TiO2. It is expected that the positively located conduction band reduce irreversible HER and achieve a long-cycle life. The influence of crystal structures and particle sizes on battery performances of rutile, anatase, and brookite TiO2 were systematically examined.
To investigate the effect of crystal structure on the electrochemical protonation of TiO2 with as little influence of particle size as possible, TiO2 was synthesized by different preparation methods for each structure (Figure 1a). The electrolyte used in this study is the citric buffer solution consisting of 1 M citric acid and 1 M trisodium citrate. The electrochemical protonation/deprotonation was investigated using three-electrode type cell with the counter electrode of activated carbon and the reference electrode of Ag/AgCl in sat. NaCl.
Figure 1b shows galvanostatic charge–discharge properties of several TiO2 synthesized by the hydrothermal method. The reaction potentials of TiO2 with protons are rutile, brookite, and anatase, in that order, reflecting the more positive conduction band level of the rutile. The XRD measurements revealed that the rutile TiO2 uptakes/releases protons while maintaining the parent phase without any phase transitions (Figure 1c). The lattice parameters, a and c reversibly and slightly expanded/contracted (Figure 1d), but did not change as much as during the insertion/extraction of alkali metal ions such as Li+ and Na+.4,5 The fact that lattice expanded and contracted clearly reveals that protons were inserted into the rutile TiO2 bulk. Meanwhile, the larger particles showed little protonation and H2 evolution was dominant regardless of the crystal structures. The particle size is an influential factor, and smaller sizes are advantageous in the electrochemical protonation.6

Acknowledgement
This work was supported by a Grant-in-Aid for Scientific Research (B) (20H02840, 24K01601) from the Japan Society for the Promotion of Science (JSPS).
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Figure 1