Electrochemical Proton Storage of Amorphous Titanium Oxide in a Highly Concentrated Phosphate Buffer

Abstract

Motivated by the inherent safety of aqueous electrolytes and the fast ionic conduction facilitated by the Grötthuss mechanism, the development of rechargeable batteries utilizing protons or hydronium ions as charge carriers is currently in progress. However, significant challenges arise from irreversible hydrogen evolution and the dissolution of active materials, which are side reactions that compete with proton insertion during charging, complicating the development of suitable materials. We have focused on the crystal structure of TiO₂ and found that the rutile structure exhibits protonation/deprotonation at a more positive potential compared to anatase- and brookite-TiO₂. This more positive reaction potential mitigates the hydrogen evolution. However, hydrogen gas generated as a side reaction compromises the mechanical durability of the composite electrode, ultimately leading to capacity degradation over cycling. In this study, we explored an approach to enhance electrode performance by shifting the reaction potential to a more positive range through the amorphization of titanium oxide. As expected, the amorphous structure enabled proton insertion at a higher potential in the buffer solution composed of 1 M H₃PO₄/1 M Na₃PO₄, resulting in a discharge capacity of 160 mA h g^–1 at the first cycle, which is approximately twice that of rutile TiO₂. Additionally, the initial Coulombic efficiency improved from 61% to 80%. However, dissolution was more pronounced in amorphous TiO_x, leading to significant capacity decay. To suppress this, a highly concentrated phosphate buffer solution was applied in which the molar ratio of H₃PO₄/Na₃PO₄/H₂O in the electrolyte was adjusted from 1/1/51 to 1/1/13 to reduce the activity of water, which is responsible for the dissolution. As a result, the discharge capacity remarkably improved from 67 to 120 mA h g^–1 after 50 cycles.