Enhanced Cycle\nLife and Expanded Voltage Window in\nAqueous Proton Full Cells Using TiO<sub>2</sub> Negative Electrodes\nwith Cationic Vacancies

Aqueous batteries face the challenge of limited energy\ndensity\ndue to parasitic gas production from hydrogen and oxygen evolution\nreactions, particularly at the negative electrode. This study investigates\nthe electrochemical properties and mechanisms of proton intercalation\nin anatase TiO<sub>2</sub> featuring vacancies (Vac-TiO<sub>2</sub>), stabilized via a low-temperature sol–gel process. XRD refinement\nanalysis, supported by thermal analysis, estimated 17% cationic vacancies,\nwhile <sup>1</sup>H MAS NMR spectroscopy revealed stabilization of\nthese vacancies by OH groups. The presence of cationic vacancies led\nto changes in the oxide anion sublattice, which accommodate proton\ninsertion. Electrochemical assessments in acetate buffer electrolyte\ndemonstrated Vac-TiO<sub>2</sub>’s ability to delay the hydrogen\nevolution reaction and enhance proton capacity, validated by pH-dependent\nstudies, DFT calculations, and kinetic analyses. Notably, the occurrence\nof undercoordinated oxide anions was shown to induce the insertion\nof H<sup>+</sup> at higher potential values, and the insertion mechanism\nwas suggested to occur via a solid-solution mechanism. Owing to these\nfeatures, Vac-TiO<sub>2</sub> exhibited superior cyclability and performance\ncompared to pure anatase TiO<sub>2</sub>, highlighting its potential\nfor sustainable proton intercalation processes. In half-cell configurations,\nVac-TiO<sub>2</sub> showed a high Coulombic efficiency (CE exceeding\n90% after 48 cycles), while full cells (MnO<sub>2</sub>||Vac-TiO<sub>2</sub>) demonstrated an excellent cycling stability (CE exceeding\n95.4% over 1000 cycles), high power density (10.5 kW·kg<sup>–1</sup> vs 6.2 kW·kg<sup>–1</sup>), and improved self-discharge.\nThis study paves the way for innovative approaches to improving proton\nintercalation materials, positioning Vac-TiO<sub>2</sub> as a viable\ncandidate for next-generation energy storage solutions.