Enhanced Cycle Life and Expanded Voltage Window in Aqueous Proton Full Cells Using TiO₂ Negative Electrodes with Cationic Vacancies

Aqueous batteries face the challenge of limited energy density due to parasitic gas production from hydrogen and oxygen evolution reactions, particularly at the negative electrode. This study investigates the electrochemical properties and mechanisms of proton intercalation in anatase TiO2 featuring vacancies (Vac-TiO2), stabilized via a low-temperature sol–gel process. XRD refinement analysis, supported by thermal analysis, estimated 17% cationic vacancies, while 1H MAS NMR spectroscopy revealed stabilization of these vacancies by OH groups. The presence of cationic vacancies led to changes in the oxide anion sublattice, which accommodate proton insertion. Electrochemical assessments in acetate buffer electrolyte demonstrated Vac-TiO2’s ability to delay the hydrogen evolution reaction and enhance proton capacity, validated by pH-dependent studies, DFT calculations, and kinetic analyses. Notably, the occurrence of undercoordinated oxide anions was shown to induce the insertion of H+ at higher potential values, and the insertion mechanism was suggested to occur via a solid-solution mechanism. Owing to these features, Vac-TiO2 exhibited superior cyclability and performance compared to pure anatase TiO2, highlighting its potential for sustainable proton intercalation processes. In half-cell configurations, Vac-TiO2 showed a high Coulombic efficiency (CE exceeding 90% after 48 cycles), while full cells (MnO2||Vac-TiO2) demonstrated an excellent cycling stability (CE exceeding 95.4% over 1000 cycles), high power density (10.5 kW·kg–1 vs 6.2 kW·kg–1), and improved self-discharge. This study paves the way for innovative approaches to improving proton intercalation materials, positioning Vac-TiO2 as a viable candidate for next-generation energy storage solutions.