Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

Crystalline structures and lattice water molecules are believed to strongly influence the ability of metal oxides to reversibly and rapidly insert protons in aqueous batteries. In the present work, we performed a systematic analysis of the electrochemical charge storage properties of nanostructured TiO₂ electrodes composed of either anatase or amorphous TiO₂ in a mild buffered aqueous electrolyte. We demonstrate that both materials allow reversible bulk proton insertion up to a maximal reversible gravimetric capacity of ~150 mA·h·g^-1. We also show that the TiO₂ crystallinity governs the energetics of the charge storage process, with a phase transition for anatase, while having little effect on either the interfacial charge-transfer kinetics or the apparent rate of proton diffusivity within the metal oxide. Finally, with both TiO₂ electrodes, reversible proton insertion leads to gravimetric capacities as high as 95 mA·h·g^-1 at 75 C. We also reveal two competitive reactions decreasing the Coulombic efficiency at low rates, i.e. hydrogen evolution and a non-faradaic self-discharge reaction. Overall, this work provides a comprehensive overview of the proton-coupled electrochemical reactivity of TiO₂ and highlights the key issues to be solved in order to truly benefit from the unique properties of protons as fast charge carriers in metal oxides.