Protonation/Deprotonation of Rutile TiO2 in Acid Aqueous Electrolytes

The diversity of carrier ions in electrochemical devices has attracted much attention with regard to the use of protons. If the fast proton conduction originating from the Grötthuss mechanism can be utilized, the development of water-based rechargeable batteries that combine safety and rapid charge–discharge performance is expected. However, the narrow potential window of acid-based aqueous solutions limits the operation of active materials. In this work, the effect of the crystal structure on electrochemical protonation/deprotonation of TiO2 as a negative electrode material was systematically investigated using slurry-based composite electrodes. In the galvanostatic charge–discharge tests using a buffer electrolyte consisting of 1 M citric acid and 1 M trisodium citrate, the protonation of the rutile TiO2 proceeded at a higher potential than those of the anatase and brookite TiO2, resulting in reversible capacities of 102 and 87 mA h g–1 at the first cycle and 50th cycle, respectively, due to the decrease in irreversible hydrogen evolution. X-ray diffraction revealed that the protonation occurred inside the bulk, although the changes in the a and c axes during protonation/deprotonation were much smaller than those of Li+ and Na+ insertion. On the other hand, in particle sizes larger than 100 nm, the hydrogen evolution was dominant and the deprotonation was less in the anatase TiO2, and neither charging/discharging nor even the hydrogen generation occurred in the larger rutile TiO2. This is presumably due to the slow proton diffusion in the solid phases, which stalls the proton storage and induces the reduction of protons at the electrode–electrolyte interface. These obstacles should be overcome by nanosizing the particles and optimizing electrolyte compositions.