Influence of Dopant Atoms in TiO2 for Its Electron Structure and Activity of Oxygen Reduction Reaction in Acidic Environmental
Authors: Tsuzuki Haruto , Maruyama Takahiro , Saida Takahiro 
Published: December 2024

Polymer electrolyte fuel cells (PEFCs) are attracting attention, but their widespread commercialization has various challenges. In particle, the Pt-based cathode catalyst has two significant problems: durability and cost. These problems will persist as long as the Pt-based catalyst is used in PEFCs. Therefore, we focused on TiO2 as the alternative Pt-based catalyst, which is highly durable and low-cost. However, TiO2 has low electron conductivity and must obtain the electron conduction path to progress the oxygen reduction reaction (ORR). In general, doping transition metal and/or introducing the oxygen vacancy into the crystal structure is conducted to enhance the electron conductivity of TiO2. It was reported that Nb-doped TiOx having oxygen vacancies showed relatively higher ORR activity. Experimentally, it is known that other metal doping and oxygen vacancies bring in high ORR activity. However, the detailed enhancement mechanism for ORR activity is still unclear.

Some research groups were using theoretical calculations to investigate the mechanism of ORR, such as the reaction path and active site on the TiO2 surface [1, 2]. These reports suggest one of the potentials for the ORR mechanism but don’t completely describe the ORR on the TiO2-based catalyst because it is hard to compute the state of a realistic, accurate ORR system. Therefore, we planned the combination of experiment data and theoretical calculations, and the balance of experimental data was heavier.

In this study, the 3d transition metal atom was doped to TiO2 and conducted for the reduction treatment for doped TiO2 to introduce oxygen vacancies. Their ORR activities and electron structure were evaluated experimentally. In addition, the electron structures of all samples were computed using DFT calculation and compared with experimental data.

5 at% of transition metal-doped TiO2 and TiO2 were synthesized by sol-gel method. The reduction treatment in this study consisted of calcining at 800°C for 10 min under the H2+Ar mixed gas flow conditions. The electrochemical measurements used a three-electrode cell with a working electrode, a carbon plate counter electrode, an Ag/AgCl reference electrode, and 0.1 M HClO4(60°C). The potential in this study was converted from versus Ag/AgCl to versus RHE. The ORR current was obtained by subtracting currents under inert gas flow conditions from currents under O2 flow conditions. The ORR activity was judged by the potential at 5 mA g-1 of ORR current. DFT calculation was performed with Quantum ESPRESSO.

All the synthesized samples had the anatase phase structure, but crystallinity was low since they formed nanoparticles or might not react with precursors completely. XPS results suggested that the dopant atom was probably introduced into the TiO2 crystal structure. The ORR activity of TiO2 was improved by doping the 3d transition metal atoms.

The density of states (DOS) of modeled our doped TiO2 samples showed a new intermediate band in the forbidden band of TiO2. Now, we expect that the location, amount, density, and other circumstances of this new intermediate band may be related to ORR activity.

[1] F. Zhang, et al. Applied Surface Science, 598 (2022) 153873.

[2] Y. Yamamoto, et al. J.phys.Chem.C, 123 (2019) 19486-19492.