The Activity of Oxygen Reduction Reaction on the Noble Metal Doped Titanium Oxide in Acidic Electrolyte
Authors: Saida Takahiro , Niwa Etsuko , Maruyama Takahiro 
Published: May 2020

Titanium oxide with the oxygen vacancies is expected as one of the candidates for the non-platinum catalyst of polymer electrolyte fuel cells. The reasons why the activity of titanium oxide improves by the presence of oxygen vacancies are assumed the formation of the active site and the micro electron-conducting path [1, 2]. From the viewpoint of enhancement of the electron conductivity, it is well known that doping a transition metal to titanium oxide is also effectiveness method. Thus, many papers had reported about the niobium doped titanium oxide to increase the electron conductivity. However, not only the doping transition metals but also the oxygen vacancies are necessary to obtain high activity of the titanium oxide. We put forward a possibility that the crystal distortion acts as the active site [3]. Now, we think that the majority role of transition metal doping is the formation of the micro electro-conducting path to the active site. Hence, we were attending to the electron conductivity of transition metal-doped titanium oxide. From the results of a theoretical calculation, the electron conductivity of niobium doped titanium oxide, which is researched well as the non-platinum catalyst, is relatively low [4]. And, ruthenium and iridium as doping species give high electron conductivity to titanium oxide. Ruthenium and iridium are famous for interacting with titanium oxide in the electrolysis field. In this study, the activity of oxygen reduction reaction (ORR) of ruthenium or iridium doped titanium oxide was evaluated.

Ruthenium doped titanium oxide (Ru-TiO2) and iridium doped titanium oxide (Ir-TiO2) were synthesized by the sol-gel method, and both doping titanium oxides were supported on the Ketjen Black 300 J. The loading of both doping titanium oxides was less than 20 wt%. In addition, the molar ratio of titanium and ruthenium or iridium was 95:5. Electrochemical studies were conducted in Ar- or O2-saturated 0.1 M HClO4 at 60ºC with carbon plate counter-electrode and Ag/AgCl reference. The ORR activity was evaluated by the difference current between the current in Ar-saturated condition from the current in O2-saturated condition.

The crystal structure of Ru-TiO2 was almost the same as the anatase structure of TiO2. The intensity of diffraction peaks of Ir-TiO2 was weaker than that of Ru-TiO2, although its crystal structure was the anatase. Both samples showed no diffraction peaks attributed to the noble metal oxide and metallic phase. The atomic valence of ruthenium in Ru-TiO2 was mainly tetravalent from the results of XPS analysis. But a full width at half maximum of Ru 2p was wide. Thus, the ruthenium atom may have many oxidation states. Titanium atom at the surface showed tetravalent, and it inside of a particle indicated the reduction state. Now, we thought that the ruthenium atom was atomically mixing with titanium oxide structure because no evidence observed ruthenium oxide and ruthenium metal by our analysis method.

ORR activity of titanium oxide improved by the addition of 5 mol% ruthenium. ORR activity of titanium oxide is well known to increase by introducing the oxygen vacancies. ORR activity of Ru-TiO2 was higher than that of titanium oxide with the oxygen vacancies. Additionally, Ru-TiO2 showed also high ORR activity after the reduction process using hydrogen gas as well as titanium oxide. Ir-TiO2 showed the highest ORR current in this study, and its onset potential of ORR was also a high value at about 0.87 V.

From the above results, the addition of a small quantity of noble metal was very effective for the enhancement of ORR activity of titanium oxide.

[1] A. Ishihara, et al., J. Phys. Chem. C, 117, 18837 (2013).

[2] A. Ishihara, et al., J. Phys. Chem. C,
123, 18150 (2019).

[3] T. Saida, et al., MRS Advances, 4, 1851 (2019).

[4] M.-C. Tsai, et al., Mol. Syst. Des. Eng., 2, 449 (2017).