Emergence of Oxygen Reduction Activity in Partially Oxidized Tantalum Carbonitrides: Roles of Deposited Carbon for Oxygen-Reduction-Reaction-Site Creation and Surface Electron Conduction

Abstract

Tantalum-oxide-based oxygen-reduction-reaction (ORR) catalysts have the ORR activity only when they are synthesized from tantalum carbonitrides. Namely pure Ta 2 O 5 does not show ORR activity, and carbon or nitrogen in precursors is inevitable to form ORR active sites and to promote ORR. To clarify this reason, we investigated structural and surface electronic properties of tantalum-oxide-based catalysts, which were synthesized by gradually oxidizing tantalum carbonitride under low partial pressure of oxygen at 1000 °C, by using X-ray absorption spectroscopy, transmission electron microscopy, and Raman spectroscopy. The results indicate that oxidized tantalum carbonitrides with high ORR activity had oxygen vacancies near the surface, indicating that the vacancies could work as an active site for ORR. We also found that carbon was deposited on the oxide's surface during oxidation of tantalum carbonitrides. The deposited carbon seems to play two important roles in formation of oxygen vacancies (ORR active sites) providing reductive atmosphere, and in producing electron conduction paths on rather insulating oxides' surface.

Titanium-Niobium Oxides as Non-Noble Metal Cathodes for Polymer Electrolyte Fuel Cells

“…Therefore, the phase transition was probably caused by a change in the valence of titanium. We previously demonstrated that tantalum and zirconium oxide-based catalysts had some oxygen vacancies that acted as active sites for the ORR [6]. In case of the titanium-niobium oxide system, the low valence state of the metal ions does not always indicate the presence of oxygen vacancies.…”

“…Based on the high stability of Group 4 and 5 metal oxide-based compounds in acidic media, low cost, [5,6] and lower solubility in acid solution compared to platinum-based catalysts, these compounds have piqued our interest as they are expected to be stable even under the conditions encountered at the PEFC cathode. Recently, we successfully synthesized oxide-based nanoparticles using oxy-metal phthalocyanines (MeOPc; Me = Ta, Zr, and Ti) as the starting material and multi-walled carbon nanotubes (MWCNTs) as the support as well as the electro-conductive material [7,8].…”

Morphology-Controlled Nitrogen-Containing Polymers as Synthetic Precursors for Electrochemical Oxygen Reduction Fe/N/C Cathode Catalysts

“…For both reaction pathways, non-precious metal (NPM) catalysts are desired, considering the cost and scarcity of precious metals. The catalysts obtained by pyrolyzing Fe, N, and C-containing precursors have been widely studied, while oxide-based catalysts also constitute another class of NPM catalysts [4][5][6]. This review article describes an effective approach to obtain a highly active Fe/N/C catalyst.…”

Kinetic Study of Oxygen Reduction Reaction in Acid Solution Using Titanium Oxide-based Catalysts Prepared from Oxy-titanium Tetra-pyrazino-porphyrazine

“…We previously developed non-platinum cathode electrocatalysts based on group 4 and 5 transition metal oxides that exhibited high stability in acidic media. 1 Among these transition metal oxides, titanium is advantageous because it is a more abundant resource. Our previous studies revealed that titanium oxide-based compounds with multi-walled carbon nanotubes (TiC x N y O z / MWCNTs) prepared by oxidation of a titanium organic complex in low p O2 atmosphere exhibited high ORR activity.…”