Difference of Active Sites between Titanium and Zirconium Oxide-Based Cathodes for Polymer Electrolyte Fuel Cells

Abstract

We have been engaged in development of group 4 and 5 oxide-based compounds as non-PGM cathodes for polymer electrolyte fuel cells because of their high stability in acidic and oxidative atmosphere. Recently, we focused on group 4 oxide-based compounds such as titanium and zirconium oxides. From theoretical consideration, these oxides might have different active sites. Theoretical consideration revealed that the active sites of zirconium and titanium oxide-based cathodes are stabilized oxygen vacancies and complex site (doped metal and lattice oxygen), respectively. Thus, we need to perform the experiments to clarify the active sites.With respect to zirconium oxide-based compounds, we prepared zirconium oxide-based electrocatalysts from oxy-zirconium phthalocyanine with multi-walled carbon nanotubes (MWCNTs) as electroconductive supports via heat-treatment under a low oxygen partial pressure. We evaluated the density of the active sites, i.e., oxygen vacancies, using X-ray photon spectroscopy, the average particle size of the oxide particles using transmission electron microscope (TEM), and the surface area utilization of the oxide particles using TEM image analysis. We successfully determined that the increase in the oxygen reduction reaction (ORR) activity was due to the increase in the density of oxygen vacancies and the effective surface area, and that the decrease in the effective surface area was responsible for the decrease in the ORR activity. According to the experimental results, the active sites of the zirconium oxide-based catalysts might be oxygen vacancies stabilized by nitrogen doping or nano-sizing.With respect to titanium oxide-based compounds, we investigated factors affecting the use of niobium-doped titanium oxides for the ORR. MWCNTs were used as support to maintain sufficient electrical conductivity. Nb-doped titanium oxide/MWCNTs prepared by hydrolysis was heat-treated at the desired temperature for a certain time under argon containing 4% hydrogen to investigate the relationship between ORR activity and physicochemical properties such as crystalline structure and electronic state. We confirmed that the density of the low-valence state of titanium ions, Ti3+, affected ORR activity. However, theoretical consideration indicated that Ti3+ located at the top surface of the oxides must be adsorbed by oxygen molecule strongly to form TiO2. This means that the Ti3+ could not be active sites. On the other hand, according to a good relationship between the cell volume of anatase phase in the catalyst and the ORR current at 0.7 V, we discovered that crystalline distortion of the anatase phase might produce Ti3+ on the surface and lead to ORR activity. This means that the active sites of the titanium oxide-based catalysts might be crystalline distortion by niobium doping or phase transition. Both theoretical consideration and experimental results revealed that the active sites of zirconium and titanium oxide-based cathodes are stabilized oxygen vacancies and complex site (doped metal and lattice oxygen), respectively. This difference affected the strategy of the catalyst design for titanium and zirconium oxide-based cathodes in order to enhance their ORR activities.