The Effect of Particle Size on the Oxygen Reduction Reaction Activity of Carbon-Supported Niobium Oxide-Based Nanoparticle Catalysts

One of the alternatives to platinum catalysts for the cathode of polymer electrolyte fuel cells is transition metal oxide-based catalysts, such as TiO<i>_x</i>, ZrO<i>_x</i>, and NbO<i>_x</i>. Although these catalysts have been considered for their potential because of their high chemical stability under acidic conditions, their low oxygen reduction reaction (ORR) activity requires significant improvement. A contributing factor to this low activity is the poor conductivity of oxides. To improve this, it is believed necessary to control the crystal phase, the size of the oxides, and the local conduction paths near the oxide particles. Generally, it is difficult to separate and discuss these factors independently, making it unclear which factor is the most influential. In this study, niobium oxide nanoparticles with significantly different sizes were synthesized using either the irradiation method or impregnation method for the preparation of catalysts precursors, followed by heat treatment. Here, we report on the comparison of ORR activity, focusing on the differences in the size of the niobium oxide nanoparticles in the catalyst.<br/>The irradiation method is a simple one-pot process, in which a glass vial containing ultra-pure water together with conductive carbon nanopowder and metal source (Nb₂(C₂O₄)₅) are irradiated with gamma-ray from a cobalt-60 source. The impregnation method places conductive carbon nanopowder and the metal source in ethanol, followed by a drying process to obtain the sample. Thus-prepared composite nanoparticles, acquired in powder form, were used as precursors. Polyacrylonitrile was added and the mixture was heat-treated in an ammonia atmosphere to prepare the catalyst. It is expected that polyacrylonitrile will be graphitized upon heat treatment, forming local conduction paths near the oxide particles.<br/>These samples were characterized by the techniques of TEM, XRD, and LSV. When the precursor prepared by the irradiation method was heat-treated, TEM observation revealed that the Nb-based nanoparticles on the surface of the carbon support had a particle size of 7 nm. Following heat treatment of the impregnation-prepared precursor, the size of the Nb-based nanoparticles on the surface of the support was 13 nm, indicating that each method of precursor preparation successfully produced Nb-based nanoparticles of different sizes. In both cases, XRD showed diffraction patterns corresponding to niobium oxynitride. Additionally, composition ratios of niobium oxynitride determined from Vegard’s law, assuming NbO<i>_x</i>N_1-<i>x</i>, were consistent regardless of the precursor preparation method.<br/>These catalytic activities were evaluated using Linear Sweep Voltammetry in an acidic medium, based on the open circuit potential in oxygen and current density. After heat-treating the irradiation-prepared precursor, the open circuit potential and current density were slightly higher compared to the impregnation method; however, no significant difference was observed. Using a model based on several assumptions, it was suggested that the dominant factor affecting ORR activity is not the size of the metal oxide nanoparticles, but the length of the local conduction paths near the oxide nanoparticles. The present study revealed that the ORR activity in niobium oxide nanoparticle catalysts does not significantly depend on the size of the oxide nanoparticles.