Effect of Synthesis Route on Oxygen Reduction Reaction Activity of Carbon-Supported Hafnium Oxynitride in Acid Media

The effect of the synthesis route on the oxygen reduction reaction (ORR) activity of carbon-supported hafnium oxynitride (HfO x N y -C) catalysts in acid media was investigated. Four types of HfO x N y -C catalysts were synthesized by heating carbon-supported hafnium oxide (HfO x -C) prepared via three different impregnation methods and a polymerized complex method at 1023–1223 K for 24–50 h under NH 3 gas. Field emission transmission electron microscope images, X-ray diffraction patterns, X-ray photoelectron spectra, cyclic voltammograms, and rotation disk electrode voltammograms were analyzed. Nanosized HfO x N y particles with a cubic Hf 2 ON 2 phase were dispersed onto C powders by any of the four synthesis routes. However, the level and uniformity of the HfO x N y particle size and the crystallinity depended on the synthesis routes rather than the synthesis conditions. The ORR activity of the HfO x N y -C catalysts increased with the increasing crystallinity of the Hf 2 ON 2 phase in the HfO x N y particles. Catalysts prepared via an impregnation method using hydrolysis of hafnium tetrachloride in the dispersion, consisting of nitric acid-refluxed C powders and water, showed (a) the most uniform and smallest HfO x N y particle sizes, in the order of a few nanometers, (b) the highest crystallinity, and (c) the highest ORR current and activity levels.