Carbon-supported hafnium oxynitride as cathode catalyst for polymer electrolyte membrane fuel cells

Highly stable carbon-supported hafnium oxynitride (HfOxNy-C) was synthesized by heating carbon-supported hafnium oxide, prepared using an impregnation method, under NH3 gas in various conditions. X-ray diffraction patterns, X-ray photoelectron spectra, and field-emission transmission electron microscope images confirmed that HfOxNy nanoparticles were dispersed onto commercial carbon black, Vulcan XC-72. The stability of HfOxNy-C in 0.1 mol dm−3 H2SO4 at 303 K was evaluated by measuring the mass ratio of dissolved hafnium to immersed HfOxNy-C using inductively coupled plasma atomic emission spectroscopy. It saturated at a low level of 0.8–4.0 mg g−1 with increasing immersion time up to ∼24 h. The oxygen reduction reaction (ORR) activity and rate were evaluated by obtaining cyclic voltammograms and rotating disk electrode voltammograms, respectively. The HfOxNy-C exhibited higher ORR activity and a lower Tafel slope than NH3-treated C under identical conditions, demonstrating that HfOxNy is active toward ORR. The ORR activity most depended on the heating temperature. The ORR rate increased with increasing the heating time at 1223 K which could be due to the increased y in HfOxNy-C. The maximum onset potential for ORR was 0.78 V vs. standard hydrogen electrode, which is 0.18 V lower than that of carbon-supported platinum.

► Carbon-supported hafnium oxynitride (HfOxNy-C) as oxygen reduction reaction (ORR) catalyst in acid media.
► The mass ratio of dissolved hafnium from HfOxNy-C immersed in 0.1 mol dm−3 H2SO4 to HfOxNy-C saturated at a low level of 0.8–4.0 mg g−1, demonstrating the high stability.
► The ORR activity and rate increased with increasing NH3-treatment temperature and time, respectively.
► The maximum onset potential for ORR was 0.78 V vs. standard hydrogen electrode, which is 0.18 V lower than that of carbon-supported platinum.