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

► Carbon-supported hafnium oxynitride (HfO x N y -C) as oxygen reduction reaction (ORR) catalyst in acid media. ► The mass ratio of dissolved hafnium from HfO x N y -C immersed in 0.1 mol dm −3 H 2SO 4 to HfO x N y -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 NH 3-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. Highly stable carbon-supported hafnium oxynitride (HfO x N y -C) was synthesized by heating carbon-supported hafnium oxide, prepared using an impregnation method, under NH 3 gas in various conditions. X-ray diffraction patterns, X-ray photoelectron spectra, and field-emission transmission electron microscope images confirmed that HfO x N y nanoparticles were dispersed onto commercial carbon black, Vulcan XC-72. The stability of HfO x N y -C in 0.1 mol dm −3 H 2SO 4 at 303 K was evaluated by measuring the mass ratio of dissolved hafnium to immersed HfO x N y -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 HfO x N y -C exhibited higher ORR activity and a lower Tafel slope than NH 3-treated C under identical conditions, demonstrating that HfO x N y 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 HfO x N y -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.