Oxygen Reduction on Hafnium/Hafnium Carbide Nanoparticles

Non-precious metal nanoparticles are attractive candidates for oxygen electrocatalytic reduction in electrochemical energy conversion devices, such as fuel cells and batteries. Here we report for the first time electrocatalytic activity of hafnium/hafnium carbide nanoparticles towards the oxygen reduction reaction (ORR). While activity of hafnium oxynitride nanoparticles towards the ORR in acid has previously been reported   [1] a little was known about the ability of Hf/HfC nanoparticles to reduce molecular oxygen. Here we demonstrate that molecular oxygen can be reduced on Hf/HfC nanoparticles both in acidic and alkaline environment. However, the reaction mechanism changes from 4 - to 2 - electron pathway, when switching from acidic to alkaline environment. Although the ORR in base occurs through the two-electron parthway, this reaction has a potential for a practical application because Hf/HfC catalyst demonstrates an activity toward the disproportionation of hydrogen peroxide to O₂ and H₂O. Onset potential for the ORR in 0.1 M KOH for non-optimized catalyst (-0.10 V vs AgAgCl ) is superior to that of the best reported to date MnO₂ catalyst (-0.13 V vs Ag/AgCl). In acid, moderate catalytic activity towards the ORR is observed. The results suggests that Hf nanomaterials can be considered as a promising candidate for replacing noble metal catalysts in alkaline air electrodes.

[1] CHISAKA, M. & MURAMOTO, H. 2014. Suppression and Evaluation of Hydrogen Peroxide Formation on Carbon-Supported Hafnium Oxynitride. ECS Electrochemistry Letters, 3, F1-F4.