Active sites for oxygen reduction reaction and the reaction mechanism in carbon-supported hafnium oxynitride

Abstract

Active sites for an oxygen reduction reaction (ORR) and the reaction mechanism in hafnium oxynitride nanoparticles supported on carbon black (HfOxNy-C) were investigated by using several heat-treatment steps under H2/Ar (5% H2) gas, NH3 gas, or both. Nitrogen-free monoclinic-phase hafnium oxide nanoparticles with oxygen defects were found to be active toward ORR for the first time. Nonetheless, doping of nitrogen into the hafnium oxides produced more active sites. Furthermore, the process proceeded primarily via a 4-electron reaction in which oxygen molecules were directly reduced to water. Therefore, at least, nitrogen atoms in HfOxNy are necessary for maximizing the ORR activity. © 2012 The Electrochemical Society.