Monoclinic hafnium oxynitride supported on reduced graphene oxide to catalyse the oxygen reduction reaction in acidic mediaElectronic supplementary information (ESI) available: Mass activity versus potential curves, RRDE voltammograms, fitting results of a N 1s spectrum, C 1s spectra. See DOI: 10.1039/c4cp03210a

Monoclinic HfO 2 nanoparticles were doped with nitrogen via hydrothermal treatment that avoided high-cost pyrolysis with NH 3 gas in order to develop a novel oxygen reduction reaction catalyst for use in acidic media. Catalyst size reduction was achieved using a reduced graphene oxide support, and activity above 0.8 V was obtained. Nitrogen-doped monoclinic-HfO 2 nanoparticles were incorporated onto reduced graphene-oxide-sheets via hydrothermal-treatment to show excellent oxygen reduction reaction activity in acidic media.