Oxygen reduction on platinum in aqueous sulphuric acid in the presence of ammonium

The oxygen reduction reaction (ORR) was studied using a rotating disk electrode. The kinetic current density (cd) at 0.7 VRHE was reduced by more than a factor of 10 on pre-reduced Pt and a factor of 2.5 on pre-oxidised Pt when comparing ORR rate for pure sulphuric acid to that in 0.18 M H2SO4 + 0.1 M NH3 solution which was the highest ammonium concentration studied. The Tafel slope at low cd on pre-reduced Pt is about −90−90  mV/decade in the presence of ammonium compared to about −74−74  mV/decade in pure acidic solution. The Tafel slopes at high cd is not affected by ammonium. The Tafel slope of ORR on pre-oxidised Pt surface is not significantly affected by ammonium. The exchange cd is lower in the presence of ammonium both for pre-reduced and pre-oxidised Pt. The direct origin of the loss of ORR activity has not been firmly resolved. Adsorption and oxidation of ammonium including intermediate species as well as nitrogen–oxygen species formed at high potentials may be involved.