Oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of trace quantities of ammonium ions: An RRDE study

The effect of trace quantities of ammonia on oxygen reduction reaction (ORR) on carbon-supported platinum catalysts in perchloric acid solutions is assessed using rotating ring disk electrode (RRDE) technique. The study demonstrates that ammonia has detrimental effects on ORR. The most significant effect takes place in the potential region above 0.7 V vs RHE. The effect is explained by the electrochemical oxidation of ammonia, which blocks Pt active sites and increases the formation of H2O2. This leads to losses in the disk currents and increments in the ring currents. The apparent losses in ORR currents may occur in two ways, namely, through the blocking of the active sites for ORR as well as by generating a small anodic current, which is believed to have a lower contribution. In addition, a detrimental effect of sodium cations in the potential range below 0.75 V vs RHE was demonstrated. This effect is most likely due to the co-adsorption of sodium cations and perchlorate anions on the Pt surface.

Changes in disk (a) and ring (b) current densities for 20% Pt-C catalyst (Pt loading: 20 μg cm−2) in 0.1 mol dm−3 HClO4 solution saturated with oxygen resulting from the addition of trace quantities of ammonium perchlorate (NH4ClO4): () 10 ppm; () 50 ppm; () 100 ppm and () 500 ppm. Temperature 30 °C. Scan rate 1 mV s−1, negative-going scans.Figure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideHighlights
► Trace quantities of ammonia have detrimental effects on ORR in HClO4.
► Significant effects of ammonium take place at E ≥ 0.7 V vs RHE; Na+ and ClO4− exert no effect on ORR in this potential range.
► The electrochemical oxidation of NH3 and ORR compete for Pt active sites.
► The competition results in both ORR current losses and increased H2O2 formation.