Reduction of Oxygen on Dispersed Nanocrystalline CoS2

The electrocatalytic properties of nanocrystalline CoS 2 have been investigated for the oxygen reduction reaction (ORR) in 0.1 M HClO 4 . CoS 2 with pyrite structure was prepared by hydrothermal synthesis and attached to a glassy carbon electrode from solution with a mixture of carbon and Nafion. The prepared CoS 2 electrode layers showed high activity toward the ORR and very good stability under oxygen reducing conditions. Selectivity of the ORR toward H 2 O 2 was determined by rotating (ring) disk electrode measurements, and relatively high selectivity was obtained with up to 80% H 2 O 2 formation around 0.4 V (vs Ag/AgCl), but this dropped to zero for potentials below 0.0 V. The amount of H 2 O 2 produced between 0.6 and 0.0 V was dependent on the quality of the CoS 2 dispersion within the electrode layer, and decreasing CoS 2 particle size resulted in significant improvement in the ORR electrocatalytic activity, both by increasing the turnover frequency and through decreasing the selectivity toward H 2 O 2 production.