The working electrode was prepared as follows: prior to modification, GCE
was carefully polished with 1.0 m, 0.3 m and 0.05 m -alumina powders in
sequence, rinsed thoroughly with double distilled water between each polishing
step, sonicated in ethanol and double distilled water for 5 min respectively and dried
with nitrogen. 15 L of 0.5% Nafion solution diluted in double distilled water was
placed on the surface of the GCE, and it was allowed to dry at room temperature
(drop coating). Then, about 20 mg of samples were dispersed in 2 mL double distilled
water with a few minutes of ultrasonication to form well-dispersed mixture.
The surface of the Nafion/GCE was casted by 10 L of the mixture and dried at room
temperature (25 ◦C). All electrochemical experiments were carried out at room temperature
using 6 M KOH as electrolyte solution. The electrodes were cycled from 0
to −0.8 V (CuO/MS) or −0.6 V (NiO/MS or Co3O4/MS) at a scan rate of 50 mV/s.
3. R