The catalyst ink was prepared by mixing of 1 mL water, 0.25 mL isopropanol, 0.01 mL 5 wt% Nafion solution and 3 mg catalysts. The ink was sonicated for at least 2 h. Then 10 or 40 µL of the ink was uniformly loaded onto a glassy carbon electrode (GC, freshly polished, 0.07065 cm 2 ) or a carbon cloth electrode (CC, plasma treated, 0.25 cm 2 ), respectively. The electrodes were dried in a 75 °C oven for 30 mins before measurements.