Ice crystals growth driving assembly of porous nitrogen-doped graphene for catalyzing oxygen reduction probed by in situ fluorescence electrochemistry

In recent years, doped carbonaceous materials as alternative catalysts for oxygen reduction reaction (ORR) have received considerable attention due to the low cost and high CO tolerance capability. Different theoretical studies have suggested that oxygen is reduced in a rapid sequence intermediated by diverse oxygen-containing reactive intermediates (ORI). However, due to the short lifetimes of the possible ORI, direct experimental evidence is very difficult to be obtained. Here, we report the synthesis of an ultralight and porous nitrogen-doped graphene (NG) by annealing graphite oxide (GO)-melamine scaffold shaped in ice template. The resultant NG exhibits excellent electrocatalytic activity toward 4e-reduction of oxygen with the onset potential as low as −0.05 V vs. Ag/AgCl in alkaline media. Using this material as model study, sensitive in situ fluorescence spectroelectrochemistry is applied to demonstrate the presence the reactive ORI. The global ORR pathway is unraveled as stepwise electron transfer involving hydroxyl radical as the important intermediate via both inner- and outer-sphere process. This result would likely provide a new insight into the further understanding of ORR mechanism on those intrinsic carbonaceous materials.