Abstract
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.
Highlights
Ice crystals growth driving assembly of porous nitrogen-doped graphene for catalyzing oxygen reduction probed by in situ fluorescence electrochemistry
We take advantage of the faster kinetics of radical reaction to compete with the transient oxygen reduction reaction (ORR) at catalyst surface, showing that in situ www.nature.com/scientificreports fluorescence spectroelectrochemistry is effective to demonstrate the contained oxygen-containing reactive intermediates (ORI) during catalyzing ORR by carbonaceous materials
The controlled sample of N-doped graphene (NNG, which is synthesized without ice template but pyrolyzing the mixture of solid graphite oxide (GO) and melamine) presents the thick deposition, seriously stacking with layer-by-layer (Figure 2d)
Summary
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. We take advantage of the faster kinetics of radical reaction to compete with the transient ORR at catalyst surface, showing that in situ www.nature.com/scientificreports fluorescence spectroelectrochemistry is effective to demonstrate the contained oxygen-containing reactive intermediates (ORI) during catalyzing ORR by carbonaceous materials. This approach provides a new insight into the probe of mechanism with very high sensitivity. For the activity improvement towards ORR, the onset potential is shifted to 20.1 V vs Ag/AgCl with 3.4e- to 3.6e-transfer in alkaline medium This N doping strategy brings a serious structural degrade that the specific surface area of graphene drastically decrease to , 6 m2 g21 16. Melamine may be viewed as a second solid template since the excess amount is simultaneously gasified by heating (Figure S4b)
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