Identifying the key N species for electrocatalytic oxygen reduction reaction on N-doped graphene

The state of nitrogen in nitrogen-doped graphene (NG) promoting the conversion of molecular oxygen to hydrogen peroxide was investigated. The oxygen reduction reaction (ORR) reactivity of graphitic-N, pyrrolic-N, and pyridinic-N in NG was predicted by density functional theory (DFT). A series of NG samples with different contents of these doped nitrogen types were prepared by the low-temperature thermal reduction method and used for the ORR evaluation. The H2O2 yield, 2e− ORR current efficiency, H2O2 selectivity, and electron transfer number (n) were systematically studied. The 2e− ORR selectivity was positively correlated with the N content, approaching 100% with increasing N content (0.40 V vs. reversible hydrogen electrode (RHE)), whereas the comparative energy efficiency showed a volcano-type trend related to N content, reaching a maximum of 94%. In addition, N species validation experiments proved the key role of pyrrolic-N in the synthesis of H2O2. Compared with a pure graphene catalyst, further contaminant degradation studies on NG electrodes with different pyrrolic-N contents revealed that the lower pyrrolic-N the higher removal of p-nitrophenol (PNP). This work provides insight into the mechanism of ORR on metal-free catalysts and a facile approach to optimize this important environmental catalytic strategy.