N species tuning strategy in N, S co-doped graphene nanosheets for electrocatalytic activity and selectivity of oxygen redox reactions

The precise regulation of N, S doping and their synergetic effect is essential for N, S co-doped carbon materials as efficient metal-free electrocatalysts for oxygen redox reaction (Oxygen reduction reaction (ORR) and Oxygen evolution reaction (OER)). Herein, an effective precursor modulated active sites engineering strategy of N, S co-doped graphene nanosheets (NSG) were developed by one step pyrolysis of 5-aminouracil (ANA) as N-containing precursor, ammonium persulfate (AP) and 2, 5-dithiobiurea (DBA) as S source, respectively. The results indicate that the specific N doping species in NSG and their synergetic effect with S dopants is strongly dependent on the S sources, which induces huge divergence of electrocatalytic activity and selectivity of NSG nanosheets for ORR and OER. The NSG prepared by ANA and AP as precursors with dominant graphitic N dopant coordinated with S possess the best ORR performance with half-wave potential, E1/2 of 0.87 V vs. RHE in 0.1 M KOH and poor OER performance with a high potential of 1.67 V at 10.0 mA cm−2, Ej=10. On the other hand NSG derived from ANA and DBA with dominant pyridinic N and pyrrolic N dopants exhibits the highest bifunctional activity for both OER and ORR with ΔE (ΔE = Ej=10-E1/2) of 0.73 V and the performance has been verified on a rechargeable Zn-Air battery fabricated by NSG with a peak power density of 146 mW·cm−2, specific capacity of 796 mAh·gZn−1, higher than that with state-of-the art Pt/C and IrO2 (1:1 wt%) air electrode at the same catalyst loading. These excellent performance fundamentally originates from the optimized intermediates energy of ORR or/and OER via the constructed configuration of S and different N species in graphene nanosheets prepared by the specific N and S precursors. The dependence of electrocatalytic selectivity and activity for ORR or/and OER on different N, S configurations revealed in this study provides a facile strategy to achieve specific active sites configurations for developing bifunctional metal-free electrocatalysts.