Oxygen Reduction Performance of the Nitrogen-Doped Carbon Materials Pyrolyzed from Polyaniline/Carbon Nanocage Composites

Fuel cells can efficiently convert chemical energy of fuels into electrical energy in a green manner, representing one of the most promising movable power sources. The main bottleneck for the wide application of fuel cells is the sluggish oxygen reduction reaction (ORR) which is usually catalyzed by expensive and unstable Pt catalysts. Hence, the exploration of cost-efficient and long-life ORR electrocatalysts is of great significance. Recently carbon-based metal-free ORR electrocatalysts have attracted much interest due to their superior activity and stability as well as the abundance and low cost. It is generally accepted that nitrogen doping of carbon materials boost the ORR activity by breaking the electroneutrality of carbon layer and activating It electrons. Considering that the embedded N doping sites are useless for ORR but significantly affect electron conduction, the carbon materials with N-enriched surface of high activity and pristine carbon bulk of high conductivity should present better ORR performance than the bulk N-doped counterpart. In this contribution, we report an efficient strategy for the synthesis of ORR electrocatalysts with surface enriched N doping and high conductive bulk by dispersing polyaniline (PAM) on the surface of 3D hierarchical carbon nanocages (CNC), followed by heat treatment in Ar. The N content and conductivity of the pyrolyzed products are regulated by changing the pyrolysis temperature and the weight ratio of PANI/CNC composite. The optimized electrocatalyst with the N content of 2.21 at% and the conductivity of 203 S.m(-1) exhibits excellent ORR catalytic performance with high onset potential of -46 mV vs Ag/AgCl, obvious superior to the bulk N-doped CNC (-105 mV). This catalyst also possesses high stability (96% activity retention after 10 h continuous tests) and dominated 2-electron pathway for ORR with transferred electron number of 2.8. This result suggests an efficient route to improve the ORR activity of carbon-based materials by increasing the N content enriched at the surface while keeping their high conductivity.