Branched Poly(l-lysine)-Derived Nitrogen-Containing Porous Carbon Flake as the Metal-Free Electrocatalyst toward Efficient Oxygen Reduction Reaction

Amino acids are recently being regarded as highly attractive candidates to fabricate thoroughly metal-free and rich nitrogen-containing mesoporous carbons. Nevertheless, the carbonization yield of the lysine is usually just around 5% due to the small molecular size and harsh polymerization condition for lysine to form the stable carbonization counterpart. Renewable biomass branched poly(l-lysine) (BPL) is hereby exploited to synthesize nitrogen-containing porous carbon flake (NPCF). Notably, the fabrication process is essentially an in situ thermal polymerization and pyrolysis with mesoporous silica template strategy, which avoids the usage of metal catalysts. Indeed, a metal-free mesoporous carbon with a high carbon yield of 20.1% and improved nitrogen-doping content of up to 9.6 at. % is achieved. As a result, the as-prepared NPCFs serve as metal-free electrocatalysts toward the efficient oxygen reduction reaction (ORR). In particular, one of the samples (NPCF-700/900) exhibits highly efficient ORR performance of oxygen-reduction voltage at 0.809 V in cyclic voltammogram measurement, onset peak at 0.93 V, and semi wave peak of 0.83 V in rotating disk electrode linear sweep voltammetry curves. The associated kinetic-limiting and diffusion-limiting current densities are 18.4 and −4.89 mA cm–2, respectively. This can be attributed to the abundance of nitrogen dopant of 8.3 at. % in a high graphitization carbon framework that possesses a large specific surface area (1431 m2 g–1). More importantly, NPCF-700/900 delivers excellent cycle stability and endurance with regards to methanol permeation and CO poisoning, making it a hopeful metal-free ORR electrocatalyst superior to that of the Pt/C in basic conditions.