Porous Ultrathin Carbon Nanoshells Embedded with Abundant Fe–N–C Sites toward Oxygen Reduction Reaction

Highly active and durable Fe–N–C electrocatalysts toward acidic oxygen reduction reaction (ORR) remain challenging due to their inferior intrinsic activity, low density, and insufficient exposure of active sites. Herein, we report the pyrolysis of coassembled hemin and copolymer capping on Zn(OH)2 nanosucculent plants, leading to the synthesis of highly porous ultrathin carbon nanoshells comprised of rich atomically dispersed Fe–N–C sites. The nanoshell is about 5.5 ± 0.8 nm thick with a pore volume of 0.5 cm3 g–1, allowing sufficient exposure of active sites. The nanoshell shows a remarkable ORR half-wave potential (E1/2) of 0.871 V (vs a reversible hydrogen electrode, RHE). The activity originates from the highest intrinsic activity with a turnover frequency of 11.7 e– site–1 s–1 at 0.8 V (vs RHE) and abundant accessible active sites (2.37 × 1020 g–1). Density functional theory elucidates that the presence of about 4 Å micropores neighboring to Fe–N–C lowers the Gibbs free energy of the ORR rate-determining step (O* + H+ + e– = OH*), which is beneficial for the improvement of intrinsic activity. Moreover, the nanoshell demonstrates a durability with 36 mV of E1/2 decay superior to that of commercial Pt/C (47 mV) during accelerated durability tests. Eventually, the remarkable activity was embodied by a peak power density of 450.6 mW cm–2 in H2–air single cells.