N-doped carbon aerogel (CA) was prepared by one-pot template-free synthesis and chemically activated with potassium hydroxide (KOH). KOH activation highlighted the structural superiority with increased BET surface area and hierarchically porous structure (especially micropores), and also significantly influenced active nitrogen species and oxygen functional groups (C-O-C and COOH) content. With KOH increasing, pyridinic N content increased, while pyrrolic N content decreased. Oxygen reduction reaction (ORR) activity showed a positive linear correlation with pyridinic N content but decreased with pyrrolic N content increasing. C-O-C and COOH content increased after activation, and then decreased with KOH increasing. The reduced C-O-C and COOH content of CA-KOH guaranteed a lower H2O2 yield for higher microbial fuel cell (MFC) efficiency. CA-10KOH (mass ratio of KOH to CA was 10) exhibited outstanding performance with the highest maximum power density (967 +/- 34 mW m(-2)), which was 37.4%, 55.2% and 283.7% higher than platinum (Pt/C), activated carbon (BET surface area of 2276 m(2) g(-1)) and CA. The outstanding performance of CA-10KOH was attributed to the conjunct effect of superior BET surface area (1827 m(2) g(-1)), increased micropores, the highest pyridinic N (0.26 at.%), the lowest pyrrolic N content and reduced C-O-C and COOH content. Overall, KOH activated CA is a promising metal-free cathode catalyst for high performance in MFCs.