Fig. 2. Morphological features of the N-G-CNT electrodes with and without the addition of Ketjenblack.

(A to D) Cross-section SEM images of (A and B) the densely packed catalyst layer of N-G-CNT/Nafion (0.5/0.5 mg cm⁻²) and (C and D) the porous catalyst layer of N-G-CNT/KB/Nafion (0.5/2/2.5 mg cm⁻²). Purple arrows in (D) indicate the parallelly separated N-G-CNT sheets with interdispersed porous KB agglomerates. (E and F) BET surface areas (E) and pore volume distributions (F) of a piece of 5-cm² GDL, GDL with KB (2 mg cm⁻²), GDL with N-G-CNT (0.5 mg cm⁻²), and GDL with N-G-CNT/KB (0.5/2 mg cm⁻²) as indicated in the figures. (G and H) Schematic drawings of the MEA catalyst layer cross section, showing that O₂ efficiently diffused through the carbon black separated N-G-CNT sheets (G) but not the densely packed N-G-CNT sheets (H).