Fig. 2.

SEM images, TEM images, and EDX mapping of a 3D NC, b 3D Mn–NC, c 3D Fe–NC, d 3D Cu–NC, e 3D Ni–NC, and f 3D Co–NC. Scale bars: a–f 1 μm; a1–f1 500 nm; a2–f2 200 nm