Fig. 1.

Porous characterization of the 3D noncontinuous carbon matrix in CoFe@PCS. a-d SEM images with the pores of 200 nm–15 μm. e–f TEM images with pores of 10–200 nm. g Scheme of formation of the pores in the CoFe@PCS. h Pore diameter distribution curves. i-l The XPS spectra of Co 2p (i), Fe 2p (j), C 1 s (k), N 1 s (l) for Fe@PCS, Co@PCS, CoFe@PCS