fig4

Figure 4. SEM images of PBF (A), PBF@N (B), and PBF@N@S (C-D) and TEM images of the PBF@N@S (E-F). PBF: Bare biochar fibers; PBF@N: N-doped porous biochar fibers; PBF@N@S: N, S dual-doped porous biochar fibers; SEM: scanning electronic microscopy; TEM: transmission electron microscopy.