Figure 11.

A) SEM images of a,e) cobalt oxalate microtubes and Co–N–C HMMTs with a controlled conversion reaction time of b,f) 4 h, c,g) 8 h, and d,h) 24 h; B) Schematic illustration of the synthesis method for the Co–N–C microtubular structure composed of hollow nanospheres and nanotubes. Cobalt oxalate microtube, which is used as both the solid precursor and the self‐template (a). Co/C hybrids microtube after a conversion reaction to ZIF‐67 microtube and a subsequent pyrolysis process at 800 °C for 3 h under argon flow (inset: inactive Co core@Co–N–C shells) (b). Co–N–C microtube composed of hollow subunits, nanopsheres, and nanotubes after an acid etching process (inset: Co–N–C hollow subunits) (c). Reproduced with permission.172