Fig. 1. Schematic of the experimental process for the fabrication of ZIF-8, ZIF-67, and ZIF-8/ZIF-67 loaded on PAN/2MI fibers, in the counterclockwise direction, respectively.

Fig. 2. (a) XRD patterns and (b) Raman spectra of annealed samples.

Fig. 3. SEM images of (a, b) ZnO/C, (c, d) ZnCoO/C (2:1), and (e, f) CoO/C. Note that ZnO/C is pristine ZIF-8 and CoO/C is pristine ZIF-67.

Fig. 4. TEM image of the ZnCoO/C (2:1) sample at (a) low (100 nm) and (b) high magnification (50 nm) (inset shows 20 nm); (c) elemental maps of C, Co, and Zn.

Fig. 5. (a) XPS survey spectrum of ZnCoO/C (2:1); core XPS spectra of (b) Zn 2p, (c) Co 2p, and (d) N 1s.

Fig. 6. (a, c, and e) Discharge-charge profiles and (b, d, and f) variations in differential capacity for (a, b) ZnO/C, (c, d) ZnCoO/C (2:1), and (e, f) CoO/C after 1st, 2nd, and 300th cycles at a current density of 100 mA·g-1.

Fig. 7. Electrochemical analyses of samples: (a) rate capability; long-term cycling at (b) low current density (100 mA·g-1) and (c) high current density (1000 mA·g-1). Here, N represents the number of cycles and CE is the coulombic efficiency. Relative capacity retention at (d) low current density (100 mA·g-1) and (e) high current density (1000 mA·g-1). Here, the shaded and solid bars indicate the capacity retention at N = 1 and N = 2, respectively.

Fig. 8. (a) Nyquist plots obtained from EIS for ZIF-derived bimetallic nanocage-decorated CNF electrodes (all samples). The inset shows the Randles equivalent circuit. (b) Z′ vs. the inverse square root of angular frequency in the Warburg diffusion tail region, used to calculate the Warburg coefficient (σw).