*3.1. Characterization*

To study the surface morphologies of prepared catalysts, SEM analysis was conducted with VEGA3 TESCON at the voltage of 20 kV. For elemental analysis of prepared catalysts, EDS analysis was conducted. Moreover, the crystal structure and phase purity of the prepared catalyst was established by XRD analysis (D8 Advanced Diffractometer) by using Jade 6.0 with diffraction angle (2θ), at a range of 10–70◦, with the step size of 4◦/s.

#### *3.2. Synthesis of ZIF-67*

A quantity of 1.97 g of 2-methylimidazole was dissolved in a 40 mL of 50/50 (*v/v* %) of ethanol and methanol. Furthermore, 1.746 g of Co(NO3)2·6H2O were mixed with ethanol and methanol mixture, keeping the ratios as before. The two solutions were then stirred together for 20 min and kept at room temperature for 20 h. After centrifugation, washing and drying, a purple precipitate was obtained [33].

#### *3.3. Synthesis of Mesoporous Carbon*

ZIF-67 was heated to 350 ◦C and was maintained at that temperature for 1.5 h using a tube furnace under reducing atmosphere (H2/Ar). The temperature was then increased to 750 ◦C with a ramp rate of 2 ◦C/min and was sustained at that temperature for 3.5 h. The furnace was naturally allowed to cool down. The sample was then treated with H2SO4, centrifuged, washed, and dried [51].

#### *3.4. Synthesis of MnO2-Doped Mesoporous Carbon*

Next, 1 M solution of KMnO4 was prepared in deionized water. Nano-porous carbon particles derived from ZIF-67 were dispersed in the 100 mL of solution using a bath sonicator for 15 min. The mixture was then stirred for 30 min and HCl (30%) was added dropwise to the mixture. The mixture was then transferred into a Teflon lined autoclave and heated in a box furnace at 80 ◦C for 3 h. The heated suspension was then filtered and washed using ethanol/water mixture and eventually dried in a vacuum oven at 80 ◦C overnight. The dried sample is the desired product (ZIF-67 derived nano-porous carbon and MnO2 hybrid). Figure 14 illustrates the synthesis route of ZCNT and ZCNT-M.

**Figure 14.** Synthesis route of ZCNT and ZCNT-M.
