*2.3. Functionalization of HCS with Metal Oxides (FexOy/MnO2/HCS)*

To store metal oxide in hollow carbon spheres, iron (III) nitrate nonahydrate (100 mg, CHEMPUR, Piekary Slaskie, Poland) was dissolved in ethanol; added dropwise to the 100 mg HCS, stirring and heating to 50 ◦C; and then, placed in a furnace for 2 h at 400 ◦C. Next, FexOy/HCS (40 mg) was added to a KMnO4 (40 mg, CHEMPUR, Piekary Slaskie, Poland) solution in a round bottom flask. The reaction was carried out for 0.5 h at 70 ◦C. Then, the product was collected by filtration, washed two times with water and ethanol and dried at 80 ◦C in a vacuum for 24 h.
