*2.2. Synthesis of Hollow Carbon Spheres (HCS)*

A chemical vapor deposition (CVD) process was applied to prepared HCS. The coreshell SiO2@mSiO2 was placed in an alumina boat in a tube furnace (Carbolite GERO, Hope, UK) and a CVD process using C2H4 as the carbon source occurred for 1 h at 800 ◦C. After synthesis, SiO2@mSiO2 covered by carbon was obtained (SiO2@mSiO2\_C). To obtain HCS, SiO2@mSiO2\_C was treated by hydrofluoric acid (CHEMPUR, Piekary Slaskie, Poland) to remove silica and then washed with water and ethanol several times. Finally, the carbon product was obtained by drying the sample in a vacuum at 80 ◦C for 12 h.
