## 3 Results and Discussion
### Spherical Carbon  
Because of the advantages of large specific surface areas, good fluidity and high mechanical strength, carbon spheres are widely used in gas storage [29], catalyst support [30] and drug delivery [31]. As previously described, the active nickel site originates from anodic corrosion, which can be used as an effective catalyst and for the growth of nucleated carbon products [28; 32]. The formation of carbon material with special nanostructures in the molten carbonate system can be attributed to the nucleation of nickel particles [33]. Under the synergistic effect of high temperature and electric field, the surface of the nickel anode is oxidized to NiO. And the solubility of NiO in molten carbonate is very low, only 1 \(\times\) 10\({}^{-5}\) mol/mol [34]. At the initial stage of electrolysis, a small current density is applied to the electrolytic system, about 15-25 mA/cm\({}^{2}\) for only 5 min. Subsequently, a current density of over 100 mA/cm\({}^{2}\) is applied to reduce the CO\({}_{3}\)\({}^{2-}\) in the molten salt electrolyte to carbon. The metal nickel particles on the cathode surface act as nucleation sites in the process of carbon deposition. Carbon atoms accumulate on the surface of nickel particles, form bonds, and synthesize carbon materials with special structures. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**Electrolyte (Weight Ratio)**} & **Temperature** & **Current Density** & **Specific Surface** \\  & & **(‘C)** & **(mA/cm\({}^{2}\))** & **Area (m\({}^{2}\)/g)** \\ \hline \multirow{2}{*}{1} & Li\({}_{2}\)CO\({}_{3}\) + Na\({}_{2}\)CO\({}_{3}\) + K\({}_{2}\)CO\({}_{3}\) & 450 & 100 & 127 \\  & (61: 22:17) & 550 & 100 & 89.5 \\  & (61: 22:17) & 650 & 100 & 56. Using the same electrode system as before, nickel wire (effective area 10 cm\({}^{2}\)) and galvanized iron (effective area 5 cm\({}^{2}\)) are employed as anode and cathode. The carbon products display a spherical structure generated in the molten salt containing calcium or barium carbonate via electrolysis at relatively high temperatures (650 \({}^{\circ}\)C or 750 \({}^{\circ}\)C). The microstructure of cathode carbon products, separated from the cathode surface in the electrolyte of \(\mathrm{Li_{2}CO_{3}+CaCO_{3}+Na_{2}CO_{3}}\), \(\mathrm{Li_{2}CO_{3}+CaCO_{3}+K_{2}CO_{3}}\), \(\mathrm{Li_{2}CO_{3}+BaCO_{3}+Na_{2}CO_{3}}\), \(\mathrm{Li_{2}CO_{3}+BaCO_{3}+K_{2}CO_{3}}\) and \(\mathrm{Li_{2}CO_{3}+CaCO_{3}+BaCO_{3}}\), is shown in Figure 4 in detail. Different from the amorphous carbon products synthesized in \(\mathrm{Li_{2}CO_{3}+Na_{2}CO_{3}+K_{2}CO_{3}}\) molten salt system, the morphology of carbon products of \(\mathrm{Li_{2}CO_{3}+CaCO_{3}+Na_{2}CO_{3}}\) and \(\mathrm{Li_{2}CO_{3}+CaCO_{3}+K_{2}CO_{3}}\) electrolytic systems at 750 \({}^{\circ}\)C shows a tufted sphere structure. The sphere diameter ranges from 300 to 500 nm, and the carbon spheres are superimposed. A current density of 25 mA/cm\({}^{2}\) is first applied to make nickel particles deposited on the cathode surface in the process of electrolysis. And then 200 mA/cm\({}^{2}\) is used to make the  
Figure 4: Carbon spheres obtained from different molten eutectic carbonates. carbon deposition reaction proceed at a fast speed. It has been proved in previous reports that Na\({}_{2}\)CO\({}_{3}\) and K\({}_{2}\)CO\({}_{3}\) are more likely to form corresponding alkali metals, which have a particular interference effect on the carbon deposition process catalyzed by nickel particles in the electrolysis process [17; 32; 33]. In molten salts containing divalent carbonates, the structure of carbon products is obviously changed from amorphous to spherical. Taking calcium carbonate as an example, CaO generated by the decomposition of CaCO\({}_{3}\) plays a certain interface modification role in the process of carbon deposition, leading to the electrolytic formation of carbon sphere morphology.