## 3 Results and Discussion
### The Catholic Production of Carbon  
It is well known that the melting points of pure Li\({}_{2}\)CO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), and K\({}_{2}\)CO\({}_{3}\) are relatively high, up to 723 \({}^{\circ}\)C, 851 \({}^{\circ}\)C, and 891 \({}^{\circ}\)C, respectively [26], signifying that over 950 \({}^{\circ}\)C is required for electrolysis (at least 50 \({}^{\circ}\)C higher than the melting point). As stated in our previous works, the pure carbonates will decompose to metallic oxide and carbon dioxide at excessively high temperatures [24; 25]. The high operating temperature inevitably leads to unwanted thermal decomposition and the waste of raw materials [25; 27; 28]. To reduce the working temperature and enable less energy consumption, we employed the ternary eutectic mixture of Li\({}_{2}\)CO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), and K\({}_{2}\)CO\({}_{3}\) as the electrolyte to transform tetravalent carbon to solid carbon at a lower temperature, such as Li\({}_{1.43}\)Na\({}_{0.26}\)K\({}_{0.21}\)CO\({}_{3}\) (61:22:17, mass ratio), which melts at 390 \({}^{\circ}\)C, and Li\({}_{0.85}\)Na\({}_{0.61}\)K\({}_{0.54}\)CO\({}_{3}\) (31:32:37, mass ratio), which melts at 393 \({}^{\circ}\)C.  
The composition of molten salts has been reported to affect the carbon electrodeposition from CO\({}_{3}^{2}\)-[29]. In our previous works, no amorphous carbon was obtained in pure Na\({}_{2}\)CO\({}_{3}\), K\({}_{2}\)CO\({}_{3}\), or a blend of both [23], clearly suggesting that the role of individual carbonates in the CO\({}_{2}\) electrochemical process is quite different. The thermodynamic calculation in Table 1 can explain why the reduction of carbon from Li\({}_{2}\)CO\({}_{3}\) is more favorable than that from Na\({}_{2}\)CO\({}_{3}\) or K\({}_{2}\)CO\({}_{3}\) under certain conditions. \begin{table}
\begin{tabular}{c c c c c} \hline
**T (K)** & **Carbonate** & \(\text{E}^{\circ}\text{{}_{M}}\) **(V)** & \(\text{E}^{\circ}\text{{}_{C}}\) **(V)** & \(\text{E}^{\circ}\text{{}_{CO}}\) **(V)** \\ \hline \multirow{4}{*}{700} & Li\({}_{2}\)CO\({}_{3}\) & 3.218 & 1.901 & 2.318 \\  & Na\({}_{2}\)CO\({}_{3}\) & 2.791 & 2.706 & 3.392 \\  & K\({}_{2}\)CO\({}_{3}\) & 2.853 & 3.249 & 4.116 \\ \hline \multirow{4}{*}{750} & Li\({}_{2}\)CO\({}_{3}\) & 3.145 & 1.845 & 2.221 \\  & Na\({}_{2}\)CO\({}_{3}\) & 2.719 & 2.655 & 3.301 \\  & K\({}_{2}\)CO\({}_{3}\) & 2.778 & 3.191 & 4.014 \\ \hline \multirow{4}{*}{800} & Li\({}_{2}\)CO\({}_{3}\) & 3.071 & 1.789 & 2.124 \\  & Na\({}_{2}\)CO\({}_{3}\) & 2.649 & 2.603 & 3.209 \\  & K\({}_{2}\)CO\({}_{3}\) & 2.703 & 3.132 & 3.913 \\ \hline \multirow{4}{*}{850} & Li\({}_{2}\)CO\({}_{3}\) & 2.998 & 1.735 & 2.029 \\  & Na\({}_{2}\)CO\({}_{3}\) & 2.579 & 2.552 & 3.118 \\  & K\({}_{2}\)CO\({}_{3}\) & 2.631 & 3.074 & 3.813 \\ \hline \multirow{4}{*}{900} & Li\({}_{2}\)CO\({}_{3}\) & 2.926 & 1.681 & 1.933 \\  & Na\({}_{2}\)CO\({}_{3}\) & 2.508 & 2.502 & 3.027 \\ \cline{1-1}  & K\({}_{2}\)CO\({}_{3}\) & 2.557 & 3.015 & 3.712 \\ \hline \end{tabular}
\end{table} 
Table 1: Standard potentials for the conversion of alkali metal carbonate to pure alkali metal, carbon, and carbon dioxide (Li\({}_{2}\)CO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), or K\({}_{2}\)CO\({}_{3}\) at different temperatures). The relevant thermodynamic data deposited to metal, carbon, or carbon dioxide from a single Li\({}_{2}\)CO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), or K\({}_{2}\)CO\({}_{3}\) at different temperatures is given in Table 1. After analyzing the above calculation results, we found that the potential required to deposit carbon (E\({}^{\circ}\)c) or CO (E\({}^{\circ}\)c) is lower than that for Li metal deposition (E\({}^{\circ}\)M) in pure molten Li\({}_{2}\)CO\({}_{3}\), revealing a stronger tendency for the carbon electrodeposition. However, the E\({}^{\circ}\)c of Na\({}_{2}\)CO\({}_{3}\) and K\({}_{2}\)CO\({}_{3}\) is comparable or more positive than the E\({}^{\circ}\)M, indicating that metal deposition is more preferred than carbon in Na\({}_{2}\)CO\({}_{3}\) or K\({}_{2}\)CO\({}_{3}\).