high partial pressures of CO2 required for the desired application. At higher values of P(CO2), thermodynamically the most favorable phenomenon is the electroreduction of CO2 into CO, this tendency being increased at P(CO2) = 1 atm. The second reduction step, occurring at lower potentials, is the reduction of CO into C. This is only one example of how thermodynamic diagrams allow predicting the conditions in which the electrochemical processes might occur. Of course, useful in high-temperature electrochemistry (where reactions are favored thermodynamically), this vision does not take into account kinetics, which can be analyzed through cyclic voltammetry, electrochemical impedance spectroscopy. Figure 5 shows cyclic voltammograms relative to CO2 in the same ternary eutectic at a gold flag electrode at 600°C, under P(CO2) = 1 atm. The three-electrode setup associating the gold working electrode with a gold auxiliary electrode and an Ag+/Ag reference was fully described in the literature (Chery et al., 2014). A slight pre-electrolysis, carried out during 360 s at −1.1 V vs. Ag+/Ag, was found to be important for conditioning the working gold electrode. The potential scan was initiated from the pre-electrolysis potential up to −0.2 V vs. Ag+/Ag and then reverses until reaching −1.35 V vs. Ag+/Ag. The reduction peak around −1.1 V vs. Ag+/Ag depicts the global reduction of CO2 into CO (2 CO2 + 2 e− = CO + CO32−). This peak is at least a combination of two reactions: the first forming an unstable species which is up to now undetermined: CO2-: 2 CO2 + 2 e− = 2 CO2- (Peelen et al., 1997), CO22- (Novoselova et al., 2008), C2O52- (Sangster and Pelton, 1987), or C2O42-? The second step is the reduction of the unstable species into CO, i.e., 2 CO2− = CO + CO32−. The oxidation roughly shows a peak around −0.8 V vs. Ag+/Ag, which is probably the reoxidation of CO, and another around −0.5 V vs. Ag+/Ag, which might be attributed to the reoxidation of adsorbed CO. More insight is given in the literature (Chery et al., 2014), but no sharp conclusion can be given on the mechanistic process. According to the evolution of the CO2 peak with the scan rate, the reduction potential is slightly moved toward negative potentials, showing a rapid to quasi-rapid system. It should be noted that the same system when using another electrolyte, for instance Li–K, appears slower (Chery et al., 2014). Thus, the properties of the system depend on the nature of the electrolyte and, of course, the electrocatalytic properties of the electrode. Analysis of the reduction peak also shows that it is diffusion-limited.
Conclusion
Among the different techniques used for the valorization of carbon dioxide, electrochemical reduction through electrolysis processes has an important position because it is a relatively low-cost technique that is beginning to be associated with renewable energies, such as solar. Moreover, the variety of electrolytic media plays in favor of this approach. High-temperature electrolysis, either with solid oxides or molten salts, appears as interesting from an electrocatalytic and energetic viewpoint. As the MCFCs electrolytes state of art, molten carbonates are able to capture and dissolve CO2; thus, they are very favourable media for transforming this greenhouse effect gas into valuable chemicals and/or fuels. A rational use of molten carbonates requires a perfect control of the properties and reactivity on CO2 in such melts, in order to optimize the electrolysis process, making it more efficient and less expensive. This field is very attractive from both scientific and technological sides. It is one of the significant challenges in the energetic and environmental field.
Acknowledgments
The authors acknowledge the French Program “Défi ENRS – Projets Exploratoires Émergence CO2” for financial support. This work was also supported by the French program PLENEX ANR-11-EQPX-0-01.
Footnotes
