processing this molecule into valuable fuels by electrolysis is a more recent challenge. The literature pointing out the feasibility and interest of electroreduction of CO2 in molten carbonates into elemental C or CO is recent and significant, but is lacking a systematic approach and rigorous data (Groult et al., 2003, 2006; Kaplan et al., 2002, 2010; Le Van et al. 2009; Yin et al., 2013; Chery et al., 2014, 2015; Ijije et al., 2014). Kaplan et al. and Groult et al. were the first in conceiving the conversion of CO2 in molten carbonates into energy-storage materials (Groult et al., 2003; Kaplan et al., 2002). These authors obtained carbon nanopowders by electrochemical reduction of CO2 in Li–Na–K carbonates at 450°C, a relatively low temperature, on nickel and glassy carbon. The amorphous carbon obtained had a high specific surface area (450–850 m2 g−1) and was tested for lithium-ion intercalation in view of its application in batteries (Groult et al., 2006). Ge et al. used an inert platinum anode and a tungsten cathode CO2 into amorphous carbon and oxygen in molten LiCl–Li2CO3 salt at 700°C (Ge et al., 2015). Yin et al. also used the ternary carbonate eutectic at 500°C, with a Ni cathode and a SnO2 anode, captured and converted carbon dioxide into a carbon material, exhibiting high BET surface areas of more than 400 m2 g−1; SnO2 anode was found efficient for oxygen production (Yin et al., 2013). In similar conditions, but optimizing the electrolysis cell voltage and over a temperature range of 450–650°C, Tang et al. obtained carbon powder at 450°C with a low energy consumption of 35.6 kWh, a current efficiency of 87.86%, and under a cell voltage of 3.5 V (Tang et al., 2013). Over the production of carbon in molten salts, Ijije et al. developed a review including electrolysis in molten chlorides and molten carbonates (Ijije et al., 2014).
Another route was tested by Kaplan et al. focusing on the conversion of carbon dioxide into carbon monoxide by continuous electrolysis in Li2CO3 at 900°C using a titanium cathode and graphite anode; these authors obtained current densities superior to 100 mA cm (Kaplan et al., 2010). Licht and his coworkers (Licht, 2009, 2011; Licht et al., 2011, 2013) explored the concept of a large-scale development of CO2 electrolysers combining the production of CO or C with solar thermal energy. Evidence was given on the feasibility of producing valuable molecules, such as CO and C, with a solar efficiency reaching 50% using the so-called STEP (Solar Thermal Electrochemical Production) process. Figure 3 shows the capability of driving three in series molten carbonate electrolysis (Li2CO3) with only the maximum power point photovoltage of one Spectrolab CPV (Licht et al., 2011). According to the electrolysis temperature, either C (T < 900°C) or CO (T > 950°C) is formed.
Apart from CO2 electrochemical reduction, another important rising application of molten carbonates is water electrolysis yielding hydrogen and oxygen. Hu et al. have very recently given the proof of concept of such approach. CO2 valorization can also be combined with water transformation into H2 through coelectrolysis of CO2 and CO in order to produce syngas (Hu et al., 2014). Another possible goal with a significant importance in the present energetic field would also be the production of CH4 through methanation reactions (CO2 + 4H2 → CH4 + 2H2O; CO + 3H2 → CH4 + H2O). High-temperature electrolysis of CO2 is probably one of the very promising routes for valorizing this molecule into CO. This benefit could be enhanced by water electrolysis leading to the combination between CO and H2. The possibility of producing graphite, used also as a fuel in the direct carbon fuel cell (DCFC), is then possible as well.
Although the proof of concept of high-temperature electrolysis processes in molten carbonates has been proven up to a certain level, chemistry and electrochemistry of molten carbonates is complex and requires a deeper understanding, scarcely existing in the present literature, in order to optimize the materials and operation conditions, from the nature and conductivity of the electrolyte to the electrocatalytic properties of the electrodes. Thus, it is compulsory to select the carbonate melt and the precise running conditions according to the required process. For example, succeeding in preparing CO or C, at lower potentials and temperatures (avoiding deterioration of the materials and the cell), is beneficial from an energetic point of view and requires a thorough comparison of the main molten carbonate eutectics. In this sense, we have focused our efforts in two recent articles on a thorough analysis of the electrochemical behavior of CO2 in different molten carbonate eutectics, from a thermodynamic predictive approach and an experimental one (Chery et al., 2014, 2015). Some significant results concerning this approach will be given.
Figure 4 depicts a potential oxoacidity diagram of the ternary molten carbonate eutectic at two temperatures. The full comprehension of such diagrams with an explanation of all the selected conditions can be found in the literature (Chery et al., 2015). All the potentials are referred to Li2O/O2 system. The oxoacidity domain is limited on the oxoacidic side by a P(CO2) pressure, arbitrarily taken as 1 atm. (higher pressures can be also selected) and on the oxobasic side by the precipitation of the less soluble oxide among the three alkali cations of the melt: Li2O. The oxidation limit of the diagram is always due to the oxidation of oxide ions into molecular oxygen in this specific case (or other species such as peroxide or superoxide ions under different melts and conditions). The direct reduction of CO2 into elemental carbon may occur only at relatively oxobasic media, with P(CO2) < 10−3 atm, which is not realistic under
