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Mechanism for electrochemical CO₂ reduction on metal surfaces in water (Jones et al., 2014).

The pH of the solution impacts the CO₂ solubility as well as the first CO₂ reduction competition reaction, the hydrogen evolution reaction. The selection of the electrolyte is also a very important parameter for electrochemical reduction of carbon dioxide (Gatrell et al., 2007; Hori, 2008; Oloman and Li, 2008). Many researchers found that lower temperatures suppress hydrogen evolution and increase CO₂ reduction efficiency (Lote, 2014). High-pressure CO₂ electrolysis (till 60 atm) could be the most feasible method for achieving a commercial electrochemical process. The high current density and efficiency observed in this case on different electrodes are comparable with those obtained with high-temperature solid oxide electrolysers.

Molten Carbonates: Strategic Electrolytes

Physicochemical Properties

The interest of molten carbonates proceeds from their unique chemical and physical properties described by many authors (Yuh et al., 1995; Janz and Lorenz, 1961; Ward and Janz, 1965; Kojima et al., 2008; Lair et al., 2012). First of all, they cover a large domain of temperatures according to their nature and combinations, roughly from 450°C to more than 1,000°C. They are constituted either by single alkali carbonates, such as Li₂CO₃, Na₂CO₃, and K₂CO₃, with melting points of 730, 901, and 858°C, respectively, or more commonly by eutectic mixtures, Li₂CO₃–K₂CO₃ (62–38 mol% and 42.8–57.2 mol%), Li₂CO₃–Na₂CO₃ (52–48 mol%), Li₂CO₃–Na₂CO₃–K₂CO₃ (43.5–31.5–25 mol%), or Na₂CO₃–K₂CO₃ (58–42 mol%) with lower melting points, respectively, 488, 499, 501, 397, and 710°C (Janz and Lorenz, 1961; Sangster and Pelton, 1987). Their conductivities are another major feature depending on the nature of the alkali cation, knowing that smaller ions imply higher conductivity: for Li₂CO₃, Na₂CO₃, and K₂CO₃, the values are, respectively, 5.4, 2.8, and 2 S cm$^{-1}$ at 900°C (Tanase et al., 1987).

The self-ionization constant of a molten carbonate is characterized by the equilibrium:

${\ce {M2CO3(l)\Leftrightarrow M2O(s){}+CO2}}$ (1)

With (l), (s), and (g) as the liquid, solid, and gas phase, respectively.

This equilibrium can be simplified in terms of a simple ionic form, and considering a molten carbonate as a strong electrolyte, the equilibrium can be given in an ionic form:

${\ce {CO3^{2}\Leftrightarrow O^{2}{}+CO2}}$ (2)

The oxoacidic concept derives from this equilibrium, where O^2- is an electron pair donor which is associated with an oxoacid CO₂ and forms an oxobase CO₃^2- (Flood and Forland, 1947; Yamada and Uchida, 1994). The oxoacidity can be fixed by imposing a carbon dioxide partial pressure or by adding oxides in the melt. High partial pressures of CO₂ represent, for instance, highly oxoacidic media.

Present Applications

Fuel Cells

Molten Carbonate Fuel Cells

If molten carbonate media have gained their reputation, it is surely due to their use in MCFCs that have nowadays reached an advanced state of maturity and a first step toward commercial market entry (Cassir et al., 2012). In the last 3 years, more power has been produced by MCFCs, between 70 and 100 MW/year, than with any other fuel cells, including polymer electrolyte membrane fuel cells (PEMFCs), used for transport and, in particular, electric vehicles. The global cell reaction in an MCFC is the following:

${\ce {H2 + 1/2O2 + CO2 (cathode) -> H2O + CO2 (anode)}}$ (3)

CO₂ formed at the anode is recycled and consumed at the cathode. The fuel introduced in the anode compartment is H₂, resulting from natural gas conversion usually by reforming. The anode reaction is:

${\ce {H2 + CO3^2 -> H2O + CO2 + 2e}}$ (4)

The oxidant introduced at the cathode side is constituted by a mixture of air and CO₂. The cathode reaction is:

${\ce {1/2O2 + CO2 + 2e -> CO3^2}}$ (5)