Déborah Chery, Virginie Lair, Michel Cassir

“ 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. ”

“ Inspired by nature, a large body of research has been devoted to new materials catalyzing CO2 functionalization in either electrochemical or chemical ways. In 2009, the US start-up Carbon Science [2] has produced liquid fuels thanks to a prototype. The demonstration prototype uses Carbon Science innovative biocatalytic process to break down CO2 and water and then combines carbon and hydrogen to form ethanol, a liquid fuel. Enzymes from microorganisms transform CO2 and H2O to methane, methanol, and butanol. ”

“ In the last decade, there is a growing and significant attention on electrolysis processes in molten carbonates with different goals, from water electrolytic transformation into hydrogen, which can also be extended to water and CO2 coelectrolysis yielding syngas, to CO2 electroreduction into C or CO in view of producing CH4 as a second step. ”

“ The molten carbonate fuel cell system can be considered as a CO2 separator and concentrator because CO2 can be transferred from the cathode to the anode stream while producing electricity from fuels at the anode side. CO2 can be extracted from the flue gas of a combined cycle power plant while generating electricity, avoiding loss in plant efficiency and consequent increase in primary energy consumption. ”

“ Hybrid Fuel Cells (MCFC/SOFC)
Another kind of hybrid fuel cells presenting a growing interest is constituted by an electrolyte combining solid oxides with molten carbonates (Benamira et al., 2011, 2012) . Oxide ions are mostly responsible for the conductivity in the oxide phase and carbonates in the carbonate phase. ”

“ The formation of peroxodicarbonate C2O62- by oxygen reduction to superoxide in RTILs is an example. Among hydrogen, nitrogen, and carbon monoxide, carbon dioxide and water have the highest solubility in an ionic liquid (C4C1-iPF6) at 25°C. CO2 is the most soluble in fluorinated IL. CO2 can be collected from the ILs by pressure decreasing. Optimizing the design of the appropriate RTILs, as well as the electrode material, is compulsory to enhance the absorption and the reduction of CO2 (Rees and Compton, 2011) . ”

“ Copper binds the CO2•− intermediate and reduces the CO to higher reduction products such as hydrocarbons (methane, ethylene, etc.) and alcohols. The last group (Ni, Fe, Pt, and Ti) does not reduce CO2 directly, strongly binding hydrogen to produce hydrogen only. 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. ”

“ 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) . ”

“ Group 2 (Au, Ag, Zn, Ga, Pd) yields CO as the major product. Group 2 binds the CO2•− to varying degrees, but cannot reduce CO. Copper is the only commonly studied (for the CO2 reduction) metal that falls into intermediate and reduces the group 3. ”

“ STEP (solar thermal electrochemical photo) generation of energetic molecules: a solar chemical process to end anthropogenic global warming. J. Phys. Chem. C 113, 16283–16292. doi:10.1021/jp9044644 Licht, S. (2011) . Efficient solar-driven synthesis, carbon capture, and desalinization, STEP: solar thermal electrochemical production of fuels, metals, bleach. ”