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 Hybrid Direct Carbon Fuel Cell

Direct oxidation of carbon in a fuel cell is an old dream of the industrial revolution where cities were polluted by CO2 emission. Nevertheless, due to advances in electrochemical kinetics, it is only in the recent years that impressive progress has been realized in direct carbon fuel cells (DCFCs), which is an important issue the important resources in carbon spread all over the world. In a DCFC, a carbon fuel (pyrolysis of biofuels, farming products, etc.) is oxidized at the anode with an energy produced per unit volume four times higher than with methane (Zecevic et al., 2004). In such devices, molten carbonates may have two roles, either directly as the electrolyte or as a carrier of carbon fuel, acting also as an electrochemical mediator for the oxidation process, in the anode reservoir of a hybrid direct carbon fuel cell (HDFC), with a solid oxide electrolyte (Irvine et al., 2006; Pointon et al., 2006; Nabae et al., 2008).

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. These composites form highly disordered interfacial regions between the oxide phase and the carbonate phase (Zhu et al., 2010).

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. It is clear that MCFC is an interesting device for CCS, but its effectiveness still has to be proven against the other present technologies for CO2 emission mitigation (Spigarelli and Komar Kawatra, 2013). Theoretically, MCFCs can reach 90% CO2 separation efficiency while producing electricity at high generation efficiency. In fact, some systems are under investigation and seem to have interesting features (Suguira et al., 2003; Wade et al., 2007). Small power plants of 10–20 Mton CO2 per year are feasible and can find more immediate implementation. Nevertheless, although the potential market is enormous, the present capacity for producing MCFCs at the level of larger power plants of more than 100 Mton CO2 per year is not yet possible.

General Features

It is well known that high-temperature electrolysis requires less electrical energy but higher thermal energy; however, as thermal energy is cheaper, electrolysis processes are favored at high temperature (Chery et al., 2015). This question is crucial in water steam electrolysis, which is thoroughly investigated in solid oxide electrolysers (Bradley et al. 2004). Recently, it has been shown that CO2 electrolysis is also feasible in such devices. Bidrawn et al. have shown that an SOE system operating under CO–CO2 atmosphere exhibited a total cell impedance of 0.36 Ω cm2, its efficiency is similar to that of water electrolysis (Bidrawn et al., 2008). The feasibility of such processes in ceramic electrolysers has been investigated in view of NASA’s future exploration of Mars, aiming at converting CO2 from the Mars atmosphere into life-supporting oxygen and oxidant/propellant fuel (Washman, 2003). The reduction of CO2 into CO, at typically 800–900°C, can be associated to water electrolysis into H2 in a so-called co-electrolysis process producing syngas: H2 + CO. The durability of these types of electrolysis devices is still under study. In any case, it would be interesting to explore the same electrochemical approach at lower temperatures to avoid the drastic corrosion of the materials. Molten carbonates with lower operation temperatures, from 600 to 650°C, and allowing a high solubility of CO2 within the molten electrolyte, are very interesting candidates to combine the benefits of high temperature, but not too high, with the reactivity of CO2 in a liquid medium.

Electrolysis in Molten Carbonates

Before introducing molten carbonates, it should be outlined that electroreduction of CO2 into C has been evidenced in molten chlorides, i.e., NaCl–KCl eutectic; the reduction mechanism has been studied by cyclic voltammetry at a gold electrode, showing a three-step reaction, the first one involving a radical intermediary species, CO22-; the second formation of CO; and the third elemental carbon (Novoselova et al., 2007, 2008; Ijije et al., 2014). The production of C by electroreduction of CO2 in LiCl–Li2O or CaCl2–CaO has also been depicted by other authors (Otake et al., 2013).

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 main attractiveness of such melts is their capability of solubilizing carbon dioxide, from single alkali molten carbonates to a variety of carbonate eutectics (Claes et al., 1996, 1999; Chery et al., 2014). Table 1 shows some solubility data extracted from the literature, showing a relatively high solubility. Nevertheless, it is clear that the values obtained depend on the technique used and on the experimental procedure. In view of the important applications of CO2 in molten media (capture and valorization), a new set of reliable data is urgently required.

CO2 properties in molten carbonates have been analyzed early in the literature. Peelen et al. have given some useful characteristics on the CO2/CO redox system in Li–K carbonate eutectic, finding a simple charge transfer and evaluating the square root of the diffusion constant and the solubility S√D (Peelen et al., 1997). Claes et al. have found by a manometric technique that CO2 solubility was higher than predicted, attributing this fact to the production of C2O4CO2- by the reaction of CO2 with the carbonate melt (Claes et al., 1996, 1999). Other authors have also detected the reduction of CO2 directly or as a rate-limiting species in oxygen reduction (Yamada et al., 1995). Nevertheless,