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Energies 2020, 13, 420 21 of 96 Sr-doped LaFeO3 that are highly compatible with YSZ electrolyte, have good electric and ionic conductivity and thermal stability [258]. The interconnector is usually made of Fe–Cr stainless steel alloy because it has high thermal conductivity, low cost and the fabrication is not tricky. A drawback is the Cr deposition and poisoning of the oxygen evolution electrode [259]. Finally, numerous materials might be applied as a sealant, including glass and glass–ceramic joints, high temperature metal, ceramic and glass–metal composite and cement [260]. The overall co-electrolysis reaction is summarized as Equation (31). At the triple-phase boundary (TPB) of the cathode–electrolyte interface, water and carbon dioxide are electrolyzed according to Equations (32) and (33), and the reverse water gas shift (Equation (34)) occurs. The oxide ions produced at the cathode are transported through the dense electrolyte and are reduced into oxygen (Equation (35)) at the TPB of the electrolyte–anode interface. In addition two reaction occurs: the CO-methanation reaction (Equation (36)), and the undesired coke formation reaction (Equation (37)) [261]. H2O + CO2 → H2 + O2 + CO (Overall co − electrolysis reaction) (31) H2O + 2e− → H2 + O2− (Water oxidation reaction) (32) CO2 + 2e− → CO + O2− (Carbon dioxide oxidation reaction) (33) CO2 + H2 → CO + H2O (Reverse water gas shift reaction) (34) O2− → O2 + 2e− (Oxide ions reduction reaction) (35) CO + 3H2 → CH4 + H2O (CO − methanation reaction) (36) CO → C + 12 O2 (Carbon cracking reaction) (37) The electrochemical kinetics and the activation energy for dry CO2 electrolysis are remarkably higher than co-electrolysis. Higher performances are ensured by the reverse water gas shift (RWGS) that converts a considerable amount of CO2 to CO and reduces the electrical consumption for the overall reaction as the water splitting involves lower polarization resistance and overpotentials [262]. At 873 K, 25% of the CO produced is ascribable to RWGS [263]. Accordingly, the lower cell voltage required for co-electrolysis and the methanation reaction reduce the coke formation and carbon deposition on the electrolyzer surfaces and, therefore, the degradation process [264]. The product gas composition and the performance of the overall process strictly depend on cell operating conditions (inlet gas composition, operational temperature and pressure, applied voltage and current) (see Figure 8). Indeed, CO-methanation and reverse water gas shift reactions occur only at specific operating temperature and pressure. The methanation is a quite exothermal reaction, occurs at a temperature below 1073 K, quickly increases with pressure raising, and is enhanced by the commonly used Ni-based electrode that has good catalytic activity. Methane molar fraction in the outlet gas is negligible at ambient pressure but significantly increases with pressure rise at the expense of hydrogen and carbon monoxide production. However, the RWGS is slightly endothermic, is thermodynamically advantageous at 1073 K, is enhanced by the high concentration of hydrogen and is not affected by pressure variations [265]. The operating voltage also affects the gas composition: at a low voltage, the RWGS decreases and the methanation increases. However, high voltage increases the power requirements and so the efficiency declines. Thus, the operating temperature of SOECs has to be a balance between the operation at the thermal-neutral voltage in which the energy for isothermal operation is supplied by heat generated from ohmic losses, and the operation below the thermal-neutral voltage that decreases the outlet temperature and, thus increases methanation reaction and decreases the amount of CO produced via RWGS [266]. Hence, if syngas is produced for methane synthesis, the operation at low temperatures and high pressures is advantageous, even though the methanation reaction is exothermic and effective heat removal is required [267].PDF Image | Green Synthetic Fuels
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