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Energies 2020, 13, 420 20 of 96 However, the high temperature needed for ceria-doped material induces to a partial reduction of Ce4+ to Ce3+ which increases the electronic conductivity and the segregation of defects that obstructs the ion diffusion through the grain boundaries [237]. The aforementioned dopants are combined in multiple doped structures such as Ce0.85 Gd0.15-y Smy O1.925 [238], Ce1-x (Y0.5 Dy0.5 )x O2- δ [239], Sm0.075Nd0.075Ce0.85O2-δ [240] etc. Doped lanthanum gallate (LaGaO3) based perovskites are also studied as an alternative electrolyte material for intermediate temperature. The ion conductivity is comparable to ceria-based oxides, and also for this material, doping is adopted to enhance the properties of the electrolyte [241]. Good ion conductivity and stability over a wide range of oxygen partial pressure is reached with LaGaO3 doped with Sr for La, and Mg for Ga, synthesizing the La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) molecules [242]. Adding also Co for Ga of LSGM, La0.8Sr0.2Ga0.8Mg0.115Co0.085O3-δ is obtained [243]. The main drawbacks are the impurity segregation and the reactivity with nickel typically used as anode material [244]. Furthermore, the proton-conducting electrolyte is studied for the intermediate temperature range (400–700 ◦C) to overcome the problem of low conductivity of metal oxides at these temperatures [245]. BaCeO3-based proton conductors and Y-doped BaZrO3 (BZY) are widely studied. The former is highly conductive but instable in CO2 and H2O atmosphere, the latter is chemically stable, but the sintering is difficult due to the high temperatures required [246]. Co-electrolysis with proton-conducting SOECs has also been tested by Xie et al. [247] which perform the co-electrolysis introducing H2O at the oxygen evolution electrode and CO2 at the hydrogen electrode. Mainly carbon monoxide (61%), a low fraction of hydrogen (8%) and methane (1.2%) are produced [248]. Finally, in recent years, hybrid-SOEC devices have been introduced. Hybrid-SOECs are systems with an electrolyte that conducts both O2− and H+ ions. This configuration allows the water-splitting reaction occurs at both electrodes, enhancing the overall efficiency [249]. Perovskite (ABO3) compounds are widely applied as anode materials for SOECs. Perovskites are mixed ionic electronic conductor (MIEC) materials with rare-earth compounds (lanthanum and strontium) at the A-site and transition metals (Ni, Co, Fe, Mn, Cr, V) at the B-site [153]. In general, the electrochemical performance of these materials is satisfactory, but the thermal expansion—a fundamental parameter in high temperature electrolyzers-of many MIEC is non-compatible with the thermal expansion of the typically used electrolyte YSZ [250]. Sr-doped lanthanum-manganite oxide (LSM) is the most used and traditional material as the thermal expansion coefficient is similar to that of YSZ but experiences a low oxide-ion and electronic conductivity. Therefore, LSM is not an appropriate material for the operation at intermediate temperatures and new MIEC have been tested [251]. Sr-doped LaCoO3 is a good oxide-ion conductor, but the thermal expansion coefficient is too high in comparison with YSZ, and the chemical reactivity with zirconia causes the formation of a blocking layer of La2Zr2O7 [252]. Sr-doped lanthanum ferrite (LaFeO3) is another material studied for intermediate temperatures SOEC application. LaFeO3 has a lower thermal expansion coefficient and higher compatibility with TSZ electrolytes [253]. A critical degradation issue during long-term operation is the anode delamination that occurs at the electrolyte-electrode interface due to high pressure of oxygen produced by electrochemical reactions [254]. The good electrocatalytic activity, low-cost chemical stability and suitable thermal expansion and good electrical and ionic conductivity make the porous Ni-YSZ cermet the material commonly used as hydrogen evolution electrode. The high porosity allows the transport of reactants to the triple-phase-boundaries and the removal of product gases from the reaction site [255], and the fine microstructure enlarges the TPB length [256]. Anyhow, irreversible degradation processes occur on the cathode. The microstructure of the Ni-YSZ material changes due to coarsening and agglomeration of Ni compounds at the triple-phase boundary. This degradation process provokes the reduction of the length of the TPB and, thus, reduces the active site surface [257]. In addition to the common Ni-YSZ electrode, perovskites are selected as cathode materials including LSM andPDF Image | Green Synthetic Fuels
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