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Energies 2020, 13, 420 19 of 96 (Equation (28)). At the anode, CO32− ions oxidize, generating CO2 and O2 (Equation (29)) [217]. However, there is an intrinsic drawback. H2O + CO2 + 2e− → H2 + CO2− (Hydrogen evolution reaction : H2 production) (27) 3 2CO2 + 2e− → CO + CO2− (Hydrogen evolution reaction : CO production) (28) 3 CO2− → 2e− + 1 O2 + CO2 (Oxidation evolution reaction) (29) 32 Solid oxide electrolysis cells (SOECs) were first developed in the 1970s by General Electric and Brookhaven National Laboratory [154]. Solid oxide electrolyzers perform steam electrolysis, which is an efficient process due to the advantageous thermodynamic and kinetic conditions derived from high operational temperature [218]. The common electrolyte of SOECs is 8 mol% Y2O3 stabilized ZrO2 (yttria-stabilized zirconia), the electrode most used is Ni/YSZ cermet for the cathode and lanthanum strontium manganite (LSM)/YSZ composite for the anode [219]. As the electrolyte is solid, the thermal expansion coefficients of electrodes and electrolyte should be similar to avoid the material failure, and electrodes should be porous to enable gas transport and provide enough triple-phase boundary in addition to typical requirements of good chemical stability, ionic conductivity, tightness to gas permeation and poor electronic conductivity [220]. The ion conductivity is performed via two pathways: the diffusion through vacancies (bulk conductivity) and the intergranular diffusion (grain boundary conductivity) [221]. The temperature effect on conductivity follows the Arrhenius equation (Equation (30)). E σ = σo exp −kBT (30) where σo is the conductivity of pre-exponential factor, E is the activation energy, kB is the Boltzmann constant, and T is the absolute temperature. This relation states that the ion conductivity increases with high temperature [222]. The most critical element of solid oxide electrolyzers is the electrolyte. Zirconia dioxide has good ionic conductivity but suffers phase transformation from the monoclinic form, stable at low temperature, to the tetragonal and cubic structures stable at highest temperatures [223]. The monoclinic phase is stabilized to cubic fluorite structure by doping the material with alkaline earth, rare earth or lanthanide oxides. The oxides additive includes calcia (CaO), magnesia (MgO), scandia (Sc2O3) and yttria (Y2O3) [224]. The maximum ion conductivity is a function of the dopant content and corresponds to the minimum amount of the dopant required to stabilize the zirconia structure completely. When the dopant concentration further increases, the oxygen vacancy and the dopant cations introduce defects and resulting in a lower ion conductivity [225]. Moreover, the ion conductivity is enhanced with the addition of dopant size close to the Zr4+ radius since the elastic energy is lower [226]. Meng Ni et al. [220] review some experimental data of ion conductivity of doped zirconia finding this additive order: Sc2O3 > Yb2O3 > Y2O3 > MgO > CaO > La2O3. Although the ion conductivity of scandia stabilized zirconia (ScSZ) is highest, the yttria is most used due to the cost-effectiveness and the high abundance [227]. The grain size also affects the ion conductivity of zirconia. Ivanov et al. [228] demonstrate that the bulk conductivity is not controlled by grain size, but grain boundary conductivity decreases with grain size. Ceria-alkaline-earth and ceria-rare-earth oxides are suitable materials for intermediate temperature applications (500–700 ◦C). At intermediate temperature, the ion conductivity of Ce-based oxides is much higher than that of YSZ. Analogously to zirconia, the ion conductivity of doped Ce-oxides is a function of temperature, ion radius mismatch between Ce4+ and cation additives, dopant concentration and charge valence [229]. Some of dopant tested as solid electrolyte are Gd3+ [230,231], Sm3+ [230,232], Y3+ [233], Sc3+ [234], Nd3+ [235] and Lu3+ [230]. Samaria and gadolinia-doped ceria exhibit high conductivity because the ionic radii of Sm3+ and Gd3+ are close to Ce4+ ion size [236].PDF Image | Green Synthetic Fuels
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