Electrical Conduction Behavior Of Gd Doped CeO₂ Electrolyte At Low Oxygen Partial Pressure And Its Application In SOFC

Nowadays,the development and utilization of fuel cells are booming under seriously increasing global environmental pollution and energy crisis.In order to extend the SOFC operating life,the research is gradually shifted from high temperature?800-1000??to low temperature?400-700??.Gd₂O₃ doped CeO₂?GDC?has excellent thermal stability and electrochemical properties which is widely used in IT-SOFC.Doped CeO₂ electrolyte material,however,has an outstanding stability problem.The reduction of Ce⁴⁺/Ce³⁺at lower oxygen partial pressure will generate electronic leakage which mainly happen in the anode side reduction process.This problem will cause electrolyte's lattice volume expansion during reduction process,affecting the mechanical properties of the electrolyte and attenuating electrochemical performance,decreasing cerium oxide ion transference number in base material,reducing SOFC open-circuit voltage,eventually lowering the efficiency of GDC electrolyte materials applied in SOFC.Ce_1-xGd_xO_2-??x=0.05-0.40?powders were synthesized by a sol-gel method,and their structures were characterized and analyzed.The relationships of oxygen vacancy and defective species with Gd³⁺doping amount were studied.The maximum oxygen vacancy value was obtained when the doping amount was 0.2,indicating that the doping amount was most conducive to the formation of oxygen vacancy.The total conductivity variation tendency of doped GDC electrolytes in low oxygen partial pressure atmosphere?nitrogen atmosphere and hydrogen atmosphere?was studied by an AC impedance method.The results showed that the total conductivity increased at first and decreased later with the increased Gd³⁺doping in nitrogen atmosphere(oxygen partial pressure range:10^-8-10^-4 Pa)and hydrogen atmosphere(oxygen partial pressure range:10^-28-10^-18 Pa).Therefore,the total conductivity at low oxygen partial pressure was affected by the oxygen vacancy conduction and electronic conduction.According to the DFT+U caculation,it is found that the Gd₂O₃ doping reduces the dissociation energy of H₂ molecules on the surface of CeO₂?100?,improving the conductivity of CeO₂,which is consistent with the experimental results.The total conductivity of Ce_0.8Gd_0.2O_2-?in both nitrogen and hydrogen atmospheres was highest than that of the other components,which was consistent with the highest oxygen vacancy and the best ionic conductivity.The electronic conductivity variation tendency of doped GDC electrolytes in nitrogen and hydrogen atmosphere was studied by Hebb-Wagner ion-blocking method.The results showed that the electronic conductivity gradually decreased with the increasedGd³⁺doping in a nitrogen atmosphere(oxygen partial pressure range:10^-2510^-16 Pa)and a hydrogen atmosphere(oxygen partial pressure range:10^-4510^-30 Pa),indicating doped Gd³⁺could inhibit the electronic conduction at low oxygen partial pressure.Meanwhile,the electronic conductivity increased with the increase of temperature.Due to the REDOX reaction,the electronic conductivity in hydrogen atmosphere was slightly higher than that in nitrogen atmosphere.Electronic conductivity of Ce_0.8Gd_0.2O_2-?electrolyte is proportional to oxygen partial pressure of different n in different oxygen partial pressure ranges.In order to reduce the electronic conduction of GDC,a GDC/ScSZ double layer was constructed on dense GDC substrate by a pulsed laser deposition?PLD?technology.The GDC/ScSZ double layers can be well fixed to the dense GDC substrate.The mismatch degrees between GDC substrate and ScSZ layer,GDC layer and ScSZ layer in?100?crystal were both less than 5%,indicating that a coherent structure formed in GDC/ScSZ-GDC bilayer electrolyte.The coherent structure would benefit the oxygen ionic conduction.Further analysis showed that Zr element diffused not only to the GDC layer but also to the GDC substrate with the diffusion range from 0-300 nm.The conductivity of GDC/ScSZ-GDC bilayer electrolyte(0.130 S·cm^-1,750°C)is very close to that of GDC electrolyte(0.142 S·cm^-1,750°C),indicating that the GDC/ScSZ bilayers can maintain the ionic conductivity of GDC electrolyte.NiO-GDC|GDC/ScSZ-GDC|LSCF-GDC and NiO-GDC|GDC|LSCF-GDC electrolyte supported single cells were respectively assembled by a screen printing technology.By comparing the OCV and power density between two kinds of single cells,it was found that the OCV and maximum power density of the single cell supported by GDC/ScSZ-GDC bilayer electrolyte could reach 0.978 V?550°C?and 0.332 W·cm²?750°C?,while the OCV and maximum power density of the single cell supported by GDC was 0.902 V?550°C?and0.283 W·cm²?750°C?.The results indicate that ScSZ layer in the GDC/ScSZ-GDC bilayer electrolyte plays an efficient role in blocking both the reduction of Ce⁴⁺by H₂ on the anode side of SOFC and the electronic conductance generated by low oxygen partial pressure.The ScSZ layer addition effectively reduces the internal resistance of SOFC and improves cell performance of SOFC with GDC as electrolyte.