Design, Preparation And Performance Of Novel Anode For Solid Oxide Electrolysis Cells

In order to meet the requirements of planar solid oxide electrolysis cells （SOEC）,four kinds of anode materials with different compositions based on La_0.8Sr_0.2MnO₃（LSM） and La_0.8Sr_0.2FeO₃（LSF） were prepared. Their compositions, microstructuresand properties have been investigated by X-ray Diffraction （XRD）, Scanning ElectronMicroscopy （SEM）, Transmission Electron Microscopy （TEM） methods, as well asElectrochemical Impedance Spectroscopy （EIS） and X-ray Photoelectron Spectroscopy（XPS）. Also, the effects of temperature, oxygen partial pressure （pO₂） and polarizationcurrent on the electrochemical properties of the anodes were systematically discussed.A system of xLSM-（100-x）YSZ （0<x<60wt%） composite materials has beenstudied with regard to its direct current （DC） conductivity transition, connectivity andelectrochemical behaviors. The experimental results demonstrated that the DCconductivity decreases with increasing x, and increases above x=20. As a result oftemperature dependent DC conductivity, a corresponding peak of the activation energyof the DC conductivity was observed at approximately x=20. The DC conductivitytransition from ionic to electronic was found to take place in the range of10<x<30.Based on the results, a simple model schematically showing the three dimensionalconnectivity of the two phases was constructed with variation in x. Furthermore, thecorrelation between the effective conductivity of LSM and its volume fraction wasconsistent with the modified Koh-Fortini equation in the LSM-rich range （x>40）. Thepredicated fittings were comparable with the experimental measurements.The polarization resistance （R_p） of the LSM/YSZ porous composite anodesdecreased with the increase of pO₂and the radius of low frequency arcs in EISdecreased greatly with the increase of the porosity. In addition, the differential analysisof impedance spectra （DIS） indicated only the intermediate and low frequencies arcs arestrongly affected by pO₂. Meanwhile, with the increase of the porosity, thepositions of the peaks in DIS shifted to higher frequency, indicating that therelaxation-time of diffusion was shortened. For the porous LSF electrodes, theinfluence of anodic polarization current on the low frequency arcs became moreand more obvious with the increase of the porosity. Furthermore, the R_pvaluesof the LSF porous electrodes always increase with the increase of anodic polarization time.La_0.8Sr_0.2FeO₃electrodes modified with La_0.8Sr_0.2MnO₃nanoparticles （LSF/LSM）were prepared by infiltration method. The catalytically active LSM phase could beuniformly distributed. The R_pvalue of the LSF/LSM electrode was0.27·cm²at themeasuring temperature of800oC. This value was much lower than those of pure LSFand LSM electrodes. Additionally, LSF@LSM （core@shell） electrodes were preparedvia the infiltration of LSM non-aqueous-solution. The results showed that the bestperformance of the electrodes with core-shell structure was obtained at theconcentration of0.010mol L^-1for precursor solution. The R_pvalue of the electrodereached0.3·cm²at800oC. Moreover, the R_pvariation of the LSF@LSM anode wasseven times less than that of the conventional LSM-YSZ anode after anodic polarizationcurrent of1A cm-2for20h. This clearly demonstrated the LSF@LSM materials will bea promising anode with anti-polarization characteristic.A triple phase boundary （TPB） model derived from the LSF anode modified with alot of LSM nanoparticles was presented. Based on charge transfer and mass transfermechanisms, the TPB length was considered as an important factor for the calculationof concentration overpotential of the anode in SOEC. The results showed that the TPBlength in the LSF/LSM anodes was5to8orders of magnitude higher than that in theconventional composite electrodes. Moreover, the concentration overpotential decreasedwith the increase of the TPB length, oxygen partial pressure, porosity, pore radius.However, it increased with the increase of the thickness of diffusion layer in the anodeof SOEC.