High Temperature Electrical Properties And Surface Segregation Of GdBaCo₂O_5+δ-based Oxygen Electrodes

Solid oxide fuel cell(SOFC) is a novel power generation assembly. It cantransformate the chemical energy of fuel into electrical energy directly.Conversely, solidoxide electrolysis cell (SOEC) can electrolyze the water vapor into hydrogen andoxygen,using a variety of renewable energy and industrial waste heat to provide energyand heat. SOFC and SOEC have the advantages of high efficiency and environmentallyfriendly, many countries are seeking to develop this technology. GdBaCo2O5+δ(GBCO)is a kind of high-performance oxygen electrode material, which can be used as thecathode in the SOFC. Many SOFC cathode material have the phenomenon of surfacesegregation wich occurs at high temperatures, and the cathode surface is an importantpart for the catalytic reaction of oxygen, the surface segregation will change the surfacecomposition of the electrode, reducing the catalytic efficiency of the electrode and theperformance of the battery. GBCO electrode may also exist this phenomeno.Meanwhile, the battery current wich flows through the electrodes will cause thepolarization of the electrodes, many electrode materials’ properties varies significantlyafter polarization, the polarization process may also affect GBCO’s performance.GdBaCo2O5+δand GdBa0.5Sr0.5Co2O5+δ(abbreviated as GBSCO) materials wereprepared by sol-gel method. By testing the physical properties of these materials, wefound that the GBSCO material having higher conductivity, but the thermal expansioncoefficient of GBSCO is slightly larger than GBCO.We prepared GBCO and GBSCOhalf-cell, which Ag is the symmetrical electrodes and SDC is the the electrolyte.Wefound that the GBSCO has a lower electrode polarization resistance and a loweractivation energy than GBCO. We keep the GBCO slices annealing in a hightemperature air environment,then found a lot of particles deposited on the electrode’ssurface by means of electron microscopy observations(SEM). We analyze thecomposition of the electrode surface by EDS tests and found that the component of theprecipitated particles deviate significantly from the normal ratio, which the content ofBa elements is significantly higher. It’s means that the GBCO electrode also have thephenomenon of surface segregation.The precipitation phenomenon of GBSCO is morepronounced than GBCO under the same treatmen conditions,wich the Sr element ismore likely to enrich in the surface than the Ba element.We annealed the GBCO half-cell in an air atmosphere of750℃for20hours.Throughout the process, the battery’s ohm impedance changed slightly, while thepolarization resistance increased by9%. The AC impedance spectroscopy of thehalf-cell were tested at different oxygen partial pressures. The results showed that thehigh-frequency part of the impedance spectra represents the charge transfer process of oxygen the electrode surface, the low-frequency part represents the process of oxygenadsorption on the electrode surface. In an air atmosphere at750℃, the GBCO electrodewas treated by anodic polarization(600·mA·cm-2) for20h, the polarization resistanceand ohmic resistance of GBCO half-cell were continues to grow. In the same processingtime, a large polarization current density will increase the magnitude of change. Wetreated the GBCO electrode with cathodic polarization for20h, the polarizationresistance and ohmic resistance of GBCO half-cell decreases rapidly in a short time(10-20min), and then reaches a steady state.It’s means that the cathodic polarizationtreatment can improve GBCO electrode’s electrochemical properties in a shorttime.According to the treatment with different current density, the lower polarizationcurrent density(100-2·mA·cm) is hardly to improve the electrochemical performance.After the annealing treatment, there are a lot of particles precipitated on the surface ofthe electrode; After anodic polarization treatment,the surface segregation wasexacerbated,while the precipitated particles becomes less after cathodic polarization.In order to observe the stability of the electrode performance after polarizationtreatment,we keep the half-cell which was polarizated before annealed at the sametemperature. We found that the electrode’s performance gradually shift to the statebefore the polarization treatment, but the change rate of the electrode’s performanceduring the annealing is far below the rate of the polarization process. After long timeanodization, the electrode performance can not be fully restored.