Activation And Deactivation Mechanism Of PrBaCo₂O_5+? Oxygen Electrode For Reversible Solide Oxide Fuel Cell

Reversible solid oxide fuel cell?RSOFC?is a novel energy conversion device with the advantages of high efficiency,no pollution and modularization.RSOFC can be operated in solid oxide fuel cell?SOFC?mode that can directly convert chemical energy to electricity.It also can be operated in solid oxide electrolysis cell?SOEC?mode to convert electricity and heat energy into chemical energy for storage.When RSOFC is alternately operated in SOFC and SOEC mode,"hydrogen-electricity power" of sustainable energy circulation system will be built and therefore it can be used for both large-scale production of hydrogen energy and the generation of electricity from fuel cell.As a consequence,the researches on the RSOFC have an important significance due to the energy crisis and environmental pollution.Oxygen electrode is one of the most important parts in RSOFC.The electrochemical activity and stability of electrode materials are the main factor affecting RSOFC performance and operating life.The purpose of this paper is to study the suitability of high performance of Pr BaCo₂O_5+??PBCO?oxygen electrode materials for application in RSOFC.Special attention is paid to the effect of electrochemical polarization and impurity on the performance and stability.A full single cell with La0.6Sr0.4Co0.2Fe0.8O3-??LSCF?derived phases as hydrogen electrode,Sm0.2Ce0.8O1.9?SDC?as electrolyte and PBCO as oxygen electrode was fabricated using a cost-effective way of single-step sintering.During testing in reducing atmosphere,the LSCF electrode was in situ decomposed with the generation of a novel composite material.The peak power density of a cell based on this innovative anode reached 325 mW?cm-2 when used hydrogen as fuel at 800 °C.The constant current discharge result showed that the output voltage drop was only 2% over the entire testing of nearly 40 h.The scanning electron micrographs?SEM?images presented a distinct morphology evolution derived from the in situ decomposed LSCF.The XRD and electrical conductivity results indicated that the decomposition products of LSCF were consisting of nano-scaled multimetal-oxide phases with excellent electrochemical performance.The electrochemical performances of a YSZ electrolyte supported,SDC isolated layer and LSCF as hydrogen electrode RSOFC with the PBCO oxygen electrode were also measured for high temperature water electrolysis experiment.The r esults showed that the electrochemical performance of the electrolysis cell and hydrogen production increased with the increasing of temperature.T he hydrogen production rate calculated from the Faraday's law was 116 mLcm-2h-1 at 800 °C and the electrolysis voltage of 1.3 V with a steam concentration of ⁴0%.The electrochemical activity and stability of the PBCO oxygen electrode have been studied.Anodic polarization degrades the activity of PBCO electrode,while the cathodic bias could recover its performance.Further alternate anodic-cathodic polarization for 180 h confirms this behavior.Microstructure and chemical analysis clearly show that anodic bias leads to the accumulation of insulating nanosized Ba O segregation on the electrode surface while cathodic polarization depletes the surface Ba O species.Therefore,a mechanism based on the segregation/incorporation of Ba O species under electrochemical potentials is considered to be responsibl e for the observed deactivation/recovery process.The electrochemical performance of PBCO electrode can be manipulated reversibly by successive alternate anodic-cathodic polarization operations,indicating alternately working mode is advantageous to long-term stability and operating life of the electrode.Performance stability of PBCO oxygen electrode was investigated under annealing processes and in the presence of CO2.The annealing results indicated that increasing of the operating temperature could accelerate the performance degradation.Electrochemical performance and thermostability of PBCO and Gd BaCo₂O_5+??GBCO?electrodes were compared in the air.A smaller size mismatch between the host?Ba?and dopant cations?Ln=Pr,Gd?was found to enhance the structural stability.Annealing process could affect the oxygen incorporation/dissociation on the electrode surface as evidenced by impedance spectra collected at different oxygen partial pressures.The formation and accumulation of Ba CO3 insulating phase would block the oxygen incorporation path and consequently lead to the performance degradation.The influence of CO2 gas on electrochemical performance and stability of PBCO electrode was studied under the two different of CO2 concentrations.Electrochemical impedance spectra results confirmed the obvious performance degradation of the PBCO cathodes in the presence of CO2 impurity,especially in high CO2 concentration condition.Microstructure and structural analyses revealed the formation of insulating Ba CO3 nanoparticles at the PBCO surface,which was considered as the primary reason for the loss of electrode activity.