| Reversible solid oxide fuel cell(RSOFC) is the green energy conversion device, which can be operated in solid oxide fuel cell(SOFC) mode and directly convert chemical energy to electricity. In addition, it also can be operated in solid oxide electrolysis cell(SOEC) mode to convert electricity into chemical energy and 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 achieve both large-scale production of hydrogen energy and generate electricity for fuel cell. As a consequence, the researches on the RSOFC have an important significance due to the energy crisis and environmental pollution. The researches on the reducing oxygen electrode polarization loss and improving electrochemical performance and stability of oxygen electrode have important significance. In this paper, in the purpose of exploiting and developing high performance of oxygen electrode materials, Sm0.5Sr0.5Co O3- Sm0.2Ce0.8O1.9(SSC-SDC) composite oxygen electrode materials were prepared and the electrochemical performance and stability were studied.Using traditional mechanical mixing method, Sm0.5Sr0.5Co O3(SSC) powder synthesized by high temperature solid state reaction was mixed with Sm0.2Ce0.8O1.9(SDC) powder to prepare composite electrode materials. And then the electrochemical performance of different ratio SSC-SDC electrodes was studied. Meanwhile, influencing mechanism of SDC on the performance of composite electrode was also analyzed. The best proportion of SSC and SDC was found to be 70 wt.% : 30 wt.% by the AC impedance Spectrascopy and polarization curve, which showed a lower polarization resistance and overpotential both in SOFC and SOEC mode. The electrochemical performances of a Ni-YSZ hydrogen electrode supported YSZ membrane SOEC with the best proportion SSC-SDC73 oxygen electrode were also measured for high temperature water electrolysis experiment. The results showed that the electrochemical performance of electrolysis cell and hydrogen production increased with the increasing of temperature. Polarization resistance was the dominant factor on the SOEC performance.By improving the synthesis process of material, high performance SSC-SDC composite powder was co-synthesized by the one-step sol-gel method, which showed smaller grains and good dispersion of SSC and SDC phases than those of physically mixed SSC-SDC cathode. And hence, the effect of co-synthesis method on the powder microtopography and electrochemical performance of cathode were studied. And then influence mechanism was also analyzed. Co-synthesized cathode showed a better electrochemical performance than physically mixed cathode by the AC impedance Spectrascopy and polarization curve, showing a lower polarization resistance and overpotential. Co-synthesized SSC-SDC73 cathode was applied in the Ni-YSZ anode support SOFC. The result showed that co-synthesis method not only improves the morphology of initial SSC-SDC composite powder but also optimizes cathode microstructure and improves the electrochemical performance of the cathode.The electrochemical performance and polarization stability of co-synthesized SSC-SDC73 oxygen electrode in SOEC mode were studied, and the influence mechanism of anodic polarization current on oxygen electrode performance was also analyzed. The results showed that the recession ratio of co-synthesized oxygen electrode was smaller than physically mixed one under a low anodic polarization current. However, isolated aggregates among the oxygen electrode particles and the formation of micro-gap between oxygen electrode and electrolyte interface became main obstacle to the polarization stability of oxygen electrode under a high anodic polarization current. Performance on the high temperature water electrolysis with co-synthesized SSC-SDC as oxygen electrode in a Ni-YSZ hydrogen electrode supported YSZ film electrolysis cell was studied. Optimizing the microstructure, the performance of SOEC with the co-synthesized SSC-SDC oxygen electrode is higher than that of physically mixed SSC-SDC73 oxygen electrode of SOEC, demonstrating that the good potential of co-synthesized SSC-SDC as SOEC oxygen electrode.The electrochemical performance and stability of co-synthesized SSC-SDC oxygen electrode under alternant cathodic-anodic-cathodic-anodic electrochemical polarization were studied for the first time in this paper, and the influence of alternant electrochemical polarization on the microstructure of oxygen electrode. The experiment results showed that the decrease of the electrochemical activity and performance due to coarsening and agglomeration of oxygen electrode particles under the cathodic polarization. However, degraded performance caused by cathodic polarization could be compensated by the subsequent anodic polarization due to the disintegration of agglomeration. The electrode microstructure results showed that the agglomeration disintegration was related to the formation of internal stress within oxygen electrode. Therefore, the electrochemical performance degradation of SSC-SDC oxygen electrode caused by cathodic polarization was compensated by the followed anodic polarization, finally to some extent leading to relatively stable performance of oxygen electrode. Consequently, the electrochemical performance of co-synthesized SSC-SDC electrode can be manipulated reversibly by successive cathodic-anodic-cathodic-anodic polarization operations, indicating the co-synthesized SSC-SDC oxygen electrode could be used as oxygen electrode of RSOFC. |
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