Study On The Performance Of La₂NiO_4+δ Oxygen Electrodes In Solid Oxide Fuel Cell/Electrolysis Cell

Solid oxide fuel cell（SOFC）and solid oxide electrolysis cell（SOEC）are two energy conversion devices with the same structure,and SOEC is reversely operated SOFC in regenerative mode.SOFC can efficiently use hydrogen to generate electricity,while SOEC can produce hydrogen without pollution,and both of them can play an important role in the new energy vehicle field.As the important components of SOFC and SOEC,oxygen electrodes directly affect their overall performance.In this thesis,in order to develop SOFC/SOEC oxygen electrodes with excellent performance,the detailed research was carried on the basic physical and chemical properties,electrochemical performance and stability of La₂NiO_4+δ（LNO）oxygen electrodes.Based on the research results,the performance of LNO oxygen electrodes was also further optimized.In the thesis,the Ruddlesden-Popper structure LNO powder was synthesized by sol-gel method.The chemical compatibility,thermal expansion performance,electrical conductivity and oxygen surface exchange performance of LNO as oxygen electrode materials were studied.The results demonstrate that LNO has good chemical compatibility with Ce_0.9Gd_0.1O_1.95（GDC）and they are also in good match in thermal expansion behavior.Meanwhile,LNO shows excellent oxygen surface exchange performance.However,the electrical conductivity of LNO is only 80⁹1 S cm^-1 in the intermediate temperature between 600 and 800°C,which needs to be further improved.Besides,the electrochemical test and stability test were carried out on the half cells with LNO oxygen electrodes to investigate their electrochemical performance and stability.The results imply that LNO oxygen electrodes have good electrocatalytic activities,and the key electrode reaction steps that limit the electrode reaction rate are oxygen surface exchange and diffusion processes.Furthermore,the LNO oxygen electrodes have good stability under SOFC operation conditions,while showing obvious performance degradation under SOEC operation conditions.A comparative analysis of the microstructure,phase composition and surface element state of the LNO oxygen electrodes before and after working under SOEC operation conditions shows that the performance degradation is due to anodic current polarization,which accelerates the decomposition of LNO oxygen electrodes on GDC electrolyte and forms La₃Ni₂O₇,La₄Ni₃O₁₀ with lower electrocatalytic activity and La₂O₃ with low electrical conductivity.In order to improve the electrical conductivity of LNO oxygen electrodes and their stability under SOEC mode,LNO-La_0.8Sr_0.2Co_0.8Ni_0.2O_3-δ（LSCN）composite oxygen electrodes with LSCN as the electrode backbone and LNO as surfactant were prepared by solution impregnation method.Test results show that the electrical conductivity of LSCN is as high as 1302¹452 S cm^-1 in the intermediate temperature between 600 and 800°C,10 times as LNO,implying that LSCN backbones significantly increase the electrical conductivity of LNO oxygen electrodes.Besides,the electrochemical test was carried out on the half cells with LNO-LSCN composite oxygen electrodes with different LNO loadings.The results indicate that the electrocatalytic activity of LNO-LSCN composite oxygen electrodes increases firstly and then decreases with increasing LNO loadings because of the influence of oxygen surface exchange process.The electrocatalytic activity is best at the loading of 8%,which is also obviously better than that of pure LNO oxygen electrodes.In addition,the stability test results of the half cells show that LNO-LSCN composite oxygen electrodes have good stability under SOFC operation conditions,while showing slight performance degradation under SOEC operation conditions.The degradation rate is lower than that of LNO oxygen electrodes.Therefore,the performance of LNO-LSCN composite oxygen electrodes prepared by solution impregnation is better than that of LNO oxygen electrodes.The research results in this thesis have a certain guiding significance for the development of high performance SOFC/SOEC oxygen electrodes based on Ruddlesden-Popper structure oxides.