Study Of Surface Oxygen Reduction Reaction Process Of Solid Oxide Cathode

The solid oxide fuel cell?SOFC?is efficient and environmental friendly energy conversion devices,which can directly convert the chemical energy into the electrical energy.During the development of the SOFC technique,the operation temperature should be further reduced for widely commercial application.However,the electrochemical performance of SOFC is seriously restricted by the polarization loss in the cathode reaction at low operation temperature.Therefore,it's necessary to develop the cathode with high performance and further understand the reaction machnism.In this thesis,the electrical conductivity relaxation method is employed to research the enhancement effect of the added ionic conductor phase,Sm_xCe_1-xO_2-??SDC?,on the oxygen reduction reaction occurred on the cathode surface using the typical mixed ionic-electronic conductor La_0.6Sr_0.4Co_0.2Fe_0.8O_3-??LSCF?as the object.The emphasis is placed on the reaction process in the three phase boundary as well as its contribution to the whole cathode reaction.The reaction mechanism and the rate-determining step of the oxygen reduction reaction are also discussed based on the experimental results.The chapter one briefly introduced the operating principle of SOFC and the common materials for cathode,mainly focusing on the synergism enhancement phenomenon in the composite cathode and the significant role in the oxygen reduction reaction played by the three phase boundary.The possible cathode reaction mechanism steps were also discussed.Then,a comprehensive introduction to the electrical conductivity relaxation method was presented including the testing process,the theoretical method and the influencing factors.It's significant to firstly determine the theoretical expressions of the kinetics parameters for the oxygen reduction reaction occurred at the composite surface as well as the three-phase boundary.Therefore,the theoretical derivation processes of those kinetics parameters,including the reaction rate and amount of the composite and the three-phase boundary,were proposed based on the traditional surface exchange coefficient of the single phase matherial in chapter two,employing the LSCF-SDC composite as the demonstrated research object.The contribution ratio of the three phase boundary was also defined.In addition,the equivalent electromotive force,the exchange current density and the polarization resistance for the surface oxygen reduction process were calculated according to the oxygen partial pressure step change and the calculated reaction rate in the electrical conductivity relaxation measurement.The work in chapter two supplied the theoretical method and foundation for the subsequent research about the kinectics parameters and the mechanism of the oxygen reduction reaction.It's necessary to quantitatively characterize and distinguish the reaction processes occurred at the two-phase boundary and the three-phase boundary for the research of the surface synergism phenomenon and the oxygen reduction reaction for the composite cathode.In chapter three,the electrical conductivity relaxation measurement was employed to determine the surface oxygen reduction process on the LSCF-SDC composite surface.The reaction rate and amount of oxygen for both the two-phase boundary and the three-phase boundary were calculated according to the theoretical method proposed in the previous chapter.The results showed that the surface exchange coefficients increased by a factor of 5 proving that the synergism enhancement phenomenon existed between LSCF and SDC.Combining the statistical results of the surface micro-structures,more than 70%of the reduced oxygen was found to be incorporated into the oxide bulk through the three phase boundary.As a consequence,the synergism enhancement could be mainly attributed to the three phase boundary.Furthermore,the reaction rate of the three phase boundary was correlated to not only the boundary length but also the LSCF particle size.This result was possibly caused by the transport process of the adsorbed oxygen species while the maximum transport distance might be 1.5 ?m.In chapter four,the oxygen relaxation process on the LSCF-Sm_xCe_1-xO_2-?composites was determined to quantitatively characterize the effect of the conductivity of the ionic conductor on the oxygen reduction reaction of the three-phase boundary.The contribution ratio of the three phase boundary,calculated based on the method in chapter two,decreased with increasing testing temperature,indicating that the three phase boundary might be more important at low temperature.The surface reaction rate of the three phase boundary was found to be promoted with raised Sm_xCe_1-xO_2-? conductivity combining the statistical results of the surface micro-structures.The theoretical derivation based on the elementary steps proved that the reaction rate of the three phase boundary would increase with Sm_xCe_1-xO_2-?conductivity linearly when the oxygen incorporation process was the only rate-determining step.Therefore,the oxygen incorporation process should be an important rate-determining step since the reaction rate always increased.The oxygen reduction reaction at the three-phase boundary was proven to be correlated to not only the density of the three-phase boundary but also the surface micro-structure of the dual phases.As a consequence,the LSCF-SDC composites with various SDC grain sizes were prepared to reveal the influence of the surface morphology of the ionic conductor on the oxygen reduction reaction at the three-phase boundary in chapter five.The SDC grain sizes gradually increased while the statistic three phase boundary lengths decreased when the calcination temperature of the SDC powder was raised.Even though the total oxygen reduction reaction rate decreased with increasing powder calcination temperature,the reaction rate per unit three phase boundary length was found to be promoted.This was possibly resulted from the oxygen spillover process,in which the abundant adsorbed oxygen ion in the three phase boundary might diffuse to the SDC grain surface to combine with the active oxygen vacancy there.Thus,the active reaction sites for the oxygen incorporation process could be expanded from the three phase boundary to the SDC surface,which made the unit boundary length more efficient.The diffusion distance of the oxygen ion on SDC surface was at least 0.42 ?m based on the experimental results.The disturbance of both the gas flush process in the reaction vessel and the diffusion process inside the pores of the sample should be completely eliminated to directly characterize the surface oxygen reduction process of porous samples.Considering it's difficult to meet those requirements using the traditional method,a novel experimental device based on the vacuum conductivity relaxation method was designed in chapter six to determine the oxygen surface reduction performance of porous LSCF samples.A new theoretical method was developed to describe the oxygen relaxation process of porous samples,which were under the mixed control of gas transport especially the Knudsen diffusion and the surface exchange process.The characteristic thickness of porous samples is defined to judge the influence of the Knudsen diffusion process inside the pores on the whole surface reaction.A two-parameter function was derived to describe the two processes and the fitting results suggest that the surface reaction performance is degraded with increasing sintering temperature,while increasing pore former content can make contribution to the reaction rate for raised surface area.Furthermore,the oxygen reduction reaction of porous LSCF behaves a relative low activation energy?49-70 kJ mol-1?compared to the dense sample?110 kJ mol-1?.The low dependence on testing temperatures indicates the important role played by the oxygen adsorption process in the surface reduction reaction,which is also confirmed by the relaxation experiments under different oxygen pressure step changes and using samples with impregnated SDC particles.