Study On The Mechanism Of Oxygen Reduction Reaction In H~+-SOFC

With the aggravation of environmental pollution as well as the increasingly exhausted fossil fuels,people are eager to find an alternative clean energy.Under this background,Fuel cell has been regarded as an important energy conversion device in preventing air pollution and providing clean energy.Among various types of fuel cells,proton conducting solid oxide fuel cell（H⁺-SOFCS）,which works at intermediate-to-low temperature range,is an efficient and environment friendly power generation device.Therefore,H⁺-SOFCs are considered to be of great significance in the governance of environmental pollution.The purpose of this paper is to study the mechanism of oxygen reduction reaction in the cathode of H⁺-SOFCs induced by hydration protonation.The main research contents of this paper are as follows:（1）We synthesized SrSc_0.175Nb_0.025Co_0.8O_3-δ（SSNC）cathode material and Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（BZCY）electrolyte material.Then a symmetric cell with the configuration of SSNC/BZCY/SSNC was fabricated.We studied the effect of various temperatures and water pressures on the oxygen reduction reaction of SSNC cathode.The electrochemical impedance spectroscopy（EIS）of the symmetric cell with SSNC cathode at various humidity and temperatures were obtained.Furthermore,the polarization impedance was divided into high frequency impedance（R_H,10⁶-10⁴）,medium frequency impedance（R_M,10⁴-10⁰）and low frequency impedance(R_L,10⁰-10^-2),and the activation energies of different frequency ranges were calculated.Meanwhile,the EIS was further analyzed using relaxation time distribution function（DRT）.The results showed that the total polarization resistance of SSNC increased when the temperature decreased.We found that the polarization impedance of the symmetrical cell achieved the minimum value under the higher moisture pressure when the temperature was higher.As the temperature decreased gradually,the humidity corresponding to the minimum polarization resistance also decreased.For example,the humidity to the minimum polarization resistance was 10%at 650°C,while it decreased to 5%at 600°C and 3%at 550°C,respectively.In combination with the results of DRT,we further found that R_Htook the highest proportion of polarization resistance at 650°C in dry air with a value of 0.65Ωcm².However,when the water was introduced,the polarization resistance decreased a lot and R_Lwas the main resistance with a value of 0.09Ωcm².This fully showed that the dominant process of oxygen reduction reaction（ORR）was changed from oxygen adsorption process to oxygen reduction process by introducing water.These results were also consistent with that of calculated activation energies.The analysis of the O₂-TPD and XPS also showed that the valence state of Co decreased after hydration protonation,there by producing more oxygen vacancies and resulting in increased adsorption sites for the generation of proton defects.（2）We synthesized oxygen-ionic conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ（BSCF）oxide,triple conducting Ba Co_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ（BCFZY）oxide,electronic La_0.8Sr_0.2Mn（LSM）oxide and BSCF-BZCY composite cathode and applied them as the cathode of symmetric cells.The EIS tests were carried out and further fitted to study the oxygen reduction reaction of the cathode.The fitting results showed that the total polarization resistance of BCFZY reduced obviously when water was introduced at the same temperature.Meanwhile,the main resistance was changed from R_Lto R_M.For example,R_Land R_Mwere 1.3Ωcm²and 0.50Ωcm²in dry air at 600°C,respectively,while,they decreased to 0.28Ωcm²and 0.31Ωcm²in 3%H₂O-air.This indicated that the dominant oxygen reduction process changed from diffusion process to dissociation process.The effect of the hydration protonation on ORR of BSCF-BZCY cathode was similar to the BCFZY cathode.For BSCF cathode,R_Land R_Mwere always dominant,but their values decreased less than that of R_H,indicated that water on BSCF mainly influenced the adsorption process of oxygen.For LSM,R_Lincreased greatly after the introduction of water.For instance,the R_Lwas 25.35Ωcm²in the dry atmosphere at 600°C and increased to 43.35Ωcm²in 3%humidity.There was no significant change in R_Hand R_M,because the active sites of LSM were too few and the introduction of water directly covered the active sites,thereby affecting the oxygen diffusion process.The XPS analysis further demonstrated that the total valence state of Mn in LSM decreased,while the total valence state of Co in BSCF-BZCY,BCFZY and BSCF decreased.This resulted in the increase of oxygen vacancy as well as the increase of proton defect.However,the increase of oxygen vacancies in BSCF-BZCY and BCFZY were more than that in BSCF cathode due to the proton conduction.This result was also consistent with the results of EIS.In conclusion,the introduction of water brings proton conduction,while the excess water also has the negative effect with covering the active sites.Therefore,the variation degrees of influence on the various types of cathode were obtained.