Control Of Oxygen Reduction Activity And Stability Of BaCo_0.7Fe_0.3O_3-δ Cathode Based On Chemical Defects

Solid oxide fuel cells（SOFCs）are recognized as one of the most promising energy technologies in the new century,which can convert the chemical energy in clean fuels into the electric power.The SOFCs are attracting wide attention due to their salient features like safety,high energy efficiency and environmental protection.However,it remains a challenge for the commercialization of the SOFCs because of the high operating temperature（about800～1000℃）.When working at such a high temperature,both the performance and the lifetime will be seriously impaired,making it difficult to promote and apply.Currently,researchers are working on lowering the operation temperature for SOFCs,while new issues may arise at the same time such as the decrease in the activity of oxygen reduction reaction in cathode and the performance degradation of batteries in toxic environment.Therefore,in this paper,a systematic study was performed to improve the catalytic activity and stability of the cathode at low temperatures.The defects of Ba²⁺ at A site have effects on the crystal structure,oxygen reduction catalytic activity,oxygen vacancy concentration,electrical conductivity,and the stability of the BaCo_0.7Fe_0.3O_3-δcathode.The inherent hexagonal phase structure of BaCo_0.7Fe_0.3O_3-δ（B100CF）are not changed by the proper amount of Ba²⁺defects,and the concentration of oxygen vacancies and oxygen surface exchange capacity of BaCo_0.7Fe_0.3O_3-δare enhanced at the same time,which is conducive to the diffusion and migration of oxygen anions,improving the ORR（Oxygen Reduction Reaction）activity of the cathode greatly.The optimal composition is x=0.1,at which the polarization impedance is only 0.0655Ωcm²for the Ba_0.9Co_0.7Fe_0.3O_3-δ（B90CF）cathode at 700℃.Meanwhile,the anode-supported Ni O-YSZ|YSZ|B90CF has a peak power density of 682 m W cm^-2,which is improved by 12.4%compared with that in B100CF.Doping high-valence and large-radius cations at B site converts the unstable hexagonal phase of the Ba_0.9Co_0.7Fe_0.3O_3-δmaterial into a stable cubic phase.After being doped with the Zr⁴⁺,Nb⁴⁺（mainly）,and Y³⁺,a larger average metal-oxygen bond energy（ABE）and a lower concentration of the oxygen vacancies are achieved in the cathode.Also,the R_pobtained in the doped samples is slightly higher than that in B90CF.Notably,the R_p reaches 0.0719Ωcm² for Ba_0.9Co_0.7Fe_0.2Zr_0.1O_3-δ（B90CFZr）at 700℃.The stability in CO₂poisoning environment of the doped cathode is improved significantly,which is due to the larger ABE and the lower alkalinity of Zr⁴⁺,Nb⁴⁺compared with Fe ions.After staying at 700℃ in 1%CO₂ atmosphere for 1500 min,the R_p of the B90CFZr cathode is increased by only 31.1%,which is much smaller than those in B90CF（359%）and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ（BSCF,347%）.Besides,a peak power density of 667 m W cm^-2 is achieved in the Ni O-YSZ|YSZ|B90CFZr single cells at 700℃.The B90CF-x PO（x=5,10,15,20 wt%） composite cathode was prepared by impregnating the Pr₆O₁₁（PO）particles on the B90CF cathode framework.The active area of the oxygen reduction reaction is enlarged effectively,and the transfer of the oxygen anion in ORR process is enhanced.When the impregnation amount is 15 wt%,the R_p of B90CF-15PO at 700℃ is0.0332Ωcm²,which is only half of that in the B90CF cathode.Concurrently,the PO particles play a certain role in protecting the cathode of B90CF which is easily eroded by CO₂.The average growth rate of the R_p value of the B90CF-15PO in 1%CO₂ atmosphere at 700℃ is only 2.372×10^-5Ωcm²/min,which is an order of magnitude lower than that of B90CF.A high peak power density of 757 m W cm^-2 is obtained in the anode-supported single-cell Ni O-YSZ|YSZ|B90CF-15PO at 700℃.