Properties Of Ca₃Co₄O_9+δ Oxygen Electrode For Reversible Solid Oxide Cell Based On Difference Isolation Layer Materials

Reversible solid oxide cells（RSOCs）are all-solid-state electrochemical energy conversion devices that can alternately operate between solid oxide electrolysis cell（SOEC）and solid oxide fuel cell（SOFC）.With the increasingly serious environmental and energy problems,the research and development of RSOCs is of great significance.Ca₃Co₄O_9+δ（CCO）material with a misfit-layered structure was studied as a oxygen electrode for RSOCs in this work.The effects of different isolation layer materials between CCO electrode and YSZ electrolyte on the electrocatalytic activity and polarization durability of CCO oxygen electrode and the output property of corresponding RSOCs were investigated.Ce_0.9Gd_0.1O₂（GDC）was used as isolation material and coated between the YSZ electrolyte and CCO electrode.The durability of CCO oxygen electrode at±500mA·cm^-2 was evaluated at 750℃ for more than 100 hours in three-probe arrangement.Both the anodic and cathodic polarization potentials were decreased during the alternating polarization process,the ohmic and polarization resistances were also significantly reduced,demonstrated the activation of CCO electrode and an improvement of electrode/electrolyte interface.The maximum power density of the Ni-YSZ/YSZ/GDC/CCO fuel cell was 614 mW·cm^-2after polarization at 500 mA·cm^-2for4 h at 750℃,which improved by 4 times than the unpolarized cell.The maximum electrolysis current density of 749 mA·cm^-2was obtained at 800℃ and 50%water vapor,which corresponded to the hydrogen production rate of 311 m L·cm^-2·h^-1.The CCO electrode showed superior durability and excellent electrocatalytic activity for oxygen reactions.La_0.6Sr_0.4Fe O₃（LSF）was then used as isolation material.LSF isolation layer and CCO oxygen electrode were successively prepared on YSZ skeleton using solution infiltration method.The CCO electrode showed good polarization stability under the alternating polarization condition at 750℃.The Ni-YSZ/YSZ/LSF/CCO cell yielded a maximum power density of 1276 mW·cm^-2 in SOFC mode at 800℃ and a maximum electrolytic current density of 1695 mA·cm^-2 in SOEC mode.The hydrogen production rate of the electrolytic cell was 705 m L·cm^-2·h^-1,which achieved a high energy conversion.GDC isolation layer was also prepared for comparison.The polarization resistances of LSF-CCO and GDC-CCO electrodes at 750℃ were 0.25 and 0.6Ω·cm²,respectively.The maximum power density of Ni-YSZ/YSZ/LSF/CCO cell was 1.7times than that of the Ni-YSZ/YSZ/GDC/CCO cell.The electrochemical performance of CCO oxygen electrode was significantly improved by the LSF isolation layer.Finally,Er_0.4Bi_1.6O₃（ESB）was prepared between the YSZ framework and the CCO oxygen electrode as the isolation layer.It was found that the cathodic polarization potential remained stable during the alternating polarization test,and the anode potential increased first and then remained constant after polarization for 50 h.Cathodic and anodic polarization showed opposite mechanism on electrode performance.The oxygen electrode deterioration in electrolytic cell mode can be recovered through fuel cell mode.Ni-YSZ/YSZ/ESB/CCO single cell was prepared and obtained a maximum power density of 722 mW·cm^-2 in SOFC mode at 800℃ and a maximum electrolysis current density of 1204 mA·cm^-2 in SOEC mode,which proved the feasibility of ESB isolation layer for CCO electrode.