Performance of ionic-conducting ceramic/carbonate composite material as solid oxide fuel cell electrolyte and CO₂ permeation membrane

Novel composite materials composed of an oxygen ionic-conducting ceramic oxide and a molten carbonate phase have recently been reported to be the promising materials for the electrolyte of intermedaite temperature (500-700 °C) solid oxide fuel cell (ITSOFC) and the membrane for high temperature (>500 °C) CO₂ separation. This work reviews the recent progresses of these composite materials as ITSOFCs electrolyte and CO₂ permeation membrane and reports the latest results of our group. The composite materials, i.e., samarium-doped ceria (SDC)-Li/Na/K₂CO₃ (43.5/31.5/25 mol%) and SDC-Li/Na₂CO₃ (50/50 mol%), were prepared and tested as the electrolyte for SOFCs, respectively. The CO₂ in the cathode gas enhances the power output. At 650 °C, and with CO₂/O₂ used as the cathode gas, the fuel cell with SDC-Li/Na₂CO₃ (50/50 mol%) electrolyte gives a power output 1700 mW cm^-2 at a current density 3000 mA cm^-2. Another composite material made of Bi_1.5Y_0.3Sm_0.2O₃/molten carbonate (Li/Na/K₂CO₃, 43.5/31.5/25 mol%) was synthesized and used for selective permeation of CO₂ at 500-650 °C. The CO₂ permeation flux for the dual-phase membrane increases with the increase of temperature and reaches a maximum value of 6.60 × 10^-2 mL cm^-2 min^-1 at 650 °C, with apparent activation energy for CO₂ permeation of 113.4 kJ mol^-1. These results further demonstrate that the ionic-conducting ceramic/carbonate composite material is an alternative choice as the ITSOFCs electrolyte and high temperature CO₂ separation membrane material. The ionic transfer mechanism is discussed.