Electrochemical, Catalytic and O2-Permeation Studies of Iron-Doped Barium Zirconates for Membrane Reactor Applications

Recently, interest in solid-oxide electrolysis cells (SOECs) with platinum or nickel cermet electrodes capable of reducing combustion effluents to synthesis gas has driven the search for mixed ionic-electronic conductors with suitable catalytic activity for reduction of carbonaceous species. There remains a need for discovering chemically stable electrode materials combining electronic and oxygen-ionic conductivities with suitable catalytic activity for oxidation or reduction. Our research group has focused upon identifying ambivalent transition-metal dopants capable of imparting both electrochemical and catalytic properties to perovskite electrolyte materials. In this study, the authors present a combined electrochemical, catalytic and permeation investigation of iron-doped barium zirconium perovskite in the context of the electrochemical reduction of combustion gas (CO₂, H₂O) to synthesis gas (CO, H₂).

Powders of BaZr_0.90Fe_0.10O_3-δ (BZF10) were synthesized via solid-state reaction and characterized by synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS). The powders were pressed and sintered into 13mm dense button cells, and a porous Pt electrode and a dense Pt blocking electrode were attached to either face of the pellet. The resulting system was used to conduct electrical conductivity relaxation (ECR), electrochemical impedance spectroscopy (EIS), and reaction analysis over a wide range of dry O₂, CO₂, H₂O, and humidified CO, H₂ partial pressures at temperatures of 600 – 800^oC. Analysis of equilibrium electrical conductivities (σ_∞) vs. temperature and partial pressures provides critical insight into the underlying electrochemical mechanisms for surface reaction. Non-linear regression of conductivity vs. time data allows deduction of surface electrochemical reaction rates and solid-state diffusivities of charge carriers, and probing of surface oxygen-acceptor/donor kinetics. BZF10 was verified as an electrochemical stable, mixed ionic-electronic conducting ceramic suitable for SOEC application. The σ_∞ vs. O₂ partial pressure indicates a high oxygen-ion acceptor activity for Ba_0.9Fe_0.1ZrO_3+δ. Equivalent experiments conducted over a range of CO₂ partial pressures (pCO₂) indicate a net decrease in  σ_∞ with respect to pCO₂, coinciding with up to a 16% conversion of CO₂ to CO and O₂ at steady-state. Reduction of the Fe-doped perovskite suggests the formation of a CO₃^-2 intermediate during the overall conversion of CO₂ to O₂ and probing of CO₂ reduction activity indicates significant catalytic activity, which is attributed to the oxygen acceptor-donor activity of the Fe-doped perovskite. Experimental results indicate that the material behaves as a mixed oxygen ionic-electronic conductor, with significant catalytic activity for CO₂ reduction.