Calcium manganite as oxygen electrode materials for reversible solid oxide fuel cell.

Area of Science:

• Materials Science
• Electrochemistry
• Energy Conversion

Background:

• Efficient high-temperature reversible solid oxide fuel cells (RSOFCs) require highly active oxygen electrodes for oxygen anion/gas conversion.
• CaMnO(3-δ) (CM) is a perovskite known for oxygen defect formation upon reduction, making it a candidate for oxygen electrodes.
• Nb doping in CaMnO(3-δ) can enhance electrical conductivity.

Purpose of the Study:

• To evaluate CaMnO(3-δ) (CM) and CaMn(0.9)Nb(0.1)O(3-δ) (CMN) as oxygen electrodes for RSOFCs.
• To compare the electrochemical performance and material stability of CM and CMN.
• To assess compatibility with yttria-stabilized zirconia (YSZ) electrolytes.

Main Methods:

• X-ray diffraction (XRD) for phase analysis and compatibility studies.
• Thermal expansion coefficient (TEC) measurements.
• Polarization curves and electrochemical impedance spectroscopy (EIS) at 700 °C.
• Thermogravimetric analysis (TGA) and variable temperature XRD for phase stability up to 900 °C.

Main Results:

• CMN exhibits significantly higher electrical conductivity (125 S cm⁻¹ at 700 °C) than CM (1-5 S cm⁻¹).
• Both CM and CMN show excellent compatibility with YSZ electrolyte up to 950 °C and possess compatible TECs (10.8-10.9 ppm K⁻¹).
• CM demonstrates a polarization resistance of 0.059 Ω cm² at -0.4 V (fuel cell mode), while CMN shows 0.081 Ω cm² at 0.4 V (electrolysis mode).

Conclusions:

• Nb doping enhances the electrical conductivity of CaMnO(3-δ) while maintaining good thermal and chemical stability with YSZ.
• Both CM and CMN are viable oxygen electrode materials for RSOFCs, with CM showing better performance in fuel cell mode and CMN in electrolysis mode under tested conditions.
• Further optimization of CMN could lead to improved RSOFC performance.