LU Xinyang ¹, YU Hezhan ¹, ZHAN Huanghao ¹, ZHANG Wei ², ZHAO Kai ¹, CHEN Min ¹, LI Jun ¹

(1. School of Materials Science and Energy, Foshan University, Foshan 528000, Guangdong, China; 2. School of Petroleum and Chemical Engineering, Dongying Vocational Institute, Dongying 257091, Shangdong, China)

Extended abstract:

[Background and purposes] Proton-conducting solid oxide fuel cells (H-SOFCs) hold significant promise for intermediate- and low-temperature operation, due to the lower activation energy associated with proton transport compared to oxygen-ion conduction. However, their widespread application is hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, which becomes a predominant performance-limiting factor at low temperatures. Cobalt-containing perovskite cathodes, while exhibiting excellent catalytic activity, suffer from high cost, significant thermal expansion and poor stability. Therefore, the development of high-performance cobalt-free cathodes is imperative. Ba_0.5Sr_0.5FeO_3−δ (BSF) has emerged as a promising candidate, due to its favorable mixed ionic-electronic conductivity and reduced thermal expansion coefficient compared to BaFeO_3−δ. Doping at the perovskite B-site is a well-established strategy to tailor material properties. This study is aimed to systematically examine the effects of B-site doping with three elements (Y, Zr, Ce) on the structural stability, thermophysical properties, electrical conductivity, and most critically, the electrochemical performance of BSF-based cathodes in H-SOFCs, with the goal of identifying an optimal high-performance cobalt-free cathode materials.

[Methods] Ba_0.5Sr_0.5Fe_0.9M_0.1O_3−δ (M=Ce, Zr, Y) powders were synthesized by using sol-gel method. Taking Ba_0.5Sr_0.5Fe_0.9Ce_0.1O_3−δ as an example, stoichiometric contents of Ba(NO₃)₂, Sr(NO₃)₂, Fe(NO₃)₃·9H₂O and Ce(NO₃)₃·6H₂O were dissolved in deionized water, followed by the addition of EDTA and citric acid. pH value was adjusted using ammonia solution. After drying, the precursor was calcined at 950 ℃ for 2 h. The other cathode powders were prepared in a similar way. BZCYYb electrolyte powder and NiO/BZCYYb/starch anode powder were synthesized by using the solid-state reaction method. The electrolyte powders were pressed into circular discs with a diameter of 25 mm at 200 MPa and then calcined at 1450 ℃ for 4 h in a muffle furnace. Cathode slurry was coated onto both sides of the sintered electrolyte pellet and subsequently calcined at 950 ℃ for 2 h, with an effective cathode area of 0.5 cm². The anode powders were pressed into circular discs with a diameter of 19 mm at 200 MPa and then calcined at 1000 ℃ for 2 h. The electrolyte paste was sprayed on the disc through ultrasonic spraying and then sintered at 1450 ℃. The cathode paste was coated in the center of the electrolyte disc by using brushing method and then sintered at 950 ℃. The effective area of the cathode was 0.196 cm². Phase analysis of powder samples was conducted by using X-ray diffraction (XRD, D8 DISCOVER, Bruker Inc., Germany). Microstructural analysis was performed by using scanning electron microscopy (SEM, Quattro S, Thermo Fisher Inc., USA). Electrical conductivity was measured with a digital multimeter (Keithley 2110, Tektronix Inc., USA). Electrochemical performance of the symmetrical cells and single cells was evaluated by using an electrochemical workstation (CS310, Corrtest Ltd., China).

[Results] XRD patterns confirmed the formation of pure perovskite phase for BSF, BSFY and BSFZ powders. For BSFC, minor secondary CeO₂ phase was detected, indicating a limited solubility of Ce in BSF under the synthesis conditions. Doping with Y, Zr and Ce effectively reduced the thermal expansion coefficient of BSF, enhancing the thermo-mechanical compatibility with the BZCYYb electrolyte. All doped samples showed enhanced electrical conductivity compared to pristine BSF, with BSFC exhibiting the highest conductivity of 7.3 S·cm⁻¹ at 550 ℃. ECR analysis revealed that the oxygen surface exchange coefficient (kchem) was significantly increased by Ce doping (2.34×10⁻⁴ cm·s⁻¹ at 650 ℃), slightly affected by Zr doping and detrimentally impacted by Y doping (5.99×10⁻⁵ cm·s⁻¹) compared to BSF (2.06×10⁻⁴ cm·s^–1). This trend was directly reflected in the electrochemical performance. In symmetrical cell tests, BSFC demonstrated the lowest polarization resistance (Rp) of 1.10 Ω·cm² at 650 ℃, compared to 1.45, 1.85 and 1.45 Ω·cm² for BSF, BSFY and BSFZ, respectively.  The BSFC-based single cell delivered outstanding peak power densities (PPDs) of 700, 520 and 370 mW·cm⁻² at 700, 650 and 600 ℃, respectively. These values represent significant improvements of about 68%, 37% and 23% over the BSF-based cell at the corresponding temperatures. Furthermore, the BSFC single cell showed exceptional operational stability, maintaining a constant current output of 0.4 A·cm⁻² at 650 ℃ for over 100 h without observable degradation. Post-test SEM analysis confirmed a stable and well-adhered interface between the BSFC cathode and the BZCYYb electrolyte.

[Conclusions] The effect of B-site doping for the Ba_0.5Sr_0.5FeO_3−δ perovskite cathode materials has been studied. Three novel cathode materials, Ba_0.5Sr_0.5Fe_0.9Y_0.1O_3−δ, Ba_0.5Sr_0.5Fe_0.9Zr_0.1O_3−δ and Ba_0.5Sr_0.5Fe_0.9Ce_0.1O_3−δ, were successfully synthesized. Their phase structure, thermal expansion coefficient, electrical conductivity and electrochemical performance are systematically examined. It is found that Y, Zr, and Ce doping effectively reduce the thermal expansion coefficient, while enhancing the electrical conductivity of BSF. Among them, BSFC has an electrical conductivity of 7.3 S·cm⁻¹ at 550 ℃. Electrochemical analysis reveals significant differences in the catalytic activity between the dopant species. Doping with Ce significantly enhances the oxygen surface exchange ability (with a K value of 2.34×10⁻⁴ cm·s⁻¹ at 650 ℃) and reduces the polarization impedance. Doping with Zr has no obvious effect, while doping with Y leads to performance decline. The single-cell test verifies that the BSFC cathode has a peak power density of 700 mW·cm⁻² at 700 ℃, which is about 23% higher than that of BSF. It can operate stably for 100 h at 650 ℃, without performance degradation. In summary, Ce-doped BSFC cathode material exhibits excellent catalytic activity and stability, making it promising Co-free cathode material for advanced applications.

Key words: SOFC; element doping; electrochemical performance; Ba_0.5Sr_0.5FeO_3−δ