Low Area Specific Resistance La-Doped Bi2O3 Nanocomposite Thin Film Cathodes for Solid Oxide Fuel Cell Applications

Abstract

In the context of solid oxide fuel cells (SOFCs), vertically aligned nanocomposite (VAN) thin films have emerged as a leading material type to overcome performance limitations in cathodes. Such VAN films combine conventional cathodes like La_xSr_1-xCo_yFe_1-yO₃ (LSCF) and La_1-xSr_xMnO₃ (LSM) together with highly O^2- ionic conducting materials including yttria-stabilized zirconia (YSZ) or doped CeO₂. Next-generation SOFCs will benefit from the exceptionally high ionic conductivity (1 S cm^-1 at 730 °C) of Bi₂O₃-based materials. Therefore, an opportunity exists to develop Bi₂O₃-based VAN cathodes. Herein, we present the first growth and characterization of a Bi₂O₃-based VAN cathode, containing epitaxial La-doped Bi₂O₃ (LDBO) columns embedded in a LSM matrix. Our novel VANs exhibit low area specific resistance (ASR) (8.3 Ω cm² at 625 °C), representing ∼3 orders of magnitude reduction compared to planar LSM. Therefore, by demonstrating a high-performance Bi₂O₃-based cathode, this work provides an important foundation for future Bi₂O₃-based VAN SOFCs.

Keywords: bismuth oxide; energy materials; epitaxial thin film; ion conductivity; nanocomposite; solid oxide fuel cell.

Figure 1

Structural characterization of LDBO-LSM VAN films: (a) high-resolution cross-sectional STEM images showing epitaxial LSM matrix and embedded LDBO columns with enlarged regions from (bi) green and (bii) brown boxes. (ci) HAADF-STEM and (cii) bismuth EDX images confirming localized Bi as part of fast ionic conducting LDBO columns. (d) XRD pattern of LDBO-LSM VAN film on YSZ (001), indicative of the epitaxial nature of both phases, i.e., very sharp LDBO (00l) family of reflections and both the LSM (00l) and LSM (h0l) h = l families. (ei) AFM height and (eii) conducting-AFM images confirming electronically insulating LDBO columns embedded in a conductive LSM matrix.

Figure 2

Electrical impedance spectroscopy studies of LDBO-LSM VAN films on YSZ (001) substrates. (a) Nyquist plot showing pO₂ dependence of impedance features, with the circuit model inset. (b) Arrhenius plot showing pO₂ dependence of R_YSZ, R₁, and R₂ elements, with the reaction orders inset. (c) Arrhenius plot comparing ASR of LDBO-LSM VAN films (before and after degradation at 500, 600, and 700 °C for 100 h) with literature values for planar LSM and LSM-SDC VAN films from ref (1). (d) Nyquist plots measured at 625 °C of LDBO-LSM VAN films before and after degradation for 100 h at 500 °C, 600 °C, and 700 °C, respectively.

Figure 3

Schematic of (a) planar LSM and (b) LDBO-LSM VAN films detailing how the high ionic conducting LDBO phase can aid surface oxygen incorporation of LSM. The high density of triple phase boundaries at the LDBO-LSM VAN film surface helps facilitate a low ASR cathode.