Superior Durability and Activity of a Benchmark Triple-Conducting Cathode by Tuning Thermo-Mechanical Compatibility for Protonic Ceramic Fuel Cells

As a benchmark triple-conducting cathode, BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY) has been widely investigated for protonic ceramic fuel cells (PCFC) in recent years. However, the reported electrochemical performance of BCFZY cathode differs, which is determined in this work to originate from the thermal expansion mismatch between BCFZY and electrolyte. Accordingly, two strategies for enhanced thermo-mechanical compatibility are examined: impregnation and thermal expansion offset. In contrast to the impregnation of BCFZY nanoparticles on electrolyte backbones that only helps improve electrochemical performance, negative thermal expansion oxide Sm_0.85Cu_0.15MnO_3−δ (SCM)-offset BCFZY exhibits superior durability and activity simultaneously. Specifically, the polarization resistance decay rate of the SCM-offset BCFZY is only ~0.2%/100 h, compared with ~18.75%/100 h for “impregnated BCFZY.” Moreover, pure SCM generates moderate cathodic performance (area-specific resistance = 0.11 Ω cm², 700 °C), X-ray diffraction and transmission electron microscopy reveal an in-situ formed intergranular Ba₂Cu₃SmO_7−δ phase at the boundaries of BCFZY and SCM. Thus, SCM can serve as a “three-effect” additive, i) offset thermo-expansion, ii) strengthen electrode structure and adhesion, and iii) provide acceptable oxygen-reduction-reaction activity, being favorable for superior performance. A PCFC using a SCM-offset BCFZY cathode achieves the highest power density (1455 mW cm⁻²) yet recorded for PCFCs with BCFZY-based cathodes.