Tuning Exsolution and Surface Properties via Excess Free Energy and Work Function Engineering for High-Performance...

Nanoparticle exsolution offers a promising strategy to enhance the surface-mediated oxygen reduction reaction (p-ORR) in protonic ceramic fuel cells. However, the thermodynamic parameters governing exsolution and the precise role of the exsolved nanoparticles remain unclear. Here, we investigate the exsolution behavior of La_0.5Ba_0.5Co_0.4Fe_0.6O_3-δ (LBCF) through B-site doping (Ce, Pr, Zr, and Sn). Density functional theory calculations identify the excess Gibbs free energy (ΔG^ex) as a robust descriptor for exsolution propensity. Experimentally, Ce- and Pr-doped LBCFs (ΔG^ex > 0) exsolve BaCeO₃/BaPrO₃ nanoparticles, whereas Zr- and Sn-doped samples (ΔG^ex < 0) maintain a stable single-phase structure, consistent with theoretical predictions. Work function (Φ) analysis combined with CO₂/NH₃-TPD reveals that the exsolved BaCeO₃ nanoparticles facilitate electron transfer to the LBCF host. This electronic interaction results in a higher oxygen surface exchange coefficient and lower polarization resistance in the Ce-doped cathode. Consequently, a single cell utilizing Ce-doped cathode achieves a peak power density of 0.97 W cm^-2 at 650 °C, an approximately 87% improvement over the pristine LBCF cathode. This work establishes a predictive framework linking dopant thermodynamics, exsolution behavior, and catalytic performance, advancing the rational design of exsolution-engineered cathodes.