Tuning Exsolution and Surface Properties via Excess Free Energy and Work Function Engineering for High-Performance Protonic Ceramic Fuel Cells

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 0.5 Ba 0.5 Co 0.4 Fe 0.6 O 3-δ (LBCF) through B-site doping (Ce, Pr, Zr, and Sn). Density functional theory calculations identify the excess Gibbs free energy (Δ G ex ) as a robust descriptor for exsolution propensity. Experimentally, Ce- and Pr-doped LBCFs (Δ G ex > 0) exsolve BaCeO 3 /BaPrO 3 nanoparticles, whereas Zr- and Sn-doped samples (Δ G ex < 0) maintain a stable single-phase structure, consistent with theoretical predictions. Work function (Φ) analysis combined with CO 2 /NH 3 -TPD reveals that the exsolved BaCeO 3 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.