Efficient and durable air electrodes for reversible protonic ceramic electrochemical cells

Abstract

Reversible protonic ceramic electrochemical cells (R-PCECs) hold great potential as an energy conversion and storage device. However, its electrochemical performance at reduced operating temperatures is hindered by sluggish and unstable oxygen reduction/evolution reactions (ORR/OER) at conventional air electrodes. To overcome this limitation, this thesis identified multiple strategies integrating bulk anion substitution, surface nanoparticles design, and one-pot bulk-phase self-assembly to develop high-performance triple-conducting (H+/O2-/e-) nanocomposite air electrodes with excellent stability. First, anion engineering is applied to altering the oxygen sites of sublattice in Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). The results indicate that the electronegative fluorine substitution weakens metal-oxygen bonds and enhances proton uptake. Consequently, the optimized Ba0.5Sr0.5Co0.8Fe0.2O2.9-σF0.1 air electrode demonstrates accelerated surface oxygen exchange and bulk H+/O2- transport, leading to a ~70% reduction in area-specific resistance compared to pristine BSCF. Building on the first work’s foundation, a dual bulk-surface modification strategy is successfully implemented through in situ growth of nanoscale catalysts on the surface of fluorine-engineered perovskite oxides. As a result, the Ba(Co0.4Fe0.4Zr0.1Y0.1)0.95Ni0.05F0.1O2.9-δ nanocomposite air electrode exhibits a peak power density of 996 mW·cm-2 at 650 °C—60% higher than conventional electrodes—along with stable reversibility over 100 hours. Moreover, to resolve persistent challenges in steam resistance and thermomechanical compatibly, a Co/Sr-free dual-phase perovskite oxide, Ba(Zr0.1Ce0.7Y0.1Yb0.1)0.4Fe0.6F0.1O2.9-δ (BZCYYFF), is rational designed via one-pot self-assembly method. This design integrates a proton-conductive Ce-rich phase (P-BZCYYFF) with triple-conducting Fe-rich domains (M-BZCYYFF), establishing continuous proton transport pathways while eliminating phase segregation risks. Additionally, fluorine anion doping further weakens metal-oxygen bonds in both phases, thus enhancing ionic mobility without catalytic compromise. The final BZCYYFF electrode achieves an ultra-low ASR of 0.33 Ω·cm² at 550 °C, coupled with a peak power density of 0.494 W·cm⁻² in fuel cell mode and an exceptional electrolysis current density of 0.649 A·cm⁻² (electrolysis voltage of 1.3 V) at 550 °C. Long-term operation under 10% humidified air for over 160 hours and the 18 cycles spanning 180 hours demonstrates negligible degradation, further underscoring its unmatched durability. In conclusion, by harmonizing triple conductivity, hydration resistance, and thermal compatibility, this work establishes a materials design paradigm for robust R-PCEC air electrodes, advancing their viability for energy storage and hydrogen economy applications.