Triple Conducting Perovskite Oxide Nanocomposites As Oxygen Electrodes for Intermediate-Temperature Protonic Ceramic Cells

Abstract

Protonic ceramic cells (PCCs) based on proton conducting (PC) electrolyte membranes have great potential to be operated at intermediate temperatures of 300-600oC because of the low activation energy for the proton transportation, which allows much more efficient, reliable and cost-effective operation of fuel cells and electrolysis cells. However, the conventional mixed ionic and electronic conducting (MIEC) oxygen electrodes showed sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics because the involvement of water limited the ORR/OER to triple phase boundaries, which resulted in unacceptably poor PCC performance. The triple (proton, oxygen ion, and electron) (TPOE) conducting microcomposite oxygen electrodes comprised of PC and MIEC microphases were studied by mixing well-known phase-pure PC and MIEC powders. Although the ORR/OER performance was improved, the solid-state reactions between PC and MIEC microphases to arrive thermodynamic equilibrium significantly degraded the performance. Furthermore, the microsized distribution of the TPOE conductivities among the microcomposites still is the dominant obstacle for improving ORR/OER performance in the intermediate temperatures range. Recently, the new phase-pure perovskite and perovskite-related oxides with TPOE conductivities were developed as oxygen electrodes for PCCs, which showed significant improvement of the ORR/OER performance because the whole electrode surface area was activated for ORR/OER. However, it is well-known that the adjustment of multiple dopants for achieving proper TPOE conductivities is very difficult. In the current work, we designed a new type of TPOE nanocomposite electrodes comprised of PC and MIEC perovskite nanophases. The dual perovskite nanocomposite (DPNCs) were fabricated using a one-pot polymeric gelation synthesis method starting from solutions of componential metal ions. The resulted DPNCs showed thermodynamically stable nanostructures with the nanosized distribution of TPOE conductivities. The superior ORR/OER performance to the state-of-the-art PCC oxygen electrodes (i.e., phase-pure TPOE perovskite oxide of BaCo0.4Fe0.4Zr0.1Y0.1O3-δ) was demonstrated based on both symmetrical cells and single cells. The architecture of the triple conducting perovskite oxide nanocomposite oxygen electrodes from nanophase basic building blocks (N3Bs) can be extensively used for the development of other components of the energy conversion and storage devices.