(Georgia Institute of Technology, 2022-04-15)
Evans, Conor
Reversible solid oxide cells based on proton conductors (P-rSOCs) offer an efficient and clean option for energy storage and conversion. However, one issue holding back this renewable technology is the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics that take place at the air electrode. The air electrode in a P-rSOC is also subject to harsh environments (e.g., high concentration of steam) that can cause degradation over time. Catalyst infiltration into the air electrode offers a possible solution to each of these issues. Several catalyst candidates were investigated using the state-of-the-art double perovskite air electrode material, PrBa0.8Ca0.2Co2O5+δ (PBCC), as the air electrode backbone. Symmetrical cells with catalyst coated PBCC electrodes were primarily used to screen catalyst solutions and isolate the air electrode performance.
Electrochemical impedance spectroscopy (EIS) was utilized to characterize the electrochemical performance and the long-term stability of catalyst infiltrated symmetrical cells under various testing conditions containing either steam and/or Cr contaminants. Electrochemical performance of single cells with a catalyst coated PBCC electrode was measured in both the fuel cell mode and the electrolysis cell mode. X- ray diffraction (XRD), scanning electron microcopy (SEM), and Raman spectroscopy were used to characterize phase composition, electrode microstructure and morphology, as well as surface chemistry to gain better understanding of the air electrode degradation mechanism during testing. Several catalysts were screened and optimized via symmetrical cell tests, including LaNiO3, La2NiO4, BaCoO3, LaNi0.6Fe0.4O3, La2Ni0.6Fe0.4O4, and PrCoO3.
Symmetrical cells infiltrated with a PrCoO3 catalyst demonstrated particularly excellent stability and electrochemical performance (with a polarization resistance as low as 0.147 Ω cm2 and minimal degradation over 500 hours) against various sources of Cr contaminations at steam concentrations as high as 30% at 600 °C. Single cells infiltrated with PrCoO3 exhibit a peak power density of 2.02 W cm-2 at 650°C in the fuel cell mode, a 35.5% increase in performance from the single cells without catalyst modification. When run in electrolysis mode these same infiltrated single cells demonstrate a current density of 3.22 A cm-2 at 650 °C, a 22.4% improvement from the performance of the cells without catalyst modification. The single cell based on a PrCoO3 infiltrated cathode was among the best performing P-rSOCs ever reported in literature