Alkaline leaching for the surface modification and performance enhancement of air electrodes for solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) have been highlighted as next-generation energy conversion devices due to their high conversion efficiency. However, their high operation temperatures cause several issues including high system costs and limited material selection. In this manner, surface modification techniques have been explored to improve the surface activity of air electrodes, aiming to reduce the operation temperatures of SOFCs. Previous methods, such as nanocatalyst decoration, surface coating, and acid etching, have been associated with limitations in terms of cost and time efficiency. In this thesis, alkaline leaching was proposed as a more straightforward strategy compared to the methods reported earlier. Under the application of an anodic bias within an alkaline solution, alkaline earth metal cations on the A-site in perovskite oxides selectively dissolve into the alkaline medium, the so-called leaching phenomenon. As a result, the surface of the perovskite oxide becomes enriched with transition metals, providing benefits for oxygen reduction reaction at the air electrodes. For the initial case study of alkaline leaching, PrBa$_{0.8}$Ca$_{0.2}$Co$_2$O$_{5+δ}$ (PBCC) was selected as the air electrode material as it has attracted considerable attention due to its outstanding electrode performance. Following alkaline leaching, an area-specific resistance of 0.018 Ω·cm2, representing a 5-fold enhancement in activity compared to that of the pristine PBCC electrode, was obtained. Various surface analyses confirmed the cobalt-rich and amorphous surface, which is known to be advantageous for reactivity.