Alkaline Leaching: A Facile Strategy to Improve the Surface Activity of Air Electrodes for Solid Oxide Electrochemical Cells

Abstract

Solid oxide electrochemical cells (SOCs) have been focused on as next-generation energy conversion devices. However, since SOCs are operated at high temperatures (>800 °C), several issues are present, such as limited material selection and high system cost. Therefore, lowering the operating temperature and developing suitable electrode materials which have high oxygen evolution/reduction reaction (OER/ORR) activity at relatively low temperatures are crucial.In this manner, surface modification methods have been researched, such as nanocatalyst decoration, surface coating, and acid etching. However, nanocatalyst decoration and surface coating require a long annealing time and complicated vacuum devices, respectively, and acid etching is not favorable for rational control on the surface of the electrode. Therefore, a cost- and time-efficient surface modification method while maintaining reasonable controllability can have a meaningful advantage.We suggest a facile method of surface modification for electrodes of SOCs: alkaline leaching. During the water splitting in a conventional 3-electrode system with an alkaline medium, cations in perovskite oxide lattice are selectively dissolved into the alkaline solution, which is called as leaching phenomenon. As a result of leaching, the composition of the perovskite oxide surface differs from the pristine condition (e.g. the transition metal-rich surface). We adopted this phenomenon as a strategy to improve the activity of air electrodes for SOCs.In this study, we chose PrBa0.8Ca0.2Co2O5+δ (PBCC) as the air electrode material for the first case-study. Recently, PBCC has gained a lot of attention due to its outstanding OER/ORR activity even at relatively low temperatures (<650 °C). We compared the reactivity of PBCC electrodes in different alkaline leaching conditions by electrochemical impedance spectroscopy. After the surface treatment, area-specific resistance of 0.018 Ω·cm2 (at 650 °C) was recorded, corresponding to the 5-fold and 53-fold enhanced ORR reactivity compared to the pristine PBCC air electrode and commercialized La0.6Sr0.4Co0.2Fe0.8O3 air electrode, respectively.To elucidate the enhanced surface activity, various analyses were carried out. Scanning electron microscopy images showed the porous structure of the PBCC electrode and the removal of Ba-containing secondary phases on the PBCC surface, which are known to degrade surface activity. Transmission electron microscopy-energy dispersive spectroscopy results demonstrated that the surface of the PBCC electrode becomes amorphous and Co-rich, which are both known to be advantageous for ORR activity. Inductively coupled plasma-mass spectrometry results of the alkaline medium in the 3-electrode system revealed that specific cation species (i.e. Ba and Ca) were selectively dissolved into the alkaline medium and were more leached out by the applied anodic bias, resulting in the Co-rich surface. X-ray photoelectron spectroscopy results suggest that surface Ba species were decreased and defective O species were increased, leading to improved ORR activity.In this case, we showcased alkaline leaching as a strategy to enhance the PBCC electrodes for SOCs, but with the sufficient current collection, our method can be further applied to various perovskite oxides in various structures. Taking the various advantages of perovskite-oxide-based materials, the alkaline leaching method will be a novel strategy to achieve a high-performance (electro)chemical catalyst.