Performance Improvement of Proton Ceramic Fuel Cells through Surface Treatment of Cobalt Oxide Nanoparticles on Perovskite Oxide

Abstract

Ceramic fuel cells, such as solid oxide fuel cells (SOFCs) have attracted substantial attention as next-generation power generators. However, SOFCs exhibit high activation energy required for ion conduction (0.8–1 eV), which considerably increases the resistance upon decreasing the driving temperature. Therefore, the operating temperature of a typical SOFC is 800 °C or higher. However, such high-temperature driving conditions are associated to several challenges, such as heat-induced cell-structure deformation , rapid degradation of the material due to bonding stress, and the need for an insulation structure to prevent energy loss. Therefore, efforts have been made to lower the operating temperatures. Common methods use proton-conductive ceramics for proton-ceramic fuel cells (PCFCs). Unlike SOFCs, where the oxide ions pass through the electrolyte, in PCFC, protons pass through the electrolyte. Given that the activation energy (0.3–0.5 eV) of proton conduction is lower than that of oxide ions, most proton-conductive ceramics have very high ion conductivity at a lower temperature than oxygen-conductive ceramics. Therefore, compared with SOFCs, PCFCs exhibit a relatively high performance in the temperature range of 600 °C or less. As described above, efforts have been made to lower the driving temperature by promoting the oxygen reduction reaction (ORR) at the cathode, as well as by changing the ion transferred from the electrolyte.PrBa0.5Sr0.5Co2xFexO5+δ (PBSCF), a typical co-doping (Sr on the A-site and Fe on the B-site) cathode material, formed rapid oxygen diffusion and surface oxygen exchange through crystalline channels. However, most perovskite oxides, including PBSCF, exhibit poor chemical surface stability in high-temperature oxidation environments. In particular, Sr aggregation is considered a major reason for cathode degradation; however, the exact mechanism of this phenomenon has not yet been elucidated and a solution has not yet been established. Currently, it is known that elements such as Ba and Sr are larger size than other elements. During the stretch compression process inside the lattice, they are pushed out via the strain effect; moreover, the electrostatic phenomenon between the oxygen vacancies and cations may be an additional reason. Surface-treatment techniques are suitable for suppressing these phenomena. Through surface coating, the secondary phases generated by Ba and Sr aggregation may be suppressed to prevent electrode degradation. In addition, surface treatment technology is used to increase the catalytic activity of the cathode. Among the various surface treatment techniques, atomic layer deposition (ALD) is a surface thin-film coating technology suitable for ceramic cell electrodes. ALD proceeds in a chemical manner, such as chemical vapor deposition; however, it has several advantages. This may limit the deposition thickness per cycle to less than that of the atomic layer units through self-limiting reaction characteristics during material synthesis. In addition, the ALD reaction proceeded at the same rate along the surface of the substrate, which enabled uniform coating along a complex surface shape. Therefore, it may be considered an effective process to uniformly deposit a surface film and material inside a porous ceramic cell electrode structure.Herein, we attempted to improve electrode performance by applying ALD CoOx to a porous PBSCF cathode surface. CoOx has gained popularity as a high-performance electrochemical catalyst owing to its excellent ORR performance, low cost, and high chemical stability. As a result, we confirmed that the ohmic and polarization resistances were reduced by approximately 50 % and 34 %, respectively, at 600 °C by treating with CoOx, and through this, we confirmed a performance improvement of approximately 32 %. In addition, the inhibition of Ba or Sr segregation was confirmed through long-term testing. These results show that CoOx acts as an ORR catalyst in a PBSCF cathode with a perovskite structure and that ALD is a suitable method for coating porous cathode structures. In addition, H2O as a by-product of the cathode generates an oxygen reduction reaction in PBSCF, and a large amount of OH- is generated. As the number of oxygen vacancies increased, the movement of active oxygen species and electrons was promoted. This further improves the reaction with the catalyst by facilitating movement to the catalyst surface via overall electron migration and the overall spillover effect.