Development on Enhancing the Tolerance of Solid Oxide Fuel Cell Cathodes Against Cr Poisoning

Solid oxide fuel cells (SOFCs) are an extremely attractive energy conversion technology due to their high efficiency, low emissions, and flexible fuels, which have wide application prospects in distributed power generation and hydrogen production. The operating temperatures of conventional SOFCs are typically above 1000 ℃. The high operating temperatures impose stricter requirements on the materials of various components in the SOFCs system, especially in terms of mechanical strength, reactivity, thermal expansion behavior, and cell sealing. Also, the high operating temperatures lead to long start-up times and weak economic competitiveness of the SOFCs, seriously hindering the commercialization of SOFC technology. Reducing the operating temperatures to intermediate temperatures (i.e., 600–800 ℃) can significantly mitigate the chemical reactions between cell components, extend the service life, and expand the range of cheaper material options, thus enhancing its economic competitiveness. However, the kinetics of oxygen reduction reaction (ORR) related to the cathodes of SOFCs become notably sluggish with the fall in temperature, resulting in a rapid increase in polarization impedance and a significant decrease in cell performance. It is thus crucial to develop cathode materials with superior electrocatalytic activity and durability for ORR at intermediate temperatures.

Although significant progress has been made in developing cathode materials with high performance and excellent durability at intermediate temperatures, the electrochemical evaluation of developed cathode materials has been performed under relatively mild conditions in most studies. However, cathodes of SOFC stacks are inevitably exposed to harsh working conditions under realistic operating conditions. For instance, the gas supplied to the cathodes may contain steam, CO₂, sulfide, etc.. In addition, the system components of the stacks will volatilize gaseous contaminants (e.g., CrO₃ and CrO₂(OH)₂, etc.) at operating temperatures. These gaseous contaminants can deposit on the surface of cathodes or react directly with cathodes, resulting in poisoning and/or corrosion of the cathode materials. In particular, Cr poisoning on the cathode is one of the problems widely concerned.

Compared with conventional ceramic interconnects, metallic interconnects widely used at intermediate temperatures can reduce the cost, and exhibit higher electrical and thermal conductivity, as well as better machinability. However, a certain amount of Cr is usually added to metallic interconnects to improve their oxidation and corrosion resistance. At high temperatures and oxidizing atmosphere, the Cr₂O₃ layer formed on the surface of metallic interconnects will evaporate gaseous chromium vapor. These gaseous Cr species are prone to react with alkaline earth metal oxides (such as BaO and SrO) that are segregated on the cathode surface, forming solid deposits on the cathode surface and/or at the interface between the cathode and electrolyte. These inert deposits occupy the active sites for oxygen reactions on the cathode, leading to a significant decrease in electrocatalytic activity, and affect the gas diffusion in the porous cathode, resulting in rapid degradation of the cell. It is thus essential for maintaining the robustness of the SOFCs system and extending its service life to alleviate the Cr poisoning of the cathodes.

Conducting in-depth research and taking measures to mitigate Cr poisoning on the cathode materials is crucial for further advancing the commercialization of SOFCs technology. In this review, recent development in enhancing the Cr tolerance of cathodes is represented. The theoretical mechanisms of Cr deposition on cathodes are introduced, and the reported strategies to alleviate Cr poisoning on cathodes are summarized. Despite the significant progress made in the existing research, there are still some challenges to be addressed, i.e., a) it seems that a single theory cannot perfectly explain the Cr deposition on different cathode materials. For different materials under different test conditions, these mechanisms may occur simultaneously or compete with each other. In addition, in proton-conducting SOFCs (H-SOFCs), these processes may be more complicated due to the introduction of protons. Although some researchers deal with the problem of Cr poisoning in H-SOFCs, there are no detailed literature reports on the mechanism of Cr poisoning in H-SOFCs system; b) Redesigning or improving the currently commercialized interconnect materials faces high costs. Applying a protective coating on the interconnects is a feasible approach. However, the added cost of additional manufacturing processes also needs to be considered; c) The surface modification of cathodes can effectively strengthen its Cr resistance. However, isolated nanoparticles are unable to achieve complete coverage of the inherently fragile electrodes. The areas of the cathode surface that are not covered may be still poisoned by contaminants such as CO₂, Cr vapor, and steam. Moreover, if the impregnated surface catalysts are used as a preferred reactant of the gaseous Cr species, they may be continuously consumed for long periods of operation. Although the poisoning effect of the deposits is mild, excessive accumulation of the deposits will occupy the active sites for the oxygen reduction reaction and hinder the diffusion of gases in the cathode; d) The design of contaminants-tolerant high-entropy materials needs to balance electrocatalytic activity and stability. In addition, the interaction mechanisms of different metal cations in the high-entropy cathodes are still unclear and require further in-depth research; and e) Recent progress has been made in the application of SOFCs in electrolysis mode. As is known, the electrode material on the air side undergoes cathodic polarization in fuel cell mode, while in electrolysis mode, anodic polarization occurs on the electrode on the air side. Therefore, the mechanisms and degree of Cr poisoning will be different. Unfortunately, there is still a lack of relevant research on Cr deposition in electrolysis mode. In summary, the phenomenon of Cr poisoning in materials is a long-standing challenge for SOFCs and other high-temperature electrochemical devices. A future research should focus on the investigation on Cr poisoning mechanisms under various operating modes and optimization of the composition and structure of cathode materials. These efforts aim to enhance the stability and reliability of SOFCs in practical industrial applications, ultimately advancing the commercialization process of SOFCs technology.