Materials of solid oxide electrolysis cells for H ₂O and CO ₂ electrolysis: A review

Reliable and economic energy storage technologies are urgently required to ensure a sustainable energy supply. H₂ is an energy carrier that can be produced environment-friendly by using renewable power to split H₂O via electrochemical cells. This way, electric energy is stored as the chemical energy of H₂, and the storage can be large-scale and economical. Among the electrochemical technologies for H₂O electrolysis, solid oxide electrolysis cells (SOECs) operated at temperatures above 500 ℃ have the benefits of high energy conversion efficiency and economic feasibility. In addition to H₂O electrolysis, SOECs can also be employed for CO₂ electrolysis and H₂O-CO₂ co-electrolysis to produce value-added chemicals of great economic and environmental significance. However, SOEC technology is not yet fully ready for commercial deployment because of the material limitations of the key components, such as electrolytes, air electrodes, and fuel electrodes. As is well known, reactions in SOEC are, in principle, inverse to reactions in solid oxide fuel cells (SOFCs). The component materials of SOECs are currently adopted from SOFC materials. However, their performance stability issues are evident, and need to be overcome by materials development in line with the unique requirements of SOEC materials. Key topics discussed in this review include SOEC critical materials and their optimization, material degradation and its safeguards, future research directions, and commercialization challenges, from both traditional O^2--conducting SOEC and H⁺-conducting SOEC perspectives. It is worthy to believe that H₂O or/and CO₂ electrolysis by SOECs provides a viable solution for future energy storage and conversion.