Application of solid electrolytes in electrochemical reduction of CO₂ or O₂

Electrochemical conversion of abundant CO2/O2 into value-added products and fuels holds great promise to achieve sustainable energy systems. In most cases, aqueous electrolyte containing salts are used, inevitably suffering subsequent purification if pure products needed for further purpose. Instead, the application of solid electrolyte has seen a booming and that thought to be commercially attractive. Nevertheless, there is a scarcity of comprehensive reviews focusing exclusively on the application of solid electrolytes in electrochemical CO2/O2 reduction, in contrast to the more readily available reviews on battery technologies. In this manner, we review recent advance and challenges in the understanding electrochemical reduction of CO2/O2 using solid electrolytes to produce high valuable chemicals like H2O2, formic acid, etc. It begins with a brief introduction of historical development of solid electrolyte and their properties. Then it followed with summarizing of its application used in oxygen reduction reaction (ORR) by power consuming mode (2e--ORR), or by power generating mode in fuelcell. Afterward, diverse type of solid electrolytes has been systemically reviewed, including the corresponding cells, the production rate along with different roles in electrochemical CO2 reduction. Finally, challenges and prospects of solid electrolytes are discussed, aiming to guide a promising direction for directly producing highvalue chemicals in near future.