Electrochemical CO 2 conversion offers a promising route for value-added products such as formate, carbon monoxide, and hydrocarbons. As a result of the highly required overpotential for CO 2 reduction, researchers have extensively studied the development of catalyst materials in a typical H-type cell, utilizing a dissolved CO 2 reactant in the liquid phase. However, the low CO 2 solubility in an aqueous solution has critically limited productivity, thereby hindering its practical application. In efforts to realize commercially available CO 2 conversion, gas-phase reactor systems have recently attracted considerable attention. Although the achieved performance to date reflects a high feasibility, further development is still required in order for a well-established technology. Accordingly, this review aims to promote the further study of gas-phase systems for CO 2 reduction, by generally examining some previous approaches from liquid-phase to gas-phase systems. Finally, we outline major challenges, with significant lessons for practical CO 2 conversion systems.
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