Electrochemical CO₂ 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 CO₂ reduction, researchers have extensively studied the development of catalyst materials in a typical H-type cell, utilizing a dissolved CO₂ reactant in the liquid phase. However, the low CO₂ solubility in an aqueous solution has critically limited productivity, thereby hindering its practical application. In efforts to realize commercially available CO₂ 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 CO₂ reduction, by generally examining some previous approaches from liquid-phase to gas-phase systems. Finally, we outline major challenges, with significant lessons for practical CO₂ conversion systems.

Morphology-controlled Au nanostructures are fabricatedviaelectroreduction of anodized Au thin films, exhibiting efficient catalytic activity for electrochemical CO₂reduction.

A NiO–Y₂O₃ stabilized ZrO₂ (NiO–YSZ) supported tubular solid oxide cell, which consists of a La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) dip-coated electrolyte film and Sm_0.5Sr_0.5CoO_3−δ (SSC) air electrode, was prepared and its power generation and electrolysis performance in the intermediate temperature range were investigated. Compared to other rare earth oxides studied, infiltration of Ce nitrate into the substrate was the most effective for increasing cell performance in the case of 2 M infiltration. Moreover, it was found that the infiltration of a higher concentration of the Ce solution increased the maximum power density, because both IR loss and overpotential were significantly decreased. The maximum power density of the cell was 0.95 and 0.42 W cm⁻² at 873 and 773 K, respectively at 3 M Ce nitrate infiltration. The long-term stability of the cell was also measured by using the cell infiltrated with 1.5 M Ce, and a stable power generation performance was demonstrated up to 100 h. The steam electrolysis performance of the cell using Ce infiltration was further studied and it was found that Ce also contributes to higher current density in SOEC operation and 1.07 A cm⁻² at 1.6 V was achieved at 873 K using 2 M Ce infiltration.