Role of H ₂ in Solid Carbon Production in a CO ₂ –CO–C Conversion System Using Solid Oxide Electrolysis Cells and an Fe Catalyst

A hybrid system combining a solid oxide electrolysis cell (SOEC) with an iron (Fe)-based catalyst was investigated for carbon dioxide (CO 2 ) electrolysis and the formation of CO 2 -derived carbon (C). Stable electrolysis at a high current density of 1.1 A/cm 2 was achieved for 4 h at H 2 /CO 2 ratios of 0.1–0.25. The formation of C on the Fe-based catalyst depended on the H 2 /CO 2 ratio, exhibiting enhanced C efficiency at low H 2 concentrations and suppressed C formation at higher ratios. CO 2 -derived carbon fibers were formed when Fe acetate infiltration was used. DFT calculations clarify these trends by showing that the activation barrier for CH–CH coupling (CH + CH → C 2 H 2 ) is much lower than that for direct C–C coupling, while CH inhibits the subsequent carbon growth. Both experimental and DFT results demonstrate the existence of an optimal H 2 concentration for the controlled carbon formation in SOEC-integrated catalytic systems. Specifically, a low H 2 /CO 2 ratio can be used to enhance the production of CO 2 -derived carbon materials, while a high H 2 /CO 2 ratio can suppress carbon formation on the catalyst.