A comprehensive review on the conversion of CO2 into solid carbon materials

The conversion of carbon dioxide (CO2) into valuable solid carbon materials presents a promising approach for carbon utilization and climate change mitigation. This review systematically evaluates six major carbon allotropes: graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), fullerenes, diamonds, and porous carbon, with a focus on synthesis methods, operating conditions, and industrial feasibility. Among these, CNTs and CNFs show the highest potential, especially when produced via molten carbonate electrolysis or the Solar Thermal Electrochemical Process (STEP), which operate at approximately 750–770 °C and near-atmospheric pressure. These methods have demonstrated high carbon conversion efficiencies and significantly lower estimated production costs compared to conventional CVD techniques, due to their simpler operation and lower material costs. Graphene, although high in quality, requires approximately 1000 °C and expensive catalysts, making it less scalable. Fullerenes (C60) and diamonds have very low yields (<1 %) and require extreme pressures (up to 1000 atm), limiting their industrial use. Porous carbons, synthesized electrochemically or by metal/inorganic reduction at 500–850 °C, show promise for supercapacitors and adsorption, with yields up to 55.3 wt % and built-in doping capabilities. Metal-mediated methods using Mg, Zn, and NaBH4 offer simplicity, moderate conditions, and tunable structures, while new hybrid approaches provide synergistic benefits. Overall, molten salt electrochemical methods are highly promising candidates for scalable and energy-efficient processes, supporting CO2 valorization in sustainable carbon material production.