Production of elemental carbon via molten carbonate electrolysis: prospects and challenges

Abstract

Carbon capture and utilization (CCU) is a viable approach to convert atmospheric CO2 into various valuable end products such as fuels, chemicals, and construction materials. In the context of CCU, this research focuses on the conversion of CO2 to elemental carbon via molten carbonate electrolysis. By utilizing renewable energy sources, this carbon production process has the potential to become carbon negative, operating on the principle of merely consuming CO2. CCU processes are an exciting area by virtue of their potentially beneficial environmental impact and effects on waste management and utilization.

The effect of electrolyte selection, electrolysis temperature, and cathode material on the carbon morphology produced was studied in this work. The focal points of the experimental work were selected based on a lack of previous studies and/or previous studies having contradictory results. Previous studies related to CO2 conversion in molten salts indicate that the produced elemental carbon can exist in various morphologies, such as carbon nanotubes (CNTs), nanofibers (CNFs), nano-onions (CNOs), platelets, and amorphous carbon.

The experimental work was conducted with two different types of electrolyzer setups: coaxial and planar. One of the key challenges encountered in the design and selection of materials for these setups is the harsh process conditions. The combination of molten salt, high temperatures, and the generation of oxygen in the process make the process conditions extremely corrosive. Corrosion leading to metallic impurities getting mixed with the carbon is an issue of concern, as any impurities affect the product quality. The ability to accurately control the process, for example by maintaining precise temperature, is a key factor in obtaining high-value carbon products of good quality.

A comprehensive understanding of the produced carbon was achieved through the utilization of various analytical methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The results revealed that the different metals dissolved from the electrodes affect the carbon produced and its morphology. Different metals and different amounts of metals seem to have different types of effects. With little to no dissolved metals mixed with the product, spherical nano-onions were the dominant product. These spherical, onion-like structures were the main product with nickel cathode. With the presence of iron, tubular structures were dominant, as the iron acted as a nucleation seed for tube growth. Tubular structures were found when steel-based cathodes, both stainless and galvanized steel, were used. As a general rule, it seems that impurities cause inconsistent product containing various carbon morphologies. These results emphasize the importance of the electrode material selection, as the purity of the carbon is a crucial factor when considering possible applications.

Along with the experimental work, a 2D numerical investigation of current density distribution, carbon deposition, and voltage-current characteristics was conducted with COMSOL Multiphysics to obtain additional information about the process. The results indicated that the current density distribution is not as uniform as anticipated with the coaxial setup. This uneven distribution causes the carbon deposition on the cathode surface to become uneven. Also, as of now, the numerical investigation only takes into account the main reactions of the carbon deposition by only considering the carbon and oxygen formation. Experimental work revealed the presence of metallic impurities, the formation mechanisms of which remain unknown. To conclude, the lack of experimental data and knowledge of side reaction mechanisms leave room for additional improvements and optimization of the model. As this type of model is an useful tool for setup geometry optimization in terms of obtaining even current density distribution, further development with additional experimental data is worth consideration.

In addition to electrode materials, the work studied the effect of electrolyte composition and electrolysis temperature on the carbon morphology produced. Electrolytes studied were Li2CO3-BaCO3 and Li2CO3-CaCO3 80:20 mol%. The results showed that both electrolyte composition and electrolysis temperature affect the carbon morphology. The effect of the electrolyte composition was more significant at lower temperatures, as the product morphology differed considerably between the electrolytes. The amount of tubular structures increased along increasing temperature in both electrolytes, which indicates that the effect of the electrolyte is not as significant at higher temperatures. Based on XRD diffraction patterns, it can be concluded that not only the morphology of the carbon changes, but also the type and amount of metallic impurities.