Carbon Nanotube Production from Li2CO3 Via High Temperature Electrolysis
Authors: Zhu Zhixu , Jang Inyoung , Wootton Callum , Gonzalez Minaya Andrea , Hankin Anna 
Published: December 2024

Electrochemical CO2 reduction (ECR) can provide an environmentally and economically viable route for converting CO2 into value-added products. A recently developed CO2 utilisation process involves its conversion into carbon nanotubes (CNTs) via high temperature electrolysis.1,2 CNTs are tubular structures consisting of graphitic carbon. Their mechanical strength, electrical conductivity, and thermal conductivity accelerated their application in a variety of systems, including nanoelectronics, catalysts and lithium-ion batteries. However, the high cost of CNTs (about $114,000 per tonne) currently limits their use.3

Production of CNTs via high temperature electrolysis of CO2 in molten carbonate salts has the potential to significantly reduce their production cost (on the order of $660 per tonne) whilst simultaneously processing CO2 emissions.4 The method involves several key steps: (i) selective exothermic absorption of CO2 from flue gas or from the air by an alkali metal oxide in molten electrolyte (e.g., Li2O in molten Li2CO3 electrolyte), forming an alkali metal carbonate; (ii) electrolysis of metal carbonate, producing CNTs and oxygen gas, while regenerating the metal oxide; (iii) heat released by CO2 absorption maintains the high temperature required for electrolysis.
In our research, carbon nanostructures were electrochemically grown in molten Li2CO3 electrolyte at 800 °C. Carbonate ions were reduced electrochemically to a mix of carbon products, including CNTs, carbon nanofibers and carbon nanoparticles. Electrolysis was carried out at different current densities: 100, 200, 300 mA/cm2, normalised using the starting cathode surface area and using various combinations of iron and nickel as anodes/ cathodes. The resulting cell potential differences were in the range 1.65 – 2.50 V, substantially higher than the thermoneutral potential of 1.42 V at this operating temperature. The carbon products were characterised with Raman spectroscopy, scanning electron microscopy and transmission electron microscopy along with energy dispersive X-ray spectroscopy. High yield of CNTs was observed from the electrolysis products on the cathode at 200 mA/cm2 with a nickel anode and an iron cathode. The SEM and TEM images of the synthesized carbon products are shown in the below figure. We have systematically investigated and were able to relate different reaction parameters to the properties of the synthesised carbonaceous products. Experiments are now being carried out to study the micro-kinetics and reaction mechanism(s) of the CNT nucleation and growth process. A new reactor has been designed and fabricated to introduce CO2 gas into the electrolyte to regenerate the consumed Li2CO3. Our goal is to optimise the reaction process for prototype development. We shall present our experimental setup, procedure and key findings.

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A. Hankin, G. Guillen Gosalbez, G. Kelsall, N. Mac Dowell, N. Shah, S. Weider and K. Brophy, Assessing the economic and environmental value of carbon capture and utilisation in the UK, Institute for Molecular Science and Engineering Briefing Paper No. 3, 2019.

Figure 1