A new electrolyte for molten carbonate decarbonization

The molten Li2CO3 transformation of CO2 to oxygen and graphene nanocarbons (GNCs), such as carbon nanotubes, is a large scale process of CO2 removal to mitigate climate change. Sustainability benefits include the stability and storage of the products, and the GNC product value is an incentive for carbon removal. However, high Li2CO3 cost and its competitive use as the primary raw material for EV batteries are obstacles. Common alternative alkali or alkali earth carbonates are ineffective substitutes due to impure GNC products or high energy limitations. A new decarbonization chemistry utilizing a majority of SrCO3 is investigated. SrCO3 is much more abundant, and an order of magnitude less expensive, than Li2CO3. The equivalent affinities of SrCO3 and Li2CO3 for absorbing and releasing CO2 are demonstrated to be comparable, and are unlike all the other alkali and alkali earth carbonates. The temperature domain in which the CO2 transformation to GNCs can be effective is <800 °C. Although the solidus temperature of SrCO3 is 1494 °C, it is remarkably soluble in Li2CO3 at temperatures less than 800 °C, and the electrolysis energy is low. High purity CNTs are synthesized from CO2 respectively in SrCO3 based electrolytes containing 30% or less Li2CO3.

In 2015, it was shown that the growth of transition metal nuclei during this electrolysis process leads directly to the conversion of CO2 into pure graphene nanocarbons, including carbon nanofibers and carbon nanotubes (CNTs) [6].This transformation of the greenhouse gas CO2 into valuable GNC products offers a chance to convert CO2 into a form of carbon stabilized by graphene, thus aiding in mitigating climate change.Graphite is an analogous macroscopic form of layered graphene, and as a mineral graphite has an established geologic (hundreds of millions of years) lifetime.
The CO2 to nanocarbon process, including electrolyte separation and return to the electrolysis chamber, and extraction of the pure GNC product is illustrated Figure S1.As illustrated in Figure 1, during electrolysis CO2 either sourced directly from the air or industrial emissions are transformed to GNCs by electrolysis in molten carbonates.The CO2 is split into C and O2 with a GNC-electrolyte matrix growing at the electrolysis cathode.This nanocarbon/carbonate electrolyte mix has been termed a carbanogel and is refined through the separation of the electrolyte.The high electrical conductivity character of the graphene nano-allotropes supports continuous growth during the CO2 molten electrolysis at low electrolysis voltage.This cathode product grows as an interconnected matrix with electrolyte in the matrix pores.Deriving its name from aerogels, this matrix containing carbonate electrolyte has been termed a carbanogel.Some of the electrolyte in this matrix is rather loosely bound.For example, a post-electrolysis cathode lifted out of the molten electrolyte can release over 30% of the bound electrolyte by gravitational drip.Control of the electrode and electrolyte composition, and CO2 electrolysis splitting temperature and current density tunes the decarbonization process to form a range of high purity graphene nanocarbon products, including carbon nanotubes.Typical SEM, TEM and HAADF (High Angle Annular Dark-Field TEM) elemental analysis imaging of the CNTs are presented in Figure S2, and have been extensively detailed [7].Control of the CO2 electrolysis conditions is used to tune the specific GNC generated by control of the temperature, current density, and the composition of the electrolyte [8].For example, a lower temperature (725°C) is typically used in the electrolytic growth of carbon nano-onions, while higher temperature (750 to 770°C) is used in the electrolytic growth of carbon nanotubes.Lithium carbonate, a typical electrolyte, has a melting point of 723°C.Binary lithium carbonate mixtures have a lower melting point.A high sodium carbonate content in a mixed sodium/lithium carbonate electrolyte and a lower electrolysis temperature (670°C) drive the formation of a graphene scaffold nanocarbon product formation.Applied electrolysis current densities generally range from 0.03 to 0.6 A cm -2 .High current density (0.6 A cm -2 or over) is one of the principal conditions driving the formation of fascinating helical, rather than straight, carbon nanotubes.

Main
Electrode (and electrolyte additive) composition variation has been used to grow a number of other GNC allotropes from CO2.These include carbon nanobamboo, carbon nanopearl, graphene from nanocarbon platelets, carbon nanofiber, carbon nanobelt, carbon nanotree, and other specific carbon allotrope morphologies.SEM of a range of these GNC products is presented in Fig. S3, and XRD and Raman spectra of the products are presented in Figs.S4 and S5 as previously detailed [8].The solid graphene nanocarbon product from CO2 grows as a matrix directly on the cathode.Under constant current electrolysis conditions, the product formation is continuous, and the growth occurs in the direction towards the anode.Figure S6 illustrates larger vertical presses that have been scaled up, which include the transfer of applied pressure to the pressing chamber using a hydraulic ram, as described previously [9].In Figure S6A, there's a cross-sectional depiction of an intermediate scaled-up carbanogel electrolyte extraction unit, detailing the plunger, filter screen platform, and electrolyte exit chamber [9].

Figure S1 .
Figure S1.The CO2 to graphene nanocarbon process.CO2 sourced either from an anthropogenic source (CCUS) or from the air (DAC), panels A and B, is directly transformed into graphene nanocarbons, panel F. The morphology of the graphene nanocarbon is determined by tuning the electrochemical conditions of the molten carbonate CO2 electrolysis, panel C.The nanocarbon product (carbon nanotube exemplified) is separated from the molten electrolyte by a high-pressure, high-temperature extraction press, panel D, and has carbon nanotube morphology, as indicated by SEM panel F and in panel E by TGA resistance to hightemperature oxidation as characterized.With parts modified with copyright permission from X. Wang, G. Licht and S. Licht, Green and scalable separation and purification of carbon materials in molten salt by efficient high-temperature press filtration, Sep.Purif.Technol., 2021, 244 117719, DOI: 10.1016/j.seppur.2020.117719.
Figure underlying data, that is TGA in Figures1, 5 and 7), SEM in Figures1, and

5- 13 ,
Electrolysis Potential and Solubilities measured in Figures 2 and 3 are all included as presented in the the individual Figures.Equilibrium Constants in Figure 4 of the main text are determined as described by Eq 5. Underlying Data for Figure 1, ∆G, kJ/mol

Figure S2 .
Figure S2.SEM TEM and HAADF of the synthesis product of high purity, high yield carbon nanotubes by electrolytic splitting of CO2 in 770°C Li2CO3.The SEM has a scale bar of 5 µm.Panels B are TEM with scale bars decreasing from 100, 20 nm, 5 and 1 nm.Bottom rows panels C are HAADF elemental analyses with scale bars decreasing from 100 to 50 nm, and in the bottom right a HAADF elemental carbon profile analysis of the carbon nanotube cross section. in the bottom right a HAADF elemental carbon profile analysis of the carbon nanotube cross section.Modified from open access paper X.Liu, G. Licht, S. Licht, Controlled Transition Metal Nucleated Growth of Carbon Nanotubes by Molten Electrolysis of CO2 Catalysts 12 (2022) 137.https://doi.org/10.3390/catal12020137.
Figure S6B shows a larger carbanogel electrolyte extraction unit in operation, capable of pressing up to 0.25 tonnes of carbanogel.Presses in the unit with 50 kg carbanogel have already achieved over 99% electrolyte extraction efficiency.