CO₂ Utilization by Electrolytic Splitting to Carbon Nanotubes in Non‐Lithiated, Cost‐Effective, Molten Carbonate Electrolytes

Abstract

Carbon nanotubes (CNTs), have extraordinarily high tensile strength, electrical and thermal conductivity, and electrical storage capabilities. To date, CNTs have had limited use due to their high synthesis cost. The synthesis costs decrease when CNTs are prepared by transition metal nucleated molten electrolytic CO2 splitting, rather than by conventional chemical vapor deposition or arc deposition techniques. In addition to cost, a second advantage of CNT electrosynthesis is that the process consumes the greenhouse gas CO2 in its transformation to CNTs (and oxygen), providing a path to climate mitigation. However, electrolytic high yield, CNT syntheses has only been demonstrated with expensive lithiated electrolytes, such as pure or mixed Li2CO3. This study demonstrates, after several hundred failures, that carbon nanomaterials, can be synthesized from CO2 in non‐lithiated electrolytes. In the new sodium barium carbonate electrolytes, lower mobility due to the absence of lithium ions is overcome by: i) low current density; or ii) the introduction of oxides, such as BaO, to induce graphene walls to facilitate passage of larger cations during the process of growing CNTs; and/or iii) addition of specific transition metal nucleation agents, such as Fe2O3. The electrolysis uses inexpensive anodes and cathodes to form carbon nanomaterials, including straight and coiled CNTs.

Overview of CO2 capture and electrolysis technology in molten salts: operational parameters and their effects

“…Li + and Ba 2+ are expected to promote the formation of CNTs, but K + hinders the formation of carbon nanostructures. 80,91 It is further evidence that electrolytes containing more than 50 wt% of Na or more than 30 wt% K carbonates inhibited CNTs growth but supported the development of carbon nano-scaffolds (CNS). 53 Overall, previous studies supported lithium-related salts because of their exceptional qualities.…”

“…78 The metal oxide additives enable not only to capture CO 2 and to initiate the subsequent carbon deposition but also to introduce significant defects into final carbon products (e.g. CNT), 41,80,83 The introduction of over-dosed O 2− has a drawback on CNTs formation. 30 wt% BaO with 0.04 wt% Fe 2 O 3 and 0.04 wt% Cr 2 O 3 additives led to 20% CNTs yield, which was about 10% higher than 7.1 wt% BaO but 60% less than using 0.1 wt% of Fe 2 O 3 .…”

Selective CO2 Electroreduction with Enhanced Oxygen Evolution Efficiency in Affordable Borate-Mediated Molten Electrolyte

“…Nowadays, the mainstream molten electrolyte compositions for exclusively O 2– -mediated mechanisms are usually alkali/alkaline-earth metal carbonates/halides, such as Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 , CaCl 2 -CaCO 3 , Na 2 CO 3 -BaCO 3 , etc., − because the electroreduction of captured CO 2 (i.e., CO 3 2– ) can usually occur in the carbonate-containing electrolytes coexisting with Li + , Ca 2+ , and Ba 2+ due to the thermodynamically suitable electrochemical reaction sequences, where dissolved oxides (e.g., Li 2 O, CaO, BaO, etc.) generated during CO 2 electrolysis can act as both CO 2 absorbent and O 2– donors. , To sustain vigorous and constant CO 2 capture, CO 2 RR, and OER kinetics, a key point for molten salt CO 2 electrolysis is to maintain a relatively high O 2– concentration in bulk electrolyte, because an adequate amount of O 2– not only can enable a pure oxygen production (2O 2– - 4e – → O 2 (g), E 0 = 1.361 V (vs Na + /Na) at 750 °C) with a lower electrode polarization compared with that of CO 3 2– -involved OER (2CO 3 2– −4e – → 2CO 2 (g) + O 2 (g), E 0 = 2.326 V (vs Na + /Na) at 750 °C) but also can efficiently capture CO 2 (CO 2 (g) + O 2– → CO 3 2– ) either from the atmosphere or from the anode (originating from CO 3 2– -involved OER), with considerable absorption kinetics. , However, it is still challenging to achieve this attempt over a wide temperature range, not only because of the limited O 2– solubility and sluggish O 2– mass transfer in specific molten compositions , but also because of cathodic passivation by which Li + /Ca 2+ (and potentially Ba 2+ ) can easily trap O 2– to form Li 2 O/CaO (potentially BaO) solids at the cathode, decreasing the O 2– concentration in the bulk electrolyte. , Among various molten salt electrolytes, molten carbonates are promising electrolyte candidates for CO 2 RR due to their higher CO 2 solubility, which endows faster reaction kinetics; , particularly, either lithium- or barium-containing molten carbonates are most commonly used because CaO is hardly dissolved in major molten carbonates, , but the dependence on Li + further increases the overall cost due to limited lithium sources.…”

“…And apparently, the residues in the filtrate after the acid treatment are difficult to recycle. Likewise, although carbon nanotubes can also be obtained in another Li-free Na 2 CO 3 -BaCO 3 , 17 an acid leaching treatment is still needed to remove BaCO 3 due to the limited water solubility. To further verify the high purity of the carbon product obtained in NK-Na 2 B 4 O 7 -0.05, TGA results show that the impurity of the carbon sample after water scrubbing was merely ∼1 wt% (Figure S11).…”

Carbon Graphitization: Towards Greener Alternatives to Develop Nanomaterials for Targeted Drug Delivery

“…An approach that is attracting great attention is the electrolysis of CO 2 in molten salt, introduced by The Licht C2CNT (carbon dioxide to CNT) group [ 151 ]. Various CNMs were recently obtained with this method, including CNOs, CNTs, graphene, carbon nanofibers, and other more exotic structures termed carbon nanodragons or nanoflowers [ 77 , 78 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 141 ]. Briefly, experiments were carried out in an alumina crucible containing an electrolyte (typically lithium carbonate), which was heated up to melting.…”