Calcium metaborate induced thin walled carbon nanotube syntheses from CO2 by molten carbonate electrolysis

Abstract

An electrosynthesis is presented to transform the greenhouse gas CO2 into an unusually thin walled, smaller diameter morphology of Carbon Nanotubes (CNTs). The transformation occurs at high yield and coulombic efficiency of the 4-electron CO2 reduction in a molten carbonate electrolyte. The electrosynthesis is driven by an unexpected synergy between calcium and metaborate. In a pure molten lithium carbonate electrolyte, thicker walled CNTs (100–160 nm diameter) are synthesized during a 4 h CO2 electrolysis at 0.1 A cm−2. At this low current density, CO2 without pre-concentration is directly absorbed by the air (direct air capture) to renew and sustain the carbonate electrolyte. The addition of 2 wt% Li2O to the electrolyte produces thinner, highly uniform (50–80 nm diameter) walled CNTs, consisting of ~ 75 concentric, cylindrical graphene walls. The product is produced at high yield (the cathode product consists of > 98% CNTs). It had previously been demonstrated that the addition of 5–10 wt% lithium metaborate to the lithium carbonate electrolyte boron dopes the CNTs increasing their electrical conductivity tenfold, and that the addition of calcium carbonate to a molten lithium carbonate supports the electrosynthesis of thinner walled CNTs, but at low yield (only ~ 15% of the product are CNTs). Here it is shown that the same electrolysis conditions, but with the addition of 7.7 wt% calcium metaborate to lithium carbonate, produces unusually thin walled CNTs uniform (22–42 nm diameter) CNTs consisting of ~ 25 concentric, cylindrical graphene walls at a high yield of > 90% CNTs.

Spiers Memorial Lecture: CO2 utilization: why, why now, and how?

“…The needs for carbon black are large and cover products such as toner for printing, llers for polymers and concrete, and many more. [91][92][93][94][95] Higher-value products such as carbon nanotubes 96,97 or diamonds 98 can also be made from CO 2 .…”

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.…”

Carbon Dioxide Conversion to Nanomaterials: Methods, Applications, and Challenges

“…(g) Thin-walled and small-diameter CNTs obtained with calcium metaborate addition. This figure was adapted with permission from ref . Copyright 2016 Creative Commons CC BY license.…”