Calcium metaborate induced thin walled carbon nanotube syntheses from CO2 by molten carbonate electrolysis
Xirui Wang, Xinye Liu, Gad Licht, Stuart Licht
Abstract
Abstract An electrosynthesis is presented to transform the greenhouse gas CO 2 into an unusually thin walled, smaller diameter morphology of C arbon N anotubes (CNTs). The transformation occurs at high yield and coulombic efficiency of the 4-electron CO 2 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 CO 2 electrolysis at 0.1 A cm −2 . At this low current density, CO 2 without pre-concentration is directly absorbed by the air (direct air capture) to renew and sustain the carbonate electrolyte. The addition of 2 wt% Li 2 O 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.