Capture and Electrochemical Conversion of CO2 into Carbon Nanotubes Using Molten Alkali Metal Borates

Abstract

Molten alkali metal borates have recently shown promise as high-temperature sorbents for capture of CO2 and acid gases. These molten salt sorbents enable realization of thermodynamic enhancements offered by conventional solid high temperature sorbents while resolving practical challenges such as morphological degradation. Prior studies have focused on regeneration of alkali borates through steam sweeping and thermal cycling. In this work, we demonstrate that mixed sodium-lithium borate salts as CO2 sorbents can also be regenerated electrochemically, producing valuable multiwalled carbon nanotubes (MWCNT) via electroreduction of captured CO2. Effects of cathode materials and operating conditions in CO2 electroreduction in molten sodium-lithium borate are quantified. By varying relative starting compositions of alkali borates and alkali carbonates, an optimal composition of borates and carbonates is determined, achieving high coulombic efficiencies and significantly higher CO2 uptake capacities than traditionally employed carbonate salts used for conversion of CO2 into CNTs in the desirable 550-650°C range.