Molten alkali metal borates are a class of molten salts that have recently shown promise as high-temperature sorbents for capture of CO2 and other acid gases. Thermal swing systems based on molten borates have demonstrated CO2 capture capacities greater than those of amines, enabling efficient recovery of high-temperature heat in flue gas without practical concerns commonly associated with solid sorbents at these temperatures. In this work, we exploited generation of carbonates upon CO2 capture by borates to enable their use as electrolytic media for carbon nanotube (CNT) synthesis by CO2 splitting. Here, we report the conditions necessary to synthesize valuable multiwalled CNTs by CO2 capture and conversion as a sustainable alternative to conventional carbon-intensive CNT synthesis techniques. Effects of cathode materials and operating conditions are quantified in sodium lithium borate, achieving significantly higher CO2 uptake capacities than alkali metal carbonate salts for conversion of CO2 into CNTs in the 550–650 °C range.