*3.2. Electrolysis and Purification*

Specific electrolyte compositions of each electrolyte are described in the text. The electrolyte was pre-mixed by weight in the noted ratios then metal or metal oxide additives were added if used. The cathode was mounted vertically across from the anode and immersed in the electrolyte. Generally, the electrodes were immersed subsequent to electrolyte melting. For several noted electrolyses, once melted, the electrolyte was maintained at 770 ◦C ("aging" the electrolyte) prior to immersion of the electrolytes, followed by immediate electrolysis. Generally, the electrolysis was driven with a described constant current density. As noted, for some electrolyses, the current density was ramped in several steps building to the applied electrolysis current, which was then maintained at a constant current density. Instead, most of the electrolyses were initiated, and held, at a single constant current. The electrolysis temperature was 770 ◦C using CO2 directly from the air. In the C2CNT process, the electrolytic splitting can occur as direct air carbon capture without CO2 pre-concentration [31–34,36–44,47–51], or with concentrated CO2, or CO2 exhaust gas including during the scaling up of this process, in which the CO2 transformation to CNTs was awarded the 2021 Carbon XPrize XFactor award for producing the most valuable product from CO2 [32,45,46,62–64]. In this study, as the electrolysis cell directly captured CO2 from the air via Equation (1), no additional introduction of CO2 was needed. A simple measure of sufficient CO2 uptake is whether the electrolyte level falls during the course of the electrolyte. As mentioned, the 13C isotope of CO2 was previously used to track carbon through the C2CNT process from its origin (CO2 as a gas phase reactant) through its transformation to a CNT or carbon nanofiber product, and the CO2 originating from the gas phase served as the renewable C building blocks in the observed CNT product [32]. If CO2 uptake was insufficient, then the carbonate electrolyte was instead consumed in accord Equation (2), rather than renewed in accord with Equations (1) and (2) in tandem. For example, when conducting at high electrolysis rates of 1 A/cm<sup>2</sup> or greater (not the situation of this study), then gas containing CO2 needed to be bubbled into the electrolyte, otherwise the electrolyte was consumed and the level of electrolyte visibly fell [40].
