**3. Materials and Methods**

#### *3.1. Materials*

Lithium carbonate (Li2CO3, 99.5%), lithium oxide (Li2O, 99.5%), lithium phosphate Li3PO4 (Li3PO4, 99.5%), iron oxide (Fe2O3, 99.9%, Alfa Aesar) and boric acid (H3BO3, Alfa Aesar 99 + %) were used as electrolyte components in this study. For electrodes, Nichrome A (0.04-inch-thick), Nichrome C (0.04-inch-thick), Inconel 600 (0.25-inch-thick), Inconel 625 (0.25-inch-thick), Monel 400, Stainless Steel 304 (0.25-inch-thick) and Muntz Brass (0.25-inch-thick) were purchased from onlinemetals.com. Ni powder was 3–7 μm (99.9%, Alfa Aesar). Cr powder was <10 μm (99.2%, Alfa Aesar). Co powder was 1.6 μm (99.8%, Alfa Aesar). Iron oxide was 99.9% Fe2O3 (Alfa Aesar). Co powder was 1.6 μm (99.8%, Alfa Aesar). Inconel 600 (100 mesh) was purchased from Cleveland Cloth. The electrolysis was a conducted in a high form crucible >99.6% alumina (Advalue).

#### *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 are immersed subsequent to electrolyte melt. For several, as 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 is ramped in several steps building to the applied electrolysis current which is then maintained at a constant current density. Instead, most of the electrolyses are initiated, and held, at a single constant current. The electrolysis temperature was 770 ◦C.

#### *3.3. Product Characterization*

The raw product was collected from the cathode after the experiment and cooldown, followed by an aqueous wash procedure which removes electrolyte congealed with the product as the cathode cools. The washed carbon product was separated by vacuum filtration. The washed carbon product is dried overnight in a 60 ◦C oven, yielding a black powder product.

The coulombic efficiency of electrolysis is the percent of applied, constant current charge that was converted to carbon determined as:

$$100\% \times \text{C}\_{\text{experimental}} / \text{C}\_{\text{theoretical}} \tag{4}$$

This is measured by the mass of washed carbon product removed from the cathode, Cexperimental, and calculated from the theoretical mass, Ctheoretical = (Q/nF) × (12.01 g C mol<sup>−</sup>1) which is determined from Q, the time integrated charged passed during the electrolysis, F, the Faraday (96,485 As mol−<sup>1</sup> e−), and the *n* =4e<sup>−</sup> mol−<sup>1</sup> reduction of tetravalent carbon consistent with Equation (2).

Characterization: The carbon product was washed and analyzed by PHENOM Pro Pro-X SEM (with EDX), FEI Teneo LV SEM, and by FEI Teneo Talos F200X TEM (with EDX). XRD powder diffraction analyses were conducted with a Rigaku D = Max 2200 XRD diffractometer and analyzed with the Jade software package. Raman spectra were collected with a LabRAM HR800 Raman microscope (HORIBA). This Raman spectrometer/microscope uses an incident laser light with a high resolution of 0.6 cm−<sup>1</sup> at 532.14 nm wavelength. The Raman spectrometer/microscope uses an incident 0.13 mW laser light with a high resolution of 0.6 cm−<sup>1</sup> at 532.14 nm wavelength with 1800 gr/mm, 800 mm focal objective, and 100 ms integration. The PHENOM Pro SEM provides over 100,000× electron optical magnification and uses up to a 15 kV acceleration voltage for imaging and analysis. Specifications of the other instruments are available online from the manufacturers.
