Graphite, the carbon core of a humble No. 2 pencil, is also an essential component in technologies such as batteries, smartphones, laptops and industrial power equipment. Today, nearly all of this critical mineral must be mined and processed and, in the United States, imported.

But now, researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), UC Berkeley and Estonia's National Institute of Chemical Physics and Biophysics have shown a promising way to convert waste carbon pulled from the air into graphite, opening up a potential alternative to mining. The work was published recently in the journal Nature Communications.

Researchers built a custom microscope setup to watch a process known as molten-salt electrolysis, which uses electricity and hot liquid salts to turn carbon dioxide into solid carbon. For the first time, researchers were able to watch the process in real time inside corrosive molten salts heated to 500°C (932°F) while the system was running.

The observations answered a decades-old question about how the reaction occurs at the molecular level, revealing an unexpected two-step process. The researchers also found that the basic chemical reaction remained the same even when they changed the materials used for the electrodes and molten salts. Because different materials produce different carbon structures, scientists should be able to tune the process to make valuable carbon products, with the goal of making battery-grade graphite.