An interdisciplinary team of scientists has worked out a way to make electric vehicles (EVs) that are not only carbon neutral but carbon negative—by replacing the graphite electrodes used in EVs' lithium-ion batteries with carbon material recovered from the atmosphere.

The unusual pairing of carbon dioxide conversion and advanced battery technology is the result of a collaboration between the laboratory of Assistant Professor of Mechanical Engineering Cary Pint, at Vanderbilt University, and Professor of Chemistry Stuart Licht, at George Washington University. The team adapted a solar-powered process that converts carbon dioxide into carbon so that it produces carbon nanotubes, demonstrating that the nanotubes can be incorporated into both lithium-ion batteries like those used in EVs and low-cost sodium-ion batteries under development for large-scale applications, such as the electric grid.

“This approach not only produces better batteries, but it also establishes a value for carbon dioxide recovered from the atmosphere that is associated with the end-user battery cost," says Pint. Most efforts to reuse CO₂ are aimed at low-value fuels, like methanol, that cannot justify the cost required to produce them, he adds.

The project builds upon a solar thermal electrochemical process (STEP) that can create carbon nanofibers from ambient carbon dioxide. STEP uses solar energy to provide both the electrical and thermal energy necessary to break down carbon dioxide into carbon and oxygen and to produce carbon nanotubes that are stable, flexible, conductive and stronger than steel.

STEP converts atmospheric CO2 into carbon nanotubes for use in advanced batteries. Image credit: Julie Turner/Vanderbilt University. "Our climate-change solution is twofold: to transform the greenhouse gas carbon dioxide into valuable products and to provide greenhouse gas emission-free alternatives to today’s industrial and transportation fossil fuel processes," says Licht.

The two laboratories worked together to show that the multi-walled carbon nanotubes produced by the process can serve as the positive electrode in both lithium-ion and sodium-ion batteries.

In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly. In sodium-ion batteries, the researchers found that small defects in the carbon can unlock stable storage performance more than 3.5 times that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40% of a battery can be made out of recycled CO₂, Pint estimates.

The researchers estimate that with a battery cost of $325 per kWh, a kilogram of carbon dioxide has a value of about $18 as a battery material—six times more than when it is converted to methanol—a number that only increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronics. And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions.

Licht also proposes that the STEP process can be coupled to a natural gas-powered electrical generator. The generator would provide electricity, heat and a concentrated source of carbon dioxide that would boost the performance of the STEP process.

At the same time, the oxygen released in the process could be piped back to the generator where it would boost the generator’s combustion efficiency to compensate for the amount of electricity that the STEP process consumes. The end result could be a fossil fuel electrical power plant with zero net CO₂ emissions.