Conjuring batteries out of thin air

An interdisciplinary team of scientists has worked out a way to make electric vehicles not just carbon neutral, but carbon negative, capable of actually reducing the amount of atmospheric carbon dioxide as they operate. They have done this by demonstrating how the graphite electrodes used in the lithium-ion batteries that power electric automobiles can be replaced with carbon material recovered from the atmosphere. The recipe for converting carbon dioxide (CO₂) gas into batteries is described in a paper in ACS Central Science.

The unusual pairing of CO₂ conversion and advanced battery technology is the result of a collaboration between the laboratory of Cary Pint, assistant professor of mechanical engineering at Vanderbilt University, and Stuart Licht, professor of chemistry at George Washington University.

Pint, Licht and their colleagues adapted a solar-powered process that converts CO₂ into carbon so that it would produce carbon nanotubes. They then demonstrated that these nanotubes can be incorporated into the lithium-ion batteries used in electric vehicles and electronic devices and also into the low-cost sodium-ion batteries under development for large-scale energy storage applications.

"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, unlike most efforts to reuse CO₂ that are aimed at low-valued fuels, like methanol, that cannot justify the cost required to produce them," said Pint.

This project builds upon a solar thermal electrochemical process (STEP) for creating carbon nanofibers from ambient CO₂ developed by the Licht group and reported in the journal Nano Letters in August 2015. STEP uses solar energy to provide both the electrical and thermal energy needed to break down CO₂ into carbon and oxygen, and then to produce carbon nanotubes that are stable, flexible, conductive and stronger than steel.

"Our climate change solution is two-fold: (1) to transform the greenhouse gas carbon dioxide into valuable products and (2) to provide greenhouse gas emission-free alternatives to today's industrial and transportation fossil fuel processes," said Licht. "In addition to better batteries other applications for the carbon nanotubes include carbon composites for strong, lightweight construction materials, sports equipment, and car, truck and airplane bodies."

Joining forces with Pint, whose research interests are focused on using carbon nanomaterials for battery applications, the two laboratories worked together to show that the multi-walled carbon nanotubes produced by STEP 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 battery performance, with this boost amplified when the battery was charged quickly. In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned using STEP, can unlock a stable storage performance over three and a half times above that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon nanotube-based batteries were exposed to about two and a half months of continuous charging and discharging without showing any signs of fatigue.

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40% of a battery could be made out of recycled CO₂, Pint estimated. This does not include the outer protective packaging, but he suggested that processes like STEP could eventually produce the packaging as well.

The researchers estimate that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the US Department of Energy in 2013), a kilogram of CO₂ has a value of about $18 as a battery material – six times more than when it is converted to methanol. This value only increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronic devices. And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions.

Licht also proposed that the STEP process could be coupled with 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 by 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.

"Imagine a world where every new electric vehicle or grid-scale battery installation would not only enable us to overcome the environmental sins of our past, but also provide a step toward a sustainable future for our children," said Pint. "Our efforts have shown a path to achieve such a future."

This story is adapted from material from Vanderbilt University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.