A group of researchers have developed a battery that is recharged with waste CO2. The invention’s inexpensive materials and use of a waste product to produce electricity may not only lower energy prices, but also reduce carbon emissions at the same time. Given that power stations are releasing so much carbon dioxide that it is considered a pollutant, this discovery could prove revolutionary.
The breakthrough was made at Pennsylvania State University, where chemists Bruce E. Logan, Christopher A. Gorski and Taeyong Kim, were able to capture the chemical energy in the difference between industrial CO2 emissions and ambient air. Whilst this has been achieved before, previous efforts produced only low power densities and required expensive ion-exchange membranes. This new technique however, is much more efficient, such that the researchers are hopeful that the process can be scaled up. As Gorski explained, “This work offers an alternative, simpler means to capturing energy from CO2 emissions compared to existing technologies that require expensive catalyst materials and very high temperatures to convert CO2 into useful fuels.”
Publishing their results in the American Chemical Society journal Environmental Science and Technology, the research team state that, “The pH-gradient flow cell produced an average power density of 0.82 W/m2, which was nearly 200 times higher than values reported using previous approaches.”
Reporting on the breakthrough, the online science journal Phys.org describes the process as follows, “In order to harness the potential energy in this [CO2] concentration difference, the researchers first dissolved CO2 gas and ambient air in separate containers of an aqueous solution, in a process called sparging. At the end of this process, the CO2-sparged solution forms bicarbonate ions, which give it a lower pH of 7.7 compared to the air-sparged solution, which has a pH of 9.4.”
It continues by identifying how the CO2 solution is then used in a ‘flow cell’ to extract the chemical energy. “After sparging, the researchers injected each solution into one of two channels in a flow cell, creating a pH gradient in the cell. The flow cell has electrodes on opposite sides of the two channels, along with a semi-porous membrane between the two channels that prevents instant mixing while still allowing ions to pass through. Due to the pH difference between the two solutions, various ions pass through the membrane, creating a voltage difference between the two electrodes and causing electrons to flow along a wire connecting the electrodes.”
When the flow cell has been discharged, it can simply be recharged by switching the channels that the solution flows through. By alternating the solution that flows over each electrode, the charging mechanism is reversed so the electrons flow in the opposite direction. This process was found to be repeatable up to 50 times before the cell’s performance deteriorated.
The researchers also found that the higher the pH difference between the two channels, the higher the average energy density. Overall, the results were much better than other pH-gradient flow-cells, but were still some way off the power levels supplied by cells that included other fuels, such as H2. But the research team is still continuing their study, in the hope that they can prove the technology to be workable on an industrial scale.
“We are currently looking to see how the solution conditions can be optimized to maximize the amount of energy produced,” Gorski said. “We are also investigating if we can dissolve chemicals in the water that exhibit pH-dependent redox properties, thus allowing us to increase the amount of energy that can be recovered. The latter approach would be analogous to a flow battery, which reduces and oxidizes dissolved chemicals in aqueous solutions, except we are causing them to be reduced and oxidized here by changing the solution pH with CO2.”
Early results prove promising, with Gorski upbeat about the projects potential when he said, “A system containing numerous identical flow cells would be installed at power plants that combust fossil fuels. The flue gas emitted from fossil fuel combustion would need to be pre-cooled, then bubbled through a reservoir of water that can be pumped through the flow cells.”
While, this may seem some way off from a real-world application, many fuel-cell experts are already hailing the discovery as a crucial step towards both cheaper energy and reduced carbon emissions. However, what makes this new design so special is that it has many notable advantages over earlier ‘flow-cell’ models. Its use of inexpensive materials, its operation at ambient temperatures, and above all its use of CO2 as a feedstock, makes this discovery an attractive and practical possibility for existing power stations.
Photo credit: Logan, Gorski and Kim