Carbon dioxide and other greenhouse gases have been long characterized as one of the leading causes of global warming. And with the seemingly limitless sources of emission — from general breathing of countless living species to vehicular and industrial emissions — the amount of carbon dioxide seems to be ever increasing. It is then, a huge waste of a resource when you consider how comparatively limited the human use of this abundant gas is.
The paper “The O2-assisted Al/CO2 electrochemical cell: A system for CO2 capture/conversion and electric power generation”, published in Science Advances, aims to change that.
Prof. Lynden Archer, chemical and biomolecular engineering, the James A. Friend Family Distinguished Professor of Engineering, and Wajdi Al Sadat, grad — who are the authors of this paper — have created a cell which can use carbon dioxide to produce electricity via electrochemical reactions.
“What we have accomplished is demonstrating of the concept that carbon dioxide can be electrochemically converted to useful products while producing electricity,” Sadat said.
The cell works galvanostatically, meaning energy is produced when the anode and cathode are connected to a load, Sadat explained.
“There are three main parts to the cell. The anode… [is] aluminum metal. The cathode is porous carbon thats electrically conductive and allows the diffusion of O2 and CO2 gases to the electrolyte,” Sadat said. “The electrolyte [is] the the liquid between the anode and cathode. It allows the transport of different ions.”
Aluminium was chosen as the metal because it is the third most abundant metal in the earth’s crust making it inherently cheap, Sadat said. It is also safer to handle and has a high energy density as compared to other metals.
“We acknowledged early on that the selection of an appropriate metal is key to developing any economical capture technology based on electrochemical conversion,” Sadat said.
Aluminum is oxidized meaning it loses electrons to the circuit, and the aluminium ions are released into the electrolyte, while the electrons reach the cathode through an external circuit. When the electrons reach the carbon cathode, oxygen is converted to superoxides through reduction (gaining electrons). This is very important since the highly reactive superoxides enable reactions with mostly unreactive carbon dioxide.
“Through a series of reactions, the reduced CO2 forms C-C bond with other CO2 molecules and forms aluminum oxalate,” Sadat said. “If we continue feeding in aluminum [through the] anode and O2 and CO2 through the cathode and harnessing out the aluminum oxalate, the cell will continue to work producing electrical power and converting CO2 to valuable oxalate.”
The proposed cell is very flexible and can be used on both small (such as vehicular exhaust) and large (industrial and power plants) scales, Sadat said.
“Since the anode is inherently safe to handle and the battery can be configured in flow battery setup, we think increasing the size is very attainable,” Sadat said. “If we incorporate our cell in exhaust streams, the released CO2 can be captured and converted to useful products. An auxiliary stream will also come from air to supply the O2.”
However there are, drawbacks to this technology. The electrolyte in the cell is sensitive to water.
“So, water will have to be separated from exhaust streams if the cell is to be incorporated in industrial applications,” Sadat said.
There are many well established water separation technologies, but the lab is looking to use electrolyte systems that insensitive to water.
“Ongoing work in our lab is looking into different classes of electrolytes that are insensitive to water and even cheaper than the currently used one,” Sadat said. “If we managed to do that the impact will go beyond our aluminum/CO2 cell to aluminum batteries in general.”
Though still in its elementary stages, the technology could have a very real impact in the world very soon. Sadat said that if industrial or power plants were to be retrofitted with the proposed technology, they could reduce CO2 emissions while producing useful products along with electrical power, thus expanding their value chain.
“I think this is a starting platform that can be built on to engineer electrochemical systems that can tackle multiple problems at the same time,” Sadat said.