Combining capture and disposal
NANCY W. STAUFFER MAY 15, 2019 MITEI
Some power plants use materials called sorbents to remove carbon dioxide (CO2) from their exhaust so it can be sequestered from the environment. But separating the CO2from the sorbent requires high temperatures and produces CO2 gas that must be put into long-term storage—a prospect that raises safety and security concerns. In a proof-of-concept study, MIT researchers have demonstrated a battery-like system that uses the same CO2-capturing sorbent in a specially designed electrolyte that drives electrochemical reactions with three benefits: They separate the CO2 from the sorbent; they promote the discharge of electricity from the battery; and they incorporate the CO2into a solid that can serve as electrode material or be safely discarded. Their system—made of lithium and carbon electrodes plus the special electrolyte—achieves discharge voltages similar to those of other lithium-gas batteries under development. The researchers are now working to understand and optimize their lithium-based system and to see whether less-expensive, earth-abundant metals might work as well.
Reducing CO2 emissions from power plants is widely considered an essential component of any climate change mitigation plan. Many research efforts focus on developing and deploying carbon capture and sequestration (CCS) systems to keep CO2 emissions from power plants out of the atmosphere. But separating the captured CO2 and converting it back into a gas that can be stored can consume up to 25% of a plant’s power-generating capacity. In addition, the CO2 gas is generally injected into underground geological formations for long-term storage—a disposal method whose safety and reliability remain unproven.
A better approach would be to convert the captured CO2 into useful products such as value-added fuels or chemicals. To that end, attention has focused on electrochemical processes—in this case, a process in which chemical reactions release electrical energy, as in the discharge of a battery. The ideal medium in which to conduct electrochemical conversion of CO2 would appear to be water. Water can provide the protons (positively charged particles) needed to make fuels such as methane. But running such “aqueous” (water-based) systems requires large energy inputs, and only a small fraction of the products formed are typically those of interest.
Betar Gallant, an assistant professor of mechanical engineering, and her group have therefore been focusing on non-aqueous (water-free) electrochemical reactions—in particular, those that occur inside lithium-CO2 batteries.
Research into lithium-CO2 batteries is in its very early stages, according to Gallant, but interest in them is growing because CO2 is used up in the chemical reactions that occur on one of the electrodes as the battery is being discharged. However, CO2 isn’t very reactive. Researchers have tried to speed things up by using different electrolytes and electrode materials. Despite such efforts, the need to use expensive metal catalysts to elicit electrochemical activity has persisted.
Given the lack of progress, Gallant wanted to try something different. “We were interested in trying to bring a new chemistry to bear on the problem,” she says. And enlisting the help of the sorbent molecules that so effectively capture CO2 in CCS seemed like a promising way to go.
The sorbent molecule used in CCS is an amine, a derivative of ammonia. In CCS, exhaust is bubbled through an amine-containing solution, and the amine chemically binds the CO2, removing it from the exhaust gases. The CO2—now in liquid form—is then separated from the amine and converted back to a gas for disposal.
In CCS, those last steps require high temperatures, which are attained using some of the electrical output of the power plant. Gallant wondered whether her team could instead use electrochemical reactions to separate the CO2 from the amine—and then continue the reaction to make a solid, CO2-containing product. If so, the disposal process would be simpler than it is for gaseous CO2. The CO2 would be more densely packed, so it would take up less space; and it couldn’t escape, so it would be safer. Better still, additional electrical energy could be extracted from the device as it discharges and forms the solid material. “The vision was to put a battery-like device into the power plant waste stream to sequester the captured CO2 in a stable solid, while harvesting the energy released in the process,” says Gallant.
Research on CCS technology has generated a good understanding of the carbon-capture process that takes place inside a CCS system. When CO2 is added to an amine solution, molecules of the two species spontaneously combine to form an “adduct,” a new chemical species in which the original molecules remain largely intact. In this case, the adduct forms when a carbon atom in a CO2 molecule chemically bonds with a nitrogen atom in an amine molecule. As they combine, the CO2 molecule is reconfigured: It changes from its original, highly stable, linear form to a “bent” shape with a negative charge—a highly reactive form that’s ready for further reaction.
In her scheme, Gallant proposed using electrochemistry to break apart the CO2-amine adduct—right at the carbon-nitrogen bond. Cleaving the adduct at that bond would separate the two pieces: the amine in its original, unreacted state, ready to capture more CO2, and the bent, chemically reactive form of CO2, which might then react with the electrons and positively charged lithium ions that flow during battery discharge (see the diagram below). The outcome of that reaction could be the formation of lithium carbonate (Li2CO3), which would deposit on the carbon electrode.
At the same time, the reactions on the carbon electrode should promote the flow of electrons during battery discharge— even without a metal catalyst. “The discharge of the battery would occur spontaneously,” Gallant says. “And we’d break the adduct in a way that allows us to renew our CO2 absorber while taking CO2 to a stable, solid form.”
A process of discovery
In 2016, Gallant and doctoral student Aliza Khurram of mechanical engineering began to explore that idea.
Their first challenge was to develop a novel electrolyte. A lithium-CO2 battery consists of two electrodes—an anode made of lithium and a cathode made of carbon—and an electrolyte, a solution that helps carry charged particles back and forth between the electrodes as the battery is charged and discharged. For their system, they needed an electrolyte made of amine plus captured CO2 dissolved in a solvent—and it needed to promote chemical reactions on the carbon cathode as the battery discharged.
They started by testing possible solvents. They mixed their CO2-absorbing amine with a series of solvents frequently used in batteries and then bubbled CO2 through the resulting solution to see if CO2 could be dissolved at high concentrations in this unconventional chemical environment. None of the amine-solvent solutions exhibited observable changes when the CO2 was introduced, suggesting that they might all be viable solvent candidates.
However, for any electrochemical device to work, the electrolyte must be spiked with a salt to provide positively charged ions. Because it’s a lithium battery, the researchers started by adding a lithium-based salt—and the experimental results changed dramatically. With most of the solvent candidates, adding the salt instantly caused the mixture either to form solid precipitates or to become highly viscous—outcomes that ruled them out as viable solvents. The sole exception was the solvent dimethyl sulfoxide, or DMSO. Even when the lithium salt was present, the DMSO could dissolve the amine and CO2.
“We found that—fortuitously—the lithium-based salt was important in enabling the reaction to proceed,” says Gallant. “There’s something about the positively charged lithium ion that chemically coordinates