Conversion Tech Could Turn CO2 Emissions Into Battery Electrolytes
The accelerating effects of climate change have led to growing agreement that carbon capture sequestration and storage (CSS) will need to be part of the solution, Yet, so far attempts to capture CO2 from power plants have proven too expensive from both a dollar and carbon footprint perspective.
Researchers around the world are exploring solutions to simplify and reduce the energy cost of CO2 capture for storage or transformation into useful products. At MIT’s Gallant Research Group, Mechanical Engineering Assistant Professor Betar Gallant has demonstrated promising results for an electrochemical conversion technology that could directly add value to CO2 waste by enabling its use in an energy storage device.
As detailed in an Oct. 2018 paper in Joule co-authored with doctoral student Aliza Khurram and postdoc Mingfu He, Gallant’s technique pre-activates the CO2 in an aqueous amine solution before further converting it within a solid phase electrolyte environment. The resulting carbamate solution can then be used as an electrolyte to drive a CO2 battery. Alternatively, it could be used for continuous power generation within the power plant.
“We’d like to capture CO2 and ideally recycle it and convert it to something valuable,” says Gallant. “If we can convert CO2 to a solid that could be used for energy storage, we could close the carbon cycle.”
Our group is interested in future technologies that will define and drive energy storage as well as greenhouse gas capture and conversion.
The Gallant Research Group is also working on a related technology designed to capture and convert the world’s most powerful class of greenhouse gases: perfluorinated gases. For example, each molecule of sulphur hexafluoride, which is used in industrial processes such as semiconductor manufacturing, has about 30,000 times higher warming impact than CO2, primarily due to its much longer lifespan. Gallant’s technology pairs a gas cathode with a lithium anode to process the sulphur hexafluoride to create a far less emissions-intensive product such as lithium fluoride. The material can even be used to power a modest battery that currently generates 2.2 to 2.3 Volts -- enough to power sensors.
“Our group is interested in future technologies that will define and drive energy storage as well as greenhouse gas capture and conversion,” says Gallant. “As much as possible we like to explore the intersection between these two technologies.”
Despite all the talk about carbon capture and storage, very few power utilities or other CO2 producing industries are doing it. Widespread application of the incentivization mechanisms called for in Paris Agreement would make the technology more feasible, but it would still be expensive. “There are high energy penalties and selectivity challenges in both capturing the CO2 and converting it to a product,” says Gallant.
The typical CSS pilot project captures CO2 by “bubbling” gaseous emissions through an absorber that contains a chemical specifically tailored to bind only to CO2. The bound CO2 is then flowed through a thermal regeneration process that reversibly removes the CO2 to the gas phase and returns the amine to its original state, ready for more capture. Typically, the gas-phase CO2 is then compressed or converted to a liquid or gas before storing it deep underground. Researchers are also experimenting with ways to turn the gas into usable products such as fuels.
The biggest flaw in traditional CSS is that it requires up to 30 percent of a power plant’s power output to regenerate the CO2 back to the gas phase. Add to that the energy and dollar costs from drilling and pumping for storage or converting the CO2 into a product.
One approach for converting captured and separated CO2 is to perform an aqueous reduction reaction. Here, the CO2 is introduced as a dissolved reactant into an electrochemical cell, which reduces the CO2 to form a product such as methane, formic acid, or a solid mineral carbonate.
“In more conventional heterogeneous electro-catalysis, an inert CO2 module interacts with a metal catalyst that alters the electronic configuration of the CO2 and facilitates pathways for it to undergo subsequent reaction steps by electron or ion transfer,” says Gallant. “The problem is that it requires significant energy input, and the catalysts are not very selective. It’s a very complex pathway, requiring up to eight elemental reaction steps that necessitates using combinations of optimized catalysts for individual steps. There is a lot of work on developing more selective catalysts, but it’s quite challenging.”
Gallant’s alternative approach reduces this complexity by avoiding metal catalysts entirely. It starts the conversion process by introducing CO2 into an aqueous amine conversion. The capture step, which occurs within the electrolyte, can then be used for discharge in a solid-state lithium or sodium battery. The process borrows amine conversion techniques from traditional CSS but applies them differently.
“Rather than using metal electrodes for activating the CO2, we use the liquid electrolyte phase,” explains Gallant. “We shift the CO2 reaction out of the aqueous space into a nonaqueous phase where we can change the electrochemical environment to design new reactions and control the reactivity. We can also restrict the products that can be formed. By the time we’re doing the electrochemistry, the CO2 had already converted into a more reactive form.”
Gallant’s approach differs from other CO2 battery technologies, which do not use amines and instead use metal catalysts to strive for rechargeability. “Typical CO2 batteries consist of a lithium or sodium anode, a CO2 cathode, and some kind of carbon-metal composite material in which the reaction occurs,” says Gallant. “One can discharge and recharge the battery to a very limited extent. The charging is limited by the high voltage required to decompose the carbonate materials and revert it to CO2.”
Gallant has demonstrated that her conversion process can produce a usable electrolyte product to act as a discharge or reduction mechanism in a battery. As an important side benefit, the electrolyte environment could prove fertile ground for more fundamental research into improving the selectivity of CO2 conversion reactions. “It provides a better platform to help us understand fundamentally how the CO2 is activated, how it reacts, and how we can learn to manipulate its conversion for high efficiency,” says Gallant.
The ideal system would perform the electrochemistry, remove the CO2, regenerate the absorber solution, and operate on a near-continuous basis.
Ultimately, the ideal use for Gallant’s conversion technique may not be a battery, but rather the ability to augment power in a power plant. “We want to flow the combustion gases through an absorber and then circulate the resulting electrolyte through a separate electrochemical process,” says Gallant. “The reactions are downhill, so at least conceptually, we could get power out of the process on-site. The ideal system would perform the electrochemistry, remove the CO2, regenerate the absorber solution, and operate on a near-continuous basis.”
The biggest challenge here is how to manage the solid formation in a continuous process, says Gallant. “We need a way to manage and remove the solid so we can keep the electrodes clear for a sustained reaction.”
Gallant speculates that her research “may be of value to the community currently attempting to develop rechargeable Li-CO2 batteries. My hope is to one day develop a primary battery -- a battery that only discharges. This would allow CO2 to ultimately be captured and safely converted to an environmentally benign solid alkali carbonate, one that could be removed and safely and permanently stored above-ground.”
Indeed, even if the CO2 battery or continuous power generator schemes don’t pan out, the technology could lead to lower costs for carbon storage. “Solid phases store CO2 in a very dense form in which there’s no risk of accidental release like you’d have in a compressed liquid state in geological storage,” says Gallant. “If we can shift away from using lithium, which is rather costly and rare, to a cheaper, geologically compatible alkali or alkaline earth carbonate, you could dispose of it safely and affordably or even use it in building materials.”
The process of creating a complete CSS system and developing a battery that runs on carbonate is at least five years away. More work needs to be done to design a more robust CO2 conversion system that can operate for thousands of hours.
The continuous power regeneration project will take several years longer. “It depends on how many people we can get working on the project,” says Gallant. “We need two speeds: investment in fundamental research into the molecular processes that underlie these transformations, as well as taking these processes to a material and device prototyping stage.”