Each year billions of tons of carbon dioxide and other greenhouse gases are released into the atmosphere by the burning of fossil fuels, certain industrial processes, construction and other human activities, creating an urgent need to find better solutions to reduce the levels of atmospheric carbon dioxide.
A team of scientists led by Haotian Wang , associate professor in chemical and biomolecular engineering at the George R. Brown School of Engineering and Computing at Rice University, and Xiaonan Shan , associate professor of electrical and computer engineering at University of Houston, have discovered simple yet elegant solutions to address a fundamental issue in carbon capture and utilization technology — carbon dioxide reduction reaction (CO2RR). The study was published recently in Nature Energy .
"This advancement paves the way for longer-lasting and more reliable (CO2RR) systems, making the technology more practical for large-scale chemical manufacturing," Shan said. "The improvements we developed are crucial for transitioning CO 2 electrolysis from laboratory setups to commercial applications for producing sustainable fuels and chemicals."
Salt buildup a major operational issue
CO2RR is a newly emerging carbon capture and utilization technique in which electricity — preferably generated by renewable sources like solar energy — and specific chemical catalysts are used to convert carbon dioxide gas into carbon-containing compounds like alcohols, ethylene, formic acids or carbon monoxide that can be used as fuels, chemicals or as starting materials to produce other compounds.
This technology is used in commercial membrane electrode assembly (MEA) electrolyzers to convert carbon dioxide into valuable compounds. However, this technology has one major setback: Over time, bicarbonate salt crystals accumulate on the backside of the cathode gas diffusion electrode and within the gas flow channels.
These salt precipitates block the flow of carbon dioxide gas through the cathode chamber, reducing the performance and causing the eventual failure of the electrolyzers.
"Operational instability is a big hurdle in the wider adoption of this technology," Wang said. "The device functions normally for a few hundred hours after which it stops working due to the buildup of salt. Our goal in undertaking this study was to understand why and how bicarbonate salts form during this reaction, which we hoped would lead us to some preventive solutions that can extend the life of this device."
Identifying the cause
One of the primary challenges was understanding the mechanism behind salt formation and migration within the MEA reactor, Shan said.
"Salt accumulation is problematic because it leads to the formation of bicarbonate salt particles in the gas diffusion electrode (GDE) and gas flow channels," Shan said. "These precipitates block CO2diffusion pathways, impeding the flow of reactant gases to the catalyst sites, and could potentially damage the membrane of the reactor as well."
To understand why these salt crystals form, Wang and his team at Rice collaborated with Shan and his team at UH who are experts in operando Raman spectroscopy, a powerful technique that allows researchers to study the structure of materials and any precipitates that adhere to it while the device is functioning.
"Our studies revealed that during this reaction, the microenvironment at the interface of the catalyst and anion electrode membrane is always alkaline," Shan said. "This allows the hydroxide molecules to easily react with the acidic carbon dioxide molecules to form carbonate ions, which can then bind with the positively charged ions (cations) like sodium or potassium present there to form the bicarbonate salt deposits as they migrate towards the cathode."
Finding solutions
The researchers' next goal was to figure out ways to prevent these salt crystals from forming inside the gas flow channel.
"By utilizing operando Raman spectroscopy and optical microscopy, we successfully tracked the movement of bicarbonate-containing droplets and identified their migration pattern," Shan said. "This provided us the information to develop an effective strategy to manage these droplets without interrupting system stability."
The first idea they tested was whether lowering the concentration of cations like sodium or potassium in the electrolyte would slow down the salt formation.
Indeed, they found this was an effective solution. Reducing the concentration of the cations in the system prevented their crossover to the cathode, slowing down salt buildup and improving the reactor's long-term functional stability.
"As we visualized this reaction using optical microscopy, we saw an interesting phenomenon. The bicarbonate crystals formed and remained trapped in droplets initially. With time, the droplets would evaporate leaving the salt crystals behind," Wang said.
This simple but astute observation led them to think of another creative solution to solve the problem.
"Inspired by the waxy surface of the lotus leaf which causes water droplets to bead up and roll off, carrying off any dirt particles with it and leaving the leaf's surface clean, we wondered if coating the gas flow channel with a nonstick substance will prevent salt-laden droplets from staying on the surface of the electrodes for too long and, therefore, reduce salt buildup," Wang said.
To test this idea, they coated the cathode gas flow channels of the MEA electrolyzer with parylene, a synthetic polymer like Teflon, that repels water. They found that parylene-coated gas channels flushed out substantially higher amounts of cations like potassium compared to a noncoated system, which notably improved the stability of the electrolyzer.
"Currently, salt-crusted electrodes and other affected components of the MEA electrolyzer need to be replaced after a few hundred hours of run time, but with our improvements, the functional stability of this device can be extended to more than 1,000 hours," Wang said. "We are excited by this significant improvement in the device's performance and life, and we believe that the easy scalability of these solutions to commercial applications will drive wider adoption of CO2RR technology as a means to manufacture chemicals and combat climate change."
This work was supported by the Robert A. Welch Foundation, the David and Lucile Packard Foundation, the UL Research Institutes, a Department of Agriculture Small Business Innovation Research and Technology award and grants from the University Training and Research for Fossil Energy Applications and the Department of Defense's Defense University Research Instrumentation Program.
