For every ton of ethylene created, one ton of carbon dioxide is produced. With more than 300 million tons of ethylene produced each year, the production system has a huge carbon footprint that scientists and engineers are eager to reduce and eventually eliminate.
A new device developed in Ted Sargent's lab at Northwestern, takes a step toward breaking that cycle.
The device, an electrolyzer, has three innovations. It uses electricity to create ethylene from syngas, a waste gas produced from plastic. It uses a novel material to help catalyze the reaction. And it does so in an efficient way — reducing the overall energy needed for the system.
The results, published Feb. 17 in Nature Energy, can be used with renewable energy sources to help pave the way for a greener ethylene supply chain.
"Our goal is to decarbonize chemicals," Sargent said. "And this work is a big step in that direction."
Sargent is the Lynn Hopton Davis and Greg Davis Professor of Chemistry at Northwestern's Weinberg College of Arts and Sciences and a professor of electrical and computer engineering at Northwestern's McCormick School of Engineering.
"We want to create a circular system that creates chemical building blocks from waste without using fossil fuels," said Ke Xie, a research faculty member in chemistry at Weinberg. "And this system is part of that new atom-efficient and energy-efficient supply chain."
Creating energy from waste
Currently, the majority of ethylene is created through steam cracking, which converts crude oil into chemicals using high-temperature steam. Scientists are working to develop new processes that use electricity from renewable sources instead.
One idea is to convert carbon dioxide itself into ethylene. But because this process is too energy-intensive, Sargent and his team turned their focus to converting syngas. A gas available from gasifying plastic waste, syngas is made up of carbon monoxide and hydrogen. That means the electricity required to turn it into ethylene is much lower than carbon dioxide.
"A lot of syngas is made into chemicals, so finding a route to take the syngas to ethylene that's both very selective and very energy efficient is of industrial interest," Sargent said.
But to turn syngas into ethylene efficiently, Sargent's team needed to develop a new kind of electrolyzer — a cell that uses energy to drive a chemical reaction. Most such devices use water as a liquid electrolyte, one that also contains salt, including both cations and anions. Sargent's team wondered if they could develop a device that was gas-fed on both sides of the reaction — one side (the cathode) with the carbon monoxide present in syngas, the other (the anode) fed using its hydrogen.
"In initial attempts, we tried to make a gas-gas electrolyzer, but it just didn't work," he said. "And what we realized was that we didn't just need the water — we needed the salt."
A novel device that works with renewable energy
Salt provides the positive ions (cations) that the device's copper catalyst needs to stabilize key intermediates in the reaction. Bosi Peng, a postdoctoral researcher in the lab and first author on the paper, searched for the right material that could trap those ions while also keeping them loose enough to react within the system.
"We needed to find a material in this Goldilocks zone to make a successful electrolyzer," Sargent said. "And Bosi found a new way to solve this hard problem, which was really exciting."
The material, sodium polyacrylate (PANa), creates a micro-environment within the system that mimics a liquid salt bath, while keeping the system dry of liquid water. The result is a process that is more than 60% more efficient than the most energy-efficient prior electrified processes that turn carbon dioxide into ethylene.
"Bosi significantly reduced the electricity needed by lowering the voltage we have to apply across the device," Sargent said. Even further, the device works well with the intermittent nature of renewable energy sources.
"Solar and wind are very cheap sources of energy, but they come and go," he said. "We needed to create a device that could deal with intermittent energy, and we found this system could do that. A key ingredient in doing so was to take out the liquid water with the high concentration salt in the electrolyte."
Next the team plans to try to reduce the energy consumption of the device even further, so it's on par with energy used in steam cracking. They are also using artificial intelligence and machine learning tools to find catalysts that would make the device even more efficient.
Ultimately, the goal is to create a device that can scale up to be used in industry to continue to reduce ethylene's carbon footprint.
The study, "A cation-functionalized layer for ethylene electrosynthesis via CO reduction paired with H2 oxidation in a pure-water-fed solid-state electrolyser," was supported by the Dow Chemical Company.
