Chemists get closer to a greener way to make plastics | Science

By Robert F. ServiceFeb. 24, 2021, 12:15 pm

Plastics are a climate problem. By making precursors for common plastics, such as ethylene and carbon monoxide (CO), fossil fuels are used and a lot of carbon dioxide (CO₂In recent years, chemists have designed bench-top reactors called electrochemical cells that aim to reverse the process, starting with water and CO waste.₂ industrial processes and the use of renewable electricity to make raw materials for plastics. But that green vision has a practical problem: the cells often use highly alkaline additives that themselves require energy to make them.

“This has been a very challenging scientific problem,” said Peidong Yang, a chemist at the University of California, Berkeley. Now his team and a second group are reporting progress in resolving the alkalinity hurdle. One advancement connects two electrochemical cells in a row to get around the problem altogether, and another switches to an enzyme-like catalyst to generate a desired chemical without using alkaline additives. The plastics industry does not intend to abandon fossil fuels for CO₂ and renewable electricity, but “the field is picking up steam,” said Feng Jiao, an electrochemist at the University of Delaware, Newark.

Companies are currently making ethylene, a clear, sweet-smelling gas, by using superheated pressurized steam to ‘crack’ the larger hydrocarbons in oil. The process, which has been sharpened for decades, is extremely efficient and can produce ethylene for about $ 1000 per ton. But its production generates about 200 million tons of CO₂ 0.6% of global emissions annually.

Electrochemical cells, acting like reverse batteries, offer a greener alternative. Unlike batteries, which convert chemical energy into electricity, electrochemical cells supply electricity to catalysts that make chemicals.

Both types of devices rely on two electrodes separated by an electrolyte that transfers charged ions. In electrochemical cells designed to convert CO₂ on more valuable chemicals, the dissolved gas and water react at the cathode to form ethylene and other hydrocarbons. The electrolyte is usually enriched with potassium hydroxide, which allows the chemical conversions to take place at a lower voltage, increasing overall energy efficiency. And it helps channel most of the added electricity into making hydrocarbons instead of hydrogen gas, a less valuable product.

But Matthew Kanan, an electrochemist at Stanford University, notes that the hydroxide has a burst of energy of its own. The hydroxide ions react with CO₂ at the cathode, forming carbonate, which precipitates out of solution as a solid. As a result, the hydroxide has to be constantly replenished – and hydroxide itself needs energy to make, making the overall process an energy loss.

In 2019, Kanan and his colleagues reported a partial resolution. Instead of CO₂, they fed their cells with CO, which does not react with hydroxide to form carbonate. The cell itself was highly efficient: Seventy-five percent of the electrons it fed their catalyst – a measure called faradaic efficiency (FE) – went into making acetate, a simple carbonaceous compound that can be used as a raw material for industrial microbes . The problem is that the production of CO normally requires fossil fuels, which negates some of the climate benefits of the scheme.

Now, a team led by Edward Sargent, a chemist at the University of Toronto, has taken this approach a step further. They started with a commercially available device, a solid oxide electrochemical cell, that uses high temperatures to convert CO₂ to CO and can be powered by renewable electricity. The CO flows into another electrochemical cell whose catalysts are tuned to produce ethylene, a more commonly used basic chemical than acetate. The tandem reactor no longer consumes hydroxide and has an FE of 65% for energy stored in ethylene produced by the device, the researchers reported last week in Joule“That is significant progress,” said Jiao.

In the December 2020 issue Nature energy, Yang and his colleagues reported a completely different way of getting around the alkalinity problem. In an alkaline electrochemical cell, they redesigned the catalyst to exclude water and hydroxide ions where it splits CO₂The device can convert the gas into CO without generating carbonate, a great energy gain. But this cell doesn’t yet convert that CO and hydrogen from water into ethylene and other hydrocarbons, Yang notes.

Better electrochemical cells are not the only driving force behind the research. While wind and solar power generation is increasing rapidly, prices for renewable energy are plummeting. Those low energy prices mean that doubling the overall energy efficiency of tandem electrochemical cells could make them cost-competitive with the standard fossil fuel approach to ethylene production, Sargent and colleagues report in a December 2020 paper in ACS Energy Letters“We’re trying to bring that option into play,” Kanan says.