Pharmacists make a greener way to make plastic | Science

By Robert F. Service24 February 2021, 12:15

Plastic is a climate problem. Making precursors for ordinary plastics, such as ethylene and carbon monoxide (CO), consumes fossil fuels and releases a lot of carbon dioxide (CO).₂). In recent years, chemists have set up chemical reactors called electrochemical cells to reverse the process, starting with water and CO₂ of industrial processes and the use of renewable electricity to convert it into feed material for plastics. But the green vision has a practical problem: the cells consume many basic additives that themselves need energy to make.

“It was a very challenging scientific problem,” said Peidong Yang, a chemist at the University of California, Berkeley. His team and a second group are now reporting progress in resolving the alkalinity barrier. One front link connects two electrochemical cells in tandem to completely circumvent the problem, and another turns to an enzyme-like catalyst to generate a desired chemical without consuming alkaline additives. The plastics industry is not going to give up fossil fuels for CO₂ and renewable electricity, but ‘the field is taking up steam’, says Feng Jiao, an electrochemist at the University of Delaware, Newark.

Companies are currently making ethylene, a clear, sweet-smelling gas, by using underheated steam under pressure to “crack” the larger hydrocarbons in oil. The process has been extremely efficient for decades and can produce ethylene for about $ 1000 per ton. But its production generates about 200 million tons of CO₂ annually, 0.6% of world emissions.

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

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

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

In 2019, Kanan and his colleagues reported a partial solution. In place of CO₂, they fed their CO, which does not react with hydroxide to form carbonate. The cell itself was very efficient: seventy-five percent of the electrons they fed with their catalyst – a measure called faradic efficiency (FE) – made acetate, a simple carbon-containing compound used as food for industrial microbes. . The problem is that CO production usually requires fossil fuels, which undo 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 called a solid oxide electrochemical cell, which uses high temperatures to convert CO₂ to CO and can be powered by renewable electricity. The CO flows into another electrochemical cell whose catalysts have been adjusted to favor the production of ethylene, a more widely used commercial product than acetate. The tandem reactor no longer consumes hydroxide and has a 65% FE for energy stored in ethylene produced by the device, the researchers reported last week Joule. “This is an important step forward,” Jiao said.

In the December 2020 issue of Natural energy, Yang and his colleagues reported a very different way of circumventing the alkalinity problem. In an alkaline electrochemical cell, they redesigned the catalyst to exclude water and hydroxide ions at the places where it CO₂. The device can convert the gas into CO without generating carbonate, which is a big energy gain. But this cell has not yet converted the CO and hydrogen from water into ethylene and other hydrocarbons, Yang says.

Better electrochemical cells are not the only driving force behind the research. As wind and solar energy generate, the prices of renewable energy fall. These low energy prices mean that doubling the overall energy efficiency of tandem electrochemical cells could cost them competitively with the standard approach for fossil fuels for ethylene production, Sargent and his colleagues reported in a December 2020 article ACS Energy Letters. “We’re trying to put the option into play,” Kanan said.