Georgia Webber explains how University of Cambridge researchers are converting PET (polyethylene terephthalate) plastic bottles and carbon dioxide (CO2) gas into valuable products.

Two of the biggest threats contributing to the current climate crisis are the well known pollutants CO2 and plastic waste and it has been long recognised that their production needs to be reduced in the coming decades.

Primarily produced through the burning of fossil fuels, CO2 is a familiar greenhouse gas (GHG) that plays a leading role in global warming. Atmospheric pressures of CO2 continue to rapidly increase as human production far surpasses the ability of natural resources to mitigate it. Plastic waste also creates widely recognised issues due to its extreme use and persistence in the environment. Of the approximately 9.2 billion tonnes of plastic produced between 1950 and 2017, 7 billion tonnes have ended up as waste in the environment. It’s clear to see why a reactor that simultaneously converts these two damaging pollutants into useful chemicals using only energy from the sun could have such a significant impact in the fight against climate change.

“A solar-driven technology that could help to address plastic pollution and greenhouse gases at the same time could be a game-changer in the development of a circular economy”

– Subhajit Bhattacharjee, co-first author of the paper.

The reactor uses a photoelectrochemical (PEC) system which essentially means extracting electrical energy from light, and in this case sunlight. To absorb the sunlight a perovskite is used. These crystal structured compounds are commonly found in solar powered technologies thanks to their low cost and high efficiency in light absorption. The electrical energy generated through the perovskite is used to power the reactor which features two compartments – one for plastic waste and one for CO2.

The CO2 conversion reaction can generate multiple types of carbon based energy sources depending on the catalyst used in the reactor. One possible product is syngas (a mix of carbon monoxide and hydrogen gas) which can be used directly as a fuel for power production as well as in the formation of other useful chemicals. Formic acid can also be generated from CO2 in the reactor which has direct applications in fuel cells, preservatives and disinfectants.

On top of the benefits associated with consuming CO2 gas, the commercial production of syngas and formic acid often relies on high energy and highly polluting methods. As a result the reduction of GHGs in our atmosphere using this reactor is twofold.

“What’s so special about this system is the versatility and tunability – we’re making fairly simple carbon based molecules right now, but in future, we could be able to tune the system to make far more complex products, just by changing the catalyst”

– Subhajit Bhattacharjee.

On the other side of the reactor, pre-treated PET plastics are converted to glycolic acid. With uses in a variety of skin care products as well as in the textile and food processing industries, this handy chemical has many possible applications.

“Developing a circular economy, where we make useful things from waste instead of throwing it into landfill, is vital if we’re going to meaningfully address the climate crisis and protect the natural world,” said the research paper’s senior author, Prof Erwin Reisner. “And powering these solutions using the sun means that we’re doing it cleanly and sustainably.”

Other similar solar powered recycling systems exist in reducing the amount of plastic waste and greenhouse gases. However, this is the first to combine the two together into a single process.

Prof Erwin Reisner recently received new funding from the European Research Council to help in further developing their solar-powered reactor. In the coming years the group hopes to improve the process by generating more complex molecules as products. The researchers say that similar techniques could someday be used to develop an entirely solar-powered recycling plant. If that doesn’t give you hope for the future of recycling, what will?

Original research paper:

S. Bhattacharjee et al., Nat. Synth., 2023, DOI: 10.1038/s44160-022-00196-0.