In a “triple win” for green research, scientists at the University of Cambridge have developed a new sunlight-activated reactor that uses one waste stream to tackle another – all while producing clean hydrogen, and promising to be profitable at commercial scale.
According to Our World in Data, a statistical-compilation site formed in partnership with the University of Oxford, plastic production now reaches 450 million tonnes per year, a staggering increase since 1950, when only 2 million tonnes were produced. The bulk of that plastic winds up in landfills (and about 0.5% lands in our oceans) after it is used, with only about 18% getting recycled and another 24% handled by incineration. That means finding a way to deal with plastic waste is of critical importance.
Current mechanical recycling methods can handle large amounts of plastics, but they have their issues. Contamination from food and other materials can foul large batches of potentially recyclable plastics. Also, new plastic produced this way is often inferior to the original product, so the process really results in downcycling more than recycling or upcycling.
Chemical recycling methods are much more effective, especially when a photocatalyst is used to alter the constituent building blocks of plastics using light. But to reach that point, acids are often used to initially break down the plastic, and most photocatalysts can’t stand up to the harsh environment created by these acids.
However, the Cambridge study showed a path forward that makes use of both acids and photocatalysts.
Kwarteng et al / Cambridge University
A near accident
“The discovery was almost accidental,” says Cambridge’s Erwin Reisner, who led the research. “We used to think acid was completely off limits in these solar-powered systems, because it would simply dissolve everything. But our catalyst developed didn’t – and suddenly a whole new world of reactions opened up.”
As a first step inside the new reactor, plastics are broken down using sulphuric acid through a process known as hydrolysis. In this case, the acid came from discarded car batteries. When applied to the plastics, the long molecular chains from which they were made are snipped apart at their chemical joints.
Kwarteng et al / Cambridge University
In testing, when the battery acid was applied to PET plastics, such as the plastic that comprises drink bottles, it resulted in two chemical compounds: terephthalic acid (TPA), which settled to the bottom and could be easily removed, and ethylene glycol, the main component in antifreeze.
Next, the new powdered catalyst engineered by Reisner and his team was introduced. This catalyst consists of three basic components: carbon nitride, a yellowish powder good at absorbing visible light; molybdenum disulfide, a component in some greases; and small amounts of cobalt, that acted as a kind of turbocharger, boosting the conversion of the plastic components into hydrogen by a factor of three.
Kwarteng et al / Cambridge University
Once introduced to the liquid acid mixture, simply exposing the system to sunlight (well, an LED-simulated equivalent, anyway) allowed it to convert the ethylene glycol molecules into hydrogen and acetic acid, the main component of vinegar.
In testing, the team reports the reactor generated “high hydrogen yields” and ran for 260 hours without any loss in performance.
Because the catalyst doesn’t use any precious metals, as is often the case with such chemicals, it is affordable and scalable. But most critically, it was able to do its job even in the presence of the harsh acid from the discarded car batteries, which are 20-40% acid by volume. That acid usually has to be neutralized before the batteries can be properly disposed of, which is a resource-heavy procedure.
“It’s an untapped resource,” says said lead author Kay Kwarteng, a PhD candidate in Reisner’s research group, who developed the photocatalyst. “If we can collect the acid before it’s neutralized, we can use it again and again to break down plastics: it’s a real win-win, avoiding the environmental cost of neutralizing the acid, while putting it to work generating clean hydrogen.”
At the moment, the amount of hydrogen produced by the system is modest, so the study functions more as a proof-of-concept effort than a deployable reactor. However, the researchers now plan to commercialize the system with the support of Cambridge’s innovation arm, Cambridge Enterprise.
Will it be economically viable?
A techno-economic model provided in the study appears to suggest this solution will scale, practically and profitably. A treatment plant might cost around UK£7.3 million to finance, set up and run over a 20-year period. Such a plant would treat some 3,000 kg (6,614 lb) of PET plastic, generating about 9.6 kg (21 lb) of hydrogen (or a little less than two tanks of fuel for a Toyota Mirai) per day.
That’s… Not a lot of hydrogen from all that plastic. It’d sell for a few bucks, tops. So how is this thing possibly going to turn a profit? It’s in the byproducts. Every day, that 3,000 kg of plastic will become 9.6 kg of hydrogen, plus 1,17 0 kg (2,580 lb) of TPA, 259.5 kg (572 lb) of acetic acid, 22.2 kg (49 lb) of formic acid, and about 332 kg (732 lb) of leftover ethylene glycol, all of which can be sold.
On the researchers’ own cost model, the TPA sales alone would just about cover the facility’s costs. Add in the rest, including the tiny hydrogen output, and the model does start to look profitable.
“We’re not promising to fix the global plastics problem,” says Reisner. “But this shows how waste can become a resource. The fact we can create value from plastic waste using sunlight and discarded battery acid makes this a really promising process.”
The study has been published in the journal, Joule.
Source: University of Cambridge