3 sustainable uses for carbon dioxide

Here’s a challenge: “If you’re a young person and you want to save the planet, while also becoming the richest person on it, then this is the problem to solve. Using captured carbon dioxide, how do you make, say, a fuel.” Posing the question is Dr Stephen Pacala, professor of ecology and evolutionary biology at Princeton University, New Jersey, and chair of the US National Academies panel on CO₂, who adds: “That’s the green dream. We just need a lot of inventions to get there.”

But, Pacala notes, we’re already well into startup territory. Indeed, in some circles carbon capture and storage (CCS) has been rephrased as CCUS (carbon capture, use and storage). Less the climate change bogeyman, and a waste product to store, some scientists are now seeing CO₂ rather as a raw material to process into something useful. 

Aside from any resulting product, a new industry growing out of CO₂  repurposing would also help both to replace those jobs lost in closing down the fossil fuel industry and to mitigate problems in the geological storage of CO₂. 

Pacala stresses that “there’s a long development road ahead” and CCUS projects are likely to only play a minor role in the wider move towards carbon neutrality, at least in the short term. But, he says, we should embrace “the chance that using carbon could make a noticeable difference”.

1 Making new plastics

“Everyone views CO₂ as a liability and it is. But it could be viewed as a chemical feedstock, as a step to making plastic,” says Dr Edward Sargent, professor of electrical and computer engineering at the University of Toronto. “The question for us was what would it take to do this in a way that’s beneficial to the overall CO₂ strategy?”

Sargent has concluded this means viewing the repurposing of CO₂ through a strictly economic lens. Extracting CO₂ from the atmosphere and storing it underground is expensive, which is why CO₂ sequestration, he argues, hasn’t taken off as a business. 

But since ethylene, a precursor to polyethylene, used in many products from synthetic fabrics to medical devices, is already a $60-billion-a-year market, people will pay for it. It’s ethylene which his team has created by using copper as a catalyst to combine the reactants CO₂, water and electricity. 

Much of the CO₂ turns into side products like carbonate and the team is currently able to produce 75 per cent pure ethylene. Since the market wants more than 90 per cent pure, Sargent is now using artificial intelligence to accelerate the discovery of a more effective catalyst, possibly a blend of copper and aluminium. He has also scaled up the lab equipment to a “mini-van size” system that increased output by 10,000 times. 

“Right now plastics have a big carbon footprint,” says Sargent. “But this is a chance to reframe our thinking about them.”

2 New routes to battery acid

A catalytic converter developed by chemical and biomolecular engineers Chuan Xia and Haotian Wang at Rice University, Houston, Texas, uses CO₂ as a chemical feedstock to produce high concentrations of formic acid, in a way that is much more purified and so less expensive than other methods to date. 

Formic acid is an energy carrier, a fuel cell that can itself generate electricity and carbon dioxide, which can then be grabbed and recycled again. It’s also useful as a storage material for excess energy from variable sources, such as wind and solar power, and also for storing hydrogen. It can hold 1,000 times the energy of the same volume of hydrogen gas, which is hard to compress. This is one of the main challenges in the development of hydrogen-powered cars. 

The scientists’ pioneering converter is based on bismuth and a solid-state electrolyte that’s free of the salts that usually then have to be removed in such a process at a great energy cost. The duo say their catalyst can already be produced at the kilogram scale and so is readily scaled up.

3 Recycling CO₂ into new fuels

“Pulling CO₂ out of the atmosphere and just storing it underground isn’t a useful endeavour; it would be far better to do something productive with thatCO₂,” argues Dr Torben Daeneke, of RMIT University Melbourne’s engineering department. “There’s a big push in the scientific literature to repurpose CO₂ into something useful, to make, for example, a fuel that can be burnt again and we’re one step closer to that.”

Daeneke’s team has developed a low-temperature, low-pressure liquid metal electrolysis method that efficiently allows CO₂ to be converted from a gas into solid particles of carbon. The catalyst they created has specific surface properties that makes it extremely efficient at conducting electricity while chemically activating the surface. CO₂ is dissolved in an electrolyte liquid with the liquid metal, before a current is passed through it. The process converts the CO₂ into solid flakes of carbon. Before, doing this required extremely high temperatures, making the process unviable on an industrial scale.

He says the resulting flakes are both a more efficient means of storing CO₂ than in its gaseous form, but also they are of a purity that means they could work as an electrode, as part of a super-capacitor or in the production of wonder material graphene.

“I’m optimistic that the process can be scaled up, but driving the reaction is energy intensive and that’s where the challenge lies,” concedes Daeneke, who in the next year or so hope to have completed a microwave oven-style device capable of producing a few kilos of carbon a day. “It may be a question of sitting the process in a location that makes the energy cheaper, in the way the manufacture of aluminium foil is.”