He boils pure carbon dioxide out of liquid salt - and hands you the steam back.
A cold email in 2016 got him into an MIT lab. What he found there - a salt that drinks CO2 at 600 degrees and never cracks - became a company that oil majors now fund.
Inside a shipping container in Cambridge, Massachusetts, a nozzle that looks like a shower head sprays molten salt into a chamber of hot industrial exhaust. The CO2 diffuses into the falling liquid. Heat it a little more and the salt boils the carbon back off, pure, ready to be piped underground or sold. The salt goes around again. Cameron Halliday runs the company built around that loop.
The company is Mantel, and Halliday is its co-founder and CEO. The pitch is brutally simple: capture roughly 95% of the CO2 from heavy industry while spending about 3% of the net energy that conventional carbon capture demands. Conventional capture taxes a plant so heavily it can swallow a third of its output. Mantel recovers most of that energy as usable steam and sells it back to the customer.
That distinction - giving energy back instead of eating it - is why Shell, Eni, and bp are on the cap table, and why cement, steel, paper, and oil and gas operators keep taking the call.
This is a technology that could radically change the energy industry.
Most carbon-capture sorbents are solids. Run them through enough heating and cooling cycles and they crack, crumble, and quit. That brittleness has dogged the field for years. Halliday's insight, discovered during his PhD in professor Alan Hatton's lab, was to use a salt that is liquid at the temperatures heavy industry already runs at.
The material - lithium-sodium ortho-borate, a molten borate salt - absorbed more than 95% of the CO2 it met and barely degraded across more than a thousand cycles. Liquids don't crack.
The salts behave like a liquid at high temperatures, which avoids the brittle cracking responsible for degradation of many solid materials.
Most of the energy comes back as steam Mantel sells to the customer.
Molten salt is sprayed through a nozzle into a stream of hot industrial flue gas.
CO2 diffuses into the falling liquid and reacts with the salt - up to 95% captured.
A small extra bump in temperature reverses the reaction. The salt boils off pure CO2.
The process heat becomes steam for the customer. The salt cycles back to the top.
In 2016, Halliday sent a cold email to MIT professor Alan Hatton asking for a summer research spot on carbon capture. He got it. He never really left. He enrolled in a joint program, working through problems in Hatton's lab while stacking degrees - a master's in chemical engineering practice, then a PhD and an MBA at the same time.
Three of the projects he worked on in that lab eventually turned into companies. Mantel is his. The moment it clicked has a precise setting: an early morning walk home, the city's runners streaming past in the other direction.
He met co-founders Sean Robertson and Danielle Rapson through MIT's Climate and Energy Ventures course, 15.366 - the class where a lab result learns to become a business. They incorporated Mantel in 2022.
I remember walking home over the Mass Ave bridge at 5 a.m. with all the morning runners going by me. That was the moment when I realized what this meant.
There is a particular kind of validation in having the companies most exposed to carbon write the checks. Mantel's $30 million Series A was co-led by Shell Ventures and Eni Next, with bp Ventures, Engine Ventures, Vale Ventures, Hartree, New Climate Ventures, MCJ Collective, and others rounding it out.
The target Halliday keeps coming back to is cost. At scale, he has said, Mantel could theoretically capture carbon for roughly $30 to $50 a ton - the range where bolting capture onto a cement kiln or a paper mill stops being charity and starts being math.
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The first prototype fit in a shoebox. The next one fit in a shipping container. The trajectory is the whole point.
The same molten salt absorbs the CO2 and, with a little more heat, releases it pure. Drink, exhale, repeat.
The system attaches to existing power stations and factories instead of asking them to start over.
SM, PhD, and MBA from MIT - on top of an M.Eng. from Loughborough. The PhD and MBA, at once.
Mantel's three co-founders found each other in 15.366, MIT's Climate and Energy Ventures course.
The realization didn't arrive in a meeting. It arrived on a bridge, against a tide of morning runners.
Halliday's bet is that decarbonizing cement, steel, paper, and oil and gas isn't a moral argument waiting to win - it's an engineering and cost problem waiting to be solved. Get the energy penalty near zero and the price near $30 a ton, and the rest takes care of itself.