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From captured CO₂ to solid carbon: the science, the energy and the limits

Turning CO₂ back into carbon means undoing combustion. This guide explains the thermodynamics, the main routes researchers and companies use, what decides the kind of carbon you get, and what the process can and cannot claim for the climate.

In short

  • Splitting CO₂ into carbon and oxygen needs at least 394 kJ per mole, about 9 kWh per kilogram of carbon, before any losses.
  • The main routes are thermal, plasma, chemical and electrochemical; electrochemical routes are the most studied for making solid carbon.
  • Process conditions decide whether the carbon is amorphous, porous or graphitic, which is what makes it a designed material.
  • With clean electricity, the process turns emissions into a stable, useful solid; with fossil power it can emit more than it uses.

Undoing combustion

Burning carbon releases energy because forming CO₂ is strongly downhill. Every mole of carbon that burns to CO₂ releases 393.5 kJ of heat. To reverse that, the same energy has to be put back in. The minimum work needed, set by the Gibbs energy of the reaction, is 394.4 kJ per mole at room temperature:

CO₂ → C + O₂ (ΔG° = +394.4 kJ/mol)

Per kilogram of carbon that is about 33 megajoules, or 9.1 kilowatt-hours. Per tonne of CO₂ converted, it is about 2.5 megawatt-hours. These are thermodynamic minimums: no real process can use less, and practical processes use more because of losses in heat, resistance and side reactions.

Energy needed to split CO2Carbon and oxygen sit 394 kilojoules per mole above CO2. Splitting CO2 needs at least that energy: about 9.1 kWh per kilogram of carbon, or 2.5 MWh per tonne of CO2.energyCO₂C + O₂+394 kJ per moleat least 9.1 kWh per kg of carbonabout 2.5 MWh per tonne of CO₂
Energy uphill: carbon and oxygen sit 394 kJ per mole above CO₂. That energy has to be supplied to split it.

This single number explains most of what follows. Converting CO₂ to carbon is only worthwhile when the energy comes from low-carbon sources, and when the carbon produced is worth more than the energy that went into it.

The main routes

Thermal decomposition. Heated on its own, CO₂ splits only partly even at temperatures around 2,000 °C and above, and mainly into carbon monoxide and oxygen, which recombine as they cool unless they are separated quickly. That makes pure thermolysis impractical for solid carbon.

Plasma. Electrical discharges can excite and split CO₂ molecules at lower bulk temperatures. Plasma routes mainly produce carbon monoxide; solid carbon forms only under specific conditions.

Reduction with a chemical partner. CO₂ can be reduced by hydrogen or metals, but the partner itself must be made with energy, and the products are usually gases such as carbon monoxide or methane rather than solid carbon.

Electrochemistry. Electricity can drive the split directly: CO₂ is reduced to solid carbon at one electrode while oxygen is released at the other. Researchers have demonstrated several electrochemical routes to solid carbon at laboratory and pilot scale, using different electrolytes, temperatures and electrode materials; a 2023 review compares their conditions, mechanisms and products (Han et al., 2023). Electrochemical routes are attractive because the energy input is electricity, which can come from renewable sources, and because voltage and current give precise control over the reaction.

CO2 is decomposed into solid carbon and oxygenCO2 molecules enter a decomposition step powered by heat and electricity. Carbon atoms join into a solid, and oxygen is released as a gas.CO₂carbon held by two oxygen atomsdecompositionheat and electricitySolid carbonstays, becomes a materialO₂ released
One molecule at a time: CO₂ is decomposed into solid carbon and oxygen.

What decides the carbon you get

The same overall reaction can give very different carbon. Reaction conditions such as temperature, voltage or current density, the electrolyte and the electrode material all shape the product, from largely amorphous carbon to porous and more graphitic forms, and side reactions such as the formation of carbon monoxide compete with carbon deposition (Han et al., 2023).

For a material maker, that controllability is the point. The carbon is not a by-product to be stored; it is designed for a use. As activated carbon, it starts almost free of the minerals found in coal or plant material, which is why it can reach very low ash. See activated carbon, pores and ash.

Why make a solid at all

Most carbon capture projects compress CO₂ and pump it underground. That needs pipelines, suitable rock formations and long-term monitoring. Solid carbon is different: it does not leak, it can be stored in a bag, and it can be sold. As activated carbon it cleans water and air; as an additive it goes into batteries, rubber, plastics and building materials. The CO₂ becomes a product with a price, instead of a cost to bury.

From an industrial chimney to a useful materialCO2 from an industrial site is captured, decomposed into carbon and oxygen, and the solid carbon is upgraded into products such as filters, battery materials and rubber.Industrial siteemits CO₂CaptureCO₂ separatedDecompositioncarbon and oxygenSolid carbonupgraded, activatedProductsfilters, batteries, rubber
From an industrial chimney to a useful material.

What it can and cannot claim

  • Energy decides the climate effect. At 9.1 kWh per kilogram of carbon as an absolute minimum, power from a fossil-heavy grid could emit more CO₂ than the process converts. With renewable or other low-carbon power, it turns emissions into materials with a much smaller footprint.
  • It uses captured CO₂; it does not remove CO₂ from the air. Whether and for how long the carbon stays out of the atmosphere depends on the product and its end of life. Activated carbon that is reactivated and reused, or carbon locked into durable products, keeps it out longer.
  • Claims need a life cycle assessment. Capture, energy, processing, transport and end of life should be assessed to ISO 14040 and 14044 before any climate claim is made.
  • It is early in scale-up. Conventional carbons are made in millions of tonnes. CO₂-derived carbon is moving from pilot to first industrial units now.

Where Aeroborn fits

Aeroborn's CO₂ Conversion and Upgrading platform decomposes captured CO₂ into solid carbon and upgrades it to activated carbon with ash below 0.5%. Patents are pending, and we do not publish the details of our own process. A unit at technology readiness level 7 is being commissioned in an industrial Fieldlab. See how the platform works or discuss a material.

Sources

  • Han, X. et al. (2023). Electrochemical reduction of carbon dioxide to solid carbon: development, challenges, and perspectives. Energy & Fuels 37, 12665 to 12684. doi:10.1021/acs.energyfuels.3c02204
  • NIST Chemistry WebBook, carbon dioxide gas phase thermochemistry (enthalpy of formation −393.5 kJ/mol; the Gibbs energy of 394.4 kJ/mol follows from the standard entropies). NIST
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Have CO₂ to put to work?

If you capture CO₂ and want it turned into a material rather than stored, talk to us about a feedstock partnership.

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