Splitting carbon dioxide means breaking apart a very stable molecule to produce useful products, typically carbon monoxide and oxygen, or hydrocarbon fuels like methane. CO2 has two strong carbon-oxygen double bonds, and overcoming them requires significant energy input. Several methods exist today, ranging from high-heat solar reactors to electricity-driven systems, plasma devices, and even light-harvesting materials that mimic photosynthesis.
Why CO2 Is Hard to Break Apart
Carbon dioxide is one of the most thermodynamically stable molecules in chemistry. Its two carbon-oxygen bonds are strong, and the molecule sits at a low energy state, which is exactly why combustion produces it so readily. To reverse the process and pull those atoms apart, you need to pump energy back in. The minimum thermodynamic requirement to split CO2 into carbon monoxide and oxygen is around 283 kJ per mole, but real-world systems always need more than the theoretical minimum because no process is perfectly efficient.
Every splitting method described below is essentially a different strategy for delivering that energy: heat, electricity, light, or energetic electrons in a plasma. The choice of method determines what equipment you need, how efficient the process is, and what products you get out the other end.
Electrolysis: Splitting CO2 With Electricity
Electrochemical CO2 reduction (sometimes called CO2 electrolysis) uses electrical current to drive the breakup of CO2 at a cathode surface, similar to how water electrolysis splits H2O into hydrogen and oxygen. You dissolve or flow CO2 past a catalyst electrode, apply voltage, and the molecules are reduced into simpler carbon compounds while oxygen is released at the other electrode.
The catalyst material determines what you get. Copper-based catalysts are the workhorses of this field because copper can push the reaction past simple carbon monoxide all the way to methane or even two-carbon products like ethylene. The challenge is selectivity: plain copper tends to produce a messy mix of products alongside unwanted hydrogen gas. Researchers have found that adding small amounts of gold to copper (around 7% gold by weight) shifts the reaction strongly toward methane production. These gold-copper catalysts suppress hydrogen evolution and favor the key step where an adsorbed CO molecule picks up a hydrogen atom, which is the bottleneck for making methane. In lab tests, a 7% gold-copper catalyst achieved 2.7 times more methane than hydrogen, a significant improvement over copper alone.
If you want carbon monoxide instead, silver and gold catalysts work well on their own, stopping the reaction at CO rather than pushing further to hydrocarbons. The appeal of electrolysis is that it can run on renewable electricity, effectively storing solar or wind energy in chemical fuels. The main limitations are cost (precious metal catalysts aren’t cheap) and the energy penalty from overpotentials, meaning you always have to apply more voltage than thermodynamics would suggest.
Solar Thermochemical Splitting
This approach uses concentrated sunlight as a heat source to drive a two-step chemical cycle. A metal oxide, most commonly cerium oxide (ceria), is heated to extremely high temperatures (typically above 1,400°C) using a solar furnace or cavity receiver. At that temperature, the ceria releases some of its oxygen atoms, creating an “oxygen-hungry” reduced form. When the temperature drops and CO2 is introduced, the reduced ceria grabs oxygen atoms from the CO2 molecules, splitting them and releasing carbon monoxide as the product.
The beauty of this cycle is that the ceria isn’t consumed. It acts as a reusable oxygen shuttle, alternating between releasing and absorbing oxygen atoms. Researchers have demonstrated stable fuel generation over 500 cycles using a solar cavity-receiver reactor, producing both CO from CO2 and hydrogen from water using the same cerium oxide material. The same system can split water into hydrogen and oxygen, meaning you could combine the two outputs to make synthesis gas (a CO and H2 mixture), which is the starting point for producing liquid fuels through established industrial chemistry.
The drawback is the extreme temperature requirement. You need large solar concentrators capable of reaching well over 1,000°C, which limits this technology to regions with strong, consistent sunlight and makes the reactor engineering demanding.
Plasma Dissociation
Plasma-based CO2 splitting uses energized gas (plasma) to tear apart CO2 molecules. Instead of heating the entire gas to thousands of degrees, non-thermal plasma selectively energizes the electrons while keeping the bulk gas relatively cool. These hot electrons collide with CO2 molecules and vibrationally excite them, weakening the bonds until they break.
Several plasma types can do this, and they differ significantly in efficiency:
- Gliding arc plasma offers the best balance of efficiency and practicality. It works at normal atmospheric pressure and achieves 43 to 60% energy efficiency for CO2 splitting. An electric arc forms between two diverging electrodes and stretches along them in the direction of gas flow until it extinguishes, then immediately re-forms. This continuous cycling creates a reactive zone that breaks down CO2 into CO and oxygen.
- Microwave plasma applies microwave energy to a gas-filled tube, raising temperatures above 3,000 K. Under normal conditions, energy efficiency reaches about 40%, but under specialized conditions with supersonic gas flow and reduced pressure, efficiency can climb to 90%, though CO2 conversion per pass drops to 10 to 20%.
- Dielectric barrier discharge (DBD) is the simplest to build (two electrodes separated by an insulating material) and can convert about 27% of CO2 per pass with 94% selectivity for CO. However, its energy efficiency is only 2 to 10%, making it the least efficient option.
- Radiofrequency plasma generally stays below 50% energy efficiency and drops significantly at higher power levels above 100 kilowatts.
Plasma systems are attractive because they can be turned on and off quickly, making them good candidates for pairing with intermittent renewable energy. They also work at ambient pressure and don’t require rare or expensive catalysts, though adding catalysts to a plasma reactor can improve selectivity and lower energy costs.
Photocatalytic Splitting
Photocatalytic CO2 reduction uses semiconductor materials to absorb light and generate the energized electrons needed to break CO2 apart, essentially artificial photosynthesis. When light hits the semiconductor, it promotes electrons from a lower energy band to a higher one. Those energized electrons reduce CO2, while the “holes” left behind oxidize water.
For this to work, the semiconductor’s energy bands have to straddle the right voltage levels: the upper band must be energetic enough to reduce CO2, and the lower band must be positive enough to oxidize water. Traditional photocatalysts like titanium dioxide and zinc oxide have bands in the right positions but absorb only ultraviolet light, which makes up less than 5% of sunlight. That severely limits their practical output.
Newer materials with narrower band gaps can harvest visible light, which carries far more solar energy. Tin disulfide atomic layers and copper indium sulfide single-unit-cell layers are among the promising candidates. An even more creative approach introduces defects into a material’s crystal structure to create an intermediate energy level that can absorb infrared light. Oxygen-deficient tungsten oxide atomic layers, for example, can capture infrared wavelengths while still maintaining the right energy levels for CO2 reduction and water oxidation.
Photocatalysis remains the least mature of these methods. Conversion rates are still low compared to electrolysis or plasma, but the appeal of a system powered directly by sunlight with no external electricity keeps driving research forward.
Biological CO2 Splitting
Nature has been splitting CO2 for billions of years. Beyond photosynthesis in plants, certain bacteria use an enzyme called carbon monoxide dehydrogenase (CODH) that directly interconverts CO2 and carbon monoxide. The enzyme contains iron-sulfur clusters at its active sites. At one site (called center C), the mechanism works by binding both water and CO to the metal center, stripping a proton from the water to create a reactive hydroxyl group, then using that hydroxyl to attack the CO and ultimately release CO2. The reaction runs in both directions, meaning the same enzyme can also reduce CO2 back to CO.
Bacteria in the genus Clostridium use this enzyme as part of a pathway to build acetyl-CoA, a fundamental building block of metabolism. Researchers are exploring whether these enzymes or synthetic mimics of their active sites could be used in bioreactors or hybrid bio-electrochemical systems to split CO2 at low temperatures and pressures, avoiding the extreme conditions required by thermochemical or plasma approaches.
Real-World Application: Making Oxygen on Mars
The most dramatic demonstration of CO2 splitting in practice happened on another planet. NASA’s MOXIE instrument, carried aboard the Perseverance rover, used solid oxide electrolysis to split Martian atmospheric CO2 (which makes up about 96% of Mars’s thin atmosphere) into oxygen and carbon monoxide. Over its mission, MOXIE completed 16 runs. At peak performance, it produced 12 grams of oxygen per hour at 98% purity or better, double NASA’s original goal for the instrument. Its final run yielded 9.8 grams.
Twelve grams per hour is a tiny amount, roughly what a small dog breathes. But MOXIE was a technology demonstrator the size of a car battery. A scaled-up version could produce the oxygen needed for astronaut breathing and, more importantly, for rocket propellant to launch a return vehicle from the Martian surface. This is one of the clearest cases where CO2 splitting moves from laboratory curiosity to enabling technology for something that would otherwise be impossible.