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Researchers Develop Device for the Photochemical Splitting of Carbon Dioxide

Solar splitting of CO2. Click to enlarge.

Chemists at the University of California, San Diego (UCSD) have demonstrated the feasibility of the solar splitting of carbon dioxide into carbon monoxide and oxygen.

Because their device is not yet optimized, they still need to input additional energy for the process to work.  However, they hope that their results, which they presented at last month’s meeting of the American Chemical Society, will draw attention to the promise of the approach.

For every mention of CO2 splitting, there are more than 100 articles on splitting water to produce hydrogen, yet CO2 splitting uses up more of what you want to put a dent into. It also produces CO, an important industrial chemical, which is normally produced from natural gas.  So with CO2 splitting you can save fuel, produce a useful chemical and reduce a greenhouse gas.

—Clifford Kubiak, professor of chemistry and biochemistry, UCSD

Carbon monoxide and hydrogen can be processed via the Fischer Tropsch process into a variety of chemicals and fuels.

The device designed by Kubiak and his graduate student, Aaron Sathrum, to split carbon dioxide utilizes a semiconductor and two thin layers of catalysts.  It splits carbon dioxide to generate carbon monoxide and oxygen in a three-step process. 

The first step is the capture of solar energy photons by the semiconductor.  The second step is the conversion of optical energy into electrical energy by the semiconductor.  The third step is the deployment of electrical energy to the catalysts.  The catalysts convert carbon dioxide to carbon monoxide on one side of the device and to oxygen on the other side.

Because electrons are passed around in these reactions, a special type of catalyst that can convert electrical energy to chemical energy is required.   Researchers in Kubiak’s laboratory have created a large molecule with three nickel atoms at its heart that has proven to be an effective catalyst for this process.

Choosing the right semiconductor is also critical to making carbon dioxide splitting practical say the researchers.  Semiconductors have bands of energy to which electrons are confined.  Sunlight causes the electrons to leap from one band to the next creating an electrical energy potential  The energy difference between the bands—the band gap—determines how much solar energy will be absorbed and how much electrical energy is generated.

Kubiak and Sathrum initially used a silicon semiconductor to test the merits of their device because silicon is well-studied.  However, silicon absorbs in the infrared range and the researchers say it is “too wimpy” to supply enough energy.  The conversion of sunlight by silicon supplied about half of the energy needed to split carbon dioxide, and the reaction worked if the researchers supplied the other half of the energy needed.

They are now building the device using a gallium-phosphide semiconductor.  It has twice the band gap of silicon and absorbs more energetic visible light.  Therefore, they predict that it will absorb the optimal amount of energy from the sun to drive the catalytic splitting of carbon dioxide.

Kubiak had initially investigated the photochemical splitting of CO2 in the context of a manned mission to Mars. The splitting of CO2, which is abundant on Mars, would provide oxygen and CO for further processing into fuel. The current research is supported by the US Department of Energy.

(A hat-tip to Samuel!)




Sounds like exciting research to help enable the exploration of Mars. Hope they aren't trying to sell this as some sort of solution to our planet's CO2/energy problems. Gallium-phosphide semiconductors and then Fischer-Tropsch... wish the journalist covering this had asked the simple question "in your most optimistic view, 20 years from now how much do you think a gallon of gas produced using this procedure will cost?" I'm sure it might make perfect sense when compared to launching tanks of fuel to Mars.


Research into co-production of hydrogen and carbon monoxide has also taken place within the context of electrolysis using SOFC.

High Temperature Electrolysis of Steam and Carbon Dioxide

Rafael Seidl

I fail to see the point of this. Reducing carbon dioxide to carbon monoxide does produce a fuel, but if you have to expend electricity to do so it's a giant waste. Just use the PV cells to drive an electric appliance and be done with it. This is a solution in search of a problem.


I wonder what the conversion rate of this to-be-invented device should be. For scrubbing the atmosphere from CO2 this is definitely no good, as CO is very toxic (at least to all organisms using some kind of globulin transport molecule - ie. most vertebrate, including humans).

Besides, I believe we already know of a process to bind CO2 (even in very diluted concentrations, not pure) from the atmosphere without producing toxic waste in large quantities.

Strikingly, it also uses high-power electrons (and secondly, the most abundant type of photons) - namely blue and red light.

Since that catalyst doesn't have any use for green light, it's reflected off - and we like those chemical conversion fabrics a lot.

For those who haven't guessed, they are called plants...

Agreed, the conversion efficiency isn't that high, but it's going on for billions of years already, thus even the former toxic byproduct is being used nowadays (oxygen).

For applications under extreme conditions (moon, mars), where CO2 is readly available (heating of carbonates / atmosphere) in high concentration, that holds some promise, however. (Personally, I'd prefer an arboretum still).



The input energy is solar, and the fuel produced (CO gas) can be stored, so I wouldn't describe this quite as negatively as you. It remains to be seen whether this pans out or not, and we don't know anything about the efficiency, but there could be a use for it.

Secondly CO gas can be used for various things - it wouldn't have to be just a fuel to be burned.

I guess my questions have to do with the efficiency of solar energy capture, and whether it could ever be used with ambient air or whether you need a concentrated source of CO2.


Photovoltaic and surplus grid electricity is more efficient than photosynthetic reduction of carbon dioxide. We are wasting gigawatts of electricity as heat in the grid every day just keeping turbines spinning waiting for demand to occasionally spike. Makes perfect sense to me.


The process to use CO to make ethanol might work with this. If you have lots of CO2, you can use sun to make CO and then make ethanol.


People that come up with this stuff truly earn their keep on this planet as far as I am concerned.


Synthetic photosynthesis may potentially be more efficient then biological photosynthesis, but it’s far from being anywhere nears as cheap. Renewable Cellulosic biomass covers a good percentage of the earth surface (not including algae in sea water) and sucks up many terawatts even at .01% efficiency, all that is need is to harvest it, with the right plants (like mix species prairie grasses) no maintenance is required. Algea in elevated CO2 environments have achieve up to 7% efficiency which makes them promising in running off coal fumes.


I second that thought. Pipe in flute gas (perhaps from biomass fired power plants...though scrubbed emissions from fossil energy fired plants will do) and use it to make CO. Store the CO for the nights and cloudy/rainy days. Use SOFC's (with waste energy recuperators) or combined cycle (steam+gas tubine) systems to generate electricity.

_An alternative might be to completely reduce the CO2 into C and O2. Afterwards, the C can be used to create plastics, carbon fibers, or other carbon rich materials. Making solid carbon (or carbide) blocks to bury, as a form of sequestation, is another possibility. The O2 can be used for various industrial, and medical purposes.


Carbon monoxide is how you get to petrochemicals, no need to go all the way through to carbon black. Carbon fiber economy anyone? One way or another you'll be turning biosphere carbon dioxide into carbon-based materials if you don't want to (or can't) use fossil deposits of hydrocarbons. This is yet one of many options to get there, not to mention it's potential as a part of an integrated cycle.


One more thing. Converting PV output to a fuel doesn't make much sense other than for time shifting/swinging PV solar, or otherwise leveraging previous investments during the replacement cycle. In that case you'd want to convert the carbon monoxide to methane or hydrocarbon liquids.


Mark said:

Carbon monoxide is how you get to petrochemicals, no need to go all the way through to carbon black. Carbon fiber economy anyone? One way or another you'll be turning biosphere carbon dioxide into carbon-based materials if you don't want to (or can't) use fossil deposits of hydrocarbons.


This is the relevant statement to me. What is the market for CO, how much is needed, how much oil(I presume) is consumed to produce it and what does it cost reletive to conventional means of producing it. Sounds to me that they're onto a hand-in-glove method to reduce oil consumption AND clean up the CO2 at the same time. I didn't realize CO was an industrial production commodity.

richard schumacher

Any process that converts atmospheric CO2 into vehicle fuel *without using fossil energy* is a Good Thing. In the not-distant future fossil fuels will be illegal; then the greater cost of producing artificial vehicle fuels won't be a barrier. I suspect that systems which use Solar energy captured on the surface of the Earth will prove more expensive than those using nuclear, wind, or space-based Solar, but that's yet to be shown.


The key I think is that this would not be used for atmospheric CO2, where the concentration is probably too low for efficiency, but rather as a scrubber of the exhaust from fossil fuel combustion. It is an alternative to carbon capture and sequestration. Instead of catching CO2 and storing it, this device could turn it into the useful chemical CO which can be cycled back into the industrial system.



That would put it in competition with biodiesel from coal fume gas to algea, which have already achieve impressive efficiencies for a photochemical process.


This sounds just about useless for reducing atmospheric CO2 or fuel production. We would far easier do thermochemical hydrogen production and run it over CO2 from limeburning to produce synthetic fuel.



Not necessarily, if we take the CO2 from burning Coal and make into oil for cars then we have basically made two units of energy out one unit of carbon, gross calculations would say you have cut your CO2 emissions in half: now instead of burning coal for elec. and burning oil for transportation, your burning coal for elec. and transportation. So for every unit of coal fume CO2 that converted into usable fuel your cutting out a unit of oil exhaust CO2, and you’re cutting your total CO2 production units in half. For a transitional economy that pretty good.


Rick, do a search for the keywords "synthesis gas" or "syngas".


Irrespective of some of the negative comments above, this seems to be a wonderful approach to use CO2 for generating fuel.

Bravo UCSD!

To the skeptics, this device is still in the development stages and in my opinion shows promise. We should encourage research of this nature.


I admit to being greatly naive about these topics so please bear with me if I am way off base. What is the reality of this being used on an industrial level rather than trying to make a new fuel source? Seems to me the CO created from a company using this process could be sold to corporations that need it. Approaching this more like companies that extract nitrogen or other gasses from the air as a commodity rather than something to power your car.


Feed the resulting CO to Clostridium ljungdahlii that produce ethanol via this reaction. 6CO + 3H2O –> CH3CH2OH + 4CO2, recycle the 4CO2 waste back to the original CO process. Now you have carbon neutral liquid fuel made by solar power without all the fancy catalysts needed to go the F-T synthesis route. Input solar radiation output ethanol what’s not to love.




Or you could just try this metabolic pathway:

6H2O + 6CO2 + photon -> C6O6H12 + 6O2

C606H12 -> 2CH3CH2OH + 2CO2


That was funny!

But seriously, if you can have a device that can produce the same stuff without having to plant the seeds, wait for the harvest, then feed the harvest into a fermenter, wait some more, then distill it using fossil fuel as heat source, then transport it to place of consumption using fossil fuel again - wouldn't you want that?

And hey, it won't die if you forget to water it. Won't need manure either. Just food for thought.



A non-biological photosynthesis may seem attractive but it has along way to go before it can match the cheapness and efficiency of fume gas feed algae in a glass tube.

Tom Prucha

Lanzatech has a technology that makes Ethanol from CO. Imagine a process to convert C02 into Oxygen and Ethanol.

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