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Thursday, October 20, 2016

Converting carbon dioxide into ethanol

Another crazy idea to use electricity to sequester carbon ... but what if we could run it in reverse?

C hemists at the US Oak Ridge National Laboratory have invented a new process for converting carbon dioxide from the air into ethanol.[1] The chemistry is very interesting: the catalyst is copper on a graphene electrode. It's not crystalline and it sounds inexpensive and easy to make. They describe it as

... a dense nanotextured carbon film terminated by randomly oriented nanospikes approximately 50–80 nm in length, where each nanospike consists of layers of puckered carbon ending in a ~2 nm wide curled tip. The film is grown by a relatively simple direct-current plasma-enhanced chemical vapor deposition reaction using acetylene and ammonia as reagents.

Very cool stuff. People shouldn't be fooled, though. It's not a source of free energy. The process requires energy in the form of electricity, and it's only 63% efficient. That's great from a chemist's point of view. But I don't think you'll get much environmental benefit out of it.

Suppose you took the ethanol and used it for fuel. You'd get all the CO2 back. But to get that ethanol you put in 100 units of energy, lost 37 as heat doing it, so you got (at most) 63 back. The only way this sequesters CO2 if they bury the ethanol or dump it in the ocean. But if that's the plan, why not just dump our dollar bills in the ocean and cut out the middleman?

Discover Magazine says “Making our own ethanol is better than pulling it out of the ground though, and could help level off our net carbon emissions.”

I don't see how. Unless we use nuclear or windmills, we still generate CO2 making the electricity, except that now we'll waste 37% of it doing the conversion. We'd be much better off shipping the fuel directly and leaving the CO2 in the atmosphere.

The atmosphere is where it belongs. The carbon in our gas and oil was originally CO2. Without humans to recycle it back to CO2, the entire biosphere would eventually get buried and the amount of life on Earth would continue to shrink. The industrial age is the best thing that could have happened for life on this planet.

Sequestering carbon dioxide would undo that. Pull even half of it out, we risk extinction: C3 plants stop growing due to CO2 starvation at about half of current levels (i.e., 200 ppmv). The C4 plants may still survive, but we're still talking real catastrophe. Leave it all in, the earth gets a degree warmer—maybe.

The practical optimal efficiency for hydrolysis of water is about 60%. So if they can scale it up, it's about the same efficiency as using electricity to hydrolyze water. This is where the process might have real value: if we can use ethanol instead of hydrogen as fuel, that makes powering vehicles a lot easier. Ethanol fuel cells are the future for vehicles. But we'll need better catalysts for that, and we'll still need to get the energy from somewhere.

Here's the reaction

2 CO2 + 9 H2O + 12 e → C2H5OH + 12 OH

Non-chemists could think of it this way:

1 CO2 + 100 energy → ethanol + 37 energy (lost as heat)
Ethanol + oxygen → 1 CO2 + 63 energy (potentially usable energy)

Thinking about the hydroxide produced in this reaction, though, a thought occurs. A catalyzed reaction is by definition reversible. Could this catalyst be adapted to generate electricity by adding sodium hydroxide to ethanol? If so, it might make those ethanol fuel cells practical.

It would be strange (and a little disappointing) if these chemists were so focused on sequestering carbon that they missed that.

1. Yang Song Y, Peng R, Hensley DK, Bonnesen PV, Liang L, Wu Z, Meyer HM, Chi M, Ma C, Sumpter BG, Rondinone AJ (2016). High-Selectivity Electrochemical Conversion of CO2 to Ethanol using a Copper Nanoparticle/N-Doped Graphene Electrode. ChemistrySelect. doi: 10.1002/slct.201601169 Link

2. Cerling TE, Ehleringer JR, Harris JM (1998). Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution. Philos Trans R Soc Lond B Biol Sci. Jan 29; 353(1365): 159–171. Link

Last updated oct 21, 2016 7:20 am

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