There are some reports on reducing CO2 into fuel chemicals but the efficiency is quite low. What do you think is the limiting factor of CO2 reduction? We have to identify the rate-limiting factor in order to improve the efficiency.
Factors involve in this process include cathode and anode material, interface, catalyst/enzyme, ion exchange membrane, CO2 solubility, product specificity, etc.
Where will the energy source to break the carbon oxygen bond come from? If it's renewable, I think it will be some time before we can do it better than plants. However, even plants aren't very good at it and some of their enzyme systems only turn over a few tens of molecules of CO2 per second per molecule of enzyme. Hardly world shattering. Would be very interested in knowing of any fundamental rate limiting phenomenon here.
In biological systems, the electron source is usually NADPH, NADH or reduced feredoxin while the energy source is ATP that could be generated through proton gradient by the exchange of NADPH to NADP+. For plants, the rate-limiting factor is the low activity of RuBisCO carboxylase.
The limiting factor is the thermodynamics. CO2 reduction in water electrolyte can proceed under electrode potential (Eo) lower RHE, i.e. in the field of discharge of water. There is one mention about the reaction HCOOH/CO2 with Eo=0,196 V only (W.M.Latimer, "The Oxidation states of the elements and their potentials in aqueous solutions", New York, 1952). For the cathodic CO2 reduction necessary to use a) not water electrolytes with Eo less Eo of CO2 reduction and b) active cathodes.
The main problem in electrochemical reduction of CO2 is a lack of efficient catalysts of water oxidation and CO2 reduction. As pointed out by Vladimir, water reduction is thermodynamically more favorable but also requires a catalyst. You need a catalyst which reduces overpotential of CO2 reduction but does not catalyze water reduction. Keep in mind that overall process includes water oxidation on anode. .
A question to Vladimir. If you switch to "not water electrolyte," what could be a source of protons to reduce CO2?
Dear Yurii!
Among proton solvents are not only a water (ɛ=78), but and many other solvents (SH) with protons in molecules and ɛ ˃20, namely , NH3(ɛ=20), ice CH3COOH(ɛ=6), CH3OH(ɛ=32) and many others.
Yours Vladimir
Vladimir,
First, the solvents you mentioned are not suitable for electrochemistry.
Second, in aqueous solutions water is oxidized on anode to O2 producing protons. which are then used on cathode to hydrogenate CO2. A solvent is involved in overall reaction. E.g. if you use CH3OH as a solvent, you will oxidize CH3OH on anode to CO2 and reduce CO2 on cathode to CH3OH
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