Why CO₂E?While fossil fuel derived molecules produce CO₂ when burned, we do have the ability to use electricity to convert the CO₂ back into fuels with renewable generated electricity from wind, solar, etc. and thus reverse the amount of  CO₂ in the atmosphere.  This is a very creative approach to solving our current issue with regards to CO₂ induced climate change.   However as air is 400ppm of CO₂, the energy we need to upconcentrate this and then upconvert it to something like a fuel will never compete with either using this electricity directly or even using it in a battery (90% vs 40% roundtrip efficiency).

 The fundamental principle of why batteries are more efficient than electrolysis is that batteries only change the oxidation state of an element, whereas electrolysis forms covalent bonds.  Covalent bonds are great in that they can last for 1000's of years whereas a change in oxidation state in a battery could probably not maintain its charge for more than a couple of years.  Covalent bonds also have a much higher energy density than batteries.  However in terms of sustainable energy, we are fine with battery stability and the energy density they provide is sufficient for most all of our applications.

Thus why investigate CO₂ electrolysis?  Both ships and especially airplanes need high energy density fuels.  On shorter trips batteries may suffice, however a ship travelling directly from China to Europe would struggle without an energy dense fuel on board.  Airplanes are even worse as the weight of the low energy-density batteries adds substantial weight to the plane that will not go empty near the end of the trip the way liquid fuels do. Together airplanes and shipping represent roughly 5% of our greenhouse gases.  Still if our round trip efficiency is 40%, maybe 50% with great research, we are still nowhere near batteries and the fossil fuels we use now have the same economic value as batteries.  Thus if we need to go to CO₂ electrolysis field, we are looking at 2x increase in fuel costs, and as these dominate aviation and shipping costs expect shipping and aviation costs to go up substantially as we go sustainably.  There is really no easy solution for this, and not even a moderately hard solution, so we will need to plan accordingly.

 As shipping and aviation will be tough and people may try to avoid these, is there anywhere else for CO₂ electrolysis?  Yes- the organic chemicals industry.  Organic chemicals, by definition, need carbon, thus meaning batteries are non-competitors in this field.  Furthermore the chemicals industry, dominated by plastics, contributes to 5% of our CO₂ emissions.  The only competitors for CO₂ electrolysis are biomass, H₂ electrolysis + thermal catalysis with CO₂, and fossil fuel useage + carbon capture.  The greates thing about organic chemicals is that CO₂ electrolysis does not need to be the best approach for all chemicals, but if it optimal for just one, it will deemed a success.  As there are hundreds of chemicals that are produced on the kiloton scale there is a high likelyhood we will be succesful somewhere.  One important point to note is that the chemicals industry has inherently higher value products than the fuels industry because chemicals need to be both selective and functional.  For example while natural gas is cheap/low value, beause it is a combination of methane, ethane and propane, and the only reaction these molecules are good at is combusting to CO₂.

Comparison to competing technologies

 Currently the chemicals industry breaks down long carbon chain fossil fuels / biomass materials start with long chain carbons into small carbon entities (CO, H₂, ethylene, propylene), and then builds the desired chemicals from these building blocks. CO₂ electrolysis is excellent at producing these small building block materials such as CO, ethylene, ethanol, and propanol, to start with.  Thus they eliminate the initial energy and capital consuming oxidation process fossil fuels and biomass goes through. (Realize in fermentation biomass to ethanol, the reaction stochiometry entails one CO₂ molecule is produced for every ethanol made). 

In terms of CO₂ electrolysis competing with hydrogen electrolysis + thermal CO₂ catalysis, both are electrolysis based with identical O₂ reactions being produced at the anode.  Thus the competition becomes whether the additional catalytic barrier energy loss related to reacting CO₂ to products (versus H₂O to H₂) is greater than the energy loss in the thermal catalysis reactions plus the capital costs for those thermal catalysis reactions.

To put the value of CO2 electrolysis into greater perspective, we could look at the value of the major products CO2 electrolysis can produce. In order of ease, these are roughly carbon monoxide, ethylene, ethanol, methane, propene. Acetate salts can also be produced and the value of these can be debated. (my thoughts are they are very low value).  In terms of $ per ton, CO2 electrolysis products do not compare that well to hydrogen, and given CO2 electrolysis is more complicated, this does not bode appear to bode well for CO2 electrolysis.  However, ranking products in terms of $/ton is actually pointless for electrolysis.  In fossil fuels the '$' term relates to the value you get whereas the 'tons' relates to the amount of work you need to dig this material out of the ground.  Electrolysis does not dig a material out of the ground, so tons is a useless figure of merit.  The amount of work for a product relates to the amount of electricity you need to put into a product.  Thus a more useful term would be to analyze products in terms on $/MWh.  This is done in the second chart.  As we need to assume some voltage, we assume a quite efficient 2V (If it was 4V, all values woudl be halved).  From this chart the CO2 electrolysis products are substantially more valuable than something like hydrogen. The basis for this improved economics is quite simple.  If we look at converting water to hydrogen and CO2 to carbon monoxide, they both need 2 electrons.  However the molecular weight of hydrogen is 2 g/mol and the molecular weight of carbon monoxide is 28 g/mol.  Effectively for the same 2 electrons I get a product that is 14 times heavier in weight.  If I am selling things at $/ton, I am going to make a lot of money.  Electrolysis to carbon monoxide has the extra economic benefit in that shipping bulk carbon monoxide around is very expensive due to its toxic nature. Unlike thermal processes, electrolysis is very down-scalable, thus carbon monoxide can easily be produced on-site, on-demand greatly reducing safety costs.  As the second figure shows there are basically two markets for carbon monoxide- bottled carbon monoxide for companies who need small amounts and bulk carbon monoxide used in large chemical synthesis plants.  The smaller market has a much higher price, whereas chemical synthesis plants are so huge they can afford the capital costs of large thermal catalysis reactors as their massive output can overcome this. If we look at bulk products like ethanol and ethylene, these are still about 50% more valuable than hydrogen.  Even if they are need a little more voltage than hydrogen, it still seems these approaches will outcompete hydrogen for renewable energy electrons needed for the chemials industry.

One could argue that the chemicals industry is not that big, and if CO₂ electrolysis to ethylene started to take off it would quickly saturate the market.  However this would be wrong.  If all of Europe's electricity went to making ethylene at very efficient 2V, we would still only be able to produce 67% of the worlds ethylene.  A similar argument could be made for the USA.  This was just ethylene.  We still have propylene, ethanol, and many other chemicals to go.  Thus we definitely not saturate the market.  One could argue that whether this will even be able to make an impact on a scale large enough to effect global energy demand.  The first counterpoint to this is that we will need to put in this same energy no matter what the approach, so we just have to realize this is coming.  Furthermore, we also need to realize that while much of the world is based on Heat engine type devices that are limited to typically 40% by Carnot efficiency losses these will be replaced by electric motors (95% efficient) and heat pumps (300-400% efficient- effectively an entropy pump).  Thus while global energy useage for the chemicals industry is around 5% right now, this will grow as other areas switch to higher efficiency approaches and since the upconversion of chemicals is not limited by the Heat Engine loss to begin with, there is not the same potential for efficiency gains.

The second counterpoint with respect to impacting global energy is: I don't care.  If we can develop CO₂ electrolysis that is cheaper than the current method of producing bulk chemicals and the market demand is almost infinitely big, we will have created an almost infinite money machine.... and do it sustainably.  Obviously this is a major overstatement, but it does show that the potential behind this technique can be quite grand even while still playing less dominant role in global energy compared to fields such as wind and solar.

Another  question one may have is whether there will still be CO₂ sources once we switch to a sustainable society. The realistic answer is it will take us 50 years to completely get off of fossil fuels.  The perfect case scenario answer is that we still will have CO₂ production from sources such as cement production and steel production because they produce  CO₂ as part of their chemistry in creating these materials. Groups in Europe such as DECHEMA have analyzed the amount of point source CO2, and we should have sufficient amounts of highly concnetrated CO2 to capture (rather than direct air capture) for the chemicals industry up until 2050, beyond which then we may need to switch to direct air (or ocean) capture. 

Great Idea!, Now How Do You Do ECR?

First we need to understand the fundamentals.  This is on the following page