Encouragingly, the short answer is yes they will be feasible and cheap enough for industrial processes, but not for 5 years at least. The market share of electrochemical technologies may also be relatively small depending on the overall efficiency of the device and cheap, available electricity.

Overall, the standard performance metrics that dictate the capital and operating costs of electrochemical cells are increasing at a rapid pace. For example, the Faradaic Efficiency of ethylene has exceeded 60% (https://www.nature.com/articles/ncomms12123) while current densities and overpotentials are reaching competitive ranges (http://www.sciencedirect.com/science/article/pii/S0378775315303797). There are several indications that these metrics will continue to improve quickly as catalysts and systems are better combined together. Further, the scale-up process is likely to take advantage of the mature fuel cell industry which could cut years off of the commercial development process.

Almost more important than the energy efficiency of producing multicarbon products, however, is the cost of electricity to power electrochemical cells. The break-even electricity cost to reach a profit is likely less than 3 cents/kWh for ethylene of a moderately efficient system, due to the large number of electrons needed to convert it from CO2. While this price is even lower than industrial rates in most places, off-peak power production can lead to much lower electricity prices and even negative costs, which can then make production very profitable. Large industrial plants that produce polyethylene or polyethylene glycol could then likely substitute some of their costs if low-cost electricity is available to them.

So while there is much work to be done, there is reason to believe that CO2 reduction to multicarbon products will at least fill a portion of the market share in the future.

It is possible, even now, to produce carbon structures from CO2. In some cases, this is currently economical as in. for example, urea production for fertilizer. However, and this may be very important to your question, it will always take much more energy to convert CO2 to products than you will be able derive from the products. Using electricity or any other energy source to make hydrocarbons from CO2 will always consume more total energy than making the hydrocarbons from fossil fuel feedstocks. If your interest is in CO2 utilization for climate change mitigation, this sets up some clear constraints.

1. If you can use ubiquitous energy that would otherwise be wasted, for example, sunlight, then you can convert CO2 to hydrocarbons without a net increase in atmospheric CO2. Photosynthesis is an example. If, however, you use fossil-generated energy sources, you will always produce more atmospheric CO2 in converting CO2 to hydrocarbons than you are able to save by the conversion.

2. If you use energy from sources that produce no or little CO2, you can decrease atmospheric CO2 content by converting CO2 to hydrocarbons and using them in products. In doing so, you will use more total energy than would be required if you used fossil sources instead of CO2 as the feedstock for the hydrocarbons. This would generally make sense only if you were using energy that would otherwise be wasted, such as energy from grounding wind turbine production when the grid cannot accept it.

3. Finally there are serious market constraints. The amount of carbon we use in all hydrocarbons of every type except fuel represents only about 3% of the carbon we put in the atmosphere each year. The addressable market is closer to 1%. Therefore, there are no markets for carbon remotely large enough to absorb the CO2 we currently generate. 

Thanks for both informative answers.

Both mention the limited market/need for such materials. I am in fact interested in the non-fuel hydrocarbons (the < 3 % portion). Lots of people have looked at cement concrete as a CO2 sink, either naturally in the long run or by accelerated carbonation in the lab. The amount captured in the former is very limited and the  latter is slow and is limited to pre-fab like bricks.

I am trying to judge how realistic it would be to take CO2, turn it into a simple carbon product like a formate or oxalate and then use this to produce a building material which could potentially be used in large quantities. The goal would be partial replacement of cement-based materials which have high associated CO2 emissions (5-10 % of total anthropogenic). Considering the amount of concrete used, this could be a market for such products. However, economics is key in the construction world. These chemicals are quite expensive (with their current industrial production methods) which makes it cost prohibitive to use them in large quantities in construction binders. If they could be made less expensive then this could be a way to use such converted carbon products.

I am quite sure that making such materials from CO2 rather than from petroleum or natural gas, as they are currently made in most of the world, or from H2 and CO, as they are sometimes made in China, will greatly increase their costs. The CO2 is not free (currently sells for about $40/tonne in very large quantities, $150-$200/tonne in small quantities) and the process to do the conversion will be very energy intensive and therefore expensive. If the energy for the process comes from fossil fuels, as does about 80% of the energy currently used in the world, the CO2 benefit will probably be negative - that is, you will probably produce more CO2 supplying energy to make the product than the product contains. 

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