Electrochemical CO2 Reduction (ECR)

DTULogo

Why ECR?

While fossil fuel derived molecules produce CO2 when burned, we do have the ability to use electricity to convert the CO2 back into fuels 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, this is not the primary reason to do this research.  The primary reason to study this research is because it is on the verge of becoming profitable and will completely re-define the chemicals industry.  The environmental benefit is just an awesome by-product.

Spurred by the incredible drop in electricity prices from solar cell and wind power over the last 5 years, every well educated person in the energy industry knows that we are currently at a point where renewable electricity is now cheaper than fossil fuel based energy in many places.  While the fossil fuel industry needs technological improvements to mitigate the depleting quality of the reserves to maintain a stable price, any technological breakthroughs in renewable energy simply lower the price.  With this relatively straight forward analysis it is clear to all major industries that renewable electricity will soon be quite cheap.  One of industries that is most on top of this development is the chemicals industry.

By taking CO2 and water (H2O) it is possible to use cheap electricity to produce high value chemicals that can be sold on the global market.  Electochemical  CO2 reduction has a couple of advantages over fossil fuel based as well as biomass based chemicals.  The most important is that fossil fuels / biomass materials start with long chain carbons, which are broken down to the appropriate chemicals.  On the other hand electrochemical CO2 reduction starts with a single carbon and builds upon that.  Thus while it is favorable for fossil fuels to make longer chain carbon materials, CO2 reduction potentially has an oppurtunity for 1-3 carbon based chemicals.  Many of the polymers that are commonly used today (polyethylene, polypropylene, polyurethane) use short carbon based moelcules as monomer precursors, thus the market for the 1-3 carbon based chemicals is huge.  Most importantly ECR research can be done using a 'shotgun' approach.  There are a dozen molecules that have the potential to be economically viable, so if we try a catalyst it has a dozen chances to produce something valuable.

A 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. 

Great Idea!, Now How Do You Do ECR?

With ECR you need to have a hydrogen source to produce your hydrocarbons, and most people simply use H2O.  However the biggest problem with ECR is that it is very easy for hydogen atoms to form H2 gas rather than react with the carbon to for some type of hydrocarbon.  Thus many potentially good catalysts are eliminated because they produce the unwanted hydrogen rather than the desired hydrocarbon.  The Hori group led this research in the 1980's and 1990's and tested many different metals as ECR catalysts.  They only found copper to be effective as shown below:

Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O., Electrochim. Acta., 1994, doi:10.1016/0013-4686(94)85172-7

From computational modeling we are developing a relatively good insight into how this reaction is occuring.  However the reaction occurs differently on different crystals facets.  We know the  <111> facet produces mostly methane, the <100> favors ethylene, etc.  We also know that all of these reaction mechanisms go through a carbon monoxide intermediate.  Thus one could simply use a 2-step process where the first step converts CO2  to CO and the second step converts CO to hydrocarbons.  While this 2-step approach is more complicated design wise, it is simpler for designing catalysts because we can optimize diffferent catalysts for different reaction intermediates.

Fortunately we have very good catalysts for CO2  to CO reduction, thus a large amount of our focus is on carbon monoxide to hydrocarbons.  We use different techniques to make our copper such as modiying a piece of standard copper foil, sputtering Cu thin films and making nanoparticles using our cluster source.

Copper Foil Sputter deposition of copper Cluster Source Deposition of Copper
 
We primarily do our experiments in elctrochemical cells with in-situ gas chromatographs as shown below.
Electrochemical Set-up Gas Chomatograph