Electrochemical CO2 Reduction (ECR)

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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.

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.

To put this in a more direct perspective, the table on the right shows the value of each potential product from reducing CO2 and water. Since we are electrochemically reducing CO2, it is relevant to know how many electrons (i.e. electricity) we need to do this reaction. While molecules like CO only need 2 electrons to convert CO2, moelcules like ethylene need need 12 electrons. By taking into consider the market value of a product, its moelcular weight, and density, we can determine how much value we can produce from a megacolumb worth of electrons.  It reality we are more interested in how much electrical power we need for a product, and are thus interested in voltage as well.  However all of these products need approximately the same voltage (theoretically 1.2V, in reality 2.5V at high currents), thus comparitively we can just look at the $ per MC of electrons.

From this table it should be obvious that fuels such as methane and potentially hydrogen do not look economically promising. CO and formic acid look quite promising, but the global market is small, whereas ethanol and ethylene have a lower price, but a larger market.  While the prices of products may fluctuate, all products are at least double that of hydrogen or methane. 
  Materials # of e- $ / ton $ /
MC of electrons
Global Production
(million tons)
  Hydrogen 2 1000 0.010 60
Carbon Monoxide 2 743 0.110 3.8
Formic acid 2 650 0.150 0.8
  Formaldehyde 4 530 0.041 10
  Methanol 6 496 0.027 160
  Methane 8 150 0.003 4000
Acetic acid 8 460 0.036 12
Ethylene glycol 10 1000 0.065 7
Acetaldehyde 10 900 0.041 1
  Ethanol 12 600 0.024 110
Ethylene 12 1050 0.025 180
  Acetone 16 700 0.064 6

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?

In the 1980's and 1990's the Hori group tested many different metals as ECR catalysts and showed that materials could be grouped into H2 producing catalysts, CO producing catalysts and formate, all of which are 2 electron reduction processes. Cu was the only material that was able to produce significant quantities of more than 2 electron reduced products.

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

While copper is known to be the best catalyst, there are many ways to modify it to alter its reactivity.  Higher surface area, alloying, pH, operating potential and many other parameters are being investigated.  From a more fundamental catalysis perspective, the copper has different surface facets, which will bind differently to the CO2, thus effecting the selectivity.  Additionally it is well known that the first step when CO2 interacts with copper is that it forms CO, and then reacts further from there.  Since there are many catalysts that take CO2 to CO, the reduction of CO on Cu also is quite interesting.

Our Research Focus

Our research is quite diverse and focuses on both copper for CO2 and CO reduction, but also gold and silver for CO2 to CO reduction.  The actual production methods range from simply using copper foils, to sputtering, to size selected nanoparticles, to single crystals.

Copper Foil Sputter deposition of copper Cluster Source Deposition of Copper
 
We test ultra small scale reactions (100 nA/cm2 -1 mA/cm2) in our micro-reactor, research scale reactions in our 3-electrode cells with in-situ gas chromatographs (1 mA/cm2 - 10 mA/cm2), and higher scale reactions in our test cell (10 mA/cm2 - 500 mA/cm2).
Our microreactor sniffere chip is basically a porous membrane attached to a mass spectrometer. The fundamentals of the device was devloped throughout the last decade in our lab, but more recently have group members have optimized this for detecting volatile species in liquid, and we have applied this to a variety of electrochemical reactions.  The devices have world-class selectivity, which has allowed for a spin-off company, Spectroinlet.

These reactors are great for investigating monolayer and submonolayer type reactions, and the time resolution is within about a second whereas a gas chromatogrpah takes 15-20 minutes.



3 Electrode Cell Electrochemical Set-up with in+situ Gas Chomatograph


Our test cells typically run either 5 cm2 or 25 cm2 devices.  We do CO2 reduction on the cathode and oxygen evolution on the anode.  Operating the system at elevated temperatures is difficult to maintain a consistent temperature, however we operate at 30C, which is enough to mitigate changes in room temperature, but not too hot to create large temperature gradients.