How does CO₂ electrolysis work

In the 1980's and 1990's the Hori group tested many different metals as CO₂ electrolysis 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.  The figure on the right groups these materials within the periodic table

Table on the left from Nitopi et al, 2019

From the figure on the right, some clear trends emerge, but the question is why? To understand this we need think about what a catlyst does. A catalyst has species sit on their surface and this reduces their barriers to proceed further in a downhill reaction.  (While CO₂ reduction is actually an uphill process, once we put enough voltage on the electrode we can make it a downhill reaction).  Thus if we are going downhill and we don't want any barrier to our reaction, we want a catalyst that is strong enough to bond a reactant (or an intermediate product), but does not bond it too strong that it gets stuck on the catlayst.  This is known as the Sabatier principle.   CO₂ reduction uses H₂O to get hydrogen, thus we need to analyze the binding of carbon, hydrogen and oxygen to a catayst surface. P-block metals (Pb, Sn, Bi) tend to like to absorb oxygen. This means that the 2 oxygens from CO₂ will bind to these molecules leaving the carbon to get hydrogenated.  This results if a formate molecule being produced.  Unfortunately, formate is not very valuable and not know to react further, thus leaving us with a formate ion.  Furthermore one needs a corresponding cation.  This most often comes from the electrolyte potassium bicarbonate.  In doing so the, potassium bicarbonate releases a CO₂, thus the net effect is CO₂ is never really consumed, but rather a carbonate is converted to a formate. 

More interesting though is how catalyst bind to carbon and oxygen. While directly getting CO2 to bind is hard from a computational catalysis standpoint, we can use CO as a proxy.  We can also plot the ability to bind hydrogen on another axis.  The combination of a catalysts ability to bind hydrogen and CO is shown on the figure to the right.  From this, it is pretty clear that Cu is unique.  However lets first look at the group of catalysts on the bottom left.  These metals bind CO very strongly (negative is strong binding, more positive is weak binding), but bind hydrogen much weaker.  This would entail 2 things.  First of all if CO gets onto the catalyst it is hard to get them off, and secondly hydrogen has this intermediate binding energy that according to the Sabatier's principle should make it a good catalyst.  Indeed, these are some of the best hydrogen evolution catalysts.  However if they are taking electrons to produce hydrogen, then they are not using these electrons for CO2, and thus are not good CO2 electrolysis catalysts.

Figure based on the work by Bagger et al., 2017

If we now look at the catalysts in the upper right, these are very binding to hydrogen, thus they should be horrible hydrogen catalysts, which they are.  While this is a good thing we see that there binding to CO is kind of weak.  While there are some formate producing p-block metals in this group, there is also gold (Au) silver (Ag), and Zinc (Zn).  The first table on this page shows that these metals primarily produce carbon monoxide.  This makes sense.  CO2 electrochemically reduces to CO and then the CO does not bind strong enough, so it leaves the catalyst as a product.  This leads us to copper.

Copper is not that good at hydrogen, but it can hold onto the CO a little bit stronger than Au, Ag, and Zn.  This allows further reductive reactions to take place. As copper produces a lot of products though, we have to figure out how we go from CO to the various products. While much of the mechanism to products has been debated, one are that is well established is the rate limiting step is at the point of CO. The next step in this mechanism is 2 CO's combining together to form a OCCO.  While this process takes place on the electocatalyst, there is actually no electron transfer to make this transformation occur.  Then the question becomes:  how does this take place? 

What has been shown to be the issue is that a negative electrode attracts positive cations in the electrolyte, thus creating an electric field.  This CO molecule bound to the catalyst is sitting within this elecrtric field, and it is the electric field that actually creates the CO-CO coupling.  The electric field can be increase one of two ways. 1) We can increase the potential of the negative electrode.  2) We can decrease the size of the cations, thus shortening the electric field distance, and thus increasing the electric field gradient.  There is a very crazy thing with cations in that all cations have adsorbed water layers attached to them, but the smaller cations (e.g. Li) has more water molecules attached to them than larger cations (e.g. Cs).  This is due to a higher volumetric charge density on smaller ions.  The end result is that a hydrated Li cation is actually larger than a hydrated Cs cation.  Thus if you use Cs as a cation you will have a shorter distance between negative electrode and the cation, thus a larger electric field, and better catalysis.  How much better?  5000% better between Li⁺ and Cs⁺ (see Resasco et al).  That is amazing!!!  However in terms of engineering, needing to have these electrolyte salts are a little painful because that is one more thing we need to maintain.

However what happens beyond this CO-CO coupling?  The mechanism is complicated and we have only recently realized how it works.  However what makes this hard to analyze is that if the CO-CO coupling is the rate limiting step, all the processes beyond that, and the branching to different products, are hard to see.  As we are producing a wide variety of products whose ratio changes, we do know that the energy barriers between many of these products are similar.   

Our Research Focus

Our research focus from a fundamental standpoint is to try to understand the mechanism.  Furthermore, once we understand the mechanism and determine the branching point to differernt products, then we try to modify either the catalyst or the environment to give us the selectivity and activity we want.  In terms of catalysts we will look at various crystal facets via single crystal materials to determine their activity and selectivity.  An electrochemical mass spectrometer allows us very rapid product detection and has allowed us to see intermediates that are too unstable to be detected using standard product quantification methods like HPLC or NMR.  We also use surface enhanced FTIR to analyze the bound surface intermediates and how they change with potential or operating conditions.  Small changes like variation in pH, cations, pressure, ionomers and such can have a drastic impact on both activity and selectivity, thus we need to analyze these and understand the science.  Once we understand the science, only then do we look towards optimization.