(622f) Electroreduction of Captured CO2. Effects of Capture Agent, Proton Source, and Catalyst.

Since industrial revolution, the concentration of atmospheric CO₂ has been increasing. Efforts to capture and utilize this CO₂ have been of interest. Currently, CO₂ is captured by an agent, like an amine, and must be thermally decomposed to conduct the CO₂ reduction reaction (CO₂RR). However, the direct reduction of captured CO₂ (c-CO₂RR) is of interest as it could avoid the large thermal energy cost from releasing the CO₂. In this process, the captured CO₂ is directly reduced into some higher values product, such as CO, and the capture agent is reformed. In this perspective we used density function theory (DFT) to explore the effects of proton source, capture agent and catalyst on the c-CO₂RR, CO₂RR, and the competing hydrogen evolution reaction (HER).

However, the c-CO₂RR is intrinsically, a more challenging process than the CO₂RR, as there are 3 required protonations, but only 2 available electrons. This third protonation must be done chemically. This presents an interesting design challenge as a catalyst for the c-CO₂RR must not only be an effective electrocatalyst, but also an effective thermocatalyst to complete the c-CO₂RR.

We show onset and over potentials of the c-CO₂RR, HER, and the CO₂RR on various catalysts (Figure 1 on Ag(111), for example). Our calculations show that the capture agent plays a much smaller role on the activity of the c-CO₂RR as compared to the proton source. We see similar onset potentials for the c-CO₂RR using different capture agents but the similar proton sources, but vastly different onset potentials when the proton source is changed. Unfortunately, decreasing the pK_a of the proton source improved the competing HER, and ultimately on Ag we find that HER will dominate. However, this opens the opportunity to search for catalysts that suppress the HER but are still active towards c-CO₂RR.