Electroreduction of Captured CO₂ on Silver Catalysts: Influence of the Capture Agent and Proton Source

In the context of carbon reutilization, the direct electroreduction of captured CO₂ (c-CO₂RR) appears as an appealing approach since it avoids the energetically costly separation of CO₂ from the capture agent. In this process, CO₂ is directly reduced from its captured form. Here, we investigate the influence of the capture agent and proton source on that reaction from a combination of theory and experiment. Specifically, we consider methoxide-captured CO₂, NH₃-captured CO₂, and bicarbonate on silver electrocatalysts. We show that the proton source plays a key role in the interplay of the chemistries for the electroreduction of protons, free CO₂, and captured CO₂. Our density functional theory calculations, including the influence of the potential, demonstrate that a proton source with smaller pK_a improves the reactivity for c-CO₂RR, but also increases the selectivity toward the hydrogen evolution reaction (HER) on silver surfaces. Since c-CO₂RR requires an additional chemical protonation step, the influence of the proton source is stronger than that of the HER. However, c-CO₂RR cannot compete with the HER on Ag, Experimentally, the dominant product observed is H₂ with low amounts of CO being produced. Through a rotating cylinder electrode cell of well-defined mass-transport properties, we conclude that although methanol solvent exhibits a lower HER activity, HER remains dominant over c-CO₂RR. Our work suggests that methoxide is a potential alternative capture agent to NH₃ for direct reduction of captured CO₂, though challenges in catalyst design, particularly in reducing the onset potential of c-CO₂RR to surpass the HER, remain to be addressed.