Electroreduction of Captured CO<sub>2</sub> on Silver Catalysts: Influence of the Capture Agent and Proton Source
Robert M. Kowalski, Avishek Banerjee, Chudi Yue, Sara G. Gracia, Dongfang Cheng, Carlos G. Morales‐Guio, Philippe Sautet
Abstract
In the context of carbon reutilization, the direct electroreduction of captured CO 2 (c-CO 2 RR) appears as an appealing approach since it avoids the energetically costly separation of CO 2 from the capture agent. In this process, CO 2 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 CO 2, NH 3 -captured CO 2, 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 CO 2, and captured CO 2 . Our density functional theory calculations, including the influence of the potential, demonstrate that a proton source with smaller p K a improves the reactivity for c-CO 2 RR, but also increases the selectivity toward the hydrogen evolution reaction (HER) on silver surfaces. Since c-CO 2 RR requires an additional chemical protonation step, the influence of the proton source is stronger than that of the HER. However, c-CO 2 RR cannot compete with the HER on Ag, Experimentally, the dominant product observed is H 2 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-CO 2 RR. Our work suggests that methoxide is a potential alternative capture agent to NH 3 for direct reduction of captured CO 2, though challenges in catalyst design, particularly in reducing the onset potential of c-CO 2 RR to surpass the HER, remain to be addressed.