Uncovering the Active Species in Amine-Mediated CO <sub>2</sub> Reduction to CO on Ag
Graham Leverick, Elizabeth M. Bernhardt, Aisyah Ilyani Ismail, Jun Hui Law, A. Arifutzzaman, Mohamed Kheireddine Aroua, Betar M. Gallant
Abstract
Electrochemical reactive capture of CO 2, which integrates CO 2 capture with its conversion directly from amine and other capture solutions, is of growing interest to enable net zero and eventually negative greenhouse gas emissions. While integration has been proposed to mitigate certain energy penalties and inefficiencies that accrue when capture and conversion are decoupled, integration introduces considerable complexity to the electrochemical process due to the number of possible reactant participants, especially in an aqueous-based capture solution. Moreover, the influence of amine-based sorbents on CO 2 reduction (CO 2 R) mechanisms is not well-understood, making rational design elusive at present. In this work, we reveal the governing parameters and active species in amine-mediated CO 2 conversion as an essential initial step toward improving these processes. We first demonstrate the critical influence of CO 2 partial pressure of the capture stream on the resulting solution pH, which directly affects amine speciation and the Faradaic efficiency of CO production on Ag. Moreover, by considering amines of different p K a and with different propensities to form the amine-CO 2 adduct carbamate, we show that dissolved CO 2 is the active species for CO 2 R in amine-containing solutions, enabling some capture solutions to have comparable CO 2 R selectivity and kinetics to amine-free bicarbonate solutions. As a result, amines can serve as a reservoir of dissolved inorganic carbon that can replenish dissolved CO 2 as it is consumed during CO 2 R, alleviating mass transfer/transport limitations without directly participating electrochemically.