Uncovering the Active Species in Amine-Mediated CO2 Reduction to CO on Ag

G. Leverick, E. M. Bernhardt, A. I. Ismail, J. H. Law, A. Arifutzzaman, M. K. Aroua, and B. M. Gallant
ACS Catalysis 2023, 13, 12322–12337. Doi: 10.1021/acscatal.3c02500
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Abstract

Electrochemical reactive capture of CO₂, which integrates CO₂ 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 CO₂ reduction (CO₂R) 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 CO₂ conversion as an essential initial step toward improving these processes. We first demonstrate the critical influence of CO₂ 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 pK_a and with different propensities to form the amine-CO₂ adduct carbamate, we show that dissolved CO₂ is the active species for CO₂R in amine-containing solutions, enabling some capture solutions to have comparable CO₂R 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 CO₂ as it is consumed during CO₂R, alleviating mass transfer/transport limitations without directly participating electrochemically.