PNNL

Abstract

The pressing issue of climate change driven by human-induced carbon emissions necessitates immediate action to avert a critical tipping point projected for 2050. Mitigation of the surge in atmospheric CO2 levels (currently at 421 ppm, over 50% higher than pre-industrial levels of approximately 280 ppm) calls for widespread deployment of efficient carbon capture technologies with minimized energy consumption. Electrochemical carbon capture processes that have been touted to have the potential to meet these needs rely heavily on the applied electrochemical cell voltage, and on electron utilization (the number of CO2 molecules separated per electron), which has generally been asserted to have a theoretical limit of 1. Here, we introduce an electron-leveraging strategy to enhance electron utilization beyond this limit by employing a redox-active organometallic species having a ligand with multiple hemi-labile coordination sites. This enhanced electron utilization of 1.43 is attributed to the dissociation and protonation of two phenolate coordination sites upon electrochemical one-electron reduction of the metal center of an iron(3+) complex, leading to a marked increase in pH and thus a strong ability for the absorption of CO2 as a carbonate or bicarbonate; the CO2 can be released as a pure stream upon re-oxidation of the iron center. The reversibility and robustness of the system were enabled by the efficient prevention of CO2 reduction upon the introduction of nicotinamide as an iron center guardian of the iron(2+) center. The effective guarding of the iron(2+) by nicotinamide was inferred by NMR/EPR analysis. The cyclic system exhibits a minimum operational energy of 22.6 kJe/mol and an average of 63.7 kJe/mol over 29 cycles, using 15% CO2 as a simulated flue gas. Our electron-leveraging strategy holds great promise for advancing energy-efficient electrochemical carbon capture technologies, and offers an alternative to prevalent redox potential shifting methods proposed to achieve stability in redox-active materials from undesired electron transfer reactions across diverse operational conditions.