Ammonia-mediated CO\(_2\) capture and direct electroreduction to formate

Abstract

Direct electrochemical conversion of CO\(_2\) capture solutions (instead of gaseous CO\(_2\)) to valuable chemicals can circumvent the energy-intensive CO\(_2\) regeneration and pressurization steps, but the performance of such processes is limited by the sluggish release of CO\(_2\) and the use of energy-consuming bipolar membranes (BPMs). Herein, we discovered that an ammonium bicarbonate (NH\(_4\)HCO\(_3\))-fed electrolyzer outperforms the state-of-the-art KHCO\(_3\) electrolyzers largely because of its favorable thermal decomposition property, which allows for a 3-fold increase in the in situ CO\(_2\) concentration, a maximum 23% increase in formate faradaic efficiency, and a 35% reduction in cell voltage by substituting BPM with an anion exchange membrane (AEM). We then demonstrated an integrated process by combining NH\(_4\)HCO\(_3\) electrolysis with CO\(_2\) capture by on-site generated ammonia from the electroreduction of nitrate, which features a remarkable 99.8% utilization of of CO\(_2\) capturing agent. Such a multipurpose process offers a sustainable route for the simultaneous removal of N wastes and streamlined CO\(_2\) capturing and upgrading.