(Invited) Green Ammonia-Mediated CO2 Capture and Direct Electrochemical Reduction to Formate

Abstract

Direct electrochemical conversion of CO 2 capture solutions (instead of gaseous CO 2 ) into valuable chemicals can circumvent the energy-intensive CO 2 regeneration and pressurization steps. While commonly used CO 2 capture agents include alkali and amine solutions, ammonia has been rarely investigated. In another aspect, mismanagement of reactive nitrogen (Nr) in waste has emerged as a major problem in water pollution to our ecosystems, causing severe eutrophication and health concerns. Sustainably recovering Nr [such as nitrate (NO 3 − )-N] and converting it into green ammonia (NH 3 ) could mitigate the environmental impacts of Nr and reduce the NH 3 demand from the carbon-intensive Haber-Bosch process, as well as a possible CO 2 capture agent due to its alkaline nature. In this talk, we will present our rencet research on integration of electrodialysis and electrocatalysis for ammonia synthesis from dilute waste Nr sources, and green ammonia-mediated CO 2 capture (to ammonium bicarbonate, NH 4 HCO 3 ) and subsequent reduction to ammonium formate (NH 4 HCO 2 ) as a new approach to CO 2 capture and utilization (CCU). We have demonstrated a record-high NO 3 − -to-NH 3 performance in a scalable, versatile, and cost-effective membrane-free alkaline electrolyzer (MFAEL): an unprecedented NH 3 partial current density of 4.22 ± 0.25 A cm −2 with a faradaic efficiency of 84.5 ± 4.9%. We also discovered that an ammonium bicarbonate (NH 4 HCO 3 )-fed electrolyzer with an anion exchange membrane (AEM) outperforms the state-of-the-art KHCO 3 electrolyzer with a bipolar membrane (BPM) owing to 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 the AEM. Our integrated process by combining NH 4 HCO 3 electrolysis with CO 2 capturing by on-site generated green ammonia from the electro-reduction of nitrate in MFAEL has shown a remarkable 99.8% utilization of CO 2 capturing agent. Such a multi-purpose process may offer a sustainable route for the simultaneous removal of N r wastes and streamlined CO 2 capturing and upgrading to valuable chemicals.