(292c) Green Ammonia-Mediated CO2 Capture and Electrochemical Upgrading to Formate

Direct electrochemical conversion of CO₂ capture solutions (instead of gaseous CO₂) into valuable chemicals can circumvent the energy-intensive CO₂ regeneration and pressurization steps. While commonly used CO₂ 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 (NO₃^−)-N] and converting it into green ammonia (NH₃) could mitigate the environmental impacts of Nr and reduce the NH₃ demand from the carbon-intensive Haber-Bosch process, as well as a possible CO₂ 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 CO₂ capture (to ammonium bicarbonate, NH₄HCO₃) and subsequent reduction to ammonium formate (NH₄HCO₂) as a new approach to CO₂ capture and utilization (CCU). We have demonstrated a record-high NO₃^−-to-NH₃ performance in a scalable, versatile, and cost-effective membrane-free alkaline electrolyzer (MFAEL): an unprecedented NH₃ 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 (NH₄HCO₃)-fed electrolyzer with an anion exchange membrane (AEM) outperforms the state-of-the-art KHCO₃ electrolyzer with a bipolar membrane (BPM) owing to its favorable thermal decomposition property, which allows for a 3-fold increase in the in-situ CO₂ 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 NH₄HCO₃ electrolysis with CO₂ capturing by on-site generated green ammonia from the electro-reduction of nitrate in MFAEL has shown a remarkable 99.8% utilization of CO₂ capturing agent. Such a multi-purpose process may offer a sustainable route for the simultaneous removal of N_r wastes and streamlined CO₂ capturing and upgrading to valuable chemicals.