Redox-Mediated pH Swing Systems for Electrochemical Carbon Capture

The rising levels of atmospheric CO2 and their resulting impacts on the climate have necessitated the urgent development of effective carbon capture technologies. Electrochemical carbon capture systems have emerged as a potential alternative to conventional thermal systems based on amine solutions due to their modularity, energy efficiency, and lower environmental impact. Among these, aqueous electrochemical pH swing systems that capitalize on the pH dependence of dissolved inorganic carbon (CO2/HCO3-/CO32-) speciation to capture and release CO2 are of particular interest as they can be flexible in system design and in the range of electrochemical potentials used as well as being environmentally benign. In this Account, we present our recent findings in pH swing-based electrochemical carbon capture using redox-active materials, paving the way toward a sustainable solution for mitigating CO2 emissions.In the first section, we discuss the utilization of molecular redox-active organic materials in electrochemical carbon capture by the pH swing method. This electrochemical system configuration involves homogeneous aqueous electrolytes containing molecular redox-active compounds combined with inert carbon-based electrodes. We first present the development of redox-active amine and oxygen-insensitive neutral red (NR)-based systems. Notably, the discovery of 1-aminopyridinium (1-AP) as an electrochemically reversible compound enables efficient pH swing, leading to an impressive electron utilization of 1.25 mol of CO2 per mole of electrons. Additionally, we explore an oxygen-insensitive neutral red/leuconeutral red (NR/NRH2) redox system, which demonstrates potential applicability to direct air capture (DAC) systems with ambient air as a feed gas.The second section focuses on the utilization of inorganic nanomaterials for redox-active electrodes for pH swing-based electrochemical carbon capture. In this system configuration, we employ redox-active electrodes for inducing reversible pH swings in aqueous electrolytes without interrupting other ionic species, except protons. Specifically, we explore the effectiveness of manganese oxide (MnO2) electrodes for achieving selective CO2 removal from simulated flue gas. We then demonstrate a bismuth/silver (Bi/BiOCl, Ag/AgCl) nanoparticle electrode system as a sodium-insensitive pH swing system for extracting dissolved inorganic carbon (DIC) from simulated seawater with high electrochemical energy efficiency.Overall, these advances in pH swing-based electrochemical carbon capture offer promising preliminary solutions for combating climate change by capturing CO2 from dilute sources such as flue gas and ambient air as well as enabling direct carbon removal from ocean water. While these systems have demonstrated impressive energy efficiency and environmental benefits using redox-active materials, they represent only the beginning of our research journey. Further development and optimization are currently underway as we strive to unlock their full potential for large-scale implementation, paving the way toward a sustainable and carbon-neutral future.