Redox-Mediated pH Swing Systems for Electrochemical Carbon Capture.

ConspectusThe 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.

Capture and electrochemical conversion of CO2 in molten alkali metal borate-carbonate blends.

A family of blended compositions of molten mixed lithium and sodium borate (Li1.5Na1.5BO3) and eutectic lithium-potassium carbonate (Li1.24K0.76CO3) salts has been introduced as reversible carbon dioxide absorbents and as media for CO2 electrolysis for carbon conversion. Material properties, temperature effects and kinetics of CO2 uptake were examined. Li, Na borate can absorb up to 7.3 mmol g-1 CO2 at 600 °C. The blended borate-carbonate compositions are molten in the 550-600 °C temperature range, with viscosity adjustable to within a 10-1000 Pa s window depending on the borate/carbonate ratio. The blends can withstand cyclic temperature and CO2 pressure swings without significant deterioration of their CO2 uptake capabilities. Addition of eutectic carbonate into mixed Li, Na borate salts lowers overall CO2 uptake due to the lower solubility of CO2 in carbonate. However, addition of the eutectic lowers the temperature of the pressure swing operation and dramatically accelerates the CO2 uptake during the initial stage of the absorption, potentially enabling a faster cycling. Electroreduction of CO2 and carbon deposition on a galvanized steel cathode was more effective with increasing carbonate fraction in the molten alkali borate/carbonate blend. Blended borate/carbonate compositions with 50-60% borate content possessed sufficiently high loading capacity for CO2 and simultaneously enabled maximum carbon product yield and Coulombic efficiency. Most of the recovered carbon product was shown to be in the form of multiwalled carbon nanotube.

Thermocapillary dewetting-based dynamic spatial light modulator.

Dynamic spatial light modulators (SLMs) are capable of precisely modulating a beam of light by tuning the phase or intensity of an array of pixels in parallel. They can be utilized in applications ranging from image projection to beam front aberration and microscopic particle manipulation with optical tweezers. However, conventional dynamic SLMs are typically incompatible with high-power sources, as they contain easily damaged optically absorbing components. To address this, we present an SLM that utilizes a viscous film with a local thickness controlled via thermocapillary dewetting. The film is reflowable and can cycle through different patterns, representing, to the best of our knowledge, the first steps towards a dynamic optical device based on the thermocapillary dewetting mechanism.

Obtaining Thickness-Limited Electrospray Deposition for 3D Coating.

Electrospray processing utilizes the balance of electrostatic forces and surface tension within a charged spray to produce charged microdroplets with a narrow dispersion in size. In electrospray deposition, each droplet carries a small quantity of suspended material to a target substrate. Past electrospray deposition results fall into two major categories: (1) continuous spray of films onto conducting substrates and (2) spray of isolated droplets onto insulating substrates. A crossover regime, or a self-limited spray, has only been limitedly observed in the spray of insulating materials onto conductive substrates. In such sprays, a limiting thickness emerges, where the accumulation of charge repels further spray. In this study, we examined the parametric spray of several glassy polymers to both categorize past electrospray deposition results and uncover the critical parameters for thickness-limited sprays. The key parameters for determining the limiting thickness were (1) field strength and (2) spray temperature, related to (i) the necessary repulsive field and (ii) the ability for the deposited materials to swell in the carrier solvent vapor and redistribute charge. These control mechanisms can be applied to the uniform or controllably-varied microscale coating of complex three-dimensional objects.