S. Voskian, T.A. Hatton
Keywords: direct air capture, quinone, polyvinylferrocene, redox electrodes
Summary:There is increasing interest in carbon-negative technologies as means of offsetting our carbon footprint, among which Direct Air Capture (DAC) is gaining attention both as a potential mitigation strategy and to provide CO2 for specific small scale uses (such as greenhouses, beverages), or as feedstock for new electro-reduction processes for production of useful products; DAC has been implemented recently on a small commercial scale. Most sorbent materials considered or being developed for DAC and other low concentration CO2 capture applications require thermal, pressure, chemical or humidity swings for regeneration. These processes have inherent inefficiencies due to parasitic energy losses, however, which can be minimized by the use of electrochemical systems. Operating at near isothermal conditions, electrochemical processes can have significantly higher efficiencies than their thermal-swing and pressure-swing counterparts; electrochemical swing operations achieve separation of species of interest from a mixture when the adsorbents are activated at some applied potential, and they are released when the polarity is reversed. We report a solid-state Faradaic Electro-Swing Reactive Adsorption (FESRA) process comprised of an electrochemical cell that exploits the reductive addition of CO2 to quinones for carbon capture. This electrochemical cell, with a polyanthraquinone-carbon nanotube composite electrode, captures CO2 when charged via the carboxylation of reduced quinones, and releases CO2 upon discharge. The novel cell architecture sandwiches a polyvinylferrocene positive electrode (which serves as an electron source and sink for the reduction and oxidation of quinones, respectively) between two polyanthraquinone electrodes, maximizing the cathode surface area exposed to gas, and allowing for ease of stacking of the cells in a separation unit. A bench-scale prototype demonstrates capture of CO2 from inlet streams of CO2 concentrations as low as 0.1% (1000 ppm) at a Faradaic efficiency of >90% and a work of 40 – 90 kJ per mole of CO2 captured. This presentation will cover the electro-swing concept, including the electrochemical and chemical processes responsible for the efficacy of the process, and discuss the preparation of the electrodes and separation units and their operation. An energetic analysis will be provided based on the overall electrochemical cycles and modeling of the flow and mass transport in the cells.