Electrochemical Direct Air Capture of Carbon Dioxide by a Redox-Mediated Salt Splitting Process

Abstract

Direct air capture of carbon dioxide is an important emerging direction for electrochemical separation technology. Carbon dioxide removal has a critical role to play in the coming century as a complement to deep decarbonization efforts, because removal may offset emissions we cannot avoid, such as agriculture for food security. Direct air capture might even be used to decrease the atmospheric carbon dioxide level if the value at which it stabilizes is deemed too high. Electrochemical methods allow us to power a carbon removal device directly by carbon-free electricity. Here, we present a salt splitting process, based on bipolar membrane electrodialysis, which begins with a neutral electrolyte (e.g. KCl), and creates a strong base (e.g., KOH) and a strong acid (e.g., HCl). The base may then be used to absorb carbon dioxide from ambient air; the acid is then used to extract the carbon dioxide by recombining with the base, reversing the salt splitting process. The process is driven by iron(II/III) hexacyanide redox on carbon cloth electrodes, which is kinetically facile and contributes relatively little to the cell voltage.Energy required for redox-mediated salt splitting depends on contributions from water dissociation in the bipolar membrane, ohmic resistances in ion exchange membranes and electrolyte layers, and electrode reactions. We show that the largest contribution to the energy required comes from the bipolar membrane, motivating further research on bipolar membrane development. We also discuss the role of ion exchange membranes in this system: these membranes must provide high ion conductivity while providing selectivity against leakage of acid and base products. Several commercial membranes were screened for conductivity and water content, and their electrodialysis performance is compared. Finally, we use technoeconomic modeling to assess how electrodialysis operating conditions and electrolyte composition affect energy, land, and water demands of a continuously operating direct air capture process, with redox-mediated salt splitting as the centerpiece. Figure 1