Using Capacitive Deionization for Direct Air Capture of Carbon Dioxide: Theory and Demonstration of the Bicarbonate-Enriched Alkalinity Concentration Swing

Abstract

To respond to the climate crisis and avoid its harms to vulnerable communities across the world, humans' collective priority must be to aggressively reduce greenhouse gas emissions. Because some emissions associated with socially just consumption will, however, remain hard to avoid, a matching gigatonne-per-year scale of carbon dioxide removal (CDR) will be required to offset residual emissions. Direct air capture (DAC) technologies are a category of industrial CDR methods that use chemical and physical processes to capture CO2 directly from the atmosphere. This thesis introduces a framework for a new solvent-based DAC approach, the bicarbonate-enriched alkalinity concentration swing (BE-ACS), and demonstrates how the two key steps of the BE-ACS cycle, bicarbonate selection and concentration, can be experimentally implemented using membrane capacitive deionization (MCDI) technology. In Chapter 2, we introduce the foundational alkalinity concentration swing (ACS) concept, theoretically demonstrating how DAC can be achieved through a cycle of concentration and dilution of dissolved inorganic carbon in an aqueous alkaline solution that has been exposed to the atmosphere, to increase and decrease, respectively, the solution partial pressure of CO2. In Chapter 3, we experimentally implement the concentration step of the ACS using MCDI, determining that it underperforms expectations due to the selectivity of MCDI microporous electrodes for carbonate ions over bicarbonate ions. In Chapter 4, we theoretically demonstrate how the basic ACS can be enhanced through bicarbonate-enrichment, achieving the BE-ACS by adding a module before the concentration module that selects for bicarbonate ions, which also increases the solution CO2 partial pressure. In Chapter 5, we experimentally implement the separation step of the BE-ACS cycle by using MCDI for bicarbonate-enrichment, exceeding the expectations from current Donnan models for microporous MCDI electrodes. In Chapter 6, we experimentally demonstrate how the BE-ACS framework, which operates in a sluggish kinetic regime below pH 11, can be meaningfully enhanced through the use of the carbonic anhydrase enzyme as a promoter. Finally, in Chapter 7, we bring together the tools developed in Chapter 4 and the results from Chapter 5 to model the MCDI-driven BE-ACS cycle, depicting two representative cycle regimes that provide candidates for technological exploration.