(Digital Presentation) Electrochemical Regeneration of Spent Alkaline Absorbent from Direct Air Capture

Abstract

The atmospheric carbon dioxide (CO2) concentration has increased by nearly 49% compared to preindustrial levels. This increase is primarily ascribed to anthropogenic activities and has caused rapid climate change in recent years. Direct air capture (DAC) technologies, aiming to remove CO2 directly from the atmosphere, are indispensable for mitigating the climate risks.One of the most studied DAC technologies is wet scrubbing with alkaline hydroxide solutions (e.g., KOH). Recently, we developed an electrochemical system that can regenerate spent alkaline absorbents from DAC and desorb high-purity CO2 gas under atmospheric pressure. The electrochemical cell consisted of three compartments, i.e., an anode compartment, an “acidifying” compartment, and a cathode compartment (Figure 1). During the operation, protons (H+) produced from the H2 oxidation at the anode are transported to the acidifying compartment where the spent absorbent coming from the air contactor is fed. The decreasing pH of the acidifying solution facilitates the conversion of bicarbonate/carbonate ions to carbonic acid (H2CO3 *, including dissolved CO2). When the chemical potential of oversaturated H2CO3 * is higher than the partial pressure of CO2 in the gas phase, CO2 can be desorbed. In the cathode compartment, the K+ from acidifying compartment combines with the locally produced hydroxides (OH−) to regenerate the alkaline absorbent. Overall, the electrochemical cell creates a pH swing in two adjacent compartments. CO2 gas is desorbed at low pH while the alkaline absorbent is regenerated at high pH.In the acidifying compartment, the cations changed from K+ to H+ that can displace the equilibrium of the carbon species towards the desorption of CO2 gas. However, this phenomenon brings two limitations to large-scale application of the system. Firstly, the depleting of K+ and HCO3 -/CO3 2- leads to the decrease of conductivity of the acidifying solution, which induces the increase of ohmic losses in the cell. Moreover, the desorbed CO2 gas bubbles in the cell also contributes to the electrical resistance of the cell. Therefore, the current work investigates optimization strategies to reduce the energy consumption of the system.The first strategy is to desorb CO2 into a lower CO2 partial pressure gas phase instead of atmospheric pressure. A decrease in the CO2 partial pressure from 0.9 to 0.3 bar displayed a maximum increase of 13% in gas production due to higher driving force for desorption and a maximum decrease of 7% in cell potential due to reduced gas bubble presence in the cell. The second strategy adds background electrolyte (phosphate or sulfate) into the absorbent. The adding of background electrolyte provided extra conductivity in the acidifying solution when most of the K+ and CO2 have been removed. Furthermore, the background electrolyte reduced the pH of the acidifying solution, which benefits the desorption of CO2. The lowest specific energy consumption achieved in this study was 249 kJ mol-1 CO2 captured under 150 A m-2. This low energy consumption makes the technology competitive among its counterparts. Finally, an equilibrium model has been developed that simulates the performance of the system and indicates further improvement potentials.This work demonstrates the proof of concept of a novel electrochemical direct air capture process. Our main conclusions show that the process has a competent energy consumption and potential for upscaling with the tested optimization strategies. Figure 1