Promoting CO₂ Release from CO₃ ²⁻-Containing Solvents during Water Electrolysis for Direct Air Capture

Abstract

The pH swings from water electrolysis are leveraged to condition OH--based facile CO2 capture solvents using an electrochemical flow cell for direct air capture (DAC). Besides demonstrating the DAC using a membrane contactor, promoting CO2 release from a CO3 2--containing solution at the anode is specifically studied by adjusting the volumetric flow rate, anode chamber volume, residence time, and K2CO3 concentration. Through case-by-case comparisons coupled with modeled results, increasing current, reducing volumetric flow rate, and/or reducing CO3 2- concentration are the effective methods to promote CO2 release, whereas enlarging the anode chamber volume poses a minor effect. Moreover, the discrepancies between the experimental and modeled results may be caused by H+ crossover rather than K+ transport through the Nafion membrane during water electrolysis based upon the total alkalinity measurements for the K2CO3 solutions gleaned from the anode. It is believed that such results will provide guidance to design and operate an electrochemical flow cell for electrochemistry-assisted DAC and point source CO2 capture.

Numerical Modeling and Comparative Analysis of Electrolysis and Electrodialysis Systems for Direct Air Capture

“…H + ions are present in the solution, and they can also be transported by the CEM. 18 In addition, the selectivity of the CEM between counterions and co-ions is not completely ideal, 21 which means that OH − ions could also be transported through the CEM from another direction. The crossover of the other anions, CO 3 2− and HCO 3 − ions, the degree of which will be affected by their concentrations and ionic mobility, is not captured in the current simplified model.…”

“…Determining the exact K + transport efficiency under different cell operating conditions poses a difficulty. Gao et al 18 conducted experimental research on a laboratoryscale water electrolysis cell for DAC, providing detailed measured data under various operating conditions. Using established mathematical models in this study, Gao et al's system was simulated with the same operating conditions, resulting in the determination of K + transport efficiencies under different cases and conditions.…”

“…To verify the credibility of the prediction methods and the proposed system model, simulations were conducted for cases A, B, and C of the electrolysis system examined in Gao et al's work. 18 Figure 4(b)−(d) display the comparison of anode solution pH between the simulation result and the experimental data. The most significant deviation occurs when the current is 6 A in case C, where the simulated anode solution pH is 0.52 lower than the experimental outcome.…”

“…Through optimization of cell and membrane configurations, they found the energy input to be below 1.4 MJ for the synthesis of 1 mol of methanol. Gao et al 18 conducted further experimental investigations into the CO 2 release performance of K 2 CO 3 electrolysis, emphasizing the detrimental effect of H + crossover in CEM. Rau et al 19 explored the global potential of synergistic hydrogen generation and CDR using seawater electrolysis.…”

“…Figure 4. Validation of K + transport efficiency: (a) linear fitting of K + transport efficiency, (b)−(d) comparison of simulated anode solution pH with the measured data in Gao et al18 …”