(703h) Moisture-Controlled CO2 Capture and Electrochemical CO2 Capture

• Introduction: Two novel approaches for CO₂ capture from air

Moisture-controlled CO₂ sorption is a novel phenomenon observed in strong-base anion exchange materials (AEMs) with quaternary ammonium cations, so-called moisture-swing materials^1,2. These materials can sorb and desorb very diluted CO₂ even from the ambient air (~ 400 ppm) by just changing the humidity. It can reduce the cost of Direct Air Capture (DAC) of CO₂ that is necessary to limit global warming to 1.5^â—¦C³. Moisture-swing for DAC applications have been first introduced by Lackner’s group^1,2.

Recently, electrochemical CO₂ capture has also been investigated for CO₂ capture both from flue gas and from air^4,5. Moisture-controlled and electrochemical CO₂ capture have much in common but differ significantly from temperature-swing or pressure-swing CO₂ capture. For example, their mass transport is driven by potential gradients either in chemical activity or electric fields. Both can perform a loading and unloading swing at constant temperature without introduction of a high-grade heat.

In this research, we apply electrochemical techniques to moisture-controlled CO₂ sorbents to investigate their kinetics. We compare the results of the analyses for these two novel sorbent systems to discuss their relevance in a unified way.

• Research: Formulate electrochemical analysis for moisture-controlled CO₂ sorption

In electrochemistry, cyclic voltammetry (CV) is a common technique to investigate the reduction and oxidation processes or electron transfer-initiated chemical reactions⁶. The output current from the analyte is plotted against the linearly increased and decreased voltage input. CV has also been used to characterize electro-swing sorbents for CO₂ capture⁴. The Randles−Sevcik equation can be used to differentiate diffusion-dominated and reaction kinetics-dominated hysteresis by analyzing the functional dependence of the peak current I_p on sweep speed v. If adsorption is the rate determining step, I_p is expected to increase linearly with v. In the other case, I_p scales square root of v, and we can extract the diffusion coefficient of the analyte. We extend this methodology to moisture-controlled CO₂ sorbents and develop theoretical and experimental approaches to such systems.

• Theory

For moisture-controlled CO₂ sorbents, we refer to the plot corresponding to the CV for electro-swing sorption as CWA (cyclic water activity measurement). The input (x-axis of a CWA plot) should be humidity instead of voltage and CO₂ flux should be used as output (y-axis of a CWA plot) instead of electrical current.

The attached figure shows a theoretically predicted shape of the CWA plot. The main difference from the CV is that diffusive transport of water inside a sorbent precedes that of Dissolved Organic Carbon, which generates the time lag that is not seen in the CV plot. Also, we take all the other differences into consideration to formulate the theoretical CWA plots and establish the protocol to extract kinetic parameters such as ion diffusivities from the plot. Furthermore, we aim to use this plot to differentiate chemical reaction kinetics at surfaces and diffusive transport kinetics inside a sorbent.

• Experiment

We will obtain a CWA plot using a preconditioned commercial anion exchange material. An open-flow experiment will be deployed, where the input humidity can be controlled using mass flow controllers. The CO₂ concentration in the inlet gas and outlet gas will be measured by Infrared Gas Analyzers located before and after the sample chamber.

• Expected outcome

We will apply an electrochemical technique to moisture-controlled CO₂ sorbents. It will provide a protocol to analyze the kinetics of moisture-controlled CO₂ sorbents in a quantitative way. Also, it enables us to compare the kinetic behaviors of the two novel CO₂ capture methodologies using the conceptually same plots.

References:

1. Environ. Sci. Technol. 2011, 45, 6670–6675
2. Phys. Chem. Chem. Phys., 2013, 15, 504
3. V. Masson-Delmotte, et al., Cambridge University Press, 2021
4. Energy Environ. Sci., 2019, 12, 3530
5. ACS Catal. 2020, 10, 13058−13074
6. J. Chem. Educ. 2018, 95, 197−206