Enhancing Electrochemically Driven CO₂ Separator Using Hydroxide Exchange Membranes

In a hydroxide exchange membrane fuel cell (HEMFC), hydroxide produced at the cathode readily reacts with CO₂ in air due to its acid-base reaction. The CO₂ then transports across the hydroxide exchange membrane to the anode where bicarbonates build up lowering the local pH until CO₂ evolution reactions become favorable. The pH gradient across the cell results in lower voltage output; however, it can act as an effective CO₂ separator from air. By focusing on CO₂ separation, rather than power output, an electrochemically-driven CO₂ separator (EDCS) was developed using a poly(aryl piperidinium) membrane.¹ Efficient and continuous CO₂ separation technologies are needed for various applications including upstream of HEMFC stacks in vehicles² as well as life support systems in submarines³ and manned spacecrafts.⁴

Previously, the EDCS was demonstrated as an effective and continuous method to remove CO₂ from ambient air using minimal hydrogen flow to power the separation, and various operating conditions and cell build configurations were investigated.⁵ This work seeks to further optimize the EDCS through improvements in ionomer, flow fields, temperature, and relative humidity for various CO₂ concentrations. Figure 1 shows the improvements in CO₂ removal from ambient air of a 25 cm² EDCS from the initial design inspired by fuel cells to the latest design using a nickel metal mesh as the combined flow field and gas diffusion layer at the cathode. The EDCS operating conditions and design is also evaluated for different applications as their requirements for air flow, CO₂ concentrations, operating conditions, energy and size restraints differ and must be taken into account. Additionally, this work explores methods to scale the EDCS for these high throughput operations such as planar stacks or spiral wound modules.

References:

1. J. Wang et al., Nature Energy, 4, 5, 392-398 (2019).

2. N. Ziv et al., Chemsuschem, 11, 7, 1136-1150 (2018).

3. Raatschen, W., et al. 47th International Conference on Environmental Systems, Charleston, South Carolina, (2017).

4. Isobe, J., et al. 46th International Conference on Environmental Systems, Vienna, Austria, (2016).

5. Matz, S., et al. J. Electrochem. Soc. 168, 014501 (2021).

Figure 1