Electrochemical Production of Formate Directly from Amine-Based CO2 Capture Media
Authors: Bohlen Barbara , Daems Nick , Breugelmans Tom 
Published: September 2023

Introduction

There is an urgency for the development and establishment of technologies to deal with the effects of climate change and increasing temperature of the planet.1 The decrease in the CO2 emissions is a possible path, and the capture of CO2 from the atmosphere is another alternative to try and tackle the effects of climate change.2 The combination of capture and conversion of CO2 is a potential approach to achieve the net-zero emission goals and a circular economy for the future.3 Amine scrubbing is an industrially established capture technology that utilizes mainly monoethanolamine (MEA) as the capture solution to capture CO2 from post-combustion flue gases. The process has the disadvantage of a high energy demand, which prevents its wider application.2 This study proposes a novel capture and utilization (CCU) combination: the use of the MEA capture solution as electrolyte for the electrochemical CO2 reduction (eCO2R). In that way, the CO2 capture and conversion will be combined in the same medium, avoiding the energy-intensive regeneration step, thus saving energy, as well as generating products of industrial interest. Sn-based catalysts were primarily chosen due to their selectivity towards formate, one of the most straightforward reduction products from CO2.

Results

The eCO2R from the capture media (30 wt% MEA solutions, saturated with CO2) was promoted in a zero-gap type reactor, composed of an Sn-based cathode, a Ni foam anode and a bipolar membrane (BPM) separating the cathode and anode compartments. The BPM is responsible for providing protons to the cathode side of the electrolyzer, which promote the hydrolysis of the carbamate on the surface of the catalyst and thus enhances the CO2 availability and consequently the eCO2R. Figure 1 presents a scheme of the zero-gap electrolyzer, highlighting the hydrolysis of the carbamate in contact with the catalyst, and compares the faradaic efficiencies (FE) towards formate obtained by different setups of the zero-gap electrolyzer, at -50 mA cm-2.
The Sn nanoparticle (SnNP)-based catalysts show a low efficiency for the eCO2R from the capture media (up to 5%). As published in the literature, surfactants are capable to inhibit the hydrogen evolution reaction (HER) in electrochemical systems and thus promote the eCO2R.4 The surfactant cetyltrimethylammonium bromide (CTAB) was therefore added to the system and the FE towards formate increased, although merely up to 6.43%. To further increase the surface area available for the eCO2R, a metal gauze was introduced as support for the working electrode (WE). Here, a Cu gauze with electrodeposited Sn (SnED) was used as WE and the obtained FE was 70% higher than for the carbon supported SnNP catalysts, up to 8.49%, without the addition of the surfactant. The hydrophilic nature of the metal surface (in comparison to the carbon paper substrate of the NPs) and a bigger surface area could be the reasons behind this enhancement in the FE using metal WE. Future studies will focus on the further enhancement of the FE towards formate.

Conclusion

This study shows the feasibility of a novel CCU technology: the electrochemical reduction of CO2 to formate from an amine-based capture medium. Sn-based catalysts lead to an FE of up to 8.49%. The use of a metallic electrode lead to a larger enhancement of the FE, in comparison to the addition of a surfactant to the electrolyte for the SnNP-based catalyst. There is yet room for further improvement of the faradaic efficiency by the combination of the metal electrode and the use of surfactants to inhibit the HER, as well as the use of catalysts with higher selectivity towards the product, such as Bi. The use of a zero-gap electrolyzer shows the feasibility of scaling up the system to industrially relevant dimensions and the easiness of incorporating the electrochemical system to the end of pre-existing capture plants.

References

Ghiat, I., & Al-Ansari, T. (2021). Journal of CO2 Utilization, 45.

Gutiérrez-Sánchez, O., Bohlen, B., et al. (2022). ChemElectroChem, 9(5), e202101540.

Li, M., Irtem, E., et al. (2022). Nature Communications 2022 13:1, 13(1), 1–11.

Chen, L., Li, F., et al. (2017). ChemSusChem, 10(20), 4109–4118.

Figure 1