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1 0 7 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

In operando investigations of oscillatory water and carbonate effects in MEA-based CO2 electrolysis devices, A.B. Moss (a), S. Garg (a), M. Mirolo (b), C.A. Giron Rodriguez (a), R. Ilvonen (b), I. Chorkendorff (a), J. Drnec (b), B. Seger (a), Joule 7(2), 350-365 (2023); https:/doi.org/10.1016/j.joule.2023.01.013 (a) Surface Physics and Catalysis (SurfCat) Section, Department of Physics, Technical University of Denmark (Denmark) (b) ESRF

REFERENCES

[1] A.B. Moss et al., J. Power Sources 562, 232754 (2023). [2] S. Garg. et al., Energy Environ. Sci. 16, 1631-1643 (2023).

causing a shift in the CO2 reduction reaction selectivity towards the hydrogen evolution reaction (HER), which is related to a drop in cell potential.

This information made it possible to develop a comprehensive hypothesis for the oscillations, as illustrated in Figure 85b. It is thought that initially, the electrode gets slightly wetted by electrolyte, causing KHCO3 crystals to deposit, which creates a blockage in the electrode that prevents sufficient CO2 access. This is supported by observations of both a decrease in CO2 reduction selectivity in favour of HER and decreased carbonate (which is produced via CO2 equilbration with the electrolyte) crossing over the membrane. Thus, there appears to be a loss of CO2 reaching the catalyst membrane interface. This leads to a shift towards OH anions transporting through the membrane, causing an increase in membrane conductivity and thereby decreasing the overall cell potential.

Additionally, the lack of CO2 increases catalytic activity and decreases the potential. Beyond this point, the cathode gets flooded in the electrolyte. Given that the incoming cathode gas is dry CO2, this electrolyte/water must be coming from the anode, but the reason for this increased water penetration is still unclear. One factor that could lead to increased water penetration is the fact that membranes in the OH form are more hydrated than in the CO32 , and this increased swelling could potentially lead to an increased water transfer from the anode to the cathode. Another factor is that osmotic drag will naturally pull water from the anode to the cathode to hydrate the salts. In any case, water does return to the cathode.

Overall, this work identifies the relationship between selectivity, flooding and salt deposition in an MEA electrolyser and determines how this affects potential and ion selectivity crossing through the membrane to the anode. The results, along with those of follow-up work [2], suggest that proper ion management is an important key to enhanced durability throughout the device.

Fig. 85: a) Evolution of electrolyte content and salt precipitates (KHCO3) in the cathode during CO2 electrolysis at 100 mA/cm2 as a function of time. The change in colour intensity from purple to yellow indicates an

increase in X-ray signals, corresponding to the electrolyte content and KHCO3 salt precipitation respectively. b) Hypothesis of the cause of the chaotic, oscillatory nature for CO2 electrolysis reactor degradation.