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X-ray analysis reveals cause of performance oscillations in CO2 electrolysis devices

Operando wide-angle X-ray scattering was used to analyse a working CO2 electrolysis reactor in order to identify the root cause of an observed deterioration in device performance. The insights gained may aid the development of stable, more efficent electrolysers.

CO2 electrolysis is a promising technology for the carbon- neutral production of chemicals for plastics, solvents or pharmaceuticals. However, membrane electrode assembly (MEA) CO2 electrolysers often suffer from poor stability and a resulting deterioration in electrolysis performance, which is a significant barrier towards the commercialisation of these devices. The durability issues are known to be due to flooding of the cathode and the formation of salt precipitates, causing a suppression in the CO2 reduction reaction and an increase in the competing hydrogen evolution reaction (HER), but there is limited understanding of how and why the cathode floods and salt deposits occur. There is therefore an urgent need to comprehend water management in these devices and to engineer electrodes that allow both stable and efficient electrocatalytic performance.

CO2 electrolysis performance deterioration often occurs via a chaotic oscillatory process. To gain greater insight into the degradation mechanisms and to understand these oscillations, this work focused on operando X-ray diffraction (XRD) analysis of an operational MEA device with an electrode area of 64 mm2, comparable to relevant industrial sizes (Figure 84) [1]. Wide-angle X-ray scattering (WAXS) experiments were carried out at beamline ID31 to monitor the amount of electrolyte flooding and salt formation on the cathode over time during CO2 electrolysis, while in-line gas chromatography and mass spectrometry were used to correlate the changes to the cathode and anode product distribution.

Figure 85a shows the evolution of electrolyte content and salt deposition in the cathode during CO2 electrolysis at 100 mA/cm2 as a function of time. Salt precipitation is clearly observed, although, surprisingly, the salt is deposited before substantial water penetration, in contrast to previous thinking (that the substantial water penetration brings along the salt). The X-ray analysis also demonstrates that salt precipitates as bicarbonates, despite carbonates being the dominating ionic species transporting through the membrane. This is attributed to the lower solubility of bicarbonates compared to carbonates. It is therefore proposed that the salt formation causes electrolyte build-up, exacerbating the flooding and

Fig. 84: a) The operando wide-angle X-ray scattering (WAXS) characterisation of MEA CO2 electrolyser and the 2D diffraction pattern. b) Enlarged view of the highlighted area in (a) where X-rays enter into the electrolyser.

(c) Typical chaotic results seen during CO2 electrolysis reactor degradation.