Energy Environ. Sci. https://doi.org/10.1039/D0EE02219E (2020)
Control of water (H2O) availability at the cathode is a crucial aspect of the electrochemical carbon dioxide reduction reaction (CO2RR). On a basic chemical level, protons from H2O are required for the formation of the reduced carbon products. However, providing too much water will typically lead to an increase in the competitive hydrogen evolution reaction (HER). Moreover, in practical flow cell membrane electrode assemblies (MEA), the movement of water from the electrolyte can lead to flooding of the gas diffusion electrodes (GDE), as well as unwanted and harmful crossover reactions. Measuring water at the CO2 cathode presents a challenge due to the enclosed design of electrolysers, therefore studies to date have relied on data only from the in- and outlets. Now, Curtis Berlinguette and colleagues develop an analytical electrolyser to gather information directly at the cathode.
The design incorporates relative humidity (RH) and temperature (T) sensors directly into the cathode flow field (pictured). The analytical electrolyser provides in situ data on the flow of water within the cell. This data is used, in turn, to inform a computed 3D model of mass transport and fluid flow around the cathode. Interestingly, the proportion of water at the cathode–membrane interface remains constant regardless of the level of humidity in the input CO2 gas stream, even when the cell is being operated at high current density. In the case of dry CO2 streams, this effect is the result of a high flux of water through the membrane from the electrolyte that flows through the anodic chamber. The flow of water brings with it an increase in electrolyte crossover, including potassium (K+) ions from the KOH electrolyte. In the cathodic chamber, the precipitation of less-soluble carbonates (K2CO3, KHCO3) leads to rapid loss of efficiency in the cell. This hypothesis is matched experimentally by the inclusion of a window in the cell through which salt deposition and flooding can be visualised.
Humidification of the CO2 stream emerges from this study as a simple and effective control to prevent unwanted crossover reactions in flow cells. More broadly, the successful use of diagnostic sensors within MEAs can be used throughout the electrolyser and fuel cell communities to understand the root causes of issues affecting performance and stability.