Observation of the Microenvironment Around CO₂ Reduction Electrodes via Fluorescent Confocal Laser-Scanning Microscopy

Citation

Böhme, Annette Ellen (2024) Observation of the Microenvironment Around CO₂ Reduction Electrodes via Fluorescent Confocal Laser-Scanning Microscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ceqb-ew84. https://resolver.caltech.edu/CaltechTHESIS:05222024-182628149

Abstract

Electrochemical carbon dioxide reduction (CO₂R) is compelling because it enables the storage of renewable energy in the form of chemical bonds and offers the possibility to make carbon-based chemicals and fuels from a sustainable feedstock. A solid understanding of and control over the local microenvironment in and around CO₂R electrodes is crucial to optimize the device performance.

In this work, we develop and refine a technique to observe the microenvironment around CO₂R electrodes via fluorescent confocal laser scanning microscopy with three-dimensional sub-micrometer spatial as well as temporal resolution. We combine two fluorescent pH probes, DHPDS and APTS, to resolve the local pH value around operando CO₂R electrodes. The pH plays an important role in determining the CO₂R activity and selectivity. In a first step, we image the local pH value in and around CO₂R GDEs with a random pattern of trenches and find that the pH is locally enhanced inside trenches. This effect becomes more pronounced for narrower trenches, reaching a maximum at a trench width of 5 µm. With the help of multiphysics simulations we can show that the high pH inside trenches is closely related to an enhanced C₂₊ Faradaic efficiency. We harness this effect and fabricate CO₂R GDEs with tailored patterns of holes and trenches that allow a more systematic study of the influence of various micromorphology geometry parameters. We confirm experimentally that narrow holes and trenches exhibit a locally enhanced CO₂R selectivity and determine the most beneficial geometry parameters. We further use the developed technique to investigate the influence of a GDE's pore size on the local pH and with it, on the CO₂R selectivity. We observe that CO₂ transport is slower through smaller pores which can lead to switching of the reaction pathway and significantly alter the selectivity. We further investigate the importance of the microenvironment pH for CO₂R in acidic bulk electrolytes with the result that a non-acidic microenvironment pH, that can be reached at sufficiently high current densities, is required for the onset of CO₂R. Finally, we aim to extend the sensing capabilities and detect the local CO concentration in electrochemical devices but identify several challenges, including the probe reduction at the cathode.

Overall, we utilized fluorescent confocal laser-scanning microscopy to observe the microenvironment around CO₂R electrodes and correlate it with the CO₂R performance to gain a better mechanistic understanding of CO₂R and inform the design of future CO₂R electrodes.

Thesis Files