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


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.


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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:CO₂ reduction; gas diffusion electrodes; microenvironment; fluorescent confocal laser-scanning microscopy
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Atwater, Harry Albert
Thesis Committee:
  • Goddard, William A., III (chair)
  • Atwater, Harry Albert
  • Greer, Julia R.
  • Ardo, Shane
Defense Date:2 April 2024
Funding AgencyGrant Number
Office of Basic Energy Sciences (BES)DE-SC0021266
Record Number:CaltechTHESIS:05222024-182628149
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for chapter 2 1039/D2EE02607DDOIArticle adapted for chapter 3 and 4 adapted for chapter 6
Böhme, Annette Ellen0000-0003-1109-3428
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16410
Deposited By: Annette Boehme
Deposited On:24 May 2024 21:15
Last Modified:28 May 2024 17:47

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