Linking Morphology and Multi-Physical Transport in Structured Electrodes

Motivated to advance the renewable energy production and diversify the technologies for storable energy carriers, my work is concentrated on the characterization and the optimization of electrode morphologies applicable in photoelectrochemical water-splitting and electrochemical carbon dioxide reduction. The geometry of the electrode-electrolyte interface affects the multi-physical transport properties in a chemical energy conversion device. The structuring of the solid-liquid interface can improve the light management and assist to overcome the limiting charge transport in a semiconductor electrode. However, it also has implications on the species concentration, the pH and the diffusive mass transport in the electrolyte. Characterizing and quantifying the link between the morphology and those multi-physical transport processes are fundamental to design electrode geometries that enhance the energy conversion efficiency. In order to study photoelectrodes with complex and anisotropic morphologies, accurate representations of the 3D electrode structures are required. A coupled experimental-numerical approach was developed to digitalize the morphology of two photoelectrodes, a particle-based lanthanum titanium oxynitride (LTON) and a ‘cauliflower-like’ structured hematite (α-Fe2O3) electrode, using high-resolution FIB-SEM tomography. Key morphological parameters were extracted from the digital model. Simulations using the exact geometry of the LTON electrode investigated the correlation between the morphology and the multi-physical transport properties in a photoelectrochemical water-splitting device. Light absorption, local current densities and ion concentration distributions in the electrolyte have been computed to link material bulk properties to the incident-light-to-charge-transfer-rate-conversion by morphology-dependent parameters. The developed numerical tools were adapted for electrochemical carbon dioxide reduction on an inverse-opal silver electrode, where concentration gradients played a more significant role. The species mass transport in the electrolyte determined the selectivity of the competing surface reactions, where mesoporous structuring of the electrode favored the carbon dioxide reduction. The calculations reproduced experimental results from the literature, supporting and quantifying the intrinsic pH-dependency of the unwanted water-splitting reaction. Lastly, the light management in thin film metal oxide photoelectrodes for water-splitting was optimized by numerical simulations of electromagnetic wave propagation. Wedge patterns of thin film hematite with a reflective backing layer on a flexible polyimide substrate were used to enhance the light absorption by resonant and geometric light trapping. In order to fabricate the patterned thin film photoelectrodes with precise control over the microstructure, an experimental platform was developed based on a template-stripping method. In conclusion, the methods developed in this work have been proven to characterize and quantify the effects of the morphology on multi-physical transport. Furthermore, design guidelines on the morphology and the operating conditions were derived from the numerical results in order to optimize the electrode performance. Novel electrode architectures were proposed to enhance the reaction selectivity in mesoporous electrodes for the CO2 reduction and improve the light trapping in thin film water-splitting photoelectrodes.


Advisor(s):
Haussener, Sophia
Year:
2019
Publisher:
Lausanne, EPFL
Keywords:
Laboratories:
LRESE


Note: The status of this file is: EPFL only


 Record created 2019-03-27, last modified 2019-07-10

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