Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Hu, Shu

Abstract

Photoelecrochemistry (PEC) and photocatalysis (PC), where solar energy is harvested to drive chemical reactions, are very promising pathways for renewable fuel production and achieving negative carbon emissions. However, many efficient light absorbing materials used as photo absorbers in PEC/PC suffer from corrosion induced by light, shortening the lifetime of PEC/PC devices developed with such materials and thus rendering those devices economically not viable. One example of such material is the narrow bandgap III-V semiconductor. For bare narrow bandgap III-V photoelectrodes, especially photoanodes, the lifetime is always less than hundreds of hours when operated in aqueous solution environment, a common environment for photoanodes to conduct oxygen evolution reaction (OER). Even with protection layers, narrow bandgap III-V photoanodes with long-term stability (>1000 hours) in an aqueous solution are yet to be developed. My thesis work is dedicated to increasing the stability of energy conversion devices, in particular the III-V PEC photoanodes, that incorporate solid/liquid interfaces. This thesis mainly consist of three thrusts: (1) reviewing the basic photocorrosion theory and mechanism; (2) on top of that, discovering effective mitigation strategies for extending device lifetime, and (3) developing a new generation of coatings for better stabilizing PEC devices, or with multifunctionalities, for performing other solid/liquid heterogeneous energy conversion process, such as electrochromism.In Chapter 2, we reviewed the common methods used in the characterization of solid/liquid interfaces. In particular, we proposed four important metrics as the indicator for a coating’s performance as a protective layer, and developed a stepwise pathway to quantify the four metrics. In Chapter 3, we first selected GaAs/TiO2/NiOx as the model system to study how the environmental factors and the photoelectrode design parameters affect the stability of photoelectrodes. Furthermore, we explored a new stabilization strategy that adopts an electrode structure of discrete photoabsorbers on self-passivating substrate, and demonstrated that this design allows new record stability (>600 hours) for GaAs based photoanode in 1M KOH. In Chapter 4, we investigated the stabilization of another interesting photoabsorber: n-GaP, by applying a new generation coating. The results prove that annealed (Ti,Mn)Ox has defective states that align better with the GaP valence band maximum (VBM) than all other coatings, which allows for more efficient charge transfer from the photo absorber to water for OER. The photocurrent of the GaP/ (Ti,Mn)Ox/IrOx photoanode can reach the bulk recombination limit and this photoanode can performed OER stably in neutral pH for over 100 hours. In Chapter 5, we developed a electrochromic coating with ternary alloy (Ti,Cr)Ox. This coating has both tunable electronic and optical properties for electrochromic color modulation and also excellent corrosion resistant properties like other TiOx based materials mentioned before. In Chapter 6, to summarize the properties of all the coatings introduced above, we investigated the coating/catalyst interface, and how the charge transfer is affected by composition, atomic concentration, annealing conditions, and oxidation states of the transition metal element in the coating. Overall, through this thesis work, we explored methods for solid/liquid interface corrosion mitigation from the aspects of environmental and electrode structural parameters, corrosion resistant morphological design, and protective coating development. Furthermore, we demonstrated that other exotic physical properties, like electrochromic properties, can be introduced to corrosion resistant coatings for broader applications. We believe these methodologies are widely applicable to other corrosion related systems and research fields.

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