Engineering catalysts and interfaces for improved stability in photoelectrochemical water splitting
Keywords: Chemical Engineering.

This thesis describes efforts to improve the stability of photoelectrochemical (PEC) water splitting devices, studying new photoanode, photocathode, and catalyst materials and interfaces, supporting the use of PEC for renewable hydrogen generation. While renewable electricity sources like wind and solar power have become cost-competitive with fossil fuels, the inherent variability of these power sources limits their ability to completely displace fossil fuels for global energy production. Hydrogen offers promise as an energy carrier and intermediate, where energy could be stored chemically and hydrogen and then used in a range of applications (e.g. fuel cells for transportation). However, the stability of PEC water splitting systems is limited due to the challenges of stabilizing semiconductor materials, catalysts, and interfacial layers in corrosive electrolytes. This dissertation provides strategies to improve the stability of PEC water splitting systems by engineering catalysts and semiconductor-catalyst interfaces. First, we focus on electrocatalysts and photoanodes for water oxidation. We investigate the degradation mechanisms of thin-film SrIrO3, a high performance oxygen evolution reaction (OER) catalyst. Combining transmission electron microscopy to image the cross-sectional structure of the catalyst and secondary ion mass spectrometry run in a helium microscopy for lateral elemental mapping, we develop a 3D picture of the catalyst after accelerated degradation tests under OER operation. The SrIrO3 film becomes thinner and rougher but maintains a uniform lateral distribution of Sr and Ir, providing evidence for a layer-wise dissolution mechanism. We then develop a spin coating procedure to coat IrO¬¬x and biphasic strontium chloride/iridium oxide (SrCl2:IrOx) OER catalysts onto silicon for use as photoanodes. The SrCl2:IrOx photoanode produces 0.1 V more photovoltage than the IrO¬x photoanode due to an improved silicon-electrolyte interface, and post-test reveals film cracking and delamination as a primary degradation mode. The second part of this dissertation focuses on pairing MoS2 catalysts with single- and dual-junction III-V absorbers for stable photocathodes. We investigate a surface architecture for GaInP2 photocathodes consisting of an AlInP window layer (WL) to reduce surface recombination, a thin GaInP2 capping layer to protect the WL from corrosion, and an MoS2 catalyst. The MoS2/CL/WL/GaInP2 photocathode displays the highest PEC performance and most durability, producing current for > 125 h. By comparison, the MoS2/WL/GaInP2 photocathode degrades the fastest, likely due to the WL dissolving. In situ optical microscopy illustrates the progression of degradation during PEC testing. We then develop unassisted PEC water splitting devices pairing MoS2 catalysts with tandem III-V absorbers grown by inverted epitaxy, which allows for high-quality growth of non-lattice matched semiconductors. We compare the stability of GaInAsP/GaAs tandem devices coated with MoS2 or PtRu catalysts, and the MoS2-protected devices lasts 5 times as long as the PtRu-protected device. A MoS2/GaInP2/GaInAs device demonstrates an efficiency of 12.0%. Finally, we design a photoreactor platform for on-sun testing of PEC devices and demonstrate its use with a MoS2/GaInP2/GaAs tandem-absorber device. We demonstrate unassisted PEC water splitting under real world operating conditions using the sun as the illumination source and test the stability of the photoelectrodes under both sunny and partly cloudy conditions. The photoreactor setup and outdoor testing capabilities can accelerate the scale-up of PEC and other solar fuels technologies. This dissertation explores catalysts and semiconductor-catalyst interfaces in thin-film OER catalysts, silicon photoanodes, and III-V photocathodes and presents a diverse array of techniques to study degradation processes, from nanoscale microscopy and spectrometry to macroscale optical imaging. We also present a summary of stability in the field of PEC, illustrating the need for improved durability in these systems