Engineering molybdenum sulfide electrocatalysts and silicon photocathodes for hydrogen production via solar water splitting [electronic resource]
Keywords: Chemical Engineering.

Hydrogen is a critical chemical reagent and energy carrier, but it is currently produced from fossil fuels, which are limited in supply and create harmful CO2 emissions when consumed. The development of new, sustainable methods for hydrogen production represents an important research challenge. Photoelectrochemical (PEC) water splitting, a process in which H2O is split into H2 and O2 using the energy from sunlight, is a promising technology for renewable hydrogen production. To make this process viable for widespread implementation, efficient, inexpensive, stable, and scalable PEC water splitting materials and devices must be developed. Creating active catalysts, strategies for corrosion prevention, and techniques for successfully integrating all required device components are especially important barriers to overcome. The first part of this dissertation focuses on molybdenum sulfide catalysts for the electrochemical hydrogen evolution reaction (HER). We begin by analyzing a selection of state-of-the-art molybdenum sulfide HER catalysts to identify best practices for measuring catalytic activity and design principles for creating even more effective catalyst materials. Then, we perform a detailed investigation of an amorphous molybdenum sulfide (MoSx) catalyst. Using a new room temperature wet chemical procedure, we synthesize a highly active form of this amorphous MoSx. Then, we attempt to understand the physical and chemical characteristics of this material that result in its high activity. Using electrochemical measurements and ex situ spectroscopic characterization, we reveal that this material initially possess a composition of MoS3, but after catalysis, the surface is reduced to a composition and chemical state resembling MoS2. To understand more about the mechanism of the catalyst transformation and the nature of the active phase under operating conditions, we use advanced in situ characterization techniques including ambient pressure X-ray photoelectron spectroscopy and environmental transmission electron microscopy to track the transformation of amorphous MoSx nanoparticles under hydrogen evolution conditions. These experiments show that the surface of the amorphous MoSx catalyst is dynamic: the initial catalyst reduction forms the active surface of amorphous MoS2, but further transformation continues during electrochemical operation, and some portions of the catalyst are eventually converted to crystalline MoS2. This process could contribute to the deactivation of the amorphous MoSx catalyst after prolonged operation. Our efforts next shift to the development of integrated water splitting photocathodes, which incorporate both HER catalysts and semiconductor light absorbers. Silicon represents a particularly promising small band gap semiconductor for application in PEC water splitting devices, but this material possesses low catalytic activity and poor durability in aqueous electrolytes. We demonstrate that molybdenum sulfide nanomaterials can provide both corrosion protection and catalytic activity in silicon photocathodes. We engineer thin, conformal MoS2 surface coatings to protect silicon absorbers, resulting in photocathodes that can operate for 100 hours with no loss in performance. We study the atomic-scale surface structure of these devices and identify the characteristics of the MoS2 layer that provide both catalytic activity and excellent stability. To further improve the performance of these structures, we incorporate a molybdenum sulfide molecular cluster catalyst and obtain further gains in the device performance. Finally, we develop new Si photocathode architectures that address the challenge of successfully integrating multiple water splitting device components while retaining a very high photovoltage from the illuminated semiconductor. Silicon surfaces and interfaces control many key aspects of device performance. We focus on engineering these interfaces to reduce surface-mediated recombination using strategies inspired by high performance silicon photovoltaics. These efforts result in Si photocathodes with improved photovoltage and provide a platform for the fabrication of integrated, monolithic dual absorber water splitting devices. In summary, this dissertation covers fundamental studies of molybdenum sulfide HER catalysts as well as device engineering efforts to create high performance silicon photocathodes. These results represent important advancements towards large-scale renewable H2 production using PEC water splitting.