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ENGINEERING MOLYBDENUM SULFIDE ELECTROCATALYSTS AND SILICON PHOTOCATHODES FOR HYDROGEN PRODUCTION VIA SOLAR WATER SPLITTING A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY JESSE D. BENCK AUGUST 2015 © 2015 by Jesse Daniel Benck. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/df442wf0767 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Thomas Jaramillo, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Stacey Bent I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Bruce Clemens Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost for Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii iv Abstract 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. v 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. vi Acknowledgements Graduate school is a challenging experience in many ways. I would not have been able to complete this dissertation without help and support from many wonderful mentors, colleagues, friends, and family members. I would like to begin by thanking Professor Thomas Jaramillo, who has been the best doctoral advisor I could have hoped for. He is an outstanding scientist and engineer, with a remarkable knowledge of the literature and a clear, systematic approach to problem solving. He is also an unusually gifted communicator and generous scientific community member. Perhaps most unusual and impressive are his deep commitment to his students’ development and his incredibly positive and encouraging attitude. Tom has given me many amazing opportunities to interact with respected scientists, present my research results, and teach a course, all of which helped me to develop my skills as a researcher and educator. I will be forever grateful for the many ways in which he has deeply impacted my life during my time at Stanford. I am also grateful for all the advice and supported provided by my reading committee members, Professors Stacey Bent and Bruce Clemens. Stacey helped me make the transition to graduate school when she took me into her group as a rotation student during my first quarter at Stanford. I also served as a teaching assistant for her course, Chemical Engineering 25E, for two years, and learned a great deal from Stacey’s incredible organization and carefully considered approach to teaching. Bruce has also had a great influence on me due to his calm and thoughtful demeanor, insightful questions, and wonderful sense of humor. I am very thankful that Bruce allowed me to become an honorary Clemens Group member during my last two of years at Stanford. My interactions with Stacey and Bruce were always encouraging and inspiring. It would have been impossible for me to complete any work at all without the continual support of the Chemical Engineering Department staff members. Over the years, Jeanne Cosby has been an especially important source of guidance. Pam Juanes and Rob Rome always ensured that the department ran smoothly. I thank Victoria Lee, Annie Jensen, and Olayinka Popoola for providing administrative support directly to the Jaramillo Group. Many other staff members including Pamela Dixon, Sandra Handy, Andrea Hubbard, Eric Ngyuen, and others also provided critical help over the years. Several facilities staff members including Todd Eberspacher, Ray Hewlett, and Justine Sousa also helped me overcome many challenges. vii Much of the research that I completed at Stanford was performed at shared user facilities, which are truly remarkable resources, not only due to the state-of-the-art equipment they contain, but also because of the dedicated staff members that provide support to users. I would like to thank Richard Chin, Bob Jones, Chuck Hitzmann, Jeff Tok, and Arturas Vailionis of the Stanford Nano Shared Facilities for training and support on many characterization tools that were critical to my work. I also thank Mary Tang, Maurice Stevens, James Conway, Uli Thumser, Mahnaz Mansourpor, Usha Raghuram, Michelle Rincon, and Nancy Latta of the Stanford Nanofabrication Facility for training and support on semiconductor processing tools. Throughout my time at Stanford, in addition to improving my research skills, it has also been important to me to develop my abilities as

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