Nanostructured Transition Metal Sulfide Catalysts for Electrochemical Water Splitting

Open Access
Author:
Wiltrout, Alex Michael
Graduate Program:
Chemistry
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 05, 2016
Committee Members:
  • Raymond Edward Schaak, Thesis Advisor
  • Christine Dolan Keating, Thesis Advisor
  • Thomas E Mallouk, Thesis Advisor
Keywords:
  • Nanoscience
  • Metal sulfide
  • Water splitting
  • Electrocatalysis
  • Hydrogen evolution
  • Oxygen evolution
Abstract:
With the world’s population steadily on the rise, there will continue to be an ever-increasing demand for energy. However, fossil fuels, which currently supply the world with an overwhelming portion of its energy needs, are quickly becoming depleted at a much faster rate than they are being generated. Most people use fossil fuels for their everyday energy needs, namely because compared to other alternative energy sources, it is cheaper and much more readily accessible. However, if one is looking to invest in a sustainable, long-term solution to the energy crisis that we currently face, these non-renewable energy sources are less than ideal. One possible solution to this problem is to begin using hydrogen as a fuel source instead. Hydrogen is an ideal alternative for a number of reasons, namely because it possesses the largest energy density by mass of any element, and that burning it produces no harmful byproducts, only water. The current industry standard for hydrogen production is primarily limited to production via steam-methane reformation and the water-gas shift reaction. However, these processes are not ideal for large-scale hydrogen production, and are detrimental to the environment because of the large amounts of CO and CO2 that are produced. One potentially cleaner alternative is proposed through electrochemical water splitting, whereby water is decomposed in hydrogen and oxygen. However, materials that catalyze these reactions are often quite rare and expensive, examples being Pt and IrO2. For this reason, the work hereafter aims to seek out new Earth-abundant materials, with a focus on transition metal sulfide systems, which can be used as catalysts to help catalyze the decomposition of water. Our work begins by investigating the catalytic activity of CuCo2S4 nanoparticles for the oxygen evolution reaction. Much of the focus insofar has been primarily concerned with transition metal oxide-based materials, however, metal sulfide systems are slowly gaining momentum. Those that do exist and have been tested for the oxygen evolution reaction (OER), often show moderate activity. By introducing additional elements into the system, we hope to further enhance the material’s OER activity. Highly crystalline and nonagglomerated colloidal CuCo2S4 nanoparticles, which were previously inaccessible in the literature, were synthesized using low-temperature, solution-based synthetic routes. The CuCo2S4 nanoparticles were found to be highly active for OER under strongly alkaline conditions. Surface studies of the material suggest that mixed-metal sulfides, such as CuCo2S4, may in fact serve as precursors to oxides and/or hydroxides, which are likely the catalytically active species in solution. In addition to the work on the OER half reaction, a number of cobalt (Co3S4, CoS, Co9S8) and nickel sulfide (Ni3S2, α-NiS, Ni9S8, Ni3S4) nanoparticle systems were investigated for use as potential hydrogen evolution reaction (HER) electrocatalysts. These materials were the target of this study because of their relatively low cost and high abundance within the Earth’s crust, as well as because they are know hydrodesulfurization (HDS) catalysts. Both HER and HDS rely upon a process by which hydrogen reversibly binds to the surface of a material. The hope was that one could then selectively target active HER catalysts, by identifying what materials are also good HDS catalysts. However, upon testing the cobalt and nickel sulfide nanoparticles, a correlation between HER and HDS could not be discerned.